• Share
  • Email
  • Embed
  • Like
  • Save
  • Private Content
Fluidos  frank m. white- fluid mechanics

Fluidos frank m. white- fluid mechanics







Total Views
Views on SlideShare
Embed Views



0 Embeds 0

No embeds



Upload Details

Uploaded via as Adobe PDF

Usage Rights

© All Rights Reserved

Report content

Flagged as inappropriate Flag as inappropriate
Flag as inappropriate

Select your reason for flagging this presentation as inappropriate.

  • Full Name Full Name Comment goes here.
    Are you sure you want to
    Your message goes here
Post Comment
Edit your comment

    Fluidos  frank m. white- fluid mechanics Fluidos frank m. white- fluid mechanics Document Transcript

    • Fluid Mechanics
    • McGraw-Hill Series in Mechanical EngineeringCONSULTING EDITORSJack P. Holman, Southern Methodist UniversityJohn Lloyd, Michigan State UniversityAndersonComputational Fluid Dynamics: The Basics with ApplicationsAndersonModern Compressible Flow: With Historical PerspectiveAroraIntroduction to Optimum DesignBorman and RaglandCombustion EngineeringBurtonIntroduction to Dynamic Systems AnalysisCulpPrinciples of Energy ConversionDieterEngineering Design: A Materials & Processing ApproachDoebelinEngineering Experimentation: Planning, Execution, ReportingDrielsLinear Control Systems EngineeringEdwards and McKeeFundamentals of Mechanical Component DesignGebhartHeat Conduction and Mass DiffusionGibsonPrinciples of Composite Material MechanicsHamrockFundamentals of Fluid Film LubricationHeywoodInternal Combustion Engine FundamentalsHinzeTurbulenceHistand and AlciatoreIntroduction to Mechatronics and Measurement SystemsHolmanExperimental Methods for EngineersHowell and BuckiusFundamentals of Engineering ThermodynamicsJaluriaDesign and Optimization of Thermal SystemsJuvinallEngineering Considerations of Stress, Strain, and StrengthKays and CrawfordConvective Heat and Mass TransferKellyFundamentals of Mechanical VibrationsKimbrellKinematics Analysis and SynthesisKreider and RablHeating and Cooling of BuildingsMartinKinematics and Dynamics of MachinesMattinglyElements of Gas Turbine PropulsionModestRadiative Heat TransferNortonDesign of MachineryOosthuizen and CarscallenCompressible Fluid FlowOosthuizen and NaylorIntroduction to Convective Heat Transfer AnalysisPhelanFundamentals of Mechanical DesignReddyAn Introduction to Finite Element MethodRosenberg and KarnoppIntroduction to Physical Systems DynamicsSchlichtingBoundary-Layer TheoryShamesMechanics of FluidsShigleyKinematic Analysis of MechanismsShigley and MischkeMechanical Engineering DesignShigley and UickerTheory of Machines and MechanismsStifflerDesign with Microprocessors for Mechanical EngineersStoecker and JonesRefrigeration and Air ConditioningTurnsAn Introduction to Combustion: Concepts and ApplicationsUllmanThe Mechanical Design ProcessWarkAdvanced Thermodynamics for EngineersWark and RichardsThermodynamicsWhiteViscous Fluid FlowZeidCAD/CAM Theory and Practice
    • Fluid MechanicsFourth EditionFrank M. WhiteUniversity of Rhode IslandBoston Burr Ridge, IL Dubuque, IA Madison, WI New York San Francisco St. LouisBangkok Bogotá Caracas Lisbon London MadridMexico City Milan New Delhi Seoul Singapore Sydney Taipei Toronto
    • About the AuthorFrank M. White is Professor of Mechanical and Ocean Engineering at the Universityof Rhode Island. He studied at Georgia Tech and M.I.T. In 1966 he helped found, atURI, the first department of ocean engineering in the country. Known primarily as ateacher and writer, he has received eight teaching awards and has written four text-books on fluid mechanics and heat transfer.During 1979–1990 he was editor-in-chief of the ASME Journal of Fluids Engi-neering and then served from 1991 to 1997 as chairman of the ASME Board of Edi-tors and of the Publications Committee. He is a Fellow of ASME and in 1991 receivedthe ASME Fluids Engineering Award. He lives with his wife, Jeanne, in Narragansett,Rhode Island.v
    • To Jeanne
    • General ApproachxiPrefaceThe fourth edition of this textbook sees some additions and deletions but no philo-sophical change. The basic outline of eleven chapters and five appendices remains thesame. The triad of integral, differential, and experimental approaches is retained andis approached in that order of presentation. The book is intended for an undergraduatecourse in fluid mechanics, and there is plenty of material for a full year of instruction.The author covers the first six chapters and part of Chapter 7 in the introductory se-mester. The more specialized and applied topics from Chapters 7 to 11 are then cov-ered at our university in a second semester. The informal, student-oriented style is re-tained and, if it succeeds, has the flavor of an interactive lecture by the author.Approximately 30 percent of the problem exercises, and some fully worked examples,have been changed or are new. The total number of problem exercises has increasedto more than 1500 in this fourth edition. The focus of the new problems is on practi-cal and realistic fluids engineering experiences. Problems are grouped according totopic, and some are labeled either with an asterisk (especially challenging) or a com-puter-disk icon (where computer solution is recommended). A number of new pho-tographs and figures have been added, especially to illustrate new design applicationsand new instruments.Professor John Cimbala, of Pennsylvania State University, contributed many of thenew problems. He had the great idea of setting comprehensive problems at the end ofeach chapter, covering a broad range of concepts, often from several different chap-ters. These comprehensive problems grow and recur throughout the book as new con-cepts arise. Six more open-ended design projects have been added, making 15 projectsin all. The projects allow the student to set sizes and parameters and achieve good de-sign with more than one approach.An entirely new addition is a set of 95 multiple-choice problems suitable for prepar-ing for the Fundamentals of Engineering (FE) Examination. These FE problems comeat the end of Chapters 1 to 10. Meant as a realistic practice for the actual FE Exam,they are engineering problems with five suggested answers, all of them plausible, butonly one of them correct.Learning Tools
    • Content ChangesNew to this book, and to any fluid mechanics textbook, is a special appendix, Ap-pendix E, Introduction to the Engineering Equation Solver (EES), which is keyed tomany examples and problems throughout the book. The author finds EES to be an ex-tremely attractive tool for applied engineering problems. Not only does it solve arbi-trarily complex systems of equations, written in any order or form, but also it has built-in property evaluations (density, viscosity, enthalpy, entropy, etc.), linear and nonlinearregression, and easily formatted parameter studies and publication-quality plotting. Theauthor is indebted to Professors Sanford Klein and William Beckman, of the Univer-sity of Wisconsin, for invaluable and continuous help in preparing this EES material.The book is now available with or without an EES problems disk. The EES engine isavailable to adopters of the text with the problems disk.Another welcome addition, especially for students, is Answers to Selected Prob-lems. Over 600 answers are provided, or about 43 percent of all the regular problemassignments. Thus a compromise is struck between sometimes having a specific nu-merical goal and sometimes directly applying yourself and hoping for the best result.There are revisions in every chapter. Chapter 1—which is purely introductory andcould be assigned as reading—has been toned down from earlier editions. For ex-ample, the discussion of the fluid acceleration vector has been moved entirely to Chap-ter 4. Four brief new sections have been added: (1) the uncertainty of engineeringdata, (2) the use of EES, (3) the FE Examination, and (4) recommended problem-solving techniques.Chapter 2 has an improved discussion of the stability of floating bodies, with a fullyderived formula for computing the metacentric height. Coverage is confined to staticfluids and rigid-body motions. An improved section on pressure measurement discussesmodern microsensors, such as the fused-quartz bourdon tube, micromachined siliconcapacitive and piezoelectric sensors, and tiny (2 mm long) silicon resonant-frequencydevices.Chapter 3 tightens up the energy equation discussion and retains the plan thatBernoulli’s equation comes last, after control-volume mass, linear momentum, angu-lar momentum, and energy studies. Although some texts begin with an entire chapteron the Bernoulli equation, this author tries to stress that it is a dangerously restrictedrelation which is often misused by both students and graduate engineers.In Chapter 4 a few inviscid and viscous flow examples have been added to the ba-sic partial differential equations of fluid mechanics. More extensive discussion con-tinues in Chapter 8.Chapter 5 is more successful when one selects scaling variables before using the pitheorem. Nevertheless, students still complain that the problems are too ambiguous andlead to too many different parameter groups. Several problem assignments now con-tain a few hints about selecting the repeating variables to arrive at traditional pi groups.In Chapter 6, the “alternate forms of the Moody chart” have been resurrected asproblem assignments. Meanwhile, the three basic pipe-flow problems—pressure drop,flow rate, and pipe sizing—can easily be handled by the EES software, and examplesare given. Some newer flowmeter descriptions have been added for further enrichment.Chapter 7 has added some new data on drag and resistance of various bodies, notablybiological systems which adapt to the flow of wind and water.xii Preface
    • SupplementsEES SoftwareChapter 8 picks up from the sample plane potential flows of Section 4.10 and plungesright into inviscid-flow analysis, especially aerodynamics. The discussion of numeri-cal methods, or computational fluid dynamics (CFD), both inviscid and viscous, steadyand unsteady, has been greatly expanded. Chapter 9, with its myriad complex algebraicequations, illustrates the type of examples and problem assignments which can besolved more easily using EES. A new section has been added about the suborbital X-33 and VentureStar vehicles.In the discussion of open-channel flow, Chapter 10, we have further attempted tomake the material more attractive to civil engineers by adding real-world comprehen-sive problems and design projects from the author’s experience with hydropower proj-ects. More emphasis is placed on the use of friction factors rather than on the Man-ning roughness parameter. Chapter 11, on turbomachinery, has added new material oncompressors and the delivery of gases. Some additional fluid properties and formulashave been included in the appendices, which are otherwise much the same.The all new Instructor’s Resource CD contains a PowerPoint presentation of key textfigures as well as additional helpful teaching tools. The list of films and videos, for-merly App. C, is now omitted and relegated to the Instructor’s Resource CD.The Solutions Manual provides complete and detailed solutions, including prob-lem statements and artwork, to the end-of-chapter problems. It may be photocopied forposting or preparing transparencies for the classroom.The Engineering Equation Solver (EES) was developed by Sandy Klein and Bill Beck-man, both of the University of Wisconsin—Madison. A combination of equation-solvingcapability and engineering property data makes EES an extremely powerful tool for yourstudents. EES (pronounced “ease”) enables students to solve problems, especially designproblems, and to ask “what if” questions. EES can do optimization, parametric analysis,linear and nonlinear regression, and provide publication-quality plotting capability. Sim-ple to master, this software allows you to enter equations in any form and in any order. Itautomatically rearranges the equations to solve them in the most efficient manner.EES is particularly useful for fluid mechanics problems since much of the propertydata needed for solving problems in these areas are provided in the program. Air ta-bles are built-in, as are psychometric functions and JointArmy NavyAir Force (JANAF)table data for many common gases. Transport properties are also provided for all sub-stances. EES allows the user to enter property data or functional relationships writtenin Pascal, C, Cϩϩ, or Fortran. The EES engine is available free to qualified adoptersvia a password-protected website, to those who adopt the text with the problems disk.The program is updated every semester.The EES software problems disk provides examples of typical problems in this text.Problems solved are denoted in the text with a disk symbol. Each fully documentedsolution is actually an EES program that is run using the EES engine. Each programprovides detailed comments and on-line help. These programs illustrate the use of EESand help the student master the important concepts without the calculational burdenthat has been previously required.Preface xiii
    • Acknowledgments So many people have helped me, in addition to Professors John Cimbala, Sanford Klein,and William Beckman, that I cannot remember or list them all. I would like to expressmy appreciation to many reviewers and correspondents who gave detailed suggestionsand materials: Osama Ibrahim, University of Rhode Island; Richard Lessmann, Uni-versity of Rhode Island; William Palm, University of Rhode Island; Deborah Pence,University of Rhode Island; Stuart Tison, National Institute of Standards and Technol-ogy; Paul Lupke, Druck Inc.; Ray Worden, Russka, Inc.; Amy Flanagan, Russka, Inc.;Søren Thalund, Greenland Tourism a/s; Eric Bjerregaard, Greenland Tourism a/s; Mar-tin Girard, DH Instruments, Inc.; Michael Norton, Nielsen-Kellerman Co.; LisaColomb, Johnson-Yokogawa Corp.; K. Eisele, Sulzer Innotec, Inc.; Z. Zhang, SultzerInnotec, Inc.; Helen Reed, Arizona State University; F. Abdel Azim El-Sayed, ZagazigUniversity; Georges Aigret, Chimay, Belgium; X. He, Drexel University; Robert Lo-erke, Colorado State University; Tim Wei, Rutgers University; Tom Conlisk, Ohio StateUniversity; David Nelson, Michigan Technological University; Robert Granger, U.S.Naval Academy; Larry Pochop, University of Wyoming; Robert Kirchhoff, Universityof Massachusetts; Steven Vogel, Duke University; Capt. Jason Durfee, U.S. MilitaryAcademy; Capt. Mark Wilson, U.S. Military Academy; Sheldon Green, University ofBritish Columbia; Robert Martinuzzi, University of Western Ontario; Joel Ferziger,Stanford University; Kishan Shah, Stanford University; Jack Hoyt, San Diego StateUniversity; Charles Merkle, Pennsylvania State University; Ram Balachandar, Univer-sity of Saskatchewan; Vincent Chu, McGill University; and David Bogard, Universityof Texas at Austin.The editorial and production staff at WCB McGraw-Hill have been most helpfulthroughout this project. Special thanks go to Debra Riegert, Holly Stark, MargaretRathke, Michael Warrell, Heather Burbridge, Sharon Miller, Judy Feldman, and Jen-nifer Frazier. Finally, I continue to enjoy the support of my wife and family in thesewriting efforts.xiv Preface
    • Preface xiChapter 1Introduction 31.1 Preliminary Remarks 31.2 The Concept of a Fluid 41.3 The Fluid as a Continuum 61.4 Dimensions and Units 71.5 Properties of the Velocity Field 141.6 Thermodynamic Properties of a Fluid 161.7 Viscosity and Other Secondary Properties 221.8 Basic Flow-Analysis Techniques 351.9 Flow Patterns: Streamlines, Streaklines, andPathlines 371.10 The Engineering Equation Solver 411.11 Uncertainty of Experimental Data 421.12 The Fundamentals of Engineering (FE) Examination 431.13 Problem-Solving Techniques 441.14 History and Scope of Fluid Mechanics 44Problems 46Fundamentals of Engineering Exam Problems 53Comprehensive Problems 54References 55Chapter 2Pressure Distribution in a Fluid 592.1 Pressure and Pressure Gradient 592.2 Equilibrium of a Fluid Element 612.3 Hydrostatic Pressure Distributions 632.4 Application to Manometry 702.5 Hydrostatic Forces on Plane Surfaces 74viiContents2.6 Hydrostatic Forces on Curved Surfaces 792.7 Hydrostatic Forces in Layered Fluids 822.8 Buoyancy and Stability 842.9 Pressure Distribution in Rigid-Body Motion 892.10 Pressure Measurement 97Summary 100Problems 102Word Problems 125Fundamentals of Engineering Exam Problems 125Comprehensive Problems 126Design Projects 127References 127Chapter 3Integral Relations for a Control Volume 1293.1 Basic Physical Laws of Fluid Mechanics 1293.2 The Reynolds Transport Theorem 1333.3 Conservation of Mass 1413.4 The Linear Momentum Equation 1463.5 The Angular-Momentum Theorem 1583.6 The Energy Equation 1633.7 Frictionless Flow: The Bernoulli Equation 174Summary 183Problems 184Word Problems 210Fundamentals of Engineering Exam Problems 210Comprehensive Problems 211Design Project 212References 213
    • Chapter 4Differential Relations for a Fluid Particle 2154.1 The Acceleration Field of a Fluid 2154.2 The Differential Equation of Mass Conservation 2174.3 The Differential Equation of Linear Momentum 2234.4 The Differential Equation of Angular Momentum 2304.5 The Differential Equation of Energy 2314.6 Boundary Conditions for the Basic Equations 2344.7 The Stream Function 2384.8 Vorticity and Irrotationality 2454.9 Frictionless Irrotational Flows 2474.10 Some Illustrative Plane Potential Flows 2524.11 Some Illustrative Incompressible Viscous Flows 258Summary 263Problems 264Word Problems 273Fundamentals of Engineering Exam Problems 273Comprehensive Applied Problem 274References 275Chapter 5Dimensional Analysis and Similarity 2775.1 Introduction 2775.2 The Principle of Dimensional Homogeneity 2805.3 The Pi Theorem 2865.4 Nondimensionalization of the Basic Equations 2925.5 Modeling and Its Pitfalls 301Summary 311Problems 311Word Problems 318Fundamentals of Engineering Exam Problems 319Comprehensive Problems 319Design Projects 320References 321Chapter 6Viscous Flow in Ducts 3256.1 Reynolds-Number Regimes 3256.2 Internal versus External Viscous Flows 3306.3 Semiempirical Turbulent Shear Correlations 3336.4 Flow in a Circular Pipe 338viii Contents6.5 Three Types of Pipe-Flow Problems 3516.6 Flow in Noncircular Ducts 3576.7 Minor Losses in Pipe Systems 3676.8 Multiple-Pipe Systems 3756.9 Experimental Duct Flows: Diffuser Performance 3816.10 Fluid Meters 385Summary 404Problems 405Word Problems 420Fundamentals of Engineering Exam Problems 420Comprehensive Problems 421Design Projects 422References 423Chapter 7Flow Past Immersed Bodies 4277.1 Reynolds-Number and Geometry Effects 4277.2 Momentum-Integral Estimates 4317.3 The Boundary-Layer Equations 4347.4 The Flat-Plate Boundary Layer 4367.5 Boundary Layers with Pressure Gradient 4457.6 Experimental External Flows 451Summary 476Problems 476Word Problems 489Fundamentals of Engineering Exam Problems 489Comprehensive Problems 490Design Project 491References 491Chapter 8Potential Flow and Computational Fluid Dynamics 4958.1 Introduction and Review 4958.2 Elementary Plane-Flow Solutions 4988.3 Superposition of Plane-Flow Solutions 5008.4 Plane Flow Past Closed-Body Shapes 5078.5 Other Plane Potential Flows 5168.6 Images 5218.7 Airfoil Theory 5238.8 Axisymmetric Potential Flow 5348.9 Numerical Analysis 540Summary 555
    • Problems 555Word Problems 566Comprehensive Problems 566Design Projects 567References 567Chapter 9Compressible Flow 5719.1 Introduction 5719.2 The Speed of Sound 5759.3 Adiabatic and Isentropic Steady Flow 5789.4 Isentropic Flow with Area Changes 5839.5 The Normal-Shock Wave 5909.6 Operation of Converging and Diverging Nozzles 5989.7 Compressible Duct Flow with Friction 6039.8 Frictionless Duct Flow with Heat Transfer 6139.9 Two-Dimensional Supersonic Flow 6189.10 Prandtl-Meyer Expansion Waves 628Summary 640Problems 641Word Problems 653Fundamentals of Engineering Exam Problems 653Comprehensive Problems 654Design Projects 654References 655Chapter 10Open-Channel Flow 65910.1 Introduction 65910.2 Uniform Flow; the Chézy Formula 66410.3 Efficient Uniform-Flow Channels 66910.4 Specific Energy; Critical Depth 67110.5 The Hydraulic Jump 67810.6 Gradually Varied Flow 68210.7 Flow Measurement and Control by Weirs 687Summary 695Contents ixProblems 695Word Problems 706Fundamentals of Engineering Exam Problems 707Comprehensive Problems 707Design Projects 707References 708Chapter 11Turbomachinery 71111.1 Introduction and Classification 71111.2 The Centrifugal Pump 71411.3 Pump Performance Curves and Similarity Rules 72011.4 Mixed- and Axial-Flow Pumps:The Specific Speed 72911.5 Matching Pumps to System Characteristics 73511.6 Turbines 742Summary 755Problems 755Word Problems 765Comprehensive Problems 766Design Project 767References 767Appendix A Physical Properties of Fluids 769Appendix B Compressible-Flow Tables 774Appendix C Conversion Factors 791Appendix D Equations of Motion in CylindricalCoordinates 793Appendix E Introduction to EES 795Answers to Selected Problems 806Index 813
    • Hurricane Elena in the Gulf of Mexico. Unlike most small-scale fluids engineering applications,hurricanes are strongly affected by the Coriolis acceleration due to the rotation of the earth, whichcauses them to swirl counterclockwise in the Northern Hemisphere. The physical properties andboundary conditions which govern such flows are discussed in the present chapter. (Courtesy ofNASA/Color-Pic Inc./E.R. Degginger/Color-Pic Inc.)2
    • 1.1 Preliminary Remarks Fluid mechanics is the study of fluids either in motion (fluid dynamics) or at rest (fluidstatics) and the subsequent effects of the fluid upon the boundaries, which may be ei-ther solid surfaces or interfaces with other fluids. Both gases and liquids are classifiedas fluids, and the number of fluids engineering applications is enormous: breathing,blood flow, swimming, pumps, fans, turbines, airplanes, ships, rivers, windmills, pipes,missiles, icebergs, engines, filters, jets, and sprinklers, to name a few. When you thinkabout it, almost everything on this planet either is a fluid or moves within or near afluid.The essence of the subject of fluid flow is a judicious compromise between theoryand experiment. Since fluid flow is a branch of mechanics, it satisfies a set of well-documented basic laws, and thus a great deal of theoretical treatment is available. How-ever, the theory is often frustrating, because it applies mainly to idealized situationswhich may be invalid in practical problems. The two chief obstacles to a workable the-ory are geometry and viscosity. The basic equations of fluid motion (Chap. 4) are toodifficult to enable the analyst to attack arbitrary geometric configurations. Thus mosttextbooks concentrate on flat plates, circular pipes, and other easy geometries. It is pos-sible to apply numerical computer techniques to complex geometries, and specializedtextbooks are now available to explain the new computational fluid dynamics (CFD)approximations and methods [1, 2, 29].1This book will present many theoretical re-sults while keeping their limitations in mind.The second obstacle to a workable theory is the action of viscosity, which can beneglected only in certain idealized flows (Chap. 8). First, viscosity increases the diffi-culty of the basic equations, although the boundary-layer approximation found by Lud-wig Prandtl in 1904 (Chap. 7) has greatly simplified viscous-flow analyses. Second,viscosity has a destabilizing effect on all fluids, giving rise, at frustratingly small ve-locities, to a disorderly, random phenomenon called turbulence. The theory of turbu-lent flow is crude and heavily backed up by experiment (Chap. 6), yet it can be quiteserviceable as an engineering estimate. Textbooks now present digital-computer tech-niques for turbulent-flow analysis [32], but they are based strictly upon empirical as-sumptions regarding the time mean of the turbulent stress field.Chapter 1Introduction31Numbered references appear at the end of each chapter.
    • 1.2 The Concept of a FluidThus there is theory available for fluid-flow problems, but in all cases it should bebacked up by experiment. Often the experimental data provide the main source of in-formation about specific flows, such as the drag and lift of immersed bodies (Chap. 7).Fortunately, fluid mechanics is a highly visual subject, with good instrumentation [4,5, 35], and the use of dimensional analysis and modeling concepts (Chap. 5) is wide-spread. Thus experimentation provides a natural and easy complement to the theory.You should keep in mind that theory and experiment should go hand in hand in allstudies of fluid mechanics.From the point of view of fluid mechanics, all matter consists of only two states, fluidand solid. The difference between the two is perfectly obvious to the layperson, and itis an interesting exercise to ask a layperson to put this difference into words. The tech-nical distinction lies with the reaction of the two to an applied shear or tangential stress.A solid can resist a shear stress by a static deformation; a fluid cannot. Any shearstress applied to a fluid, no matter how small, will result in motion of that fluid. Thefluid moves and deforms continuously as long as the shear stress is applied. As a corol-lary, we can say that a fluid at rest must be in a state of zero shear stress, a state of-ten called the hydrostatic stress condition in structural analysis. In this condition, Mohr’scircle for stress reduces to a point, and there is no shear stress on any plane cut throughthe element under stress.Given the definition of a fluid above, every layperson also knows that there are twoclasses of fluids, liquids and gases. Again the distinction is a technical one concerningthe effect of cohesive forces. A liquid, being composed of relatively close-packed mol-ecules with strong cohesive forces, tends to retain its volume and will form a free sur-face in a gravitational field if unconfined from above. Free-surface flows are domi-nated by gravitational effects and are studied in Chaps. 5 and 10. Since gas moleculesare widely spaced with negligible cohesive forces, a gas is free to expand until it en-counters confining walls. A gas has no definite volume, and when left to itself with-out confinement, a gas forms an atmosphere which is essentially hydrostatic. The hy-drostatic behavior of liquids and gases is taken up in Chap. 2. Gases cannot form afree surface, and thus gas flows are rarely concerned with gravitational effects otherthan buoyancy.Figure 1.1 illustrates a solid block resting on a rigid plane and stressed by its ownweight. The solid sags into a static deflection, shown as a highly exaggerated dashedline, resisting shear without flow. A free-body diagram of element A on the side of theblock shows that there is shear in the block along a plane cut at an angle ␪ through A.Since the block sides are unsupported, element A has zero stress on the left and rightsides and compression stress ␴ ϭ Ϫp on the top and bottom. Mohr’s circle does notreduce to a point, and there is nonzero shear stress in the block.By contrast, the liquid and gas at rest in Fig. 1.1 require the supporting walls in or-der to eliminate shear stress. The walls exert a compression stress of Ϫp and reduceMohr’s circle to a point with zero shear everywhere, i.e., the hydrostatic condition. Theliquid retains its volume and forms a free surface in the container. If the walls are re-moved, shear develops in the liquid and a big splash results. If the container is tilted,shear again develops, waves form, and the free surface seeks a horizontal configura-4 Chapter 1 Introduction
    • tion, pouring out over the lip if necessary. Meanwhile, the gas is unrestrained and ex-pands out of the container, filling all available space. Element A in the gas is also hy-drostatic and exerts a compression stress Ϫp on the walls.In the above discussion, clear decisions could be made about solids, liquids, andgases. Most engineering fluid-mechanics problems deal with these clear cases, i.e., thecommon liquids, such as water, oil, mercury, gasoline, and alcohol, and the commongases, such as air, helium, hydrogen, and steam, in their common temperature and pres-sure ranges. There are many borderline cases, however, of which you should be aware.Some apparently “solid” substances such as asphalt and lead resist shear stress for shortperiods but actually deform slowly and exhibit definite fluid behavior over long peri-ods. Other substances, notably colloid and slurry mixtures, resist small shear stressesbut “yield” at large stress and begin to flow as fluids do. Specialized textbooks are de-voted to this study of more general deformation and flow, a field called rheology [6].Also, liquids and gases can coexist in two-phase mixtures, such as steam-water mix-tures or water with entrapped air bubbles. Specialized textbooks present the analysis1.2 The Concept of a Fluid 5StaticdeflectionFreesurfaceHydrostaticconditionLiquidSolidA A A(a) (c)(b) (d)00A AGas(1)– p – pppp= 0τθθθ21– = p – = pσσ1τστστσFig. 1.1 A solid at rest can resistshear. (a) Static deflection of thesolid; (b) equilibrium and Mohr’scircle for solid element A. A fluidcannot resist shear. (c) Containingwalls are needed; (d) equilibriumand Mohr’s circle for fluidelement A.
    • 1.3 The Fluid as a Continuumof such two-phase flows [7]. Finally, there are situations where the distinction betweena liquid and a gas blurs. This is the case at temperatures and pressures above the so-called critical point of a substance, where only a single phase exists, primarily resem-bling a gas. As pressure increases far above the critical point, the gaslike substance be-comes so dense that there is some resemblance to a liquid and the usual thermodynamicapproximations like the perfect-gas law become inaccurate. The critical temperatureand pressure of water are Tc ϭ 647 K and pc ϭ 219 atm,2so that typical problems in-volving water and steam are below the critical point. Air, being a mixture of gases, hasno distinct critical point, but its principal component, nitrogen, has Tc ϭ 126 K andpc ϭ 34 atm. Thus typical problems involving air are in the range of high temperatureand low pressure where air is distinctly and definitely a gas. This text will be concernedsolely with clearly identifiable liquids and gases, and the borderline cases discussedabove will be beyond our scope.We have already used technical terms such as fluid pressure and density without a rig-orous discussion of their definition. As far as we know, fluids are aggregations of mol-ecules, widely spaced for a gas, closely spaced for a liquid. The distance between mol-ecules is very large compared with the molecular diameter. The molecules are not fixedin a lattice but move about freely relative to each other. Thus fluid density, or mass perunit volume, has no precise meaning because the number of molecules occupying agiven volume continually changes. This effect becomes unimportant if the unit volumeis large compared with, say, the cube of the molecular spacing, when the number ofmolecules within the volume will remain nearly constant in spite of the enormous in-terchange of particles across the boundaries. If, however, the chosen unit volume is toolarge, there could be a noticeable variation in the bulk aggregation of the particles. Thissituation is illustrated in Fig. 1.2, where the “density” as calculated from molecularmass ␦m within a given volume ␦ᐂ is plotted versus the size of the unit volume. Thereis a limiting volume ␦ᐂ* below which molecular variations may be important and6 Chapter 1 IntroductionMicroscopicuncertaintyMacroscopicuncertainty01200δδ * ≈ 10-9mm3ElementalvolumeRegion containing fluid= 1000 kg/m3= 1100= 1200= 1300(a) (b)ρρρρρδFig. 1.2 The limit definition of con-tinuum fluid density: (a) an ele-mental volume in a fluid region ofvariable continuum density; (b) cal-culated density versus size of theelemental volume.2One atmosphere equals 2116 lbf/ft2ϭ 101,300 Pa.
    • 1.4 Dimensions and Unitsabove which aggregate variations may be important. The density ␳ of a fluid is bestdefined as␳ ϭ lim␦ᐂ→␦ᐂ*ᎏ␦␦ᐂmᎏ (1.1)The limiting volume ␦ᐂ* is about 10Ϫ9mm3for all liquids and for gases at atmosphericpressure. For example, 10Ϫ9mm3of air at standard conditions contains approximately3 ϫ 107molecules, which is sufficient to define a nearly constant density according toEq. (1.1). Most engineering problems are concerned with physical dimensions much largerthan this limiting volume, so that density is essentially a point function and fluid proper-ties can be thought of as varying continually in space, as sketched in Fig. 1.2a. Such afluid is called a continuum, which simply means that its variation in properties is so smooththat the differential calculus can be used to analyze the substance. We shall assume thatcontinuum calculus is valid for all the analyses in this book. Again there are borderlinecases for gases at such low pressures that molecular spacing and mean free path3are com-parable to, or larger than, the physical size of the system. This requires that the contin-uum approximation be dropped in favor of a molecular theory of rarefied-gas flow [8]. Inprinciple, all fluid-mechanics problems can be attacked from the molecular viewpoint, butno such attempt will be made here. Note that the use of continuum calculus does not pre-clude the possibility of discontinuous jumps in fluid properties across a free surface orfluid interface or across a shock wave in a compressible fluid (Chap. 9). Our calculus inChap. 4 must be flexible enough to handle discontinuous boundary conditions.A dimension is the measure by which a physical variable is expressed quantitatively.A unit is a particular way of attaching a number to the quantitative dimension. Thuslength is a dimension associated with such variables as distance, displacement, width,deflection, and height, while centimeters and inches are both numerical units for ex-pressing length. Dimension is a powerful concept about which a splendid tool calleddimensional analysis has been developed (Chap. 5), while units are the nitty-gritty, thenumber which the customer wants as the final answer.Systems of units have always varied widely from country to country, even after in-ternational agreements have been reached. Engineers need numbers and therefore unitsystems, and the numbers must be accurate because the safety of the public is at stake.You cannot design and build a piping system whose diameter is D and whose lengthis L. And U.S. engineers have persisted too long in clinging to British systems of units.There is too much margin for error in most British systems, and many an engineeringstudent has flunked a test because of a missing or improper conversion factor of 12 or144 or 32.2 or 60 or 1.8. Practicing engineers can make the same errors. The writer isaware from personal experience of a serious preliminary error in the design of an air-craft due to a missing factor of 32.2 to convert pounds of mass to slugs.In 1872 an international meeting in France proposed a treaty called the Metric Con-vention, which was signed in 1875 by 17 countries including the United States. It wasan improvement over British systems because its use of base 10 is the foundation ofour number system, learned from childhood by all. Problems still remained because1.4 Dimensions and Units 73The mean distance traveled by molecules between collisions.
    • even the metric countries differed in their use of kiloponds instead of dynes or new-tons, kilograms instead of grams, or calories instead of joules. To standardize the met-ric system, a General Conference of Weights and Measures attended in 1960 by 40countries proposed the International System of Units (SI). We are now undergoing apainful period of transition to SI, an adjustment which may take many more years tocomplete. The professional societies have led the way. Since July 1, 1974, SI units havebeen required by all papers published by the American Society of Mechanical Engi-neers, which prepared a useful booklet explaining the SI [9]. The present text will useSI units together with British gravitational (BG) units.In fluid mechanics there are only four primary dimensions from which all other dimen-sions can be derived: mass, length, time, and temperature.4These dimensions and their unitsin both systems are given in Table 1.1. Note that the kelvin unit uses no degree symbol.The braces around a symbol like {M} mean “the dimension” of mass. All other variablesin fluid mechanics can be expressed in terms of {M}, {L}, {T}, and {⌰}. For example, ac-celeration has the dimensions {LTϪ2}. The most crucial of these secondary dimensions isforce, which is directly related to mass, length, and time by Newton’s second lawF ϭ ma (1.2)From this we see that, dimensionally, {F} ϭ {MLTϪ2}. A constant of proportionalityis avoided by defining the force unit exactly in terms of the primary units. Thus wedefine the newton and the pound of force1 newton of force ϭ 1 N ϵ 1 kg и m/s2(1.3)1 pound of force ϭ 1 lbf ϵ 1 slug и ft/s2ϭ 4.4482 NIn this book the abbreviation lbf is used for pound-force and lb for pound-mass. If in-stead one adopts other force units such as the dyne or the poundal or kilopond or adoptsother mass units such as the gram or pound-mass, a constant of proportionality calledgc must be included in Eq. (1.2). We shall not use gc in this book since it is not nec-essary in the SI and BG systems.A list of some important secondary variables in fluid mechanics, with dimensionsderived as combinations of the four primary dimensions, is given in Table 1.2. A morecomplete list of conversion factors is given in App. C.8 Chapter 1 Introduction4If electromagnetic effects are important, a fifth primary dimension must be included, electric current{I}, whose SI unit is the ampere (A).Primary dimension SI unit BG unit Conversion factorMass {M} Kilogram (kg) Slug 1 slug ϭ 14.5939 kgLength {L} Meter (m) Foot (ft) 1 ft ϭ 0.3048 mTime {T} Second (s) Second (s) 1 s ϭ 1 sTemperature {⌰} Kelvin (K) Rankine (°R) 1 K ϭ 1.8°RTable 1.1 Primary Dimensions inSI and BG SystemsPrimary Dimensions
    • Part (a)Part (b)Part (c)EXAMPLE 1.1A body weighs 1000 lbf when exposed to a standard earth gravity g ϭ 32.174 ft/s2. (a) What isits mass in kg? (b) What will the weight of this body be in N if it is exposed to the moon’s stan-dard acceleration gmoon ϭ 1.62 m/s2? (c) How fast will the body accelerate if a net force of 400lbf is applied to it on the moon or on the earth?SolutionEquation (1.2) holds with F ϭ weight and a ϭ gearth:F ϭ W ϭ mg ϭ 1000 lbf ϭ (m slugs)(32.174 ft/s2)orm ϭ ᎏ3120.10704ᎏ ϭ (31.08 slugs)(14.5939 kg/slug) ϭ 453.6 kg Ans. (a)The change from 31.08 slugs to 453.6 kg illustrates the proper use of the conversion factor14.5939 kg/slug.The mass of the body remains 453.6 kg regardless of its location. Equation (1.2) applies with anew value of a and hence a new forceF ϭ Wmoon ϭ mgmoon ϭ (453.6 kg)(1.62 m/s2) ϭ 735 N Ans. (b)This problem does not involve weight or gravity or position and is simply a direct applicationof Newton’s law with an unbalanced force:F ϭ 400 lbf ϭ ma ϭ (31.08 slugs)(a ft/s2)ora ϭ ᎏ3410.008ᎏ ϭ 12.43 ft/s2ϭ 3.79 m/s2Ans. (c)This acceleration would be the same on the moon or earth or anywhere.1.4 Dimensions and Units 9Secondary dimension SI unit BG unit Conversion factorArea {L2} m2ft21 m2ϭ 10.764 ft2Volume {L3} m3ft31 m3ϭ 35.315 ft3Velocity {LTϪ1} m/s ft/s 1 ft/s ϭ 0.3048 m/sAcceleration {LTϪ2} m/s2ft/s21 ft/s2ϭ 0.3048 m/s2Pressure or stress{MLϪ1TϪ2} Pa ϭ N/m2lbf/ft21 lbf/ft2ϭ 47.88 PaAngular velocity {TϪ1} sϪ1sϪ11 sϪ1ϭ 1 sϪ1Energy, heat, work{ML2TϪ2} J ϭ N и m ft и lbf 1 ft и lbf ϭ 1.3558 JPower {ML2TϪ3} W ϭ J/s ft и lbf/s 1 ft и lbf/s ϭ 1.3558 WDensity {MLϪ3} kg/m3slugs/ft31 slug/ft3ϭ 515.4 kg/m3Viscosity {MLϪ1TϪ1} kg/(m и s) slugs/(ft и s) 1 slug/(ft и s) ϭ 47.88 kg/(m и s)Specific heat {L2TϪ2⌰Ϫ1} m2/(s2и K) ft2/(s2и °R) 1 m2/(s2и K) ϭ 5.980 ft2/(s2и °R)Table 1.2 Secondary Dimensions inFluid Mechanics
    • Part (a)Part (b)Many data in the literature are reported in inconvenient or arcane units suitable onlyto some industry or specialty or country. The engineer should convert these data to theSI or BG system before using them. This requires the systematic application of con-version factors, as in the following example.EXAMPLE 1.2An early viscosity unit in the cgs system is the poise (abbreviated P), or g/(cm и s), named afterJ. L. M. Poiseuille, a French physician who performed pioneering experiments in 1840 on wa-ter flow in pipes. The viscosity of water (fresh or salt) at 293.16 K ϭ 20°C is approximately␮ ϭ 0.01 P. Express this value in (a) SI and (b) BG units.Solution␮ ϭ [0.01 g/(cm и s)] ᎏ1010k0ggᎏ (100 cm/m) ϭ 0.001 kg/(m и s) Ans. (a)␮ ϭ [0.001 kg/(m и s)] ᎏ141.5sl9ugkgᎏ (0.3048 m/ft)ϭ 2.09 ϫ 10Ϫ5slug/(ft и s) Ans. (b)Note: Result (b) could have been found directly from (a) by dividing (a) by the viscosity con-version factor 47.88 listed in Table 1.2.We repeat our advice: Faced with data in unusual units, convert them immediatelyto either SI or BG units because (1) it is more professional and (2) theoretical equa-tions in fluid mechanics are dimensionally consistent and require no further conversionfactors when these two fundamental unit systems are used, as the following exampleshows.EXAMPLE 1.3A useful theoretical equation for computing the relation between pressure, velocity, and altitudein a steady flow of a nearly inviscid, nearly incompressible fluid with negligible heat transferand shaft work5is the Bernoulli relation, named after Daniel Bernoulli, who published a hy-drodynamics textbook in 1738:p0 ϭ p ϩ ᎏ12ᎏ␳V2ϩ ␳gZ (1)where p0 ϭ stagnation pressurep ϭ pressure in moving fluidV ϭ velocity␳ ϭ densityZ ϭ altitudeg ϭ gravitational acceleration10 Chapter 1 Introduction5That’s an awful lot of assumptions, which need further study in Chap. 3.
    • Part (a)Part (b)Part (c)(a) Show that Eq. (1) satisfies the principle of dimensional homogeneity, which states that alladditive terms in a physical equation must have the same dimensions. (b) Show that consistentunits result without additional conversion factors in SI units. (c) Repeat (b) for BG units.SolutionWe can express Eq. (1) dimensionally, using braces by entering the dimensions of each termfrom Table 1.2:{MLϪ1TϪ2} ϭ {MLϪ1TϪ2} ϩ {MLϪ3}{L2TϪ2} ϩ {MLϪ3}{LTϪ2}{L}ϭ {MLϪ1TϪ2} for all terms Ans. (a)Enter the SI units for each quantity from Table 1.2:{N/m2} ϭ {N/m2} ϩ {kg/m3}{m2/s2} ϩ {kg/m3}{m/s2}{m}ϭ {N/m2} ϩ {kg/(m и s2)}The right-hand side looks bad until we remember from Eq. (1.3) that 1 kg ϭ 1 N и s2/m.{kg/(m и s2)} ϭ ᎏ{N{mи sи2s/2m}}ᎏ ϭ {N/m2} Ans. (b)Thus all terms in Bernoulli’s equation will have units of pascals, or newtons per square meter,when SI units are used. No conversion factors are needed, which is true of all theoretical equa-tions in fluid mechanics.Introducing BG units for each term, we have{lbf/ft2} ϭ {lbf/ft2} ϩ {slugs/ft3}{ft2/s2} ϩ {slugs/ft3}{ft/s2}{ft}ϭ {lbf/ft2} ϩ {slugs/(ft и s2)}But, from Eq. (1.3), 1 slug ϭ 1 lbf и s2/ft, so that{slugs/(ft и s2)} ϭ ᎏ{l{bfftииss22/}ft}ᎏ ϭ {lbf/ft2} Ans. (c)All terms have the unit of pounds-force per square foot. No conversion factors are needed in theBG system either.There is still a tendency in English-speaking countries to use pound-force per squareinch as a pressure unit because the numbers are more manageable. For example, stan-dard atmospheric pressure is 14.7 lbf/in2ϭ 2116 lbf/ft2ϭ 101,300 Pa. The pascal is asmall unit because the newton is less than ᎏ14ᎏ lbf and a square meter is a very large area.It is felt nevertheless that the pascal will gradually gain universal acceptance; e.g., re-pair manuals for U.S. automobiles now specify pressure measurements in pascals.Note that not only must all (fluid) mechanics equations be dimensionally homogeneous,one must also use consistent units; that is, each additive term must have the same units.There is no trouble doing this with the SI and BG systems, as in Ex. 1.3, but woe unto1.4 Dimensions and Units 11Consistent Units
    • Homogeneous versusDimensionally InconsistentEquationsthose who try to mix colloquial English units. For example, in Chap. 9, we often usethe assumption of steady adiabatic compressible gas flow:h ϩ ᎏ12ᎏV2ϭ constantwhere h is the fluid enthalpy and V2/2 is its kinetic energy. Colloquial thermodynamictables might list h in units of British thermal units per pound (Btu/lb), whereas V islikely used in ft/s. It is completely erroneous to add Btu/lb to ft2/s2. The proper unitfor h in this case is ft и lbf/slug, which is identical to ft2/s2. The conversion factor is1 Btu/lb Ϸ 25,040 ft2/s2ϭ 25,040 ft и lbf/slug.All theoretical equations in mechanics (and in other physical sciences) are dimension-ally homogeneous; i.e., each additive term in the equation has the same dimensions.For example, Bernoulli’s equation (1) in Example 1.3 is dimensionally homogeneous:Each term has the dimensions of pressure or stress of {F/L2}. Another example is theequation from physics for a body falling with negligible air resistance:S ϭ S0 ϩ V0t ϩ ᎏ12ᎏgt2where S0 is initial position, V0 is initial velocity, and g is the acceleration of gravity. Eachterm in this relation has dimensions of length {L}. The factor ᎏ12ᎏ, which arises from inte-gration, is a pure (dimensionless) number, {1}. The exponent 2 is also dimensionless.However, the reader should be warned that many empirical formulas in the engi-neering literature, arising primarily from correlations of data, are dimensionally in-consistent. Their units cannot be reconciled simply, and some terms may contain hid-den variables. An example is the formula which pipe valve manufacturers cite for liquidvolume flow rate Q (m3/s) through a partially open valve:Q ϭ CV΂ᎏS⌬Gpᎏ΃1/2where ⌬p is the pressure drop across the valve and SG is the specific gravity of theliquid (the ratio of its density to that of water). The quantity CV is the valve flow co-efficient, which manufacturers tabulate in their valve brochures. Since SG is dimen-sionless {1}, we see that this formula is totally inconsistent, with one side being a flowrate {L3/T} and the other being the square root of a pressure drop {M1/2/L1/2T}. It fol-lows that CV must have dimensions, and rather odd ones at that: {L7/2/M1/2}. Nor isthe resolution of this discrepancy clear, although one hint is that the values of CV inthe literature increase nearly as the square of the size of the valve. The presentation ofexperimental data in homogeneous form is the subject of dimensional analysis (Chap.5). There we shall learn that a homogeneous form for the valve flow relation isQ ϭ Cd Aopening΂ᎏ⌬␳pᎏ΃1/2where ␳ is the liquid density and A the area of the valve opening. The discharge coeffi-cient Cd is dimensionless and changes only slightly with valve size. Please believe—un-til we establish the fact in Chap. 5—that this latter is a much better formulation of the data.12 Chapter 1 Introduction
    • Convenient Prefixes inPowers of 10Part (a)Part (b)Meanwhile, we conclude that dimensionally inconsistent equations, though theyabound in engineering practice, are misleading and vague and even dangerous, in thesense that they are often misused outside their range of applicability.Engineering results often are too small or too large for the common units, with toomany zeros one way or the other. For example, to write p ϭ 114,000,000 Pa is longand awkward. Using the prefix “M” to mean 106, we convert this to a concise p ϭ114 MPa (megapascals). Similarly, t ϭ 0.000000003 s is a proofreader’s nightmarecompared to the equivalent t ϭ 3 ns (nanoseconds). Such prefixes are common andconvenient, in both the SI and BG systems. A complete list is given in Table 1.3.EXAMPLE 1.4In 1890 Robert Manning, an Irish engineer, proposed the following empirical formula for theaverage velocity V in uniform flow due to gravity down an open channel (BG units):V ϭ ᎏ1.n49ᎏR2/3S1/2(1)where R ϭ hydraulic radius of channel (Chaps. 6 and 10)S ϭ channel slope (tangent of angle that bottom makes with horizontal)n ϭ Manning’s roughness factor (Chap. 10)and n is a constant for a given surface condition for the walls and bottom of the channel. (a) IsManning’s formula dimensionally consistent? (b) Equation (1) is commonly taken to be valid inBG units with n taken as dimensionless. Rewrite it in SI form.SolutionIntroduce dimensions for each term. The slope S, being a tangent or ratio, is dimensionless, de-noted by {unity} or {1}. Equation (1) in dimensional form isΆᎏTLᎏ·ϭΆᎏ1.n49ᎏ·{L2/3}{1}This formula cannot be consistent unless {1.49/n} ϭ {L1/3/T}. If n is dimensionless (and it isnever listed with units in textbooks), then the numerical value 1.49 must have units. This can betragic to an engineer working in a different unit system unless the discrepancy is properly doc-umented. In fact, Manning’s formula, though popular, is inconsistent both dimensionally andphysically and does not properly account for channel-roughness effects except in a narrow rangeof parameters, for water only.From part (a), the number 1.49 must have dimensions {L1/3/T} and thus in BG units equals1.49 ft1/3/s. By using the SI conversion factor for length we have(1.49 ft1/3/s)(0.3048 m/ft)1/3ϭ 1.00 m1/3/sTherefore Manning’s formula in SI becomesV ϭ ᎏ1n.0ᎏR2/3S1/2Ans. (b) (2)1.4 Dimensions and Units 13Table 1.3 Convenient Prefixesfor Engineering UnitsMultiplicativefactor Prefix Symbol1012tera T109giga G106mega M103kilo k102hecto h10 deka da10Ϫ1deci d10Ϫ2centi c10Ϫ3milli m10Ϫ6micro ␮10Ϫ9nano n10Ϫ12pico p10Ϫ15femto f10Ϫ18atto a
    • 1.5 Properties of theVelocity FieldEulerian and LagrangianDesciptionsThe Velocity Fieldwith R in m and V in m/s. Actually, we misled you: This is the way Manning, a metric user, firstproposed the formula. It was later converted to BG units. Such dimensionally inconsistent formu-las are dangerous and should either be reanalyzed or treated as having very limited application.In a given flow situation, the determination, by experiment or theory, of the propertiesof the fluid as a function of position and time is considered to be the solution to theproblem. In almost all cases, the emphasis is on the space-time distribution of the fluidproperties. One rarely keeps track of the actual fate of the specific fluid particles.6Thistreatment of properties as continuum-field functions distinguishes fluid mechanics fromsolid mechanics, where we are more likely to be interested in the trajectories of indi-vidual particles or systems.There are two different points of view in analyzing problems in mechanics. The firstview, appropriate to fluid mechanics, is concerned with the field of flow and is calledthe eulerian method of description. In the eulerian method we compute the pressurefield p(x, y, z, t) of the flow pattern, not the pressure changes p(t) which a particle ex-periences as it moves through the field.The second method, which follows an individual particle moving through the flow,is called the lagrangian description. The lagrangian approach, which is more appro-priate to solid mechanics, will not be treated in this book. However, certain numericalanalyses of sharply bounded fluid flows, such as the motion of isolated fluid droplets,are very conveniently computed in lagrangian coordinates [1].Fluid-dynamic measurements are also suited to the eulerian system. For example,when a pressure probe is introduced into a laboratory flow, it is fixed at a specific po-sition (x, y, z). Its output thus contributes to the description of the eulerian pressurefield p(x, y, z, t). To simulate a lagrangian measurement, the probe would have to movedownstream at the fluid particle speeds; this is sometimes done in oceanographic mea-surements, where flowmeters drift along with the prevailing currents.The two different descriptions can be contrasted in the analysis of traffic flow alonga freeway. A certain length of freeway may be selected for study and called the fieldof flow. Obviously, as time passes, various cars will enter and leave the field, and theidentity of the specific cars within the field will constantly be changing. The traffic en-gineer ignores specific cars and concentrates on their average velocity as a function oftime and position within the field, plus the flow rate or number of cars per hour pass-ing a given section of the freeway. This engineer is using an eulerian description of thetraffic flow. Other investigators, such as the police or social scientists, may be inter-ested in the path or speed or destination of specific cars in the field. By following aspecific car as a function of time, they are using a lagrangian description of the flow.Foremost among the properties of a flow is the velocity field V(x, y, z, t). In fact, de-termining the velocity is often tantamount to solving a flow problem, since other prop-14 Chapter 1 Introduction6One example where fluid-particle paths are important is in water-quality analysis of the fate ofcontaminant discharges.
    • erties follow directly from the velocity field. Chapter 2 is devoted to the calculation ofthe pressure field once the velocity field is known. Books on heat transfer (for exam-ple, Ref. 10) are essentially devoted to finding the temperature field from known ve-locity fields.In general, velocity is a vector function of position and time and thus has three com-ponents u, v, and w, each a scalar field in itself:V(x, y, z, t) ϭ iu(x, y, z, t) ϩ jv(x, y, z, t) ϩ kw(x, y, z, t) (1.4)The use of u, v, and w instead of the more logical component notation Vx, Vy, and Vzis the result of an almost unbreakable custom in fluid mechanics.Several other quantities, called kinematic properties, can be derived by mathemati-cally manipulating the velocity field. We list some kinematic properties here and givemore details about their use and derivation in later chapters:1. Displacement vector: r ϭ ͵V dt (Sec. 1.9)2. Acceleration: a ϭ ᎏddVtᎏ (Sec. 4.1)3. Volume rate of flow: Q ϭ ͵(V ؒ n) dA (Sec. 3.2)4. Volume expansion rate: ᎏᐂ1ᎏ ᎏddᐂtᎏ ϭ ٌ ؒ V (Sec. 4.2)5. Local angular velocity: ␻ ϭ ᎏ12ᎏٌ ϫ V (Sec. 4.8)We will not illustrate any problems regarding these kinematic properties at present. Thepoint of the list is to illustrate the type of vector operations used in fluid mechanics andto make clear the dominance of the velocity field in determining other flow properties.Note: The fluid acceleration, item 2 above, is not as simple as it looks and actually in-volves four different terms due to the use of the chain rule in calculus (see Sec. 4.1).EXAMPLE 1.5Fluid flows through a contracting section of a duct, as in Fig. E1.5. A velocity probe inserted atsection (1) measures a steady value u1 ϭ 1 m/s, while a similar probe at section (2) records asteady u2 ϭ 3 m/s. Estimate the fluid acceleration, if any, if ⌬x ϭ 10 cm.SolutionThe flow is steady (not time-varying), but fluid particles clearly increase in velocity as they passfrom (1) to (2). This is the concept of convective acceleration (Sec. 4.1). We may estimate theacceleration as a velocity change ⌬u divided by a time change ⌬t ϭ ⌬x/uavg:ax Ϸ ᎏvetliomceitychcahnagnegeᎏ Ϸ ϭ Ϸ 40 m/s2Ans.A simple estimate thus indicates that this seemingly innocuous flow is accelerating at 4 times(3.0 Ϫ 1.0 m/s)(1.0 ϩ 3.0 m/s)ᎏᎏᎏᎏ2(0.1 m)u2 Ϫ u1ᎏᎏ⌬x/[ᎏ12ᎏ(u1 ϩ u2)]1.5 Properties of the Velocity Field 15(1)(2)u1u2⌬xE1.5
    • 1.6 Thermodynamic Propertiesof a Fluidthe acceleration of gravity. In the limit as ⌬x and ⌬t become very small, the above estimate re-duces to a partial-derivative expression for convective x-acceleration:ax,convective ϭ lim⌬t→0ᎏ⌬⌬utᎏ ϭ uᎏѨѨuxᎏIn three-dimensional flow (Sec. 4.1) there are nine of these convective terms.While the velocity field V is the most important fluid property, it interacts closely withthe thermodynamic properties of the fluid. We have already introduced into the dis-cussion the three most common such properties1. Pressure p2. Density ␳3. Temperature TThese three are constant companions of the velocity vector in flow analyses. Four otherthermodynamic properties become important when work, heat, and energy balances aretreated (Chaps. 3 and 4):4. Internal energy e5. Enthalpy h ϭ û ϩ p/␳6. Entropy s7. Specific heats cp and cvIn addition, friction and heat conduction effects are governed by the two so-called trans-port properties:8. Coefficient of viscosity ␮9. Thermal conductivity kAll nine of these quantities are true thermodynamic properties which are determinedby the thermodynamic condition or state of the fluid. For example, for a single-phasesubstance such as water or oxygen, two basic properties such as pressure and temper-ature are sufficient to fix the value of all the others:␳ ϭ ␳(p, T) h ϭ h(p, T) ␮ ϭ ␮(p, T) (1.5)and so on for every quantity in the list. Note that the specific volume, so important inthermodynamic analyses, is omitted here in favor of its inverse, the density ␳.Recall that thermodynamic properties describe the state of a system, i.e., a collec-tion of matter of fixed identity which interacts with its surroundings. In most caseshere the system will be a small fluid element, and all properties will be assumed to becontinuum properties of the flow field: ␳ ϭ ␳(x, y, z, t), etc.Recall also that thermodynamics is normally concerned with static systems, whereasfluids are usually in variable motion with constantly changing properties. Do the prop-erties retain their meaning in a fluid flow which is technically not in equilibrium? Theanswer is yes, from a statistical argument. In gases at normal pressure (and even moreso for liquids), an enormous number of molecular collisions occur over a very shortdistance of the order of 1 ␮m, so that a fluid subjected to sudden changes rapidly ad-16 Chapter 1 Introduction
    • TemperatureSpecific WeightDensityjusts itself toward equilibrium. We therefore assume that all the thermodynamic prop-erties listed above exist as point functions in a flowing fluid and follow all the lawsand state relations of ordinary equilibrium thermodynamics. There are, of course, im-portant nonequilibrium effects such as chemical and nuclear reactions in flowing flu-ids which are not treated in this text.Pressure is the (compression) stress at a point in a static fluid (Fig. 1.1). Next to ve-locity, the pressure p is the most dynamic variable in fluid mechanics. Differences orgradients in pressure often drive a fluid flow, especially in ducts. In low-speed flows,the actual magnitude of the pressure is often not important, unless it drops so low as tocause vapor bubbles to form in a liquid. For convenience, we set many such problemassignments at the level of 1 atm ϭ 2116 lbf/ft2ϭ 101,300 Pa. High-speed (compressible)gas flows (Chap. 9), however, are indeed sensitive to the magnitude of pressure.Temperature T is a measure of the internal energy level of a fluid. It may vary con-siderably during high-speed flow of a gas (Chap. 9). Although engineers often use Cel-sius or Fahrenheit scales for convenience, many applications in this text require ab-solute (Kelvin or Rankine) temperature scales:°R ϭ °F ϩ 459.69K ϭ °C ϩ 273.16If temperature differences are strong, heat transfer may be important [10], but our con-cern here is mainly with dynamic effects. We examine heat-transfer principles brieflyin Secs. 4.5 and 9.8.The density of a fluid, denoted by ␳ (lowercase Greek rho), is its mass per unit vol-ume. Density is highly variable in gases and increases nearly proportionally to the pres-sure level. Density in liquids is nearly constant; the density of water (about 1000 kg/m3)increases only 1 percent if the pressure is increased by a factor of 220. Thus most liq-uid flows are treated analytically as nearly “incompressible.”In general, liquids are about three orders of magnitude more dense than gases at at-mospheric pressure. The heaviest common liquid is mercury, and the lightest gas is hy-drogen. Compare their densities at 20°C and 1 atm:Mercury: ␳ ϭ 13,580 kg/m3Hydrogen: ␳ ϭ 0.0838 kg/m3They differ by a factor of 162,000! Thus the physical parameters in various liquid andgas flows might vary considerably. The differences are often resolved by the use of di-mensional analysis (Chap. 5). Other fluid densities are listed in Tables A.3 and A.4 (inApp. A).The specific weight of a fluid, denoted by ␥ (lowercase Greek gamma), is its weightper unit volume. Just as a mass has a weight W ϭ mg, density and specific weight aresimply related by gravity:␥ ϭ ␳g (1.6)1.6 Thermodynamic Properties of a Fluid 17Pressure
    • Specific GravityPotential and Kinetic EnergiesThe units of ␥ are weight per unit volume, in lbf/ft3or N/m3. In standard earth grav-ity, g ϭ 32.174 ft/s2ϭ 9.807 m/s2. Thus, e.g., the specific weights of air and water at20°C and 1 atm are approximately␥air ϭ (1.205 kg/m3)(9.807 m/s2) ϭ 11.8 N/m3ϭ 0.0752 lbf/ft3␥water ϭ (998 kg/m3)(9.807 m/s2) ϭ 9790 N/m3ϭ 62.4 lbf/ft3Specific weight is very useful in the hydrostatic-pressure applications of Chap. 2. Spe-cific weights of other fluids are given in Tables A.3 and A.4.Specific gravity, denoted by SG, is the ratio of a fluid density to a standard referencefluid, water (for liquids), and air (for gases):SGgas ϭ ᎏ␳␳gaairsᎏ ϭ ᎏ1.20␳5gkasg/m3ᎏ (1.7)SGliquid ϭ ᎏ␳␳lwiqauteidrᎏ ϭ ᎏ99␳8likqugi/dm3ᎏFor example, the specific gravity of mercury (Hg) is SGHg ϭ 13,580/998 Ϸ 13.6. En-gineers find these dimensionless ratios easier to remember than the actual numericalvalues of density of a variety of fluids.In thermostatics the only energy in a substance is that stored in a system by molecu-lar activity and molecular bonding forces. This is commonly denoted as internal en-ergy û. A commonly accepted adjustment to this static situation for fluid flow is to addtwo more energy terms which arise from newtonian mechanics: the potential energyand kinetic energy.The potential energy equals the work required to move the system of mass m fromthe origin to a position vector r ϭ ix ϩ jy ϩ kz against a gravity field g. Its value isϪmg ؒ r, or Ϫg ؒ r per unit mass. The kinetic energy equals the work required to changethe speed of the mass from zero to velocity V. Its value is ᎏ12ᎏmV2or ᎏ12ᎏV2per unit mass.Then by common convention the total stored energy e per unit mass in fluid mechan-ics is the sum of three terms:e ϭ û ϩ ᎏ12ᎏV2ϩ (Ϫg ؒ r) (1.8)Also, throughout this book we shall define z as upward, so that g ϭ Ϫgk and g ؒ r ϭϪgz. Then Eq. (1.8) becomese ϭ û ϩ ᎏ12ᎏV2ϩ gz (1.9)The molecular internal energy û is a function of T and p for the single-phase pure sub-stance, whereas the potential and kinetic energies are kinematic properties.Thermodynamic properties are found both theoretically and experimentally to be re-lated to each other by state relations which differ for each substance. As mentioned,18 Chapter 1 IntroductionState Relations for Gases
    • we shall confine ourselves here to single-phase pure substances, e.g., water in its liq-uid phase. The second most common fluid, air, is a mixture of gases, but since the mix-ture ratios remain nearly constant between 160 and 2200 K, in this temperature rangeair can be considered to be a pure substance.All gases at high temperatures and low pressures (relative to their critical point) arein good agreement with the perfect-gas lawp ϭ ␳RT R ϭ cp Ϫ cv ϭ gas constant (1.10)Since Eq. (1.10) is dimensionally consistent, R has the same dimensions as specificheat, {L2TϪ2⌰Ϫ1}, or velocity squared per temperature unit (kelvin or degree Rank-ine). Each gas has its own constant R, equal to a universal constant ⌳ divided by themolecular weightRgas ϭ ᎏM⌳gasᎏ (1.11)where ⌳ ϭ 49,700 ft2/(s2и °R) ϭ 8314 m2/(s2и K). Most applications in this book arefor air, with M ϭ 28.97:Rair ϭ 1717 ft2/(s2и °R) ϭ 287 m2/(s2и K) (1.12)Standard atmospheric pressure is 2116 lbf/ft2, and standard temperature is 60°F ϭ520°R. Thus standard air density is␳air ϭ ᎏ(171271)1(6520)ᎏ ϭ 0.00237 slug/ft3ϭ 1.22 kg/m3(1.13)This is a nominal value suitable for problems.One proves in thermodynamics that Eq. (1.10) requires that the internal molecularenergy û of a perfect gas vary only with temperature: û ϭ û(T). Therefore the specificheat cv also varies only with temperature:cv ϭ ΂ᎏѨѨTûᎏ΃␳ϭ ᎏddTûᎏ ϭ cv(T)or dû ϭ cv(T) dT (1.14)In like manner h and cp of a perfect gas also vary only with temperature:h ϭ û ϩ ᎏp␳ᎏ ϭ û ϩ RT ϭ h(T)cp ϭ΂ᎏѨѨThᎏ΃pϭ ᎏddThᎏ ϭ cp(T) (1.15)dh ϭ cp(T) dTThe ratio of specific heats of a perfect gas is an important dimensionless parameter incompressible-flow analysis (Chap. 9)k ϭ ᎏccpvᎏ ϭ k(T) Ն 1 (1.16)1.6 Thermodynamic Properties of a Fluid 19
    • Part (a)Part (b)As a first approximation in airflow analysis we commonly take cp, cv, and k to be constantkair Ϸ 1.4cv ϭ ᎏk ϪR1ᎏ Ϸ 4293 ft2/(s2и °R) ϭ 718 m2/(s2и K)(1.17)cp ϭ ᎏkkϪR1ᎏ Ϸ 6010 ft2/(s2и °R) ϭ 1005 m2/(s2и K)Actually, for all gases, cp and cv increase gradually with temperature, and k decreasesgradually. Experimental values of the specific-heat ratio for eight common gases areshown in Fig. 1.3.Many flow problems involve steam. Typical steam operating conditions are rela-tively close to the critical point, so that the perfect-gas approximation is inaccurate.The properties of steam are therefore available in tabular form [13], but the error ofusing the perfect-gas law is sometimes not great, as the following example shows.EXAMPLE 1.6Estimate ␳ and cp of steam at 100 lbf/in2and 400°F (a) by a perfect-gas approximation and(b) from the ASME steam tables [13].SolutionFirst convert to BG units: p ϭ 100 lbf/in2ϭ 14,400 lb/ft2, T ϭ 400°F ϭ 860°R. From Table A.4the molecular weight of H2O is 2MH ϩ MO ϭ 2(1.008) ϩ 16.0 ϭ 18.016. Then from Eq. (1.11)the gas constant of steam is approximatelyR ϭ ᎏ4198,.700106ᎏ ϭ 2759 ft2/(s2и °R)whence, from the perfect-gas law,␳ Ϸ ᎏRpTᎏ ϭ ᎏ271549,4(80600)ᎏ ϭ 0.00607 slug/ft3Ans. (a)From Fig. 1.3, k for steam at 860°R is approximately 1.30. Then from Eq. (1.17),cp Ϸ ᎏkkϪR1ᎏ ϭ ᎏ11.3.300(2Ϫ7519)ᎏ ϭ 12,000 ft2/(s2и °R) Ans. (a)From Ref. 13, the specific volume v of steam at 100 lbf/in2and 400°F is 4.935 ft3/lbm. Thenthe density is the inverse of this, converted to slugs:␳ ϭ ᎏ1vᎏ ϭ ϭ 0.00630 slug/ft3Ans. (b)This is about 4 percent higher than our ideal-gas estimate in part (a).Reference 13 lists the value of cp of steam at 100 lbf/in2and 400°F as 0.535 Btu/(lbm и °F).Convert this to BG units:cp ϭ [0.535 Btu/(lbm и °R)](778.2 ft и lbf/Btu)(32.174 lbm/slug)ϭ 13,400 ft и lbf/(slug и °R) ϭ 13,400 ft2/(s2и °R) Ans. (b)1ᎏᎏᎏᎏ(4.935 ft2/lbm)(32.174 lbm/slug)20 Chapter 1 Introduction
    • Fig. 1.3 Specific-heat ratio of eightcommon gases as a function of tem-perature. (Data from Ref. 12.)This is about 11 percent higher than our ideal-gas estimate in part (a). The chief reason for thediscrepancy is that this temperature and this pressure are quite close to the critical point and sat-uration line of steam. At higher temperatures and lower pressures, say, 800°F and 50 lbf/in2, theperfect-gas law gives ␳ and cp of steam within an accuracy of Ϯ1 percent.Note that the use of pound-mass and British thermal units in the traditional steam tables re-quires continual awkward conversions to BG units. Newer tables and disks are in SI units.The writer knows of no “perfect-liquid law” comparable to that for gases. Liquids arenearly incompressible and have a single reasonably constant specific heat. Thus an ide-alized state relation for a liquid is␳ Ϸ const cp Ϸ cv Ϸ const dh Ϸ cp dT (1.18)Most of the flow problems in this book can be attacked with these simple as-sumptions. Water is normally taken to have a density of 1.94 slugs/ft3and a spe-cific heat cp ϭ 25,200 ft2/(s2и °R). The steam tables may be used if more accuracyis required.1.6 Thermodynamic Properties of a Fluid 21ArAtmospheric pressureH2COAir andN2O2SteamCO21. 3000 4000 5000=kcpc2000Temperature, °RυState Relations for Liquids
    • 1.7 Viscosity and OtherSecondary PropertiesViscosityThe density of a liquid usually decreases slightly with temperature and increasesmoderately with pressure. If we neglect the temperature effect, an empirical pressure-density relation for a liquid isᎏppaᎏ Ϸ (B ϩ 1)΂ᎏ␳␳aᎏ΃nϪ B (1.19)where B and n are dimensionless parameters which vary slightly with temperature andpa and ␳a are standard atmospheric values. Water can be fitted approximately to thevalues B Ϸ 3000 and n Ϸ 7.Seawater is a variable mixture of water and salt and thus requires three thermody-namic properties to define its state. These are normally taken as pressure, temperature,and the salinity Sˆ, defined as the weight of the dissolved salt divided by the weight ofthe mixture. The average salinity of seawater is 0.035, usually written as 35 parts per1000, or 35 ‰. The average density of seawater is 2.00 slugs/ft3. Strictly speaking,seawater has three specific heats, all approximately equal to the value for pure waterof 25,200 ft2/(s2и °R) ϭ 4210 m2/(s2и K).EXAMPLE 1.7The pressure at the deepest part of the ocean is approximately 1100 atm. Estimate the densityof seawater at this pressure.SolutionEquation (1.19) holds for either water or seawater. The ratio p/pa is given as 1100:1100 Ϸ (3001)΂ᎏ␳␳aᎏ΃7Ϫ 3000or ᎏ␳␳aᎏ ϭ΂ᎏ43100001ᎏ΃1/7ϭ 1.046Assuming an average surface seawater density ␳a ϭ 2.00 slugs/ft3, we compute␳ Ϸ 1.046(2.00) ϭ 2.09 slugs/ft3Ans.Even at these immense pressures, the density increase is less than 5 percent, which justifies thetreatment of a liquid flow as essentially incompressible.The quantities such as pressure, temperature, and density discussed in the previous sec-tion are primary thermodynamic variables characteristic of any system. There are alsocertain secondary variables which characterize specific fluid-mechanical behavior. Themost important of these is viscosity, which relates the local stresses in a moving fluidto the strain rate of the fluid element.When a fluid is sheared, it begins to move at a strain rate inversely proportional to aproperty called its coefficient of viscosity ␮. Consider a fluid element sheared in one22 Chapter 1 Introduction
    • Fig. 1.4 Shear stress causes contin-uous shear deformation in a fluid:(a) a fluid element straining at arate ␦␪/␦t; (b) newtonian shear dis-tribution in a shear layer near awall.plane by a single shear stress ␶, as in Fig. 1.4a. The shear strain angle ␦␪ will contin-uously grow with time as long as the stress ␶ is maintained, the upper surface movingat speed ␦u larger than the lower. Such common fluids as water, oil, and air show alinear relation between applied shear and resulting strain rate␶ ϰ ᎏ␦␦␪tᎏ (1.20)From the geometry of Fig. 1.4a we see thattan ␦␪ ϭ ᎏ␦u␦y␦tᎏ (1.21)In the limit of infinitesimal changes, this becomes a relation between shear strain rateand velocity gradientᎏdd␪tᎏ ϭ ᎏdduyᎏ (1.22)From Eq. (1.20), then, the applied shear is also proportional to the velocity gradientfor the common linear fluids. The constant of proportionality is the viscosity coeffi-cient ␮␶ ϭ ␮ᎏdd␪tᎏ ϭ ␮ᎏdduyᎏ (1.23)Equation (1.23) is dimensionally consistent; therefore ␮ has dimensions of stress-time:{FT/L2} or {M/(LT)}. The BG unit is slugs per foot-second, and the SI unit is kilo-grams per meter-second. The linear fluids which follow Eq. (1.23) are called newton-ian fluids, after Sir Isaac Newton, who first postulated this resistance law in 1687.We do not really care about the strain angle ␪(t) in fluid mechanics, concentratinginstead on the velocity distribution u(y), as in Fig. 1.4b. We shall use Eq. (1.23) inChap. 4 to derive a differential equation for finding the velocity distribution u(y)—and,more generally, V(x, y, z, t)—in a viscous fluid. Figure 1.4b illustrates a shear layer,or boundary layer, near a solid wall. The shear stress is proportional to the slope of the1.7 Viscosity and Other Secondary Properties 23(a) (b)θδδu δt δθδtτ ∝δu = uu = 0δxτu(y)yτ = dudydudyNo slip at wallVelocityprofileδ y0µθδ
    • The Reynolds Numbervelocity profile and is greatest at the wall. Further, at the wall, the velocity u is zerorelative to the wall: This is called the no-slip condition and is characteristic of allviscous-fluid flows.The viscosity of newtonian fluids is a true thermodynamic property and varies withtemperature and pressure. At a given state (p, T) there is a vast range of values among thecommon fluids. Table 1.4 lists the viscosity of eight fluids at standard pressure and tem-perature. There is a variation of six orders of magnitude from hydrogen up to glycerin.Thus there will be wide differences between fluids subjected to the same applied stresses.Generally speaking, the viscosity of a fluid increases only weakly with pressure. Forexample, increasing p from 1 to 50 atm will increase ␮ of air only 10 percent. Tem-perature, however, has a strong effect, with ␮ increasing with T for gases and decreas-ing for liquids. Figure A.1 (in App. A) shows this temperature variation for various com-mon fluids. It is customary in most engineering work to neglect the pressure variation.The variation ␮(p, T) for a typical fluid is nicely shown by Fig. 1.5, from Ref. 14,which normalizes the data with the critical-point state (␮c, pc, Tc). This behavior, calledthe principle of corresponding states, is characteristic of all fluids, but the actual nu-merical values are uncertain to Ϯ20 percent for any given fluid. For example, valuesof ␮(T) for air at 1 atm, from Table A.2, fall about 8 percent low compared to the“low-density limit” in Fig. 1.5.Note in Fig. 1.5 that changes with temperature occur very rapidly near the criticalpoint. In general, critical-point measurements are extremely difficult and uncertain.As we shall see in Chaps. 5 through 7, the primary parameter correlating the viscousbehavior of all newtonian fluids is the dimensionless Reynolds number:Re ϭ ᎏ␳␮VLᎏ ϭ ᎏV␯Lᎏ (1.24)where V and L are characteristic velocity and length scales of the flow. The second formof Re illustrates that the ratio of ␮ to ␳ has its own name, the kinematic viscosity:␯ ϭ ᎏ␮␳ᎏ (1.25)It is called kinematic because the mass units cancel, leaving only the dimensions {L2/T}.24 Chapter 1 Introduction␮, Ratio ␳, ␯ RatioFluid kg/(m ؒ s)†␮/␮(H2) kg/m3m2/s†␯/␯(Hg)Hydrogen 8.8 E–6 00,0001.0 00,000.084 1.05 E–4 00,920Air 1.8 E–5 0,00002.1 00,001.20 1.51 E–5 00,130Gasoline 2.9 E–4 00,0033 0,0680 4.22 E–7 00,003.7Water 1.0 E–3 00,0114 0,0998 1.01 E–6 0000,8.7Ethyl alcohol 1.2 E–3 0,00135 0,0789 1.52 E–6 000,13Mercury 1.5 E–3 00,0170 13,580 1.16 E–7 0000,1.0SAE 30 oil 0.29 033,000 0,0891 3.25 E–4 02,850Glycerin 1.5 170,000 01,264 1.18 E–3 10,300†1 kg/(m и s) ϭ 0.0209 slug/(ft и s); 1 m2/s ϭ 10.76 ft2/s.Table 1.4 Viscosity and KinematicViscosity of Eight Fluids at 1 atmand 20°C
    • Fig. 1.5 Fluid viscosity nondimen-sionalized by critical-point proper-ties. This generalized chart is char-acteristic of all fluids but is onlyaccurate to Ϯ20 percent. (FromRef. 14.)Flow between PlatesGenerally, the first thing a fluids engineer should do is estimate the Reynolds num-ber range of the flow under study. Very low Re indicates viscous creeping motion,where inertia effects are negligible. Moderate Re implies a smoothly varying laminarflow. High Re probably spells turbulent flow, which is slowly varying in the time-meanbut has superimposed strong random high-frequency fluctuations. Explicit numericalvalues for low, moderate, and high Reynolds numbers cannot be stated here. They de-pend upon flow geometry and will be discussed in Chaps. 5 through 7.Table 1.4 also lists values of ␯ for the same eight fluids. The pecking order changesconsiderably, and mercury, the heaviest, has the smallest viscosity relative to its ownweight. All gases have high ␯ relative to thin liquids such as gasoline, water, and al-cohol. Oil and glycerin still have the highest ␯, but the ratio is smaller. For a givenvalue of V and L in a flow, these fluids exhibit a spread of four orders of magnitudein the Reynolds number.A classic problem is the flow induced between a fixed lower plate and an upper platemoving steadily at velocity V, as shown in Fig. 1.6. The clearance between plates ish, and the fluid is newtonian and does not slip at either plate. If the plates are large,1.7 Viscosity and Other Secondary Properties 250.4=crµTrTc= Tµµ0. 0.8 1 2 3 4 5 76 8 9 10LiquidTwo-phaseregionCriticalpoint0.50pr= 0.2Low-density limit12351025Dense gas
    • Fig. 1.6 Viscous flow induced byrelative motion between two paral-lel plates.this steady shearing motion will set up a velocity distribution u(y), as shown, with v ϭw ϭ 0. The fluid acceleration is zero everywhere.With zero acceleration and assuming no pressure variation in the flow direction, youshould show that a force balance on a small fluid element leads to the result that theshear stress is constant throughout the fluid. Then Eq. (1.23) becomesᎏdduyᎏ ϭ ᎏ␮␶ᎏ ϭ constwhich we can integrate to obtainu ϭ a ϩ byThe velocity distribution is linear, as shown in Fig. 1.6, and the constants a and b canbe evaluated from the no-slip condition at the upper and lower walls:0 ϭ a ϩ b(0) at y ϭ 0V ϭ a ϩ b(h) at y ϭ hHence a ϭ 0 and b ϭ V/h. Then the velocity profile between the plates is given byu ϭ Vᎏhyᎏ (1.26)as indicated in Fig. 1.6. Turbulent flow (Chap. 6) does not have this shape.Although viscosity has a profound effect on fluid motion, the actual viscous stressesare quite small in magnitude even for oils, as shown in the following example.EXAMPLE 1.8Suppose that the fluid being sheared in Fig. 1.6 is SAE 30 oil at 20°C. Compute the shear stressin the oil if V ϭ 3 m/s and h ϭ 2 cm.SolutionThe shear stress is found from Eq. (1.23) by differentiating Eq. (1.26):␶ ϭ ␮ᎏdduyᎏ ϭ ᎏ␮hVᎏ (1)26 Chapter 1 Introductionyh u(y)VMovingplate:u = VViscousfluidu = Vu = 0Fixed plateu ϭ Ά
    • Variation of Viscosity withTemperatureThermal ConductivityFrom Table 1.4 for SAE 30 oil, ␮ ϭ 0.29 kg/(m и s). Then, for the given values of V and h,Eq. (1) predicts␶ ϭ ϭ 43 kg/(m и s2)ϭ 43 N/m2ϭ 43 Pa Ans.Although oil is very viscous, this is a modest shear stress, about 2400 times less than atmos-pheric pressure. Viscous stresses in gases and thin liquids are even smaller.Temperature has a strong effect and pressure a moderate effect on viscosity. The vis-cosity of gases and most liquids increases slowly with pressure. Water is anomalousin showing a very slight decrease below 30°C. Since the change in viscosity is only afew percent up to 100 atm, we shall neglect pressure effects in this book.Gas viscosity increases with temperature. Two common approximations are thepower law and the Sutherland law:΂ᎏTT0ᎏ΃npower lawᎏ(T/T0)T3/2ϩ(TS0 ϩ S)ᎏ Sutherland lawwhere ␮0 is a known viscosity at a known absolute temperature T0 (usually 273 K). Theconstants n and S are fit to the data, and both formulas are adequate over a wide range oftemperatures. For air, n Ϸ 0.7 and S Ϸ 110 K ϭ 199°R. Other values are given in Ref. 3.Liquid viscosity decreases with temperature and is roughly exponential, ␮ Ϸ aeϪbT;but a better fit is the empirical result that ln ␮ is quadratic in 1/T, where T is absolutetemperaturelnᎏ␮␮0ᎏ Ϸ a ϩ b΂ᎏTT0ᎏ΃ϩ c΂ᎏTT0ᎏ΃2(1.28)For water, with T0 ϭ 273.16 K, ␮0 ϭ 0.001792 kg/(m и s), suggested values are a ϭϪ1.94, b ϭ Ϫ4.80, and c ϭ 6.74, with accuracy about Ϯ1 percent. The viscosity ofwater is tabulated in Table A.1. Curve-fit viscosity formulas for 355 organic liquids aregiven by Yaws et al. [34]. For further viscosity data, see Refs. 28 and 36.Just as viscosity relates applied stress to resulting strain rate, there is a property calledthermal conductivity k which relates the vector rate of heat flow per unit area q to thevector gradient of temperature ٌT. This proportionality, observed experimentally forfluids and solids, is known as Fourier’s law of heat conductionq ϭ ϪkٌT (1.29a)which can also be written as three scalar equationsqx ϭ ϪkᎏѨѨTxᎏ qy ϭ ϪkᎏѨѨTyᎏ qz ϭ ϪkᎏѨѨTzᎏ (1.29b)[0.29 kg/(m и s)](3 m/s)ᎏᎏᎏ0.02 m1.7 Viscosity and Other Secondary Properties 27ᎏ␮␮0ᎏ Ϸ (1.27)
    • Nonnewtonian FluidsThe minus sign satisfies the convention that heat flux is positive in the direction of de-creasing temperature. Fourier’s law is dimensionally consistent, and k has SI units ofjoules per second-meter-kelvin. Thermal conductivity k is a thermodynamic propertyand varies with temperature and pressure in much the same way as viscosity. The ra-tio k/k0 can be correlated with T/T0 in the same manner as Eqs. (1.27) and (1.28) forgases and liquids, respectively.Further data on viscosity and thermal-conductivity variations can be found inRef. 11.Fluids which do not follow the linear law of Eq. (1.23) are called nonnewtonian andare treated in books on rheology [6]. Figure 1.7a compares four examples with a new-tonian fluid. A dilatant, or shear-thickening, fluid increases resistance with increasingapplied stress. Alternately, a pseudoplastic, or shear-thinning, fluid decreases resistancewith increasing stress. If the thinning effect is very strong, as with the dashed-linecurve, the fluid is termed plastic. The limiting case of a plastic substance is one whichrequires a finite yield stress before it begins to flow. The linear-flow Bingham plasticidealization is shown, but the flow behavior after yield may also be nonlinear. An ex-ample of a yielding fluid is toothpaste, which will not flow out of the tube until a fi-nite stress is applied by squeezing.A further complication of nonnewtonian behavior is the transient effect shown inFig. 1.7b. Some fluids require a gradually increasing shear stress to maintain a con-stant strain rate and are called rheopectic. The opposite case of a fluid which thins outwith time and requires decreasing stress is termed thixotropic. We neglect nonnewton-ian effects in this book; see Ref. 6 for further study.28 Chapter 1 IntroductionShearstressShearstressYieldstressPlasticIdeal BinghamplasticDilatantNewtonianPseudoplasticShear strain rate ddtθ0 0 Time(a) (b)Constantstrain rateRheopecticCommonfluidsThixotropicττFig. 1.7 Rheological behavior ofvarious viscous materials: (a) stressversus strain rate; (b) effect of timeon applied stress.
    • A liquid, being unable to expand freely, will form an interface with a second liquid orgas. The physical chemistry of such interfacial surfaces is quite complex, and wholetextbooks are devoted to this specialty [15]. Molecules deep within the liquid repeleach other because of their close packing. Molecules at the surface are less dense andattract each other. Since half of their neighbors are missing, the mechanical effect isthat the surface is in tension. We can account adequately for surface effects in fluidmechanics with the concept of surface tension.If a cut of length dL is made in an interfacial surface, equal and opposite forces ofmagnitude ⌼ dL are exposed normal to the cut and parallel to the surface, where ⌼ iscalled the coefficient of surface tension. The dimensions of ⌼ are {F/L}, with SI unitsof newtons per meter and BG units of pounds-force per foot. An alternate concept isto open up the cut to an area dA; this requires work to be done of amount ⌼ dA. Thusthe coefficient ⌼ can also be regarded as the surface energy per unit area of the inter-face, in N и m/m2or ft и lbf/ft2.The two most common interfaces are water-air and mercury-air. For a clean surfaceat 20°C ϭ 68°F, the measured surface tension is0.0050 lbf/ft ϭ 0.073 N/m air-water0.033 lbf/ft ϭ 0.48 N/m air-mercuryThese are design values and can change considerably if the surface contains contami-nants like detergents or slicks. Generally ⌼ decreases with liquid temperature and iszero at the critical point. Values of ⌼ for water are given in Fig. 1.8.If the interface is curved, a mechanical balance shows that there is a pressure dif-ference across the interface, the pressure being higher on the concave side, as illus-trated in Fig. 1.9. In Fig. 1.9a, the pressure increase in the interior of a liquid cylinderis balanced by two surface-tension forces2RL ⌬p ϭ 2⌼Lor ⌬p ϭ ᎏ⌼Rᎏ (1.31)We are not considering the weight of the liquid in this calculation. In Fig. 1.9b, the pres-sure increase in the interior of a spherical droplet balances a ring of surface-tension force␲R2⌬p ϭ 2␲R⌼or ⌬p ϭ ᎏ2R⌼ᎏ (1.32)We can use this result to predict the pressure increase inside a soap bubble, which hastwo interfaces with air, an inner and outer surface of nearly the same radius R:⌬pbubble Ϸ 2 ⌬pdroplet ϭ ᎏ4R⌼ᎏ (1.33)Figure 1.9c shows the general case of an arbitrarily curved interface whose principalradii of curvature are R1 and R2. A force balance normal to the surface will show thatthe pressure increase on the concave side is⌬p ϭ ⌼(R1Ϫ1ϩ R2Ϫ1) (1.34)1.7 Viscosity and Other Secondary Properties 29Surface TensionΆ⌼ ϭ (1.30)
    • Fig. 1.8 Surface tension of a cleanair-water interface. Data from TableA.5.Fig. 1.9 Pressure change across a curved interface due to surface tension: (a) interior of a liquid cylinder; (b) interior of a sphericaldroplet; (c) general curved interface.Equations (1.31) to (1.33) can all be derived from this general relation; e.g., inEq. (1.31), R1 ϭ R and R2 ϭ ϱ.A second important surface effect is the contact angle ␪ which appears when aliquid interface intersects with a solid surface, as in Fig. 1.10. The force balancewould then involve both ⌼ and ␪. If the contact angle is less than 90°, the liquid issaid to wet the solid; if ␪ Ͼ 90°, the liquid is termed nonwetting. For example, wa-ter wets soap but does not wet wax. Water is extremely wetting to a clean glass sur-face, with ␪ Ϸ 0°. Like ⌼, the contact angle ␪ is sensitive to the actual physico-chemical conditions of the solid-liquid interface. For a clean mercury-air-glassinterface, ␪ ϭ 130°.Example 1.9 illustrates how surface tension causes a fluid interface to rise or fall ina capillary tube.30 Chapter 1 Introduction00.0500.0600.0700.08010 20 30 40 50 60 70 80 90 100T, °C,N/mϒ2RL ∆p2RL(a)⌼L⌼LπR2∆p2πR⌼(b) (c)∆p dA⌼dL2⌼dL1⌼dL2⌼dL1R2R1
    • Fig. 1.10 Contact-angle effects atliquid-gas-solid interface. If ␪ Ͻ90°, the liquid “wets” the solid; if␪ Ͼ 90°, the liquid is nonwetting.EXAMPLE 1.9Derive an expression for the change in height h in a circular tube of a liquid with surface ten-sion ⌼ and contact angle ␪, as in Fig. E1.9.SolutionThe vertical component of the ring surface-tension force at the interface in the tube must bal-ance the weight of the column of fluid of height h2␲R⌼ cos ␪ ϭ ␥␲R2hSolving for h, we have the desired resulth ϭ ᎏ2⌼␥cRos ␪ᎏ Ans.Thus the capillary height increases inversely with tube radius R and is positive if ␪ Ͻ 90° (wet-ting liquid) and negative (capillary depression) if ␪ Ͼ 90°.Suppose that R ϭ 1 mm. Then the capillary rise for a water-air-glass interface, ␪ Ϸ 0°, ⌼ ϭ0.073 N/m, and ␳ ϭ 1000 kg/m3ish ϭ ϭ 0.015 (N и s2)/kg ϭ 0.015 m ϭ 1.5 cmFor a mercury-air-glass interface, with ␪ ϭ 130°, ⌼ ϭ 0.48 N/m, and ␳ ϭ 13,600 kg/m3, the cap-illary rise ish ϭ ϭ Ϫ0.46 cmWhen a small-diameter tube is used to make pressure measurements (Chap. 2), these capillaryeffects must be corrected for.Vapor pressure is the pressure at which a liquid boils and is in equilibrium with itsown vapor. For example, the vapor pressure of water at 68°F is 49 lbf/ft2, while thatof mercury is only 0.0035 lbf/ft2. If the liquid pressure is greater than the vapor2(0.48)(cos 130°)ᎏᎏᎏ13,600(9.81)(0.001)2(0.073 N/m)(cos 0°)ᎏᎏᎏᎏ(1000 kg/m3)(9.81 m/s2)(0.001 m)1.7 Viscosity and Other Secondary Properties 31NonwettingSolidLiquidGasθ θFig. E1.9θ2RhVapor Pressure
    • No-Slip and No-Temperature-Jump Conditionspressure, the only exchange between liquid and vapor is evaporation at the inter-face. If, however, the liquid pressure falls below the vapor pressure, vapor bubblesbegin to appear in the liquid. If water is heated to 212°F, its vapor pressure rises to2116 lbf/ft2, and thus water at normal atmospheric pressure will boil. When the liq-uid pressure is dropped below the vapor pressure due to a flow phenomenon, wecall the process cavitation. As we shall see in Chap. 2, if water is accelerated fromrest to about 50 ft/s, its pressure drops by about 15 lbf/in2, or 1 atm. This can causecavitation.The dimensionless parameter describing flow-induced boiling is the cavitationnumberCa ϭ (1.35)where pa ϭ ambient pressurepv ϭ vapor pressureV ϭ characteristic flow velocityDepending upon the geometry, a given flow has a critical value of Ca below which theflow will begin to cavitate. Values of surface tension and vapor pressure of water aregiven in Table A.5. The vapor pressure of water is plotted in Fig. 1.11.Figure 1.12a shows cavitation bubbles being formed on the low-pressure surfacesof a marine propeller. When these bubbles move into a higher-pressure region, theycollapse implosively. Cavitation collapse can rapidly spall and erode metallic surfacesand eventually destroy them, as shown in Fig. 1.12b.When a fluid flow is bounded by a solid surface, molecular interactions cause the fluidin contact with the surface to seek momentum and energy equilibrium with that surface.All liquids essentially are in equilibrium with the surface they contact. All gases are, too,pa Ϫ pvᎏᎏ12ᎏ␳V232 Chapter 1 Introduction100806040200pv,kPa0 20 40 60 80 100T, °CFig. 1.11 Vapor pressure of water.Data from Table A.5.
    • 1.7 Viscosity and Other Secondary Properties 33Fig. 1.12 Two aspects of cavitationbubble formation in liquid flows:(a) Beauty: spiral bubble sheetsform from the surface of a marinepropeller. (Courtesy of the GarfieldThomas Water Tunnel, Pennsylva-nia State University); (b) ugliness:collapsing bubbles erode a pro-peller surface. (Courtesy of ThomasT. Huang, David Taylor ResearchCenter.)
    • Fig. 1.13 The no-slip condition inwater flow past a thin fixed plate.The upper flow is turbulent; thelower flow is laminar. The velocityprofile is made visible by a line ofhydrogen bubbles discharged fromthe wire across the flow. [From Il-lustrated Experiments in Fluid Me-chanics (The NCFMF Book of FilmNotes), National Committee forFluid Mechanics Films, EducationDevelopment Center, Inc., copy-right 1972.]except under the most rarefied conditions [8]. Excluding rarefied gases, then, all fluidsat a point of contact with a solid take on the velocity and temperature of that surfaceVfluid ϵ Vwall Tfluid ϵ Twall (1.36)These are called the no-slip and no-temperature-jump conditions, respectively. Theyserve as boundary conditions for analysis of fluid flow past a solid surface (Chap. 6).Figure 1.13 illustrates the no-slip condition for water flow past the top and bottom sur-faces of a fixed thin plate. The flow past the upper surface is disorderly, or turbulent,while the lower surface flow is smooth, or laminar.7In both cases there is clearly noslip at the wall, where the water takes on the zero velocity of the fixed plate. The ve-locity profile is made visible by the discharge of a line of hydrogen bubbles from thewire shown stretched across the flow.To decrease the mathematical difficulty, the no-slip condition is partially relaxed inthe analysis of inviscid flow (Chap. 8). The flow is allowed to “slip” past the surfacebut not to permeate through the surfaceVnormal(fluid) ϵ Vnormal(solid) (1.37)while the tangential velocity Vt is allowed to be independent of the wall. The analysisis much simpler, but the flow patterns are highly idealized.34 Chapter 1 Introduction7Laminar and turbulent flows are studied in Chaps. 6 and 7.
    • Speed of Sound In gas flow, one must be aware of compressibility effects (significant density changescaused by the flow). We shall see in Sec. 4.2 and in Chap. 9 that compressibility be-comes important when the flow velocity reaches a significant fraction of the speed ofsound of the fluid. The speed of sound a of a fluid is the rate of propagation of small-disturbance pressure pulses (“sound waves”) through the fluid. In Chap. 9 we shallshow, from momentum and thermodynamic arguments, that the speed of sound is de-fined bya2ϭ΂ᎏѨѨp␳ᎏ΃sϭ k΂ᎏѨѨp␳ᎏ΃Tk ϭ ᎏccpvᎏ (1.38)This is true for either a liquid or a gas, but it is for gases that the problem of com-pressibility occurs. For an ideal gas, Eq. (1.10), we obtain the simple formulaaideal gas ϭ (kRT)1/2(1.39)where R is the gas constant, Eq. (1.11), and T the absolute temperature. For example,for air at 20°C, a ϭ {(1.40)[287 m2/(s2и K)](293 K)}1/2Ϸ 343 m/s (1126 ft/s ϭ 768mi/h). If, in this case, the air velocity reaches a significant fraction of a, say, 100 m/s,then we must account for compressibility effects (Chap. 9). Another way to state thisis to account for compressibility when the Mach number Ma ϭ V/a of the flow reachesabout 0.3.The speed of sound of water is tabulated in Table A.5. The speed of sound of air(or any approximately perfect gas) is simply calculated from Eq. (1.39).There are three basic ways to attack a fluid-flow problem. They are equally importantfor a student learning the subject, and this book tries to give adequate coverage to eachmethod:1. Control-volume, or integral analysis (Chap. 3)2. Infinitesimal system, or differential analysis (Chap. 4)3. Experimental study, or dimensional analysis (Chap. 5)In all cases, the flow must satisfy the three basic laws of mechanics8plus a thermo-dynamic state relation and associated boundary conditions:1. Conservation of mass (continuity)2. Linear momentum (Newton’s second law)3. First law of thermodynamics (conservation of energy)4. A state relation like ␳ ϭ ␳(p, T)5. Appropriate boundary conditions at solid surfaces, interfaces, inlets, and exitsIn integral and differential analyses, these five relations are modeled mathematicallyand solved by computational methods. In an experimental study, the fluid itself per-forms this task without the use of any mathematics. In other words, these laws are be-lieved to be fundamental to physics, and no fluid flow is known to violate them.1.8 Basic Flow-Analysis Techniques 351.8 Basic Flow-AnalysisTechniques8In fluids which are variable mixtures of components, such as seawater, a fourth basic law is required,conservation of species. For an example of salt conservation analysis, see Chap. 4, Ref. 16.
    • A control volume is a finite region, chosen carefully by the analyst, with open bound-aries through which mass, momentum, and energy are allowed to cross. The analystmakes a budget, or balance, between the incoming and outgoing fluid and the resul-tant changes within the control volume. The result is a powerful tool but a crude one.Details of the flow are normally washed out or ignored in control-volume analyses.Nevertheless, the control-volume technique of Chap. 3 never fails to yield useful andquantitative information to the engineering analyst.When the conservation laws are written for an infinitesimal system of fluid in motion,they become the basic differential equations of fluid flow. To apply them to a specificproblem, one must integrate these equations mathematically subject to the boundary con-ditions of the particular problem. Exact analytic solutions are often possible only for verysimple geometries and boundary conditions (Chap. 4). Otherwise, one attempts numeri-cal integration on a digital computer, i.e., a summing procedure for finite-sized systemswhich one hopes will approximate the exact integral calculus [1]. Even computer analy-sis often fails to provide an accurate simulation, because of either inadequate storage orinability to model the finely detailed flow structure characteristic of irregular geometriesor turbulent-flow patterns. Thus differential analysis sometimes promises more than itdelivers, although we can successfully study a number of classic and useful solutions.A properly planned experiment is very often the best way to study a practical en-gineering flow problem. Guidelines for planning flow experiments are given in Chap.5. For example, no theory presently available, whether differential or integral, calcu-lus or computer, is able to make an accurate computation of the aerodynamic drag andside force of an automobile moving down a highway with crosswinds. One must solvethe problem by experiment. The experiment may be full-scale: One can test a real au-tomobile on a real highway in real crosswinds. For that matter, there are wind tunnelsin existence large enough to hold a full-scale car without significant blockage effects.Normally, however, in the design stage, one tests a small-model automobile in a smallwind tunnel. Without proper interpretation, the model results may be poor and misleadthe designer (Chap. 5). For example, the model may lack important details such as sur-face finish or underbody protuberances. The “wind” produced by the tunnel propellersmay lack the turbulent gustiness of real winds. It is the job of the fluid-flow analyst,using such techniques as dimensional analysis, to plan an experiment which gives anaccurate estimate of full-scale or prototype results expected in the final product.It is possible to classify flows, but there is no general agreement on how to do it.Most classifications deal with the assumptions made in the proposed flow analysis.They come in pairs, and we normally assume that a given flow is eitherSteady or unsteady (1.40a)Inviscid or viscous (1.40b)Incompressible or compressible (1.40c)Gas or liquid (1.40d)As Fig. 1.14 indicates, we choose one assumption from each pair. We may have a steadyviscous compressible gas flow or an unsteady inviscid (␮ ϭ 0) incompressible liquidflow. Although there is no such thing as a truly inviscid fluid, the assumption ␮ ϭ 0gives adequate results in many analyses (Chap. 8). Often the assumptions overlap: Aflow may be viscous in the boundary layer near a solid surface (Fig. 1.13) and effec-36 Chapter 1 Introduction
    • Fig. 1.14 Ready for a flow analy-sis? Then choose one assumptionfrom each box.1.9 Flow Patterns: Streamlines,Streaklines, and Pathlinestively inviscid away from the surface. The viscous part of the flow may be laminar ortransitional or turbulent or combine patches of all three types of viscous flow. A flowmay involve both a gas and a liquid and the free surface, or interface, between them(Chap. 10). A flow may be compressible in one region and have nearly constant den-sity in another. Nevertheless, Eq. (1.40) and Fig. 1.14 give the basic binary assump-tions of flow analysis, and Chaps. 6 to 10 try to separate them and isolate the basic ef-fect of each assumption.Fluid mechanics is a highly visual subject. The patterns of flow can be visualized in adozen different ways, and you can view these sketches or photographs and learn a greatdeal qualitatively and often quantitatively about the flow.Four basic types of line patterns are used to visualize flows:1. A streamline is a line everywhere tangent to the velocity vector at a given in-stant.2. A pathline is the actual path traversed by a given fluid particle.3. A streakline is the locus of particles which have earlier passed through a pre-scribed point.4. A timeline is a set of fluid particles that form a line at a given instant.The streamline is convenient to calculate mathematically, while the other three are eas-ier to generate experimentally. Note that a streamline and a timeline are instantaneouslines, while the pathline and the streakline are generated by the passage of time. Thevelocity profile shown in Fig. 1.13 is really a timeline generated earlier by a single dis-charge of bubbles from the wire. A pathline can be found by a time exposure of a sin-gle marked particle moving through the flow. Streamlines are difficult to generate ex-perimentally in unsteady flow unless one marks a great many particles and notes theirdirection of motion during a very short time interval [17, p. 35]. In steady flow the sit-uation simplifies greatly:Streamlines, pathlines, and streaklines are identical in steady flow.In fluid mechanics the most common mathematical result for visualization purposesis the streamline pattern. Figure 1.15a shows a typical set of streamlines, and Fig. 1.15bshows a closed pattern called a streamtube. By definition the fluid within a streamtubeis confined there because it cannot cross the streamlines; thus the streamtube wallsneed not be solid but may be fluid surfaces.Figure 1.16 shows an arbitrary velocity vector. If the elemental arc length dr of astreamline is to be parallel to V, their respective components must be in proportion:Streamline: ᎏduxᎏ ϭ ᎏdvyᎏ ϭ ᎏdwzᎏ ϭ ᎏdVrᎏ (1.41)1.9 Flow Patterns: Streamlines, Streaklines, and Pathlines 37SteadyUnsteadyInviscidViscousIncompressibleCompressibleGasLiquid
    • If the velocities (u, v, w) are known functions of position and time, Eq. (1.41) can beintegrated to find the streamline passing through the initial point (x0, y0, z0, t0). Themethod is straightforward for steady flows (Example 1.10) but may be laborious forunsteady flow.The pathline, or displacement of a particle, is defined by integration of the velocitycomponents, as mentioned in Sec. 1.5:Pathline: x ϭ ͵u dt y ϭ ͵v dt z ϭ ͵w dt (1.42)Given (u, v, w) as known functions of position and time, the integration is begun at aspecified initial position (x0, y0, z0, t0). Again the integration may be laborious.Streaklines, easily generated experimentally with smoke, dye, or bubble releases,are very difficult to compute analytically. See Ref. 18 for mathematical details.38 Chapter 1 Introduction(a)V(b)No flow acrossstreamtube wallsIndividualstreamlineFig. 1.15 The most commonmethod of flow-pattern presenta-tion: (a) Streamlines are every-where tangent to the local velocityvector; (b) a streamtube is formedby a closed collection of stream-lines.zydydxdrxvdzuVVwFig. 1.16 Geometric relations fordefining a streamline.
    • EXAMPLE 1.10Given the steady two-dimensional velocity distributionu ϭ Kx v ϭ ϪKy w ϭ 0 (1)where K is a positive constant, compute and plot the streamlines of the flow, including direc-tions, and give some possible interpretations of the pattern.SolutionSince time does not appear explicitly in Eq. (1), the motion is steady, so that streamlines, path-lines, and streaklines will coincide. Since w ϭ 0 everywhere, the motion is two dimensional, inthe xy plane. The streamlines can be computed by substituting the expressions for u and v intoEq. (1.41):ᎏKdxxᎏ ϭ ϪᎏKdyyᎏor ͵ᎏdxxᎏ ϭ Ϫ͵ᎏdyyᎏIntegrating, we obtain ln x ϭ Ϫln y ϩ ln C, orxy ϭ C Ans. (2)This is the general expression for the streamlines, which are hyperbolas. The complete pat-tern is plotted in Fig. E1.10 by assigning various values to the constant C. The arrowheadscan be determined only by returning to Eqs. (1) to ascertain the velocity component direc-tions, assuming K is positive. For example, in the upper right quadrant (x Ͼ 0, y Ͼ 0), u ispositive and v is negative; hence the flow moves down and to the right, establishing the ar-rowheads as shown.1.9 Flow Patterns: Streamlines, Streaklines, and Pathlines 3900C = –3–2–1C = 00+ 1+ 2C = + 3 – 3– 2– 1xC = 0+1+2+3yFig. E1.10 Streamlines for the ve-locity distribution given byEq. (1), for K Ͼ 0.
    • Note that the streamline pattern is entirely independent of constant K. It could represent theimpingement of two opposing streams, or the upper half could simulate the flow of a single down-ward stream against a flat wall. Taken in isolation, the upper right quadrant is similar to the flowin a 90° corner. This is definitely a realistic flow pattern and is discussed again in Chap. 8.Finally note the peculiarity that the two streamlines (C ϭ 0) have opposite directions and in-tersect. This is possible only at a point where u ϭ v ϭ w ϭ 0, which occurs at the origin in thiscase. Such a point of zero velocity is called a stagnation point.A streakline can be produced experimentally by the continuous release of marked par-ticles (dye, smoke, or bubbles) from a given point. Figure 1.17 shows two examples. Theflow in Fig. 1.17b is unsteady and periodic due to the flapping of the plate against theoncoming stream. We see that the dash-dot streakline does not coincide with either thestreamline or the pathline passing through the same release point. This is characteristicof unsteady flow, but in Fig. 1.17a the smoke filaments form streaklines which are iden-tical to the streamlines and pathlines. We noted earlier that this coincidence of lines isalways true of steady flow: Since the velocity never changes magnitude or direction atany point, every particle which comes along repeats the behavior of its earlier neighbors.Methods of experimental flow visualization include the following:1. Dye, smoke, or bubble discharges2. Surface powder or flakes on liquid flows3. Floating or neutral-density particles4. Optical techniques which detect density changes in gas flows: shadowgraph,schlieren, and interferometer5. Tufts of yarn attached to boundary surfaces6. Evaporative coatings on boundary surfaces7. Luminescent fluids or additives40 Chapter 1 IntroductionFig. 1.17 Experimental visualization of steady and unsteady flow: (a) steady flow past an airfoilvisualized by smoke filaments (C. A. A. SCIENTIFIC—Prime Movers Laboratory Systems);(b) unsteady flow past an oscillating plate with a point bubble release (from an experiment inRef. 17).(b)PeriodicflappingplateReleasepointStreamlinePathlineStreaklineUniformapproachflow(a)
    • The mathematical implications of flow-pattern analysis are discussed in detail in Ref.18. References 19 and 20 are beautiful albums of photographs. References 21 and 22are monographs on flow visualization.Most of the examples and exercises in this text are amenable to direct calculation with-out guessing or iteration or looping. Until recently, only such direct problem assign-ments, whether “plug-and-chug” or more subtle, were appropriate for undergraduateengineering courses. However, the recent introduction of computer software solversmakes almost any set of algebraic relations viable for analysis and solution. The solverrecommended here is the Engineering Equation Solver (EES) developed by Klein andBeckman [33] and described in Appendix E.Any software solver should handle a purely mathematical set of relations, such asthe one posed in Ref. 33: X ln (X) ϭ Y3, X1/2ϭ 1/Y. Submit that pair to any commer-cial solver and you will no doubt receive the answer: X ϭ 1.467, Y ϭ 0.826. However,for engineers, in the author’s opinion, EES is superior to most solvers because (1) equa-tions can be entered in any order; (2) scores of mathematical formulas are built-in, suchas the Bessel functions; and (3) thermophysical properties of many fluids are built-in,such as the steam tables [13]. Both metric and English units are allowed. Equationsneed not be written in the traditional BASIC or FORTRAN style. For example, X ϪY ϩ 1 ϭ 0 is perfectly satisfactory; there is no need to retype this as X ϭ Y Ϫ 1.For example, reconsider Example 1.7 as an EES exercise. One would first enter thereference properties p0 and ␳0 plus the curve-fit constants B and n:Pz ϭ 1.0Rhoz ϭ 2.0B ϭ 3000n ϭ 7Then specify the given pressure ratio and the curve-fit relation, Eq. (1.19), for the equa-tion of state of water:P ϭ 1100*PzP/Pz ϭ (B ϩ 1)*(Rho/Rhoz)^n Ϫ BIf you request an initial opinion from the CHECK/FORMAT menu, EES states thatthere are six equations in six unknowns and there are no obvious difficulties. Thenrequest SOLVE from the menu and EES quickly prints out Rho ϭ 2.091, the correctanswer as seen already in Ex. 1.7. It also prints out values of the other five variables.Occasionally EES reports “unable to converge” and states what went wrong (divisionby zero, square root of a negative number, etc.). One needs only to improve the guessesand ranges of the unknowns in Variable Information to assist EES to the solution.In subsequent chapters we will illustrate some implicit (iterative) examples by us-ing EES and will also assign some advanced problem exercises for which EES is anideal approach. The use of an engineering solver, notably EES, is recommended to allengineers in this era of the personal computer.1.10 The Engineering Equation Solver 411.10 The Engineering EquationSolverEES
    • Earlier in this chapter we referred to the uncertainty of the principle of correspondingstates in discussing Fig. 1.5. Uncertainty is a fact of life in engineering. We rarely knowany engineering properties or variables to an extreme degree of accuracy. Therefore,we need to know the uncertainty U of our data, usually defined as the band withinwhich the experimenter is 95 percent confident that the true value lies (Refs. 30, 31).In Fig. 1.5, we were given that the uncertainty of ␮/␮c is U Ϸ Ϯ20 percent.Fluid mechanics is heavily dependent upon experimentation, and the data uncer-tainty is needed before we can use it for prediction or design purposes. Sometimes un-certainty completely changes our viewpoint. As an offbeat example, suppose that as-tronomers reported that the length of the earth year was 365.25 days “give or take acouple of months.” First, that would make the five-figure accuracy ridiculous, and theyear would better be stated as Y Ϸ 365 Ϯ 60 days. Second, we could no longer planconfidently or put together accurate calendars. Scheduling Christmas vacation wouldbe chancy.Multiple variables make uncertainty estimates cumulative. Suppose a given result Pdepends upon N variables, P ϭ P(x1, x2, x3, . . . , xN), each with its own uncertainty;for example, x1 has uncertainty ␦x1. Then, by common agreement among experimenters,the total uncertainty of P is calculated as a root-mean-square average of all effects:␦P ϭ΄΂ᎏѨѨxP1ᎏ␦x1΃2ϩ΂ᎏѨѨxP2ᎏ␦x2΃2ϩ иии ϩ΂ᎏѨѨxPNᎏ ␦xN΃2΅1/2(1.43)This calculation is statistically much more probable than simply adding linearly thevarious uncertainties ␦xi, thereby making the unlikely assumption that all variablessimultaneously attain maximum error. Note that it is the responsibility of the ex-perimenter to establish and report accurate estimates of all the relevant uncertain-ties ␦xi.If the quantity P is a simple power-law expression of the other variables, for ex-ample, P ϭ Const x1n1x2n2x3n3. . . , then each derivative in Eq. (1.43) is proportional toP and the relevant power-law exponent and is inversely proportional to that variable.If P ϭ Const x1n1x2n2x3n3. . . , thenᎏѨѨxP1ᎏ ϭ ᎏnx11Pᎏ, ᎏѨѨxP2ᎏ ϭ ᎏnx22Pᎏ, ᎏѨѨxP3ᎏ ϭ ᎏnx33Pᎏ, иииThus, from Eq. (1.43),ᎏ␦PPᎏ ϭ΄΂n1ᎏ␦xx11ᎏ΃2ϩ΂n2ᎏ␦xx22ᎏ΃2ϩ΂n3ᎏ␦xx33ᎏ΃2ϩ иии΅1/2(1.44)Evaluation of ␦P is then a straightforward procedure, as in the following example.EXAMPLE 1.11The so-called dimensionless Moody pipe-friction factor f, plotted in Fig. 6.13, is calculated inexperiments from the following formula involving pipe diameter D, pressure drop ⌬p, density␳, volume flow rate Q, and pipe length L:f ϭ ᎏ3␲22ᎏ ᎏD␳Q5⌬2Lpᎏ42 Chapter 1 Introduction1.11 Uncertainty ofExperimental Data
    • Measurement uncertainties are given for a certain experiment: for D: 0.5 percent, ⌬p: 2.0 per-cent, ␳: 1.0 percent, Q: 3.5 percent, and L: 0.4 percent. Estimate the overall uncertainty of thefriction factor f.SolutionThe coefficient ␲2/32 is assumed to be a pure theoretical number, with no uncertainty. The othervariables may be collected using Eqs. (1.43) and (1.44):U ϭ ᎏ␦ffᎏ ϭ΄΂5ᎏ␦DDᎏ΃2ϩ΂1ᎏ␦⌬⌬ppᎏ΃2ϩ΂1ᎏ␦␳␳ᎏ΃2ϩ΂2ᎏ␦QQᎏ΃2ϩ΂1ᎏ␦LLᎏ΃2΅1/2ϭ [{5(0.5%)}2ϩ (2.0%)2ϩ (1.0%)2ϩ {2(3.5%)}2ϩ (0.4%)2]1/2Ϸ 7.8% Ans.By far the dominant effect in this particular calculation is the 3.5 percent error in Q, which isamplified by doubling, due to the power of 2 on flow rate. The diameter uncertainty, which isquintupled, would have contributed more had ␦D been larger than 0.5 percent.The road toward a professional engineer’s license has a first stop, the Fundamentalsof Engineering Examination, known as the FE exam. It was formerly known asthe Engineer-in-Training (E-I-T) Examination. This 8-h national test will probablysoon be required of all engineering graduates, not just for licensure, but as a student-assessment tool. The 120-problem morning session covers many general studies:Chemistry Computers DynamicsElectric circuits Engineering economics Fluid MechanicsMaterials science Mathematics Mechanics of materialsStatics Thermodynamics EthicsFor the 60-problem afternoon session you may choose chemical, civil, electrical, in-dustrial, or mechanical engineering or take more general-engineering problems for re-maining disciplines. As you can see, fluid mechanics is central to the FE exam. There-fore, this text includes a number of end-of-chapter FE problems where appropriate.The format for the FE exam questions is multiple-choice, usually with five selec-tions, chosen carefully to tempt you with plausible answers if you used incorrect unitsor forgot to double or halve something or are missing a factor of ␲, etc. In some cases,the selections are unintentionally ambiguous, such as the following example from aprevious exam:Transition from laminar to turbulent flow occurs at a Reynolds number of(A) 900 (B) 1200 (C) 1500 (D) 2100 (E) 3000The “correct” answer was graded as (D), Re ϭ 2100. Clearly the examiner was think-ing, but forgot to specify, Red for flow in a smooth circular pipe, since (see Chaps. 6and 7) transition is highly dependent upon geometry, surface roughness, and the lengthscale used in the definition of Re. The moral is not to get peevish about the exam butsimply to go with the flow (pun intended) and decide which answer best fits an1.12 The Fundamentals of Engineering (FE) Examination 431.12 The Fundamentals ofEngineering (FE) Examination
    • undergraduate-training situation. Every effort has been made to keep the FE exam ques-tions in this text unambiguous.Fluid flow analysis generates a plethora of problems, 1500 in this text alone! To solvethese problems, one must deal with various equations, data, tables, assumptions, unitsystems, and numbers. The writer recommends these problem-solving steps:1. Gather all the given system parameters and data in one place.2. Find, from tables or charts, all needed fluid property data: ␳, ␮, cp, k, ⌼, etc.3. Use SI units (N, s, kg, m) if possible, and no conversion factors will be neces-sary.4. Make sure what is asked. It is all too common for students to answer the wrongquestion, for example, reporting mass flow instead of volume flow, pressure in-stead of pressure gradient, drag force instead of lift force. Engineers are ex-pected to read carefully.5. Make a detailed sketch of the system, with everything clearly labeled.6. Think carefully and then list your assumptions. Here knowledge is power; youshould not guess the answer. You must be able to decide correctly if the flowcan be considered steady or unsteady, compressible or incompressible, one-dimensional, or multidimensional, viscous or inviscid, and whether a control vol-ume or partial differential equations are needed.7. Based on steps 1 to 6 above, write out the appropriate equations, data correla-tions, and fluid state relations for your problem. If the algebra is straightforward,solve for what is asked. If the equations are complicated, e.g., nonlinear or tooplentiful, use the Engineering Equation Solver (EES).8. Report your solution clearly, with proper units listed and to the proper numberof significant figures (usually two or three) that the overall uncertainty of thedata will allow.Like most scientific disciplines, fluid mechanics has a history of erratically occurringearly achievements, then an intermediate era of steady fundamental discoveries in theeighteenth and nineteenth centuries, leading to the twentieth-century era of “modernpractice,” as we self-centeredly term our limited but up-to-date knowledge. Ancientcivilizations had enough knowledge to solve certain flow problems. Sailing ships withoars and irrigation systems were both known in prehistoric times. The Greeks producedquantitative information. Archimedes and Hero of Alexandria both postulated the par-allelogram law for addition of vectors in the third century B.C. Archimedes (285–212B.C.) formulated the laws of buoyancy and applied them to floating and submergedbodies, actually deriving a form of the differential calculus as part of the analysis. TheRomans built extensive aqueduct systems in the fourth century B.C. but left no recordsshowing any quantitative knowledge of design principles.From the birth of Christ to the Renaissance there was a steady improvement in thedesign of such flow systems as ships and canals and water conduits but no recordedevidence of fundamental improvements in flow analysis. Then Leonardo da Vinci(1452–1519) derived the equation of conservation of mass in one-dimensional steady44 Chapter 1 Introduction1.13 Problem-SolvingTechniques1.14 History and Scope ofFluid Mechanics
    • 1.14 History and Scope of Fluid Mechanics 45flow. Leonardo was an excellent experimentalist, and his notes contain accurate de-scriptions of waves, jets, hydraulic jumps, eddy formation, and both low-drag (stream-lined) and high-drag (parachute) designs. A Frenchman, Edme Mariotte (1620–1684),built the first wind tunnel and tested models in it.Problems involving the momentum of fluids could finally be analyzed after Isaac New-ton (1642–1727) postulated his laws of motion and the law of viscosity of the linear flu-ids now called newtonian. The theory first yielded to the assumption of a “perfect” orfrictionless fluid, and eighteenth-century mathematicians (Daniel Bernoulli, LeonhardEuler, Jean d’Alembert, Joseph-Louis Lagrange, and Pierre-Simon Laplace) producedmany beautiful solutions of frictionless-flow problems. Euler developed both the differ-ential equations of motion and their integrated form, now called the Bernoulli equation.D’Alembert used them to show his famous paradox: that a body immersed in a friction-less fluid has zero drag. These beautiful results amounted to overkill, since perfect-fluidassumptions have very limited application in practice and most engineering flows aredominated by the effects of viscosity. Engineers began to reject what they regarded as atotally unrealistic theory and developed the science of hydraulics, relying almost entirelyon experiment. Such experimentalists as Chézy, Pitot, Borda, Weber, Francis, Hagen,Poiseuille, Darcy, Manning, Bazin, and Weisbach produced data on a variety of flowssuch as open channels, ship resistance, pipe flows, waves, and turbines. All too often thedata were used in raw form without regard to the fundamental physics of flow.At the end of the nineteenth century, unification between experimental hydraulicsand theoretical hydrodynamics finally began. William Froude (1810–1879) and his sonRobert (1846–1924) developed laws of model testing, Lord Rayleigh (1842–1919) pro-posed the technique of dimensional analysis, and Osborne Reynolds (1842–1912) pub-lished the classic pipe experiment in 1883 which showed the importance of the di-mensionless Reynolds number named after him. Meanwhile, viscous-flow theory wasavailable but unexploited, since Navier (1785–1836) and Stokes (1819–1903) had suc-cessfully added newtonian viscous terms to the equations of motion. The resultingNavier-Stokes equations were too difficult to analyze for arbitrary flows. Then, in 1904,a German engineer, Ludwig Prandtl (1875–1953), published perhaps the most impor-tant paper ever written on fluid mechanics. Prandtl pointed out that fluid flows withsmall viscosity, e.g., water flows and airflows, can be divided into a thin viscous layer,or boundary layer, near solid surfaces and interfaces, patched onto a nearly inviscidouter layer, where the Euler and Bernoulli equations apply. Boundary-layer theory hasproved to be the single most important tool in modern flow analysis. The twentieth-century foundations for the present state of the art in fluid mechanics were laid in aseries of broad-based experiments and theories by Prandtl and his two chief friendlycompetitors, Theodore von Kármán (1881–1963) and Sir Geoffrey I. Taylor (1886–1975). Many of the results sketched here from a historical point of view will, of course,be discussed in this textbook. More historical details can be found in Refs. 23 to 25.Since the earth is 75 percent covered with water and 100 percent covered with air,the scope of fluid mechanics is vast and touches nearly every human endeavor. Thesciences of meteorology, physical oceanography, and hydrology are concerned withnaturally occurring fluid flows, as are medical studies of breathing and blood circula-tion. All transportation problems involve fluid motion, with well-developed specialtiesin aerodynamics of aircraft and rockets and in naval hydrodynamics of ships and sub-marines. Almost all our electric energy is developed either from water flow or from
    • P1.4 A beaker approximates a right circular cone of diameter 7 inand height 9 in. When filled with liquid, it weighs 70 oz.When empty, it weighs 14 oz. Estimate the density of thisliquid in both SI and BG units.P1.5 The mean free path of a gas, ᐍ, is defined as the averagedistance traveled by molecules between collisions. A pro-posed formula for estimating ᐍ of an ideal gas isᐍ ϭ 1.26ᎏ␳͙␮RෆTෆᎏWhat are the dimensions of the constant 1.26? Use the for-mula to estimate the mean free path of air at 20°C and 7 kPa.Would you consider air rarefied at this condition?P1.6 In the {MLT⌰} system, what is the dimensional represen-tation of (a) enthalpy, (b) mass rate of flow, (c) bending mo-ment, (d) angular velocity, (e) modulus of elasticity;(f) Poisson’s ratio?P1.7 A small village draws 1.5 acre и ft/day of water from itsreservoir. Convert this average water usage to (a) gallonsper minute and (b) liters per second.P1.8 Suppose we know little about the strength of materialsbut are told that the bending stress ␴ in a beam is pro-portional to the beam half-thickness y and also dependsupon the bending moment M and the beam area momentof inertia I. We also learn that, for the particular caseM ϭ 2900 in и lbf, y ϭ 1.5 in, and I ϭ 0.4 in4, the pre-dicted stress is 75 MPa. Using this information and di-mensional reasoning only, find, to three significant fig-ures, the only possible dimensionally homogeneousformula ␴ ϭ y f(M, I).ProblemsMost of the problems herein are fairly straightforward. More diffi-cult or open-ended assignments are labeled with an asterisk as inProb. 1.18. Problems labeled with an EES icon (for example, Prob.2.62), will benefit from the use of the Engineering Equation Solver(EES), while problems labeled with a computer disk may requirethe use of a computer. The standard end-of-chapter problems 1.1 to1.85 (categorized in the problem list below) are followed by fun-damentals of engineering (FE) exam problems FE3.1 to FE3.10,and comprehensive problems C1.1 to C1.4.Problem DistributionSection Topic Problems1.1, 1.2, 1.3 Fluid-continuum concept 1.1–1.31.4 Dimensions, units, dynamics 1.4–1.201.5 Velocity field 1.21–1.231.6 Thermodynamic properties 1.24–1.371.7 Viscosity; no-slip condition 1.38–1.611.7 Surface tension 1.62–1.711.7 Vapor pressure; cavitation 1.72–1.751.7 Speed of sound; Mach number 1.76–1.781.8,9 Flow patterns, streamlines, pathlines 1.79–1.841.10 History of fluid mechanics 1.85P1.1 A gas at 20°C may be considered rarefied, deviating fromthe continuum concept, when it contains less than 1012mol-ecules per cubic millimeter. If Avogadro’s number is 6.023E23 molecules per mole, what absolute pressure (in Pa) forair does this represent?P1.2 Table A.6 lists the density of the standard atmosphere as afunction of altitude. Use these values to estimate, crudely—say, within a factor of 2—the number of molecules of airin the entire atmosphere of the earth.P1.3 For the triangular element in Fig. P1.3, show that a tiltedfree liquid surface, in contact with an atmosphere at pres-sure pa, must undergo shear stress and hence begin to flow.Hint: Account for the weight of the fluid and show that ano-shear condition will cause horizontal forces to be out ofbalance.steam flow through turbine generators. All combustion problems involve fluid motion,as do the more classic problems of irrigation, flood control, water supply, sewage dis-posal, projectile motion, and oil and gas pipelines. The aim of this book is to presentenough fundamental concepts and practical applications in fluid mechanics to prepareyou to move smoothly into any of these specialized fields of the science of flow—andthen be prepared to move out again as new technologies develop.46 Chapter 1 IntroductionpaFluid density ␳␪P1.3
    • where Q is the volume rate of flow and ⌬p is the pressurerise produced by the pump. Suppose that a certain pumpdevelops a pressure rise of 35 lbf/in2when its flow rate is40 L/s. If the input power is 16 hp, what is the efficiency?*P1.14 Figure P1.14 shows the flow of water over a dam. The vol-ume flow Q is known to depend only upon crest width B,acceleration of gravity g, and upstream water height Habove the dam crest. It is further known that Q is propor-tional to B. What is the form of the only possible dimen-sionally homogeneous relation for this flow rate?P1.9 The kinematic viscosity of a fluid is the ratio of viscosityto density, ␯ ϭ ␮/␳. What is the only possible dimension-less group combining ␯ with velocity V and length L? Whatis the name of this grouping? (More information on thiswill be given in Chap. 5.)P1.10 The Stokes-Oseen formula [18] for drag force F on asphere of diameter D in a fluid stream of low velocity V,density ␳, and viscosity ␮, isF ϭ 3␲␮DV ϩ ᎏ91␲6ᎏ␳V2D2Is this formula dimensionally homogeneous?P1.11 Engineers sometimes use the following formula for thevolume rate of flow Q of a liquid flowing through a holeof diameter D in the side of a tank:Q ϭ 0.68 D2͙gෆhෆwhere g is the acceleration of gravity and h is the heightof the liquid surface above the hole. What are the dimen-sions of the constant 0.68?P1.12 For low-speed (laminar) steady flow through a circularpipe, as shown in Fig. P1.12, the velocity u varies with ra-dius and takes the formu ϭ Bᎏ⌬␮pᎏ(r02Ϫ r2)where ␮ is the fluid viscosity and ⌬p is the pressure dropfrom entrance to exit. What are the dimensions of the con-stant B?Problems 47r = 0ru (r)Pipe wallr = r0P1.13 The efficiency ␩ of a pump is defined as the (dimension-less) ratio of the power developed by the flow to the powerrequired to drive the pump:␩ ϭ ᎏinpuQt⌬popwerᎏP1.12HBQWater levelDamP1.15 As a practical application of Fig. P1.14, often termed asharp-crested weir, civil engineers use the following for-mula for flow rate: Q Ϸ 3.3BH3/2, with Q in ft3/s and Band H in feet. Is this formula dimensionally homogeneous?If not, try to explain the difficulty and how it might be con-verted to a more homogeneous form.P1.16 Algebraic equations such as Bernoulli’s relation, Eq. (1)of Ex. 1.3, are dimensionally consistent, but what aboutdifferential equations? Consider, for example, the bound-ary-layer x-momentum equation, first derived by LudwigPrandtl in 1904:␳uᎏѨѨuxᎏ ϩ ␳vᎏѨѨuyᎏ ϭ ϪᎏѨѨpxᎏ ϩ ␳gx ϩ ᎏѨѨ␶yᎏwhere ␶ is the boundary-layer shear stress and gx is thecomponent of gravity in the x direction. Is this equationdimensionally consistent? Can you draw a general con-clusion?P1.17 The Hazen-Williams hydraulics formula for volume rate offlow Q through a pipe of diameter D and length L is given byQ Ϸ 61.9 D2.63΂ᎏ⌬Lpᎏ΃0.54P1.14
    • Q/Across-section and the dimensionless Reynolds number of theflow, Re ϭ ␳VavgD/␮. Comment on your results.P1.24 Air at 1 atm and 20°C has an internal energy of approxi-mately 2.1 E5 J/kg. If this air moves at 150 m/s at an al-titude z ϭ 8 m, what is its total energy, in J/kg, relative tothe datum z ϭ 0? Are any energy contributions negligible?P1.25 A tank contains 0.9 m3of helium at 200 kPa and 20°C.Estimate the total mass of this gas, in kg, (a) on earth and(b) on the moon. Also, (c) how much heat transfer, in MJ,is required to expand this gas at constant temperature to anew volume of 1.5 m3?P1.26 When we in the United States say a car’s tire is filled “to32 lb,” we mean that its internal pressure is 32 lbf/in2abovethe ambient atmosphere. If the tire is at sea level, has avolume of 3.0 ft3, and is at 75°F, estimate the total weightof air, in lbf, inside the tire.P1.27 For steam at 40 lbf/in2, some values of temperature andspecific volume are as follows, from Ref. 13:T, °F 400 500 600 700 800v, ft3/lbm 12.624 14.165 15.685 17.195 18.699Is steam, for these conditions, nearly a perfect gas, or is itwildly nonideal? If reasonably perfect, find a least-squares†value for the gas constant R, in m2/(s2и K), estimate the per-cent error in this approximation, and compare with TableA.4.P1.28 Wet atmospheric air at 100 percent relative humidity con-tains saturated water vapor and, by Dalton’s law of partialpressures,patm ϭ pdry air ϩ pwater vaporSuppose this wet atmosphere is at 40°C and 1 atm. Cal-culate the density of this 100 percent humid air, and com-pare it with the density of dry air at the same conditions.P1.29 A compressed-air tank holds 5 ft3of air at 120 lbf/in2“gage,” that is, above atmospheric pressure. Estimate theenergy, in ft-lbf, required to compress this air from the at-mosphere, assuming an ideal isothermal process.P1.30 Repeat Prob. 1.29 if the tank is filled with compressed wa-ter instead of air. Why is the result thousands of times lessthan the result of 215,000 ft и lbf in Prob. 1.29?*P1.31 The density of (fresh) water at 1 atm, over the tempera-ture range 0 to 100°C, is given in Table A.1. Fit these val-ues to a least-squares†equation of the form ␳ ϭ a ϩ bT ϩcT2, with T in °C, and estimate its accuracy. Use your for-mula to compute the density of water at 45°C, and com-pare your result with the accepted experimental value of990.1 kg/m3.where ⌬p is the pressure drop required to drive the flow.What are the dimensions of the constant 61.9? Can this for-mula be used with confidence for various liquids and gases?*P1.18 For small particles at low velocities, the first term in theStokes-Oseen drag law, Prob. 1.10, is dominant; hence,F Ϸ KV, where K is a constant. Suppose a particle of massm is constrained to move horizontally from the initial po-sition x ϭ 0 with initial velocity V0. Show (a) that its ve-locity will decrease exponentially with time and (b) that itwill stop after traveling a distance x ϭ mV0/K.*P1.19 For larger particles at higher velocities, the quadratic termin the Stokes-Oseen drag law, Prob. 1.10, is dominant;hence, F Ϸ CV2, where C is a constant. Repeat Prob. 1.18to show that (a) its velocity will decrease as 1/(1 ϩ CV0t/m)and (b) it will never quite stop in a finite time span.P1.20 A baseball, with m ϭ 145 g, is thrown directly upward fromthe initial position z ϭ 0 and V0 ϭ 45 m/s. The air drag onthe ball is CV2, as in Prob. 1.19, where C Ϸ 0.0013 N иs2/m2. Set up a differential equation for the ball motion, andsolve for the instantaneous velocity V(t) and position z(t).Find the maximum height zmax reached by the ball, andcompare your results with the classical case of zero air drag.P1.21 A velocity field is given by V ϭ Kxti Ϫ Kytj ϩ 0k, whereK is a positive constant. Evaluate (a) ٌ ؒ ١ and (b)ٌ ؋ V.*P1.22 According to the theory of Chap. 8, as a uniform streamapproaches a cylinder of radius R along the symmetry lineAB in Fig. P1.22, the velocity has only one component:u ϭ Uϱ΂1 Ϫ ᎏRx22ᎏ΃ for Ϫϱ Ͻ x Յ Ϫ Rwhere Uؕ is the stream velocity far from the cylinder. Us-ing the concepts from Ex. 1.5, find (a) the maximum flowdeceleration along AB and (b) its location.48 Chapter 1 IntroductionEESyxuRBAP1.22P1.23 Experiment with a faucet (kitchen or otherwise) to determinetypical flow rates Q in m3/h, perhaps timing the discharge ofa known volume. Try to achieve an exit jet condition whichis (a) smooth and round and (b) disorderly and fluctuating.Measure the supply-pipe diameter (look under the sink). Forboth cases, calculate the average flow velocity, Vavg ϭ†The concept of “least-squares” error is very important and should belearned by everyone.
    • P1.32 A blimp is approximated by a prolate spheroid 90 m longand 30 m in diameter. Estimate the weight of 20°C gaswithin the blimp for (a) helium at 1.1 atm and (b) air at1.0 atm. What might the difference between these two val-ues represent (see Chap. 2)?*P1.33 Experimental data for the density of mercury versus pres-sure at 20°C are as follows:p, atm 1 500 1,000 1,500 2,000␳, kg/m313,545 13,573 13,600 13,625 13,653Fit this data to the empirical state relation for liquids,Eq. (1.22), to find the best values of B and n for mercury.Then, assuming the data are nearly isentropic, use thesevalues to estimate the speed of sound of mercury at 1 atmand compare with Table 9.1.P1.34 If water occupies 1 m3at 1 atm pressure, estimate the pres-sure required to reduce its volume by 5 percent.P1.35 In Table A.4, most common gases (air, nitrogen, oxygen,hydrogen) have a specific heat ratio k Ϸ 1.40. Why do ar-gon and helium have such high values? Why does NH3have such a low value? What is the lowest k for any gasthat you know of?P1.36 The isentropic bulk modulus B of a fluid is defined as theisentropic change in pressure per fractional change in den-sity:B ϭ ␳΂ᎏѨѨp␳ᎏ΃sWhat are the dimensions of B? Using theoretical p(␳) re-lations, estimate the bulk modulus of (a) N2O, assumed tobe an ideal gas, and (b) water, at 20°C and 1 atm.P1.37 A near-ideal gas has a molecular weight of 44 and a spe-cific heat cv ϭ 610 J/(kg и K). What are (a) its specific heatratio, k, and (b) its speed of sound at 100°C?P1.38 In Fig. 1.6, if the fluid is glycerin at 20°C and the width be-tween plates is 6 mm, what shear stress (in Pa) is requiredto move the upper plate at 5.5 m/s? What is the Reynoldsnumber if L is taken to be the distance between plates?P1.39 Knowing ␮ for air at 20°C from Table 1.4, estimate its vis-cosity at 500°C by (a) the power law and (b) the Suther-land law. Also make an estimate from (c) Fig. 1.5. Com-pare with the accepted value of ␮ Ϸ 3.58 E-5 kg/m и s.*P1.40 For liquid viscosity as a function of temperature, a sim-plification of the log-quadratic law of Eq. (1.31) is An-drade’s equation [11], ␮ Ϸ A exp (B/T), where (A, B) arecurve-fit constants and T is absolute temperature. Fit thisrelation to the data for water in Table A.1 and estimate thepercent error of the approximation.P1.41 Some experimental values of the viscosity of argon gas at1 atm are as follows:T, K 300 400 500 600 700 800␮, kg/(m и s) 2.27 E-5 2.85 E-5 3.37 E-5 3.83 E-5 4.25 E-5 4.64 E-5Fit these value to either (a) a power law or (b) the Suther-land law, Eq. (1.30).P1.42 Experimental values for the viscosity of helium at 1 atmare as follows:T, K 200 400 600 800 1000 1200␮, kg/(m и s) 1.50 E-5 2.43 E-5 3.20 E-5 3.88 E-5 4.50 E-5 5.08 E-5Fit these values to either (a) a power law or (b) the Suther-land law, Eq. (1.30).*P1.43 Yaws et al. [34] suggest the following curve-fit formulafor viscosity versus temperature of organic liquids:log10 ␮ Ϸ A ϩ ᎏBTᎏ ϩ CT ϩ DT2with T in absolute units. (a) Can this formula be criticizedon dimensional grounds? (b) Disregarding (a), indicate an-alytically how the curve-fit constants A, B, C, D could befound from N data points (␮i, Ti) using the method of leastsquares. Do not actually carry out a calculation.P1.44 The values for SAE 30 oil in Table 1.4 are strictly “rep-resentative,” not exact, because lubricating oils vary con-siderably according to the type of crude oil from whichthey are refined. The Society ofAutomotive Engineers [26]allows certain kinematic viscosity ranges for all lubricat-ing oils: for SAE 30, 9.3 Ͻ ␯ Ͻ 12.5 mm2/s at 100°C. SAE30 oil density can also vary Ϯ2 percent from the tabulatedvalue of 891 kg/m3. Consider the following data for an ac-ceptable grade of SAE 30 oil:T, °C 0 20 40 60 80 100␮, kg/(m и s) 2.00 0.40 0.11 0.042 0.017 0.0095How does this oil compare with the plot in Appendix Fig.A.1? How well does the data fit Andrade’s equation inProb. 1.40?P1.45 A block of weight W slides down an inclined plane whilelubricated by a thin film of oil, as in Fig. P1.45. The filmcontact area is A and its thickness is h. Assuming a linearvelocity distribution in the film, derive an expression forthe “terminal” (zero-acceleration) velocity V of the block.P1.46 Find the terminal velocity of the block in Fig. P1.45 if theblock mass is 6 kg, A ϭ 35 cm2, ␪ ϭ 15°, and the film is1-mm-thick SAE 30 oil at 20°C.P1.47 A shaft 6.00 cm in diameter is being pushed axiallythrough a bearing sleeve 6.02 cm in diameter and 40 cmlong. The clearance, assumed uniform, is filled with oilProblems 49EES
    • whose properties are ␯ ϭ 0.003 m2/s and SG ϭ 0.88. Es-timate the force required to pull the shaft at a steady ve-locity of 0.4 m/s.P1.48 A thin plate is separated from two fixed plates by very vis-cous liquids ␮1 and ␮2, respectively, as in Fig. P1.48. Theplate spacings h1 and h2 are unequal, as shown. The con-tact area is A between the center plate and each fluid.(a) Assuming a linear velocity distribution in each fluid,derive the force F required to pull the plate at velocity V.(b) Is there a necessary relation between the two viscosi-ties, ␮1 and ␮2?sured torque is 0.293 N и m, what is the fluid viscosity?Suppose that the uncertainties of the experiment are as fol-lows: L (Ϯ0.5 mm), M (Ϯ0.003 N и m), ⍀ (Ϯ1 percent),and ri or ro (Ϯ0.02 mm). What is the uncertainty in themeasured viscosity?P1.52 The belt in Fig. P1.52 moves at a steady velocity V andskims the top of a tank of oil of viscosity ␮, as shown. As-suming a linear velocity profile in the oil, develop a sim-ple formula for the required belt-drive power P as a func-tion of (h, L, V, b, ␮). What belt-drive power P, in watts,is required if the belt moves at 2.5 m/s over SAE 30W oilat 20°C, with L ϭ 2 m, b ϭ 60 cm, and h ϭ 3 cm?50 Chapter 1 IntroductionLiquid film ofthickness hWV Block contactarea AθP1.45P1.48h1h2µ1µ2F, VP1.49 The shaft in Prob. 1.47 is now fixed axially and rotated in-side the sleeve at 1500 r/min. Estimate (a) the torque(N и m) and (b) the power (kW) required to rotate the shaft.P1.50 An amazing number of commercial and laboratory deviceshave been developed to measure the viscosity of fluids, asdescribed in Ref. 27. The concentric rotating shaft of Prob.1.49 is an example of a rotational viscometer. Let the in-ner and outer cylinders have radii ri and ro, respectively,with total sleeve length L. Let the rotational rate be ⍀(rad/s) and the applied torque be M. Derive a theoreticalrelation for the viscosity of the clearance fluid, ␮, in termsof these parameters.P1.51 Use the theory of Prob. 1.50 (or derive an ad hoc expres-sion if you like) for a shaft 8 cm long, rotating at 1200r/min, with ri ϭ 2.00 cm and ro ϭ 2.05 cm. If the mea-LVOil, depth hMoving belt, width bP1.52*P1.53 A solid cone of angle 2␪, base r0, and density ␳c is rotat-ing with initial angular velocity ␻0 inside a conical seat,as shown in Fig. P1.53. The clearance h is filled with oilof viscosity ␮. Neglecting air drag, derive an analytical ex-pression for the cone’s angular velocity ␻(t) if there is noapplied torque.OilhBaseradius r0ω(t)2θP1.53*P1.54 A disk of radius R rotates at an angular velocity ⍀ insidea disk-shaped container filled with oil of viscosity ␮, asshown in Fig. P1.54. Assuming a linear velocity profileand neglecting shear stress on the outer disk edges, derivea formula for the viscous torque on the disk.
    • *P1.55 The device in Fig. P1.54 is called a rotating disk viscometer[27]. Suppose that R ϭ 5 cm and h ϭ 1 mm. If the torquerequired to rotate the disk at 900 r/min is 0.537 N и m,what is the viscosity of the fluid? If the uncertainty in eachparameter (M, R, h, ⍀) is Ϯ1 percent, what is the overalluncertainty in the viscosity?*P1.56 The device in Fig. P1.56 is called a cone-plate viscometer[27]. The angle of the cone is very small, so that sin ␪ Ϸ␪, and the gap is filled with the test liquid. The torque Mto rotate the cone at a rate ⍀ is measured. Assuming a lin-ear velocity profile in the fluid film, derive an expressionfor fluid viscosity ␮ as a function of (M, R, ⍀, ␪).␮ ϭ ᎏ␲8rL04Q⌬pᎏPipe end effects are neglected [27]. Suppose our capillaryhas r0 ϭ 2 mm and L ϭ 25 cm. The following flow rateand pressure drop data are obtained for a certain fluid:Q, m3/h 0.36 0.72 1.08 1.44 1.80⌬p, kPa 159 318 477 1274 1851What is the viscosity of the fluid? Note: Only the first threepoints give the proper viscosity. What is peculiar about thelast two points, which were measured accurately?P1.59 A solid cylinder of diameter D, length L, and density ␳sfalls due to gravity inside a tube of diameter D0. The clear-ance, D0 Ϫ D ϽϽ D, is filled with fluid of density ␳ andviscosity ␮. Neglect the air above and below the cylinder.Derive a formula for the terminal fall velocity of the cylin-der. Apply your formula to the case of a steel cylinder,D ϭ 2 cm, D0 ϭ 2.04 cm, L ϭ 15 cm, with a film of SAE30 oil at 20°C.P1.60 For Prob. 1.52 suppose that P ϭ 0.1 hp when V ϭ 6 ft/s,L ϭ 4.5 ft, b ϭ 22 in, and h ϭ 7/8 in. Estimate the vis-cosity of the oil, in kg/(m и s). If the uncertainty in eachparameter (P, L, b, h, V) is Ϯ1 percent, what is the over-all uncertainty in the viscosity?*P1.61 An air-hockey puck has a mass of 50 g and is 9 cm in di-ameter. When placed on the air table, a 20°C air film, of0.12-mm thickness, forms under the puck. The puck isstruck with an initial velocity of 10 m/s. Assuming a lin-ear velocity distribution in the air film, how long will ittake the puck to (a) slow down to 1 m/s and (b) stop com-pletely? Also, (c) how far along this extremely long tablewill the puck have traveled for condition (a)?P1.62 The hydrogen bubbles which produced the velocity pro-files in Fig. 1.13 are quite small, D Ϸ 0.01 mm. If the hy-drogen-water interface is comparable to air-water and thewater temperature is 30°C estimate the excess pressurewithin the bubble.P1.63 Derive Eq. (1.37) by making a force balance on the fluidinterface in Fig. 1.9c.P1.64 At 60°C the surface tension of mercury and water is 0.47and 0.0662 N/m, respectively. What capillary heightchanges will occur in these two fluids when they are incontact with air in a clean glass tube of diameter 0.4 mm?P1.65 The system in Fig. P1.65 is used to calculate the pressurep1 in the tank by measuring the 15-cm height of liquid inthe 1-mm-diameter tube. The fluid is at 60°C (see Prob.1.64). Calculate the true fluid height in the tube and thepercent error due to capillarity if the fluid is (a) water and(b) mercury.Problems 51R RClearancehOilΩP1.54ΩFluidRθ θ*P1.57 For the cone-plate viscometer of Fig. P1.56, suppose thatR ϭ 6 cm and ␪ ϭ 3°. If the torque required to rotate thecone at 600 r/min is 0.157 N и m, what is the viscosity ofthe fluid? If the uncertainty in each parameter (M, R, ⍀,␪) is Ϯ1 percent, what is the overall uncertainty in the vis-cosity?*P1.58 The laminar-pipe-flow example of Prob. 1.12 can be usedto design a capillary viscometer [27]. If Q is the volumeflow rate, L is the pipe length, and ⌬p is the pressure dropfrom entrance to exit, the theory of Chap. 6 yields a for-mula for viscosity:P1.56
    • P1.66 A thin wire ring, 3 cm in diameter, is lifted from a watersurface at 20°C. Neglecting the wire weight, what is theforce required to lift the ring? Is this a good way to mea-sure surface tension? Should the wire be made of any par-ticular material?P1.67 Experiment with a capillary tube, perhaps borrowed fromthe chemistry department, to verify, in clean water, the risedue to surface tension predicted by Example 1.9.Add smallamounts of liquid soap to the water, and report to the classwhether detergents significantly lower the surface tension.What practical difficulties do detergents present?*P1.68 Make an analysis of the shape ␩(x) of the water-air inter-face near a plane wall, as in Fig. P1.68, assuming that theslope is small, RϪ1Ϸ d2␩/dx2. Also assume that the pres-sure difference across the interface is balanced by the spe-cific weight and the interface height, ⌬p Ϸ ␳g␩. Theboundary conditions are a wetting contact angle ␪ at x ϭ 0and a horizontal surface ␩ ϭ 0 as x → ϱ. What is the max-imum height h at the wall?mula for the maximum diameter dmax able to float in theliquid. Calculate dmax for a steel needle (SG ϭ 7.84) inwater at 20°C.P1.70 Derive an expression for the capillary height change h fora fluid of surface tension Y and contact angle ␪ betweentwo vertical parallel plates a distance W apart, as in Fig.P1.70. What will h be for water at 20°C if W ϭ 0.5 mm?52 Chapter 1 Introduction15 cmp1P1.65(x)θx = 0y = hyxηP1.68P1.69 A solid cylindrical needle of diameter d, length L, and den-sity ␳n may float in liquid of surface tension Y. Neglectbuoyancy and assume a contact angle of 0°. Derive a for-WhθP1.70*P1.71 A soap bubble of diameter D1 coalesces with another bub-ble of diameter D2 to form a single bubble D3 with thesame amount of air. Assuming an isothermal process, de-rive an expression for finding D3 as a function of D1, D2,patm, and Y.P1.72 Early mountaineers boiled water to estimate their altitude.If they reach the top and find that water boils at 84°C, ap-proximately how high is the mountain?P1.73 A small submersible moves at velocity V, in fresh waterat 20°C, at a 2-m depth, where ambient pressure is 131kPa. Its critical cavitation number is known to be Ca ϭ0.25.At what velocity will cavitation bubbles begin to formon the body? Will the body cavitate if V ϭ 30 m/s and thewater is cold (5°C)?P1.74 A propeller is tested in a water tunnel at 20°C as in Fig.1.12a. The lowest pressure on the blade can be estimatedby a form of Bernoulli’s equation (Ex. 1.3):pmin Ϸ p0 Ϫ ᎏ12ᎏ␳V2where p0 ϭ 1.5 atm and V ϭ tunnel velocity. If we run thetunnel at V ϭ 18 m/s, can we be sure that there will be nocavitation? If not, can we change the water temperatureand avoid cavitation?P1.75 Oil, with a vapor pressure of 20 kPa, is delivered througha pipeline by equally spaced pumps, each of which in-creases the oil pressure by 1.3 MPa. Friction losses in thepipe are 150 Pa per meter of pipe. What is the maximumpossible pump spacing to avoid cavitation of the oil?EES
    • P1.76 An airplane flies at 555 mi/h. At what altitude in the stan-dard atmosphere will the airplane’s Mach number be ex-actly 0.8?*P1.77 The density of 20°C gasoline varies with pressure ap-proximately as follows:p, atm 1 500 1000 1500␳, lbm/ft342.45 44.85 46.60 47.98Use these data to estimate (a) the speed of sound (m/s)and (b) the bulk modulus (MPa) of gasoline at 1 atm.P1.78 Sir Isaac Newton measured the speed of sound by timingthe difference between seeing a cannon’s puff of smokeand hearing its boom. If the cannon is on a mountain 5.2 miaway, estimate the air temperature in degrees Celsius if thetime difference is (a) 24.2 s and (b) 25.1 s.P1.79 Examine the photographs in Figs. 1.12a, 1.13, 5.2a, 7.14a,and 9.10b and classify them according to the boxes in Fig.1.14.*P1.80 A two-dimensional steady velocity field is given by u ϭx2Ϫ y2, v ϭ Ϫ2xy. Derive the streamline pattern andsketch a few streamlines in the upper half plane. Hint: Thedifferential equation is exact.P1.81 Repeat Ex. 1.10 by letting the velocity components in-crease linearly with time:V ϭ Kxti Ϫ Kytj ϩ 0kFind and sketch, for a few representative times, the in-stantaneous streamlines. How do they differ from thesteady flow lines in Ex. 1.10?P1.82 A velocity field is given by u ϭ V cos ␪, v ϭ V sin ␪, andw ϭ 0, where V and ␪ are constants. Derive a formula forthe streamlines of this flow.*P1.83 A two-dimensional unsteady velocity field is given by u ϭx(1 ϩ 2t), v ϭ y. Find the equation of the time-varyingstreamlines which all pass through the point (x0, y0) atsome time t. Sketch a few of these.*P1.84 Repeat Prob. 1.83 to find and sketch the equation of thepathline which passes through (x0, y0) at time t ϭ 0.P1.85 Do some reading and report to the class on the life andachievements, especially vis-à-vis fluid mechanics, of(a) Evangelista Torricelli (1608–1647)(b) Henri de Pitot (1695–1771)(c) Antoine Chézy (1718–1798)(d) Gotthilf Heinrich Ludwig Hagen (1797–1884)(e) Julius Weisbach (1806–1871)(f) George Gabriel Stokes (1819–1903)(g) Moritz Weber (1871–1951)(h) Theodor von Kármán (1881–1963)(i) Paul Richard Heinrich Blasius (1883–1970)(j) Ludwig Prandtl (1875–1953)(k) Osborne Reynolds (1842–1912)(l) John William Strutt, Lord Rayleigh (1842–1919)(m) Daniel Bernoulli (1700–1782)(n) Leonhard Euler (1707–1783)Fundamentals of Engineering Exam Problems 53Fundamentals of Engineering Exam ProblemsFE1.1 The absolute viscosity ␮ of a fluid is primarily a func-tion of(a) Density, (b) Temperature, (c) Pressure, (d) Velocity,(e) Surface tensionFE1.2 If a uniform solid body weighs 50 N in air and 30 N inwater, its specific gravity is(a) 1.5, (b) 1.67, (c) 2.5, (d) 3.0, (e) 5.0FE1.3 Helium has a molecular weight of 4.003. What is theweight of 2 m3of helium at 1 atm and 20°C?(a) 3.3 N, (b) 6.5 N, (c) 11.8 N, (d) 23.5 N, (e) 94.2 NFE1.4 An oil has a kinematic viscosity of 1.25 E-4 m2/s and aspecific gravity of 0.80. What is its dynamic (absolute)viscosity in kg/(m и s)?(a) 0.08, (b) 0.10, (c) 0.125, (d) 1.0, (e) 1.25FE1.5 Consider a soap bubble of diameter 3 mm. If the surfacetension coefficient is 0.072 N/m and external pressure is0 Pa gage, what is the bubble’s internal gage pressure?(a) Ϫ24 Pa, (b) ϩ48 Pa, (c) ϩ96 Pa, (d) ϩ192 Pa,(e) Ϫ192 PaFE1.6 The only possible dimensionless group which combinesvelocity V, body size L, fluid density ␳, and surface ten-sion coefficient ␴ is(a) L␳␴/V, (b) ␳VL2/␴, (c) ␳␴V2/L, (d) ␴LV2/␳,(e) ␳LV2/␴FE1.7 Two parallel plates, one moving at 4 m/s and the otherfixed, are separated by a 5-mm-thick layer of oil of spe-cific gravity 0.80 and kinematic viscosity 1.25 E-4 m2/s.What is the average shear stress in the oil?(a) 80 Pa, (b) 100 Pa, (c) 125 Pa, (d) 160 Pa, (e) 200 PaFE1.8 Carbon dioxide has a specific heat ratio of 1.30 and agas constant of 189 J/(kg и °C). If its temperature risesfrom 20 to 45°C, what is its internal energy rise?(a) 12.6 kJ/kg, (b) 15.8 kJ/kg, (c) 17.6 kJ/kg, (d) 20.5kJ/kg, (e) 25.1 kJ/kgEES
    • FE1.9 A certain water flow at 20°C has a critical cavitationnumber, where bubbles form, Ca Ϸ 0.25, where Ca ϭ2(pa Ϫ pvap)/␳V2. If pa ϭ 1 atm and the vapor pressureis 0.34 pounds per square inch absolute (psia), for whatwater velocity will bubbles form?(a) 12 mi/h, (b) 28 mi/h, (c) 36 mi/h, (d) 55 mi/h,(e) 63 mi/hFE1.10 A steady incompressible flow, moving through a con-traction section of length L, has a one-dimensional av-erage velocity distribution given by u Ϸ U0(1 ϩ 2x/L).What is its convective acceleration at the end of the con-traction, x ϭ L?(a) U02/L, (b) 2U02/L, (c) 3U02/L, (d) 4U02/L, (e) 6U02/L54 Chapter 1 IntroductionComprehensive ProblemsC1.1 Sometimes equations can be developed and practical prob-lems can be solved by knowing nothing more than the di-mensions of the key parameters in the problem. For ex-ample, consider the heat loss through a window in abuilding. Window efficiency is rated in terms of “R value”which has units of (ft2и h и °F)/Btu. A certain manufac-turer advertises a double-pane window with an R value of2.5. The same company produces a triple-pane windowwith an R value of 3.4. In either case the window dimen-sions are 3 ft by 5 ft. On a given winter day, the temper-ature difference between the inside and outside of thebuilding is 45°F.(a) Develop an equation for the amount of heat lost in agiven time period ⌬t, through a window of area A, withR value R, and temperature difference ⌬T. How muchheat (in Btu) is lost through the double-pane windowin one 24-h period?(b) How much heat (in Btu) is lost through the triple-panewindow in one 24-h period?(c) Suppose the building is heated with propane gas, whichcosts $1.25 per gallon. The propane burner is 80 per-cent efficient. Propane has approximately 90,000 Btuof available energy per gallon. In that same 24-h pe-riod, how much money would a homeowner save perwindow by installing triple-pane rather than double-pane windows?(d) Finally, suppose the homeowner buys 20 such triple-pane windows for the house. A typical winter has theequivalent of about 120 heating days at a temperaturedifference of 45°F. Each triple-pane window costs $85more than the double-pane window. Ignoring interestand inflation, how many years will it take the home-owner to make up the additional cost of the triple-panewindows from heating bill savings?C1.2 When a person ice skates, the surface of the ice actuallymelts beneath the blades, so that he or she skates on a thinsheet of water between the blade and the ice.(a) Find an expression for total friction force on the bot-tom of the blade as a function of skater velocity V, bladelength L, water thickness (between the blade and theice) h, water viscosity ␮, and blade width W.(b) Suppose an ice skater of total mass m is skating alongat a constant speed of V0 when she suddenly stands stiffwith her skates pointed directly forward, allowing her-self to coast to a stop. Neglecting friction due to air re-sistance, how far will she travel before she comes to astop? (Remember, she is coasting on two skate blades.)Give your answer for the total distance traveled, x, asa function of V0, m, L, h, ␮, and W.(c) Find x for the case where V0 ϭ 4.0 m/s, m ϭ 100 kg, L ϭ30 cm, W ϭ 5.0 mm, and h ϭ 0.10 mm. Do you think ourassumption of negligible air resistance is a good one?C1.3 Two thin flat plates, tilted at an angle ␣, are placed in atank of liquid of known surface tension ⌼ and contact an-gle ␪, as shown in Fig. C1.3. At the free surface of the liq-uid in the tank, the two plates are a distance L apart andhave width b into the page. The liquid rises a distance hbetween the plates, as shown.(a) What is the total upward (z-directed) force, due to sur-face tension, acting on the liquid column between theplates?(b) If the liquid density is ␳, find an expression for surfacetension ⌼ in terms of the other variables.hzgL␪␣ ␣␪C1.3
    • References 55C1.4 Oil of viscosity ␮ and density ␳ drains steadily down theside of a tall, wide vertical plate, as shown in Fig. C1.4.In the region shown, fully developed conditions exist; thatis, the velocity profile shape and the film thickness ␦ areindependent of distance z along the plate. The vertical ve-locity w becomes a function only of x, and the shear re-sistance from the atmosphere is negligible.(a) Sketch the approximate shape of the velocity profilew(x), considering the boundary conditions at the wall andat the film surface.(b) Suppose film thickness ␦, and the slope of the veloc-ity profile at the wall, (dw/dx)wall, are measured by a laserDoppler anemometer (to be discussed in Chap. 6). Find anexpression for the viscosity of the oil as a function of ␳,␦, (dw/dx)wall, and the gravitational accleration g. Notethat, for the coordinate system given, both w and(dw/dx)wall are negative.z␦xgOil filmAirPlateReferences1. J. C. Tannehill, D. A. Anderson, and R. H. Pletcher, Compu-tational Fluid Mechanics and Heat Transfer, 2d ed., Taylorand Francis, Bristol, PA, 1997.2. S. V. Patankar, Numerical Heat Transfer and Fluid Flow,McGraw-Hill, New York, 1980.3. F. M. White, Viscous Fluid Flow, 2d ed., McGraw-Hill, NewYork, 1991.4. R. J. Goldstein (ed.), Fluid Mechanics Measurements, 2d ed.,Taylor and Francis, Bristol, PA, 1997.5. R. A. Granger, Experiments in Fluid Mechanics, Oxford Uni-versity Press, 1995.6. H. A. Barnes, J. F. Hutton, and K. Walters, An Introductionto Rheology, Elsevier, New York, 1989.7. A. E. Bergeles and S. Ishigai, Two-Phase Flow Dynamics andReactor Safety, McGraw-Hill, New York, 1981.8. G. N. Patterson, Introduction to the Kinetic Theory of GasFlows, University of Toronto Press, Toronto, 1971.9. ASME Orientation and Guide for Use of Metric Units, 9th ed.,American Society of Mechanical Engineers, NewYork, 1982.10. J. P. Holman, Heat Transfer, 8th ed., McGraw-Hill, NewYork,1997.11. R. C. Reid, J. M. Prausnitz, and T. K. Sherwood, The Prop-erties of Gases and Liquids, 4th ed., McGraw-Hill, NewYork,1987.12. J. Hilsenrath et al., “Tables of Thermodynamic and TransportProperties,” U. S. Nat. Bur. Stand. Circ. 564, 1955; reprintedby Pergamon, New York, 1960.13. R. A. Spencer et al., ASME Steam Tables with Mollier Chart,6th ed., American Society of Mechanical Engineers, NewYork, 1993.14. O. A. Hougen and K. M. Watson, Chemical Process Princi-ples Charts, Wiley, New York, 1960.15. A. W. Adamson, Physical Chemistry of Surfaces, 5th ed., In-terscience, New York, 1990.16. J. A. Knauss, Introduction to Physical Oceanography, Pren-tice-Hall, Englewood Cliffs, NJ, 1978.17. National Committee for Fluid Mechanics Films, IllustratedExperiments in Fluid Mechanics, M.I.T. Press, Cambridge,MA, 1972.18. I. G. Currie, Fundamental Mechanics of Fluids, 2d ed.,McGraw-Hill, New York, 1993.19. M. van Dyke, An Album of Fluid Motion, Parabolic Press,Stanford, CA, 1982.20. Y. Nakayama (ed.), Visualized Flow, Pergamon Press, Ox-ford, 1988.21. W. J. Yang (ed.), Handbook of Flow Visualization, Hemi-sphere, New York, 1989.22. W. Merzkirch, Flow Visualization, 2d ed., Academic, NewYork, 1987.23. H. Rouse and S. Ince, History of Hydraulics, Iowa Instituteof Hydraulic Research, Univ. of Iowa, Iowa City, 1957;reprinted by Dover, New York, 1963.24. H. Rouse, Hydraulics in the United States 1776–1976, IowaInstitute of Hydraulic Research, Univ. of Iowa, Iowa City, 1976.25. G. Garbrecht, Hydraulics and Hydraulic Research: An His-torical Review, Gower Pub., Aldershot, UK, 1987.26. 1986 SAE Handbook, 4 vols., Society of Automotive Engi-neers, Warrendale, PA.27. J. R. van Wazer, Viscosity and Flow Measurement, Inter-science, New York, 1963.28. SAE Fuels and Lubricants Standards Manual, Society of Au-tomotive Engineers, Warrendale, PA, 1995.29. John D. Anderson, Computational Fluid Dynamics: The Ba-sics with Applications, McGraw-Hill, New York, 1995.C1.4
    • 34. C. L. Yaws, X. Lin, and L. Bu, “Calculate Viscosities for 355Compounds. An Equation Can Be Used to Calculate LiquidViscosity as a Function of Temperature,” Chemical Engi-neering, vol. 101, no. 4, April 1994, pp. 119–128.35. Frank E. Jones, Techniques and Topics in Flow Measurement,CRC Press, Boca Raton, FL, 1995.36. Carl L. Yaws, Handbook of Viscosity, 3 vols., Gulf Publish-ing, Houston, TX, 1994.30. H. W. Coleman and W. G. Steele, Experimentation and Un-certainty Analysis for Engineers, John Wiley, New York,1989.31. R. J. Moffatt, “Describing the Uncertainties in ExperimentalResults,” Experimental Thermal and Fluid Science, vol., 1,1988, pp. 3–17.32. Paul A. Libby, An Introduction to Turbulence, Taylor andFrancis, Bristol, PA, 1996.33. Sanford Klein and William Beckman, Engineering EquationSolver (EES), F-Chart Software, Middleton, WI, 1997.56 Chapter 1 Introduction
    • Roosevelt Dam in Arizona. Hydrostatic pressure, due to the weight of a standing fluid, can causeenormous forces and moments on large-scale structures such as a dam. Hydrostatic fluid analy-sis is the subject of the present chapter. (Courtesy of Dr. E.R. Degginger/Color-Pic Inc.)58
    • 2.1 Pressure and PressureGradientMotivation. Many fluid problems do not involve motion. They concern the pressuredistribution in a static fluid and its effect on solid surfaces and on floating and sub-merged bodies.When the fluid velocity is zero, denoted as the hydrostatic condition, the pressurevariation is due only to the weight of the fluid. Assuming a known fluid in a givengravity field, the pressure may easily be calculated by integration. Important applica-tions in this chapter are (1) pressure distribution in the atmosphere and the oceans, (2)the design of manometer pressure instruments, (3) forces on submerged flat and curvedsurfaces, (4) buoyancy on a submerged body, and (5) the behavior of floating bodies.The last two result in Archimedes’ principles.If the fluid is moving in rigid-body motion, such as a tank of liquid which has beenspinning for a long time, the pressure also can be easily calculated, because the fluidis free of shear stress. We apply this idea here to simple rigid-body accelerations inSec. 2.9. Pressure measurement instruments are discussed in Sec. 2.10. As a matter offact, pressure also can be easily analyzed in arbitrary (nonrigid-body) motions V(x, y,z, t), but we defer that subject to Chap. 4.In Fig. 1.1 we saw that a fluid at rest cannot support shear stress and thus Mohr’s cir-cle reduces to a point. In other words, the normal stress on any plane through a fluidelement at rest is equal to a unique value called the fluid pressure p, taken positive forcompression by common convention. This is such an important concept that we shallreview it with another approach.Figure 2.1 shows a small wedge of fluid at rest of size ⌬x by ⌬z by ⌬s and depth binto the paper. There is no shear by definition, but we postulate that the pressures px, pz,and pn may be different on each face. The weight of the element also may be important.Summation of forces must equal zero (no acceleration) in both the x and z directions.⌺ Fx ϭ 0 ϭ pxb ⌬z Ϫ pnb ⌬s sin ␪(2.1)⌺ Fz ϭ 0 ϭ pzb ⌬x Ϫ pnb ⌬s cos ␪ Ϫ ᎏ12ᎏ ␥b ⌬x ⌬zChapter 2Pressure Distributionin a Fluid59
    • Fig. 2.1 Equilibrium of a smallwedge of fluid at rest.Pressure Force on a FluidElementbut the geometry of the wedge is such that⌬s sin ␪ ϭ ⌬z ⌬s cos ␪ ϭ ⌬x (2.2)Substitution into Eq. (2.1) and rearrangement givepx ϭ pn pz ϭ pn ϩ ᎏ12ᎏ␥ ⌬z (2.3)These relations illustrate two important principles of the hydrostatic, or shear-free, con-dition: (1) There is no pressure change in the horizontal direction, and (2) there is avertical change in pressure proportional to the density, gravity, and depth change. Weshall exploit these results to the fullest, starting in Sec. 2.3.In the limit as the fluid wedge shrinks to a “point,’’ ⌬z → 0 and Eqs. (2.3) becomepx ϭ pz ϭ pn ϭ p (2.4)Since ␪ is arbitrary, we conclude that the pressure p at a point in a static fluid is inde-pendent of orientation.What about the pressure at a point in a moving fluid? If there are strain rates in amoving fluid, there will be viscous stresses, both shear and normal in general (Sec.4.3). In that case (Chap. 4) the pressure is defined as the average of the three normalstresses ␴ii on the elementp ϭ Ϫ ᎏ13ᎏ(␴xx ϩ ␴yy ϩ ␴zz) (2.5)The minus sign occurs because a compression stress is considered to be negativewhereas p is positive. Equation (2.5) is subtle and rarely needed since the great ma-jority of viscous flows have negligible viscous normal stresses (Chap. 4).Pressure (or any other stress, for that matter) causes no net force on a fluid elementunless it varies spatially.1To see this, consider the pressure acting on the two x facesin Fig. 2.2. Let the pressure vary arbitrarilyp ϭ p(x, y, z, t) (2.6)60 Chapter 2 Pressure Distribution in a Fluid∆s∆zOθ∆x θpxpzxz (up)Element weight:dW = g( b ∆x ∆z)12Width b into paperpnρ1An interesting application for a large element is in Fig. 3.7.
    • Fig. 2.2 Net x force on an elementdue to pressure variation.2.2 Equilibrium of a FluidElementThe net force in the x direction on the element in Fig. 2.2 is given bydFx ϭ p dy dz Ϫ΂p ϩ ᎏѨѨpxᎏ dx΃ dy dz ϭ ϪᎏѨѨpxᎏ dx dy dz (2.7)In like manner the net force dFy involves ϪѨp/Ѩy, and the net force dFz concernsϪѨp/Ѩz. The total net-force vector on the element due to pressure isdFpress ϭ΂Ϫi ᎏѨѨpxᎏ Ϫ j ᎏѨѨpyᎏ Ϫ k ᎏѨѨpzᎏ΃ dx dy dz (2.8)We recognize the term in parentheses as the negative vector gradient of p. Denoting fas the net force per unit element volume, we rewrite Eq. (2.8) asfpress ϭ Ϫ∇p (2.9)Thus it is not the pressure but the pressure gradient causing a net force which must bebalanced by gravity or acceleration or some other effect in the fluid.The pressure gradient is a surface force which acts on the sides of the element. Theremay also be a body force, due to electromagnetic or gravitational potentials, acting onthe entire mass of the element. Here we consider only the gravity force, or weight ofthe elementdFgrav ϭ ␳g dx dy dz(2.10)or fgrav ϭ ␳gIn general, there may also be a surface force due to the gradient, if any, of the vis-cous stresses. For completeness, we write this term here without derivation and con-sider it more thoroughly in Chap. 4. For an incompressible fluid with constant viscos-ity, the net viscous force isfVS ϭ ␮΂ᎏѨѨ2xV2ᎏ ϩ ᎏѨѨ2yV2ᎏ ϩ ᎏѨѨ2zV2ᎏ΃ϭ ␮∇2V (2.11)where VS stands for viscous stresses and ␮ is the coefficient of viscosity from Chap.1. Note that the term g in Eq. (2.10) denotes the acceleration of gravity, a vector act-2.2 Equilibrium of a Fluid Element 61( p + dx) dy dz∂p∂xxdydxzp dy dzydz
    • ing toward the center of the earth. On earth the average magnitude of g is 32.174 ft/s2ϭ9.807 m/s2.The total vector resultant of these three forces—pressure, gravity, and viscousstress—must either keep the element in equilibrium or cause it to move with acceler-ation a. From Newton’s law, Eq. (1.2), we have␳a ϭ ⌺ f ϭ fpress ϩ fgrav ϩ fvisc ϭ Ϫ∇p ϩ ␳g ϩ ␮∇2V (2.12)This is one form of the differential momentum equation for a fluid element, and it isstudied further in Chap. 4. Vector addition is implied by Eq. (2.12): The accelerationreflects the local balance of forces and is not necessarily parallel to the local-velocityvector, which reflects the direction of motion at that instant.This chapter is concerned with cases where the velocity and acceleration are known,leaving one to solve for the pressure variation in the fluid. Later chapters will take upthe more general problem where pressure, velocity, and acceleration are all unknown.Rewrite Eq. (2.12) as∇p ϭ ␳(g Ϫ a) ϩ ␮∇2V ϭ B(x, y, z, t) (2.13)where B is a short notation for the vector sum on the right-hand side. If V and a ϭdV/dt are known functions of space and time and the density and viscosity are known,we can solve Eq. (2.13) for p(x, y, z, t) by direct integration. By components, Eq. (2.13)is equivalent to three simultaneous first-order differential equationsᎏѨѨpxᎏ ϭ Bx(x, y, z, t) ᎏѨѨpyᎏ ϭ By(x, y, z, t) ᎏѨѨpzᎏ ϭ Bz(x, y, z, t) (2.14)Since the right-hand sides are known functions, they can be integrated systematicallyto obtain the distribution p(x, y, z, t) except for an unknown function of time, whichremains because we have no relation for Ѩp/Ѩt. This extra function is found from a con-dition of known time variation p0(t) at some point (x0, y0, z0). If the flow is steady (in-dependent of time), the unknown function is a constant and is found from knowledgeof a single known pressure p0 at a point (x0, y0, z0). If this sounds complicated, it isnot; we shall illustrate with many examples. Finding the pressure distribution from aknown velocity distribution is one of the easiest problems in fluid mechanics, whichis why we put it in Chap. 2.Examining Eq. (2.13), we can single out at least four special cases:1. Flow at rest or at constant velocity: The acceleration and viscous terms vanishidentically, and p depends only upon gravity and density. This is the hydrostaticcondition. See Sec. 2.3.2. Rigid-body translation and rotation: The viscous term vanishes identically,and p depends only upon the term ␳(g Ϫ a). See Sec. 2.9.3. Irrotational motion (ٌ ؋ V ϵ 0): The viscous term vanishes identically, andan exact integral called Bernoulli’s equation can be found for the pressure distri-bution. See Sec. 4.9.4. Arbitrary viscous motion: Nothing helpful happens, no general rules apply, butstill the integration is quite straightforward. See Sec. 6.4.Let us consider cases 1 and 2 here.62 Chapter 2 Pressure Distribution in a Fluid
    • Fig. 2.3 Illustration of absolute,gage, and vacuum pressure read-ings.Gage Pressure and VacuumPressure: Relative Terms2.3 Hydrostatic PressureDistributionsBefore embarking on examples, we should note that engineers are apt to specify pres-sures as (1) the absolute or total magnitude or (2) the value relative to the local am-bient atmosphere. The second case occurs because many pressure instruments are ofdifferential type and record, not an absolute magnitude, but the difference between thefluid pressure and the atmosphere. The measured pressure may be either higher or lowerthan the local atmosphere, and each case is given a name:1. p Ͼ pa Gage pressure: p(gage) ϭ p Ϫ pa2. p Ͻ pa Vacuum pressure: p(vacuum) ϭ pa Ϫ pThis is a convenient shorthand, and one later adds (or subtracts) atmospheric pressureto determine the absolute fluid pressure.A typical situation is shown in Fig. 2.3. The local atmosphere is at, say, 90,000 Pa,which might reflect a storm condition in a sea-level location or normal conditions atan altitude of 1000 m. Thus, on this day, pa ϭ 90,000 Pa absolute ϭ 0 Pa gage ϭ 0 Pavacuum. Suppose gage 1 in a laboratory reads p1 ϭ 120,000 Pa absolute. This valuemay be reported as a gage pressure, p1 ϭ 120,000 Ϫ 90,000 ϭ 30,000 Pa gage. (Onemust also record the atmospheric pressure in the laboratory, since pa changes gradu-ally.) Suppose gage 2 reads p2 ϭ 50,000 Pa absolute. Locally, this is a vacuum pres-sure and might be reported as p2 ϭ 90,000 Ϫ 50,000 ϭ 40,000 Pa vacuum. Occasion-ally, in the Problems section, we will specify gage or vacuum pressure to keep youalert to this common engineering practice.If the fluid is at rest or at constant velocity, a ϭ 0 and ∇2V ϭ 0. Equation (2.13) forthe pressure distribution reduces to∇p ϭ ␳g (2.15)This is a hydrostatic distribution and is correct for all fluids at rest, regardless of theirviscosity, because the viscous term vanishes identically.Recall from vector analysis that the vector ∇p expresses the magnitude and direc-tion of the maximum spatial rate of increase of the scalar property p. As a result, ∇p2.3 Hydrostatic Pressure Distributions 63Absolute zero reference:p = 0 Pa abs = 90,000 Pa vacuum120,00090,00050,0000p (Pascals)High pressure:p = 120,000 Pa abs = 30,000 Pa gageLocal atmosphere:p = 90,000 Pa abs = 0 Pa gage = 0 Pa vacuumVacuum pressure:p = 50,000 Pa abs = 40,000 Pa vacuum40,00030,00050,000(Tension)
    • Fig. 2.4 Hydrostatic-pressure distri-bution. Points a, b, c, and d are atequal depths in water and thereforehave identical pressures. Points A,B, and C are also at equal depths inwater and have identical pressureshigher than a, b, c, and d. Point Dhas a different pressure from A, B,and C because it is not connectedto them by a water path.is perpendicular everywhere to surfaces of constant p. Thus Eq. (2.15) states that a fluidin hydrostatic equilibrium will align its constant-pressure surfaces everywhere normalto the local-gravity vector. The maximum pressure increase will be in the direction ofgravity, i.e., “down.’’ If the fluid is a liquid, its free surface, being at atmospheric pres-sure, will be normal to local gravity, or “horizontal.’’ You probably knew all this be-fore, but Eq. (2.15) is the proof of it.In our customary coordinate system z is “up.’’Thus the local-gravity vector for small-scale problems isg ϭ Ϫgk (2.16)where g is the magnitude of local gravity, for example, 9.807 m/s2. For these coordi-nates Eq. (2.15) has the componentsᎏѨѨpxᎏ ϭ 0 ᎏѨѨpyᎏ ϭ 0 ᎏѨѨpzᎏ ϭ Ϫ␳g ϭ Ϫ␥ (2.17)the first two of which tell us that p is independent of x and y. Hence Ѩp/Ѩz can be re-placed by the total derivative dp/dz, and the hydrostatic condition reduces toᎏddpzᎏ ϭ Ϫ␥or p2 Ϫ p1 ϭ Ϫ͵21␥ dz (2.18)Equation (2.18) is the solution to the hydrostatic problem. The integration requires anassumption about the density and gravity distribution. Gases and liquids are usuallytreated differently.We state the following conclusions about a hydrostatic condition:Pressure in a continuously distributed uniform static fluid varies only with verticaldistance and is independent of the shape of the container. The pressure is the sameat all points on a given horizontal plane in the fluid. The pressure increases withdepth in the fluid.An illustration of this is shown in Fig. 2.4. The free surface of the container is atmos-pheric and forms a horizontal plane. Points a, b, c, and d are at equal depth in a horizon-64 Chapter 2 Pressure Distribution in a FluidAtmospheric pressure:Depth 1Depth 2WaterMercuryFree surfacea b c dA B C D
    • Effect of Variable GravityHydrostatic Pressure in Liquidstal plane and are interconnected by the same fluid, water; therefore all points have thesame pressure. The same is true of points A, B, and C on the bottom, which all have thesame higher pressure than at a, b, c, and d. However, point D, although at the same depthas A, B, and C, has a different pressure because it lies beneath a different fluid, mercury.For a spherical planet of uniform density, the acceleration of gravity varies inverselyas the square of the radius from its centerg ϭ g0΂ ΃2(2.19)where r0 is the planet radius and g0 is the surface value of g. For earth, r0 Ϸ 3960statute mi Ϸ 6400 km. In typical engineering problems the deviation from r0 extendsfrom the deepest ocean, about 11 km, to the atmospheric height of supersonic transportoperation, about 20 km. This gives a maximum variation in g of (6400/6420)2, or 0.6percent. We therefore neglect the variation of g in most problems.Liquids are so nearly incompressible that we can neglect their density variation in hy-drostatics. In Example 1.7 we saw that water density increases only 4.6 percent at thedeepest part of the ocean. Its effect on hydrostatics would be about half of this, or 2.3percent. Thus we assume constant density in liquid hydrostatic calculations, for whichEq. (2.18) integrates toLiquids: p2 Ϫ p1 ϭ Ϫ␥(z2 Ϫ z1) (2.20)or z1 Ϫ z2 ϭ ᎏp␥2ᎏ Ϫ ᎏp␥1ᎏWe use the first form in most problems. The quantity ␥ is called the specific weight ofthe fluid, with dimensions of weight per unit volume; some values are tabulated inTable 2.1. The quantity p/␥ is a length called the pressure head of the fluid.For lakes and oceans, the coordinate system is usually chosen as in Fig. 2.5, withz ϭ 0 at the free surface, where p equals the surface atmospheric pressure pa. Whenr0ᎏr2.3 Hydrostatic Pressure Distributions 65Table 2.1 Specific Weight of SomeCommon FluidsSpecific weight ␥at 68°F ϭ 20°CFluid lbf/ft3N/m3Air (at 1 atm) 000.0752 000,011.8Ethyl alcohol 049.2 007,733SAE 30 oil 055.5 008,720Water 062.4 009,790Seawater 064.0 010,050Glycerin 078.7 012,360Carbon tetrachloride 099.1 015,570Mercury 846 133,100
    • Fig. 2.5 Hydrostatic-pressure distri-bution in oceans and atmospheres.The Mercury Barometerwe introduce the reference value (p1, z1) ϭ (pa, 0), Eq. (2.20) becomes, for p at any(negative) depth z,Lakes and oceans: p ϭ pa Ϫ ␥z (2.21)where ␥ is the average specific weight of the lake or ocean. As we shall see, Eq. (2.21)holds in the atmosphere also with an accuracy of 2 percent for heights z up to 1000 m.EXAMPLE 2.1Newfound Lake, a freshwater lake near Bristol, New Hampshire, has a maximum depth of 60m, and the mean atmospheric pressure is 91 kPa. Estimate the absolute pressure in kPa at thismaximum depth.SolutionFrom Table 2.1, take ␥ Ϸ 9790 N/m3. With pa ϭ 91 kPa and z ϭ Ϫ60 m, Eq. (2.21) predicts thatthe pressure at this depth will bep ϭ 91 kN/m2Ϫ (9790 N/m3)(Ϫ60 m) ᎏ1100k0NNᎏϭ 91 kPa ϩ 587 kN/m2ϭ 678 kPa Ans.By omitting pa we could state the result as p ϭ 587 kPa (gage).The simplest practical application of the hydrostatic formula (2.20) is the barometer(Fig. 2.6), which measures atmospheric pressure. A tube is filled with mercury and in-verted while submerged in a reservoir. This causes a near vacuum in the closed upperend because mercury has an extremely small vapor pressure at room temperatures (0.16Pa at 20°C). Since atmospheric pressure forces a mercury column to rise a distance hinto the tube, the upper mercury surface is at zero pressure.66 Chapter 2 Pressure Distribution in a Fluidg0+b–hZp ≈ pa– bγairFree surface: Z = 0, p = paAirWaterp ≈ pa+ hγwater
    • Fig. 2.6 A barometer measures local absolute atmospheric pressure: (a) the height of a mercury column is pro-portional to patm; (b) a modern portable barometer, with digital readout, uses the resonating silicon element ofFig. 2.28c. (Courtesy of Paul Lupke, Druck Inc.)Hydrostatic Pressure in GasesFrom Fig. 2.6, Eq. (2.20) applies with p1 ϭ 0 at z1 ϭ h and p2 ϭ pa at z2 ϭ 0:pa Ϫ 0 ϭ Ϫ␥M(0 Ϫ h)or h ϭ (2.22)At sea-level standard, with pa ϭ 101,350 Pa and ␥M ϭ 133,100 N/m3from Table 2.1,the barometric height is h ϭ 101,350/133,100 ϭ 0.761 m or 761 mm. In the UnitedStates the weather service reports this as an atmospheric “pressure’’ of 29.96 inHg(inches of mercury). Mercury is used because it is the heaviest common liquid. A wa-ter barometer would be 34 ft high.Gases are compressible, with density nearly proportional to pressure. Thus density mustbe considered as a variable in Eq. (2.18) if the integration carries over large pressurechanges. It is sufficiently accurate to introduce the perfect-gas law p ϭ ␳RT in Eq.(2.18)ϭ Ϫ␳g ϭ Ϫ gpᎏRTdpᎏdzpaᎏ␥M2.3 Hydrostatic Pressure Distributions 67p1≈ 0(Mercury has a verylow vapor pressure.)z1= hh =paMzMercuryz2= 0p2≈ pa(The mercury is incontact with theatmosphere.) paMγρ(a) (b)
    • Separate the variables and integrate between points 1 and 2:͵21ϭ ln ϭ Ϫ ͵21(2.23)The integral over z requires an assumption about the temperature variation T(z). Onecommon approximation is the isothermal atmosphere, where T ϭ T0:p2 ϭ p1 exp΄Ϫ΅ (2.24)The quantity in brackets is dimensionless. (Think that over; it must be dimensionless,right?) Equation (2.24) is a fair approximation for earth, but actually the earth’s meanatmospheric temperature drops off nearly linearly with z up to an altitude of about36,000 ft (11,000 m):T Ϸ T0 Ϫ Bz (2.25)Here T0 is sea-level temperature (absolute) and B is the lapse rate, both of which varysomewhat from day to day. By international agreement [1] the following standard val-ues are assumed to apply from 0 to 36,000 ft:T0 ϭ 518.69°R ϭ 288.16 K ϭ 15°CB ϭ 0.003566°R/ft ϭ 0.00650 K/m (2.26)This lower portion of the atmosphere is called the troposphere. Introducing Eq. (2.25)into (2.23) and integrating, we obtain the more accurate relationp ϭ pa΂1 Ϫ΃g/(RB)where ᎏRgBᎏ ϭ 5.26 (air) (2.27)in the troposphere, with z ϭ 0 at sea level. The exponent g/(RB) is dimensionless (againit must be) and has the standard value of 5.26 for air, with R ϭ 287 m2/(s2и K).The U.S. standard atmosphere [1] is sketched in Fig. 2.7. The pressure is seen to benearly zero at z ϭ 30 km. For tabulated properties see Table A.6.EXAMPLE 2.2If sea-level pressure is 101,350 Pa, compute the standard pressure at an altitude of 5000 m, us-ing (a) the exact formula and (b) an isothermal assumption at a standard sea-level temperatureof 15°C. Is the isothermal approximation adequate?SolutionUse absolute temperature in the exact formula, Eq. (2.27):p ϭ pa΄1 Ϫ΅5.26ϭ (101,350 Pa)(0.8872)5.26ϭ 101,350(0.52388) ϭ 54,000 Pa Ans. (a)This is the standard-pressure result given at z ϭ 5000 m in Table A.6.(0.00650 K/m)(5000 m)ᎏᎏᎏ288.16 KBzᎏT0g(z2 Ϫ z1)ᎏᎏRT0dzᎏTgᎏRp2ᎏp1dpᎏp68 Chapter 2 Pressure Distribution in a FluidPart (a)
    • Is the Linear Formula Adequatefor Gases?If the atmosphere were isothermal at 288.16 K, Eq. (2.24) would apply:p Ϸ pa exp΂ϪᎏRgTzᎏ΃ϭ (101,350 Pa) expΆϪ·ϭ (101,350 Pa) exp( Ϫ 0.5929) Ϸ 60,100 Pa Ans. (b)This is 11 percent higher than the exact result. The isothermal formula is inaccurate in the tro-posphere.The linear approximation from Eq. (2.20) or (2.21), ⌬p Ϸ ␥ ⌬z, is satisfactory for liq-uids, which are nearly incompressible. It may be used even over great depths in theocean. For gases, which are highly compressible, it is valid only over moderate changesin altitude.The error involved in using the linear approximation (2.21) can be evaluated by ex-panding the exact formula (2.27) into a series΂1 Ϫ΃nϭ 1 Ϫ n ϩ΂ ΃2Ϫ иии (2.28)where n ϭ g/(RB). Introducing these first three terms of the series into Eq. (2.27) andrearranging, we obtainp ϭ pa Ϫ ␥az΂1 Ϫ ϩ иии΃ (2.29)BzᎏT0n Ϫ 1ᎏ2BzᎏT0n(n Ϫ 1)ᎏ2!BzᎏT0BzᎏT0(9.807 m/s2)(5000 m)ᎏᎏᎏ[287 m2/(s2и K)](288.16 K)2.3 Hydrostatic Pressure Distributions 69Fig. 2.7 Temperature and pressuredistribution in the U.S. standard at-mosphere. (From Ref. 1.)60504030201006050403020100–60 –40 –20 0 +2020.1 km11.0 km–56.5°CTroposphereEq. (2.26)15°CTemperature, °C Pressure, kPa40 80 120Altitudez,kmAltitudez,km1.20 kPaEq. (2.27)101.33 kPaEq. (2.24)Part (b)
    • 2.4 Application to ManometryA Memory Device: Up VersusDownFig. 2.8 Evaluating pressurechanges through a column of multi-ple fluids.Thus the error in using the linear formula (2.21) is small if the second term in paren-theses in (2.29) is small compared with unity. This is true ifz Ӷ ϭ 20,800 m (2.30)We thus expect errors of less than 5 percent if z or ␦z is less than 1000 m.From the hydrostatic formula (2.20), a change in elevation z2 Ϫ z1 of a liquid is equiv-alent to a change in pressure (p2 Ϫ p1)/␥. Thus a static column of one or more liquidsor gases can be used to measure pressure differences between two points. Such a de-vice is called a manometer. If multiple fluids are used, we must change the density inthe formula as we move from one fluid to another. Figure 2.8 illustrates the use of theformula with a column of multiple fluids. The pressure change through each fluid iscalculated separately. If we wish to know the total change p5 Ϫ p1, we add the suc-cessive changes p2 Ϫ p1, p3 Ϫ p2, p4 Ϫ p3, and p5 Ϫ p4. The intermediate values of pcancel, and we have, for the example of Fig. 2.8,p5 Ϫ p1 ϭ Ϫ ␥0(z2 Ϫ z1) Ϫ ␥w(z3 Ϫ z2) Ϫ ␥G(z4 Ϫ z3) Ϫ ␥M(z5 Ϫ z4) (2.31)No additional simplification is possible on the right-hand side because of the dif-ferent densities. Notice that we have placed the fluids in order from the lighteston top to the heaviest at bottom. This is the only stable configuration. If we attemptto layer them in any other manner, the fluids will overturn and seek the stablearrangement.The basic hydrostatic relation, Eq. (2.20), is mathematically correct but vexing to en-gineers, because it combines two negative signs to have the pressure increase down-ward. When calculating hydrostatic pressure changes, engineers work instinctively bysimply having the pressure increase downward and decrease upward. Thus they use thefollowing mnemonic, or memory, device, first suggested to the writer by Professor John2T0ᎏ(n Ϫ 1)B70 Chapter 2 Pressure Distribution in a FluidKnown pressure p1Oil, oρWater, wρGlycerin, GρMercury, Mρz = z1z2z3z4z5zp2– p1= – og(z2– z1)ρp3– p2= – wg(z3– z2)ρp4– p3= – Gg(z4– z3)ρp5– p4= – Mg(z5– z4)ρSum = p5– p1
    • Fig. 2.9 Simple open manometerfor measuring pA relative to atmos-pheric pressure.Foss of Michigan State University:pdown ϭ pupϩ ␥⌬z (2.32)Thus, without worrying too much about which point is “z1” and which is “z2”, the for-mula simply increases or decreases the pressure according to whether one is movingdown or up. For example, Eq. (2.31) could be rewritten in the following “multiple in-crease” mode:p5 ϭ p1 ϩ ␥0z1 Ϫ z2 ϩ ␥wz2 Ϫ z3 ϩ ␥Gz3 Ϫ z4 ϩ ␥Mz4 Ϫ z5That is, keep adding on pressure increments as you move down through the layeredfluid. A different application is a manometer, which involves both “up” and “down”calculations.Figure 2.9 shows a simple open manometer for measuring pA in a closed chamberrelative to atmospheric pressure pa, in other words, measuring the gage pressure. Thechamber fluid ␳1 is combined with a second fluid ␳2, perhaps for two reasons: (1) toprotect the environment from a corrosive chamber fluid or (2) because a heavier fluid␳2 will keep z2 small and the open tube can be shorter. One can, of course, apply thebasic hydrostatic formula (2.20). Or, more simply, one can begin at A, apply Eq. (2.32)“down” to z1, jump across fluid 2 (see Fig. 2.9) to the same pressure p1, and then useEq. (2.32) “up” to level z2:pA ϩ ␥1zA Ϫ z1 Ϫ ␥2z1 Ϫ z2 ϭ p2 Ϸ patm (2.33)The physical reason that we can “jump across” at section 1 in that a continuous lengthof the same fluid connects these two equal elevations. The hydrostatic relation (2.20)requires this equality as a form of Pascal’s law:Any two points at the same elevation in a continuous mass of the same static fluidwill be at the same pressure.This idea of jumping across to equal pressures facilitates multiple-fluid problems.EXAMPLE 2.3The classic use of a manometer is when two U-tube legs are of equal length, as in Fig. E2.3,and the measurement involves a pressure difference across two horizontal points. The typical ap-2.4 Application to Manometry 71zA, pA Aρ2ρ1z1, p1Jump acrossOpen, paz2, p2≈ pap = p1at z = z1in fluid 2
    • Fig. 2.10 A complicated multiple-fluid manometer to relate pA to pB.This system is not especially prac-tical but makes a good homeworkor examination problem.plication is to measure pressure change across a flow device, as shown. Derive a formula for thepressure difference pa Ϫ pb in terms of the system parameters in Fig. E2.3.SolutionUsing our “up-down” concept as in Eq. (2.32), start at (a), evaluate pressure changes around theU-tube, and end up at (b):pa ϩ ␳1gL ϩ ␳1gh Ϫ ␳2gh Ϫ ␳1gL ϭ pbor pa Ϫ pb ϭ (␳2 Ϫ ␳1)gh Ans.The measurement only includes h, the manometer reading. Terms involving L drop out. Note theappearance of the difference in densities between manometer fluid and working fluid. It is a com-mon student error to fail to subtract out the working fluid density ␳1—a serious error if bothfluids are liquids and less disastrous numerically if fluid 1 is a gas. Academically, of course,such an error is always considered serious by fluid mechanics instructors.Although Ex. 2.3, because of its popularity in engineering experiments, is some-times considered to be the “manometer formula,” it is best not to memorize it butrather to adapt Eq. (2.20) or (2.32) to each new multiple-fluid hydrostatics problem.For example, Fig. 2.10 illustrates a multiple-fluid manometer problem for finding the72 Chapter 2 Pressure Distribution in a FluidzA, pAz1, p1z1, p1Jump acrossAρ1ρ2z2, p2z2, p2ρ3Jump acrossJump acrossz3, p3z3, p3ρ4B zB, pBFlow device␳1␳2(a)Lh(b)E2.3
    • E2.4difference in pressure between two chambers A and B. We repeatedly apply Eq. (2.20),jumping across at equal pressures when we come to a continuous mass of the samefluid. Thus, in Fig. 2.10, we compute four pressure differences while making three jumps:pA Ϫ pB ϭ (pA Ϫ p1) ϩ (p1 Ϫ p2) ϩ (p2 Ϫ p3) ϩ (p3 Ϫ pB)ϭ Ϫ␥1(zA Ϫ z1) Ϫ ␥2(z1 Ϫ z2) Ϫ ␥3(z2 Ϫ z3) Ϫ ␥4(z3 Ϫ zB) (2.34)The intermediate pressures p1,2,3 cancel. It looks complicated, but really it is merelysequential. One starts at A, goes down to 1, jumps across, goes up to 2, jumps across,goes down to 3, jumps across, and finally goes up to B.EXAMPLE 2.4Pressure gage B is to measure the pressure at point A in a water flow. If the pressure at B is 87kPa, estimate the pressure at A, in kPa. Assume all fluids are at 20°C. See Fig. E2.4.2.4 Application to Manometry 73AWaterflow5 cm4 cmMercurySAE 30 oil Gage B6 cm11 cmSolutionFirst list the specific weights from Table 2.1 or Table A.3:␥water ϭ 9790 N/m3␥mercury ϭ 133,100 N/m3␥oil ϭ 8720 N/m3Now proceed from A to B, calculating the pressure change in each fluid and adding:pA Ϫ ␥W(⌬z)W Ϫ ␥M(⌬z)M Ϫ ␥O(⌬z)O ϭ pBor pA Ϫ (9790 N/m3)(Ϫ 0.05 m) Ϫ (133,100 N/m3)(0.07 m) Ϫ (8720 N/m3)(0.06 m)ϭ pA ϩ 489.5 Pa Ϫ 9317 Pa Ϫ 523.2 Pa ϭ pB ϭ 87,000 Pawhere we replace N/m2by its short name, Pa. The value ⌬zM ϭ 0.07 m is the net elevationchange in the mercury (11 cm Ϫ 4 cm). Solving for the pressure at point A, we obtainpA ϭ 96,351 Pa ϭ 96.4 kPa Ans.The intermediate six-figure result of 96,351 Pa is utterly fatuous, since the measurementscannot be made that accurately.
    • Fig. 2.11 Hydrostatic force andcenter of pressure on an arbitraryplane surface of area A inclined atan angle ␪ below the free surface.In making these manometer calculations we have neglected the capillary-heightchanges due to surface tension, which were discussed in Example 1.9. These effectscancel if there is a fluid interface, or meniscus, on both sides of the U-tube, as in Fig.2.9. Otherwise, as in the right-hand U-tube of Fig. 2.10, a capillary correction can bemade or the effect can be made negligible by using large-bore ( Ն 1 cm) tubes.A common problem in the design of structures which interact with fluids is the com-putation of the hydrostatic force on a plane surface. If we neglect density changes inthe fluid, Eq. (2.20) applies and the pressure on any submerged surface varies linearlywith depth. For a plane surface, the linear stress distribution is exactly analogous tocombined bending and compression of a beam in strength-of-materials theory. The hy-drostatic problem thus reduces to simple formulas involving the centroid and momentsof inertia of the plate cross-sectional area.Figure 2.11 shows a plane panel of arbitrary shape completely submerged in a liq-uid. The panel plane makes an arbitrary angle ␪ with the horizontal free surface, sothat the depth varies over the panel surface. If h is the depth to any element area dAof the plate, from Eq. (2.20) the pressure there is p ϭ pa ϩ ␥h.To derive formulas involving the plate shape, establish an xy coordinate system inthe plane of the plate with the origin at its centroid, plus a dummy coordinate ␰ downfrom the surface in the plane of the plate. Then the total hydrostatic force on one sideof the plate is given byF ϭ ͵p dA ϭ ͵(pa ϩ ␥h) dA ϭ paA ϩ ␥ ͵h dA (2.35)The remaining integral is evaluated by noticing from Fig. 2.11 that h ϭ ␰ sin ␪ and,74 Chapter 2 Pressure Distribution in a FluidFree surface p = paθh(x, y)hCGResultantforce:F = pCGASide viewCPxyCGPlan view of arbitrary plane surfacedA = dx dyξ = hsin θ2.5 Hydrostatic Forces onPlane Surfaces
    • by definition, the centroidal slant distance from the surface to the plate is␰CG ϭ ᎏA1ᎏ ͵␰ dA (2.36)Therefore, since ␪ is constant along the plate, Eq. (2.35) becomesF ϭ paA ϩ ␥ sin ␪ ͵␰ dA ϭ paA ϩ ␥ sin ␪ ␰CGA (2.37)Finally, unravel this by noticing that ␰CG sin ␪ ϭ hCG, the depth straight down fromthe surface to the plate centroid. ThusF ϭ paA ϩ ␥hCGA ϭ (pa ϩ ␥hCG)A ϭ pCGA (2.38)The force on one side of any plane submerged surface in a uniform fluid equals thepressure at the plate centroid times the plate area, independent of the shape of the plateor the angle ␪ at which it is slanted.Equation (2.38) can be visualized physically in Fig. 2.12 as the resultant of a lin-ear stress distribution over the plate area. This simulates combined compression andbending of a beam of the same cross section. It follows that the “bending’’ portion ofthe stress causes no force if its “neutral axis’’ passes through the plate centroid of area.Thus the remaining “compression’’ part must equal the centroid stress times the platearea. This is the result of Eq. (2.38).However, to balance the bending-moment portion of the stress, the resultant forceF does not act through the centroid but below it toward the high-pressure side. Its lineof action passes through the center of pressure CP of the plate, as sketched in Fig. 2.11.To find the coordinates (xCP, yCP), we sum moments of the elemental force p dA aboutthe centroid and equate to the moment of the resultant F. To compute yCP, we equateFyCP ϭ ͵yp dA ϭ ͵y(pa ϩ ␥␰ sin ␪) dA ϭ ␥ sin ␪ ͵y␰ dA (2.39)The term ͐ pay dA vanishes by definition of centroidal axes. Introducing ␰ ϭ ␰CG Ϫ y,2.5 Hydrostatic Forces on Plane Surfaces 75Fig. 2.12 The hydrostatic-pressureforce on a plane surface is equal,regardless of its shape, to the resul-tant of the three-dimensional linearpressure distribution on that surfaceF ϭ pCGA.Pressure distributionp (x, y)Centroid of the plane surfaceArbitraryplane surfaceof area Apav= pCG
    • we obtainFyCP ϭ ␥ sin ␪΂␰CG ͵y dA Ϫ ͵y2dA΃ϭ Ϫ ␥ sin ␪ Ixx (2.40)where again ͐ y dA ϭ 0 and Ixx is the area moment of inertia of the plate area about itscentroidal x axis, computed in the plane of the plate. Substituting for F gives the resultyCP ϭ Ϫ␥ sin ␪ ᎏpCIxGxAᎏ (2.41)The negative sign in Eq. (2.41) shows that yCP is below the centroid at a deeper leveland, unlike F, depends upon angle ␪. If we move the plate deeper, yCP approaches thecentroid because every term in Eq. (2.41) remains constant except pCG, which increases.The determination of xCP is exactly similar:FxCP ϭ ͵xp dA ϭ ͵x[pa ϩ ␥(␰CG Ϫ y) sin ␪] dAϭ Ϫ␥ sin ␪ ͵xy dA ϭ Ϫ␥ sin ␪ Ixy (2.42)where Ixy is the product of inertia of the plate, again computed in the plane of theplate. Substituting for F givesxCP ϭ Ϫ␥ sin ␪ (2.43)For positive Ixy, xCP is negative because the dominant pressure force acts in the third,or lower left, quadrant of the panel. If Ixy ϭ 0, usually implying symmetry, xCP ϭ 0and the center of pressure lies directly below the centroid on the y axis.IxyᎏpCGA76 Chapter 2 Pressure Distribution in a Fluidxyb2b2L2L2(a)A = bLIxx =bL312Ixy = 0xyRRA = R2Ixx =R44Ixy = 0ππ(b)xyIxx =bL336A =2Ixy =b(b – 2s)L272b2b2L32L3(c)xyR RA =Ixx = 0.10976R4Ixy = 0π24R3π(d)sbL R2Fig. 2.13 Centroidal moments ofinertia for various cross sections:(a) rectangle, (b) circle, (c) trian-gle, and (d) semicircle.
    • Part (a)Part (b)In most cases the ambient pressure pa is neglected because it acts on both sides of theplate; e.g., the other side of the plate is inside a ship or on the dry side of a gate or dam.In this case pCG ϭ ␥hCG, and the center of pressure becomes independent of specific weightF ϭ ␥hCGA yCP ϭ ϪᎏIxhxCsGinA␪ᎏ xCP ϭ ϪᎏIxhyCsGinA␪ᎏ (2.44)Figure 2.13 gives the area and moments of inertia of several common cross sectionsfor use with these formulas.EXAMPLE 2.5The gate in Fig. E2.5a is 5 ft wide, is hinged at point B, and rests against a smooth wall at pointA. Compute (a) the force on the gate due to seawater pressure, (b) the horizontal force P exertedby the wall at point A, and (c) the reactions at the hinge B.2.5 Hydrostatic Forces on Plane Surfaces 77Gage-Pressure Formulas15 ftWall6 ft8 ftθGateHingeBASeawater:64 lbf/ft3papaSolutionBy geometry the gate is 10 ft long from A to B, and its centroid is halfway between, or at eleva-tion 3 ft above point B. The depth hCG is thus 15 Ϫ 3 ϭ 12 ft. The gate area is 5(10) ϭ 50 ft2. Ne-glect pa as acting on both sides of the gate. From Eq. (2.38) the hydrostatic force on the gate isF ϭ pCGA ϭ ␥hCGA ϭ (64 lbf/ft3)(12 ft)(50 ft2) ϭ 38,400 lbf Ans. (a)First we must find the center of pressure of F. A free-body diagram of the gate is shown in Fig.E2.5b. The gate is a rectangle, henceIxy ϭ 0 and Ixx ϭ ᎏb1L23ᎏ ϭ ᎏ(5 ft)1(120 ft)3ᎏ ϭ 417 ft4The distance l from the CG to the CP is given by Eq. (2.44) since pa is neglected.l ϭ ϪyCP ϭ ϩ ᎏIxhxCsGinA␪ᎏ ϭ ϭ 0.417 ft(417 ft4)(ᎏ160ᎏ)ᎏᎏ(12 ft)(50 ft2)E2.5a
    • The distance from point B to force F is thus 10 Ϫ l Ϫ 5 ϭ 4.583 ft. Summing moments coun-terclockwise about B givesPL sin ␪ Ϫ F(5 Ϫ l) ϭ P(6 ft) Ϫ (38,400 lbf)(4.583 ft) ϭ 0or P ϭ 29,300 lbf Ans. (b)With F and P known, the reactions Bx and Bz are found by summing forces on the gate⌺ Fx ϭ 0 ϭ Bx ϩ F sin ␪ Ϫ P ϭ Bx ϩ 38,400(0.6) Ϫ 29,300or Bx ϭ 6300 lbf⌺ Fz ϭ 0 ϭ Bz Ϫ F cos ␪ ϭ Bz Ϫ 38,400(0.8)or Bz ϭ 30,700 lbf Ans. (c)This example should have reviewed your knowledge of statics.EXAMPLE 2.6A tank of oil has a right-triangular panel near the bottom, as in Fig. E2.6. Omitting pa, find the(a) hydrostatic force and (b) CP on the panel.78 Chapter 2 Pressure Distribution in a Fluidpa5 m11 m30°6 m4 mOil: = 800 kg/m3ρpaCG CP4 m2 m8 m4 mPart (c)E2.6PAFBBxBzθ CPCGl5 ftL = 10 ftE2.5b
    • 2.6 Hydrostatic Forces onCurved SurfacesFig. 2.14 Computation of hydro-static force on a curved surface:(a) submerged curved surface; (b)free-body diagram of fluid abovethe curved surface.SolutionThe triangle has properties given in Fig. 2.13c. The centroid is one-third up (4 m) and one-thirdover (2 m) from the lower left corner, as shown. The area isᎏ12ᎏ(6 m)(12 m) ϭ 36 m2The moments of inertia areIxx ϭ ᎏb3L63ᎏ ϭ ϭ 288 m4and Ixy ϭ ᎏb(b Ϫ722s)L2ᎏ ϭ ϭ Ϫ72 m4The depth to the centroid is hCG ϭ 5 ϩ 4 ϭ 9 m; thus the hydrostatic force from Eq. (2.44) isF ϭ ␳ghCGA ϭ (800 kg/m3)(9.807 m/s2)(9 m)(36 m2)ϭ 2.54 ϫ 106(kg и m)/s2ϭ 2.54 ϫ 106N ϭ 2.54 MN Ans. (a)The CP position is given by Eqs. (2.44):yCP ϭ ϪᎏIxhxCsGinA␪ᎏ ϭ Ϫ ϭ Ϫ0.444 mxCP ϭ ϪᎏIxhyCsGinA␪ᎏ ϭ Ϫ ϭ ϩ0.111 m Ans. (b)The resultant force F ϭ 2.54 MN acts through this point, which is down and to the right of thecentroid, as shown in Fig. E2.6.The resultant pressure force on a curved surface is most easily computed by separat-ing it into horizontal and vertical components. Consider the arbitrary curved surfacesketched in Fig. 2.14a. The incremental pressure forces, being normal to the local areaelement, vary in direction along the surface and thus cannot be added numerically. We(Ϫ72 m4)(sin 30°)ᎏᎏ(9 m)(36 m2)(288 m4)(sin 30°)ᎏᎏ(9 m)(36 m2)(6 m)[6 m Ϫ 2(6 m)](12 m)2ᎏᎏᎏ72(6 m)(12 m)3ᎏᎏ362.6 Hydrostatic Forces on Curved Surfaces 79Part (a)Curved surfaceprojection ontovertical planeFVFHFH(a)F1F1FHFHFV(b)dcabeWairW2W1Part (b)
    • could sum the separate three components of these elemental pressure forces, but it turnsout that we need not perform a laborious three-way integration.Figure 2.14b shows a free-body diagram of the column of fluid contained in the ver-tical projection above the curved surface. The desired forces FH and FV are exerted bythe surface on the fluid column. Other forces are shown due to fluid weight and hori-zontal pressure on the vertical sides of this column. The column of fluid must be instatic equilibrium. On the upper part of the column bcde, the horizontal componentsF1 exactly balance and are not relevant to the discussion. On the lower, irregular por-tion of fluid abc adjoining the surface, summation of horizontal forces shows that thedesired force FH due to the curved surface is exactly equal to the force FH on the ver-tical left side of the fluid column. This left-side force can be computed by the plane-surface formula, Eq. (2.38), based on a vertical projection of the area of the curvedsurface. This is a general rule and simplifies the analysis:The horizontal component of force on a curved surface equals the force on the planearea formed by the projection of the curved surface onto a vertical plane normal tothe component.If there are two horizontal components, both can be computed by this scheme.Summation of vertical forces on the fluid free body then shows thatFV ϭ W1 ϩ W2 ϩ Wair (2.45)We can state this in words as our second general rule:The vertical component of pressure force on a curved surface equals in magnitudeand direction the weight of the entire column of fluid, both liquid and atmosphere,above the curved surface.Thus the calculation of FV involves little more than finding centers of mass of a col-umn of fluid—perhaps a little integration if the lower portion abc has a particularlyvexing shape.EXAMPLE 2.7A dam has a parabolic shape z/z0 ϭ (x/x0)2as shown in Fig. E2.7a, with x0 ϭ 10 ft and z0 ϭ 24ft. The fluid is water, ␥ ϭ 62.4 lbf/ft3, and atmospheric pressure may be omitted. Compute the80 Chapter 2 Pressure Distribution in a Fluidz = z0( xx0(2;;;;;;pa = 0 lbf/ft2 gageFVFHzCPx0xz0E2.7a
    • E2.7bforces FH and FV on the dam and the position CP where they act. The width of the dam is50 ft.SolutionThe vertical projection of this curved surface is a rectangle 24 ft high and 50 ft wide, with itscentroid halfway down, or hCG ϭ 12 ft. The force FH is thusFH ϭ ␥hCGAproj ϭ (62.4 lbf/ft3)(12 ft)(24 ft)(50 ft)ϭ 899,000 lbf ϭ 899 ϫ 103lbf Ans.The line of action of FH is below the centroid by an amountyCP ϭ ϪᎏhIxCxGsAinpro␪jᎏ ϭ Ϫ ϭ Ϫ4 ftThus FH is 12 ϩ 4 ϭ 16 ft, or two-thirds, down from the free surface or 8 ft from the bottom,as might have been evident by inspection of the triangular pressure distribution.The vertical component FV equals the weight of the parabolic portion of fluid above thecurved surface. The geometric properties of a parabola are shown in Fig. E2.7b. The weight ofthis amount of water isFV ϭ ␥(ᎏ23ᎏx0z0b) ϭ (62.4 lbf/ft3)(ᎏ23ᎏ)(10 ft)(24 ft)(50 ft)ϭ 499,000 lbf ϭ 499 ϫ 103lbf Ans.ᎏ112ᎏ(50 ft)(24 ft)3(sin 90°)ᎏᎏᎏ(12 ft)(24 ft)(50 ft)2.6 Hydrostatic Forces on Curved Surfaces 813z05Area =2x0z03z00 3x08x0 = 10 ftParabolaFVThis acts downward on the surface at a distance 3x0/8 ϭ 3.75 ft over from the origin of coordi-nates. Note that the vertical distance 3z0/5 in Fig. E2.7b is irrelevant.The total resultant force acting on the dam isF ϭ (F2H ϩ F2V)1/2ϭ [(499)2ϩ (899)2]1/2ϭ 1028 ϫ 103lbfAs seen in Fig. E2.7c, this force acts down and to the right at an angle of 29° ϭ tanϪ1 ᎏ489999ᎏ. Theforce F passes through the point (x, z) ϭ (3.75 ft, 8 ft). If we move down along the 29° line un-til we strike the dam, we find an equivalent center of pressure on the dam atxCP ϭ 5.43 ft zCP ϭ 7.07 ft Ans.This definition of CP is rather artificial, but this is an unavoidable complication of dealing witha curved surface.
    • E2.7c2.7 Hydrostatic Forces inLayered FluidsThe formulas for plane and curved surfaces in Secs. 2.5 and 2.6 are valid only for afluid of uniform density. If the fluid is layered with different densities, as in Fig. 2.15,a single formula cannot solve the problem because the slope of the linear pressure dis-tribution changes between layers. However, the formulas apply separately to each layer,and thus the appropriate remedy is to compute and sum the separate layer forces andmoments.Consider the slanted plane surface immersed in a two-layer fluid in Fig. 2.15. Theslope of the pressure distribution becomes steeper as we move down into the denser82 Chapter 2 Pressure Distribution in a FluidzResultant = 1028 × 1031bf acts along z = 10.083 – 0.5555xParabola z = 0.24x2CPCG 29°7.07 ft5.43 ft8993.75 ftx8 ft4990p2= p1– 2g(z2– z1)zz = 0z2, p2p = p1– 2g(z – z1)pa1< 2Fluid 1p = pa– 1gzp1= pa– 1gz12Fluid 2PlanesurfaceF1= pCG1A1z1, p1F2= pCG2A2ρρ ρρρρρFig. 2.15 Hydrostatic forces on asurface immersed in a layered fluidmust be summed in separate pieces.
    • Part (a)E2.8second layer. The total force on the plate does not equal the pressure at the centroidtimes the plate area, but the plate portion in each layer does satisfy the formula, so thatwe can sum forces to find the total:F ϭ ⌺ Fi ϭ ⌺ pCGiAi (2.46)Similarly, the centroid of the plate portion in each layer can be used to locate the cen-ter of pressure on that portionyCPiϭ Ϫᎏ␳igpsCinGi␪AiiIxxiᎏ xCPiϭ Ϫᎏ␳igpsCinGi␪AiiIxyiᎏ (2.47)These formulas locate the center of pressure of that particular Fi with respect to thecentroid of that particular portion of plate in the layer, not with respect to the centroidof the entire plate. The center of pressure of the total force F ϭ ⌺ Fi can then be foundby summing moments about some convenient point such as the surface. The follow-ing example will illustrate.EXAMPLE 2.8A tank 20 ft deep and 7 ft wide is layered with 8 ft of oil, 6 ft of water, and 4 ft of mercury.Compute (a) the total hydrostatic force and (b) the resultant center of pressure of the fluid onthe right-hand side of the tank.SolutionDivide the end panel into three parts as sketched in Fig. E2.8, and find the hydrostatic pressureat the centroid of each part, using the relation (2.38) in steps as in Fig. E2.8:2.7 Hydrostatic Forces in Layered Fluids 83pa = 0z = 04 ft11 ft16 ft7 ft8 ft6 ftWater (62.4)Oil: 55.0 lbf/ft 3(1)(2)4 ftMercury (846)(3)PCG1ϭ (55.0 lbf/ft3)(4 ft) ϭ 220 lbf/ft2pCG2ϭ (55.0)(8) ϩ 62.4(3) ϭ 627 lbf/ft2pCG3ϭ (55.0)(8) ϩ 62.4(6) ϩ 846(2) ϭ 2506 lbf/ft2
    • 2.8 Buoyancy and StabilityThese pressures are then multiplied by the respective panel areas to find the force on each portion:F1 ϭ pCG1A1 ϭ (220 lbf/ft2)(8 ft)(7 ft) ϭ 12,300 lbfF2 ϭ pCG2A2 ϭ 627(6)(7) ϭ 26,300 lbfF3 ϭ pCG3A3 ϭ 2506(4)(7) ϭ 70,200 lbfF ϭ ⌺ Fi ϭ 108,800 lbf Ans. (a)Equations (2.47) can be used to locate the CP of each force Fi, noting that ␪ ϭ 90° and sin ␪ ϭ1 for all parts. The moments of inertia are Ixx1ϭ (7 ft)(8 ft)3/12 ϭ 298.7 ft4, Ixx2ϭ 7(6)3/12 ϭ126.0 ft4, and Ixx3ϭ 7(4)3/12 ϭ 37.3 ft4. The centers of pressure are thus atyCP1ϭ Ϫᎏ␳1FgI1xx1ᎏ ϭ Ϫ ϭ Ϫ1.33 ftyCP2ϭ Ϫᎏ622.46(,132060.0)ᎏ ϭ Ϫ0.30 ft yCP3ϭ Ϫ ϭ Ϫ0.45 ftThis locates zCP1ϭ Ϫ4 Ϫ 1.33 ϭ Ϫ5.33 ft, zCP2ϭ Ϫ11 Ϫ 0.30 ϭ Ϫ11.30 ft, and zCP3ϭϪ16 Ϫ 0.45 ϭ Ϫ16.45 ft. Summing moments about the surface then gives⌺ FizCPiϭ FzCPor 12,300(Ϫ5.33) ϩ 26,300(Ϫ11.30) ϩ 70,200(Ϫ16.45) ϭ 108,800zCPor zCP ϭ Ϫ ϭ Ϫ13.95 ft Ans. (b)The center of pressure of the total resultant force on the right side of the tank lies 13.95 ft be-low the surface.The same principles used to compute hydrostatic forces on surfaces can be applied tothe net pressure force on a completely submerged or floating body. The results are thetwo laws of buoyancy discovered by Archimedes in the third century B.C.:1. A body immersed in a fluid experiences a vertical buoyant force equal to theweight of the fluid it displaces.2. A floating body displaces its own weight in the fluid in which it floats.These two laws are easily derived by referring to Fig. 2.16. In Fig. 2.16a, the bodylies between an upper curved surface 1 and a lower curved surface 2. From Eq. (2.45)for vertical force, the body experiences a net upward forceFB ϭ FV(2) Ϫ FV (1)ϭ (fluid weight above 2) Ϫ (fluid weight above 1)ϭ weight of fluid equivalent to body volume (2.48)Alternatively, from Fig. 2.16b, we can sum the vertical forces on elemental verticalslices through the immersed body:FB ϭ ͵body(p2 Ϫ p1) dAH ϭ Ϫ␥ ͵(z2 Ϫ z1) dAH ϭ (␥)(body volume) (2.49)1,518,000ᎏᎏ108,800846(37.3)ᎏᎏ70,200(55.0 lbf/ft3)(298.7 ft4)ᎏᎏᎏ12,300 lbf84 Chapter 2 Pressure Distribution in a FluidPart (b)
    • These are identical results and equivalent to law 1 above.Equation (2.49) assumes that the fluid has uniform specific weight. The line of ac-tion of the buoyant force passes through the center of volume of the displaced body;i.e., its center of mass is computed as if it had uniform density. This point throughwhich FB acts is called the center of buoyancy, commonly labeled B or CB on a draw-ing. Of course, the point B may or may not correspond to the actual center of mass ofthe body’s own material, which may have variable density.Equation (2.49) can be generalized to a layered fluid (LF) by summing the weightsof each layer of density ␳i displaced by the immersed body:(FB)LF ϭ ⌺ ␳ig(displaced volume)i (2.50)Each displaced layer would have its own center of volume, and one would have to summoments of the incremental buoyant forces to find the center of buoyancy of the im-mersed body.Since liquids are relatively heavy, we are conscious of their buoyant forces, but gasesalso exert buoyancy on any body immersed in them. For example, human beings havean average specific weight of about 60 lbf/ft3. We may record the weight of a personas 180 lbf and thus estimate the person’s total volume as 3.0 ft3. However, in so doingwe are neglecting the buoyant force of the air surrounding the person. At standard con-ditions, the specific weight of air is 0.0763 lbf/ft3; hence the buoyant force is approxi-mately 0.23 lbf. If measured in vacuo, the person would weigh about 0.23 lbf more.For balloons and blimps the buoyant force of air, instead of being negligible, is thecontrolling factor in the design. Also, many flow phenomena, e.g., natural convectionof heat and vertical mixing in the ocean, are strongly dependent upon seemingly smallbuoyant forces.Floating bodies are a special case; only a portion of the body is submerged, withthe remainder poking up out of the free surface. This is illustrated in Fig. 2.17, wherethe shaded portion is the displaced volume. Equation (2.49) is modified to apply to thissmaller volumeFB ϭ (␥)(displaced volume) ϭ floating-body weight (2.51)2.8 Buoyancy and Stability 85Surface1Surface2FV (1)Horizontalelementalarea dAHz1 – z2p1p2(a)FV (2)(b)Fig. 2.16 Two different approachesto the buoyant force on an arbitraryimmersed body: (a) forces on up-per and lower curved surfaces; (b)summation of elemental vertical-pressure forces.
    • E2.9Not only does the buoyant force equal the body weight, but also they are collinearsince there can be no net moments for static equilibrium. Equation (2.51) is the math-ematical equivalent of Archimedes’ law 2, previously stated.EXAMPLE 2.9A block of concrete weighs 100 lbf in air and “weighs’’ only 60 lbf when immersed in fresh wa-ter (62.4 lbf/ft3). What is the average specific weight of the block?SolutionA free-body diagram of the submerged block (see Fig. E2.9) shows a balance between the ap-parent weight, the buoyant force, and the actual weight⌺ Fz ϭ 0 ϭ 60 ϩ FB Ϫ 100or FB ϭ 40 lbf ϭ (62.4 lbf/ft3)(block volume, ft3)Solving gives the volume of the block as 40/62.4 ϭ 0.641 ft3. Therefore the specific weight ofthe block is␥block ϭ ᎏ01.60401lbftf3ᎏ ϭ 156 lbf/ft3Ans.Occasionally, a body will have exactly the right weight and volume for its ratio toequal the specific weight of the fluid. If so, the body will be neutrally buoyant and willremain at rest at any point where it is immersed in the fluid. Small neutrally buoyantparticles are sometimes used in flow visualization, and a neutrally buoyant body calleda Swallow float [2] is used to track oceanographic currents. A submarine can achievepositive, neutral, or negative buoyancy by pumping water in or out of its ballast tanks.A floating body as in Fig. 2.17 may not approve of the position in which it is floating.If so, it will overturn at the first opportunity and is said to be statically unstable, likea pencil balanced upon its point. The least disturbance will cause it to seek anotherequilibrium position which is stable. Engineers must design to avoid floating instabil-86 Chapter 2 Pressure Distribution in a FluidFBW = 100 lbf60 lbfStabilityFig. 2.17 Static equilibrium of afloating body.CGWFBB(Displaced volume) × ( γ of fluid) = body weightNeglect the displaced air up here.
    • Stability Related to WaterlineAreaity. The only way to tell for sure whether a floating position is stable is to “disturb’’the body a slight amount mathematically and see whether it develops a restoring mo-ment which will return it to its original position. If so, it is stable; if not, unstable. Suchcalculations for arbitrary floating bodies have been honed to a fine art by naval archi-tects [3], but we can at least outline the basic principle of the static-stability calcula-tion. Figure 2.18 illustrates the computation for the usual case of a symmetric floatingbody. The steps are as follows:1. The basic floating position is calculated from Eq. (2.51). The body’s center ofmass G and center of buoyancy B are computed.2. The body is tilted a small angle ⌬␪, and a new waterline is established for thebody to float at this angle. The new position BЈ of the center of buoyancy is cal-culated. A vertical line drawn upward from BЈ intersects the line of symmetry ata point M, called the metacenter, which is independent of ⌬␪ for small angles.3. If point M is above G, that is, if the metacentric height MෆGෆ is positive, a restor-ing moment is present and the original position is stable. If M is below G (nega-tive MෆGෆ, the body is unstable and will overturn if disturbed. Stability increaseswith increasing MෆGෆ.Thus the metacentric height is a property of the cross section for the given weight, andits value gives an indication of the stability of the body. For a body of varying crosssection and draft, such as a ship, the computation of the metacenter can be very in-volved.Naval architects [3] have developed the general stability concepts from Fig. 2.18 intoa simple computation involving the area moment of inertia of the waterline area aboutthe axis of tilt. The derivation assumes that the body has a smooth shape variation (nodiscontinuities) near the waterline and is derived from Fig. 2.19.The y-axis of the body is assumed to be a line of symmetry. Tilting the body a smallangle ␪ then submerges small wedge Obd and uncovers an equal wedge cOa, as shown.2.8 Buoyancy and Stability 87Fig. 2.18 Calculation of the meta-center M of the floating bodyshown in (a). Tilt the body a smallangle ⌬␪. Either (b) BЈ moves farout (point M above G denotes sta-bility); or (c) BЈ moves slightly(point M below G denotes instabil-ity).Line ofsymmetryGWFBBMGWBFB∆GW MFBBSmalldisturbanceangleSmalldisturbanceangle(b)Either Restoring moment or Overturning moment(a) (c)θ∆θ
    • 88 Chapter 2 Pressure Distribution in a FluidcOdea Obd cOa Obd cOaObd cOa waterlinexෆ υabOde ϭ ͵x dυ ϩ ͵x dυ Ϫ ͵x dυ ϭ 0 ϩ ͵x (L dA) Ϫ ͵x (L dA)The new position BЈ of the center of buoyancy is calculated as the centroid of the sub-merged portion aObde of the body:ycaed xxbMOBGGGdA = x tan ␪ dxTilted floating body␪␪Variable-widthL(x) into paperOriginalwaterlinearea␦␪BЈFig. 2.19 A floating body tiltedthrough a small angle ␪. The move-ment xෆ of the center of buoyancy Bis related to the waterline area mo-ment of inertia.ϭ 0 ϩ ͵x L (x tan ␪ dx) Ϫ ͵xL (Ϫx tan ␪ dx) ϭ tan ␪ ͵x2dAwaterline ϭ IO tan ␪where IO is the area moment of inertia of the waterline footprint of the body about itstilt axis O. The first integral vanishes because of the symmetry of the original sub-merged portion cOdea. The remaining two “wedge” integrals combine into IO whenwe notice that L dx equals an element of waterline area. Thus we determine the de-sired distance from M to B:ϭ MෆBෆ ϭ ϭ MෆGෆ ϩ GෆBෆ or MෆGෆ ϭ ᎏυIsOubᎏ Ϫ GෆBෆ (2.52)The engineer would determine the distance from G to B from the basic shape anddesign of the floating body and then make the calculation of IO and the submergedvolume υsub. If the metacentric height MG is positive, the body is stable for smalldisturbances. Note that if GෆBෆ is negative, that is, B is above G, the body is alwaysstable.EXAMPLE 2.10A barge has a uniform rectangular cross section of width 2L and vertical draft of height H, asin Fig. E2.10. Determine (a) the metacentric height for a small tilt angle and (b) the range ofratio L/H for which the barge is statically stable if G is exactly at the waterline as shown.IOᎏᎏυsubmergedxෆᎏ
    • SolutionIf the barge has length b into the paper, the waterline area, relative to tilt axis O, has a base band a height 2L; therefore, IO ϭ b(2L)3/12. Meanwhile, υsub ϭ 2LbH. Equation (2.52) predictsMෆGෆ ϭ ᎏυIsOubᎏ Ϫ GෆBෆ ϭ Ϫ ᎏH2ᎏ ϭ ᎏ3LH2ᎏ Ϫ ᎏH2ᎏ Ans. (a)The barge can thus be stable only ifL2Ͼ 3H2/2 or 2L Ͼ 2.45H Ans. (b)The wider the barge relative to its draft, the more stable it is. Lowering G would help also.Even an expert will have difficulty determining the floating stability of a buoyantbody of irregular shape. Such bodies may have two or more stable positions. For ex-ample, a ship may float the way we like it, so that we can sit upon the deck, or it mayfloat upside down (capsized). An interesting mathematical approach to floating stabil-ity is given in Ref. 11. The author of this reference points out that even simple shapes,e.g., a cube of uniform density, may have a great many stable floating orientations, notnecessarily symmetric. Homogeneous circular cylinders can float with the axis of sym-metry tilted from the vertical.Floating instability occurs in nature. Living fish generally swim with their planeof symmetry vertical. After death, this position is unstable and they float with theirflat sides up. Giant icebergs may overturn after becoming unstable when their shapeschange due to underwater melting. Iceberg overturning is a dramatic, rarely seenevent.Figure 2.20 shows a typical North Atlantic iceberg formed by calving from a Green-land glacier which protruded into the ocean. The exposed surface is rough, indicatingthat it has undergone further calving. Icebergs are frozen fresh, bubbly, glacial waterof average density 900 kg/m3. Thus, when an iceberg is floating in seawater, whoseaverage density is 1025 kg/m3, approximately 900/1025, or seven-eighths, of its vol-ume lies below the water.In rigid-body motion, all particles are in combined translation and rotation, and thereis no relative motion between particles. With no relative motion, there are no strains8bL3/12ᎏ2LbH2.9 Pressure Distribution in Rigid-Body Motion 89GB HOL LGE2.102.9 Pressure Distribution inRigid-Body Motion
    • or strain rates, so that the viscous term ␮∇2V in Eq. (2.13) vanishes, leaving a balancebetween pressure, gravity, and particle acceleration∇p ϭ ␳(g Ϫ a) (2.53)The pressure gradient acts in the direction g Ϫ a, and lines of constant pressure (in-cluding the free surface, if any) are perpendicular to this direction. The general caseof combined translation and rotation of a rigid body is discussed in Chap. 3, Fig. 3.12.If the center of rotation is at point O and the translational velocity is V0 at this point,the velocity of an arbitrary point P on the body is given by2V ϭ V0 ϩ ⍀ ؋ r0where ⍀ is the angular-velocity vector and r0 is the position of point P. Differentiat-ing, we obtain the most general form of the acceleration of a rigid body:a ϭ ϩ ⍀ ؋ (⍀ ؋ r0) ϩ ؋ r0 (2.54)Looking at the right-hand side, we see that the first term is the translational accel-eration; the second term is the centripetal acceleration, whose direction is from pointd⍀ᎏdtdV0ᎏdt90 Chapter 2 Pressure Distribution in a FluidFig. 2.20 A North Atlantic icebergformed by calving from a Green-land glacier. These, and their evenlarger Antarctic sisters, are thelargest floating bodies in the world.Note the evidence of further calv-ing fractures on the front surface.(Courtesy of S/oren Thalund, Green-land tourism a/s Iiulissat, Green-land.)2For a more detailed derivation of rigid-body motion, see Ref. 4, Sec. 2.7.
    • P perpendicular toward the axis of rotation; and the third term is the linear accelera-tion due to changes in the angular velocity. It is rare for all three of these terms to ap-ply to any one fluid flow. In fact, fluids can rarely move in rigid-body motion unlessrestrained by confining walls for a long time. For example, suppose a tank of wateris in a car which starts a constant acceleration. The water in the tank would begin toslosh about, and that sloshing would damp out very slowly until finally the particlesof water would be in approximately rigid-body acceleration. This would take so longthat the car would have reached hypersonic speeds. Nevertheless, we can at least dis-cuss the pressure distribution in a tank of rigidly accelerating water. The following isan example where the water in the tank will reach uniform acceleration rapidly.EXAMPLE 2.11A tank of water 1 m deep is in free fall under gravity with negligible drag. Compute the pres-sure at the bottom of the tank if pa ϭ 101 kPa.SolutionBeing unsupported in this condition, the water particles tend to fall downward as a rigid hunkof fluid. In free fall with no drag, the downward acceleration is a ϭ g. Thus Eq. (2.53) for thissituation gives ∇p ϭ ␳(g Ϫ g) ϭ 0. The pressure in the water is thus constant everywhere andequal to the atmospheric pressure 101 kPa. In other words, the walls are doing no service in sus-taining the pressure distribution which would normally exist.In this general case of uniform rigid-body acceleration, Eq. (2.53) applies, a havingthe same magnitude and direction for all particles. With reference to Fig. 2.21, the par-allelogram sum of g and Ϫa gives the direction of the pressure gradient or greatest rateof increase of p. The surfaces of constant pressure must be perpendicular to this andare thus tilted at a downward angle ␪ such that␪ ϭ tanϪ1ᎏg ϩaxazᎏ (2.55)2.9 Pressure Distribution in Rigid-Body Motion 91Uniform Linear Accelerationp = p1p2p3xz∆p µg – aaxaaz Fluidat restS–a θ = tan –1axg + azgazaxθFig. 2.21 Tilting of constant-pressure surfaces in a tank ofliquid in rigid-body acceleration.
    • One of these tilted lines is the free surface, which is found by the requirement that thefluid retain its volume unless it spills out. The rate of increase of pressure in the di-rection g Ϫ a is greater than in ordinary hydrostatics and is given byϭ ␳G where G ϭ [a2x ϩ (g ϩ az)2]1/2(2.56)These results are independent of the size or shape of the container as long as thefluid is continuously connected throughout the container.EXAMPLE 2.12A drag racer rests her coffee mug on a horizontal tray while she accelerates at 7 m/s2. The mugis 10 cm deep and 6 cm in diameter and contains coffee 7 cm deep at rest. (a) Assuming rigid-body acceleration of the coffee, determine whether it will spill out of the mug. (b) Calculate thegage pressure in the corner at point A if the density of coffee is 1010 kg/m3.SolutionThe free surface tilts at the angle ␪ given by Eq. (2.55) regardless of the shape of the mug. Withaz ϭ 0 and standard gravity,␪ ϭ tanϪ1ᎏagxᎏ ϭ tanϪ1ᎏ97..801ᎏ ϭ 35.5°If the mug is symmetric about its central axis, the volume of coffee is conserved if the tilted sur-face intersects the original rest surface exactly at the centerline, as shown in Fig. E2.12.dpᎏds92 Chapter 2 Pressure Distribution in a FluidPart (a)Part (b)3 cmax= 7 m/s2A∆z3 cm7 cmθE2.12Thus the deflection at the left side of the mug isz ϭ (3 cm)(tan ␪) ϭ 2.14 cm Ans. (a)This is less than the 3-cm clearance available, so the coffee will not spill unless it was sloshedduring the start-up of acceleration.When at rest, the gage pressure at point A is given by Eq. (2.20):pA ϭ ␳g(zsurf Ϫ zA) ϭ (1010 kg/m3)(9.81 m/s2)(0.07 m) ϭ 694 N/m2ϭ 694 Pa
    • During acceleration, Eq. (2.56) applies, with G ϭ [(7.0)2ϩ (9.81)2]1/2ϭ 12.05 m/s2. The dis-tance ∆s down the normal from the tilted surface to point A is⌬s ϭ (7.0 ϩ 2.14)(cos ␪) ϭ 7.44 cmThus the pressure at point A becomespA ϭ ␳G ⌬s ϭ 1010(12.05)(0.0744) ϭ 906 Pa Ans. (b)which is an increase of 31 percent over the pressure when at rest.As a second special case, consider rotation of the fluid about the z axis without anytranslation, as sketched in Fig. 2.22. We assume that the container has been rotatinglong enough at constant ⍀ for the fluid to have attained rigid-body rotation. The fluidacceleration will then be the centripetal term in Eq. (2.54). In the coordinates of Fig.2.22, the angular-velocity and position vectors are given by⍀ ϭ k⍀ r0 ϭ irr (2.57)Then the acceleration is given by⍀ ؋ (⍀ ؋ r0) ϭ Ϫr⍀2ir (2.58)as marked in the figure, and Eq. (2.53) for the force balance becomes∇p ϭ ir ϩ k ϭ ␳(g Ϫ a) ϭ ␳(Ϫgk ϩ r⍀2ir) (2.59)Equating like components, we find the pressure field by solving two first-order partialdifferential equationsϭ ␳r⍀2ϭ Ϫ␥ (2.60)This is our first specific example of the generalized three-dimensional problem de-scribed by Eqs. (2.14) for more than one independent variable. The right-hand sides ofѨpᎏѨzѨpᎏѨrѨpᎏѨzѨpᎏѨr2.9 Pressure Distribution in Rigid-Body Motion 93z, kr, irp = paΩa = –rΩ2ir –ag g–aStill-waterlevelAxis ofrotationp = p1p2p3Fig. 2.22 Development of parabo-loid constant-pressure surfaces in afluid in rigid-body rotation. Thedashed line along the direction ofmaximum pressure increase is anexponential curve.Rigid-Body Rotation
    • (2.60) are known functions of r and z. One can proceed as follows: Integrate the firstequation “partially,’’ i.e., holding z constant, with respect to r. The result isp ϭ ᎏ12ᎏ␳r2⍀2ϩ f(z) (2.61)where the “constant’’ of integration is actually a function f(z).†Now differentiate thiswith respect to z and compare with the second relation of (2.60):ϭ 0 ϩ fЈ(z) ϭ Ϫ␥or f(z) ϭ Ϫ␥z ϩ C (2.62a)where C is a constant. Thus Eq. (2.61) now becomesp ϭ const Ϫ ␥z ϩ ᎏ12ᎏ␳r2⍀2(2.62b)This is the pressure distribution in the fluid. The value of C is found by specifying thepressure at one point. If p ϭ p0 at (r, z) ϭ (0, 0), then C ϭ p0. The final desired dis-tribution isp ϭ p0 Ϫ ␥z ϩ ᎏ12ᎏ␳r2⍀2(2.63)The pressure is linear in z and parabolic in r. If we wish to plot a constant-pressuresurface, say, p ϭ p1, Eq. (2.63) becomesz ϭ ϩ ϭ a ϩ br2(2.64)Thus the surfaces are paraboloids of revolution, concave upward, with their minimumpoint on the axis of rotation. Some examples are sketched in Fig. 2.22.As in the previous example of linear acceleration, the position of the free surface isfound by conserving the volume of fluid. For a noncircular container with the axis ofrotation off-center, as in Fig. 2.22, a lot of laborious mensuration is required, and asingle problem will take you all weekend. However, the calculation is easy for a cylin-der rotating about its central axis, as in Fig. 2.23. Since the volume of a paraboloid isr2⍀2ᎏ2gp0 Ϫ p1ᎏ␥ѨpᎏѨz94 Chapter 2 Pressure Distribution in a FluidVolume =π2R2hStill -waterlevelR RΩh =Ω2R22gh2h2Fig. 2.23 Determining the free-surface position for rotation of acylinder of fluid about its centralaxis.†This is because f(z) vanishes when differentiated with respect to r. If you don’t see this, you shouldreview your calculus.
    • one-half the base area times its height, the still-water level is exactly halfway betweenthe high and low points of the free surface. The center of the fluid drops an amounth/2 ϭ ⍀2R2/(4g), and the edges rise an equal amount.EXAMPLE 2.13The coffee cup in Example 2.12 is removed from the drag racer, placed on a turntable, and ro-tated about its central axis until a rigid-body mode occurs. Find (a) the angular velocity whichwill cause the coffee to just reach the lip of the cup and (b) the gage pressure at point A for thiscondition.SolutionThe cup contains 7 cm of coffee. The remaining distance of 3 cm up to the lip must equal thedistance h/2 in Fig. 2.23. Thusᎏh2ᎏ ϭ 0.03 m ϭ ᎏ⍀42gR2ᎏ ϭSolving, we obtain⍀2ϭ 1308 or ⍀ ϭ 36.2 rad/s ϭ 345 r/min Ans. (a)To compute the pressure, it is convenient to put the origin of coordinates r and z at the bottomof the free-surface depression, as shown in Fig. E2.13. The gage pressure here is p0 ϭ 0, andpoint A is at (r, z) ϭ (3 cm, Ϫ4 cm). Equation (2.63) can then be evaluatedpA ϭ 0 Ϫ (1010 kg/m3)(9.81 m/s2)(Ϫ0.04 m)ϩ ᎏ12ᎏ(1010 kg/m3)(0.03 m)2(1308 rad2/s2)ϭ 396 N/m2ϩ 594 N/m2ϭ 990 Pa Ans. (b)This is about 43 percent greater than the still-water pressure pA ϭ 694 Pa.Here, as in the linear-acceleration case, it should be emphasized that the paraboloidpressure distribution (2.63) sets up in any fluid under rigid-body rotation, regardlessof the shape or size of the container. The container may even be closed and filled withfluid. It is only necessary that the fluid be continuously interconnected throughout thecontainer. The following example will illustrate a peculiar case in which one can vi-sualize an imaginary free surface extending outside the walls of the container.EXAMPLE 2.14A U-tube with a radius of 10 in and containing mercury to a height of 30 in is rotated about itscenter at 180 r/min until a rigid-body mode is achieved. The diameter of the tubing is negligi-ble. Atmospheric pressure is 2116 lbf/ft2. Find the pressure at point A in the rotating condition.See Fig. E2.14.⍀2(0.03 m)2ᎏᎏ4(9.81 m/s2)2.9 Pressure Distribution in Rigid-Body Motion 95Part (a)Part (b)3 cmA3 cm7 cm3 cmΩzr0E2.13
    • 96 Chapter 2 Pressure Distribution in a FluidSolutionConvert the angular velocity to radians per second:⍀ ϭ (180 r/min) ϭ 18.85 rad/sFrom Table 2.1 we find for mercury that ␥ ϭ 846 lbf/ft3and hence ␳ ϭ 846/32.2 ϭ 26.3 slugs/ft3.At this high rotation rate, the free surface will slant upward at a fierce angle [about 84°; checkthis from Eq. (2.64)], but the tubing is so thin that the free surface will remain at approximatelythe same 30-in height, point B. Placing our origin of coordinates at this height, we can calcu-late the constant C in Eq. (2.62b) from the condition pB ϭ 2116 lbf/ft2at (r, z) ϭ (10 in, 0):pB ϭ 2116 lbf/ft2ϭ C Ϫ 0 ϩ ᎏ12ᎏ(26.3 slugs/ft3)(ᎏ1102ᎏ ft)2(18.85 rad/s)2or C ϭ 2116 Ϫ 3245 ϭ Ϫ1129 lbf/ft2We then obtain pA by evaluating Eq. (2.63) at (r, z) ϭ (0, Ϫ30 in):pA ϭ Ϫ1129 Ϫ (846 lbf/ft3)(Ϫᎏ3102ᎏ ft) ϭ Ϫ1129 ϩ 2115 ϭ 986 lbf/ft2Ans.This is less than atmospheric pressure, and we can see why if we follow the free-surface pa-raboloid down from point B along the dashed line in the figure. It will cross the horizontal por-tion of the U-tube (where p will be atmospheric) and fall below point A. From Fig. 2.23 the ac-tual drop from point B will beh ϭ ᎏ⍀22gR2ᎏ ϭ ϭ 3.83 ft ϭ 46 inThus pA is about 16 inHg below atmospheric pressure, or about ᎏ1162ᎏ(846) ϭ 1128 lbf/ft2belowpa ϭ 2116 lbf/ft2, which checks with the answer above. When the tube is at rest,pA ϭ 2116 Ϫ 846(Ϫᎏ3102ᎏ) ϭ 4231 lbf/ft2Hence rotation has reduced the pressure at point A by 77 percent. Further rotation can reducepA to near-zero pressure, and cavitation can occur.An interesting by-product of this analysis for rigid-body rotation is that the lineseverywhere parallel to the pressure gradient form a family of curved surfaces, assketched in Fig. 2.22. They are everywhere orthogonal to the constant-pressure sur-faces, and hence their slope is the negative inverse of the slope computed from Eq.(2.64):GLϭ Ϫ ϭ Ϫwhere GL stands for gradient lineor ϭ Ϫ (2.65)gᎏr⍀2dzᎏdr1ᎏr⍀2/g1ᎏᎏ(dz/dr)pϭconstdzᎏdr(18.85)2(ᎏ1102ᎏ)2ᎏᎏ2(32.2)2␲ rad/rᎏ60 s/minzΩrImaginaryfree surface10 in30 in0ABE2.14
    • 2.10 Pressure Measurement 97Separating the variables and integrating, we find the equation of the pressure-gradientsurfacesr ϭ C1 exp΂Ϫ΃ (2.66)Notice that this result and Eq. (2.64) are independent of the density of the fluid. In theabsence of friction and Coriolis effects, Eq. (2.66) defines the lines along which the ap-parent net gravitational field would act on a particle. Depending upon its density, a smallparticle or bubble would tend to rise or fall in the fluid along these exponential lines,as demonstrated experimentally in Ref. 5. Also, buoyant streamers would align them-selves with these exponential lines, thus avoiding any stress other than pure tension. Fig-ure 2.24 shows the configuration of such streamers before and during rotation.Pressure is a derived property. It is the force per unit area as related to fluid molecu-lar bombardment of a surface. Thus most pressure instruments only infer the pressureby calibration with a primary device such as a deadweight piston tester. There are manysuch instruments, both for a static fluid and a moving stream. The instrumentation textsin Refs. 7 to 10, 12, and 13 list over 20 designs for pressure measurement instruments.These instruments may be grouped into four categories:1. Gravity-based: barometer, manometer, deadweight piston.2. Elastic deformation: bourdon tube (metal and quartz), diaphragm, bellows,strain-gage, optical beam displacement.3. Gas behavior: gas compression (McLeod gage), thermal conductance (Pirani gage),molecular impact (Knudsen gage), ionization, thermal conductivity, air piston.4. Electric output: resistance (Bridgman wire gage), diffused strain gage, capacita-tive, piezoelectric, magnetic inductance, magnetic reluctance, linear variable dif-ferential transformer (LVDT), resonant frequency.The gas-behavior gages are mostly special-purpose instruments used for certain scien-tific experiments. The deadweight tester is the instrument used most often for calibra-tions; for example, it is used by the U.S. National Institute for Standards and Tech-nology (NIST). The barometer is described in Fig. 2.6.The manometer, analyzed in Sec. 2.4, is a simple and inexpensive hydrostatic-principle device with no moving parts except the liquid column itself. Manometer mea-surements must not disturb the flow. The best way to do this is to take the measure-ment through a static hole in the wall of the flow, as illustrated for the two instrumentsin Fig. 2.25. The hole should be normal to the wall, and burrs should be avoided. Ifthe hole is small enough (typically 1-mm diameter), there will be no flow into the mea-suring tube once the pressure has adjusted to a steady value. Thus the flow is almostundisturbed. An oscillating flow pressure, however, can cause a large error due to pos-sible dynamic response of the tubing. Other devices of smaller dimensions are used fordynamic-pressure measurements. Note that the manometers in Fig. 2.25 are arrangedto measure the absolute pressures p1 and p2. If the pressure difference p1 Ϫ p2 is de-⍀2zᎏg2.10 Pressure Measurement
    • 98 Chapter 2 Pressure Distribution in a FluidFig. 2.24 Experimental demonstra-tion with buoyant streamers of thefluid force field in rigid-body rota-tion: (top) fluid at rest (streamershang vertically upward); (bottom)rigid-body rotation (streamers arealigned with the direction of maxi-mum pressure gradient). (From Ref.5, courtesy of R. Ian Fletcher.)
    • Fig. 2.25 Two types of accuratemanometers for precise measure-ments: (a) tilted tube with eye-piece; (b) micrometer pointer withammeter detector.sired, a significant error is incurred by subtracting two independent measurements, andit would be far better to connect both ends of one instrument to the two static holes p1and p2 so that one manometer reads the difference directly. In category 2, elastic-deformation instruments, a popular, inexpensive, and reliable device is the bourdontube, sketched in Fig. 2.26. When pressurized internally, a curved tube with flattenedcross section will deflect outward. The deflection can be measured by a linkage at-tached to a calibrated dial pointer, as shown. Or the deflection can be used to driveelectric-output sensors, such as a variable transformer. Similarly, a membrane or dia-phragm will deflect under pressure and can either be sensed directly or used to driveanother sensor.2.10 Pressure Measurement 99Flow Flowp2p1(a) (b)Fig. 2.26 Schematic of a bourdon-tube device for mechanical mea-surement of high pressures.BourdontubePointer fordial gageSection AAAALinkageHigh pressureFlattened tube deflectsoutward under pressure
    • Fig. 2.27 The fused-quartz, force-balanced bourdon tube is the mostaccurate pressure sensor used incommercial applications today.(Courtesy of Ruska InstrumentCorporation, Houston, TX.)100 Chapter 2 Pressure Distribution in a FluidAn interesting variation of Fig. 2.26 is the fused-quartz, forced-balanced bourdontube, shown in Fig. 2.27, whose deflection is sensed optically and returned to a zeroreference state by a magnetic element whose output is proportional to the fluid pres-sure. The fused-quartz, forced-balanced bourdon tube is reported to be one of the mostaccurate pressure sensors ever devised, with uncertainty of the order of Ϯ0.003 per-cent.The last category, electric-output sensors, is extremely important in engineeringbecause the data can be stored on computers and freely manipulated, plotted, and an-alyzed. Three examples are shown in Fig. 2.28, the first being the capacitive sensorin Fig. 2.28a. The differential pressure deflects the silicon diaphragm and changesthe capacitancce of the liquid in the cavity. Note that the cavity has spherical endcaps to prevent overpressure damage. In the second type, Fig. 2.28b, strain gages andother sensors are diffused or etched onto a chip which is stressed by the applied pres-sure. Finally, in Fig. 2.28c, a micromachined silicon sensor is arranged to deformunder pressure such that its natural vibration frequency is proportional to the pres-sure. An oscillator excites the element’s resonant frequency and converts it into ap-propriate pressure units. For further information on pressure sensors, see Refs. 7 to10, 12, and 13.This chapter has been devoted entirely to the computation of pressure distributions andthe resulting forces and moments in a static fluid or a fluid with a known velocity field.All hydrostatic (Secs. 2.3 to 2.8) and rigid-body (Sec. 2.9) problems are solved in thismanner and are classic cases which every student should understand. In arbitrary vis-cous flows, both pressure and velocity are unknowns and are solved together as a sys-tem of equations in the chapters which follow.Summary
    • Seal diaphragmFilling liquidSensing diaphragmCover flangeLow-pressure sideHigh-pressure side(a)Fig. 2.28 Pressure sensors withelectric output: (a) a silicon dia-phragm whose deflection changesthe cavity capacitance (Courtesy ofJohnson-Yokogawa Inc.); (b) a sili-con strain gage which is stressedby applied pressure; (c) a microma-chined silicon element which res-onates at a frequency proportionalto applied pressure. [(b) and (c)are courtesy of Druck, Inc., Fair-field, CT.]101(Druck, Inc., Fairfield, Connecticut)Strain gagesDiffused into integratedsilicon chipWire bondingStitch bondedconnections fromchip to body plugEtched cavityMicromachinedsilicon sensorTemperature sensorOn-chip diode foroptimum temperatureperformance(c)(b)
    • ProblemsMost of the problems herein are fairly straightforward. Moredifficult or open-ended assignments are indicated with an as-terisk, as in Prob. 2.8. Problems labeled with an EES icon (forexample, Prob. 2.62), will benefit from the use of the Engi-neering Equation Solver (EES), while problems labeled with adisk icon may require the use of a computer. The standard end-of-chapter problems 2.1 to 2.158 (categorized in the problemlist below) are followed by word problems W2.1 to W2.8, fun-damentals of engineering exam problems FE2.1 to FE2.10, com-prehensive problems C2.1 to C2.4, and design projects D2.1 andD2.2.Problem DistributionSection Topic Problems2.1, 2.2 Stresses; pressure gradient; gage pressure 2.1–2.62.3 Hydrostatic pressure; barometers 2.7–2.232.3 The atmosphere 2.24–2.292.4 Manometers; multiple fluids 2.30–2.472.5 Forces on plane surfaces 2.48–2.812.6 Forces on curved surfaces 2.82–2.1002.7 Forces in layered fluids 2.101–2.1022.8 Buoyancy; Archimedes’ principles 2.103–2.1262.8 Stability of floating bodies 2.127–2.1362.9 Uniform acceleration 2.137–2.1512.9 Rigid-body rotation 2.152–2.1582.10 Pressure measurements NoneP2.1 For the two-dimensional stress field shown in Fig. P2.1 itis found that␴xx ϭ 3000 lbf/ft2␴yy ϭ 2000 lbf/ft2␴xy ϭ 500 lbf/ft2Find the shear and normal stresses (in lbf/ft2) acting onplane AA cutting through the element at a 30° angle asshown.102 Chapter 2 Pressure Distribution in a FluidP2.2 For the two-dimensional stress field shown in Fig. P2.1suppose that␴xx ϭ 2000 lbf/ft2␴yy ϭ 3000 lbf/ft2␴n(AA) ϭ 2500 lbf/ft2Compute (a) the shear stress ␴xy and (b) the shear stresson plane AA.P2.3 Derive Eq. (2.18) by using the differential element in Fig.2.2 with z “up,’’no fluid motion, and pressure varying onlyin the z direction.P2.4 In a certain two-dimensional fluid flow pattern the linesof constant pressure, or isobars, are defined by the ex-pression P0 Ϫ Bz ϩ Cx2ϭ constant, where B and C areconstants and p0 is the (constant) pressure at the origin,(x, z) ϭ (0, 0). Find an expression x ϭ f(z) for the familyof lines which are everywhere parallel to the local pres-sure gradient Vෆp.P2.5 Atlanta, Georgia, has an average altitude of 1100 ft. On astandard day (Table A.6), pressure gage A in a laboratoryexperiment reads 93 kPa and gage B reads 105 kPa. Ex-press these readings in gage pressure or vacuum pressure(Pa), whichever is appropriate.P2.6 Any pressure reading can be expressed as a length or head,h ϭ p/␳g. What is standard sea-level pressure expressed in(a) ft of ethylene glycol, (b) in Hg, (c) m of water, and (d)mm of methanol? Assume all fluids are at 20°C.P2.7 The deepest known point in the ocean is 11,034 m in theMariana Trench in the Pacific. At this depth the specificweight of seawater is approximately 10,520 N/m3. At thesurface, ␥ Ϸ 10,050 N/m3. Estimate the absolute pressureat this depth, in atm.P2.8 Dry adiabatic lapse rate (DALR) is defined as the nega-tive value of atmospheric temperature gradient, dT/dz,when temperature and pressure vary in an isentropic fash-ion. Assuming air is an ideal gas, DALR ϭ ϪdT/dz whenT ϭ T0(p/p0)a, where exponent a ϭ (k Ϫ 1)/k, k ϭ cp/cv isthe ratio of specific heats, and T0 and p0 are the tempera-ture and pressure at sea level, respectively. (a) Assumingthat hydrostatic conditions exist in the atmosphere, showthat the dry adiabatic lapse rate is constant and is given byDALR ϭ g(kϪ 1)/(kR), where R is the ideal gas constantfor air. (b) Calculate the numerical value of DALR for airin units of °C/km.*P2.9 For a liquid, integrate the hydrostatic relation, Eq. (2.18),by assuming that the isentropic bulk modulus, B ϭ␳(Ѩp/Ѩ␳)s, is constant—see Eq. (9.18). Find an expressionfor p(z) and apply the Mariana Trench data as in Prob. 2.7,using Bseawater from Table A.3.P2.10 A closed tank contains 1.5 m of SAE 30 oil, 1 m of wa-ter, 20 cm of mercury, and an air space on top, all at 20°C.The absolute pressure at the bottom of the tank is 60 kPa.What is the pressure in the air space?30°AAσxxσxyσyxσyyσxxσyyσyxσxy==P2.1
    • P2.16 A closed inverted cone, 100 cm high with diameter 60 cmat the top, is filled with air at 20°C and 1 atm. Water at20°C is introduced at the bottom (the vertex) to compressthe air isothermally until a gage at the top of the cone reads30 kPa (gage). Estimate (a) the amount of water needed(cm3) and (b) the resulting absolute pressure at the bottomof the cone (kPa).P2.11 In Fig. P2.11, pressure gage A reads 1.5 kPa (gage). Thefluids are at 20°C. Determine the elevations z, in meters,of the liquid levels in the open piezometer tubes B and C.Problems 103P2.12 In Fig. P2.12 the tank contains water and immiscible oilat 20°C. What is h in cm if the density of the oil is 898kg/m3?1 m1.5 m2 mz=0GasolineGlycerinAB CAirP2.116 cm12 cmh8 cmOilWaterP2.12P2.13 In Fig. P2.13 the 20°C water and gasoline surfaces areopen to the atmosphere and at the same elevation. What isthe height h of the third liquid in the right leg?P2.14 The closed tank in Fig. P2.14 is at 20°C. If the pressureat point A is 95 kPa absolute, what is the absolute pres-sure at point B in kPa? What percent error do you makeby neglecting the specific weight of the air?P2.15 The air-oil-water system in Fig. P2.15 is at 20°C. Know-ing that gage A reads 15 lbf/in2absolute and gage B reads1.25 lbf/in2less than gage C, compute (a) the specificweight of the oil in lbf/ft3and (b) the actual reading ofgage C in lbf/in2absolute.1.5 m1 mhWaterGasolineLiquid, SG = 1.60P2.13AAirBAir4 m2 m2 m4 mWaterP2.14AirOilWater1 ft1 ft2 ft2 ftABC15 lbf/in2absP2.15
    • P2.19 The U-tube in Fig. P2.19 has a 1-cm ID and contains mer-cury as shown. If 20 cm3of water is poured into the right-hand leg, what will the free-surface height in each leg beafter the sloshing has died down?P2.20 The hydraulic jack in Fig. P2.20 is filled with oil at 56lbf/ft3. Neglecting the weight of the two pistons, what forceF on the handle is required to support the 2000-lbf weightfor this design?P2.21 At 20°C gage A reads 350 kPa absolute. What is the heighth of the water in cm? What should gage B read in kPa ab-solute? See Fig. P2.21.P2.18 The system in Fig. P2.18 is at 20°C. If atmospheric pres-sure is 101.33 kPa and the pressure at the bottom of thetank is 242 kPa, what is the specific gravity of fluid X?P2.17 The system in Fig. P2.17 is at 20°C. If the pressure at pointA is 1900 lbf/ft2, determine the pressures at points B, C,and D in lbf/ft2.104 Chapter 2 Pressure Distribution in a FluidAir AirAirWaterABC2 ftD4 ft3 ft2 ft5 ftP2.170.5 mSAE 30 oilWaterFluid X1 m2 m3 mMercuryP2.18Mercury10 cm10 cm10 cmP2.19Oil3-in diameter1 in 15 in1-in diameterF2000lbfMercury80 cmA Bh?Air: 180 kPa absWaterP2.20P2.21P2.22 The fuel gage for a gasoline tank in a car reads propor-tional to the bottom gage pressure as in Fig. P2.22. If thetank is 30 cm deep and accidentally contains 2 cm of wa-ter plus gasoline, how many centimeters of air remain atthe top when the gage erroneously reads “full’’?P2.23 In Fig. P2.23 both fluids are at 20°C. If surface tension ef-fects are negligible, what is the density of the oil, in kg/m3?P2.24 In Prob. 1.2 we made a crude integration of the densitydistribution ␳(z) in Table A.6 and estimated the mass ofthe earth’s atmosphere to be m Ϸ 6 E18 kg. Can this re-
    • mental observations. (b) Find an expression for the pres-sure at points 1 and 2 in Fig. P2.27b. Note that the glassis now inverted, so the original top rim of the glass is atthe bottom of the picture, and the original bottom of theglass is at the top of the picture. The weight of the cardcan be neglected.Problems 105sult be used to estimate sea-level pressure on the earth?Conversely, can the actual sea-level pressure of 101.35 kPabe used to make a more accurate estimate of the atmos-pheric mass?P2.25 Venus has a mass of 4.90 E24 kg and a radius of 6050 km.Its atmosphere is 96 percent CO2, but let us assume it tobe 100 percent. Its surface temperature averages 730 K,decreasing to 250 K at an altitude of 70 km. The averagesurface pressure is 9.1 MPa. Estimate the atmosphericpressure of Venus at an altitude of 5 km.P2.26 Investigate the effect of doubling the lapse rate on atmos-pheric pressure. Compare the standard atmosphere (TableA.6) with a lapse rate twice as high, B2 ϭ 0.0130 K/m.Find the altitude at which the pressure deviation is (a) 1percent and (b) 5 percent. What do you conclude?P2.27 Conduct an experiment to illustrate atmospheric pressure.Note: Do this over a sink or you may get wet! Find a drink-ing glass with a very smooth, uniform rim at the top. Fillthe glass nearly full with water. Place a smooth, light, flatplate on top of the glass such that the entire rim of theglass is covered. A glossy postcard works best. A small in-dex card or one flap of a greeting card will also work. SeeFig. P2.27a.(a) Hold the card against the rim of the glass and turn theglass upside down. Slowly release pressure on the card.Does the water fall out of the glass? Record your experi-GasolineSG = 0.6830 cmWaterh?2 cmAirpgageVentP2.228 cm6 cmWaterOil10 cmP2.23Card Top of glassBottom of glassCard Original top of glassOriginal bottom of glass1 G2 GP2.27aP2.27b(c) Estimate the theoretical maximum glass height suchthat this experiment could still work, i.e., such that the wa-ter would not fall out of the glass.P2.28 Earth’s atmospheric conditions vary somewhat. On a cer-tain day the sea-level temperature is 45°F and the sea-levelpressure is 28.9 inHg. An airplane overhead registers anair temperature of 23°F and a pressure of 12 lbf/in2. Esti-mate the plane’s altitude, in feet.P2.29 Under some conditions the atmosphere is adiabatic, p Ϸ(const)(␳k), where k is the specific heat ratio. Show that,for an adiabatic atmosphere, the pressure variation isgiven byp ϭ p0΄1 Ϫ΅k/(kϪ1)Compare this formula for air at z ϭ 5000 m with the stan-dard atmosphere in Table A.6.P2.30 In Fig. P2.30 fluid 1 is oil (SG ϭ 0.87) and fluid 2 is glyc-erin at 20°C. If pa ϭ 98 kPa, determine the absolute pres-sure at point A.(k Ϫ 1)gzᎏᎏkRT0*
    • P2.34 Sometimes manometer dimensions have a significant ef-fect. In Fig. P2.34 containers (a) and (b) are cylindrical andconditions are such that pa ϭ pb. Derive a formula for thepressure difference pa Ϫ pb when the oil-water interface onthe right rises a distance ⌬h Ͻ h, for (a) d Ӷ D and (b) d ϭ0.15D. What is the percent change in the value of ⌬p?P2.31 In Fig. P2.31 all fluids are at 20°C. Determine the pres-sure difference (Pa) between points A and B.106 Chapter 2 Pressure Distribution in a FluidP2.33 In Fig. P2.33 the pressure at point A is 25 lbf/in2. All flu-ids are at 20°C. What is the air pressure in the closed cham-ber B, in Pa?P2.30P2.31P2.32P2.33P2.34ρ1ρ2paA10 cm32 cmP2.35 Water flows upward in a pipe slanted at 30°, as in Fig.P2.35. The mercury manometer reads h ϭ 12 cm. Both flu-ids are at 20°C. What is the pressure difference p1 Ϫ p2 inthe pipe?P2.36 In Fig. P2.36 both the tank and the tube are open to theatmosphere. If L ϭ 2.13 m, what is the angle of tilt ␪ ofthe tube?P2.37 The inclined manometer in Fig. P2.37 contains Meriamred manometer oil, SG ϭ 0.827. Assume that the reservoir20 cm40 cm8 cm9 cm14 cmABKerosineAirWaterMercuryBenzene18 cmH35 cmMercuryWaterMeriamred oil,SG = 0.827ABA4 cm3 cm6 cm8 cm5 cm3 cmAirLiquid, SG = 1.45BWaterSAE 30 oil(a)(b)dLhD DWaterSAE 30 oilHP2.32 For the inverted manometer of Fig. P2.32, all fluids are at20°C. If pB Ϫ pA ϭ 97 kPa, what must the height H bein cm?*
    • with manometer fluid ␳m. One side of the manometer is opento the air, while the other is connected to new tubing whichextends to pressure measurement location 1, some height Hhigher in elevation than the surface of the manometer liquid.For consistency, let ␳a be the density of the air in the room,␳t be the density of the gas inside the tube, ␳m be the den-sity of the manometer liquid, and h be the height differencebetween the two sides of the manometer. See Fig. P2.38.(a) Find an expression for the gage pressure at the mea-surement point. Note: When calculating gage pressure, usethe local atmospheric pressure at the elevation of the mea-surement point. You may assume that h Ӷ H; i.e., assumethe gas in the entire left side of the manometer is of den-sity ␳t. (b) Write an expression for the error caused by as-suming that the gas inside the tubing has the same densityas that of the surrounding air. (c) How much error (in Pa)is caused by ignoring this density difference for the fol-lowing conditions: ␳m ϭ 860 kg/m3, ␳a ϭ 1.20 kg/m3,␳t ϭ 1.50 kg/m3, H ϭ 1.32 m, and h ϭ 0.58 cm? (d) Canyou think of a simple way to avoid this error?is very large. If the inclined arm is fitted with graduations1 in apart, what should the angle ␪ be if each graduationcorresponds to 1 lbf/ft2gage pressure for pA?Problems 107P2.38 An interesting article appeared in the AIAA Journal (vol. 30,no. 1, January 1992, pp. 279–280). The authors explain thatthe air inside fresh plastic tubing can be up to 25 percentmore dense than that of the surroundings, due to outgassingor other contaminants introduced at the time of manufacture.Most researchers, however, assume that the tubing is filledwith room air at standard air density, which can lead to sig-nificant errors when using this kind of tubing to measurepressures. To illustrate this, consider a U-tube manometerh(1)(2)30Њ2 mP2.35P2.39 An 8-cm-diameter piston compresses manometer oil intoan inclined 7-mm-diameter tube, as shown in Fig. P2.39.When a weight W is added to the top of the piston, the oilrises an additional distance of 10 cm up the tube, as shown.How large is the weight, in N?P2.40 A pump slowly introduces mercury into the bottom of theclosed tank in Fig. P2.40. At the instant shown, the airpressure pB ϭ 80 kPa. The pump stops when the air pres-sure rises to 110 kPa. All fluids remain at 20°C. What willbe the manometer reading h at that time, in cm, if it is con-nected to standard sea-level ambient air patm?50 cm50 cmOilSG = 0.8WaterSG = 1.0L␪P2.361 inReservoirθ D =516inpAP2.37hH1U-tubemanometer ␳m␳t(tubing gas)␳a(air)paat location 1p1P2.38
    • P2.44 Water flows downward in a pipe at 45°, as shown in Fig.P2.44. The pressure drop p1 Ϫ p2 is partly due to gravityand partly due to friction. The mercury manometer readsa 6-in height difference. What is the total pressure dropp1 Ϫ p2 in lbf/in2? What is the pressure drop due to fric-tion only between 1 and 2 in lbf/in2? Does the manome-ter reading correspond only to friction drop? Why?108 Chapter 2 Pressure Distribution in a FluidP2.41 The system in Fig. P2.41 is at 20°C. Compute the pres-sure at point A in lbf/ft2absolute.D = 8 cmd = 7 mmMeriam redoil, SG = 0.82710 cm15˚PistonW8 cm9 cmAir: pBWaterMercuryPumppatmh2 cmHg10 cmP2.39P2.40WaterWater5 in10 in6 inMercuryAOil, SG = 0.85pa= 14.7 lbf/in2P2.41h1pA1pB1h2h1ρ ρρP2.425 ftFlow1245˚6 inMercuryWaterP2.44P2.42 Very small pressure differences pA Ϫ pB can be measuredaccurately by the two-fluid differential manometer in Fig.P2.42. Density ␳2 is only slightly larger than that of theupper fluid ␳1. Derive an expression for the proportional-ity between h and pA Ϫ pB if the reservoirs are very large.*P2.43 A mercury manometer, similar to Fig. P2.35, records h Ϸ1.2, 4.9, and 11.0 mm when the water velocities in the pipeare V ϭ 1.0, 2.0, and 3.0 m/s, respectively. Determine ifthese data can be correlated in the form p1 Ϫ p2 Ϸ Cf␳V2,where Cf is dimensionless.P2.45 In Fig. P2.45, determine the gage pressure at point A inPa. Is it higher or lower than atmospheric?P2.46 In Fig. P2.46 both ends of the manometer are open to theatmosphere. Estimate the specific gravity of fluid X.P2.47 The cylindrical tank in Fig. P2.47 is being filled with wa-ter at 20°C by a pump developing an exit pressure of 175kPa. At the instant shown, the air pressure is 110 kPa andH ϭ 35 cm. The pump stops when it can no longer raisethe water pressure. For isothermal air compression, esti-mate H at that time.P2.48 Conduct the following experiment to illustrate air pres-sure. Find a thin wooden ruler (approximately 1 ft inEES
    • Problems 10945 cm30 cm15 cm40 cmpatmAirOil,SG = 0.85Water MercuryAa karate chop on the portion of the ruler sticking out overthe edge of the desk. Record your results. (c) Explainyour results.P2.49 A water tank has a circular panel in its vertical wall. Thepanel has a radius of 50 cm, and its center is 2 m belowthe surface. Neglecting atmospheric pressure, determinethe water force on the panel and its line of action.P2.50 A vat filled with oil (SG ϭ 0.85) is 7 m long and 3 m deepand has a trapezoidal cross section 2 m wide at the bot-tom and 4 m wide at the top. Compute (a) the weight ofoil in the vat, (b) the force on the vat bottom, and (c) theforce on the trapezoidal end panel.P2.51 Gate AB in Fig. P2.51 is 1.2 m long and 0.8 m into thepaper. Neglecting atmospheric pressure, compute the forceF on the gate and its center-of-pressure position X.*P2.52 Suppose that the tank in Fig. P2.51 is filled with liquid X,not oil. Gate AB is 0.8 m wide into the paper. Suppose thatliquid X causes a force F on gate AB and that the momentof this force about point B is 26,500 N и m. What is thespecific gravity of liquid X?P2.457 cm4 cm6 cm9 cm5 cm12 cmSAE 30 oilWaterFluid X10 cm75 cmH50 cmAir20˚ CWaterPumpNewspaperRulerDeskP2.46P2.48P2.47length) or a thin wooden paint stirrer. Place it on the edgeof a desk or table with a little less than half of it hang-ing over the edge lengthwise. Get two full-size sheets ofnewspaper; open them up and place them on top of theruler, covering only the portion of the ruler resting on thedesk as illustrated in Fig. P2.48. (a) Estimate the totalforce on top of the newspaper due to air pressure in theroom. (b) Careful! To avoid potential injury, make surenobody is standing directly in front of the desk. Perform
    • P2.53 Panel ABC in the slanted side of a water tank is an isosce-les triangle with the vertex at A and the base BC ϭ 2 m,as in Fig. P2.53. Find the water force on the panel and itsline of action.110 Chapter 2 Pressure Distribution in a Fluid8 m6 m1.2 mF40°X1 mAB4 mOil,SG = 0.82P2.51P2.54 If, instead of water, the tank in Fig. P2.53 is filled with liq-uid X, the liquid force on panel ABC is found to be 115 kN.What is the density of liquid X? The line of action is foundto be the same as in Prob. 2.53. Why?P2.55 Gate AB in Fig. P2.55 is 5 ft wide into the paper, hingedat A, and restrained by a stop at B. The water is at 20°C.Compute (a) the force on stop B and (b) the reactions atA if the water depth h ϭ 9.5 ft.P2.56 In Fig. P2.55, gate AB is 5 ft wide into the paper, and stopB will break if the water force on it equals 9200 lbf. Forwhat water depth h is this condition reached?P2.57 In Fig. P2.55, gate AB is 5 ft wide into the paper. Supposethat the fluid is liquid X, not water. Hinge A breaks whenits reaction is 7800 lbf, and the liquid depth is h ϭ 13 ft.What is the specific gravity of liquid X?P2.58 In Fig. P2.58, the cover gate AB closes a circular opening80 cm in diameter. The gate is held closed by a 200-kgmass as shown. Assume standard gravity at 20°C. At whatwater level h will the gate be dislodged? Neglect the weightof the gate.WaterB, C 3 m4 mApaWaterpa4 ftBAhP2.53P2.55Water30 cm3 mm200 kghB AP2.58P2.59␪BhHingeAPL*P2.59 Gate AB has length L, width b into the paper, is hinged atB, and has negligible weight. The liquid level h remainsat the top of the gate for any angle ␪. Find an analytic ex-pression for the force P, perpendicular to AB, required tokeep the gate in equilibrium in Fig. P2.59.*P2.60 Find the net hydrostatic force per unit width on the rec-tangular gate AB in Fig. P2.60 and its line of action.*P2.61 Gate AB in Fig. P2.61 is a homogeneous mass of 180 kg,1.2 m wide into the paper, hinged at A, and resting on asmooth bottom at B. All fluids are at 20°C. For what wa-ter depth h will the force at point B be zero?
    • P2.63 The tank in Fig. P2.63 has a 4-cm-diameter plug at thebottom on the right. All fluids are at 20°C. The plug willpop out if the hydrostatic force on it is 25 N. For this con-dition, what will be the reading h on the mercury manome-ter on the left side?P2.62 Gate AB in Fig. P2.62 is 15 ft long and 8 ft wide into thepaper and is hinged at B with a stop at A. The water is at20°C. The gate is 1-in-thick steel, SG ϭ 7.85. Computethe water level h for which the gate will start to fall.Problems 111WaterGlycerinAB1.8 m1.2 m2 m2 mP2.60*P2.64 Gate ABC in Fig. P2.64 has a fixed hinge line at B and is2 m wide into the paper. The gate will open at A to releasewater if the water depth is high enough. Compute the depthh for which the gate will begin to open.*P2.65 Gate AB in Fig. P2.65 is semicircular, hinged at B, andheld by a horizontal force P at A. What force P is requiredfor equilibrium?P2.66 Dam ABC in Fig. P2.66 is 30 m wide into the paper andmade of concrete (SG ϭ 2.4). Find the hydrostatic forceon surface AB and its moment about C. Assuming no seep-age of water under the dam, could this force tip the damover? How does your argument change if there is seepageunder the dam?WaterBAGlycerin1 mh2 m60°P2.61WaterhB15 ft60˚10,000 lbPulleyAP2.62h50°2 cmHWaterPlug,D = 4 cmMercuryP2.63EESWater at 20°CA 20 cm BC1mhP2.64
    • **P2.67 Generalize Prob. 2.66 as follows. Denote length AB as H,length BC as L, and angle ABC as ␪. Let the dam mater-ial have specific gravity SG. The width of the dam is b.Assume no seepage of water under the dam. Find an an-alytic relation between SG and the critical angle ␪c forwhich the dam will just tip over to the right. Use your re-lation to compute ␪c for the special case SG ϭ 2.4 (con-crete).P2.68 Isosceles triangle gate AB in Fig. P2.68 is hinged at A andweighs 1500 N. What horizontal force P is required at pointB for equilibrium?**P2.69 The water tank in Fig. P2.69 is pressurized, as shown bythe mercury-manometer reading. Determine the hydrosta-tic force per unit depth on gate AB.P2.70 Calculate the force and center of pressure on one side ofthe vertical triangular panel ABC in Fig. P2.70. Neglectpatm.**P2.71 In Fig. P2.71 gate AB is 3 m wide into the paper and isconnected by a rod and pulley to a concrete sphere (SG ϭ112 Chapter 2 Pressure Distribution in a FluidPAB5 mWater3 m Gate:Side view60 mC;;;;ABWater 20˚C80 mDamP2.65P2.66AP3 mGate50˚ B1 mOil, SG = 0.832 mP2.681 m5 m2 mWater,20˚CHg, 20˚CAB80 cmP2.69CWater4 ftBA2 ft6 ftP2.70
    • ***P2.74 In “soft’’ liquids (low bulk modulus ␤), it may be neces-sary to account for liquid compressibility in hydrostaticcalculations. An approximate density relation would bedp Ϸ ᎏ␤␳ᎏ d␳ ϭ a2d␳ or p Ϸ p0 ϩ a2(␳ Ϫ ␳0)where a is the speed of sound and (p0, ␳0) are the condi-tions at the liquid surface z ϭ 0. Use this approximationto show that the density variation with depth in a soft liq-uid is ␳ ϭ ␳0eϪgz/a2where g is the acceleration of gravityand z is positive upward. Then consider a vertical wall ofwidth b, extending from the surface (z ϭ 0) down to depthz ϭ Ϫ h. Find an analytic expression for the hydrostaticforce F on this wall, and compare it with the incompress-ible result F ϭ ␳0gh2b/2. Would the center of pressure bebelow the incompressible position z ϭ Ϫ 2h/3?*P2.75 Gate AB in Fig. P2.75 is hinged at A, has width b into thepaper, and makes smooth contact at B. The gate has den-sity ␳s and uniform thickness t. For what gate density ␳s,expressed as a function of (h, t, ␳, ␪), will the gate just be-gin to lift off the bottom? Why is your answer indepen-dent of gate length L and width b?2.40). What diameter of the sphere is just sufficient to keepthe gate closed?Problems 113P2.72 The V-shaped container in Fig. P2.72 is hinged at A andheld together by cable BC at the top. If cable spacing is1 m into the paper, what is the cable tension?P2.73 Gate AB is 5 ft wide into the paper and opens to let freshwater out when the ocean tide is dropping. The hinge at Ais 2 ft above the freshwater level. At what ocean level hwill the gate first open? Neglect the gate weight.A4 mBConcretesphere, SG = 2.4;;8 m6 mWaterCableA3 m1 mBC110˚WaterP2.71P2.72**P2.76 Consider the angled gate ABC in Fig. P2.76, hinged at Cand of width b into the paper. Derive an analytic formulafor the horizontal force P required at the top for equilib-rium, as a function of the angle ␪.P2.77 The circular gate ABC in Fig. P2.77 has a 1-m radius andis hinged at B. Compute the force P just sufficient to keepthe gate from opening when h ϭ 8 m. Neglect atmosphericpressure.P2.78 Repeat Prob. 2.77 to derive an analytic expression for Pas a function of h. Is there anything unusual about yoursolution?P2.79 Gate ABC in Fig. P2.79 is 1 m square and is hinged at B.It will open automatically when the water level h becomeshigh enough. Determine the lowest height for which theABhStop10 ftTiderangeSeawater, SG = 1.025P2.73ALth␪␳BP2.75
    • 114 Chapter 2 Pressure Distribution in a Fluidgate will open. Neglect atmospheric pressure. Is this resultindependent of the liquid density?P2.80 For the closed tank in Fig. P2.80, all fluids are at 20°C, andthe airspace is pressurized. It is found that the net outwardhydrostatic force on the 30-by 40-cm panel at the bottom ofthe water layer is 8450 N. Estimate (a) the pressure in theairspace and (b) the reading h on the mercury manometer.WaterABC1mh1mPpapaP2.77P2.81 Gate AB in Fig. P2.81 is 7 ft into the paper and weighs3000 lbf when submerged. It is hinged at B and restsagainst a smooth wall at A. Determine the water level h atthe left which will just cause the gate to open.WaterABCh60 cm40 cmP2.79P2.82 The dam in Fig. P2.82 is a quarter circle 50 m wide intothe paper. Determine the horizontal and vertical compo-nents of the hydrostatic force against the dam and the pointCP where the resultant strikes the dam.P2.83 Gate AB in Fig. P2.83 is a quarter circle 10 ft wide intothe paper and hinged at B. Find the force F just sufficientto keep the gate from opening. The gate is uniform andweighs 3000 lbf.P2.84 Determine (a) the total hydrostatic force on the curved sur-face AB in Fig. P2.84 and (b) its line of action. Neglect at-mospheric pressure, and let the surface have unit width.WaterWater4 ft6 ft8 ftAhBAirSAE 30 oilWater60 cm20 cm80 cmPanel, 30 cm high, 40 cm wideMercury2 m1 atmhP2.80P2.81h;;;;;;Specific weight γθθPBACP2.76**
    • Problems 115P2.85 Compute the horizontal and vertical components of the hy-drostatic force on the quarter-circle panel at the bottom ofthe water tank in Fig. P2.85.Water20 m;;;;;;;;;20 mCPpa= 0P2.82WaterABFr = 8 ftABWater at 20° C1 mxzz = x3P2.87 The bottle of champagne (SG ϭ 0.96) in Fig. P2.87 is un-der pressure, as shown by the mercury-manometer read-ing. Compute the net force on the 2-in-radius hemispher-ical end cap at the bottom of the bottle.P2.88 Gate ABC is a circular arc, sometimes called a Tainter gate,which can be raised and lowered by pivoting about pointO. See Fig. P2.88. For the position shown, determine (a)the hydrostatic force of the water on the gate and (b) itsline of action. Does the force pass through point O?P2.83P2.84Water6 m2 m5 m2 mP2.85Water10 ft2 ftP2.86P2.86 Compute the horizontal and vertical components of the hy-drostatic force on the hemispherical bulge at the bottomof the tank in Fig. P2.86.4 in2 in6 inMercuryr = 2 inP2.87WaterBCOR = 6 m6 m6 mAP2.88*
    • P2.91 The hemispherical dome in Fig. P2.91 weighs 30 kN andis filled with water and attached to the floor by six equallyspaced bolts. What is the force in each bolt required tohold down the dome?P2.92 A 4-m-diameter water tank consists of two half cylinders,each weighing 4.5 kN/m, bolted together as shown in Fig.P2.92. If the support of the end caps is neglected, deter-mine the force induced in each bolt.*P2.93 In Fig. P2.93, a one-quadrant spherical shell of radius Ris submerged in liquid of specific gravity ␥ and depthh Ͼ R. Find an analytic expression for the resultant hydro-static force, and its line of action, on the shell surface.P2.89 The tank in Fig. P2.89 contains benzene and is pressur-ized to 200 kPa (gage) in the air gap. Determine the ver-tical hydrostatic force on circular-arc section AB and itsline of action.116 Chapter 2 Pressure Distribution in a FluidP2.90 A 1-ft-diameter hole in the bottom of the tank in Fig. P2.90is closed by a conical 45° plug. Neglecting the weight ofthe plug, compute the force F required to keep the plug inthe hole.60 cm60 cmp = 200 kPaBenzeneat 20ЊCAB30 cmP2.89Air:Water45˚coneF1 ftp = 3 lbf/in2gage3 ft1 ftP2.90P2.94 The 4-ft-diameter log (SG ϭ 0.80) in Fig. P2.94 is 8 ftlong into the paper and dams water as shown. Computethe net vertical and horizontal reactions at point C.Water4 m3cmSixbolts2 mP2.912 m2 mWaterBolt spacing 25 cmP2.92z, γRRxzRhρP2.93
    • wall at A. Compute the reaction forces at points Aand B.***P2.95 The uniform body A in Fig. P2.95 has width b into the pa-per and is in static equilibrium when pivoted about hingeO. What is the specific gravity of this body if (a) h ϭ 0and (b) h ϭ R?Problems 117P2.96 The tank in Fig. P2.96 is 3 m wide into the paper. Ne-glecting atmospheric pressure, compute the hydrostatic (a)horizontal force, (b) vertical force, and (c) resultant forceon quarter-circle panel BC.2ftLog2ftWaterCWaterP2.94P2.98 Gate ABC in Fig. P2.98 is a quarter circle 8 ft wide intothe paper. Compute the horizontal and vertical hydrostaticforces on the gate and the line of action of the resultantforce.ARRhOWaterP2.95P2.99 A 2-ft-diameter sphere weighing 400 lbf closes a 1-ft-di-ameter hole in the bottom of the tank in Fig. P2.99. Com-pute the force F required to dislodge the sphere from thehole.4 m4 m6 mWaterCBAP2.96P2.97 Gate AB in Fig. P2.97 is a three-eighths circle, 3 m wideinto the paper, hinged at B, and resting against a smooth4 mSeawater, 10,050 N/m32 m45°ABP2.9745°45°r = 4 ftBCWaterAP2.98Water3 ft1 ftF1 ftP2.99
    • whether his new crown was pure gold (SG ϭ 19.3).Archimedes measured the weight of the crown in air to be11.8 N and its weight in water to be 10.9 N. Was it puregold?P2.106 It is found that a 10-cm cube of aluminum (SG ϭ 2.71)will remain neutral under water (neither rise nor fall) if itis tied by a string to a submerged 18-cm-diameter sphereof buoyant foam. What is the specific weight of the foam,in N/m3?P2.107 Repeat Prob. 2.62, assuming that the 10,000-lbf weight isaluminum (SG ϭ 2.71) and is hanging submerged in thewater.P2.108 A piece of yellow pine wood (SG ϭ 0.65) is 5 cm squareand 2.2 m long. How many newtons of lead (SG ϭ 11.4)should be attached to one end of the wood so that it willfloat vertically with 30 cm out of the water?P2.109 A hydrometer floats at a level which is a measure of thespecific gravity of the liquid. The stem is of constant di-ameter D, and a weight in the bottom stabilizes the bodyto float vertically, as shown in Fig. P2.109. If the positionh ϭ 0 is pure water (SG ϭ 1.0), derive a formula for h asa function of total weight W, D, SG, and the specific weight␥0 of water.P2.100 Pressurized water fills the tank in Fig. P2.100. Computethe net hydrostatic force on the conical surface ABC.118 Chapter 2 Pressure Distribution in a FluidP2.101 A fuel truck has a tank cross section which is approxi-mately elliptical, with a 3-m horizontal major axis and a2-m vertical minor axis. The top is vented to the atmos-phere. If the tank is filled half with water and half withgasoline, what is the hydrostatic force on the flat ellipti-cal end panel?P2.102 In Fig. P2.80 suppose that the manometer reading is h ϭ25 cm. What will be the net hydrostatic force on the com-plete end wall, which is 160 cm high and 2 m wide?P2.103 The hydrogen bubbles in Fig. 1.13 are very small, lessthan a millimeter in diameter, and rise slowly. Their dragin still fluid is approximated by the first term of Stokes’expression in Prob. 1.10: F ϭ 3␲␮VD, where V is the risevelocity. Neglecting bubble weight and setting bubblebuoyancy equal to drag, (a) derive a formula for the ter-minal (zero acceleration) rise velocity Vterm of the bubbleand (b) determine Vterm in m/s for water at 20°C if D ϭ30 ␮m.P2.104 The can in Fig. P2.104 floats in the position shown. Whatis its weight in N?P2.105 It is said that Archimedes discovered the buoyancy lawswhen asked by King Hiero of Syracuse to determine2 mA CB150 kPagage4 m7 mWaterP2.100Water3 cm8 cmD = 9 cmP2.104P2.110 An average table tennis ball has a diameter of 3.81 cm anda mass of 2.6 g. Estimate the (small) depth at which thisball will float in water at 20°C and sea level standard airif air buoyancy is (a) neglected and (b) included.P2.111 A hot-air balloon must be designed to support basket, cords,and one person for a total weight of 1300 N. The balloonmaterial has a mass of 60 g/m2. Ambient air is at 25°C and1 atm. The hot air inside the balloon is at 70°C and 1 atm.What diameter spherical balloon will just support the totalweight? Neglect the size of the hot-air inlet vent.P2.112 The uniform 5-m-long round wooden rod in Fig. P2.112is tied to the bottom by a string. Determine (a) the tensionhFluid, SG > 1WDSG = 1.0P2.109
    • P2.116 The homogeneous 12-cm cube in Fig. 2.116 is balancedby a 2-kg mass on the beam scale when the cube is im-mersed in 20°C ethanol. What is the specific gravity of thecube?in the string and (b) the specific gravity of the wood. Is itpossible for the given information to determine the incli-nation angle ␪? Explain.Problems 119Water at 20°CString1 m4 mθD = 8 cmP2.112P2.113 A spar buoy is a buoyant rod weighted to float and protrudevertically, as in Fig. P2.113. It can be used for measurementsor markers. Suppose that the buoy is maple wood (SG ϭ0.6), 2 in by 2 in by 12 ft, floating in seawater (SG ϭ 1.025).How many pounds of steel (SG ϭ 7.85) should be added tothe bottom end so that h ϭ 18 in?WsteelhP2.1138 mHinge␪ = 30Њ2 kg of leadD = 4 cmBP2.114P2.117 The balloon in Fig. P2.117 is filled with helium and pres-surized to 135 kPa and 20°C. The balloon material has aP2.114 The uniform rod in Fig. P2.114 is hinged at point B on thewaterline and is in static equilibrium as shown when 2 kgof lead (SG ϭ 11.4) are attached to its end. What is thespecific gravity of the rod material? What is peculiar aboutthe rest angle ␪ ϭ 30?P2.115 The 2-in by 2-in by 12-ft spar buoy from Fig. P2.113 has 5lbm of steel attached and has gone aground on a rock, as inFig. P2.115. Compute the angle ␪ at which the buoy willlean, assuming that the rock exerts no moments on the spar.RockSeawaterWoodAB8 ft θ SG = 0.6P2.11512 cm2 kgP2.116
    • P2.120 A uniform wooden beam (SG ϭ 0.65) is 10 cm by 10 cmby 3 m and is hinged at A, as in Fig. P2.120. At what an-gle ␪ will the beam float in the 20°C water?mass of 85 g/m2. Estimate (a) the tension in the mooringline and (b) the height in the standard atmosphere to whichthe balloon will rise if the mooring line is cut.120 Chapter 2 Pressure Distribution in a FluidP2.118 A 14-in-diameter hollow sphere is made of steel (SG ϭ7.85) with 0.16-in wall thickness. How high will thissphere float in 20°C water? How much weight must beadded inside to make the sphere neutrally buoyant?P2.119 When a 5-lbf weight is placed on the end of the uniformfloating wooden beam in Fig. P2.119, the beam tilts at anangle ␪ with its upper right corner at the surface, as shown.Determine (a) the angle ␪ and (b) the specific gravity ofthe wood. (Hint: Both the vertical forces and the momentsabout the beam centroid must be balanced.)Air:100 kPa at20°CD = 10 mP2.117Water 9 ft5 lbf4 in × 4 inθP2.119P2.121 The uniform beam in Fig. P2.121, of size L by h by b andwith specific weight ␥b, floats exactly on its diagonal whena heavy uniform sphere is tied to the left corner, as shown.Show that this can only happen (a) when ␥b ϭ ␥/3 and (b)when the sphere has sizeD ϭ΄ ΅1/3Lhbᎏᎏ␲(SG Ϫ 1)θWater1 mAP2.120P2.122 A uniform block of steel (SG ϭ 7.85) will “float’’ at amercury-water interface as in Fig. P2.122. What is theratio of the distances a and b for this condition?;LDiameter DγWidth b << LγbSG > 1h << LP2.121SteelblockMercury: SG = 13.56abWaterP2.122EESP2.123 In an estuary where fresh water meets and mixes with sea-water, there often occurs a stratified salinity condition withfresh water on top and salt water on the bottom, as in Fig.P2.123. The interface is called a halocline. An idealizationof this would be constant density on each side of the halo-cline as shown. A 35-cm-diameter sphere weighing 50 lbfwould “float’’ near such a halocline. Compute the sphereposition for the idealization in Fig. P2.123.P2.124 A balloon weighing 3.5 lbf is 6 ft in diameter. It is filledwith hydrogen at 18 lbf/in2absolute and 60°F and is re-leased. At what altitude in the U.S. standard atmospherewill this balloon be neutrally buoyant?
    • Fig. P2.128 suppose that the height is L and the depth intothe paper is L, but the width in the plane of the paper isH Ͻ L. Assuming S ϭ 0.88 for the iceberg, find the ratioH/L for which it becomes neutrally stable, i.e., about tooverturn.P2.130 Consider a wooden cylinder (SG ϭ 0.6) 1 m in diameterand 0.8 m long. Would this cylinder be stable if placed tofloat with its axis vertical in oil (SG ϭ 0.8)?P2.131 A barge is 15 ft wide and 40 ft long and floats with a draftof 4 ft. It is piled so high with gravel that its center of grav-ity is 2 ft above the waterline. Is it stable?P2.132 A solid right circular cone has SG ϭ 0.99 and floats ver-tically as in Fig. P2.132. Is this a stable position for thecone?P2.125 Suppose that the balloon in Prob. 2.111 is constructed tohave a diameter of 14 m, is filled at sea level with hot airat 70°C and 1 atm, and is released. If the air inside the bal-loon remains constant and the heater maintains it at 70°C,at what altitude in the U.S. standard atmosphere will thisballoon be neutrally buoyant?*P2.126 A cylindrical can of weight W, radius R, and height H isopen at one end. With its open end down, and while filledwith atmospheric air (patm, Tatm), the can is eased downvertically into liquid, of density ␳, which enters and com-presses the air isothermally. Derive a formula for the heighth to which the liquid rises when the can is submerged withits top (closed) end a distance d from the surface.P2.127 Consider the 2-in by 2-in by 10-ft spar buoy of Prob. 2.113.How many pounds of steel (SG ϭ 7.85) should be addedat the bottom to ensure vertical floating with a metacen-tric height MෆGෆ of (a) zero (neutral stability) or (b) 1 ft(reasonably stable)?P2.128 An iceberg can be idealized as a cube of side length L, asin Fig. P2.128. If seawater is denoted by S ϭ 1.0, thenglacier ice (which forms icebergs) has S ϭ 0.88. Deter-mine if this “cubic’’iceberg is stable for the position shownin Fig. P2.128.Problems 121Salinity IdealizationHaloclineSG = 1.02535°/°°SG = 1.00P2.123P2.129 The iceberg idealization in Prob. 2.128 may become un-stable if its sides melt and its height exceeds its width. InWaterS = 1.0M?GBSpecific gravityL= ShP2.133 Consider a uniform right circular cone of specific gravityS Ͻ 1, floating with its vertex down in water (S ϭ 1). Thebase radius is R and the cone height is H. Calculate andplot the stability MMෆGෆ of this cone, in dimensionless form,versus H/R for a range of S Ͻ 1.P2.134 When floating in water (SG ϭ 1.0), an equilateral trian-gular body (SG ϭ 0.9) might take one of the two positionsshown in Fig. P2.134. Which is the more stable position?Assume large width into the paper.P2.128*Water :SG = 1.0SG = 0.99P2.132(a) (b)P2.134P2.135 Consider a homogeneous right circular cylinder of lengthL, radius R, and specific gravity SG, floating in water(SG ϭ 1). Show that the body will be stable with its axisvertical ifϾ [2SG(1 Ϫ SG)]1/2RᎏL
    • P2.136 Consider a homogeneous right circular cylinder of lengthL, radius R, and specific gravity SG ϭ 0.5, floating in wa-ter (SG ϭ 1). Show that the body will be stable with itsaxis horizontal if L/R Ͼ 2.0.P2.137 A tank of water 4 m deep receives a constant upward ac-celeration az. Determine (a) the gage pressure at the tankbottom if az ϭ 5 m2/s and (b) the value of az which causesthe gage pressure at the tank bottom to be 1 atm.P2.138 A 12-fl-oz glass, of 3-in diameter, partly full of water, isattached to the edge of an 8-ft-diameter merry-go-roundwhich is rotated at 12 r/min. How full can the glass be be-fore water spills? (Hint: Assume that the glass is muchsmaller than the radius of the merry-go-round.)P2.139 The tank of liquid in Fig. P2.139 accelerates to the rightwith the fluid in rigid-body motion. (a) Compute ax inm/s2. (b) Why doesn’t the solution to part (a) depend uponthe density of the fluid? (c) Determine the gage pressureat point A if the fluid is glycerin at 20°C.122 Chapter 2 Pressure Distribution in a FluidFig. P2.139P2.141P2.140 Suppose that the elliptical-end fuel tank in Prob. 2.101 is10 m long and filled completely with fuel oil (␳ ϭ 890kg/m3). Let the tank be pulled along a horizontal road. Forrigid-body motion, find the acceleration, and its direction,for which (a) a constant-pressure surface extends from thetop of the front end wall to the bottom of the back end and(b) the top of the back end is at a pressure 0.5 atm lowerthan the top of the front end.P2.141 The same tank from Prob. 2.139 is now moving with con-stant acceleration up a 30° inclined plane, as in Fig.P2.141. Assuming rigid-body motion, compute (a) thevalue of the acceleration a, (b) whether the acceleration isup or down, and (c) the gage pressure at point A if the fluidis mercury at 20°C.P2.142 The tank of water in Fig. P2.142 is 12 cm wide into thepaper. If the tank is accelerated to the right in rigid-bodymotion at 6.0 m/s2, compute (a) the water depth on sideAB and (b) the water-pressure force on panel AB. Assumeno spilling.P2.143 The tank of water in Fig. P2.143 is full and open to the at-mosphere at point A. For what acceleration ax in ft/s2will thepressure at point B be (a) atmospheric and (b) zero absolute?28 cm100 cm15 cmVAa?30°zxA28 cm100 cmax15 cmP2.144 Consider a hollow cube of side length 22 cm, filled com-pletely with water at 20°C. The top surface of the cube ishorizontal. One top corner, point A, is open through a smallhole to a pressure of 1 atm. Diagonally opposite to pointA is top corner B. Determine and discuss the various rigid-body accelerations for which the water at point B beginsto cavitate, for (a) horizontal motion and (b) vertical mo-tion.P2.145 A fish tank 14 in deep by 16 by 27 in is to be carriedin a car which may experience accelerations as high as6 m/s2. What is the maximum water depth which will avoidWater at 20°C24 cm9 cmABP2.142Water2ft2ft1ft1ftABaxpa= 15 lbf/in2absP2.143
    • with the child, which way will the balloon tilt, forward orbackward? Explain. (b) The child is now sitting in a carwhich is stopped at a red light. The helium-filled balloonis not in contact with any part of the car (seats, ceiling,etc.) but is held in place by the string, which is in turn heldby the child. All the windows in the car are closed. Whenthe traffic light turns green, the car accelerates forward. Ina frame of reference moving with the car and child, whichway will the balloon tilt, forward or backward? Explain.(c) Purchase or borrow a helium-filled balloon. Conduct ascientific experiment to see if your predictions in parts (a)and (b) above are correct. If not, explain.P2.149 The 6-ft-radius waterwheel in Fig. P2.149 is being used tolift water with its 1-ft-diameter half-cylinder blades. If thewheel rotates at 10 r/min and rigid-body motion is as-sumed, what is the water surface angle ␪ at position A?spilling in rigid-body motion? What is the proper align-ment of the tank with respect to the car motion?P2.146 The tank in Fig. P2.146 is filled with water and has a venthole at point A. The tank is 1 m wide into the paper. In-side the tank, a 10-cm balloon, filled with helium at 130kPa, is tethered centrally by a string. If the tank acceler-ates to the right at 5 m/s2in rigid-body motion, at whatangle will the balloon lean? Will it lean to the right or tothe left?Problems 123P2.147 The tank of water in Fig. P2.147 accelerates uniformly byfreely rolling down a 30° incline. If the wheels are fric-tionless, what is the angle ␪? Can you explain this inter-esting result?Water at 20°C60 cm40 cm20 cmD = 10 cmString1 atmAHeP2.146θ30°P2.147P2.148 A child is holding a string onto which is attached a he-lium-filled balloon. (a) The child is standing still and sud-denly accelerates forward. In a frame of reference moving10 r/minAθ1 ft6 ftP2.149DaxL12LL12h Rest levelP2.150P2.150 A cheap accelerometer, probably worth the price, can bemade from a U-tube as in Fig. P2.150. If L ϭ 18 cm andD ϭ 5 mm, what will h be if ax ϭ 6 m/s2? Can the scalemarkings on the tube be linear multiples of ax?P2.151 The U-tube in Fig. P2.151 is open at A and closed at D.If accelerated to the right at uniform ax, what acceleration
    • will cause the pressure at point C to be atmospheric? Thefluid is water (SG ϭ 1.0).124 Chapter 2 Pressure Distribution in a Fluid1 ft1 ft1 ftDCBAP2.151ABC20 cm10 cm 5 cm12 cmΩP2.155P2.156 Suppose that the U-tube of Fig. P2.151 is rotated aboutaxis DC. If the fluid is water at 122°F and atmosphericpressure is 2116 lbf/ft2absolute, at what rotation rate willthe fluid within the tube begin to vaporize? At what pointwill this occur?P2.157 The 45° V-tube in Fig. P2.157 contains water and is openat A and closed at C. What uniform rotation rate in r/minabout axis AB will cause the pressure to be equal at pointsB and C? For this condition, at what point in leg BC willthe pressure be a minimum?30 cm45˚A CBP2.157P2.158 It is desired to make a 3-m-diameter parabolic telescopemirror by rotating molten glass in rigid-body motion un-til the desired shape is achieved and then cooling the glassto a solid. The focus of the mirror is to be 4 m from themirror, measured along the centerline. What is the propermirror rotation rate, in r/min, for this task?P2.152 A 16-cm-diameter open cylinder 27 cm high is full of wa-ter. Compute the rigid-body rotation rate about its centralaxis, in r/min, (a) for which one-third of the water willspill out and (b) for which the bottom will be barely ex-posed.P2.153 Suppose the U-tube in Fig. P2.150 is not translated butrather rotated about its right leg at 95 r/min. What will bethe level h in the left leg if L ϭ 18 cm and D ϭ 5 mm?P2.154 A very deep 18-cm-diameter can contains 12 cm of wateroverlaid with 10 cm of SAE 30 oil. If the can isrotated in rigid-body motion about its central axis at150 r/min, what will be the shapes of the air-oil andoil-water interfaces? What will be the maximum fluid pres-sure in the can in Pa (gage)?P2.155 For what uniform rotation rate in r/min about axis C willthe U-tube in Fig. P2.155 take the configuration shown?The fluid is mercury at 20°C.EES*
    • Fundamentals of Engineering Exam ProblemsFE2.1 A gage attached to a pressurized nitrogen tank reads agage pressure of 28 in of mercury. If atmospheric pres-sure is 14.4 psia, what is the absolute pressure in the tank?(a) 95 kPa, (b) 99 kPa, (c) 101 kPa, (d) 194 kPa,(e) 203 kPaFE2.2 On a sea-level standard day, a pressure gage, moored be-low the surface of the ocean (SG ϭ 1.025), reads an ab-solute pressure of 1.4 MPa. How deep is the instrument?(a) 4 m, (b) 129 m, (c) 133 m, (d) 140 m, (e) 2080 mFE2.3 In Fig. FE2.3, if the oil in region B has SG ϭ 0.8 and theabsolute pressure at point A is 1 atm, what is the absolutepressure at point B?(a) 5.6 kPa, (b) 10.9 kPa, (c) 106.9 kPa, (d) 112.2 kPa,(e) 157.0 kPaWord ProblemsW2.1 Consider a hollow cone with a vent hole in the vertex atthe top, along with a hollow cylinder, open at the top, withthe same base area as the cone. Fill both with water to thetop. The hydrostatic paradox is that both containers havethe same force on the bottom due to the water pressure, al-though the cone contains 67 percent less water. Can youexplain the paradox?W2.2 Can the temperature ever rise with altitude in the real at-mosphere? Wouldn’t this cause the air pressure to increaseupward? Explain the physics of this situation.W2.3 Consider a submerged curved surface which consists of atwo-dimensional circular arc of arbitrary angle, arbitrarydepth, and arbitrary orientation. Show that the resultant hy-drostatic pressure force on this surface must pass throughthe center of curvature of the arc.W2.4 Fill a glass approximately 80 percent with water, and add alarge ice cube. Mark the water level. The ice cube, havingSG Ϸ 0.9, sticks up out of the water. Let the ice cube meltwith negligible evaporation from the water surface. Will thewater level be higher than, lower than, or the same as before?Fundamentals of Engineering Exam Problems 1255 cm3 cm4 cm8 cmABOilMercurySG = 13.56WaterSG = 1FE2.3W2.5 A ship, carrying a load of steel, is trapped while floatingin a small closed lock. Members of the crew want to getout, but they can’t quite reach the top wall of the lock. Acrew member suggests throwing the steel overboard in thelock, claiming the ship will then rise and they can climbout. Will this plan work?W2.6 Consider a balloon of mass m floating neutrally in the at-mosphere, carrying a person/basket of mass M Ͼ m. Dis-cuss the stability of this system to disturbances.W2.7 Consider a helium balloon on a string tied to the seat ofyour stationary car. The windows are closed, so there is noair motion within the car. The car begins to accelerate for-ward. Which way will the balloon lean, forward or back-ward? (Hint: The acceleration sets up a horizontal pressuregradient in the air within the car.)W2.8 Repeat your analysis of Prob. W2.7 to let the car move atconstant velocity and go around a curve. Will the balloonlean in, toward the center of curvature, or out?FE2.4 In Fig. FE2.3, if the oil in region B has SG ϭ 0.8 and theabsolute pressure at point B is 14 psia, what is the ab-solute pressure at point B?(a) 11 kPa, (b) 41 kPa, (c) 86 kPa, (d) 91 kPa, (e) 101 kPaFE2.5 A tank of water (SG ϭ 1,.0) has a gate in its vertical wall5 m high and 3 m wide. The top edge of the gate is 2 mbelow the surface. What is the hydrostatic force on the gate?(a) 147 kN, (b) 367 kN, (c) 490 kN, (d) 661 kN,(e) 1028 kNFE2.6 In Prob. FE2.5 above, how far below the surface is thecenter of pressure of the hydrostatic force?(a) 4.50 m, (b) 5.46 m, (c) 6.35 m, (d) 5.33 m, (e) 4.96 mFE2.7 A solid 1-m-diameter sphere floats at the interface betweenwater (SG ϭ 1.0) and mercury (SG ϭ 13.56) such that 40 per-cent is in the water. What is the specific gravity of the sphere?(a) 6.02, (b) 7.28, (c) 7.78, (d) 8.54, (e) 12.56FE2.8 A 5-m-diameter balloon contains helium at 125 kPa absoluteand 15°C, moored in sea-level standard air. If the gas con-stant of helium is 2077 m2/(s2иK) and balloon material weightis neglected, what is the net lifting force of the balloon?(a) 67 N, (b) 134 N, (c) 522 N, (d) 653 N, (e) 787 NFE2.9 A square wooden (SG ϭ 0.6) rod, 5 cm by 5 cm by 10 mlong, floats vertically in water at 20°C when 6 kg of steel(SG ϭ 7.84) are attached to one end. How high above thewater surface does the wooden end of the rod protrude?(a) 0.6 m, (b) 1.6 m, (c) 1.9 m, (d) 2.4 m, (e) 4.0 m
    • of buoyancy is above its metacenter, (d) metacenter isabove its center of buoyancy, (e) metacenter is above itscenter of gravityFE2.10 A floating body will be stable when its(a) center of gravity is above its center of buoyancy,(b) center of buoyancy is below the waterline, (c) center126 Chapter 2 Pressure Distribution in a FluidDhdZero pressure levelTo pressure measurement locationp1␳a(air)pa␳mC2.1Comprehensive ProblemsC2.1 Some manometers are constructed as in Fig. C2.1, whereone side is a large reservoir (diameter D) and the other sideis a small tube of diameter d, open to the atmosphere. Insuch a case, the height of manometer liquid on the reservoirside does not change appreciably. This has the advantagethat only one height needs to be measured rather than two.The manometer liquid has density ␳m while the air has den-sity ␳a. Ignore the effects of surface tension. When there isno pressure difference across the manometer, the elevationson both sides are the same, as indicated by the dashed line.Height h is measured from the zero pressure level as shown.(a) When a high pressure is applied to the left side, themanometer liquid in the large reservoir goes down, whilethat in the tube at the right goes up to conserve mass. Writean exact expression for p1gage, taking into account the move-ment of the surface of the reservoir. Your equation shouldgive p1gage as a function of h, ␳m, and the physical para-meters in the problem, h, d, D, and gravity constant g.(b) Write an approximate expression for p1gage, neglectingthe change in elevation of the surface of the reservoir liq-uid. (c) Suppose h ϭ 0.26 m in a certain application. If pa ϭ101,000 Pa and the manometer liquid has a density of 820kg/m3, estimate the ratio D/d required to keep the errorof the approximation of part (b) within 1 percent of the ex-act measurement of part (a). Repeat for an error within 0.1percent.U-tube is still useful as a pressure-measuring device. It isattached to a pressurized tank as shown in the figure. (a)Find an expression for h as a function of H and other pa-rameters in the problem. (b) Find the special case of yourresult in (a) when ptank ϭ pa. (c) Suppose H ϭ 5.0 cm, pais 101.2kPa, ptank is 1.82 kPa higher than pa, and SG0 ϭ0.85. Calculate h in cm, ignoring surface tension effects andneglecting air density effects.OilWaterHhPressurized air tank,with pressure ϭ ptankpaC2.2C2.3 Professor F. Dynamics, riding the merry-go-round with hisson, has brought along his U-tube manometer. (You neverknow when a manometer might come in handy.) As shownin Fig. C2.3, the merry-go-round spins at constant angularvelocity and the manometer legs are 7 cm apart. Themanometer center is 5.8 m from the axis of rotation. De-termine the height difference h in two ways: (a) approxi-mately, by assuming rigid body translation with a equal tothe average manometer acceleration; and (b) exactly, usingrigid-body rotation theory. How good is the approximation?C2.4 A student sneaks a glass of cola onto a roller coaster ride.The glass is cylindrical, twice as tall as it is wide, and filledto the brim. He wants to know what percent of the cola heshould drink before the ride begins, so that none of it spillsduring the big drop, in which the roller coaster achieves0.55-g acceleration at a 45° angle below the horizontal.Make the calculation for him, neglecting sloshing and as-suming that the glass is vertical at all times.C2.2 A prankster has added oil, of specific gravity SG0, to theleft leg of the manometer in Fig. C2.2. Nevertheless, the
    • 8. R. P. Benedict, Fundamentals of Temperature, Pressure, andFlow Measurement, 3d ed., Wiley, New York, 1984.9. T. G. Beckwith and R. G. Marangoni, Mechanical Measure-ments, 4th ed., Addison-Wesley, Reading, MA, 1990.10. J. W. Dally, W. F. Riley, and K. G. McConnell, Instrumenta-tion for Engineering Measurements, Wiley, New York, 1984.11. E. N. Gilbert, “How Things Float,’’ Am. Math. Monthly, vol.98, no. 3, pp. 201–216, 1991.12. R. J. Figliola and D. E. Beasley, Theory and Design for Me-chanical Measurements, 2d ed., Wiley, New York, 1994.13. R. W. Miller, Flow Measurement Engineering Handbook, 3ded., McGraw-Hill, New York, 1996.References1. U.S. Standard Atmosphere, 1976, Government Printing Of-fice, Washington, DC, 1976.2. G. Neumann and W. J. Pierson, Jr., Principles of PhysicalOceanography, Prentice-Hall, Englewood Cliffs, NJ, 1966.3. T. C. Gillmer and B. Johnson, Introduction to Naval Archi-tecture, Naval Institute Press, Annapolis, MD, 1982.4. D. T. Greenwood, Principles of Dynamics, 2d ed., Prentice-Hall, Englewood Cliffs, NJ, 1988.5. R. I. Fletcher, “The Apparent Field of Gravity in a RotatingFluid System,’’Am. J. Phys., vol. 40, pp. 959–965, July 1972.6. National Committee for Fluid Mechanics Films, IllustratedExperiments in Fluid Mechanics, M.I.T. Press, Cambridge,MA, 1972.7. J. P. Holman, Experimental Methods for Engineers, 6th ed.,McGraw-Hill, New York, 1993.h, m F, N h, m F, N6.00 400 7.25 5546.25 437 7.50 5736.50 471 7.75 5896.75 502 8.00 6007.00 530bhYLWCircular arc blockFluid: ␳Pivot armPivotCounterweightSide viewof block faceRD2.2Design ProjectsD2.1 It is desired to have a bottom-moored, floating systemwhich creates a nonlinear force in the mooring line as thewater level rises. The design force F need only be accuratein the range of seawater depths h between 6 and 8 m, asshown in the accompanying table. Design a buoyant sys-tem which will provide this force distribution. The systemshould be practical, i.e., of inexpensive materials and sim-ple construction.D2.2 A laboratory apparatus used in some universities is shownin Fig. D2.2. The purpose is to measure the hydrostaticforce on the flat face of the circular-arc block and com-pare it with the theoretical value for given depth h. Thecounterweight is arranged so that the pivot arm is hori-zontal when the block is not submerged, whence the weightW can be correlated with the hydrostatic force when thesubmerged arm is again brought to horizontal. First showthat the apparatus concept is valid in principle; then derivea formula for W as a function of h in terms of the systemparameters. Finally, suggest some appropriate values of Y,L, etc., for a suitable appartus and plot theoretical W ver-sus h for these values.R ϭ 5.80 m (to center of manometer)hCenter ofrotation⍀ ϭ 6.00 rpmWater7.00 cmC2.3127
    • 128Table tennis ball suspended by an air jet. The control volume momentum principle, studied inthis chapter, requires a force to change the direction of a flow. The jet flow deflects around theball, and the force is the ball’s weight. (Courtesy of Paul Silverman/Fundamental Photographs)
    • 3.1 Basic Physical Lawsof Fluid MechanicsMotivation. In analyzing fluid motion, we might take one of two paths: (1) seeking todescribe the detailed flow pattern at every point (x, y, z) in the field or (2) workingwith a finite region, making a balance of flow in versus flow out, and determining grossflow effects such as the force or torque on a body or the total energy exchange. Thesecond is the “control-volume” method and is the subject of this chapter. The first isthe “differential” approach and is developed in Chap. 4.We first develop the concept of the control volume, in nearly the same manner asone does in a thermodynamics course, and we find the rate of change of an arbitrarygross fluid property, a result called the Reynolds transport theorem. We then apply thistheorem, in sequence, to mass, linear momentum, angular momentum, and energy, thusderiving the four basic control-volume relations of fluid mechanics. There are manyapplications, of course. The chapter then ends with a special case of frictionless, shaft-work-free momentum and energy: the Bernoulli equation. The Bernoulli equation is awonderful, historic relation, but it is extremely restrictive and should always be viewedwith skepticism and care in applying it to a real (viscous) fluid motion.It is time now to really get serious about flow problems. The fluid-statics applicationsof Chap. 2 were more like fun than work, at least in my opinion. Statics problems ba-sically require only the density of the fluid and knowledge of the position of the freesurface, but most flow problems require the analysis of an arbitrary state of variablefluid motion defined by the geometry, the boundary conditions, and the laws of me-chanics. This chapter and the next two outline the three basic approaches to the analy-sis of arbitrary flow problems:1. Control-volume, or large-scale, analysis (Chap. 3)2. Differential, or small-scale, analysis (Chap. 4)3. Experimental, or dimensional, analysis (Chap. 5)The three approaches are roughly equal in importance, but control-volume analysis is“more equal,” being the single most valuable tool to the engineer for flow analysis. Itgives “engineering” answers, sometimes gross and crude but always useful. In princi-129Chapter 3Integral Relationsfor a Control Volume
    • Systems versus Control Volumesple, the differential approach of Chap. 4 can be used for any problem, but in practicethe lack of mathematical tools and the inability of the digital computer to model small-scale processes make the differential approach rather limited. Similarly, although thedimensional analysis of Chap. 5 can be applied to any problem, the lack of time andmoney and generality often makes experimentation a limited approach. But a control-volume analysis takes about half an hour and gives useful results. Thus, in a trio of ap-proaches, the control volume is best. Oddly enough, it is the newest of the three. Dif-ferential analysis began with Euler and Lagrange in the eighteenth century, anddimensional analysis was pioneered by Lord Rayleigh in the late nineteenth century,but the control volume, although proposed by Euler, was not developed on a rigorousbasis as an analytical tool until the 1940s.All the laws of mechanics are written for a system, which is defined as an arbitraryquantity of mass of fixed identity. Everything external to this system is denoted by theterm surroundings, and the system is separated from its surroundings by its bound-aries. The laws of mechanics then state what happens when there is an interaction be-tween the system and its surroundings.First, the system is a fixed quantity of mass, denoted by m. Thus the mass of thesystem is conserved and does not change.1This is a law of mechanics and has a verysimple mathematical form, called conservation of mass:msyst ϭ constor ᎏddmtᎏ ϭ 0(3.1)This is so obvious in solid-mechanics problems that we often forget about it. In fluidmechanics, we must pay a lot of attention to mass conservation, and it takes a littleanalysis to make it hold.Second, if the surroundings exert a net force F on the system, Newton’s second lawstates that the mass will begin to accelerate2F ϭ ma ϭ m ᎏddVtᎏ ϭ ᎏddtᎏ (mV) (3.2)In Eq. (2.12) we saw this relation applied to a differential element of viscous incom-pressible fluid. In fluid mechanics Newton’s law is called the linear-momentum rela-tion. Note that it is a vector law which implies the three scalar equations Fx ϭ max,Fy ϭ may, and Fz ϭ maz.Third, if the surroundings exert a net moment M about the center of mass of thesystem, there will be a rotation effectM ϭ ᎏddHtᎏ (3.3)where H ϭ ⌺(r ؋ V) ␦m is the angular momentum of the system about its center of130 Chapter 3 Integral Relations for a Control Volume1We are neglecting nuclear reactions, where mass can be changed to energy.2We are neglecting relativistic effects, where Newton’s law must be modified.
    • mass. Here we call Eq. (3.3) the angular-momentum relation. Note that it is also a vec-tor equation implying three scalar equations such as Mx ϭ dHx /dt.For an arbitrary mass and arbitrary moment, H is quite complicated and containsnine terms (see, e.g., Ref. 1, p. 285). In elementary dynamics we commonly treat onlya rigid body rotating about a fixed x axis, for which Eq. (3.3) reduces toMx ϭ Ix ᎏddtᎏ (␻x) (3.4)where ␻x is the angular velocity of the body and Ix is its mass moment of inertia aboutthe x axis. Unfortunately, fluid systems are not rigid and rarely reduce to such a sim-ple relation, as we shall see in Sec. 3.5.Fourth, if heat dQ is added to the system or work dW is done by the system, thesystem energy dE must change according to the energy relation, or first law of ther-modynamics,dQ Ϫ dW ϭ dEor ᎏddQtᎏ Ϫ ᎏddWtᎏ ϭ ᎏddEtᎏ(3.5)Like mass conservation, Eq. (3.1), this is a scalar relation having only a single com-ponent.Finally, the second law of thermodynamics relates entropy change dS to heat addeddQ and absolute temperature T:dS Ն ᎏdTQᎏ (3.6)This is valid for a system and can be written in control-volume form, but there are al-most no practical applications in fluid mechanics except to analyze flow-loss details(see Sec. 9.5).All these laws involve thermodynamic properties, and thus we must supplementthem with state relations p ϭ p(␳, T) and e ϭ e(␳, T) for the particular fluid being stud-ied, as in Sec. 1.6.The purpose of this chapter is to put our four basic laws into the control-volumeform suitable for arbitrary regions in a flow:1. Conservation of mass (Sec. 3.3)2. The linear-momentum relation (Sec. 3.4)3. The angular-momentum relation (Sec. 3.5)4. The energy equation (Sec. 3.6)Wherever necessary to complete the analysis we also introduce a state relation such asthe perfect-gas law.Equations (3.1) to (3.6) apply to either fluid or solid systems. They are ideal for solidmechanics, where we follow the same system forever because it represents the productwe are designing and building. For example, we follow a beam as it deflects under load.We follow a piston as it oscillates. We follow a rocket system all the way to Mars.But fluid systems do not demand this concentrated attention. It is rare that we wishto follow the ultimate path of a specific particle of fluid. Instead it is likely that the3.1 Basic Physical Laws of Fluid Mechanics 131
    • Fig. 3.1 Volume rate of flowthrough an arbitrary surface: (a) anelemental area dA on the surface;(b) the incremental volume sweptthrough dA equals V dt dA cos ␪.fluid forms the environment whose effect on our product we wish to know. For thethree examples cited above, we wish to know the wind loads on the beam, the fluidpressures on the piston, and the drag and lift loads on the rocket. This requires that thebasic laws be rewritten to apply to a specific region in the neighborhood of our prod-uct. In other words, where the fluid particles in the wind go after they leave the beamis of little interest to a beam designer. The user’s point of view underlies the need forthe control-volume analysis of this chapter.Although thermodynamics is not at all the main topic of this book, it would be ashame if the student did not review at least the first law and the state relations, as dis-cussed, e.g., in Refs. 6 and 7.In analyzing a control volume, we convert the system laws to apply to a specific re-gion which the system may occupy for only an instant. The system passes on, and othersystems come along, but no matter. The basic laws are reformulated to apply to thislocal region called a control volume. All we need to know is the flow field in this re-gion, and often simple assumptions will be accurate enough (e.g., uniform inlet and/oroutlet flows). The flow conditions away from the control volume are then irrelevant.The technique for making such localized analyses is the subject of this chapter.All the analyses in this chapter involve evaluation of the volume flow Q or mass flowm˙ passing through a surface (imaginary) defined in the flow.Suppose that the surface S in Fig. 3.1a is a sort of (imaginary) wire mesh throughwhich the fluid passes without resistance. How much volume of fluid passes through Sin unit time? If, typically, V varies with position, we must integrate over the elementalsurface dA in Fig. 3.1a. Also, typically V may pass through dA at an angle ␪ off thenormal. Let n be defined as the unit vector normal to dA. Then the amount of fluid sweptthrough dA in time dt is the volume of the slanted parallelopiped in Fig. 3.1b:dᐂ ϭ V dt dA cos ␪ ϭ (V ؒ n) dA dtThe integral of dᐂ/dt is the total volume rate of flow Q through the surface SQ ϭ ͵S(V и n) dA ϭ ͵SVn dA (3.7)132 Chapter 3 Integral Relations for a Control VolumeθSdA1VUnit normal ndAθnVV dt(a) (b)Volume and Mass Rate of Flow
    • 3.2 The Reynolds TransportTheoremWe could replace V ؒ n by its equivalent, Vn, the component of V normal to dA, butthe use of the dot product allows Q to have a sign to distinguish between inflow andoutflow. By convention throughout this book we consider n to be the outward normalunit vector. Therefore V ؒ n denotes outflow if it is positive and inflow if negative. Thiswill be an extremely useful housekeeping device when we are computing volume andmass flow in the basic control-volume relations.Volume flow can be multiplied by density to obtain the mass flow m˙ . If densityvaries over the surface, it must be part of the surface integralm˙ ϭ ͵S␳(Vؒ n) dA ϭ ͵S␳Vn dAIf density is constant, it comes out of the integral and a direct proportionality results:Constant density: m˙ ϭ ␳QTo convert a system analysis to a control-volume analysis, we must convert our math-ematics to apply to a specific region rather than to individual masses. This conversion,called the Reynolds transport theorem, can be applied to all the basic laws. Examin-ing the basic laws (3.1) to (3.3) and (3.5), we see that they are all concerned with thetime derivative of fluid properties m, V, H, and E. Therefore what we need is to relatethe time derivative of a system property to the rate of change of that property withina certain region.The desired conversion formula differs slightly according to whether the control vol-ume is fixed, moving, or deformable. Figure 3.2 illustrates these three cases. The fixedcontrol volume in Fig. 3.2a encloses a stationary region of interest to a nozzle designer.The control surface is an abstract concept and does not hinder the flow in any way. Itslices through the jet leaving the nozzle, circles around through the surrounding at-mosphere, and slices through the flange bolts and the fluid within the nozzle. This par-ticular control volume exposes the stresses in the flange bolts, which contribute to ap-plied forces in the momentum analysis. In this sense the control volume resembles thefree-body concept, which is applied to systems in solid-mechanics analyses.Figure 3.2b illustrates a moving control volume. Here the ship is of interest, not theocean, so that the control surface chases the ship at ship speed V. The control volumeis of fixed volume, but the relative motion between water and ship must be considered.3.2 The Reynolds Transport Theorem 133(a)Controlsurface(c)ControlsurfaceVV(b)ControlsurfaceVFig. 3.2 Fixed, moving, and de-formable control volumes: (a) fixedcontrol volume for nozzle-stressanalysis; (b) control volume mov-ing at ship speed for drag-forceanalysis; (c) control volume de-forming within cylinder for tran-sient pressure-variation analysis.
    • One-Dimensional Fixed ControlVolumeIf V is constant, this relative motion is a steady-flow pattern, which simplifies the analy-sis.3If V is variable, the relative motion is unsteady, so that the computed results aretime-variable and certain terms enter the momentum analysis to reflect the noninertialframe of reference.Figure 3.2c shows a deforming control volume. Varying relative motion at the bound-aries becomes a factor, and the rate of change of shape of the control volume entersthe analysis. We begin by deriving the fixed-control-volume case, and we consider theother cases as advanced topics.As a simple first example, consider a duct or streamtube with a nearly one-dimensionalflow V ϭ V(x), as shown in Fig. 3.3. The selected control volume is a portion of theduct which happens to be filled exactly by system 2 at a particular instant t. At timet ϩ dt, system 2 has begun to move out, and a sliver of system 1 has entered from theleft. The shaded areas show an outflow sliver of volume AbVb dt and an inflow volumeAaVa dt.Now let B be any property of the fluid (energy, momentum, etc.), and let ␤ ϭ dB/dmbe the intensive value or the amount of B per unit mass in any small portion of thefluid. The total amount of B in the control volume is thusBCV ϭ ͵CV␤␳ dᐂ ␤ ϭ ᎏddmBᎏ (3.8)134 Chapter 3 Integral Relations for a Control VolumeFig. 3.3 Example of inflow andoutflow as three systems passthrough a control volume: (a) Sys-tem 2 fills the control volume attime t; (b) at time t ϩ dt system 2begins to leave and system 1enters.3A wind tunnel uses a fixed model to simulate flow over a body moving through a fluid. A tow tankuses a moving model to simulate the same situation.;;; ;;;;;;;;;System 1System 2System 3x, V(x)ab1 2 3Controlvolumefixed inspacedᐂin = AaVa dtout = AbVb dtdᐂ(a)(b)SectionaSectionb1 2
    • Arbitrary Fixed Control Volumewhere ␳ dᐂ is a differential mass of the fluid. We want to relate the rate of change ofBCV to the rate of change of the amount of B in system 2 which happens to coincidewith the control volume at time t. The time derivative of BCV is defined by the calcu-lus limitᎏddtᎏ (BCV) ϭ ᎏd1tᎏ BCV(t ϩ dt) Ϫ ᎏd1tᎏ BCV(t)ϭ ᎏd1tᎏ [B2(t ϩ dt) Ϫ (␤␳ dᐂ)out ϩ (␤␳dᐂ)in] Ϫ ᎏd1tᎏ [B2(t)]ϭ ᎏd1tᎏ [B2(t ϩ dt) Ϫ B2(t)] Ϫ (␤␳AV)out ϩ (␤␳AV)inThe first term on the right is the rate of change of B within system 2 at the instant itoccupies the control volume. By rearranging the last line of the above equation, wehave the desired conversion formula relating changes in any property B of a local sys-tem to one-dimensional computations concerning a fixed control volume which in-stantaneously encloses the system.ᎏddtᎏ (Bsyst) ϭ ᎏddtᎏ ΂͵CV␤␳ dᐂ΃ϩ (␤␳AV)out Ϫ (␤␳AV)in (3.9)This is the one-dimensional Reynolds transport theorem for a fixed volume. The threeterms on the right-hand side are, respectively,1. The rate of change of B within the control volume2. The flux of B passing out of the control surface3. The flux of B passing into the control surfaceIf the flow pattern is steady, the first term vanishes. Equation (3.9) can readily be gen-eralized to an arbitrary flow pattern, as follows.Figure 3.4 shows a generalized fixed control volume with an arbitrary flow patternpassing through. The only additional complication is that there are variable slivers ofinflow and outflow of fluid all about the control surface. In general, each differentialarea dA of surface will have a different velocity V making a different angle ␪ with thelocal normal to dA. Some elemental areas will have inflow volume (VA cos ␪)in dt, andothers will have outflow volume (VA cos ␪)out dt, as seen in Fig. 3.4. Some surfacesmight correspond to streamlines (␪ ϭ 90°) or solid walls (V ϭ 0) with neither inflownor outflow. Equation (3.9) generalizes toᎏddtᎏ (Bsyst) ϭ ᎏddtᎏ΂͵CV␤␳dᐂ΃ϩ ͵CS␤␳V cos ␪ dAout Ϫ ͵CS␤␳V cos ␪ dAin (3.10)This is the Reynolds transport theorem for an arbitrary fixed control volume. By let-ting the property B be mass, momentum, angular momentum, or energy, we can rewriteall the basic laws in control-volume form. Note that all three of the control-volume in-tegrals are concerned with the intensive property ␤. Since the control volume is fixedin space, the elemental volumes dᐂ do not vary with time, so that the time derivativeof the volume integral vanishes unless either ␤ or ␳ varies with time (unsteady flow).3.2 The Reynolds Transport Theorem 135
    • Fig. 3.4 Generalization of Fig. 3.3to an arbitrary control volume withan arbitrary flow pattern.Equation (3.10) expresses the basic formula that a system derivative equals the rateof change of B within the control volume plus the flux of B out of the control surfaceminus the flux of B into the control surface. The quantity B (or ␤) may be any vectoror scalar property of the fluid. Two alternate forms are possible for the flux terms. Firstwe may notice that V cos ␪ is the component of V normal to the area element of thecontrol surface. Thus we can writeFlux terms ϭ ͵CS␤␳Vn dAout Ϫ ͵CS␤␳Vn dAin ϭ ͵CS␤ dm˙ out Ϫ ͵CS␤ dm˙ in (3.11a)where dm˙ ϭ ␳Vn dA is the differential mass flux through the surface. Form (3.11a)helps visualize what is being calculated.A second alternate form offers elegance and compactness as advantages. If n is de-fined as the outward normal unit vector everywhere on the control surface, then V ؒn ϭ Vn for outflow and V ؒ n ϭ ϪVn for inflow. Therefore the flux terms can be rep-resented by a single integral involving V ؒ n which accounts for both positive outflowand negative inflowFlux terms ϭ ͵CS␤␳(V ؒ n) dA (3.11b)The compact form of the Reynolds transport theorem is thusᎏddtᎏ (Bsyst) ϭ ᎏddtᎏ΂͵CV␤␳ dᐂ΃ ϩ ͵CV␤␳(V ؒ n) dA (3.12)This is beautiful but only occasionally useful, when the coordinate system is ideallysuited to the control volume selected. Otherwise the computations are easier when theflux of B out is added and the flux of B in is subtracted, according to (3.10) or (3.11a).136 Chapter 3 Integral Relations for a Control VolumeSystem attime t + dtSystem attime td Aθn, Unit outwardnormal to dAArbitraryfixedcontrolsurfaceCSVindAVoutn, Unit outwardnormal to dAFixedcontrolvolumeCVθdᐂin = Vin dAin cos in dt= –V• n dA dtθ dᐂout = V dAout cos out dt= V • n dA dtout θ
    • Control Volume of ConstantShape but Variable Velocity4Arbitrarily Moving andDeformable Control Volume5Control Volume Moving atConstant VelocityThe time-derivative term can be written in the equivalent formᎏddtᎏ΂͵CV␤␳ dᐂ΃ϭ͵CVᎏѨѨtᎏ (␤␳) dᐂ (3.13)for the fixed control volume since the volume elements do not vary.If the control volume is moving uniformly at velocity Vs, as in Fig. 3.2b, an observerfixed to the control volume will see a relative velocity Vr of fluid crossing the controlsurface, defined byVr ‫؍‬ V ؊ Vs (3.14)where V is the fluid velocity relative to the same coordinate system in which the con-trol volume motion Vs is observed. Note that Eq. (3.14) is a vector subtraction. Theflux terms will be proportional to Vr, but the volume integral is unchanged because thecontrol volume moves as a fixed shape without deforming. The Reynolds transport the-orem for this case of a uniformly moving control volume isᎏddtᎏ (Bsyst) ϭ ᎏddtᎏ ΂͵CV␤␳ dᐂ΃ϩ ͵CS␤␳(Vr ؒ n) dA (3.15)which reduces to Eq. (3.12) if Vs ϵ 0.If the control volume moves with a velocity Vs(t) which retains its shape, then the vol-ume elements do not change with time but the boundary relative velocity Vr ϭV(r, t) Ϫ Vs(t) becomes a somewhat more complicated function. Equation (3.15) is un-changed in form, but the area integral may be more laborious to evaluate.The most general situation is when the control volume is both moving and deformingarbitrarily, as illustrated in Fig. 3.5. The flux of volume across the control surface isagain proportional to the relative normal velocity component Vr ؒ n, as in Eq. (3.15).However, since the control surface has a deformation, its velocity Vs ϭ Vs(r, t), so thatthe relative velocity Vr ϭ V(r, t) Ϫ Vs(r, t) is or can be a complicated function, eventhough the flux integral is the same as in Eq. (3.15). Meanwhile, the volume integralin Eq. (3.15) must allow the volume elements to distort with time. Thus the time de-rivative must be applied after integration. For the deforming control volume, then, thetransport theorem takes the formᎏddtᎏ (Bsyst) ϭ ᎏddtᎏ΂͵CV␤␳ dᐂ΃ϩ ͵CS␤␳(Vr ؒ n) dA (3.16)This is the most general case, which we can compare with the equivalent form for afixed control volume3.2 The Reynolds Transport Theorem 1374This section may be omitted without loss of continuity.5This section may be omitted without loss of continuity.
    • Fig. 3.5 Relative-velocity effectsbetween a system and a controlvolume when both move and de-form. The system boundaries moveat velocity V, and the control sur-face moves at velocity Vs.ᎏddtᎏ (Bsyst) ϭ ͵CVᎏѨѨtᎏ (␤␳) dᐂ ϩ ͵CS␤␳(V ؒ n) dA (3.17)The moving and deforming control volume, Eq. (3.16), contains only two complica-tions: (1) The time derivative of the first integral on the right must be taken outside,and (2) the second integral involves the relative velocity Vr between the fluid systemand the control surface. These differences and mathematical subtleties are best shownby examples.In many applications, the flow crosses the boundaries of the control surface only at cer-tain simplified inlets and exits which are approximately one-dimensional; i.e., the flowproperties are nearly uniform over the cross section of the inlet or exit. Then the double-integral flux terms required in Eq. (3.16) reduce to a simple sum of positive (exit) andnegative (inlet) product terms involving the flow properties at each cross section͵CS␤␳(Vr ؒ n) dA ϭ Α(␤i␳iVriAi)out Ϫ Α(␤i␳iVriAi)in (3.18)An example of this situation is shown in Fig. 3.6. There are inlet flows at sections 1and 4 and outflows at sections 2, 3, and 5. For this particular problem Eq. (3.18) wouldbe͵CS␤␳(Vr ؒ n) dA ϭ ␤2␳2Vr2A2 ϩ ␤3␳3Vr3A3ϩ ␤5␳5Vr5A5 Ϫ ␤1␳1Vr1A1 Ϫ ␤4␳4Vr4A4 (3.19)138 Chapter 3 Integral Relations for a Control VolumeVrVsVndᐂin = –(Vr • n) dA dtdᐂout = (Vr • n) dA dtnVVsVr = V – VsSystem attime t + dtCV at time t + dtSystem andCV at time tOne-Dimensional Flux-TermApproximations
    • Fig. 3.6 A control volume withsimplified one-dimensional inletsand exits.with no contribution from any other portion of the control surface because there is noflow across the boundary.EXAMPLE 3.1A fixed control volume has three one-dimensional boundary sections, as shown in Fig. E3.1. Theflow within the control volume is steady. The flow properties at each section are tabulated be-low. Find the rate of change of energy of the system which occupies the control volume at thisinstant.Section Type ␳, kg/m3V, m/s A, m2e, J/kg1 Inlet 800 5.0 2.0 3002 Inlet 800 8.0 3.0 1003 Outlet 800 17.0 2.0 150SolutionThe property under study here is energy, and so B ϭ E and ␤ ϭ dE/dm ϭ e, the energy per unitmass. Since the control volume is fixed, Eq. (3.17) applies:΂ᎏddEtᎏ΃systϭ ͵CVᎏѨѨtᎏ (e␳) dᐂ ϩ ͵CSe␳(V ؒ n) dAThe flow within is steady, so that Ѩ(e␳)/Ѩt ϵ 0 and the volume integral vanishes. The area inte-gral consists of two inlet sections and one outlet section, as given in the table΂ᎏddEtᎏ΃systϭ Ϫe1␳1A1V1 Ϫ e2␳2A2V2 ϩ e3␳3A3V33.2 The Reynolds Transport Theorem 139CV12345All sections i:Vri approximatelynormal to area AiCSSection 2:uniform Vr2, A2, 2, 2, etc.ρ βE3.1321CV
    • E3.2Introducing the numerical values from the table, we have΂ᎏddEtᎏ΃systϭ Ϫ(300 J/kg)(800 kg/m3)(2 m2)(5 m/s) Ϫ 100(800)(3)(8) ϩ 150(800)(2)(17)ϭ (Ϫ2,400,000 Ϫ 1,920,000 ϩ 4,080,000) J/sϭ Ϫ240,000 J/s ϭ Ϫ0.24 MJ/s Ans.Thus the system is losing energy at the rate of 0.24 MJ/s ϭ 0.24 MW. Since we have accountedfor all fluid energy crossing the boundary, we conclude from the first law that there must be heatloss through the control surface or the system must be doing work on the environment throughsome device not shown. Notice that the use of SI units leads to a consistent result in joules persecond without any conversion factors. We promised in Chap. 1 that this would be the case.Note: This problem involves energy, but suppose we check the balance of mass also.Then B ϭ mass m, and B ϭ dm/dm ϭ unity. Again the volume integral vanishes for steady flow,and Eq. (3.17) reduces to΂ᎏddmtᎏ΃systϭ ͵CS␳(V ؒ n) dA ϭ Ϫ␳1A1V1 Ϫ ␳2A2V2 ϩ ␳3A3V3ϭ Ϫ(800 kg/m3)(2 m2)(5 m/s) Ϫ 800(3)(8) ϩ 800(17)(2)ϭ (Ϫ8000 Ϫ 19,200 ϩ 27,200) kg/s ϭ 0 kg/sThus the system mass does not change, which correctly expresses the law of conservation ofsystem mass, Eq. (3.1).EXAMPLE 3.2The balloon in Fig. E3.2 is being filled through section 1, where the area is A1, velocity is V1,and fluid density is ␳1. The average density within the balloon is ␳b(t). Find an expression forthe rate of change of system mass within the balloon at this instant.SolutionIt is convenient to define a deformable control surface just outside the balloon, expanding atthe same rate R(t). Equation (3.16) applies with Vr ϭ 0 on the balloon surface and Vr ϭ V1at the pipe entrance. For mass change, we take B ϭ m and ␤ ϭ dm/dm ϭ 1. Equation (3.16)becomes΂ᎏddmtᎏ΃systϭ ᎏddtᎏ΂͵CS␳ dᐂ΃ϩ ͵CS␳(Vr ؒ n) dAMass flux occurs only at the inlet, so that the control-surface integral reduces to the single neg-ative term Ϫ␳1A1V1. The fluid mass within the control volume is approximately the average den-sity times the volume of a sphere. The equation thus becomes΂ᎏddmtᎏ΃systϭ ᎏddtᎏ΂␳b ᎏ43ᎏ ␲R3΃Ϫ ␳1A1V1 Ans.This is the desired result for the system mass rate of change. Actually, by the conservation law140 Chapter 3 Integral Relations for a Control Volume1R(t)PipeAveragedensity: b(t)CS expands outwardwith balloon radius R(t)ρ
    • 3.3 Conservation of Mass(3.1), this change must be zero. Thus the balloon density and radius are related to the inlet massflux byᎏddtᎏ (␳bR3) ϭ ᎏ43␲ᎏ ␳1A1V1This is a first-order differential equation which could form part of an engineering analysis ofballoon inflation. It cannot be solved without further use of mechanics and thermodynamics torelate the four unknowns ␳b, ␳1, V1, and R. The pressure and temperature and the elastic prop-erties of the balloon would also have to be brought into the analysis.For advanced study, many more details of the analysis of deformable control vol-umes can be found in Hansen [4] and Potter and Foss [5].The Reynolds transport theorem, Eq. (3.16) or (3.17), establishes a relation betweensystem rates of change and control-volume surface and volume integrals. But systemderivatives are related to the basic laws of mechanics, Eqs. (3.1) to (3.5). Eliminatingsystem derivatives between the two gives the control-volume, or integral, forms of thelaws of mechanics of fluids. The dummy variable B becomes, respectively, mass, lin-ear momentum, angular momentum, and energy.For conservation of mass, as discussed in Examples 3.1 and 3.2, B ϭ m and ␤ ϭdm/dm ϭ 1. Equation (3.1) becomes΂ᎏddmtᎏ΃systϭ 0 ϭ ᎏddtᎏ ΂͵CV␳ dᐂ΃ϩ ͵CS␳(Vr ؒ n) dA (3.20)This is the integral mass-conservation law for a deformable control volume. For a fixedcontrol volume, we have͵CVᎏѨѨ␳tᎏ dᐂϩ ͵CS␳(V ؒ n) dA ϭ 0 (3.21)If the control volume has only a number of one-dimensional inlets and outlets, we canwrite͵CVᎏѨѨ␳tᎏ dᐂ ϩ Αi(␳i AiVi)out ϪΑi(␳i AiVi)in ϭ 0 (3.22)Other special cases occur. Suppose that the flow within the control volume is steady;then Ѩ␳/Ѩt ϵ 0, and Eq. (3.21) reduces to͵CS␳(V ؒ n) dA ϭ 0 (3.23)This states that in steady flow the mass flows entering and leaving the control volumemust balance exactly.6If, further, the inlets and outlets are one-dimensional, we have3.3 Conservation of Mass 1416Throughout this section we are neglecting sources or sinks of mass which might be embedded in thecontrol volume. Equations (3.20) and (3.21) can readily be modified to add source and sink terms, but thisis rarely necessary.
    • Incompressible Flowfor steady flowΑi(␳i AiVi)in ϭ Αi(␳i AiVi)out (3.24)This simple approximation is widely used in engineering analyses. For example, re-ferring to Fig. 3.6, we see that if the flow in that control volume is steady, the threeoutlet mass fluxes balance the two inlets:Outflow ϭ inflow␳2A2V2 ϩ ␳3A3V3 ϩ ␳5A5V5 ϭ ␳1A1V1 ϩ ␳4A4V4 (3.25)The quantity ␳AV is called the mass flow m˙ passing through the one-dimensional crosssection and has consistent units of kilograms per second (or slugs per second) for SI(or BG) units. Equation (3.25) can be rewritten in the short formm˙ 2 ϩ m˙ 3 ϩ m˙ 5 ϭ m˙ 1 ϩ m˙ 4 (3.26)and, in general, the steady-flow–mass-conservation relation (3.23) can be written asΑi(m˙ i)out ϭ Αi(m˙ i)in (3.27)If the inlets and outlets are not one-dimensional, one has to compute m˙ by integrationover the sectionm˙ cs ϭ ͵cs␳(V ؒ n) dA (3.28)where “cs’’ stands for cross section. An illustration of this is given in Example 3.4.Still further simplification is possible if the fluid is incompressible, which we may de-fine as having density variations which are negligible in the mass-conservation re-quirement.7As we saw in Chap. 1, all liquids are nearly incompressible, and gas flowscan behave as if they were incompressible, particularly if the gas velocity is less thanabout 30 percent of the speed of sound of the gas.Again consider the fixed control volume. If the fluid is nearly incompressible, Ѩ␳/Ѩtis negligible and the volume integral in Eq. (3.21) may be neglected, after which thedensity can be slipped outside the surface integral and divided out since it is nonzero.The result is a conservation law for incompressible flows, whether steady or unsteady:͵CS(V ؒ n) dA ϭ 0 (3.29)If the inlets and outlets are one-dimensional, we haveΑi(Vi Ai) out ϭ Αi(Vi Ai)in (3.30)or Α Qout ϭ Α Qinwhere Qi ϭ Vi Ai is called the volume flow passing through the given cross section.142 Chapter 3 Integral Relations for a Control Volume7Be warned that there is subjectivity in specifying incompressibility. Oceanographers consider a 0.1percent density variation very significant, while aerodynamicists often neglect density variations in highlycompressible, even hypersonic, gas flows. Your task is to justify the incompressible approximation whenyou make it.
    • E3.3Again, if consistent units are used, Q ϭ VA will have units of cubic meters per second(SI) or cubic feet per second (BG). If the cross section is not one-dimensional, we haveto integrateQCS ϭ ͵CS(V ؒ n) dA (3.31)Equation (3.31) allows us to define an average velocity Vav which, when multiplied bythe section area, gives the correct volume flowVav ϭ ᎏQAᎏ ϭ ᎏA1ᎏ ͵ (V ؒ n) dA (3.32)This could be called the volume-average velocity. If the density varies across the sec-tion, we can define an average density in the same manner:␳av ϭ ᎏA1ᎏ ͵ ␳ dA (3.33)But the mass flow would contain the product of density and velocity, and the averageproduct (␳V)av would in general have a different value from the product of the aver-ages(␳V)av ϭ ᎏA1ᎏ ͵ ␳(V ؒ n) dA Ϸ ␳avVav (3.34)We illustrate average velocity in Example 3.4. We can often neglect the difference or,if necessary, use a correction factor between mass average and volume average.EXAMPLE 3.3Write the conservation-of-mass relation for steady flow through a streamtube (flow everywhereparallel to the walls) with a single one-dimensional exit 1 and inlet 2 (Fig. E3.3).SolutionFor steady flow Eq. (3.24) applies with the single inlet and exitm˙ ϭ ␳1A1V1 ϭ ␳2A2V2 ϭ constThus, in a streamtube in steady flow, the mass flow is constant across every section of the tube.If the density is constant, thenQ ϭ A1V1 ϭ A2V2 ϭ const or V2 ϭ ᎏAA12ᎏ V1The volume flow is constant in the tube in steady incompressible flow, and the velocity increasesas the section area decreases. This relation was derived by Leonardo da Vinci in 1500.EXAMPLE 3.4For steady viscous flow through a circular tube (Fig. E3.4), the axial velocity profile is givenapproximately by3.3 Conservation of Mass 14321Streamtubecontrol volumeV • n = 0V1V2
    • E3.4E3.5u ϭ U0΂1 Ϫ ᎏRrᎏ΃mso that u varies from zero at the wall (r ϭ R), or no slip, up to a maximum u ϭ U0 at the cen-terline r ϭ 0. For highly viscous (laminar) flow m Ϸ ᎏ12ᎏ, while for less viscous (turbulent) flowm Ϸ ᎏ17ᎏ. Compute the average velocity if the density is constant.SolutionThe average velocity is defined by Eq. (3.32). Here V ϭ iu and n ϭ i, and thus V ؒ n ϭ u. Sincethe flow is symmetric, the differential area can be taken as a circular strip dA ϭ 2 ␲r dr. Equa-tion (3.32) becomesVav ϭ ᎏA1ᎏ ͵u dA ϭ ᎏ␲1R2ᎏ ͵R0U0΂1 Ϫ ᎏRrᎏ΃m2␲r dror Vav ϭ U0 ᎏ(1 ϩ m)2(2 ϩ m)ᎏ Ans.For the laminar-flow approximation, m Ϸ ᎏ12ᎏ and Vav Ϸ 0.53U0. (The exact laminar theory in Chap.6 gives Vav ϭ 0.50U0.) For turbulent flow, m Ϸ ᎏ17ᎏ and Vav Ϸ 0.82U0. (There is no exact turbu-lent theory, and so we accept this approximation.) The turbulent velocity profile is more uniformacross the section, and thus the average velocity is only slightly less than maximum.EXAMPLE 3.5Consider the constant-density velocity fieldu ϭ ᎏVL0xᎏ ␷ ϭ 0 w ϭ ϪᎏVL0zᎏsimilar to Example 1.10. Use the triangular control volume in Fig. E3.5, bounded by (0, 0),(L, L), and (0, L), with depth b into the paper. Compute the volume flow through sections 1, 2,and 3, and compare to see whether mass is conserved.SolutionThe velocity field everywhere has the form V ϭ iu ϩ kw. This must be evaluated along eachsection. We save section 2 until last because it looks tricky. Section 1 is the plane z ϭ L withdepth b. The unit outward normal is n ϭ k, as shown. The differential area is a strip of depth bvarying with x: dA ϭ b dx. The normal velocity is(V ؒ n)1 ϭ (iu ϩ kw) ؒ k ϭ w|1 ϭ ϪᎏVL0zᎏzϭLϭ ϪV0The volume flow through section 1 is thus, from Eq. (3.31),Q1 ϭ ͵01(V ؒ n) dA ϭ ͵L0(ϪV0)b dx ϭ ϪV0bL Ans. 1144 Chapter 3 Integral Relations for a Control Volumen = k(L, L)Lzn = –i00xn = ?312CVDepth b into paperrr = Rxu(r)U0u = 0 (no slip)
    • E3.6Since this is negative, section 1 is a net inflow. Check the units: V0bL is a velocity times anarea; OK.Section 3 is the plane x ϭ 0 with depth b. The unit normal is n ϭ Ϫi, as shown, anddA ϭ b dz. The normal velocity is(V ؒ n)3 ϭ (iu ϩ kw) и (Ϫi) ϭ Ϫu|3 ϭ ϪᎏVL0xᎏsϭ0ϭ 0 Ans. 3Thus Vn ϵ 0 all along section 3; hence Q3 ϭ 0.Finally, section 2 is the plane x ϭ z with depth b. The normal direction is to the right i anddown Ϫk but must have unit value; thus n ϭ (1/͙2ෆ)(i Ϫ k). The differential area is either dA ϭ͙2ෆb dx or dA ϭ ͙2ෆb dz. The normal velocity is(V ؒ n)2 ϭ (iu ϩ kw) ؒ ᎏ͙12ෆᎏ (i Ϫ k) ϭ ᎏ͙12ෆᎏ (u Ϫ w)2ϭ ᎏ͙12ෆᎏ΄V0 ᎏLxᎏ Ϫ΂ϪV0 ᎏLzᎏ΃΅xϭzϭ ᎏ͙2ෆLV0xᎏ or ᎏ͙2ෆLV0zᎏThen the volume flow through section 2 isQ2 ϭ ͵02(V ؒ n) dA ϭ ͵L0ᎏ͙2ෆLV0xᎏ (͙2ෆb dx) ϭ V0bL Ans. 2This answer is positive, indicating an outflow. These are the desired results. We should note thatthe volume flow is zero through the front and back triangular faces of the prismatic control vol-ume because Vn ϭ ␷ ϭ 0 on those faces.The sum of the three volume flows isQ1 ϩ Q2 ϩ Q3 ϭ ϪV0bL ϩ V0bL ϩ 0 ϭ 0Mass is conserved in this constant-density flow, and there are no net sources or sinks within thecontrol volume. This is a very realistic flow, as described in Example 1.10EXAMPLE 3.6The tank in Fig. E3.6 is being filled with water by two one-dimensional inlets. Air is trapped atthe top of the tank. The water height is h. (a) Find an expression for the change in water heightdh/dt. (b) Compute dh/dt if D1 ϭ 1 in, D2 ϭ 3 in, V1 ϭ 3 ft/s, V2 ϭ 2 ft/s, and At ϭ 2 ft2, as-suming water at 20°C.SolutionA suggested control volume encircles the tank and cuts through the two inlets. The flow withinis unsteady, and Eq. (3.22) applies with no outlets and two inlets:ᎏddtᎏ΂͵0CV␳ dᐂ΃Ϫ ␳1A1V1 Ϫ ␳2A2V2 ϭ 0 (1)Now if At is the tank cross-sectional area, the unsteady term can be evaluated as follows:ᎏddtᎏ΂͵0CV␳ dᐂ΃ϭ ᎏddtᎏ (␳wAth) ϩ ᎏddtᎏ [␳aAt(H Ϫ h)] ϭ ␳wAt ᎏddhtᎏ (2)3.3 Conservation of Mass 14512Tank area AtawHhFixed CSρρPart (a)
    • Part (b)3.4 The Linear MomentumEquationThe ␳a term vanishes because it is the rate of change of air mass and is zero because the air istrapped at the top. Substituting (2) into (1), we find the change of water heightᎏddhtᎏ ϭ ᎏ␳1A1V1␳ϩwA␳t2A2V2ᎏ Ans. (a)For water, ␳1 ϭ ␳2 ϭ ␳w, and this result reduces toᎏddhtᎏ ϭ ᎏA1V1 ϩAtA2V2ᎏ ϭ ᎏQ1 ϩAtQ2ᎏ (3)The two inlet volume flows areQ1 ϭ A1V1 ϭ ᎏ14ᎏ␲(ᎏ112ᎏ ft)2(3 ft/s) ϭ 0.016 ft3/sQ2 ϭ A2V2 ϭ ᎏ14ᎏ␲(ᎏ132ᎏ ft)2(2 ft/s) ϭ 0.098 ft3/sThen, from Eq. (3),ᎏddhtᎏ ϭ ϭ 0.057 ft/s Ans. (b)Suggestion: Repeat this problem with the top of the tank open.An illustration of a mass balance with a deforming control volume has already beengiven in Example 3.2.The control-volume mass relations, Eq. (3.20) or (3.21), are fundamental to all fluid-flow analyses. They involve only velocity and density. Vector directions are of no con-sequence except to determine the normal velocity at the surface and hence whether theflow is in or out. Although your specific analysis may concern forces or moments orenergy, you must always make sure that mass is balanced as part of the analysis; oth-erwise the results will be unrealistic and probably rotten. We shall see in the exampleswhich follow how mass conservation is constantly checked in performing an analysisof other fluid properties.In Newton’s law, Eq. (3.2), the property being differentiated is the linear momentummV. Therefore our dummy variable is B ϭ mV and ␤ ϭ dB/dm ϭ V, and applicationof the Reynolds transport theorem gives the linear-momentum relation for a deformablecontrol volumeᎏddtᎏ (mV)syst ϭ ΑF ϭ ᎏddtᎏ΂͵CVV␳ dᐂ΃ϩ ͵CSV␳(Vr ؒ n) dA (3.35)The following points concerning this relation should be strongly emphasized:1. The term V is the fluid velocity relative to an inertial (nonaccelerating) coordi-nate system; otherwise Newton’s law must be modified to include noninertialrelative-acceleration terms (see the end of this section).2. The term ⌺ F is the vector sum of all forces acting on the control-volume mate-rial considered as a free body; i.e., it includes surface forces on all fluids and(0.016 ϩ 0.098) ft3/sᎏᎏᎏ2 ft2146 Chapter 3 Integral Relations for a Control Volume
    • One-Dimensional MomentumFluxNet Pressure Force on a ClosedControl Surfacesolids cut by the control surface plus all body forces (gravity and electromag-netic) acting on the masses within the control volume.3. The entire equation is a vector relation; both the integrals are vectors due to theterm V in the integrands. The equation thus has three components. If we wantonly, say, the x component, the equation reduces toΑFx ϭ ᎏddtᎏ΂͵CVu␳ dᐂ΃ϩ ͵CSu␳(Vr ؒ n) dA (3.36)and similarly, ⌺ Fy and ⌺ Fz would involve v and w, respectively. Failure to ac-count for the vector nature of the linear-momentum relation (3.35) is probably thegreatest source of student error in control-volume analyses.For a fixed control volume, the relative velocity Vr ϵ V, andΑF ϭ ᎏddtᎏ΂͵CVV␳ dᐂ΃ϩ ͵CSV␳(V ؒ n) dA (3.37)Again we stress that this is a vector relation and that V must be an inertial-frame ve-locity. Most of the momentum analyses in this text are concerned with Eq. (3.37).By analogy with the term mass flow used in Eq. (3.28), the surface integral in Eq.(3.37) is called the momentum-flux term. If we denote momentum by M, thenM˙ CS ϭ ͵0secV␳(V ؒ n) dA (3.38)Because of the dot product, the result will be negative for inlet momentum flux andpositive for outlet flux. If the cross section is one-dimensional, V and ␳ are uniformover the area and the integrated result isM˙ seci ϭ Vi(␳iVniAi) ϭ m˙ iVi (3.39)for outlet flux and Ϫm˙ iVi for inlet flux. Thus if the control volume has only one-dimensional inlets and outlets, Eq. (3.37) reduces toΑF ϭ ᎏddtᎏ΂͵CVV␳ dᐂ΃ϩ Α(m˙ iVi)out ϪΑ(m˙ iVi)in (3.40)This is a commonly used approximation in engineering analyses. It is crucial to real-ize that we are dealing with vector sums. Equation (3.40) states that the net vector forceon a fixed control volume equals the rate of change of vector momentum within thecontrol volume plus the vector sum of outlet momentum fluxes minus the vector sumof inlet fluxes.Generally speaking, the surface forces on a control volume are due to (1) forces ex-posed by cutting through solid bodies which protrude through the surface and (2) forcesdue to pressure and viscous stresses of the surrounding fluid. The computation of pres-sure force is relatively simple, as shown in Fig. 3.7. Recall from Chap. 2 that the ex-ternal pressure force on a surface is normal to the surface and inward. Since the unitvector n is defined as outward, one way to write the pressure force isFpress ϭ ͵CSp(Ϫn) dA (3.41)3.4 The Linear Momentum Equation 147
    • Fig. 3.7 Pressure-force computationby subtracting a uniform distribu-tion: (a) uniform pressure, F ϭϪpa ͵n dA ϵ 0; (b) nonuniformpressure, F ϭ Ϫ͵(p Ϫ pa)n dA.Now if the pressure has a uniform value pa all around the surface, as in Fig. 3.7a, thenet pressure force is zeroFUP ϭ͵pa(Ϫn) dA ϭ Ϫpa ͵ n dA ϵ 0 (3.42)where the subscript UP stands for uniform pressure. This result is independent of theshape of the surface8as long as the surface is closed and all our control volumes areclosed. Thus a seemingly complicated pressure-force problem can be simplified by sub-tracting any convenient uniform pressure pa and working only with the pieces of gagepressure which remain, as illustrated in Fig. 3.7b. Thus Eq. (3.41) is entirely equiva-lent toFpress ϭ ͵CS(p Ϫ pa)(Ϫn) dA ϭ͵CSpgage(Ϫn) dAThis trick can mean quite a saving in computation.EXAMPLE 3.7A control volume of a nozzle section has surface pressures of 40 lbf/in2absolute at section 1 andatmospheric pressure of 15 lbf/in2absolute at section 2 and on the external rounded part of thenozzle, as in Fig. E3.7a. Compute the net pressure force if D1 ϭ 3 in and D2 ϭ 1 in.SolutionWe do not have to bother with the outer surface if we subtract 15 lbf/in2from all surfaces.This leaves 25 lbf/in2gage at section 1 and 0 lbf/in2gage everywhere else, as in Fig. E3.7b.148 Chapter 3 Integral Relations for a Control VolumeClosedCSpanpapapapgage= p – paClosedCS pgage= 0pgagepgagepa(a) (b)npa8Can you prove this? It is a consequence of Gauss’ theorem from vector analysis.
    • E3.7Then the net pressure force is computed from section 1 onlyF ϭ pg1(Ϫn)1A1 ϭ (25 lbf/in2) ᎏ␲4ᎏ (3 in)2i ϭ 177i lbf Ans.Notice that we did not change inches to feet in this case because, with pressure in pounds-forceper square inch and area in square inches, the product gives force directly in pounds. More of-ten, though, the change back to standard units is necessary and desirable. Note: This problemcomputes pressure force only. There are probably other forces involved in Fig. E3.7, e.g.,nozzle-wall stresses in the cuts through sections 1 and 2 and the weight of the fluid within thecontrol volume.Figure E3.7 illustrates a pressure boundary condition commonly used for jet exit-flowproblems. When a fluid flow leaves a confined internal duct and exits into an ambient“atmosphere,” its free surface is exposed to that atmosphere. Therefore the jet itselfwill essentially be at atmospheric pressure also. This condition was used at section 2in Fig. E3.7.Only two effects could maintain a pressure difference between the atmosphere anda free exit jet. The first is surface tension, Eq. (1.31), which is usually negligible. Thesecond effect is a supersonic jet, which can separate itself from an atmosphere withexpansion or compression waves (Chap. 9). For the majority of applications, therefore,we shall set the pressure in an exit jet as atmospheric.EXAMPLE 3.8A fixed control volume of a streamtube in steady flow has a uniform inlet flow (␳1, A1, V1) anda uniform exit flow (␳2, A2, V2), as shown in Fig. 3.8. Find an expression for the net force onthe control volume.SolutionEquation (3.40) applies with one inlet and exitΑF ϭ m˙ 2V2 Ϫ m˙ 1V1 ϭ (␳2A2V2)V2 Ϫ (␳1A1V1)V13.4 The Linear Momentum Equation 149212140 lbf/in2abs 15 lbf/in2absJet exit pressure is atmospheric25 lbf/in2gage15 lbf/in2abs(a) (b)15 lbf/in2absFlow Flow0 lbf/in2gage0 lbf/in2gage0 lbf/in2gagePressure Condition at a Jet Exit
    • Fig. 3.8 Net force on a one-dimen-sional streamtube in steady flow:(a) streamtube in steady flow; (b)vector diagram for computing netforce.Fig. 3.9 Net applied force on afixed jet-turning vane: (a) geometryof the vane turning the water jet;(b) vector diagram for the netforce.The volume-integral term vanishes for steady flow, but from conservation of mass in Example3.3 we saw thatm˙ 1 ϭ m˙ 2 ϭ m˙ ϭ constTherefore a simple form for the desired result isΑF ϭ m˙ (V2 Ϫ V1) Ans.This is a vector relation and is sketched in Fig. 3.8b. The term ⌺ F represents the net force act-ing on the control volume due to all causes; it is needed to balance the change in momentum ofthe fluid as it turns and decelerates while passing through the control volume.EXAMPLE 3.9As shown in Fig. 3.9a, a fixed vane turns a water jet of area A through an angle ␪ without chang-ing its velocity magnitude. The flow is steady, pressure is pa everywhere, and friction on thevane is negligible. (a) Find the components Fx and Fy of the applied vane force. (b) Find ex-pressions for the force magnitude F and the angle ␾ between F and the horizontal; plot themversus ␪.SolutionThe control volume selected in Fig. 3.9a cuts through the inlet and exit of the jet and throughthe vane support, exposing the vane force F. Since there is no cut along the vane-jet interface,150 Chapter 3 Integral Relations for a Control VolumeθFixedcontrolvolumem V2ΣF = m (V2 – V1)(a) (b)12V • n = 0 V2θV1.m = constantm V1yxVV12paθCVFFθFyFxmVmVφ(a) (b)Part (a)
    • Part (b)vane friction is internally self-canceling. The pressure force is zero in the uniform atmosphere.We neglect the weight of fluid and the vane weight within the control volume. Then Eq. (3.40)reduces toFvane ϭ m˙ 2V2 Ϫ m˙ 1V1But the magnitude V1 ϭ V2 ϭ V as given, and conservation of mass for the streamtube requiresm˙ 1 ϭ m˙ 2 ϭ m˙ ϭ ␳AV. The vector diagram for force and momentum change becomes an isosce-les triangle with legs m˙V and base F, as in Fig. 3.9b. We can readily find the force componentsfrom this diagramFx ϭ m˙V(cos ␪ Ϫ 1) Fy ϭ m˙V sin ␪ Ans. (a)where m˙V ϭ ␳AV2for this case. This is the desired result.The force magnitude is obtained from part (a):F ϭ (Fx2ϩ Fy2)1/2ϭ m˙V[sin2␪ ϩ (cos ␪ Ϫ 1)2]1/2ϭ 2m˙V sin ᎏ␪2ᎏ Ans. (b)From the geometry of Fig. 3.9b we obtain␾ ϭ 180° Ϫ tanϪ1ᎏFFyxᎏ ϭ 90° ϩ ᎏ␪2ᎏ Ans. (b)3.4 The Linear Momentum Equation 151Fm VFm Vφφ0 45˚ 90˚ 135˚ 180˚180˚1.02.0θ90˚E3.9These can be plotted versus ␪ as shown in Fig. E3.9. Two special cases are of interest. First, themaximum force occurs at ␪ ϭ 180°, that is, when the jet is turned around and thrown back inthe opposite direction with its momentum completely reversed. This force is 2m˙V and acts to theleft; that is, ␾ ϭ 180°. Second, at very small turning angles (␪ Ͻ 10°) we obtain approximatelyF Ϸ m˙V␪ ␾ Ϸ 90°The force is linearly proportional to the turning angle and acts nearly normal to the jet. This isthe principle of a lifting vane, or airfoil, which causes a slight change in the oncoming flow di-rection and thereby creates a lift force normal to the basic flow.EXAMPLE 3.10A water jet of velocity Vj impinges normal to a flat plate which moves to the right at velocityVc, as shown in Fig. 3.10a. Find the force required to keep the plate moving at constant veloc-ity if the jet density is 1000 kg/m3, the jet area is 3 cm2, and Vj and Vc are 20 and 15 m/s, re-
    • Fig. 3.10 Force on a plate movingat constant velocity: (a) jet strikinga moving plate normally; (b) con-trol volume fixed relative to theplate.spectively. Neglect the weight of the jet and plate, and assume steady flow with respect to themoving plate with the jet splitting into an equal upward and downward half-jet.SolutionThe suggested control volume in Fig. 3.10a cuts through the plate support to expose the desiredforces Rx and Ry. This control volume moves at speed Vc and thus is fixed relative to the plate,as in Fig. 3.10b. We must satisfy both mass and momentum conservation for the assumed steady-flow pattern in Fig. 3.10b. There are two outlets and one inlet, and Eq. (3.30) applies for massconservationm˙ out ϭ m˙ inor ␳1A1V1 ϩ ␳2A2V2 ϭ ␳jAj(Vj Ϫ Vc) (1)We assume that the water is incompressible ␳1 ϭ ␳2 ϭ ␳j, and we are given that A1 ϭ A2 ϭ ᎏ12ᎏAj.Therefore Eq. (1) reduces toV1 ϩ V2 ϭ 2(Vj Ϫ Vc) (2)Strictly speaking, this is all that mass conservation tells us. However, from the symmetry of thejet deflection and the neglect of fluid weight, we conclude that the two velocities V1 and V2 mustbe equal, and hence (2) becomesV1 ϭ V2 ϭ Vj Ϫ Vc (3)For the given numerical values, we haveV1 ϭ V2 ϭ 20 Ϫ 15 ϭ 5 m/sNow we can compute Rx and Ry from the two components of momentum conservation. Equa-tion (3.40) applies with the unsteady term zeroΑ Fx ϭ Rx ϭ m˙1u1 ϩ m˙2u2 Ϫ m˙ juj (4)where from the mass analysis, m˙1 ϭ m˙2 ϭ ᎏ12ᎏm˙ j ϭ ᎏ12ᎏ␳jAj(Vj Ϫ Vc). Now check the flow directionsat each section: u1 ϭ u2 ϭ 0, and uj ϭ Vj Ϫ Vc ϭ 5 m/s. Thus Eq. (4) becomesRx ϭ Ϫm˙ juj ϭ Ϫ[␳jAj(Vj Ϫ Vc)](Vj Ϫ Vc) (5)152 Chapter 3 Integral Relations for a Control VolumeNozzleVjp = paVcCSVcVj – VcAj jCS1RyRxA1 = Aj122 A2 = Aj12(a) (b)
    • Fig. 3.11 Control-volume analysisof drag force on a flat plate due toboundary shear.For the given numerical values we haveRx ϭ Ϫ(1000 kg/m3)(0.0003 m2)(5 m/s)2ϭ Ϫ7.5 (kg и m)/s2ϭ Ϫ7.5 N Ans.This acts to the left; i.e., it requires a restraining force to keep the plate from accelerating to theright due to the continuous impact of the jet. The vertical force isFy ϭ Ry ϭ m˙1␷1 ϩ m˙2␷2 Ϫ m˙ j␷jCheck directions again: ␷1 ϭ V1, ␷2 ϭ ϪV2, ␷j ϭ 0. ThusRy ϭ m˙1(V1) ϩ m˙2(ϪV2) ϭ ᎏ12ᎏm˙ j(V1 Ϫ V2) (6)But since we found earlier that V1 ϭ V2, this means that Ry ϭ 0, as we could expect from thesymmetry of the jet deflection.9Two other results are of interest. First, the relative velocity atsection 1 was found to be 5 m/s up, from Eq. (3). If we convert this to absolute motion by addingon the control-volume speed Vc ϭ 15 m/s to the right, we find that the absolute velocity V1 ϭ15i ϩ 5j m/s, or 15.8 m/s at an angle of 18.4° upward, as indicated in Fig. 3.10a. Thus the ab-solute jet speed changes after hitting the plate. Second, the computed force Rx does not changeif we assume the jet deflects in all radial directions along the plate surface rather than just upand down. Since the plate is normal to the x axis, there would still be zero outlet x-momentumflux when Eq. (4) was rewritten for a radial-deflection condition.EXAMPLE 3.11The previous example treated a plate at normal incidence to an oncoming flow. In Fig. 3.11 theplate is parallel to the flow. The stream is not a jet but a broad river, or free stream, of uniformvelocity V ϭ U0i. The pressure is assumed uniform, and so it has no net force on the plate. Theplate does not block the flow as in Fig. 3.10, so that the only effect is due to boundary shear,which was neglected in the previous example. The no-slip condition at the wall brings the fluidthere to a halt, and these slowly moving particles retard their neighbors above, so that at the endof the plate there is a significant retarded shear layer, or boundary layer, of thickness y ϭ ␦. The3.4 The Linear Momentum Equation 1539Symmetry can be a powerful tool if used properly. Try to learn more about the uses and misuses ofsymmetry conditions. Here we doggedly computed the results without invoking symmetry.yU0Oncomingstreamparallelto plate01y = hStreamline justoutside theshear-layer region2Plate of width b4Boundary layerwhere shear stressis significantp = pay = δ3U0u(y)xL
    • viscous stresses along the wall can sum to a finite drag force on the plate. These effects are il-lustrated in Fig. 3.11. The problem is to make an integral analysis and find the drag force D interms of the flow properties ␳, U0, and ␦ and the plate dimensions L and b.†SolutionLike most practical cases, this problem requires a combined mass and momentum balance. Aproper selection of control volume is essential, and we select the four-sided region from 0 to hto ␦ to L and back to the origin 0, as shown in Fig. 3.11. Had we chosen to cut across horizon-tally from left to right along the height y ϭ h, we would have cut through the shear layer andexposed unknown shear stresses. Instead we follow the streamline passing through (x, y) ϭ(0, h), which is outside the shear layer and also has no mass flow across it. The four control-volume sides are thus1. From (0, 0) to (0, h): a one-dimensional inlet, V ؒ n ϭ ϪU02. From (0, h) to (L, ␦): a streamline, no shear, V ؒ n ϵ 03. From (L, ␦) to (L, 0): a two-dimensional outlet, V ؒ n ϭ ϩu(y)4. From (L, 0) to (0, 0): a streamline just above the plate surface, V ؒ n ϭ 0, shear forcessumming to the drag force ϪDi acting from the plate onto the retarded fluidThe pressure is uniform, and so there is no net pressure force. Since the flow is assumed in-compressible and steady, Eq. (3.37) applies with no unsteady term and fluxes only across sec-tions 1 and 3:ΑFx ϭ ϪD ϭ ␳ ͵01u(V ؒ n) dA ϩ ␳͵03u(V ؒ n) dAϭ ␳ ͵h0U0(ϪU0)b dy ϩ ␳͵␦0u(ϩu)b dyEvaluating the first integral and rearranging giveD ϭ ␳U02bh Ϫ ␳b͵␦0u2dy (1)This could be considered the answer to the problem, but it is not useful because the height h isnot known with respect to the shear-layer thickness ␦. This is found by applying mass conser-vation, since the control volume forms a streamtube␳ ͵0CS(V ؒ n) dA ϭ 0 ϭ ␳͵h0(ϪU0)b dy ϩ ␳͵␦0ub dyor U0h ϭ͵␦0u dy (2)after canceling b and ␳ and evaluating the first integral. Introduce this value of h into Eq. (1) fora much cleaner resultD ϭ ␳b͵␦0u(U0 Ϫ u) dyxϭLAns. (3)This result was first derived by Theodore von Kármán in 1921.10It relates the friction drag on154 Chapter 3 Integral Relations for a Control Volume†The general analysis of such wall-shear problems, called boundary-layer theory, is treated in Sec. 7.3.10The autobiography of this great twentieth-century engineer and teacher [2] is recommended for itshistorical and scientific insight.
    • Momentum-Flux CorrectionFactorone side of a flat plate to the integral of the momentum defect u(U0 Ϫ u) across the trailing crosssection of the flow past the plate. Since U0 Ϫ u vanishes as y increases, the integral has a finitevalue. Equation (3) is an example of momentum-integral theory for boundary layers, which istreated in Chap. 7. To illustrate the magnitude of this drag force, we can use a simple parabolicapproximation for the outlet-velocity profile u(y) which simulates low-speed, or laminar, shearflowu Ϸ U0΂ᎏ2␦yᎏ Ϫ ᎏ␦y22ᎏ΃ for 0 Յ y Յ ␦ (4)Substituting into Eq. (3) and letting ␩ ϭ y/␦ for convenience, we obtainD ϭ ␳bU02␦ ͵10(2␩ Ϫ ␩2)(1 Ϫ 2␩ ϩ␩2) d␩ ϭ ᎏ125ᎏ␳U02b␦ (5)This is within 1 percent of the accepted result from laminar boundary-layer theory (Chap. 7) inspite of the crudeness of the Eq. (4) approximation. This is a happy situation and has led to thewide use of Kármán’s integral theory in the analysis of viscous flows. Note that D increases withthe shear-layer thickness ␦, which itself increases with plate length and the viscosity of the fluid(see Sec. 7.4).For flow in a duct, the axial velocity is usually nonuniform, as in Example 3.4. Forthis case the simple momentum-flux calculation ͵u␳(V ؒ n) dA ϭ m˙ V ϭ ␳AV2is some-what in error and should be corrected to ␤␳AV2, where ␤ is the dimensionlessmomentum-flux correction factor, ␤ Ն 1.The factor ␤ accounts for the variation of u2across the duct section. That is, we com-pute the exact flux and set it equal to a flux based on average velocity in the duct␳͵ u2dA ϭ ␤m˙ Vav ϭ ␤␳AVav2or ␤ ϭ ᎏA1ᎏ͵ ΂ᎏVuavᎏ΃2dA (3.43a)Values of ␤ can be computed based on typical duct velocity profiles similar to thosein Example 3.4. The results are as follows:Laminar flow: u ϭ U0΂1 Ϫ ᎏRr22ᎏ΃ ␤ ϭ ᎏ43ᎏ (3.43b)Turbulent flow: u Ϸ U0΂1 Ϫ ᎏRrᎏ΃mᎏ19ᎏ Յ m Յ ᎏ15ᎏ␤ ϭ (3.43c)The turbulent correction factors have the following range of values:Turbulent flow:(1 ϩ m)2(2 ϩ m)2ᎏᎏᎏ2(1 ϩ 2m)(2 ϩ 2m)3.4 The Linear Momentum Equation 155m ᎏ15ᎏ ᎏ16ᎏ ᎏ17ᎏ ᎏ18ᎏ ᎏ19ᎏ␤ 1.037 1.027 1.020 1.016 1.013
    • Noninertial Reference Frame11These are so close to unity that they are normally neglected. The laminar correctionmay sometimes be important.To illustrate a typical use of these correction factors, the solution to Example 3.8for nonuniform velocities at sections 1 and 2 would be given asΑF ϭ m˙ (␤2V2 Ϫ ␤1V1) (3.43d)Note that the basic parameters and vector character of the result are not changed at allby this correction.All previous derivations and examples in this section have assumed that the coordinatesystem is inertial, i.e., at rest or moving at constant velocity. In this case the rate ofchange of velocity equals the absolute acceleration of the system, and Newton’s lawapplies directly in the form of Eqs. (3.2) and (3.35).In many cases it is convenient to use a noninertial, or accelerating, coordinate sys-tem. An example would be coordinates fixed to a rocket during takeoff. A second ex-ample is any flow on the earth’s surface, which is accelerating relative to the fixed starsbecause of the rotation of the earth. Atmospheric and oceanographic flows experiencethe so-called Coriolis acceleration, outlined below. It is typically less than 10Ϫ5g, whereg is the acceleration of gravity, but its accumulated effect over distances of many kilo-meters can be dominant in geophysical flows. By contrast, the Coriolis acceleration isnegligible in small-scale problems like pipe or airfoil flows.Suppose that the fluid flow has velocity V relative to a noninertial xyz coordinatesystem, as shown in Fig. 3.12. Then dV/dt will represent a noninertial accelerationwhich must be added vectorially to a relative acceleration arel to give the absolute ac-celeration ai relative to some inertial coordinate system XYZ, as in Fig. 3.12. Thusai ϭ ᎏddVtᎏ ϩ arel (3.44)156 Chapter 3 Integral Relations for a Control Volume11This section may be omitted without loss of continuity.Fig. 3.12 Geometry of fixed versusaccelerating coordinates.ParticleVrel =drdtyxzXZYRNoninertial, moving,rotating coordinatesInertialcoordinatesr
    • Since Newton’s law applies to the absolute acceleration,ΑF ϭ mai ϭ m΂ᎏddVtᎏ ϩ arel΃or ΑF Ϫ marel ϭ m ᎏddVtᎏ (3.45)Thus Newton’s law in noninertial coordinates xyz is equivalent to adding more “force”terms Ϫmarel to account for noninertial effects. In the most general case, sketched inFig. 3.12, the term arel contains four parts, three of which account for the angular ve-locity ⍀(t) of the inertial coordinates. By inspection of Fig. 3.12, the absolute dis-placement of a particle isSi ϭ r ϩ R (3.46)Differentiation gives the absolute velocityVi ϭ V ϩ ᎏddRtᎏ ϩ⍀ ؋ r (3.47)A second differentiation gives the absolute acceleration:ai ϭ ᎏddVtᎏ ϩ ᎏdd2tR2ᎏ ϩ ᎏdd⍀tᎏ ؋ r ϩ 2⍀ ؋ V ϩ ⍀ ؋ (⍀ ؋ r) (3.48)By comparison with Eq. (3.44), we see that the last four terms on the right representthe additional relative acceleration:1. d2R/dt2is the acceleration of the noninertial origin of coordinates xyz.2. (d⍀/dt) ؋ r is the angular-acceleration effect.3. 2⍀ ؋ V is the Coriolis acceleration.4. ⍀ ؋ (⍀ ؋ r) is the centripetal acceleration, directed from the particle normal tothe axis of rotation with magnitude ⍀2L, where L is the normal distance to theaxis.12Equation (3.45) differs from Eq. (3.2) only in the added inertial forces on the left-hand side. Thus the control-volume formulation of linear momentum in noninertial co-ordinates merely adds inertial terms by integrating the added relative acceleration overeach differential mass in the control volumeΑF Ϫ͵0CVarel dm ϭ ᎏddtᎏ΂͵0CVV␳ dᐂ΃ϩ ͵0CSV␳(Vr ؒ n) dA (3.49)where arel ϭ ᎏdd2tR2ᎏ ϩ ᎏdd⍀tᎏ ؋ r ϩ 2⍀ ؋ V ϩ ⍀ ؋ (⍀ ؋ r)This is the noninertial equivalent to the inertial form given in Eq. (3.35). To analyzesuch problems, one must have knowledge of the displacement R and angular velocity⍀ of the noninertial coordinates.If the control volume is nondeformable, Eq. (3.49) reduces to3.4 The Linear Momentum Equation 15712A complete discussion of these noninertial coordinate terms is given, e.g., in Ref. 4, pp. 49 – 51.
    • E3.12ΑF Ϫ͵0CVarel dm ϭ ᎏddtᎏ΂͵0CVV␳ dᐂ΃ϩ͵CSV␳(V ؒ n) dA (3.50)In other words, the right-hand side reduces to that of Eq. (3.37).EXAMPLE 3.12A classic example of an accelerating control volume is a rocket moving straight up, as in Fig.E3.12. Let the initial mass be M0, and assume a steady exhaust mass flow m˙ and exhaust ve-locity Ve relative to the rocket, as shown. If the flow pattern within the rocket motor is steadyand air drag is neglected, derive the differential equation of vertical rocket motion V(t) and in-tegrate using the initial condition V ϭ 0 at t ϭ 0.SolutionThe appropriate control volume in Fig. E3.12 encloses the rocket, cuts through the exit jet, andaccelerates upward at rocket speed V(t). The z-momentum equation (3.49) becomesΑFz Ϫ ͵arel dm ϭ ᎏddtᎏ΂͵CVw dm˙΃ϩ (m˙ w)eor Ϫmg Ϫ m ᎏddVtᎏ ϭ 0 ϩ m˙ Ve with m ϭ m(t) ϭ M0 Ϫ m˙ tThe term arel ϭ dV/dt of the rocket. The control volume integral vanishes because of the steadyrocket-flow conditions. Separate the variables and integrate, assuming V ϭ 0 at t ϭ 0:͵V0dV ϭ m˙ Ve ͵t0ᎏM0dϪtm˙ tᎏ Ϫ g ͵t0dt or V(t) ϭ ϪVeln΂1 Ϫ ᎏMm˙0tᎏ΃Ϫ gt Ans.This is a classic approximate formula in rocket dynamics. The first term is positive and, if thefuel mass burned is a large fraction of initial mass, the final rocket velocity can exceed Ve.A control-volume analysis can be applied to the angular-momentum relation, Eq. (3.3),by letting our dummy variable B be the angular-momentum vector H. However, sincethe system considered here is typically a group of nonrigid fluid particles of variable ve-locity, the concept of mass moment of inertia is of no help and we have to calculate theinstantaneous angular momentum by integration over the elemental masses dm. If O isthe point about which moments are desired, the angular momentum about O is given byHO ϭ͵syst(r ؋ V) dm (3.51)where r is the position vector from 0 to the elemental mass dm and V is the velocityof that element. The amount of angular momentum per unit mass is thus seen to be␤ ϭ ᎏddHmOᎏ ϭ r ؋ V158 Chapter 3 Integral Relations for a Control VolumeAcceleratingcontrol volumeGmDatumV(t)V(t)Vezg3.5 The Angular-MomentumTheorem1313This section may be omitted without loss of continuity.m˙
    • The Reynolds transport theorem (3.16) then tells us thatᎏdHdtOᎏsystϭ ᎏddtᎏ΄͵CV(r ؋ V)␳ dᐂ΅ϩ͵CS(r ؋ V)␳(Vr ؒ n) dA (3.52)for the most general case of a deformable control volume. But from the angular-momentum theorem (3.3), this must equal the sum of all the moments about point Oapplied to the control volumeᎏdHdtOᎏ ϭ ΑMO ϭ Α(r ؋ F)ONote that the total moment equals the summation of moments of all applied forcesabout point O. Recall, however, that this law, like Newton’s law (3.2), assumes that theparticle velocity V is relative to an inertial coordinate system. If not, the momentsabout point O of the relative acceleration terms arel in Eq. (3.49) must also be includedΑMO ϭ Α(r ؋ F)O Ϫ͵CV(r ؋ arel) dm (3.53)where the four terms constituting arel are given in Eq. (3.49). Thus the most generalcase of the angular-momentum theorem is for a deformable control volume associatedwith a noninertial coordinate system. We combine Eqs. (3.52) and (3.53) to obtainΑ(r ؋ F)0 Ϫ͵CV(r ؋ arel) dm ϭ ᎏddtᎏ΄͵CV(r ؋ V)␳ dᐂ΅ϩ͵CS(r ؋ V)␳(Vr ؒ n) dA(3.54)For a nondeformable inertial control volume, this reduces toΑM0 ϭ ᎏѨѨtᎏ΄͵CV(r ؋ V)␳ dᐂ΅ϩ ͵CS(r ؋ V)␳(V ؒ n) dA (3.55)Further, if there are only one-dimensional inlets and exits, the angular-momentum fluxterms evaluated on the control surface become͵CS(r ؋ V)␳(V ؒ n) dA ϭ Α(r ؋ V)out m˙ out ϪΑ(r ؋ V)in m˙ in (3.56)Although at this stage the angular-momentum theorem can be considered to be a sup-plementary topic, it has direct application to many important fluid-flow problems in-volving torques or moments. A particularly important case is the analysis of rotatingfluid-flow devices, usually called turbomachines (Chap. 11).EXAMPLE 3.13As shown in Fig. E3.13a, a pipe bend is supported at point A and connected to a flow systemby flexible couplings at sections 1 and 2. The fluid is incompressible, and ambient pressure pais zero. (a) Find an expression for the torque T which must be resisted by the support at A, interms of the flow properties at sections 1 and 2 and the distances h1 and h2. (b) Compute thistorque if D1 ϭ D2 ϭ 3 in, p1 ϭ 100 lbf/in2gage, p2 ϭ 80 lbf/in2gage, V1 ϭ 40 ft/s, h1 ϭ 2 in,h2 ϭ 10 in, and ␳ ϭ 1.94 slugs/ft3.3.5 The Angular-Momentum Theorem 159
    • E3.13aSolutionThe control volume chosen in Fig. E3.13b cuts through sections 1 and 2 and through the sup-port at A, where the torque TA is desired. The flexible-couplings description specifies that thereis no torque at either section 1 or 2, and so the cuts there expose no moments. For the angular-momentum terms r ؋ V, r should be taken from point A to sections 1 and 2. Note that the gagepressure forces p1A1 and p2A2 both have moments about A. Equation (3.55) with one-dimen-sional flux terms becomesΑMA ϭ TA ϩ r1 ؋ (Ϫp1A1n1) ϩ r2 ؋ (Ϫp2A2n2)ϭ (r2 ؋ V2)(ϩm˙ out) ϩ (r1 ؋ V1)(Ϫm˙ in) (1)Figure E3.13c shows that all the cross products are associated either with r1 sin ␪1 ϭ h1 orr2 sin ␪2 ϭ h2, the perpendicular distances from point A to the pipe axes at 1 and 2. Rememberthat m˙ in ϭ m˙ out from the steady-flow continuity relation. In terms of counterclockwise moments,Eq. (1) then becomesTA ϩ p1A1h1 Ϫ p2A2h2 ϭ m˙ (h2V2 Ϫ h1V1) (2)Rewriting this, we find the desired torque to beTA ϭ h2(p2A2 ϩ m˙ V2) Ϫ h1(p1A1 ϩ m˙ V1) Ans. (a) (3)160 Chapter 3 Integral Relations for a Control Volumep1, V1, A11pa = 0= constantV2 , A2, p2h2h1A2ρPart (a)CVV2TAV1Ar1r2p1A1p2A2r1V1= h1V1r2V2= h2V2θθ21r2r1V2V1h2= r2sinh1= r1sinθθ21E3.13b E3.13c
    • Part (b)counterclockwise. The quantities p1 and p2 are gage pressures. Note that this result is indepen-dent of the shape of the pipe bend and varies only with the properties at sections 1 and 2 andthe distances h1 and h2.†The inlet and exit areas are the same:A1 ϭ A2 ϭ ᎏ␲4ᎏ (3)2ϭ 7.07 in2ϭ 0.0491 ft2Since the density is constant, we conclude from continuity that V2 ϭ V1 ϭ 40 ft/s. The massflow ism˙ ϭ ␳A1V1 ϭ 1.94(0.0491)(40) ϭ 3.81 slug/sEquation (3) can be evaluated asTA ϭ (ᎏ1102ᎏ ft)[80(7.07) lbf ϩ 3.81(40) lbf] Ϫ (ᎏ122ᎏ ft)[100(7.07) lbf ϩ 3.81(40) lbf]ϭ 598 Ϫ 143 ϭ 455 ft и lbf counterclockwise Ans. (b)We got a little daring there and multiplied p in lbf/in2gage times A in in2to get lbf withoutchanging units to lbf/ft2and ft2.EXAMPLE 3.14Figure 3.13 shows a schematic of a centrifugal pump. The fluid enters axially and passes throughthe pump blades, which rotate at angular velocity ␻; the velocity of the fluid is changed fromV1 to V2 and its pressure from p1 to p2. (a) Find an expression for the torque TO which must beapplied to these blades to maintain this flow. (b) The power supplied to the pump would be P ϭ␻TO. To illustrate numerically, suppose r1 ϭ 0.2 m, r2 ϭ 0.5 m, and b ϭ 0.15 m. Let the pumprotate at 600 r/min and deliver water at 2.5 m3/s with a density of 1000 kg/m3. Compute the ide-alized torque and power supplied.SolutionThe control volume is chosen to be the angular region between sections 1 and 2 where the flowpasses through the pump blades (see Fig. 3.13). The flow is steady and assumed incompress-ible. The contribution of pressure to the torque about axis O is zero since the pressure forces at1 and 2 act radially through O. Equation (3.55) becomesΑMO ϭ TO ϭ (r2 ؋ V2)m˙ out Ϫ (r1 ؋ V1)m˙ in (1)where steady-flow continuity tells us thatm˙ in ϭ ␳Vn12␲r1b ϭ m˙ out ϭ ␳Vn2␲r2b ϭ ␳QThe cross product r ؋ V is found to be clockwise about O at both sections:r2 ؋ V2 ϭ r2Vt2 sin 90° k ϭ r2Vt2k clockwiser1 ؋ V1 ϭ r1Vt1k clockwiseEquation (1) thus becomes the desired formula for torqueTO ϭ ␳Q(r2Vt2 Ϫ r1Vt1)k clockwise Ans. (a) (2a)3.5 The Angular-Momentum Theorem 161†Indirectly, the pipe-bend shape probably affects the pressure change from p1 to p2.Part (a)
    • Fig. 3.13 Schematic of a simplifiedcentrifugal pump.This relation is called Euler’s turbine formula. In an idealized pump, the inlet and outlet tan-gential velocities would match the blade rotational speeds Vt1 ϭ ␻r1 and Vt2 ϭ ␻r2. Then theformula for torque supplied becomesTO ϭ ␳Q␻(r22Ϫ r12) clockwise (2b)Convert ␻ to 600(2␲/60) ϭ 62.8 rad/s. The normal velocities are not needed here but followfrom the flow rateVn1 ϭ ᎏ2␲Qr1bᎏ ϭ ᎏ2␲(0.22.5mm)(30/.s15 m)ᎏ ϭ 13.3 m/sVn2 ϭ ᎏ2␲Qr2bᎏ ϭ ᎏ2␲(0.25.)5(0.15)ᎏ ϭ 5.3 m/sFor the idealized inlet and outlet, tangential velocity equals tip speedVt1 ϭ ␻r1 ϭ (62.8 rad/s)(0.2 m) ϭ 12.6 m/sVt2 ϭ ␻r2 ϭ 62.8(0.5) ϭ 31.4 m/sEquation (2a) predicts the required torque to beTO ϭ (1000 kg/m3)(2.5 m3/s)[(0.5 m)(31.4 m/s) Ϫ (0.2 m)(12.6 m/s)]ϭ 33,000 (kg и m2)/s2ϭ 33,000 N и m Ans.The power required isP ϭ ␻TO ϭ (62.8 rad/s)(33,000 N и m) ϭ 2,070,000 (N и m)/sϭ 2.07 MW (2780 hp) Ans.In actual practice the tangential velocities are considerably less than the impeller-tip speeds, andthe design power requirements for this pump may be only 1 MW or less.162 Chapter 3 Integral Relations for a Control Volume12ωBladePumpbladeshapeWidth bCVInflowz,kr1r2rOBladeVn2Vt2Vn1Vt1Part (b)
    • Fig. 3.14 View from above of asingle arm of a rotating lawnsprinkler.EXAMPLE 3.15Figure 3.14 shows a lawn-sprinkler arm viewed from above. The arm rotates about O at con-stant angular velocity ␻. The volume flux entering the arm at O is Q, and the fluid is incom-pressible. There is a retarding torque at O, due to bearing friction, of amount ϪTOk. Find an ex-pression for the rotation ␻ in terms of the arm and flow properties.SolutionThe entering velocity is V0k, where V0 ϭ Q/Apipe. Equation (3.55) applies to the control volumesketched in Fig. 3.14 only if V is the absolute velocity relative to an inertial frame. Thus the exitvelocity at section 2 isV2 ϭ V0i Ϫ R␻iEquation (3.55) then predicts that, for steady flow,Α MO ϭ Ϫ TOk ϭ (r2 ؋ V2)m˙ out Ϫ (r1 ؋ V1)m˙ in (1)where, from continuity, m˙ out ϭ m˙ in ϭ ␳Q. The cross products with reference to point O arer2 ؋ V2 ϭ Rj ؋ (V0 Ϫ R␻)i ϭ (R2␻ Ϫ RV0)kr1 ؋ V1 ϭ 0j ؋ V0k ϭ 0Equation (1) thus becomesϪTOk ϭ ␳Q(R2␻ Ϫ RV0)k␻ ϭ ᎏVROᎏ Ϫ ᎏ␳QTOR2ᎏ Ans.The result may surprise you: Even if the retarding torque TO is negligible, the arm rotationalspeed is limited to the value V0/R imposed by the outlet speed and the arm length.As our fourth and final basic law, we apply the Reynolds transport theorem (3.12) tothe first law of thermodynamics, Eq. (3.5). The dummy variable B becomes energy E,and the energy per unit mass is ␤ ϭ dE/dm ϭ e. Equation (3.5) can then be written fora fixed control volume as follows:15ᎏddQtᎏ Ϫ ᎏddWtᎏ ϭ ᎏddEtᎏ ϭ ᎏddtᎏ΂͵CVe␳ dᐂ΃ϩ ͵CSe␳(V ؒ n) dA (3.57)Recall that positive Q denotes heat added to the system and positive W denotes workdone by the system.The system energy per unit mass e may be of several types:e ϭ einternal ϩ ekinetic ϩ epotential ϩ eother3.6 The Energy Equation 163yxRω2 Absolute outletvelocityV2 = V0i – RωiCVRetardingtorque T0OV0 =QApipekInlet velocity3.6 The Energy Equation1414This section should be read for information and enrichment even if you lack formal background inthermodynamics.15The energy equation for a deformable control volume is rather complicated and is not discussedhere. See Refs. 4 and 5 for further details.
    • where eother could encompass chemical reactions, nuclear reactions, and electrostaticor magnetic field effects. We neglect eother here and consider only the first three termsas discussed in Eq. (1.9), with z defined as “up”:e ϭ û ϩ ᎏ12ᎏV2ϩ gz (3.58)The heat and work terms could be examined in detail. If this were a heat-transferbook, dQ/dT would be broken down into conduction, convection, and radiation effectsand whole chapters written on each (see, e.g., Ref. 3). Here we leave the term un-touched and consider it only occasionally.Using for convenience the overdot to denote the time derivative, we divide the workterm into three parts:W˙ ϭ W˙ shaft ϩ W˙ press ϩ W˙ viscous stresses ϭ W˙ s ϩ W˙ p ϩ W˙ ␷The work of gravitational forces has already been included as potential energy in Eq.(3.58). Other types of work, e.g., those due to electromagnetic forces, are excludedhere.The shaft work isolates that portion of the work which is deliberately done by amachine (pump impeller, fan blade, piston, etc.) protruding through the control sur-face into the control volume. No further specification other than W˙ s is desired atthis point, but calculations of the work done by turbomachines will be performedin Chap. 11.The rate of work W˙ p done on pressure forces occurs at the surface only; all workon internal portions of the material in the control volume is by equal and oppositeforces and is self-canceling. The pressure work equals the pressure force on a smallsurface element dA times the normal velocity component into the control volumedW˙ p ϭ Ϫ(p dA)Vn,in ϭ Ϫp(ϪV ؒ n) dAThe total pressure work is the integral over the control surfaceW˙ p ϭ ͵CSp(V ؒ n) dA (3.59)A cautionary remark: If part of the control surface is the surface of a machine part, weprefer to delegate that portion of the pressure to the shaft work term W˙s, not to W˙p,which is primarily meant to isolate the fluid-flow pressure-work terms.Finally, the shear work due to viscous stresses occurs at the control surface, the in-ternal work terms again being self-canceling, and consists of the product of each vis-cous stress (one normal and two tangential) and the respective velocity componentdW˙ ␷ ϭ Ϫ␶ ؒ V dAor W˙ ␷ ϭ Ϫ ͵CS␶ ؒ V dA (3.60)where ␶ is the stress vector on the elemental surface dA. This term may vanish or benegligible according to the particular type of surface at that part of the control volume:Solid surface. For all parts of the control surface which are solid confining walls,V ϭ 0 from the viscous no-slip condition; hence W˙␷ ϭ zero identically.164 Chapter 3 Integral Relations for a Control Volume
    • One-Dimensional Energy-FluxTermsSurface of a machine. Here the viscous work is contributed by the machine, andso we absorb this work in the term W˙ s.An inlet or outlet. At an inlet or outlet, the flow is approximately normal to theelement dA; hence the only viscous-work term comes from the normal stress␶nnVn dA. Since viscous normal stresses are extremely small in all but rarecases, e.g., the interior of a shock wave, it is customary to neglect viscouswork at inlets and outlets of the control volume.Streamline surface. If the control surface is a streamline such as the upper curvein the boundary-layer analysis of Fig. 3.11, the viscous-work term must beevaluated and retained if shear stresses are significant along this line. In theparticular case of Fig. 3.11, the streamline is outside the boundary layer, andviscous work is negligible.The net result of the above discussion is that the rate-of-work term in Eq. (3.57)consists essentially ofW˙ ϭ W˙ s ϩ͵CSp(V и n) dA Ϫ ͵CS(␶ ؒ V)SS dA (3.61)where the subscript SS stands for stream surface. When we introduce (3.61) and (3.58)into (3.57), we find that the pressure-work term can be combined with the energy-fluxterm since both involve surface integrals of V ؒ n. The control-volume energy equationthus becomesQ˙ Ϫ W˙ s Ϫ (W˙ υ)SS ϭ ᎏѨѨtᎏ΂͵CVep dᐂ΃ϩ ͵CS(e ϩ ᎏp␳ᎏ)␳(V ؒ n) dA (3.62)Using e from (3.58), we see that the enthalpy hˆ ϭ û ϩ p/␳ occurs in the control-sur-face integral. The final general form for the energy equation for a fixed control vol-ume becomesQ˙ Ϫ W˙ s Ϫ W˙ υ ϭ ᎏѨѨtᎏ ΄͵CV ΂û ϩ ᎏ12ᎏ V2ϩ gz΃␳ dᐂ΅ϩ ͵CS ΂hˆ ϩ ᎏ12ᎏ V2ϩ gz΃␳(V ؒ n) dA(3.63)As mentioned above, the shear-work term W˙␷ is rarely important.If the control volume has a series of one-dimensional inlets and outlets, as in Fig.3.6, the surface integral in (3.63) reduces to a summation of outlet fluxes minus in-let fluxes͵CS(hˆ ϩ ᎏ12ᎏV2ϩ gz)␳(V ؒ n) dAϭΑ(hˆ ϩ ᎏ12ᎏV2ϩ gz)outm˙ out ϪΑ(hˆ ϩ ᎏ12ᎏV2ϩ gz)inm˙ in (3.64)where the values of hˆ, ᎏ12ᎏV2, and gz are taken to be averages over each cross section.3.6 The Energy Equation 165
    • EXAMPLE 3.16A steady-flow machine (Fig. E3.16) takes in air at section 1 and discharges it at sections 2 and3. The properties at each section are as follows:Section A, ft2Q, ft3/s T, °F p, lbf/in2abs z, ft1 0.4 100 70 20 1.02 1.0 40 100 30 4.03 0.25 50 200 ? 1.5Work is provided to the machine at the rate of 150 hp. Find the pressure p3 in lbf/in2absoluteand the heat transfer Q˙ in Btu/s. Assume that air is a perfect gas with R ϭ 1715 and cp ϭ 6003ft и lbf/(slug и °R).SolutionThe control volume chosen cuts across the three desired sections and otherwise follows the solidwalls of the machine. Therefore the shear-work term W␷ is negligible. We have enough infor-mation to compute Vi ϭ Qi /Ai immediatelyV1 ϭ ᎏ100.40ᎏ ϭ 250 ft/s V2 ϭ ᎏ14.00ᎏ ϭ 40 ft/s V3 ϭ ᎏ05.205ᎏ ϭ 200 ft/sand the densities ␳i ϭ pi/(RTi)␳1 ϭ ᎏ17152(07(014ϩ4)460)ᎏ ϭ 0.00317 slug/ft3␳2 ϭ ᎏ13701(51(4546)0)ᎏ ϭ 0.00450 slug/ft3but ␳3 is determined from the steady-flow continuity relation:m˙ 1 ϭ m˙ 2 ϩ m˙ 3␳1Q1 ϭ ␳2Q2 ϩ ␳3Q3 (1)0.00317(100) ϭ 0.00450(40) ϩ ␳3(50)or 50␳3 ϭ 0.317 Ϫ 0.180 ϭ 0.137 slug/s␳3 ϭ ᎏ0.51037ᎏ ϭ 0.00274 slug/ft3ϭ ᎏ1711454(6p630)ᎏp3 ϭ 21.5 lbf/in2absolute Ans.Note that the volume flux Q1 Q2 ϩ Q3 because of the density changes.For steady flow, the volume integral in (3.63) vanishes, and we have agreed to neglect vis-cous work. With one inlet and two outlets, we obtainQ˙ Ϫ W˙ s ϭ Ϫm˙ 1(hˆ1 ϩ ᎏ12ᎏV12ϩ gz1) ϩ m˙ 2(hˆ2 ϩ ᎏ12ᎏV22ϩ gz2) ϩ m˙ 3(hˆ3 ϩ ᎏ12ᎏV32ϩ gz3) (2)where W˙ s is given in hp and can be quickly converted to consistent BG units:W˙ s ϭ Ϫ150 hp [550 ft и lbf/(s и hp)]ϭ Ϫ82,500 ft и lbf/s negative work on system166 Chapter 3 Integral Relations for a Control Volume(1) (3)(2)Q = ?150 hpCVE3.16
    • The Steady-Flow Energy EquationFor a perfect gas with constant cp, hˆ ϭ cpT plus an arbitrary constant. It is instructive to sepa-rate the flux terms in Eq. (2) above to examine their magnitudes:Enthalpy flux:cp(Ϫm˙ 1T1 ϩ m˙ 2T2 ϩ m˙ 3T3) ϭ [6003 ft и lbf/(slug и °R)][(Ϫ0.317 slug/s)(530 °R)ϩ 0.180(560) ϩ 0.137(660)]ϭ Ϫ1,009,000 ϩ 605,000 ϩ 543,000ϭ ϩ139,000 ft и lbf/sKinetic-energy flux:Ϫm˙ 1(ᎏ12ᎏV12) ϩ m˙ 2(ᎏ12ᎏV22) ϩ m˙ 3(ᎏ12ᎏV32) ϭ ᎏ12ᎏ[Ϫ0.317(250)2ϩ 0.180(40)2ϩ 0.137(200)2]ϭ Ϫ9900 ϩ 150 ϩ 2750 ϭ Ϫ7000 ft и lbf/sPotential-energy flux:g(Ϫm˙ 1z1 ϩ m˙ 2z2 ϩ m˙ 3z3) ϭ 32.2[Ϫ0.317(1.0) ϩ 0.180(4.0) ϩ 0.137(1.5)]ϭ Ϫ10 ϩ 23 ϩ 7 ϭ ϩ20 ft и lbf/sThese are typical effects: The potential-energy flux is negligible in gas flows, the kinetic-energyflux is small in low-speed flows, and the enthalpy flux is dominant. It is only when we neglectheat-transfer effects that the kinetic and potential energies become important. Anyway, we cannow solve for the heat fluxQ˙ ϭ Ϫ82,500 ϩ 139,000 Ϫ 7000 ϩ 20 ϭ 49,520 ft и lbf/s (3)Converting, we getQ˙ ϭ ᎏ778.249ft,5и2lb0f/Btuᎏ ϭ ϩ63.6 Btu/s Ans.For steady flow with one inlet and one outlet, both assumed one-dimensional, Eq. (3.63)reduces to a celebrated relation used in many engineering analyses. Let section 1 bethe inlet and section 2 the outlet. ThenQ˙ Ϫ W˙s Ϫ W˙␷ ϭ Ϫm˙ 1(hˆ1 ϩ ᎏ12ᎏV12ϩ gz1) ϩ m˙ 2(hˆ2 ϩ ᎏ12ᎏV22ϩ gz2) (3.65)But, from continuity, m˙ 1 ϭ m˙ 2 ϭ m˙ , and we can rearrange (3.65) as follows:hˆ1 ϩ ᎏ12ᎏV12ϩ gz1 ϭ (hˆ2 ϩ ᎏ12ᎏV22ϩ gz2) Ϫ q ϩ ws ϩ wυ (3.66)where q ϭ Q˙ /m˙ ϭ dQ/dm, the heat transferred to the fluid per unit mass. Similarly,ws ϭ W˙s/m˙ ϭ dWs/dm and wυ ϭ W˙υ/m˙ ϭ dWυ/dm. Equation (3.66) is a general formof the steady-flow energy equation, which states that the upstream stagnation enthalpyH1 ϭ (hˆ ϩ ᎏ12ᎏV2ϩ gz)1 differs from the downstream value H2 only if there is heat trans-fer, shaft work, or viscous work as the fluid passes between sections 1 and 2. Recallthat q is positive if heat is added to the control volume and that ws and w␷ are positiveif work is done by the fluid on the surroundings.3.6 The Energy Equation 167
    • Friction Losses in Low-SpeedFlowEach term in Eq. (3.66) has the dimensions of energy per unit mass, or velocitysquared, which is a form commonly used by mechanical engineers. If we divide throughby g, each term becomes a length, or head, which is a form preferred by civil engi-neers. The traditional symbol for head is h, which we do not wish to confuse with en-thalpy. Therefore we use internal energy in rewriting the head form of the energy re-lation:ᎏp␥1ᎏ ϩ ᎏûg1ᎏ ϩ ᎏV2g12ᎏ ϩ z1 ϭ ᎏp␥2ᎏ ϩ ᎏûg2ᎏ ϩ ᎏV2g12ᎏ ϩ z2 Ϫ hq ϩ hs ϩ h␷ (3.67)where hq ϭ q/g, hs ϭ ws/g, and hυ ϭ wu/g are the head forms of the heat added, shaftwork done, and viscous work done, respectively. The term p/␥ is called pressure headand the term V2/2g is denoted as velocity head.A very common application of the steady-flow energy equation is for low-speed flowwith no shaft work and negligible viscous work, such as liquid flow in pipes. For thiscase Eq. (3.67) may be written in the formᎏp␥1ᎏ ϩ ᎏV2g12ᎏ ϩ z1 ϭ ΂ᎏp␥2ᎏ ϩ ᎏV2g22ᎏ ϩ z2΃ϩ ᎏû2 Ϫ ûg1 Ϫ qᎏ (3.68)The term in parentheses is called the useful head or available head or total head ofthe flow, denoted as h0. The last term on the right is the difference between the avail-able head upstream and downstream and is normally positive, representing the loss inhead due to friction, denoted as hf. Thus, in low-speed (nearly incompressible) flowwith one inlet and one exit, we may write΂ᎏ␥pᎏ ϩ ᎏV2g2ᎏ ϩ z΃inϭ΂ᎏ␥pᎏ ϩ ᎏV2g2ᎏ ϩ z΃outϩ hfriction Ϫ hpump ϩ hturbine (3.69)Most of our internal-flow problems will be solved with the aid of Eq. (3.69). The hterms are all positive; that is, friction loss is always positive in real (viscous) flows, apump adds energy (increases the left-hand side), and a turbine extracts energy from theflow. If hp and/or ht are included, the pump and/or turbine must lie between points 1and 2. In Chaps. 5 and 6 we shall develop methods of correlating hf losses with flowparameters in pipes, valves, fittings, and other internal-flow devices.EXAMPLE 3.17Gasoline at 20°C is pumped through a smooth 12-cm-diameter pipe 10 km long, at a flow rateof 75 m3/h (330 gal/min). The inlet is fed by a pump at an absolute pressure of 24 atm. The exitis at standard atmospheric pressure and is 150 m higher. Estimate the frictional head loss hf, andcompare it to the velocity head of the flow V2/(2g). (These numbers are quite realistic forliquid flow through long pipelines.)SolutionFor gasoline at 20°C, from Table A.3, ␳ ϭ 680 kg/m3, or ␥ ϭ (680)(9.81) ϭ 6670 N/m3. Thereis no shaft work; hence Eq. (3.69) applies and can be evaluated:168 Chapter 3 Integral Relations for a Control Volume
    • ᎏp␥inᎏ ϩ ᎏV2gin2ᎏ ϩ zin ϭ ᎏp␥outᎏ ϩ ᎏV22goutᎏ ϩ zout ϩ hf (1)The pipe is of uniform cross section, and thus the average velocity everywhere isVin ϭ Vout ϭ ᎏQAᎏ ϭ ᎏ((7␲5//43)6(000.1)2mm3/)s2ᎏ Ϸ 1.84 m/sBeing equal at inlet and exit, this term will cancel out of Eq. (1) above, but we are asked to com-pute the velocity head of the flow for comparison purposes:ᎏV2g2ᎏ ϭ ᎏ2((19.8.841mm//ss)22)ᎏ Ϸ 0.173 mNow we are in a position to evaluate all terms in Eq. (1) except the friction head loss:ϩ 0.173 m ϩ 0 m ϭ ᎏ10616,73050NN/m/m32ᎏ ϩ 0.173 m ϩ 150 m ϩ hfor hf ϭ 364.7 Ϫ 15.2 Ϫ 150 Ϸ 199 m Ans.The friction head is larger than the elevation change ⌬z, and the pump must drive the flow againstboth changes, hence the high inlet pressure. The ratio of friction to velocity head isᎏV2/h(f2g)ᎏ Ϸ ᎏ01.19793mmᎏ Ϸ 1150 Ans.This high ratio is typical of long pipelines. (Note that we did not make direct use of the10,000-m pipe length, whose effect is hidden within hf.) In Chap. 6 we can state this problemin a more direct fashion: Given the flow rate, fluid, and pipe size, what inlet pressure is needed?Our correlations for hf will lead to the estimate pinlet Ϸ 24 atm, as stated above.EXAMPLE 3.18Air [R ϭ 1715, cp ϭ 6003 ft и lbf/(slug и °R)] flows steadily, as shown in Fig. E3.18, through aturbine which produces 700 hp. For the inlet and exit conditions shown, estimate (a) the exit ve-locity V2 and (b) the heat transferred Q˙ in Btu/h.(24)(101,350 N/m2)ᎏᎏᎏ6670 N/m33.6 The Energy Equation 1691 2Turbomachinews = 700 hp⋅D1 = 6 inp1 = 150 lb/in2T1 = 300° FV1 = 100 ft/sD2 = 6 inp2 = 40 lb/in2T2 = 35° FQ ?⋅E3.18
    • Part (b)SolutionThe inlet and exit densities can be computed from the perfect-gas law:␳1 ϭ ᎏRpT11ᎏ ϭ ᎏ17151(5406(014ϩ4)300)ᎏ ϭ 0.0166 slug/ft3␳2 ϭ ᎏRpT22ᎏ ϭ ᎏ17154(04(61044ϩ)35)ᎏ ϭ 0.00679 slug/ft3The mass flow is determined by the inlet conditionsm˙ ϭ ␳1A1V1 ϭ (0.0166) ᎏ␲4ᎏ΂ᎏ162ᎏ΃2(100) ϭ 0.325 slug/sKnowing mass flow, we compute the exit velocitym˙ ϭ 0.325 ϭ ␳2A2V2 ϭ (0.00679) ᎏ␲4ᎏ΂ᎏ162ᎏ΃2V2or V2 ϭ 244 ft/s Ans. (a)The steady-flow energy equation (3.65) applies with W˙ ␷ ϭ 0, z1 ϭ z2, and hˆ ϭ cpT:Q˙ Ϫ W˙s ϭ m˙ (cpT2 ϩ ᎏ12ᎏV22Ϫ cpT1 Ϫ ᎏ12ᎏV12)Convert the turbine work to foot-pounds-force per second with the conversion factor 1 hp ϭ550 ft и lbf/s. The turbine work is positiveQ˙ Ϫ 700(550) ϭ 0.325[6003(495) ϩ ᎏ12ᎏ(244)2Ϫ 6003(760) Ϫ ᎏ12ᎏ(100)2]ϭ Ϫ510,000 ft и lbf/sor Q˙ ϭ Ϫ125,000 ft и lbf/sConvert this to British thermal units as follows:Q˙ ϭ (Ϫ125,000 ft и lbf/s) ᎏ778.326f0t0иslb/hf/Btuᎏϭ Ϫ576,000 Btu/h Ans. (b)The negative sign indicates that this heat transfer is a loss from the control volume.Often the flow entering or leaving a port is not strictly one-dimensional. In particular,the velocity may vary over the cross section, as in Fig. E3.4. In this case the kinetic-energy term in Eq. (3.64) for a given port should be modified by a dimensionless cor-rection factor ␣ so that the integral can be proportional to the square of the averagevelocity through the port͵port(ᎏ12ᎏV2)␳(V ؒ n) dA ϵ ␣(ᎏ12ᎏVav2)m˙where Vav ϭ ᎏA1ᎏ ͵ u dA for incompressible flow170 Chapter 3 Integral Relations for a Control VolumePart (a)Kinetic-Energy Correction Factor
    • If the density is also variable, the integration is very cumbersome; we shall not treatthis complication. By letting u be the velocity normal to the port, the first equationabove becomes, for incompressible flow,ᎏ12ᎏ␳ ͵ u3dA ϭ ᎏ12ᎏ␳␣Vav3Aor ␣ ϭ ᎏA1ᎏ ͵ ΂ᎏVuavᎏ΃3dA (3.70)The term ␣ is the kinetic-energy correction factor, having a value of about 2.0 for fullydeveloped laminar pipe flow and from 1.04 to 1.11 for turbulent pipe flow. The com-plete incompressible steady-flow energy equation (3.69), including pumps, turbines,and losses, would generalize to΂ᎏ␳pgᎏ ϩ ᎏ2␣gᎏ V2ϩ z΃inϭ΂ᎏ␳pgᎏ ϩ ᎏ2␣gᎏ V2ϩ z΃outϩ hturbine Ϫ hpump ϩ hfriction (3.71)where the head terms on the right (ht, hp, hf) are all numerically positive. All additiveterms in Eq. (3.71) have dimensions of length {L}. In problems involving turbulentpipe flow, it is common to assume that ␣ Ϸ 1.0. To compute numerical values, we canuse these approximations to be discussed in Chap. 6:Laminar flow: u ϭ U0΄1 Ϫ΂ᎏRrᎏ΃2΅from which Vav ϭ 0.5U0and ␣ ϭ 2.0 (3.72)Turbulent flow: u Ϸ U0΂1 Ϫ ᎏRrᎏ΃mm Ϸ ᎏ17ᎏfrom which, in Example 3.4,Vav ϭ ᎏ(1 ϩ m2)U(20ϩ m)ᎏSubstituting into Eq. (3.70) gives␣ ϭ (3.73)and numerical values are as follows:Turbulent flow:These values are only slightly different from unity and are often neglected in elemen-tary turbulent-flow analyses. However, ␣ should never be neglected in laminar flow.(1 ϩ m)3(2 ϩ m)3ᎏᎏᎏ4(1 ϩ 3m)(2 ϩ 3m)3.6 The Energy Equation 171m ᎏ15ᎏ ᎏ16ᎏ ᎏ17ᎏ ᎏ18ᎏ ᎏ19ᎏ␣ 1.106 1.077 1.058 1.046 1.037
    • E3.19EXAMPLE 3.19A hydroelectric power plant (Fig. E3.19) takes in 30 m3/s of water through its turbine and dis-charges it to the atmosphere at V2 ϭ 2 m/s. The head loss in the turbine and penstock system ishf ϭ 20 m. Assuming turbulent flow, ␣ Ϸ 1.06, estimate the power in MW extracted by the tur-bine.172 Chapter 3 Integral Relations for a Control VolumeSolutionWe neglect viscous work and heat transfer and take section 1 at the reservoir surface (Fig. E3.19),where V1 Ϸ 0, p1 ϭ patm, and z1 ϭ 100 m. Section 2 is at the turbine outlet. The steady-flow en-ergy equation (3.71) becomes, in head form,ᎏp␥1ᎏ ϩ ᎏ␣21Vg12ᎏ ϩ z1 ϭ ᎏp␥2ᎏ ϩ ᎏ␣22Vg22ᎏ ϩ z2 ϩ ht ϩ hfᎏp␥aᎏ ϩ ᎏ12.(096.(801))2ᎏ ϩ 100 m ϭ ᎏp␥aᎏ ϩ ᎏ12.0(96.(821.0mm/s/s2))2ᎏ ϩ 0 m ϩ ht ϩ 20 mThe pressure terms cancel, and we may solve for the turbine head (which is positive):ht ϭ 100 Ϫ 20 Ϫ 0.2 Ϸ 79.8 mThe turbine extracts about 79.8 percent of the 100-m head available from the dam. The totalpower extracted may be evaluated from the water mass flow:P ϭ m˙ ws ϭ (␳Q)(ght) ϭ (998 kg/m3)(30 m3/s)(9.81 m/s2)(79.8 m)ϭ 23.4 E6 kg и m2/s3ϭ 23.4 E6 N и m/s ϭ 23.4 MW Ans. 7The turbine drives an electric generator which probably has losses of about 15 percent, so thenet power generated by this hydroelectric plant is about 20 MW.EXAMPLE 3.20The pump in Fig. E3.20 delivers water (62.4 lbf/ft3) at 3 ft3/s to a machine at section 2, whichis 20 ft higher than the reservoir surface. The losses between 1 and 2 are given by hf ϭ KV22/(2g),Water30 m3/sz1 = 100 mz2 = 0 m2 m/sTurbine1
    • E3.20where K Ϸ 7.5 is a dimensionless loss coefficient (see Sec. 6.7). Take ␣ Ϸ 1.07. Find the horse-power required for the pump if it is 80 percent efficient.SolutionIf the reservoir is large, the flow is steady, with V1 Ϸ 0. We can compute V2 from the given flowrate and the pipe diameter:V2 ϭ ᎏAQ2ᎏ ϭ ϭ 61.1 ft/sThe viscous work is zero because of the solid walls and near-one-dimensional inlet and exit. Thesteady-flow energy equation (3.71) becomesᎏp␥1ᎏ ϩ ᎏ␣21Vg12ᎏ ϩ z1 ϭ ᎏp␥2ᎏ ϩ ᎏ␣22Vg22ᎏ ϩ z2 ϩ hs ϩ hfIntroducing V1 Ϸ 0, z1 ϭ 0, and hf ϭ KV22/(2g), we may solve for the pump head:hs ϭ ᎏp1 Ϫ␥p2ᎏ Ϫ z2 Ϫ (␣2 ϩ K)΂ᎏV2g22ᎏ΃The pressures should be in lbf/ft2for consistent units. For the given data, we obtainhs ϭ Ϫ 20 ft Ϫ (1.07 ϩ 7.5) ᎏ2((6312.1.2fftt//ss)22)ᎏϭ 11 Ϫ 20 Ϫ 497 ϭ Ϫ506 ftThe pump head is negative, indicating work done on the fluid. As in Example 3.19, the powerdelivered is computed fromP ϭ m˙ ws ϭ ␳Qghs ϭ (1.94 slug/ft3)(3.0 ft3/s)(32.2 ft/s2)(Ϫ507 ft) ϭ Ϫ94,900 ft и lbf/sor hp ϭ ᎏ55904,f9t0и0lbftf/и(slbиfh/sp)ᎏ Ϸ 173 hp(14.7 Ϫ 10.0)(144) lbf/ft2ᎏᎏᎏ62.4 lbf/ft33 ft3/sᎏᎏ(␲/4)(ᎏ132ᎏ ft)23.6 The Energy Equation 17321z1 = 0Pumphs (negative)WaterMachineD2 = 3 inz2 = 20 ftp2 = 10 lbf/in2p1 = 14.7 lbf/in2abs
    • 3.7 Frictionless Flow:The Bernoulli EquationWe drop the negative sign when merely referring to the “power” required. If the pump is 80 per-cent efficient, the input power required to drive it isPinput ϭ ᎏefficPiencyᎏ ϭ ᎏ1703.8hpᎏ Ϸ 216 hp Ans.The inclusion of the kinetic-energy correction factor ␣ in this case made a difference of about1 percent in the result.Closely related to the steady-flow energy equation is a relation between pressure, ve-locity, and elevation in a frictionless flow, now called the Bernoulli equation. It wasstated (vaguely) in words in 1738 in a textbook by Daniel Bernoulli. A complete der-ivation of the equation was given in 1755 by Leonhard Euler. The Bernoulli equationis very famous and very widely used, but one should be wary of its restrictions—allfluids are viscous and thus all flows have friction to some extent. To use the Bernoulliequation correctly, one must confine it to regions of the flow which are nearly fric-tionless. This section (and, in more detail, Chap. 8) will address the proper use of theBernoulli relation.Consider Fig. 3.15, which is an elemental fixed streamtube control volume of vari-able area A(s) and length ds, where s is the streamline direction. The properties (␳, V,p) may vary with s and time but are assumed to be uniform over the cross section A.The streamtube orientation ␪ is arbitrary, with an elevation change dz ϭ ds sin ␪. Fric-tion on the streamtube walls is shown and then neglected—a very restrictive assump-tion.Conservation of mass (3.20) for this elemental control volume yieldsᎏddtᎏ΂͵CV␳ dᐂ΃ϩ m˙ out Ϫ m˙ in ϭ 0 Ϸ ᎏѨѨ␳tᎏ dᐂ ϩ dm˙where m˙ ϭ ␳AV and dᐂ Ϸ A ds. Then our desired form of mass conservation isdm˙ ϭ d(␳AV) ϭ ϪᎏѨѨ␳tᎏ A ds (3.74)174 Chapter 3 Integral Relations for a Control Volumep+AA + dAτ = 0, VθdsCVdz+ dV+ dVp + dpdp00dpdpdFs ≈ 12dp dAdW ≈ g dᐂ(a) (b)ρ ρρρpSFig. 3.15 The Bernoulli equationfor frictionless flow along a stream-line: (a) forces and fluxes; (b) netpressure force after uniform sub-traction of p.
    • This relation does not require an assumption of frictionless flow.Now write the linear-momentum relation (3.37) in the streamwise direction:ΑdFs ϭ ᎏddtᎏ΂͵CVV␳ dᐂ΃ϩ (m˙ V)out Ϫ (m˙ V)in Ϸ ᎏѨѨtᎏ (␳V) A ds ϩ d(m˙ V)where Vs ϭ V itself because s is the streamline direction. If we neglect the shear forceon the walls (frictionless flow), the forces are due to pressure and gravity. The stream-wise gravity force is due to the weight component of the fluid within the control vol-ume:dFs,grav ϭ ϪdW sin ␪ ϭ Ϫ␥A ds sin ␪ ϭ Ϫ␥A dzThe pressure force is more easily visualized, in Fig. 3.15b, by first subtracting a uni-form value p from all surfaces, remembering from Fig. 3.7 that the net force is notchanged. The pressure along the slanted side of the streamtube has a streamwise com-ponent which acts not on A itself but on the outer ring of area increase dA. The netpressure force is thusdFs,press ϭ ᎏ12ᎏ dp dA Ϫ dp(A ϩ dA) Ϸ ϪA dpto first order. Substitute these two force terms into the linear-momentum relation:ΑdFs ϭ Ϫ␥A dz Ϫ A dp ϭ ᎏѨѨtᎏ (␳V) A ds ϩ d(m˙ V)ϭ ᎏѨѨ␳tᎏ VA ds ϩ ᎏѨѨVtᎏ ␳A ds ϩ m˙ dV ϩ V dm˙The first and last terms on the right cancel by virtue of the continuity relation (3.74).Divide what remains by ␳A and rearrange into the final desired relation:ᎏѨѨVtᎏ ds ϩ ᎏd␳pᎏ ϩ V dV ϩ g dz ϭ 0 (3.75)This is Bernoulli’s equation for unsteady frictionless flow along a streamline. It is indifferential form and can be integrated between any two points 1 and 2 on the stream-line:͵21ᎏѨѨVtᎏ ds ϩ ͵21ᎏd␳pᎏ ϩ ᎏ12ᎏ (V22Ϫ V12) ϩ g(z2 Ϫ z1) ϭ 0 (3.76)To evaluate the two remaining integrals, one must estimate the unsteady effect ѨV/Ѩt andthe variation of density with pressure. At this time we consider only steady (ѨV/Ѩt ϭ 0)incompressible (constant-density) flow, for which Eq. (3.76) becomesᎏp2 Ϫ␳p1ᎏ ϩ ᎏ12ᎏ (V22Ϫ V12) ϩ g(z2 Ϫ z1) ϭ 0or ᎏp␳1ᎏ ϩ ᎏ12ᎏ V12ϩ gz1 ϭ ᎏp␳2ᎏ ϩ ᎏ12ᎏ V22ϩ gz2 ϭ const (3.77)This is the Bernoulli equation for steady frictionless incompressible flow along astreamline.3.7 Frictionless Flow: The Bernoulli Equation 175
    • Relation between the Bernoulliand Steady-Flow EnergyEquationsEquation (3.77) is a widely used form of the Bernoulli equation for incompressiblesteady frictionless streamline flow. It is clearly related to the steady-flow energy equa-tion for a streamtube (flow with one inlet and one outlet), from Eq. (3.66), which westate as follows:ᎏp␳1ᎏ ϩ ᎏ␣12V12ᎏ ϩ gz1 ϭ ᎏp␳2ᎏ ϩ ᎏ␣22V22ᎏ ϩ gz2 ϩ (û2 Ϫ û1 Ϫ q) ϩ ws ϩ wv (3.78)This relation is much more general than the Bernoulli equation, because it allows for(1) friction, (2) heat transfer, (3) shaft work, and (4) viscous work (another frictionaleffect).If we compare the Bernoulli equation (3.77) with the energy equation (3.78), wesee that the Bernoulli equation contains even more restrictions than might first be re-alized. The complete list of assumptions for Eq. (3.77) is as follows:1. Steady flow—a common assumption applicable to many flows.2. Incompressible flow—acceptable if the flow Mach number is less than 0.3.3. Frictionless flow—very restrictive, solid walls introduce friction effects.4. Flow along a single streamline—different streamlines may have different“Bernoulli constants” w0 ϭ p/␳ ϩ V2/2 ϩ gz, depending upon flow conditions.5. No shaft work between 1 and 2—no pumps or turbines on the streamline.6. No heat transfer between 1 and 2—either added or removed.Thus our warning: Be wary of misuse of the Bernoulli equation. Only a certain lim-ited set of flows satisfies all six assumptions above. The usual momentum or “me-chanical force” derivation of the Bernoulli equation does not even reveal items 5 and6, which are thermodynamic limitations. The basic reason for restrictions 5 and 6 isthat heat transfer and work transfer, in real fluids, are married to frictional effects,which therefore invalidate our assumption of frictionless flow.Figure 3.16 illustrates some practical limitations on the use of Bernoulli’s equation(3.77). For the wind-tunnel model test of Fig. 3.16a, the Bernoulli equation is valid inthe core flow of the tunnel but not in the tunnel-wall boundary layers, the model sur-face boundary layers, or the wake of the model, all of which are regions with high fric-tion.In the propeller flow of Fig. 3.16b, Bernoulli’s equation is valid both upstreamand downstream, but with a different constant w0 ϭ p/␳ ϩ V2/2 ϩ gz, caused by thework addition of the propeller. The Bernoulli relation (3.77) is not valid near thepropeller blades or in the helical vortices (not shown, see Fig. 1.12a) shed down-stream of the blade edges. Also, the Bernoulli constants are higher in the flowing“slipstream” than in the ambient atmosphere because of the slipstream kinetic en-ergy.For the chimney flow of Fig. 3.16c, Eq. (3.77) is valid before and after the fire, butwith a change in Bernoulli constant that is caused by heat addition. The Bernoulli equa-tion is not valid within the fire itself or in the chimney-wall boundary layers.The moral is to apply Eq. (3.77) only when all six restrictions can be satisfied: steadyincompressible flow along a streamline with no friction losses, no heat transfer, andno shaft work between sections 1 and 2.176 Chapter 3 Integral Relations for a Control Volume
    • A useful visual interpretation of Bernoulli’s equation is to sketch two grade lines of aflow. The energy grade line (EGL) shows the height of the total Bernoulli constanth0 ϭ z ϩ p/␥ ϩ V2/(2g). In frictionless flow with no work or heat transfer, Eq. (3.77),the EGL has constant height. The hydraulic grade line (HGL) shows the height corre-sponding to elevation and pressure head z ϩ p/␥, that is, the EGL minus the velocityhead V2/(2g). The HGL is the height to which liquid would rise in a piezometer tube(see Prob. 2.11) attached to the flow. In an open-channel flow the HGL is identical tothe free surface of the water.Figure 3.17 illustrates the EGL and HGL for frictionless flow at sections 1 and 2of a duct. The piezometer tubes measure the static-pressure head z ϩ p/␥ and thus out-line the HGL. The pitot stagnation-velocity tubes measure the total head z ϩ p/␥ ϩV2/(2g), which corresponds to the EGL. In this particular case the EGL is constant, andthe HGL rises due to a drop in velocity.In more general flow conditions, the EGL will drop slowly due to friction lossesand will drop sharply due to a substantial loss (a valve or obstruction) or due to workextraction (to a turbine). The EGL can rise only if there is work addition (as from apump or propeller). The HGL generally follows the behavior of the EGL with respectto losses or work transfer, and it rises and/or falls if the velocity decreases and/or in-creases.3.7 Frictionless Flow: The Bernoulli Equation 177InvalidModelValidValid(a) (b)ValidValid,newconstantAmbientairInvalidValid, newconstantInvalid(c)ValidFig. 3.16 Illustration of regions ofvalidity and invalidity of theBernoulli equation: (a) tunnelmodel, (b) propeller, (c) chimney.Hydraulic and Energy GradeLines
    • Fig. 3.17 Hydraulic and energygrade lines for frictionless flow in aduct.E3.21As mentioned before, no conversion factors are needed in computations with theBernoulli equation if consistent SI or BG units are used, as the following exampleswill show.In all Bernoulli-type problems in this text, we consistently take point 1 upstreamand point 2 downstream.EXAMPLE 3.21Find a relation between nozzle discharge velocity V2and tank free-surface height h as in Fig.E3.21. Assume steady frictionless flow.178 Chapter 3 Integral Relations for a Control VolumeFlow12Energy grade lineHydraulic grade lineArbitrary datum (z = 0)z12gV22V122gp1gz2p2gConstantBernoulliheadρρ12V1EGLHGLV2Open jet:p2 = paV122gh = z1 – z2
    • SolutionAs mentioned, we always choose point 1 upstream and point 2 downstream. Try to choose points1 and 2 where maximum information is known or desired. Here we select point 1 as the tankfree surface, where elevation and pressure are known, and point 2 as the nozzle exit, where againpressure and elevation are known. The two unknowns are V1 and V2.Mass conservation is usually a vital part of Bernoulli analyses. If A1 is the tank cross sectionand A2 the nozzle area, this is approximately a one-dimensional flow with constant density, Eq.(3.30),A1V1 ϭ A2V2 (1)Bernoulli’s equation (3.77) givesᎏp␳1ᎏ ϩ ᎏ12ᎏV12ϩ gz1 ϭ ᎏp␳2ᎏ ϩ ᎏ12ᎏV22ϩ gz2But since sections 1 and 2 are both exposed to atmospheric pressure p1 ϭ p2 ϭ pa, the pressureterms cancel, leavingV22Ϫ V12ϭ 2g(z1 Ϫ z2) ϭ 2gh (2)Eliminating V1 between Eqs. (1) and (2), we obtain the desired result:V22ϭ ᎏ1 Ϫ2gAh22/A12ᎏ Ans. (3)Generally the nozzle area A2 is very much smaller than the tank area A1, so that the ratio A22/A12is doubly negligible, and an accurate approximation for the outlet velocity isV2 Ϸ (2gh)1/2Ans. (4)This formula, discovered by Evangelista Torricelli in 1644, states that the discharge velocityequals the speed which a frictionless particle would attain if it fell freely from point 1 to point2. In other words, the potential energy of the surface fluid is entirely converted to kinetic energyof efflux, which is consistent with the neglect of friction and the fact that no net pressure workis done. Note that Eq. (4) is independent of the fluid density, a characteristic of gravity-drivenflows.Except for the wall boundary layers, the streamlines from 1 to 2 all behave in the same way,and we can assume that the Bernoulli constant h0 is the same for all the core flow. However, theoutlet flow is likely to be nonuniform, not one-dimensional, so that the average velocity is onlyapproximately equal to Torricelli’s result. The engineer will then adjust the formula to includea dimensionless discharge coefficient cd(V2)av ϭ ᎏAQ2ᎏ ϭ cd(2gh)1/2(5)As discussed in Sec. 6.10, the discharge coefficient of a nozzle varies from about 0.6 to 1.0 asa function of (dimensionless) flow conditions and nozzle shape.Before proceeding with more examples, we should note carefully that a solution byBernoulli’s equation (3.77) does not require a control-volume analysis, only a selec-tion of two points 1 and 2 along a given streamline. The control volume was used toderive the differential relation (3.75), but the integrated form (3.77) is valid all along3.7 Frictionless Flow: The Bernoulli Equation 179
    • the streamline for frictionless flow with no heat transfer or shaft work, and a controlvolume is not necessary.EXAMPLE 3.22Rework Example 3.21 to account, at least approximately, for the unsteady-flow condition causedby the draining of the tank.SolutionEssentially we are asked to include the unsteady integral term involving ѨV/Ѩt from Eq. (3.76).This will result in a new term added to Eq. (2) from Example 3.21:2 ͵21ᎏѨѨVtᎏ ds ϩ V22Ϫ V12ϭ 2gh (1)Since the flow is incompressible, the continuity equation still retains the simple form A1V1 ϭA2V2 from Example 3.21. To integrate the unsteady term, we must estimate the acceleration allalong the streamline. Most of the streamline is in the tank region where ѨV/Ѩt Ϸ dV1/dt. Thelength of the average streamline is slightly longer than the nozzle depth h. A crude estimate forthe integral is thus͵21ᎏѨѨVtᎏ ds Ϸ ͵21ᎏddVt1ᎏ ds Ϸ ϪᎏddVt1ᎏ h (2)But since A1 and A2 are constant, dV1/dt Ϸ (A2/A1)(dV2/dt). Substitution into Eq. (1) givesϪ2h ᎏAA21ᎏ ᎏddVt2ᎏ ϩ V22΂1 Ϫ ᎏAA1222ᎏ΃Ϸ 2gh (3)This is a first-order differential equation for V2(t). It is complicated by the fact that the depth his variable; therefore h ϭ h(t), as determined by the variation in V1(t)h(t) ϭ h0 Ϫ ͵t0V1 dt (4)Equations (3) and (4) must be solved simultaneously, but the problem is well posed and can behandled analytically or numerically. We can also estimate the size of the first term in Eq. (3) byusing the approximation V2 Ϸ (2gh)1/2from the previous example. After differentiation, we ob-tain2h ᎏAA21ᎏ ᎏddVt2ᎏ Ϸ Ϫ΂ᎏAA21ᎏ΃2V22(5)which is negligible if A2 Ӷ A1, as originally postulated.EXAMPLE 3.23A constriction in a pipe will cause the velocity to rise and the pressure to fall at section 2 in thethroat. The pressure difference is a measure of the flow rate through the pipe. The smoothlynecked-down system shown in Fig. E3.23 is called a venturi tube. Find an expression for themass flux in the tube as a function of the pressure change.180 Chapter 3 Integral Relations for a Control Volume
    • E3.23SolutionBernoulli’s equation is assumed to hold along the center streamlineᎏp␳1ᎏ ϩ ᎏ12ᎏV12ϩ gz1 ϭ ᎏp␳2ᎏ ϩ ᎏ12ᎏV22ϩ gz2If the tube is horizontal, z1 ϭ z2 and we can solve for V2:V22Ϫ V12ϭ ᎏ2␳⌬pᎏ ⌬p ϭ p1 Ϫ p2 (1)We relate the velocities from the incompressible continuity relationA1V1 ϭ A2V2or V1 ϭ ␤2V2 ␤ ϭ ᎏDD21ᎏ (2)Combining (1) and (2), we obtain a formula for the velocity in the throatV2 ϭ΄ᎏ␳(12Ϫ⌬p␤4)ᎏ΅1/2(3)The mass flux is given bym˙ ϭ ␳A2V2 ϭ A2΂ᎏ12␳Ϫ⌬␤p4ᎏ΃1/2(4)This is the ideal frictionless mass flux. In practice, we measure m˙ actual ϭ cd m˙ ideal and correlatethe discharge coefficient cd.EXAMPLE 3.24A 10-cm fire hose with a 3-cm nozzle discharges 1.5 m3/min to the atmosphere. Assuming fric-tionless flow, find the force FB exerted by the flange bolts to hold the nozzle on the hose.SolutionWe use Bernoulli’s equation and continuity to find the pressure p1 upstream of the nozzle andthen we use a control-volume momentum analysis to compute the bolt force, as in Fig. E3.24.The flow from 1 to 2 is a constriction exactly similar in effect to the venturi in Example 3.23for which Eq. (1) gavep1 ϭ p2 ϩ ᎏ12ᎏ␳(V22Ϫ V12) (1)3.7 Frictionless Flow: The Bernoulli Equation 18121p1HGL p2
    • E3.24The velocities are found from the known flow rate Q ϭ 1.5 m3/min or 0.025 m3/s:V2 ϭ ᎏAQ2ᎏ ϭ ᎏ(␲0/4.0)2(05.0m33m/s)2ᎏ ϭ 35.4 m/sV1 ϭ ᎏAQ1ᎏ ϭ ᎏ(␲0/.40)2(50.m13m/s)2ᎏ ϭ 3.2 m/sWe are given p2 ϭ pa ϭ 0 gage pressure. Then Eq. (1) becomesp1 ϭ ᎏ12ᎏ(1000 kg/m3)[(35.42Ϫ 3.22) m2/s2]ϭ 620,000 kg/(m и s2) ϭ 620,000 Pa gageThe control-volume force balance is shown in Fig. E3.24b:ΑFx ϭ ϪFB ϩ p1A1and the zero gage pressure on all other surfaces contributes no force. The x-momentum flux isϩm˙V2 at the outlet and Ϫm˙V1 at the inlet. The steady-flow momentum relation (3.40) thus givesϪFB ϩ p1A1 ϭ m˙ (V2 Ϫ V1)or FB ϭ p1A1 Ϫ m˙ (V2 Ϫ V1) (2)Substituting the given numerical values, we findm˙ ϭ ␳Q ϭ (1000 kg/m3)(0.025 m3/s) ϭ 25 kg/sA1 ϭ ᎏ␲4ᎏ D12ϭ ᎏ␲4ᎏ (0.1 m)2ϭ 0.00785 m2FB ϭ (620,000 N/m2)(0.00785 m2) Ϫ (25 kg/s)[(35.4 Ϫ 3.2) m/s]ϭ 4872 N Ϫ 805 (kg и m)/s2ϭ 4067 N (915 lbf) Ans.This gives an idea of why it takes more than one firefighter to hold a fire hose at full discharge.Notice from these examples that the solution of a typical problem involvingBernoulli’s equation almost always leads to a consideration of the continuity equation182 Chapter 3 Integral Relations for a Control Volume2Water:1000 kg/m31D1 = 10 cm(a)CVpa = 0 (gage)12FB12FBp1000xControl volume(b)D2 = 3 cm
    • Summaryas an equal partner in the analysis. The only exception is when the complete velocitydistribution is already known from a previous or given analysis, but that means thatthe continuity relation has already been used to obtain the given information. The pointis that the continuity relation is always an important element in a flow analysis.This chapter has analyzed the four basic equations of fluid mechanics: conservation of(1) mass, (2) linear momentum, (3) angular momentum, and (4) energy. The equationswere attacked “in the large,” i.e., applied to whole regions of a flow. As such, the typ-ical analysis will involve an approximation of the flow field within the region, givingsomewhat crude but always instructive quantitative results. However, the basic control-volume relations are rigorous and correct and will give exact results if applied to theexact flow field.There are two main points to a control-volume analysis. The first is the selection ofa proper, clever, workable control volume. There is no substitute for experience, butthe following guidelines apply. The control volume should cut through the place wherethe information or solution is desired. It should cut through places where maximuminformation is already known. If the momentum equation is to be used, it should notcut through solid walls unless absolutely necessary, since this will expose possible un-known stresses and forces and moments which make the solution for the desired forcedifficult or impossible. Finally, every attempt should be made to place the control vol-ume in a frame of reference where the flow is steady or quasi-steady, since the steadyformulation is much simpler to evaluate.The second main point to a control-volume analysis is the reduction of the analy-sis to a case which applies to the problem at hand. The 24 examples in this chaptergive only an introduction to the search for appropriate simplifying assumptions. Youwill need to solve 24 or 124 more examples to become truly experienced in simplify-ing the problem just enough and no more. In the meantime, it would be wise for thebeginner to adopt a very general form of the control-volume conservation laws andthen make a series of simplifications to achieve the final analysis. Starting with thegeneral form, one can ask a series of questions:1. Is the control volume nondeforming or nonaccelerating?2. Is the flow field steady? Can we change to a steady-flow frame?3. Can friction be neglected?4. Is the fluid incompressible? If not, is the perfect-gas law applicable?5. Are gravity or other body forces negligible?6. Is there heat transfer, shaft work, or viscous work?7. Are the inlet and outlet flows approximately one-dimensional?8. Is atmospheric pressure important to the analysis? Is the pressure hydrostatic onany portions of the control surface?9. Are there reservoir conditions which change so slowly that the velocity and timerates of change can be neglected?In this way, by approving or rejecting a list of basic simplifications like those above,one can avoid pulling Bernoulli’s equation off the shelf when it does not apply.Summary 183
    • ProblemsMost of the problems herein are fairly straightforward. More diffi-cult or open-ended assignments are labeled with an asterisk. Prob-lems labeled with an EES icon, for example, Prob. 3.5, will benefitfrom the use of the Engineering Equation Solver (EES), while fig-ures labeled with a computer disk may require the use of a computer.The standard end-of-chapter problems 3.1 to 3.182 (categorized inthe problem list below) are followed by word problemsW3.1 toW3.7,fundamentals of engineering (FE) exam problems FE3.1 to FE3.10,comprehensive problems C3.1 to C3.4, and design project D3.1.Problem DistributionSection Topic Problems3.1 Basic physical laws; volume flow 3.1–3.83.2 The Reynolds transport theorem 3.9–3.113.3 Conservation of mass 3.12–3.383.4 The linear momentum equation 3.39–3.1093.5 The angular momentum theorem 3.110–3.1253.6 The energy equation 3.126–3.1463.7 The Bernoulli equation 3.147–3.182P3.1 Discuss Newton’s second law (the linear-momentum rela-tion) in these three forms:ΑF ϭ ma ΑF ϭ ᎏddtᎏ (mV)ΑF ϭ ᎏddtᎏ΂͵systemV␳ dᐂ΃Are they all equally valid? Are they equivalent? Are someforms better for fluid mechanics as opposed to solid me-chanics?P3.2 Consider the angular-momentum relation in the formΑMO ϭ ᎏddtᎏ΄͵system(r ؋ V)␳ dᐂ΅What does r mean in this relation? Is this relation valid inboth solid and fluid mechanics? Is it related to the linear-momentum equation (Prob. 3.1)? In what manner?P3.3 For steady low-Reynolds-number (laminar) flow througha long tube (see Prob. 1.12), the axial velocity distributionis given by u ϭ C(R2Ϫ r2), where R is the tube radius andr Յ R. Integrate u(r) to find the total volume flow Qthrough the tube.P3.4 Discuss whether the following flows are steady or un-steady: (a) flow near an automobile moving at 55 mi/h,(b) flow of the wind past a water tower, (c) flow in a pipeas the downstream valve is opened at a uniform rate, (d)river flow over the spillway of a dam, and (e) flow in theocean beneath a series of uniform propagating surfacewaves. Elaborate if these questions seem ambiguous.*P3.5 A theory proposed by S. I. Pai in 1953 gives the follow-ing velocity values u(r) for turbulent (high-Reynolds-num-ber) airflow in a 4-cm-diameter tube:r, cm 0 0.25 0.5 0.75 1.0 1.25 1.5 1.75 2.0u, m/s 6.00 5.97 5.88 5.72 5.51 5.23 4.89 4.43 0.00Comment on these data vis-à-vis laminar flow, Prob. 3.3.Estimate, as best you can, the total volume flow Q throughthe tube, in m3/s.P3.6 When a gravity-driven liquid jet issues from a slot in atank, as in Fig. P3.6, an approximation for the exit veloc-ity distribution is u Ϸ ͙2ෆgෆ(hෆ Ϫෆ zෆ)ෆ, where h is the depthof the jet centerline. Near the slot, the jet is horizontal,two-dimensional, and of thickness 2L, as shown. Find ageneral expression for the total volume flow Q issuingfrom the slot; then take the limit of your result if L Ӷ h.184 Chapter 3 Integral Relations for a Control Volumez = +Lxz = –LhzFig. P3.6Fig. P3.7P3.7 Consider flow of a uniform stream U toward a circularcylinder of radius R, as in Fig. P3.7. An approximate the-ory for the velocity distribution near the cylinder is devel-oped in Chap. 8, in polar coordinates, for r Ն R:υr ϭ U cos ␪ ΂1 Ϫ ᎏRr22ᎏ΃ υ␪ ϭ ϪU sin ␪ ΂1 ϩ ᎏRr22ᎏ΃where the positive directions for radial (υr) and circum-ferential (υ␪) velocities are shown in Fig. P3.7. Computethe volume flow Q passing through the (imaginary) sur-face CC in the figure. (Comment: If CC were far upstreamof the cylinder, the flow would be Q ϭ 2URb.)U R2RCCRrθvrvθImaginary surface:Width b into paper
    • P3.8 Consider the two-dimensional stagnation flow of Example1.10, where u ϭ Kx and v ϭ ϪKy, with K Ͼ 0. Evaluatethe volume flow Q, per unit depth into the paper, passingthrough the rectangular surface normal to the paper whichstretches from (x, y) ϭ (0, 0) to (1, 1).P3.9 A laboratory test tank contains seawater of salinity S anddensity ␳. Water enters the tank at conditions (S1, ␳1, A1,V1) and is assumed to mix immediately in the tank. Tankwater leaves through an outlet A2 at velocity V2. If salt isa “conservative” property (neither created nor destroyed),use the Reynolds transport theorem to find an expressionfor the rate of change of salt mass Msalt within the tank.P3.10 Laminar steady flow, through a tube of radius R and lengthL, is being heated at the wall. The fluid entered the tubeat uniform temperature T0 ϭ Tw/3. As the fluid exits thetube, its axial velocity and enthalpy profiles are approxi-mated byu ϭ U0΂1 Ϫ ᎏRr22ᎏ΃ h ϭ ᎏcp2Twᎏ΂1 ϩ ᎏRr22ᎏ΃cp ϭ const(a) Sketch these profiles and comment on their physicalrealism. (b) Compute the total flux of enthalpy through theexit section.P3.11 A room contains dust of uniform concentration C ϭ␳dust/␳. It is to be cleaned up by introducing fresh air atvelocity Vi through a duct of area Ai on one wall and ex-hausting the room air at velocity V0 through a duct A0 onthe opposite wall. Find an expression for the instantaneousrate of change of dust mass within the room.P3.12 Water at 20°C flows steadily through a closed tank, as inFig. P3.12. At section 1, D1 ϭ 6 cm and the volume flowis 100 m3/h. At section 2, D2 ϭ 5 cm and the average ve-locity is 8 m/s. If D3 ϭ 4 cm, what is (a) Q3 in m3/h and(b) average V3 in m/s?P3.14 The open tank in Fig. P3.14 contains water at 20°C and isbeing filled through section 1. Assume incompressibleflow. First derive an analytic expression for the water-levelchange dh/dt in terms of arbitrary volume flows (Q1, Q2,Q3) and tank diameter d. Then, if the water level h is con-stant, determine the exit velocity V2 for the given dataV1 ϭ 3 m/s and Q3 ϭ 0.01 m3/s.Problems 185123WaterP3.12P3.13P3.14P3.15P3.13 Water at 20°C flows steadily at 40 kg/s through the noz-zle in Fig. P3.13. If D1 ϭ 18 cm and D2 ϭ 5 cm, computethe average velocity, in m/s, at (a) section 1 and (b) sec-tion 2.21WaterD2 = 7 cmQ3 = 0.01 m3/sD1 = 5 cmd231hP3.15 Water, assumed incompressible, flows steadily through theround pipe in Fig. P3.15. The entrance velocity is constant,u ϭ U0, and the exit velocity approximates turbulent flow,u ϭ umax(1 Ϫ r/R)1/7. Determine the ratio U0 /umax for thisflow.rr = Rx = 0U0x = Lu(r)P3.16 An incompressible fluid flows past an impermeable flatplate, as in Fig. P3.16, with a uniform inlet profile u ϭ U0and a cubic polynomial exit profile
    • u Ϸ U0΂ᎏ3␩ Ϫ2␩3ᎏ΃ where ␩ ϭ ᎏ␦yᎏCompute the volume flow Q across the top surface of thecontrol volume.P3.17 Incompressible steady flow in the inlet between parallelplates in Fig. P3.17 is uniform, u ϭ U0 ϭ 8 cm/s, whiledownstream the flow develops into the parabolic laminarprofile u ϭ az(z0 Ϫ z), where a is a constant. If z0 ϭ 4 cmand the fluid is SAE 30 oil at 20°C, what is the value ofumax in cm/s?P3.19 A partly full water tank admits water at 20°C and 85 N/sweight flow while ejecting water on the other side at 5500cm3/s. The air pocket in the tank has a vent at the top andis at 20°C and 1 atm. If the fluids are approximately in-compressible, how much air in N/h is passing through thevent? In which direction?P3.20 Oil (SG ϭ 0.89) enters at section 1 in Fig. P3.20 at aweight flow of 250 N/h to lubricate a thrust bearing. Thesteady oil flow exits radially through the narrow clearancebetween thrust plates. Compute (a) the outlet volume fluxin mL/s and (b) the average outlet velocity in cm/s.186 Chapter 3 Integral Relations for a Control VolumeP3.17P3.20P3.18 P3.22z = z0umaxz = 0U0h = 2 mmD = 10 cm212D1 = 3 mm12AirD1 = 1 cmD2 = 2.5 cmP3.18 An incompressible fluid flows steadily through the rec-tangular duct in Fig. P3.18. The exit velocity profile isgiven approximately byu ϭ umax΂1 Ϫ ᎏby22ᎏ΃΂1 Ϫ ᎏhz22ᎏ΃(a) Does this profile satisfy the correct boundary condi-tions for viscous fluid flow? (b) Find an analytical expres-sion for the volume flow Q at the exit. (c) If the inlet flowis 300 ft3/min, estimate umax in m/s for b ϭ h ϭ 10 cm.P3.21 A dehumidifier brings in saturated wet air (100 percent rel-ative humidity) at 30°C and 1 atm, through an inlet of 8-cm diameter and average velocity 3 m/s. After some of thewater vapor condenses and is drained off at the bottom,the somewhat drier air leaves at approximately 30°C, 1atm, and 50 percent relative humidity. For steady opera-tion, estimate the amount of water drained off in kg/h. (Thisproblem is idealized from a real dehumidifier.)P3.22 The converging-diverging nozzle shown in Fig. P3.22 ex-pands and accelerates dry air to supersonic speeds at theexit, where p2 ϭ 8 kPa and T2 ϭ 240 K. At the throat, p1 ϭ284 kPa, T1 ϭ 665 K, and V1 ϭ 517 m/s. For steady com-pressible flow of an ideal gas, estimate (a) the mass flowin kg/h, (b) the velocity V2, and (c) the Mach number Ma2.P3.16y = 0 CVU0 U0y = δ Q?Solid plate, width b into paperCubicInlet flowL2h2bzx, uy
    • P3.23 The hypodermic needle in Fig. P3.23 contains a liquidserum (SG ϭ 1.05). If the serum is to be injected steadilyat 6 cm3/s, how fast in in/s should the plunger be advanced(a) if leakage in the plunger clearance is neglected and (b)if leakage is 10 percent of the needle flow?P3.26 A thin layer of liquid, draining from an inclined plane, asin Fig. P3.26, will have a laminar velocity profile u ϷU0(2y/h Ϫ y2/h2), where U0 is the surface velocity. If theplane has width b into the paper, determine the volumerate of flow in the film. Suppose that h ϭ 0.5 in and theflow rate per foot of channel width is 1.25 gal/min. Esti-mate U0 in ft/s.Problems 187yxθu (y)ghP3.26P3.27P3.25P3.23P3.24D1 = 0.75 inD2 = 0.030 inV2U0U0uWidth binto paperDead air (negligible velocity)Exponential curveU + ∆UzL2CL*P3.24 Water enters the bottom of the cone in Fig. P3.24 at a uni-formly increasing average velocity V ϭ Kt. If d is verysmall, derive an analytic formula for the water surface riseh(t) for the condition h ϭ 0 at t ϭ 0. Assume incompress-ible flow.*P3.27 The cone frustum in Fig. P3.27 contains incompressibleliquid to depth h. A solid piston of diameter d penetratesthe surface at velocity V. Derive an analytic expression forthe rate of rise dh/dt of the liquid surface.P3.28 Consider a cylindrical water tank of diameter D and wa-ter depth h. According to elementary theory, the flow ratefrom a small hole of area A in the bottom of the tank wouldbe Q Ϸ CA ͙2ෆgෆhෆ, where C Ϸ 0.61. If the initial waterlevel is h0 and the hole is opened, derive an expression forthe time required for the water level to drop to ᎏ13ᎏh0.P3.29 In elementary compressible-flow theory (Chap. 9), com-pressed air will exhaust from a small hole in a tank at themass flow rate m˙ Ϸ C␳, where ␳ is the air density in thetank and C is a constant. If ␳0 is the initial density in atank of volume ᐂ, derive a formula for the density change␳(t) after the hole is opened. Apply your formula to thefollowing case: a spherical tank of diameter 50 cm, withinitial pressure 300 kPa and temperature 100°C, and a holewhose initial exhaust rate is 0.01 kg/s. Find the time re-quired for the tank density to drop by 50 percent.h(t)V = KtDiameter dCone␪␪PistonVR␪ dhConeP3.25 As will be discussed in Chaps. 7 and 8, the flow of a streamU0 past a blunt flat plate creates a broad low-velocity wakebehind the plate. A simple model is given in Fig. P3.25,with only half of the flow shown due to symmetry. Thevelocity profile behind the plate is idealized as “dead air”(near-zero velocity) behind the plate, plus a higher veloc-ity, decaying vertically above the wake according to thevariation u Ϸ U0 ϩ ⌬U eϪz/L, where L is the plate heightand z ϭ 0 is the top of the wake. Find ⌬U as a function ofstream speed U0.
    • *P3.30 The V-shaped tank in Fig. P3.30 has width b into the pa-per and is filled from the inlet pipe at volume flow Q. De-rive expressions for (a) the rate of change dh/dt and (b)the time required for the surface to rise from h1 to h2.P3.33 In some wind tunnels the test section is perforated to suckout fluid and provide a thin viscous boundary layer. Thetest section wall in Fig. P3.33 contains 1200 holes of5-mm diameter each per square meter of wall area. Thesuction velocity through each hole is Vs ϭ 8 m/s, and thetest-section entrance velocity is V1 ϭ 35 m/s. Assumingincompressible steady flow of air at 20°C, compute (a) V0,(b) V2, and (c) Vf, in m/s.188 Chapter 3 Integral Relations for a Control VolumeP3.33P3.34P3.35Df = 2.2 mD0 = 2.5 mVf V2 V1 V0L = 4 mTest sectionDs = 0.8 mUniform suction132Liquid oxygen:0.5 slug/sLiquid fuel:0.1 slug/s1100° F15 lbf/in2D2 = 5.5 in4000° R400 lbf/in2PropellantPropellantCombustion:1500 K, 950 kPaExit sectionDe = 18 cmpe = 90 kPaVe = 1150 m/sTe = 750 KP3.31 A bellows may be modeled as a deforming wedge-shapedvolume as in Fig. P3.31. The check valve on the left(pleated) end is closed during the stroke. If b is the bel-lows width into the paper, derive an expression for outletmass flow m˙ 0 as a function of stroke ␪(t).P3.32 Water at 20°C flows steadily through the piping junctionin Fig. P3.32, entering section 1 at 20 gal/min. The aver-age velocity at section 2 is 2.5 m/s. A portion of the flowis diverted through the showerhead, which contains 100holes of 1-mm diameter. Assuming uniform shower flow,estimate the exit velocity from the showerhead jets.P3.34 A rocket motor is operating steadily, as shown in Fig.P3.34. The products of combustion flowing out the exhaustnozzle approximate a perfect gas with a molecular weightof 28. For the given conditions calculate V2 in ft/s.P3.35 In contrast to the liquid rocket in Fig. P3.34, the solid-propellant rocket in Fig. P3.35 is self-contained and hasno entrance ducts. Using a control-volume analysis for theconditions shown in Fig. P3.35, compute the rate of massloss of the propellant, assuming that the exit gas has a mo-lecular weight of 28.P3.30P3.31P3.3220˚ 20˚QhhhStrokeLd <<hm0(t)θ(t)θ(3)(2) (1)d = 4 cmd = 1.5 cmd = 2 cm
    • P3.36 The jet pump in Fig. P3.36 injects water at U1 ϭ 40 m/sthrough a 3-in-pipe and entrains a secondary flow of waterU2 ϭ 3 m/s in the annular region around the small pipe.The two flows become fully mixed downstream, where U3is approximately constant. For steady incompressible flow,compute U3 in m/s.P3.40 The water jet in Fig. P3.40 strikes normal to a fixed plate.Neglect gravity and friction, and compute the force F innewtons required to hold the plate fixed.Problems 189FPlateDj= 10 cmVj= 8 m/sF00, V0, D0ρP3.40P3.41P3.36P3.38P3.39InletMixingregionFullymixedU1D1 = 3 inU2U3D2 = 10 inrh(t)VCV CVV(r)?V0Fixed circular disk1230°D1= 10 cmD2= 6 cmP3.37 A solid steel cylinder, 4.5 cm in diameter and 12 cm long,with a mass of 1500 g, falls concentrically through a5-cm-diameter vertical container filled with oil (SG ϭ0.89). Assuming the oil is incompressible, estimate the oilaverage velocity in the annular clearance between cylin-der and container (a) relative to the container and (b) rel-ative to the cylinder.P3.38 An incompressible fluid in Fig. P3.38 is being squeezedoutward between two large circular disks by the uniformdownward motion V0 of the upper disk. Assuming one-dimensional radial outflow, use the control volume shownto derive an expression for V(r).P3.39 For the elbow duct in Fig. P3.39, SAE 30 oil at 20°C en-ters section 1 at 350 N/s, where the flow is laminar, andexits at section 2, where the flow is turbulent:u1 Ϸ Vav,1΂1 Ϫ ᎏRr212ᎏ΃ u2 Ϸ Vav,2΂1 Ϫ ᎏRr2ᎏ΃1/ 7Assuming steady incompressible flow, compute the force,and its direction, of the oil on the elbow due to momen-tum change only (no pressure change or friction effects)for (a) unit momentum-flux correction factors and (b) ac-tual correction factors ␤1 and ␤2.P3.41 In Fig. P3.41 the vane turns the water jet completelyaround. Find an expression for the maximum jet velocityV0 if the maximum possible support force is F0.P3.42 A liquid of density ␳ flows through the sudden contractionin Fig. P3.42 and exits to the atmosphere. Assume uniformconditions (p1, V1, D1) at section 1 and (p2, V2, D2) at sec-
    • *P3.44 When a uniform stream flows past an immersed thickcylinder, a broad low-velocity wake is created downstream,idealized as a V shape in Fig. P3.44. Pressures p1 and p2are approximately equal. If the flow is two-dimensionaland incompressible, with width b into the paper, derive aP3.46 When a jet strikes an inclined fixed plate, as in Fig. P3.46,it breaks into two jets at 2 and 3 of equal velocity V ϭ Vjetbut unequal fluxes ␣Q at 2 and (1 Ϫ ␣)Q at section 3, ␣being a fraction. The reason is that for frictionless flow thefluid can exert no tangential force Ft on the plate. The con-dition Ft ϭ 0 enables us to solve for ␣. Perform this analy-sis, and find ␣ as a function of the plate angle ␪. Whydoesn’t the answer depend upon the properties of the jet?190 Chapter 3 Integral Relations for a Control Volume123θFt = 0(1-α)Q, VαQ, VFn, Q, A, VρP3.43 Water at 20°C flows through a 5-cm-diameter pipe whichhas a 180° vertical bend, as in Fig. P3.43. The total lengthof pipe between flanges 1 and 2 is 75 cm. When the weightflow rate is 230 N/s, p1 ϭ 165 kPa and p2 ϭ 134 kPa. Ne-glecting pipe weight, determine the total force which theflanges must withstand for this flow.Water jetD0 = 5 cmWP3.45P3.46formula for the drag force F on the cylinder. Rewrite yourresult in the form of a dimensionless drag coefficient basedon body length CD ϭ F/(␳U2bL).P3.45 In Fig. P3.45 a perfectly balanced weight and platform aresupported by a steady water jet. If the total weight sup-ported is 700 N, what is the proper jet velocity?P3.47 A liquid jet of velocity Vj and diameter Dj strikes a fixedhollow cone, as in Fig. P3.47, and deflects back as a con-ical sheet at the same velocity. Find the cone angle ␪ forwhich the restraining force F ϭ ᎏ32ᎏ␳Aj Vj2.P3.48 The small boat in Fig. P3.48 is driven at a steady speedV0 by a jet of compressed air issuing from a 3-cm-diame-ter hole at Ve ϭ 343 m/s. Jet exit conditions are pe ϭ 1 atmand Te ϭ 30°C. Air drag is negligible, and the hull drag iskV02, where k Ϸ 19 N и s2/m2. Estimate the boat speed V0,in m/s.Atmospherepap12121P3.42P3.43P3.4421UU2 LLUUL2tion 2. Find an expression for the force F exerted by thefluid on the contraction.EES
    • P3.49 The horizontal nozzle in Fig. P3.49 has D1 ϭ 12 in andD2 ϭ 6 in, with inlet pressure p1 ϭ 38 lbf/in2absolute andV2 ϭ 56 ft/s. For water at 20°C, compute the horizontalforce provided by the flange bolts to hold the nozzle fixed.tion 2 at atmospheric pressure and higher temperature,where V2 ϭ 900 m/s and A2 ϭ 0.4 m2. Compute the hori-zontal test stand reaction Rx needed to hold this enginefixed.P3.51 A liquid jet of velocity Vj and area Aj strikes a single 180°bucket on a turbine wheel rotating at angular velocity ⍀,as in Fig. P3.51. Derive an expression for the power P de-livered to this wheel at this instant as a function of the sys-tem parameters. At what angular velocity is the maximumpower delivered? How would your analysis differ if therewere many, many buckets on the wheel, so that the jet wascontinually striking at least one bucket?Problems 191P3.48P3.49P3.50P3.51P3.52Conical sheetθJetFCompressedairV0VeDe= 3 cmHull drag kV0212OpenjetWaterPa = 15 lbf/in2 abs1 2Rxm fuelCombustionchamberJetBucketWheel, radius RΩθθV1V2Top view2whSide viewP3.47P3.50 The jet engine on a test stand in Fig. P3.50 admits air at20°C and 1 atm at section 1, where A1 ϭ 0.5 m2and V1 ϭ250 m/s. The fuel-to-air ratio is 1:30. The air leaves sec-P3.52 The vertical gate in a water channel is partially open, asin Fig. P3.52. Assuming no change in water level and ahydrostatic pressure distribution, derive an expression forthe streamwise force Fx on one-half of the gate as a func-tion of (␳, h, w, ␪, V1). Appl