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Numerical simulation of wind flow around building complex with different software approaches

Numerical simulation of wind flow around 4-building complex in hot climate conditions is described in order to assess a wind comfort in the courtyard area. The solution of the air flow around the multiply building complex geometry including the using of various numeral simulation software packages is presented in this article. The article also provides the subsequent comparison of the results obtained from specialized and simplified software packages (ANSYS FLUENT, Autodesk Flow design). Use of the numeral simulation in conditions of technogenic relief and complicated construction is relevant at the present time. This method allows significantly reducing the costs of undoing mistakes and to speed up the problem solving. An explicit demonstration of the results is also a very important criterion for choosing the methods and numeral simulation software. Use of modern methods of CFD-data visualization in real time can greatly simplify the interpretation of simulation results for non-professional users.


Introduction
Planning and design issue of large cities, in connection with environmental and climatic factors is relevant for the urban planning theory and practice. This problem is related to increasing urbanization. The air pollution just as the wind and heating comfort is an acute problem for the modern city inhabitants living in conditions of high-density. The Urban Science advanced applications also concern the regions with a hot climate. An average annual temperature in these regions is about 20 ºC. This climate is rather common, and it is more related to southern and near the equator countries [1].
Degradation of microclimate in modern cities is explained by the fact that the area climate specificities were not carefully taken into account during design phase. One of the most effective and sustainable methods of microclimate improvement and regulation in hot countries is mainstreaming of aeration at the beginning of buildings and architectural complexes design. It is necessary to forecast and calculate the impacts of taken design decisions that were based on the natural and technological landscape analysis [2].
Aeration is the process of natural and regulated ventilation of the cities territories and other human settlements [3]. There are various methodologies approaches to study and forecast the aeration schemes of residential development. These methodologies were highlighted in several works that demonstrated the theoretical and experimental researches of the aeration processes of various territories [4,5].
The most progressive approaches to the research of aeration regime and microclimate forecasting for the architectural complexes are based on methods of numerical simulation and computer analysis of air masses movement in the area studied [6,7].
Its main objective is to resolve the tasks of CFD simulation (computational fluid dynamics) with the aid of software complexes [8]. At present, there are many software solutions which are intended for computer simulation of wind influence on buildings and structures by the CFD methods. Some complexes focus on narrowly defined tasks, while others imply a more general approach.
In addition, computer modeling of the experiments with the use of different software packages is an interesting and relevant issue. This process also involves comparison of results and evaluations of their matches taking into account the calculation time, and optimal choice. This article describes the solution of a particular problem of simulation the air flow around the architectural complex of buildings in the city of Erbil (Iraqi Kurdistan) in order to review its optimal location as well as to assess the quality of temperature comfort and parameters of courtyard microclimate.

The software review and selection variants for experimentation.
In this paper, the simulation will be implemented through two distinct software complexes. There are relatively large numbers of CFD packages exist currently on the market. These software packages have different specializations and characteristics. The major solutions of CFD simulation were reviewed in Broekhuizen's work in 2016 [9]. It is possible to select the two main software packages through their capabilities for the urban planning application as the professional and simplified variants: Ansys fluent as a reference method of simulation due to highest prevalence and well described calculation methods; Autodesk Flow Design as a software which is oriented on less advanced researchers. This software also implies quicker experimentation cycles and simple configuration with acceptable. These complexes were selected for simulation and qualitative comparison of results.

The baseline data review for numerical experiment.
Experimentation methodology can be described in terms of the following stages: 1) Obtaining of baseline data on building and environmental parameters; 2) Modelling of building geometry in two versions; 3) Adjust settings for each software package; 4) Obtaining and visualization of results; 5) Qualitative image comparison and numerical values analysis; 6) Definition of desired outcomes and verification of conformity.
The blueprints of the building located in Erbil (Kurdistan) were used as the baseline data. This construction is an architectural complex comprising a group of four combined buildings. It is necessary to figure out a forecast microclimate in the residential area between the buildings considering a person's height, and using the method of experimental modeling [10][11][12]. Evaluation the level of wind comfort in this zone is needed as well. Building orientation was defined according to plan. It helped to construct the original model in the right way. And after that the data were superimposed on the wind rose.
The geometrical dimensions of buildings with a dominant wind direction mark for this point were taken from the plan, and are shown in figure 1-a. On the basis of given baseline data, the starting point for experiment will be the following values of the experiment parameters: geometrical dimensions in the plan -117 meters x 95 meters; predominant wind direction is Northern; average wind speeds -6 International Conference on Construction, Architecture and Technosphere Safety  Let's consider the first reference case with the use of ANSYS Fluent software package. Calculation for this software was carried out according to the following algorithm: 1.
The preparation of the calculation model:  Optimization of the polygon model with the aim of creating a solid-state parametric model.
 Importing of the polygon model into the Space Claim (solid-state modelling editor).
 Building a geometric area of the air flow around the building for creating computational domain.
 Generation of mesh model based on the constructed geometry in ANSYS MESHING module.
 Setting up the boundary and initial conditions, physical model of calculation -processing.

2.
Solution of problem in the calculation module.

3.
Viewing and evaluation of results -post-processing.
Construction of calculating area is based on the rules described in the article written by Poddaeva [13]. The purpose is to reduce the impact of the region boundaries on calculation. Center of coordinates remain in the center of the researched block of buildings. The block of buildings was rotated 45 degrees in order to be compatible with the researched wind rose of windy object.
А ≥ 5Нmax distance from the center of the object to the edge of the calculating area. В ≥ 5Нmax distance from the center of the object to the Inlet boundary. С ≥ 15Нmax the distance from the center of the object to the Outlet boundary. Hmax -The largest dimensional object size.
Quality of the finite element mesh construction is crucial in accurate calculation results achievement. This quality is important during the process of finite element modeling.
Mesh density was regulating by indication of the maximum value for finite element edge, while the experiment was carried out. (Max Face Size Parameter). Set point for parameter was 10 meters. The mesh density was increased (Face Sizing Function) because of the need for accurate calculation near the buildings. The maximum edge size was 2 meters (*near the buildings). As a result the finite element mesh consisting of 4925989 components was obtained.  Figure 2. Calculating area sizes.
Quality of the finite element mesh construction is crucial in accurate calculation results achievement. This quality is important during the process of finite element modeling.
Mesh density was regulating by indication of the maximum value for finite element edge, while the experiment was carried out. (Max Face Size Parameter). Set point for parameter was 10 meters. The mesh density was increased (Face Sizing Function) because of the need for accurate calculation near the buildings. The maximum edge size was 2 meters (*near the buildings). As a result the finite element mesh consisting of 4925989 components was obtained.
Experience of research in the wind tube demonstrates that it is necessary to follow the geometric, cinematic and dynamic similarity during the experiment [14][15][16]. The experiment reproduces completely and geometrically the real building sizes. Cinematic and dynamic similarity of an oncoming wind air flow is provided by the ANSYS software package and Fluent Flow analysis. The conditions for a change speed profile and height in calculation module ANSYS Fluent are set through the connection to boundary conditions of data tables including the rate of speed (csv format) [17]. Average flow speed is 6 meters per second. The flow is directed at an angle to building in accordance with the wind direction to wind rose.

For Autodesk Flow Design case.
Let's consider the setup process for modelling a similar task using the Autodesk Flow Design. This software package supports dynamic modelling of gas processing in real time. The software also allows viewing results as a cutting plane and three-dimensional demonstration with the help of supporting particles or visual presentation of air flows.
This software relates to the «meshless» type. This type provides number of advantages in speed and simplicity of work. Universal approach of visualization was chosen for comparison of all results of purging. This visualization is presented in the form of counter plane.
Calculation is carried out according to the following algorithm: 1) Preparation of polygonal model in the original scale; 2) Downloading models in program; 3)Setup of parameters for virtual wind tube according to recommended speed -6 meters per second; 4)Setting the initial conditions, choice of purging area sizes. It is unnecessary to comply with the requirements concerning the size of calculating area in accordance with ANSYS, but it is recommended to maintain the significant distances around model to minimize the impact of boundary areas.
The following settings for setup were chosen using the empirical selection (on the basis of analysis of area sizes & cell size & modelling speed): size of the modeling area around the object: 1600x1000x300 meters; size of the voxel: 3.3 meters on the side.

Results and analysis of simulation data for verification of environmental comfort in the builtup area
During the course of numerical simulation in Ansys Fluent the following results were obtained: contour scheme displaying 10 levels of wind speed (parameter contours count) in a horizontal projection at the level of 2 meters above the ground with a range limit of visualized speeds from 0 to Unlike Ansys Fluent, the results in the second case can be obtained at any time as a flat image, animated vectors, or video. In this case, a variant of the same plane was chosen with a cut at a height of 2 meters. Character visualization in this software package is different from that in the other packages, but is sufficiently informative. Figure 3. shows a snapshot of the results of modeling the flow velocity at a height of 2 m for both packages overlaid with mirror horizontal separation in graphics editor.
Flow Design does not have visualization settings for the contours and their quantities, as well as the built-in tool for point retrieval of characteristics by coordinates, therefore, the speed values can be estimated only visually. In view of the difficulty of characterization, it was decided to reduce the simulation area to the immediate surroundings of the building in an area of reduced detalization.  The results of Ansys Fluent were used as a reference result to check the quantitative characteristics of the speed of wind flow between buildings. According to table 1 the average speed of the wind in the area between the buildings can be considered to be more than 3 m / s. At the same time, according to [14,18], a speed of up to 4 m / s can be considered a comfortable wind speed in a residential zone. Consequently, the simulation results can be assumed that the wind speed in this zone is considered comfortable for people, it is worth noting that with height variation average velocity increases significantly, and in the field of flow separation near the wall and can reach high impulses to 8-10 m/s. Also, for a qualitative analysis of the suitability of simulation results in flow design, the images were combined in a graphical editor and each placed along the axis of symmetry with different location, Fluent results are places at the bottom (Figure 3-a) and at the Flow Design [19] at Figure 3-b. These images clearly show a significant visual correspondence of the nature of the flows, especially in the areas of the borders of abrupt changes in velocity (contrast transition), which repeat exactly the contours in both images. It should be noted that for a detailed analysis of the flow rate in the interval between buildings, the equivalent setting of the zone parameters is not enough, due to the low detail of the color contours in the zone of interest.

Findings
In this article, the practical implementation of setting up a computer model of air streams wrap around the complex of four buildings of complex shape was considered.
Mathematical modeling and simulation were performed on the source data at real scales using different modeling software packages and the data obtained were combined on the same graphic image to reveal the visual similarity of the graphic image of the flow map. In summary the following conclusions can be drawn: after analyzing the flow rate using classical CFD modeling tools, it was concluded that the wind level is comfortable in the built-up area; at the same time, a similar result could be obtained using a much simpler Autodesk Flow Design package; flow design modeling package was the easiest to set up and allows you to quickly get the initial estimated results sufficient to quickly test hypotheses at an early stage, followed by modeling in professional packages. These experiments and datasets may be useful for further research on methods of simplified modeling of wind comfort in town planning tasks for non-professional CFD users [20].