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ON THE USE OF COMPUTATIONAL FLUID DYNAMICS FOR MODELLING BUOYANCY DRIVEN FLOWS

Buoyancy driven flows have always been, and still are, difficult to model using CFD programs. Much validation work is required along with guidelines for the CFD practitioner about how to model such flows. This paper makes a contribution to these two areas by considering buoyancy driven displacement ventilation in a very simple geometry. The nature of buoyancy driven displacement ventilation is briefly described and guidance offered on how to represent the problem in a CFD program. Simulations were carried out using two eddy viscosity turbulence models. The results show good qualitative agreement with experimental data and analytical results but small discrepancies in some of the quantitative results. INTRODUCTION Buildings must be provided with adequate ventilation. Environmental concerns together with demands for energy efficiency are prompting designers to investigate natural ventilation as an alternative to mechanical ventilation or full air conditioning. Buoyancy driven displacement ventilation is proving to be a popular approach, and it is the modelling of this phenomenon, using Computational Fluid Dynamics (CFD), that is addressed in this paper. Buoyancy driven displacement ventilation occurs when thermal plumes produced above occupants and equipment cause a layer of warm, buoyant air to form close to the ceiling of the space. This layer drives a flow through high-level openings and causes fresh, ambient air to be drawn in at low level. A prominent feature of this flow is the formation of a horizontal interface separating the warm, buoyant air above, from the ambient, cooler air below. A theory for this type of flow was described by Linden et al. (1990) who considered displacement ventilation of a simple rectangular box. To visualise these flows, the salt bath modelling technique was employed. The results of these experiments compared favourably with the theory. The CFD approach is being used to explore the same idealistic conditions. Buoyancy driven flows are notoriously difficult to model using CFD techniques for the following reasons: small driving forces lead to numerical instabilities; there is uncertainty regarding how to accurately model turbulence; and the flow is implicitly specified (i.e. sources of buoyancy are specified from which the program must calculate a velocity field). Consequently, much validation work is needed to evaluate the ability of these codes to model such flows. It is the intention of this paper to offer guidance on how to model buoyancy driven flows and the accuracy which can be expected from a commercial CFD package. The simulations reported here comprise simple geometries with a single source of buoyancy, but it is hoped that the conclusions drawn and the experience gathered can be used in modelling more complex, real buildings. THEORY OF BUOYANCY DRIVEN DISPLACEMENT VENTILATION A good example of a buoyancy driven flow is displacement ventilation in which use is made of the buoyancy forces produced by naturally occuring heat sources such as occupants and equipment (e.g. computer terminals, projector units, etc). The theory for this type of flow was elucidated by Linden et al. (1990) who considered displacement ventilation of a simple rectangular box (fig. 1). The heat source produces a rising plume which entrains fresh (ambient) air taking it upwards into the upper region of the space where it is recirculated. As more and more light air accumulates in the upper region of the space, the rising plume begins to entrain this lighter air until a steady-state is achieved (fig. 1), in which a constant interface forms (at y h = ) separating the dense, cool