J. Fluid Mech. (2001), vol. 426, pp. 355–386. Printed in the United Kingdomc 2001 Cambridge University Press355Steady-state flows in an enclosure ventilated bybuoyancy forces assisted by windBy G. R. HUNT† AND P. F. LINDEN‡Department of Applied Mathematics and Theoretical Physics, University of Cambridge,Silver Street, Cambridge, CB3 9EW, UK(Received 10 March 1998 and in revised form 7 July 2000)We examine ventilation driven by a point source of buoyancy on the floor of anenclosure in the presence of wind. Ventilation openings connecting the internal andexternal environment are at high level on the leeward fac¸ade and at low level on thewindward fac¸ade, so that the wind-driven flow in the enclosure is in the same senseas the buoyancy-driven flow. We describe laboratory experiments that determine theparameters controlling the ventilation under these conditions and compare the resultswith predictions of a theoretical model.Previous work has shown that when ventilation is driven solely by a single localizedsource of buoyancy flux B, a stable, two-layer stratification and displacement flowforms. The steady height of the interface, between the buoyant upper layer and thelower layer at ambient density ρ, is independent of B and depends only on the‘effective’ area A∗of the openings, the height H of the enclosure and entrainment intothe plume.For wind-assisted flows, the ventilation is increased owing to the wind pressuredrop ∆ between the windward and leeward openings. The two-layer stratificationand displacement flow are maintained over a range of wind speeds, even when thewind-induced flow far exceeds the flow induced by the buoyancy force. The steadyheight of the interface depends upon the Froude number Fr =(∆/ρ)1/2(H/B)1/3andthe dimensionless area of the openings A∗/H2. Increasing the wind speed raises theposition of the interface and decreases the temperature of the upper layer (as doesincreasing A∗/H2), while increasing B lowers the level of the interface and increasesthe temperature of the upper layer. For significantly larger Fr, the displacement flowbreaks down and we investigate some aspects of this breakdown. The implications ofthese flows to passive cooling of a building by natural ventilation are discussed.1. IntroductionA major part of energy expenditure in modern buildings is due to air condi-tioning and other mechanical means of ventilation. Operating correctly, naturallyventilated buildings typically consume less than a third of the energy of comparableair-conditioned buildings (Energy Consumption Guide 19, Best Practice Programme,Energy Efficiency in Offices, BRESCU, Energy Efficiency Office, October 1991). Nat-ural ventilation is driven by wind, and buoyancy forces associated with density† Present address: Department of Civil & Environmental Engineering, Imperial College ofScience, Technology and Medicine, London SW7 2BU, UK.‡ Present address: Department of Mechanical & Aerospace Engineering, University of California,San Diego, 9500 Gilman Drive, La Jolla, CA 92093-0411, USA.356 G. R. Hunt and P. F. Lindendifferences between the internal and external environment. In buildings, the densitycontrast is usually generated by temperature differences but may also arise due toa difference in composition, e.g. in a building filled with smoke, or due to a releaseof a chemical substance, e.g. natural gas. Flows in buildings driven by density dif-ferences are commonly referred to as stack-driven flows, and they may be enhancedby increasing the density contrast between the internal and external environment, byincreasing the height between the inlet and outlet openings (the stack), by increasingthe area of the vents, or by suitably harnessing the force of the wind. The flow ofwind around a building produces a dynamic (or wind) pressure distribution over itsexternal surface with, in general, positive wind pressures on the windward fac¸adesand negative wind pressures in the lee and in regions of flow separation. By locatingventilation openings in regions of positive and negative pressures, the wind may beharnessed to drive a flow in the interior. The driving force produced by the windis free and buoyancy-driven ventilation is typically produced by heat gains fromsolar radiation, occupants, machinery, electrical equipment, etc., or as a consequenceof warming the air inside the space in order to provide comfort for the occupants.Natural ventilation has, therefore, the potential to provide an energy efficient meansof ventilating a space.To be effective, a natural ventilation system must meet specific requirements for thesupply of fresh air throughout the year. These requirements are normally to providefresh air for respiration and for the removal of carbon-dioxide, odours and excessheat. In winter these requirements can usually be met with relatively low ventilationflow rates, and in office spaces typically 8 l s−1of air per person are recommended(Building Regulations: part F1 (Ventilation) HMSO, 1991). In summer the mainrequirement of a natural ventilation system is to remove excess heat in order tomaintain internal air temperatures at comfortable levels. Non-domestic buildingstypically experience the largest heat gains during the occupied daylight hours, whenthe need for ventilation is greatest, and minimal gains at night when the space isunoccupied. Different natural ventilation strategies may be adopted depending uponthe seasonal or diurnal requirements. For example, if cool air is introduced at highlevel it will tend to mix with the air in the space. This mixing ventilation can beused to temper the ambient air and it provides fairly low ventilation rates. If coolair is introduced at low level, displacing warm air out through openings at highlevels – displacement ventilation – larger ventilation rates are produced.In this paper, we focus attention on displacement ventilation for which the ventila-tion rate is determined by the areas of the upper and lower openings and the verticaldistance separating them, the temperature and depth of the warm upper zone, andthe driving produced by the wind. A key consideration in the design of stack-drivennaturally ventilated buildings is the depth and temperature of the warm upper zone;it must be maintained at a depth sufficient to drive the required flow through thebuilding, while ensuring it is above the occupied levels. If the openings are too smallor if their locations are not appropriately chosen, the