Steady-state flows in an enclosure ventilated by buoyancy forces assisted by wind

Abstract

We examine ventilation driven by a point source of buoyancy on the floor of an enclosure in the presence of wind. Ventilation openings connecting the internal and external environment are at high level on the leeward façade and at low level on the windward façade, so that the wind-driven flow in the enclosure is in the same sense as the buoyancy-driven flow. We describe laboratory experiments that determine the parameters controlling the ventilation under these conditions and compare the results with predictions of a theoretical model.Previous work has shown that when ventilation is driven solely by a single localized source of buoyancy flux B, a stable, two-layer stratification and displacement flow forms. The steady height of the interface, between the buoyant upper layer and the lower 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 into the plume.For wind-assisted flows, the ventilation is increased owing to the wind pressure drop δ between the windward and leeward openings. The two-layer stratification and displacement flow are maintained over a range of wind speeds, even when the wind-induced flow far exceeds the flow induced by the buoyancy force. The steady height of the interface depends upon the Froude number Fr = (Δ/ρ)1/2(H/B)1/3 and the dimensionless area of the openings A*/H2. Increasing the wind speed raises the position of the interface and decreases the temperature of the upper layer (as does increasing A*/H2), while increasing B lowers the level of the interface and increases the temperature of the upper layer. For significantly larger Fr, the displacement flow breaks down and we investigate some aspects of this breakdown. The implications of these flows to passive cooling of a building by natural ventilation are discussed.