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.
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