The fluid mechanics of natural ventilation - displacement ventilation by buoyancy-driven flows assisted by wind

This paper describes the fluid mechanics of natural ventilation by the comb ined effects of buoyancy and wind. Attention is restricted to transient dra ining flows in a space containing buoyant fluid, when the wind and buoyancy forces reinforce one another. The flows have been studied theoretically an d the results compared with small-scale laboratory experiments. Connections between the enclosure and the surrounding fluid are with high-level and lo w-level openings on both windward and leeward faces. Dense fluid enters thr ough windward openings at low levels and displaces the lighter fluid within the enclosure through high-level, leeward openings. A strong, stable strat ification develops in this case and a displacement flow is maintained for a range of Froude numbers. The rate at which the enclosure drains increases as the wind-induced pressure drop between the inlet and outlet is increased and as the density difference between the exterior and interior environmen t is increased. A major result of this work is the identification of the fo rm of the nonlinear relationship between the buoyancy and wind effects. It is shown that there is a Pythagorean relationship between the combined buoy ancy and wind-driven velocity and the velocities which are produced by buoy ancy and wind forces acting in isolation. This study has particular relevan ce to understanding and predicting the air flow in a building which is nigh t cooled by natural ventilation, and to the flushing of gas from a building after a leak. (C) 1999 Elsevier Science Ltd. All rights reserved.