On transitions in natural ventilation flow driven by changes in the wind

The interaction between wind and buoyancy in naturally ventilated buildings can lead to multiple steady states. Previous studies have shown that there may exist a stable wind-dominated regime, an unstable wind-dominated regime, and a stable buoyancy-dominated regime. Here we examine the transient evolution of the flow between these regimes as a result of changes in the wind forcing. We show that the transition from the wind-dominated regime to the buoyancy-dominated regime occurs as the wind forcing decreases below a critical value. However, the reverse transition, from the buoyancy-dominated regime to the wind-dominated regime, only occurs if there is a sufficiently large and rapid increase in the wind forcing. We calculate the instantaneous increase in wind forcing required for the system to evolve from the buoyancy-dominated regime to the wind-dominated regime, and we also determine the minimum rate of increase of the wind forcing for such a transition to occur. We discuss the implication of our results for building design.