Multiple Steady States in Buoyancy Driven Natural Ventilation

When designing natural ventilation for complex buildings, it is key to understand whether there is a unique solution for the flow, or if multiple flow regimes are possible. Here, we show that in a 2 storey open plan office type building, in which the floors are connected to a common atrium, two stable flow regimes may occur under identical conditions. In one stable flow regime, the upper floor acts as part of the outflow stack for the lower floor, whereas in the complementary stable flow regime, both floors draw air from the exterior and this is then heated and vents from a central stack at the top of the atrium. We show that the existence of these flow regimes depends on the relative heat load on each floor, as well as the size of the vents connecting each floor to the exterior. We also identify a strategy to control the environment and enable a transition from one regime to the other regime.