The effect of gradual changes in wind speed or heat load on natural ventilation in a thermally massive building

We examine the transitions in flow regime which can occur in naturally ventilated thermally massive buildings subject to changes in the wind and buoyancy forcing. For a range of heat loads there are both wind-dominated and buoyancy-dominated flow regimes. However, outside this range, only the steady state wind-dominated or buoyancy-dominated flow can develop. As a result of this non-linearity, and the different timescales for the evolution of the air and of the thermal mass, the transient evolution of the system caused by changes in either the heat load or the wind forcing can be complex. We develop a simplified model to identify the influence of the thermal mass on transitions in flow regime caused by changes in heat load or wind forcing. We show that the interior air responds rapidly to changes in the forcing, and as a result, the thermal mass can then act as a slowly evolving heat source or heat sink. In some situations this can lead to temporary buffering of the interior temperature, followed by a second, rapid transition in the interior temperature and ventilation regime as the system adjusts to the new steady state.