Wind fluctuations alter expectations of natural ventilation: experimental evidence and stochastic modelling

This work experimentally investigates the natural ventilation of an emptying filling box under stochastic forcing in an opposing wind. A point buoyancy source on the floor generates a turbulent plume that rises towards the ceiling, producing a buoyant layer. The resulting stack effect induces a volume flux exiting through the high-level opening, equal to that entering through the low-level opening. The buoyancy-induced pressure is reduced by an opposing wind. If the wind velocity exceeds a critical threshold, two different states may occur: either the stratification is preserved although the buoyant layer thickens, or a well-mixed regime takes place. We find that if wind undergoes stochastic fluctuations, a noise-induced phenomenon occurs: when the stratification persists, the height of the interface fluctuates around a mean value significantly lower than that exhibited in the constant-wind scenario. The deviation from the constant-wind equilibrium increases as the mean wind velocity and the noise intensity grow. Moreover, the transition to the well-mixed regime may occur for wind velocity lower than the critical threshold under constant wind conditions. These results confirm the predictions given by a theoretical model, where the system is forced by stochastic wind fluctuations modelled with an Ornstein–Uhlenbeck process. This modelling choice is supported by the experimental characterisation of the pressure field. Lastly, we provide an approximate analytical solution for the system’s mean behaviour.