Transitions between the multiple steady states in a natural ventilation system with combined buoyancy and wind driven flows

Natural ventilation systems may have multiple steady states in the combined buoyancy and wind driven mode due to the nonlinearity of the systems. Previous studies have shown that some of the steady states are locally stable for small disturbances. However, the system can flip over from one stable steady state to another under sufficiently strong perturbations. In this paper, the mechanism of such state transitions is quantitatively investigated by a dynamical system approach.The transition dynamics between the stable steady states is examined by the system's responses to two types of perturbations—heat source fluctuations and wind variations. Two important parameters—the minimum perturbation magnitude and the minimum perturbation time to switch from one stable steady state to another—are defined to describe the transition requirements. The result from a previous experimental study was discussed and explained by these state transition behaviors.The transition dynamics between two stable steady states under perturbations are found important to the robustness of the stable steady states, which can be quantitatively described by the minimum perturbation time and the minimum perturbation magnitude. The experimental and numerical simulation results from another existing study are successfully explained by these two parameters. The applications of the developed perturbation method are further discussed.