Passive flow control on suppressing wake switch dynamics of a square-back vehicle

The square-back vehicle with a specific width/height ratio exhibits a pronounced bi-stable wake characterized by random switching between two asymmetric states, which strongly influences its base pressure and aerodynamic forces. This study investigates the suppression of wake bi-stability using two passive flow control strategies: a rear base cavity and a top-mounted flap. The unsteady wake is resolved using the improved delayed detached-eddy simulation method. The baseline configuration displays persistent low-frequency switching associated with large-scale vortex asymmetry and a significant base pressure deficit. Both the base cavity and top flap stabilize the near-wake flow by weakening the large-scale recirculation structures responsible for bi-stable behavior. The suppression of wake switching is accompanied by a substantial reduction in turbulent kinetic energy within the recirculation region and a clear recovery of the mean base pressure, particularly over the central base area. As a direct consequence of bi-stability suppression and wake stabilization, a reduction in aerodynamic drag is achieved. The results demonstrate the strong potential of passive geometric modifications for suppressing wake bi-stability and improving overall the aerodynamic performance of the vehicle.