Wake bi-modal switching and its dependence on the upstream bluff body boundary layers

The wake of the squareback Ahmed body exhibits a random bi-modal switching in one of the crossflow directions. This study investigates numerically the sensitivity of wake bi-modality to upstream disturbances related to the boundary layers (BL) on the longitudinal Ahmed body surfaces. Close to the body nose, BL on the top and the side surfaces separate and reattach before reaching the base, where a massive separation occurs forming the wake. Hairpin vortices are generated close to the reattachment points. These vortices grow along the surfaces reaching a critical point -beyond which their interaction with the freestream leads to vortex breakdown. The resulting smaller vortices from all the surfaces interact periodically as they convect downstream. The associated flow disturbances are suggested to be the main trigger of the wake switching. Here, we investigate suppressing the near-nose BL separation using a suction velocity. When only the top-surface BL is suppressed, bi-modal switching still occurs. The BL suppression interrupts the interaction of the vortices upstream of the base and increases the fluctuations of the wake around its asymmetric position. Suppressing BL on both sides is found to completely suppress bi-modal switching and lock the wake in an asymmetric position.