Effects of approach flow conditions on the unsteady three-dimensional wake structure of a square-back Ahmed body

This paper investigates the effects of approach flow conditions on the unsteady three-dimensional wake characteristics of a square-back Ahmed body using improved delayed detached eddy simulations. Two approach flow conditions were investigated: a reference uniform flow (case A) and a thick turbulent boundary layer (TBL) (case B), where the Ahmed body is fully submerged in the TBL, $\delta >H$, where $\delta$ is the boundary layer thickness and $H$ is the total height of the body from the ground. Case A is studied extensively in the literature, whereas case B is less studied. The present results showed that the wake structure of case B is significantly different from that of case A. In particular, the wake of case A is dominated by a strong downwash flow from the top surface, but case B exhibits a much stronger upwash flow, which is induced by lower momentum of the fluid that emanates from the ground clearance and the pressure imbalance generated behind the body. For case A, conditional averaging of the flow field based on the sign of the drift force demonstrated the occurrence of the well-known wake switching (bimodality) event in the spanwise direction. However, the bimodality was completely suppressed in the wake of the Ahmed body submerged in the TBL. Both time-averaged turbulence statistics such as the Reynolds stresses and production terms, and time-resolved statistics including spectral analysis and temporal cross correlations are used to explore the differences between the wake structure of the Ahmed body in a uniform flow and thick TBL.