This work aims to investigate the comparative effect of two passive flow controls in modifying the mean wake topology around a simplified square back vehicle model. The two passive flow controls are (i) Single cavity and (ii) Multi-cavity. A straight cavity with an optimum depth at the rear base of a vehicle is a well-known technique used to alter the mean wake topology and achieve drag reduction[1]. For two dimensional bluff bodies, a multi-cavity is known to deliver better drag reduction at shorter cavity depths in comparison to a single cavity[2]. With this viewpoint, a numerical investigation is carried out to examine the performance of a multi-cavity over a single cavity in drag reduction for a three-dimensional bluff body vehicle model. The numerical simulations are performed at Reynolds Number (Re) = 1 × 105 using the k-ω SSTSAS turbulence model in a Finite volume open-source code OpenFOAM. The investigations revealed, for any cavity depth, a single cavity always performed better than multi-cavity in reducing drag. However, at optimum cavity depth equal to 33% of the body height, the drag reduction magnitude was identical for both the flow controls. The plausible mechanisms responsible for their relative difference in performance will be explored by analyzing the base pressure distribution, wake mean topology, and the temporal behavior of the wake.

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