Drag reduction in a spanwise-infinite boat-tailed body through multiple transverse grooves

Abstract

Appropriately-designed spanwise-extruded small grooves, oriented transverse to the freestream flow, may delay flow separation in adverse pressure gradients by generating local and stable recirculation zones. In this study, we explore the potential of multiple grooves to reduce the drag on a boat-tailed bluff body characterized by vortex shedding. Numerical simulations reveal that the presence of two consecutive transverse grooves results in a maximum boat-tail drag reduction of 23.2%, further decreasing the drag by 15% when compared to the performance of the same boat-tailed bluff body with a single groove. This flow-control mechanism proves effective when the flow reattachment takes place downstream of both grooves throughout the entire vortex-shedding cycle, leading to the formation of two distinct local recirculation zones within the grooves. Lower frictional losses are observed along the streamline bounding the recirculation within the grooves as compared to the friction losses caused by the no-slip boundary conditions along the lateral walls of a plain boat tail (i.e., without grooves). Consequently, the boundary layer exhibits higher momentum downstream of the grooves, resulting in a delayed flow separation and an improved reduction in body drag for the boat tail.