• drag reduction
• boundary layer separation
• flow control

The present work investigates on the performance of a passive control method to delay boundary layer separation, consisting of the introduction of properly contoured transverse grooves on the surface of a two-dimensional boat-tailed bluff body. This technique has already shown to improve the drag-reducing performance of axisymmetric boat tails thanks to the recirculation region forming inside the groove, which causes the relaxation of the no-slip condition, and thus reduces the near-wall momentum losses and delays boundary layer separation.
The aim is now to assess the effectiveness of this strategy even in presence of a strong vortex-shedding phenomenon, characterizing the highly fluctuating wake of a two-dimensional case.
The considered body’s cross-section is composed by a 3:1 elliptical forebody and a rectangular main part followed by a circular-arc boat tail, while the shape of the grooves and their depth – which must be significantly smaller than the thickness of the incoming boundary layer – are chosen to ensure that the flow recirculation within the cavity remains steady and stable.
Variational Multiscale Large Eddy Simulations (VMS-LES) are carried out at R_e=Lu_∞ ν=5.5×〖10〗^5 and show a base-drag reduction of the order of 9.7 %.
A sensitivity analysis, by means of supplementary simulations, is also discussed in order to prove the robustness of the grooves’ effect to the variation of the geometrical parameters defining their shape and position.