The present work focuses on a passive strategy consisting in the introduction of properly contoured transverse grooves to delay the flow separation occurring on a boat-tailed bluff body before its sharp-edged base. We consider a two-dimensional body having a cross-section with a 3:1 elliptical forebody and a rectangular main part followed by a circular-arc boat tail. We carry out Variational Multiscale Large Eddy Simulations at Re = Du/v = 9.6 × 104. A boat-tail drag reduction of the order of 9.7% is produced by the significant delay of the flow separation caused by the groove and by the consequent increase of the base pressure. This effect is mainly due to the relaxation of the no-slip condition over the small and steady recirculation region inside the groove, which reduces the momentum losses near the wall and thus delays boundary layer separation. The flow control device is also robust to small variations of the groove location and depth.

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