• Bluff bodies
• boat tail
• drag reduction
• experiments
• flow separation
• grooves
• optimization
• passive control
• simulations

In the present work the possibility of controlling flow separation in boat-tailed bluff bodies through optimized contoured grooves is investigated by means of experiments and numerical methodologies. The considered geometry is an axisymmetric blunt based model, having a forebody with a 3:1 elliptical contour and a cylindrical main body followed by a circular-arc boat-tail. The diameter-to-length ratio is 0.175 and the boat-tail is half diameter long. The Reynolds number, based on the model diameter, is .
The grooves start with a sharp edge, have an upstream part with a circular-arc shape and end with a spline leading to a point on the original boat-tail contour.
The effectiveness of the contoured grooves in moving downstream the separation point is assessed by using both VMS-LES simulations and experiments. In order to identify the best possible values of the parameters defining the groove geometry, a preliminary optimization procedure has been developed by using the RANS approach for the numerical evaluation of the device performance. In all the considered boat-tails, the presence of the contoured groove improves the performance by reducing the aerodynamic drag. The success of the proposed flow control strategy is due to the relaxation of the no-slip condition in the small recirculation region inside the groove, which reduces the momentum losses near the wall and thus delays the boundary layer separation. The maximum depth of the contoured grooves has been limited to one third of the incoming boundary layer thickness in order to avoid possible problems linked to the stability of the flow configuration. In order to delay as much as possible the flow separation, the contoured groove needs to be placed slightly upstream of the separation point of the reference boat-tail.