• flow control
• experimental
• boat tail
• grooves
• numerical

The present work describes the results of an experimental and numerical investigation aimed at characterizing the performance of a recently introduced passive method to delay flow separation. The technique is based on the use of suitably-shaped transverse grooves.
In previous works small contoured cavities were shown to be able to delay flow separation and to increase the efficiency of diffusers in both laminar and turbulent flow. Now we apply the same flow control strategy to an external flow, by introducing one transverse groove, i.e. a small cavity, over different boat tails applied on the rear portion of an axisymmetric body whose configuration without boat-tail had already been investigated experimentally and numerically. Boat-tailing is a well-known drag reduction method and consists in a gradual decrease of the body cross-section before a sharp-edged base, where the pressure is consequently increased. The drag-reducing performance of boat tails improves with the reduction of the base cross-section, but only as long as no significant boundary-layer separation is present over the lateral surface. Hence, we combine boat-tailing with properly contoured transverse grooves to delay boundary-layer separation and thus to further reduce the pressure drag.
The effectiveness of the proposed strategy is assessed through a synergic use of wind tunnel tests and Variational MultiScale Large-Eddy Simulations (VMS-LES).