Flow Separation Delay and Drag Reduction Through Contoured Transverse Grooves

The results of several investigations aimed at assessing the performance of contoured transverse grooves as a method to delay flow separation are described. The physical mechanism at the basis of this passive technique is the local relaxation of the no-slip boundary condition, with a consequent reduction of the viscous losses and an increase of the downstream near-wall momentum. Numerical simulations and experiments showed that the application of one groove transverse to the flow direction may delay boundary layer separation both in laminar and turbulent conditions. As a consequence, significant increases of the pressure recovery in plane diffusers and decreases of the drag of boat-tailed axisymmetric and two-dimensional bluff bodies were obtained. It is shown that robust configurations may be devised provided the shape and dimension of the grooves are suitably chosen in order to assure the formation of steady and stable local flow recirculations.