Effect of Wall Transpiration and Heat Transfer on Gortler Vortices in High-Speed Flows

Gortler vortices in boundary-layer flows over concave surfaces are caused by the imbalance between centrifugal effects and radial pressure gradients. Depending on various geometrical and/or freestream flow conditions, vortex breakdown via secondary instabilities leads to early transition to turbulence. It is desirable, therefore, to reduce vortex energy in an attempt to delay the transition from laminar to turbulent flow, and thereby achieve a reduced frictional drag. To this end, a proportional control algorithm was applied, aimed at reducing the wall shear stress and the energy of Gortler vortices evolving in high-speed boundary layers. The active control scheme is based on wall transpiration with sensors placed either in the flow or at the wall. In addition, the effect of wall cooling and heating on Gortler vortices evolving in high-speed boundary layers was evaluated, by reducing or increasing the upstream wall temperature. The numerical results are obtained by solving the full Navier-Stokes equations in generalized curvilinear coordinates, using a high-order numerical algorithm. The results show that the active control based on wall transpiration reduces both the wall shear stress and the energy of the Gortler vortices; the passive control based on wall cooling or heating reduces the wall shear stress, but slightly increases the energy of the vortices in both supersonic and hypersonic regimes.