Investigation of the Growth and Breakdown of Gortler Vortices Evolving in High-Speed Boundary Layers

We study the transition in hypersonic boundary layer flows developing over concave surfaces, involving the so-called Gortler vortices, using direct numerical simulation. We focus our efforts on understanding how these vortices grow, experience secondary instabilities and breakdown into turbulence under various conditions. In our analysis, a hypersonic boundary layer at freestream Mach numbers of 6 and 7 is considered, under either adiabatic or isothermal wall-temperature conditions, different spanwise separations. In the downstream, high- and low-speed streaks develop in the flow, accompanied by counter-rotating pairs of streamwise vortices that breakdown nonlinearly and transition into turbulence. From our numerical simulations, we observe that the breakdown process is dominated by both varicose-type and sinuous-type instabilities, initiated and manifested at the base of the mushroom shapes. Multiple results in terms of iso-surfaces of Q-criterion, contours of different variables and statistical quantities will be outlined and discussed.