Unsteady effects in a hypersonic compression ramp flow with laminar separation

Direct numerical simulations (DNS) are performed to investigate a hypersonic flow over a compression ramp with a free stream Mach number of 7.7 and a free stream Reynolds number of based on the flat plate length. The DNS results are validated by comparison with experimental data and theoretical predictions. It is shown that even in the absence of external disturbances, streamwise heat flux streaks form on the ramp surface downstream of reattachment, and that they are non-uniformly distributed in the spanwise direction. The surface heat flux exhibits a low-frequency unsteadiness, which propagates in the streamwise direction. Additionally, the unsteadiness of the heat flux streaks downstream of reattachment is coupled with a pulsation of the reattachment position. By conducting a dynamic mode decomposition (DMD) analysis, several oscillatory modes, characterised by streamwise low-frequency periodicity, are revealed in the separation bubble flow. The DNS results are further explained by a global stability analysis (GSA). Particularly, the flow structure of the leading DMD modes is consistent with that of the oscillatory unstable modes identified by the GSA. It is therefore concluded that the global instabilities are responsible for the unsteadiness of the considered compression ramp flow.