Understanding instability-wave selectivity of hypersonic compression ramp laminar flow

The hypersonic flow stability over a two-dimensional compression corner is studied using Resolvent analysis, linear stability theory (LST), and parabolized stability equation. The authors find that the interaction between upstream convective-type disturbances and the laminar separation bubble can be divided into two regimes. First, two-dimensional (2D) high-frequency Mack (second, third, etc.) modes neutrally oscillate with the presence of alternating stable and unstable regions inside the separation bubble, which arises from repeated synchronizations between discrete modes with evolving branches. Meanwhile, the second modes upstream and downstream of the separation bubble can differ significantly from each other. Second, the 2D low-frequency shear-layer mode is stable, whereas multiple unstable three-dimensional (3D) eigenmodes are identified by LST. These modes are found to be sensitive to the streamline curvature effect. The locally dominant modes agree with the Resolvent response in terms of the preferential spanwise wave number, the disturbance shape, and the growth rate of energy. Thus, a combination of global and local analyses demonstrates that the separation bubble tends to selectively amplify low-frequency 3D disturbances and freeze high-frequency Mack-mode disturbances in an explainable manner. These findings facilitate the understanding of the early evolution of low- and high-frequency instabilities in hypersonic separated flows.