Effect of angle of attack on the instability-wave selectivity in hypersonic compression ramp laminar flow

This study investigates the effect of angle of attack (AOA) on the convective instability of a hypersonic flow over a compression ramp at Mach 7.7, with particular emphasis on high-frequency two-dimensional Mack modes and low-frequency three-dimensional shear-layer instabilities. Combining linear stability theory (LST) and resolvent analysis, we examine how the change in AOA affects the convective instability mechanisms associated with separation bubbles. Results show that as AOA decreases from zero to negative, the separation bubble elongates, leading to increased growth rates and spatial extent of higher-order Mack modes. The negative AOA also promotes the emergence of additional shear-layer instabilities within concave high-curvature regions near the aft portion of the bubble. In contrast, positive AOAs compress the separation bubble, suppress higher-order modes, and reduce both the number and growth rates of shear-layer instability modes. Notably, at large positive AOA, the separation bubble acts as a broadband perturbation amplifier in the vicinity of the separation point for high-frequency Mack modes. For both large positive or negative AOAs, the low-frequency shear layer modes are shown to be unsteady Görtler modes. A comparison between LST and resolvent analysis demonstrated good agreement, confirming that the parallel-flow assumption underlying LST remains largely valid across multiple AOAs. These results indicate that, despite the changing bubble morphology with AOA, LST remains a valid tool for stability studies of the shock wave–boundary layer interaction (laminar flow).