A new receptivity mechanism for the second-mode instability in hypersonic boundary layers

Direct numerical simulations of the flow over a compound delta-wing model reveal a novel receptivity mechanism for the second-mode instability in hypersonic boundary layers. This mechanism involves disturbances that propagate obliquely upward from the wall, interact with the shock wave and reflect back into the boundary layer. Detailed analysis indicates that this process strongly modulates high-frequency instabilities, such as the second mode, while exerting minimal influence on low-frequency cross-flow and first-mode disturbances. Upon re-entry into the boundary layer, the reflected disturbances cause rapid, stepwise amplification of the internal second mode, demonstrating that the mechanism modulates and enhances boundary-layer instability. Further analysis shows that a phase-locked mechanism is involved in these interactions. The identification of this receptivity mechanism offers new insights into hypersonic boundary-layer transition.