Generation of the second mode in the hypersonic boundary layer by wavy wall.

The present study experimentally and numerically investigates the evolution of the original dominant second mode in a Mach 6 hypersonic boundary layer over a flared cone, together with two additional disturbances that arise over a wavy wall. The results show that these additional disturbances are dominated by dilatational motion and are identified as second-mode instabilities rather than first-mode waves. Owing to the excitation of these additional second-mode components, the original dominant second mode is effectively suppressed, leading to a substantial reduction in the overall disturbance amplitude and the complete elimination of the associated localized surface-heating peak, in agreement with the experimental observations of Si et al. [(2019). Phys. Fluids 31(5), 051702] and Zhu et al. [(2018). J. Fluid Mech. 855, 152-180]. This effect is attributed to a redistribution of disturbance energy from the original dominant second mode into two additional, lower-frequency second-mode components induced by the wavy-wall modulation of the mean boundary-layer profile.