Spatial evolution of second-mode instabilities over controlled wall conditions in a Mach 6 boundary layer

The spatial evolution of second-mode instability over a Mach-6 flared cone is investigated using scanning focused laser differential interferometry. For smooth, permeable, wavy, and filmed surfaces, the instability peaks near the critical layer, but its amplitude and distribution are strongly modified. The filmed surface yields the strongest attenuation. The permeable surface suppresses near-wall disturbances while sustaining or enhancing fluctuations near the critical layer, indicating redistribution. The wavy surface induces streamwise modulation and excites low-frequency modes. Thus, tailored surface conditions control hypersonic boundary-layer transition by redistributing the energy of disturbances spatially and spectrally, informing passive transition control.