Effect of wall mass injection on receptivity of high-enthalpy hypersonic boundary layer

Boundary-layer receptivity plays a fundamental role in hypersonic transition and can be strongly influenced by wall mass injection associated with ablative thermal protection systems. Direct numerical simulations are performed to investigate the effects of wall-injection composition on the receptivity of a thermochemically nonequilibrium hypersonic blunt-cone boundary layer to free-stream slow acoustic waves. Four wall conditions are considered: non-catalytic no injection, fully catalytic no injection, air injection, and pyrolysis-gas injection. Wall catalysis and air injection reduce the boundary-layer thickness through near-wall recombination and the associated modification of temperature and Mach-number distributions, whereas pyrolysis-gas injection thickens the boundary layer because of the high H2 content and the resulting increase in the effective gas constant and thermal conductivity. The modified mean flow substantially changes the receptivity mechanism. In the non-catalytic no-injection case, the disturbance evolves from leading-edge modal synchronization to an unstable supersonic mode and finally to downstream second-mode amplification. In contrast, the fully catalytic and air-injection cases remain dominated by damped F2 modes. For pyrolysis-gas injection, entropy/vorticity waves dominate the intermediate region, followed by earlier second-mode excitation downstream. Frequency-dependent results over 700–1100 kHz further show that wall catalysis and air injection suppress unstable-mode growth, while high-frequency excitation appears in the pyrolysis-gas case. The receptivity coefficient is largest for the non-catalytic no-injection case and smallest for the pyrolysis-gas-injection case among the cases with identifiable unstable-mode growth. These results demonstrate that wall chemistry and injection-gas composition can regulate hypersonic boundary-layer receptivity by altering the thermochemical mean flow and the dominant disturbance-mode pathway.