Direct numerical simulation of subharmonic second-mode breakdown in hypersonic boundary layers with finite-rate chemistry

This study explores the turbulent breakdown of high-enthalpy hypersonic boundary layers under adiabatic wall conditions using direct numerical simulations, with a focus on finite-rate chemistry effects. By subjecting a Mach 10 boundary layer to controlled perturbations via suction and blowing, the investigation shows the evolution from laminar to fully turbulent regime, via second-mode transition. In contrast to conventional understanding, this work identifies subharmonic breakdown scenario as significant contributor to turbulent transition, with energy transfer to secondary disturbances driving the process. The influence of finite-rate chemistry on growth rates is analyzed, revealing that chemical non-equilibrium has a stabilizing effect on secondary instabilities. Dynamic mode decomposition analysis further elucidates the modes predominantly involved in turbulent breakdown.