Breakdown mechanisms induced by stationary crossflow vortices in hypersonic three-dimensional boundary layers

This study investigates the crossflow breakdown of a Mach 6 flow over a swept flat plate by direct numerical simulation (DNS) considering three cases with different spanwise wavenumbers of stationary vortices. Transition in these cases is initiated by the linear and nonlinear evolution of these vortices, followed by secondary instabilities and breakdown due to type-I, type-II modes, and wall blowing/suction perturbations, respectively. The results showed that amplified secondary instabilities significantly distort the mean flow, causing a steep rise in the wall friction coefficient. Fourier analysis shows that, in this fast-varying flow region, the low-frequency disturbances undergo significantly greater amplifications than high-frequency disturbances. Moreover, the stability characteristics of the time- and spanwise-averaged mean flow were examined to elucidate the breakdown mechanisms. It was found that the unstable region initially contracts to a lower frequency band and then expands significantly in the spanwise wavenumber range at low frequencies. This suggests the significant amplifications of low-frequency disturbances, consistent with the observations from DNS. These amplified low-frequency disturbances, in turn, modify the mean flow, leading to the final breakdown. The presented mechanisms, highlighting the critical role of low-frequency disturbances in the breakdown process, are likely to be universally relevant across various parameter regimes.