Secondary instability of the hypersonic high-enthalpy boundary layers with thermal–chemical nonequilibrium effects

Abstract

Compared to the widely studied linear instabilities, the nonlinear analyses of the high-enthalpy and hypersonic boundary-layer transition had received much less attention. In this work, secondary instabilities of high-enthalpy boundary layers are studied to highlight the impacts of thermal–chemical nonequilibrium (TCNE). The numerical tools adopted here are the nonlinear parabolized stability equation (NPSE) and the Floquet analysis (or the secondary instability theory, SIT). The flow over a blunt cone is computed at a free-stream Mach number of 20. The fundamental resonance is found to dominate over the subharmonic resonance. The development of secondary waves from SIT agrees well with that from NPSE, suggesting satisfactory cross-validation of both solvers. It is also illustrated that the disturbances of the species mass and vibrational temperature are mainly generated by the product term of the wall-normal velocity disturbance and the mean-flow gradient. The typical rope-like structures are observed near the boundary-layer edge. The effects of TCNE on the secondary instability are to cause larger maximum secondary growth rate and the corresponding azimuthal wavenumber.