Spatial linear stability of a hypersonic shear layer with nonequilibrium thermochemistry

Abstract

We examine the spatial linear stability of a shear layer in a hypervelocity flow where high temperature effects such as chemical dissociation and vibrational excitation are present. A shock triple point is used to generate a free shear layer in a model problem which also occurs in several aerodynamic applications such as shock-boundary layer interaction. Calculations were performed using a state-resolved, three-dimensional forced harmonic oscillator thermochemical model. An extension of an existing molecular-molecular energy transfer rate model to higher collisional energies is presented and verified. Nonequilibrium model results are compared with calculations assuming equilibrium and frozen flows over a range of (frozen) convective Mach numbers from 0.341 to 1.707. A substantial difference in two- and three-dimensional perturbation growth rates is observed among the three models. Thermochemical nonequilibrium has a destabilizing effect on shear-layer perturbations for all convective Mach numbers considered. The analysis considers the evolution of the molecular vibrational quantum distribution during the instability growth by examining the perturbation eigenfunctions. Oxygen and nitrogen preserve a Boltzmann distribution of vibrational energy, while nitric oxide shows a significant deviation from equilibrium. The difference between translational and vibrational temperature eigenfunctions increases with the convective Mach number. Dissociation and vibration transfer effects on the perturbation evolution remain closely correlated at all convective Mach numbers.