Hypersonic boundary-layer stability with chemical reactions using PSE

Abstract

Linear stability of reacting flows in hypersonic boundary layers is studied via the parabolized stability equations (PSE) method. Three gas models are considered in the basic flow as well as stability calculations: perfect gas, chemical equilibrium and non-equilibrium (finite-rate chemistry). The finite-rate chemistry model for air contains five species, eight reactants and six reactions. The equilibrium calculation is performed by a table look-up procedure. Amplifying supersonic modes characterized by an oscillating disturbance structure outside the boundary-layer and a relative phase velocity faster than the free-stream sonic speed were found to appear in a Mach-20 flow past a 6° wedge when either equilibrium or finite-rate chemistry model is incorporated. These supersonic modes emerge just downstream of the unstable subsonic second-mode region and they propagate into the free-stream with a phase speed different from the corresponding acoustic wave and decay at a finite distance outside the boundary layer. The Rankine-Hugoniot (shock) conditions applied at the shock have very little effect on the supersonic modes studied here since the mode structure decays before the shock is reached. Due to the presence of supersonic modes, which are enhanced by the chemistry effect, the transition onset (based on N = 10) for the Mach-20 wedge flow is estimated to be at 14 ft, 24ft and 39 ft if one uses equilibrium, non-equilibrium and perfect gas models, respectively. It is therefore very important to account for the chemistry effect in future transition prediction for hypersonic vehicles.