Diffusion and chemical non-equilibrium effects on hypersonic boundary-layer stability

Boundary-layer transition prediction has a dramatic impact on the optimization of hypersonic cruise and entry vehicle design. Linear Stability Theory approaches laminar-to-turbulent transition in a simplified yet physics-based manner. This work investigates the stability of Mach 10 air flow over an adiabatic flat plate in chemical non-equilibrium and in frozen conditions, analyzing the effect of diffusion by varying the Schmidt number. Air chemistry was observed to be destabilizing, due to the associated wall-cooling. The results also suggest that flows are most unstable when the Schmidt number is of order 0.01, mean- ing that the molecular diffusion rate is 100 times faster than that associated to viscous diffusion. The thermodynamic and transport models used for the base-flow quantities were observed to have a much larger impact on the stability characteristics than those used for the perturbations.