| An accurate prediction of boundary layer transition is necessary for an optimum design of a hypersonic cruise vehicle or an aerobrabe for Earth and Mars entry missions. Stability analysis can identify fundamental causes of transition by predicting the frequency and amplification of disturbances in the mean flow. For flight at high Mach numbers, thermal and chemical nonequilibrium may exist in the mean flow and thus affect the stability of the flow. In this dissertation, the processes leading to transition and the manner in which they are affected by thermal and chemical nonequilibrium are assessed.; A computational tool was developed to analyze a hypersonic mean flow and its stability. The mean flow analysis employs the Navier-Stokes equations with a translational/vibrational temperature model for thermal nonequilibrium and a five species reacting air model for chemical nonequilibrium. A modified Steger-Warming, flux-vector splitting upwind numerical technique is used to solve the mean flow. The stability analysis employs linear stability theory to describe the spatial amplification of two and three-dimensional disturbances. Global and local boundary value methods are used to solve the resulting non-linear eigenvalue problem. The vibrational relaxation times and chemical reaction rates are adjusted in both the mean flow and stability analyses to simulate thermal and/or chemical equilibrium.; The computational tool was verified with computations in the literature for perfect gas, equilibrium, and chemical nonequilibrium/thermal equilibrium. It was then applied to determine the effects of thermal and chemical nonequilibrium on the stability of a Mach 10 and Mach 15 flow over a cold wall flat plate and an adiabatic plate. For oblique first mode disturbances, the cold surface was stabilizing and both thermal and chemical non-equilibrium were destabilizing. For two-dimensional second mode disturbances, the cold wall was destabilizing and neither thermal nor chemical nonequilibrium had a significant effect on the spatial amplification rates, despite the different mean flow profiles. Thus, the effect of the nonequilibrium gas model on stability depends on the disturbance mode. |
