Computation of hypersonic low-density flows with thermochemical nonequilibrium

Recent interest in hypersonic transitional flows, transitional referring to the flow regime between continuum and free-molecular flows, is motivated by the current and projected flight activities at high altitudes and at high speeds. Simulation of such flows involves consideration of the thermochemical nonequilibrium processes in the continuum Navier-Stokes formulation. There is considerable uncertainty whether the Navier-Stokes formulation adequately describes the underlying physics. Uncertainties particularly arise in the constitutive relations and the boundary conditions at solid walls. The constitutive relations are the Navier-Stokes diffusion of momentum, Fourier's law of heat conduction, and Fick's law of diffusion of mass. The conventional boundary conditions at the solid wall are the so-called no-slip conditions for velocity and temperature.; These uncertainties led some researchers to take a completely different point of view, that is, to simulate flows directly using Monte Carlo methods in conjunction with the molecular theory of gases. However, Monte Carlo simulation methods have the disadvantage of being computationally intensive and not being efficient except for very rarefied flows. Therefore, it would be of practical and theoretical importance to resolve the above uncertainties and to extend the range of the Navier-Stokes equations to high speed low density flows with thermochemical nonequilibrium.; In this dissertation, a new thermochemical nonequilibrium formulation appropriate to hypersonic transitional flows of air has been developed. The present nonequilibrium gas model for air consists of five chemical species: diatomic species; molecular nitrogen {dollar}Nsb2{dollar}, molecular oxygen {dollar}Osb2{dollar}, nitric oxide {dollar}NO{dollar}, and atomic species; atomic nitrogen {dollar}N{dollar} and atomic oxygen {dollar}O{dollar}. The local thermodynamic state of the gas is described by three temperatures corresponding to three internal energy modes, i.e., translational temperature {dollar}T{dollar}, vibrational temperature {dollar}Tsb{lcub}v{rcub}{dollar}, and rotational temperature {dollar}Tsb{lcub}r{rcub}{dollar}. The thermal and chemical nonequilibrium processes are vibrational and rotational relaxation for the diatomic species and chemical reactions among the five chemical species. Slip and catalytic wall boundary conditions are included in the formulation. The governing partial differential equations with the proper boundary conditions are solved numerically using an implicit time marching finite volume technique.; The computed results are compared with the existing Monte Carlo simulations in terms of surface quantities such as drag and heat transfer and in terms of nonequilibrium flow structure. These extensive comparisons are used (1) to determine when and to what degree the continuum Navier-Stokes description coupled with thermochemical nonequilibrium processes is accurate for high speed low density flows; (2) to explore the idea that the continuum Navier-Stokes equations with the proper slip boundary conditions will be adequate for an extended range of low density hypersonic flow problems.