Simulate Hypersonic Nonequlibrium Flow Using Kinetic Models

Abstract

Nonequilibrium thermodynamic state and chemical reactions are common features of hypersonic flows. At an altitude above 60 km, the flow field around a typical blunt body with a flight speed exceeding 2 km/s cannot attain chemically equilibrium state. On the framework of continuum mechanics, the bridge linking the Boltzmann equation and conservative laws of aerodynamics is the kinetics theory of gas through Chapman-Enskog expansion [1]. Since the kinetic theory does not consider the internal structure of atom and molecule, the higher degrees of excitation beyond translation motion must be resolved by approximations. Traditionally the connection between molecular and atomic structures and thermodynamic behavior of high-temperature gas is described by statistic mechanics and augmented by quantum physics. Therefore, the internal degrees of freedom of atomic and molecular species were studied via the partition function and assumed the gas mixture in local equilibrium [2]. A substantial amount of research results have been derived from this approach [3] and more recently even extended to include the ablation phenomenon [4]. However, significant discrepancy has been noted from this group of results form a limited amount of flight data. A part of the discrepancy is due to inaccurate computation and experimental measurement but a major portion of this disparity has been incurred by the modeling of physics [57]. The physical-chemical phenomena of nonequilibrium hypersonic flows are extremely complex and a thorough understanding of the basics has been beyond our reached for the past sixty years. All the interactions of internal degrees of freedom are taken placed in the molecular and atomic scales therefore must be modeled and verified by sparse validating data for better understanding. The present investigation is focused on the blunt-body problem which is dominated by the strong bow shock. In this flowfield the energy content in the translational mode is initially much greater than the vibrational excitations and the vibrational quanta occupy only the first few levels