Multiquantum Transitions in Oxygen and Nitrogen Molecules in Hypersonic Nonequilibrium Flows

Abstract

A computational fluid dynamics analysis was performed with state-to-state kinetics of oxygen and nitrogen for assessing the influence of vibrational excitation and dissociation on hypersonic flowfields in the range of Mach 7–20. Flows around an axisymmetric forebody geometry were considered by using multiquantum vibrational–translation and dissociation rate coefficients from an ab initio database. The nonequilibrium vibrational energy distributions were modeled by the master kinetic equation. Output quantities of interest included shock standoff distance, translational and vibrational temperatures, and population fractions in the quantum energy states. Both a two-temperature model and a state-to-state kinetics model were used to compare with existing experimental data. The objective of the study was to understand the energy exchange process of multiquantum transitions in the shock layer of a blunt body flow and to assess the maximum number of multiquantum jumps required in describing the nonequilibrium behavior. Multiquantum transitions caused an increase in population in upper energy states due to more efficient energy exchanges. The greater dissociation for the higher multiquantum jumps resulted in reduced shock standoff distance. At the higher Mach numbers, population depletion from the upper energy states demonstrated preferential dissociation. At high temperatures, results indicated that multiquantum transitions had a large influence on the macroscopic parameters of the gas flow behind the shock, especially on the temperature profiles as well as on vibrational population distributions in the upper energy states.