Nonequilibrium Radiation Modeling for a Low Enthalpy Hypersonic Shock Layer

A new technique is presented for inclusion of nonequilibrium electronic excitation and radiative emission in direct simulation Monte Carlo (DSMC) calculations for non-ionized hypersonic flows. This technique follows earlier DSMC implementations involving assignment of quantized energy distributions to each simulated particle, and expands on previous work through physically consistent collisional relaxation routines which dramatically reduce statistical scatter in computed emission coefficients. Procedures are outlined for the use of experimentally derived rate coefficients to model collision-induced transitions between individual electronic energy states, and an extension of this work for state-to-state DSMC modeling of vibrational energy is proposed. A Mach 10 flow of reacting oxygen is used as a demonstration case, and additional simulations are performed to evaluate sensitivities to freestream Mach number, electronic collision numbers and various approximations employed for electronic energy exchange. It is shown that oxygen species emission is extremely sensitive to Mach number under conditions of interest, with a change in maximum local emission strength by around four orders of magnitude as the Mach number is varied between 8 and 12.