Direct simulation of high altitude ultraviolet emission from the hydroxyl radical

Ultra-violet emissions radiated by hydroxyl (OH) are computed for hypersonic nonequilibrium flow conditions corresponding to the Bow-Shock Ultra-Violet-2 flight experiment. The flow field is analyzed using the direct simulation Monte Carlo method. These computations include direct analysis of the electronically excited state of hydroxyl. Ultra-violet emission is estimated using a nonequilibrium radiation code. New algorithms are described that improve the numerical resolution of the excited state which occurs at mole fractions as low as 10^-13. Results are presented for the altitude range from 80 to 100 km. It is shown that the high altitude emission is sensitive to modeling of the interaction of the gas with the vehicle surface. Sensitivity of emission predictions to free stream concentrations of hydrogen-bearing species is also considered. It is found that the quasi-steady-state assumption often employed in the nonequilibrium radiation code is invalid at high altitude. Comparison of the predicted values for peak OH emission with flight measurements indicates good agreement. Detailed comparisons of the spectra, however, indicate that the simulations fail to include strong nonequilibrium effects observed in the measured data.