DIRECT SIMULATION OF HIGH-ALTITUDE ULTRAVIOLET EMISSION FROM THE HYDROXYL RADICAL

Ultraviolet emissions radiated by hydroxyl (OH) are computed for hyper sonic nonequilibrium now conditions corresponding to the Bow-Shock Ult ra-Violet-2 flight experiment. The flowfield is analyzed using the dir ect simulation Monte Carlo method. These computations include direct a nalysis of the electronically excited state of hydroxyl. Ultraviolet e mission is estimated using a nonequilibrium radiation code. New algori thms are described that improve the numerical resolution of the excite d state that occurs at number densities as low as 10(-3) cm(-3). Resul ts 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 inter action of the gas with the vehicle surface. Sensitivity of emission pr edictions to freestream concentrations of hydrogen-bearing species is also considered. It is found that the quasi-steady-state assumption of ten employed in the nonequilibrium radiation code is invalid at high a ltitude. 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 st rong nonequilibrium effects observed in the measured data.