Modeling of Nitric Oxide Vibrational Level Populations for High Mach Number Flows

Recent measurements of nitric oxide (NO) infrared emission from a hypersonic shock suggest that this spectral region may provide important information about nonequilibrium flow chemistry. This work considers a number of fundamental aspects related to the modeling of the spatial distributions of NO vibrational states in the ground electronic state that need to be considered in the interpretation of such experiments. The hypersonic steady state stagnation, expansion, and wake flow regions over a cylinder, a test article that can be employed in ground based measurements, is examined using the direct simulation Monte Carlo (DSMC) approach. Using quasi-classical trajectory derived relaxation cross sections for the most important vibrational relaxation mechanism of NO-O, we observe that the faster rates, compared to O2-O, lead to vibrationally colder NO molecules in the expansion regions of the flow with the maximum decrease in NO vibrational temperatures close to 400 K. We propose a new collisional radiative model to characterize the state-to-state transitions of vibrational states of NO through collisional and radiative processes and compare the difference in NO vibrational state populations obtained when it is implemented directly into the DSMC versus an overlay, quasi-steady state, and Boltzmann approaches. Finally, using those NO vibrational state populations, we perform emission simulations to quantify the differences in emission spectra resulting from the use of Boltzmann and non-Boltzmann distributions for vibrational state populations of NO.