Effect of dissociation modeling in strongly nonequilibrium flows at high altitude

Vibration-dissociation coupling in low-density, hypersonic flows of air is investigated. Radiative emission data for nitric oxide measured by the second Bow-Shock Ultra-Violet (BSUV-2) flight experiment is used to assess this phenomenon. Flow field computations are performed using the direct simulation Monte Carlo (DSMC) method. Due to the relatively small number of collisions under high altitude, low-density flow conditions, an overlay approach is used to simulate changes in chemical composition of trace species including nitric oxide. Radiative emission is calculated using a nonequilibrium radiation code. It is found that the strong degree of thermal nonequilibrium that occurs in high-altitude, hypersonic flows makes the chemistry very sensitive to the vibration-dissociation coupling model. A number of such models based on continuum and particle representations of the flow are assessed. A variation in dissociation rate of up to nine orders of magnitude between these models is found for the BSUV-2 flight conditions. By using a detailed dissociation model, the emission calculated at the highest altitude for which measurements are available is improved from being a factor of 220 too low to be within a factor of 4 too low. Dissociation effects are also briefly assessed for a new flight experiment.