Vibration-dissociation coupling in low-density, hypersonic flows of air is investigated. Radiative emission data for nitric oxide and for atomic oxygen measured by a reentry flight experiment are used to assess the modeling of this phenomenon. Flow field computations are performed using the direct simulation Monte Carlo 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 both nitric oxide and atomic oxygen. Radiative emission is calculated using a nonequilibrium radiation method. 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 among these models is found for the lowest-density flight conditions. By using a sophisticated dissociation model, the emission calculated at the highest altitude for which measurements are available is improved from a factor of 220 too low to within a factor of 4 too low. With the same model, improvement by a factor of 50 is also obtained for the computation of emission from atomic oxygen. This is the first time that the observed dependence of the flight data on the free-stream density has been predicted correctly.
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