State-Specific Study of Air in the Expansion Tunnel Nozzle and Test Section

Thermochemical nonequilibrium in expansion tunnel nozzles is investigated numerically using a state-to-state description in one dimension for representative air conditions. Limiting the multiquantum jumps of vibration–vibration–translation (VVT) transitions to three in both N2 and O2 can accurately simulate the nonequilibrium nozzle flow. The reduction of VVT transitions to vibration–translation transitions works well. State-to-state modeling of an actual expansion tunnel nozzle condition yielded agreement with the measured static pressure. A study on the influence of different thermochemical excitations in the freestream at the test section shows that the postshock radiation emissions can differ by more than 50%. However, the non-Boltzmann distributions in the freestream have no influence. An evaluation of the discrepancy between the two-temperature and state-to-state models shows that the former generally predicts a faster thermochemical relaxation. Furthermore, the state-to-state results indicate that, in general, the molecular species all have a different vibrational temperature.