Accurate calculations of radiation on and from transatmospheric flight vehicles are currently a challenge to computational aerodynamicists. Due to combined effects of low density and hypersonic flight conditions (high velocity and temperature) that characterize flows around such vehicles, the gas in the shock-layer is in a state of thermal and chemical nonequilibrium. The present work aims at gathering existing ideas together about how such flows should be modeled and comparing them to recent, more accurate experiments that probe the separate energy modes of the different species of the gas in a more direct way than previously reported.

Two recent Bow-Shock-Ultra-Violet flight experiments (BSUV₁ and BSUV₂), and two recent shock-tube experiments, one at the East Facility of NASA Ames Research Center, the other at Calspan Research Center of the University of Buffalo are used to test the validity of the flow-field models implemented in the current state-of-the-art numerical codes. They involve highly non-equilibrium flow regimes in nitrogen and air with negligible ionization at speeds below 6 km/s and altitudes ranging from 40 km to 80 km. They all provide detailed spectra emitted by the hot gas.

A recent plasma torch experiment at Stanford, and the Cochise experiments at the Geophysics Directorate laboratories, have been the ideal experimental counterpart to test and improve the radiation calculation in the UV-visible spectral range and the IR region respectively. Each spectral region is used to probe several different aspects of the thermal and chemical nonequilibrium.

A hierarchy of flow-field codes was therefore developed in conjunction with a greatly enhanced radiation code, termed NEQAIR2, to simulate these experiments. The flow-field codes involve axisymmetric Navier-Stokes and Burnett simulations around blunt-nose cones (geometry of the flight experiments) and quasi-ID Euler simulations for the shock-tube experiments. They include between 5 and 8 chemical species and between 3 and 6 separate internal energy modes. The corresponding system of conservation equations are solved with finite volume, flux split algorithms. For the axisymmetric simulations, Gauss-Siedel line relaxation is used to increase efficiency of the fully-implicit method. In all simulations exact numerical jacobians have been derived to increase the rate of convergence. The radiation code involves a collisional-radiative model based on a quasi-steady-state (QSS) approximation and a detailed line-by-line calculation for several atomic systems and molecular band systems.

The developed NEQAIR2 code provides an improvement in numerical resolution and in the scope of radiation physics that can be treated. Its accurate diagnostics enabled us to compare with fine spectra measurements and test the proposed state-of-the-art flow-field models for the first time. It is concluded that the rotational nonequilibrium in nitrogen at very high temperatures is unexpected large, and that the role of internal energy modes in the Zeldovich reaction mechanism producing NO is very significant at high altitudes. Comparisons with the flight experiments show, however, that an adequate quantification of the forementioned phenomena is still missing and that more unresolved physics are likely still buried in the measured stagnation line radiance and spectra (UV signatures). These comparisons hint at the necessary experimental and theoretical feedbacks to further proceed in the investigation and refine the present critical review of flow-field and radiation models.