Detailed Computation of Ultra-Violet Spectra in Rarefied Hypersonic Flow

Koffi K. Kossi and Iain D. Boyd Sibley School of Mechanical and Aerospace Engineering Cornell University, Ithaca, NY 14853

Abstract

A new nonequilibrium radiation model is described that predicts the ultra-violet spectrum of hydroxyl (OH) from the electronic transition A --> X for flow conditions corresponding to the Bow-Shock Ultra-Violet-2 flight experiment. Unlike previous studies, the new model includes the direct analysis of the electronically and vibrationally excited states, OH(A,v'=0,1,2) of hydroxyl in the flow field simulations. The flow field is analyzed using the direct simulation Monte Carlo method. Results are presented for the altitude range from 80 to 100 km, where the Knudsen number varies from 0.036 to 1.3. The computation uses algorithms that improve the numerical resolution of the excited states which typically occur at mole fractions of 1e-15. The collisional transfer of rotational and vibrational energies of the electronic state, OH(A), are also studied in detail. It is demonstrated that the usual assumption made in continuum radiation models that the rotational and vibrational temperatures of the electronically excited state are the same as those of the ground state of the bulk flow fails. An important improvement is achieved in the spectral prediction using the new nonequilibrium radiation model and good agreement is obtained between flight data and emission predictions over a range of altitudes.

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