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The detailed description of the vibrational energy content is important for many gas flow processes including experimental gas measurement techniques, shock layer vibration-dissociation coupling, and vibrational energy freezing in strong expansions. Various vibrational relaxation models have been developed for use within the direct simulation Monte Carlo (DSMC) method. This study compares the Larsen-Borgnakke and Forced Harmonic Oscillator vibrational relaxation models through comparison with detailed quasi-classical trajectory (QCT) results for an atom-diatom system with focus on the effects of the form of the relaxation model. These QCT results have been extensively compared with measured data and provide a unique opportunity to evaluate these inelastic models. This work also compares the results from each model for a vibrationally exciting relaxation problem. Even when the two models have consistent vibrational collision numbers to match bulk temperature predictions for relaxation problems, the predicted evolution of the vibrational distribution function differs. This disagreement results in different prediction of radiative emission spectra., The original document contains color images. Presented at the 52nd AIAA Aerospace Sciences Meeting, held at National Harbor, MD, on January 13-17, 2014.

Often, rarefied flows of interest in aerospace applications are embedded within largely continuum flow fields. Neither continuum nor kinetic methods provide both physically accurate and numerically efficient techniques to simulate the entire flow field. Instead, multi-scale methods can provide the capability of achieving the efficiency of continuum methods in regions where the degree of collisional nonequilibrium is small, while maintaining the physical accuracy of kinetic methods in rarefied portions of the flow. This work begins with an outline of typical aerospace flows that may require a multi-scale analysis in order to make accurate and efficient predictions. It then provides an overview of the research performed in developing hybrid methods for partially rarefied gas flows. Finally, some results derived from current state of the art hybrid codes are presented and emerging developments are described., See also ADA579248. Models and Computational Methods for Rarefied Flows (Modeles et methodes de calcul des coulements de gaz rarefies). RTO-EN-AVT-194.