Vibrational Excitation, Thermal Nonuniformities, and Unsteady Effects on Supersonic Blunt Bodies

Abstract

We have performed a computational study of the experiments performed by Lowry et al. at the Arnold Engineering Development Center. In these experiments, an rf discharge is used to weakly ionize a volume of air; then a projectileis e red through this plasma. Relativeto the conditionswithout the discharge, theshock standoff distance is observed to increase substantially, and the bow shock becomes e atter. We have modeled the rf discharge and the resulting thermochemical state of the air within the discharge region. Based on these conditions, the projectile e owe eld wassimulatedto determinewhethertherelaxation of thestored internalenergy causestheobservedshock movement. The results indicate that the stored internal energy does not relax fast enough to reproduce the experimental results, and, therefore, vibrational energy storage is not responsible for the observed shock movement. We considertwo additional mechanisms to explaintheexperiments: modie cationof theelectrice eld by thepresenceof the metallic projectile, and thermal nonuniformities in theplasma. The latter effect appears to provide the best explanation for the observations. We have also modeled experiments in which microwave-discharge excited air e ows over a model. Unsteady thermal effects in the pulsed discharge can account for most of the observed drag change.