Modeling of RF plasma kinetics and aerodynamics of the AEDC ballistic range experiments

Abstract

We have performed a computational study of the experiments performed by Lowry et al.' at the Arnold Engineering Development Center (AEDC). In these experiments, a RF discharge is used to weakly ionize a volume of air; then a projectile is fired through this plasma. Relative to the conditions without the discharge, the shock standoff distance is observed to increase substantially, and the bow shock becomes flatter. We have modeled the RF discharge and the resulting thermo-chemical state of the air within the discharge region. Based on these conditions, the projectile flow field was simulated to determine if the relaxation of the stored internal energy causes the observed shock movement. The results obtained to date 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 consider two additional mechanisms to explain the experiments: modification of the electric field by the presence of the metallic projectile, and non-uniformities in the plasma. The latter effect appears to be the leading candidate. * Professor, Senior Member AIAA ** Research Scientist, Associate Fellow AIAA t Research Staff Member, Member AIAA t Assistant Professor, Senior Member AIAA il Boeing Technical Fellow, Member AIAA Copyright ©2001 by Graham V. Candler. Published by the American Institute of Aeronautics and Astronautics, Inc. with permission. Introduction A series of experiments at the Arnold Engineering Development Center (AEDC) has been carried out over the past several years. This work was originally planned to reproduce experiments performed in Russia that showed significant increases in shock standoff distances on spheres flying through a weakly ionized gas.' In the AEDC experiments,' an RF plasma generator similar to that used in the Russian experiments was installed in the AEDC SI Hypervelocity Impact Range. | and | diameter spheres were fired through air and Argon at pressures of 30 and 40torr. The bow shock shape was measured with holographic interferometry methods, and the temperature within the discharge was measured with a variety of techniques. Figure 1 shows a schematic of the RF plasma generator used in the experiments, and Fig. 2 shows pictures of the generator operating in Argon. The glowing regions indicate the primary current carrying paths of the plasma; note that there is substantial nonuniformity within the generator. The bow shock shape is measured when the projectile is nominally centered in the generator. The AEDC experiments show that the bow shock tends to flatten and its standoff distance increases when the plasma generator is turned on. There is a substantial amount of scatter of the data, but the shock standoff distance, A, is measured to be up to A/rn = 0.63, where rn is the projectile radius. This compares to the shock standoff distance of A/rn = 0.31 for a sphere flying through air at the range conditions using the average measured temperature of (c)2001 American Institute of Aeronautics & Astronautics or Published with Permission of Author(s) and/or Author(s)' Sponsoring Organization.