A Plausible Non-thermal Plasma Effect on Shock Waves

Shock waves have been the main obstacle for the development of supersonic/hypersonic vehicle. The very high wave drag associated with shock wave formation around a vehicle imposes restrictive vehicle shape requirements, which limit the flexibility of the vehicle design. Moreover, shock waves formed around the supersonic/hypersonic vehicle have the side effect of producing sonic booms on the ground. Introducing an energy source at somewhere upstream of the object to the supersonic incoming flow, the gas heating results in a local reduction of Mach number that in turn causes the shock to move upstream. The usually strong bow shock front is then replaced by a weaker oblique shock with significantly lower wave drag. However. This process requires a large power density to significantly elevate the gas temperature.Plasma can effectively convert electric energy to thermal energy for gas heating. More importantly, it has the potential to possibly offer non-thermal modification effects on the structure of shock waves. Recently, Gordeev et al. have carried out a pioneer experiment, which applied a plasma jet injected through the nozzle of a cone-cylinder model to against an up-coming supersonic flow. Significant reduction of drag to the model was reported. Subsequently, Ganguly et al. carried out an experiment in the shock tube using laser diagnostics to distinguish the non-thermal effect from the thermal effect of glow-discharge plasma on shock waves. The non-thermal plasma effect was also manifested by the long-lasting perturbation on the shock structure in decaying discharge plasma. The observed effect was attributed to the existence of long-lived excited states of atoms and molecules in the gas.In a wind tunnel experiment by Kuo et al.4, plasma was introduced by an on-board discharge to interact with the flow. Complete elimination of the shock wave was observed. The peak and average power for plasma generation in this experiment were less than 1.2 kW and 100 W, respectively. Therefore, the thermal effect was not likely to be the cause mechanism. It was also found, that significant plasma effect on shock wave was observed under two conditions: 1. plasma is generated in the region upstream of the baseline shock front and 2. plasma has a symmetrical spatial distribution with respect to the axis of the model.In this work, a plausible physical mechanism explaining the observed nonthermal plasma effect on shock waves is presented. Analysis shows that a symmetrically distributed plasma spike in front of the shock can effectively deflect the incoming flow symmetrically. It is the increase of the deflection angle of the flow that modifies the shock wave structure.