Plasma Spikes for the Mitigation of Shock Waves

Ever since Chuck Yeager and his Bell X-l first broke the sound barrier in 1947, aircraft designers have dreamed of building a passenger airplane that is supersonic, fuel efficient and economical. However, the agitated flow disturbances by the flying object at supersonic/hypersonic speed coalesce into a shock wave appearing in the form of a steep pressure gradient in front of the object, The background pressure behind the shock front increases considerably, leading to significant enhancement of the flow drag and friction on the object. Moreover, shock wave produces notorious sonic boom on the ground. It occurs when flight conditions are changing to cause shock waves unstable. The faster the aircraft flies, the larger the boom. The noise issue raises environmental concerns, which have precluded for example, the Concorde supersonic jetliner from flying overland. Plasma spikes for the reduction of sonic boom and wave drag in supersonic flights are being developed. Experiments have been conducted in a Mach 2.5 wind tunnel. A cone-shaped model having a 60°-cone angle was used as the shock wave generator, which resembled a supersonic vehicle. The tip and the body of the model were designed as two electrodes with the tip of the model designated as the cathode for gaseous discharges, which produced a plasma in the nose region of the model where shock waves were formed in a supersonic flow. The produced plasma acted as a spatially distributed spike, which interacted with the incoming flow prior to the wind-tunnel model. The effect of this plasma spike on the shock wave formation was explored by the shadowgraphs and Schlieren images of the flow field taken during wind tunnel runs. Two types of discharges, 60 Hz self-sustained diffused arc discharge and pulsed dc discharge, have been applied for the on-board plasma generation. The results^1-3 show that in both cases the introduced plasma spike has drastically modified the shock structure. Drag and temperature measurements were performed only in the later experiments using pulsed dc discharges for plasma generation. Significant drag reduction, which reduces wave drag by more than 50%, has been measured. The results also show that the plasma effect on shock structure lasts much longer than the discharge period. A thermocouple probe measured the gas temperature on the conic surface of the model. It is shown that during each pulsed dc discharge the gas temperature does not increase significantly to account for the observed plasma effect on the modification of the shock wave structure.