Shock wave control by nonequilibrium plasmas in cold supersonic gas flows

Experimental studies of shock modification in weakly ionized supersonic gas flows are discussed. In these experiments, a supersonic nonequilibrium pla sma wind tunnel which produces a highly nonequilibrium plasma flow with the low gas kinetic temperature at M = 2, is used. Supersonic flow is maintain ed at complete steady state. The flow is ionized by a high-pressure aerodyn amically stabilized do discharge in the tunnel plenum and by a transverse r f discharge in the supersonic test section. The do discharge is primarily u sed for the supersonic flow visualization, whereas the rf discharge provide s high electron density in the supersonic test section. High-pressure flow visualization produced by the plasma makes all features of the supersonic f low, including shocks, boundary layers, expansion waves, and wakes, clearly visible. Attached oblique shock structure on the nose of a 35-deg wedge wi th and without rf ionization in a If = 2 flow is studied in various nitroge n-helium mixtures. It is Found that the use of the rf discharge increases t he shock angle by 14 deg, from 99 to 113 deg, which corresponds to a Mach n umber reduction from M=2.0 to 1.8. Time-dependent measurements of the obliq ue shock angle show that the time for the shock weakening by the rf plasma, as well as the shock recovery time after the plasma is turned off, is of t he order of seconds. Because the flow residence time in the test section is of the order of 10 mus, this result suggests a purely thermal mechanism of shock weakening due to heating of the boundary layers and the nozzle walls by the rf discharge. Gas flow temperature measurements in the test section using infrared emission spectroscopy, with carbon monoxide as a thermometr ic element, are consistent with the observed shock angle change. This shows that shock weakening by the plasma is a purely thermal effect. The results demonstrate the feasibility of both sustaining uniform ionization in cold supersonic nitrogen and airflows and the use of nonequilibrium plasmas For supersonic flow control. This opens a possibility for the use of transverse stable rf discharges for magnetohydrodynamic energy extraction and/or acce leration of supersonic airflows.