Shock Wave Control by Nonequilibrium Plasmas in Cold Supersonic Gas Flows

Abstract

Experimental studies of shock modi cation in weakly ionized supersonic gas ows are discussed. In these experiments, a supersonic nonequilibrium plasma wind tunnel, which produces a highly nonequilibrium plasma ow with the low gas kinetic temperature at M = 2, is used. Supersonic ow is maintained at complete steady state. The ow is ionized by a high-pressure aerodynamically stabilized dc discharge in the tunnel plenum and by a transverse rf discharge in the supersonic test section. The dc discharge is primarily used for the supersonic ow visualization, whereas the rf discharge provides high electron density in the supersonic test section. High-pressure ow visualization produced by the plasma makes all features of the supersonic ow, including shocks, boundary layers, expansion waves, and wakes, clearly visible. Attached oblique shock structure on the nose of a 35-deg wedge with and without rf ionization in a M = 2 ow is studied in various nitrogen-helium mixtures. It is found that the use of the rf discharge increases the shock angle by 14 deg, from 99 to 113 deg, which corresponds to a Mach number reduction from M = 2:0 to 1:8. Time-dependent measurements of the oblique 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 the order of seconds. Because the ow residence time in the test section is of the order of 10 ¹s, 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 ow temperature measurements in the test section using infrared emission spectroscopy, with carbon monoxide as a thermometric 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 air ows and the use of nonequilibrium plasmas for supersonic ow control. This opens a possibility for the use of transverse stable rf discharges for magnetohydrodynamic energy extraction and/or acceleration of supersonic air ows.

MHD Flow Control and Power Generation in Low-Temperature Supersonic Flows

“…Contribution of warm boundary layer regions (with the recovery temperature of Tr= 2 7 0 K) into the spectrum results in raising the "tail" of the vibrational band, thereby increasing the apparent rotational temperature. This effect has also been observed in our previous work on shock wave control in M-2 low-temperature RF plasma flows [20]. We emphasize that the most important result is that the temperatures measured with and without the DC sustainer discharge turn out to be very close.…”

Characterization of a Direct-Current Glow Discharge Plasma Actuator in Low-Pressure Supersonic Flow

“…T HERE is much recent interest in plasma-based localized aerodynamic flow actuation using single dielectric-barrier discharges (DBD) [1,2], direct-current (DC) glow discharges [3][4][5], DC filamentary discharges [6,7], and RF discharges [8]. The use of plasmas for flow-control applications is particularly promising owing to the potential for achieving high-bandwidth actuation without moving mechanical parts.…”

Effect of electron density on shock wave propagation in optically pumped plasmas

“…Shock wave propagation in weakly ionized glow discharge plasmas ͑with an ionization fraction of n e /N ϳ10 Ϫ8 -10 Ϫ6 ͒ has been extensively studied over the last 15 years, both in Russia [1][2][3][4][5][6][7][8][9][10][11] and in the U.S. [12][13][14][15][16][17][18][19][20] A number of anomalous effects occurring at these conditions, such as shock acceleration, weakening, and dispersion, have been reported. These effects have been observed in discharges in various gases ͑air, N 2 , Ar͒ at pressures up to Pϭ30 Torr, and for shock wave Mach numbers Mϭ1.5-4.5.…”