Shock Wave Control by Plasmas

(Sponsored by the air Force Office of Scientific Research and MSE, Inc. under a NASA prime)

Photograph of the supersonic nitrogen plasma flow over a wedge in a DC discharge afterglow. P0=2/3 atm, M=3 Photograph of the supersonic nitrogen plasma flow over a wedge in an RF discharge afterglow. P 0 =1/3 atm, M=2

Reports of anomalous shock wave behavior in weakly ionized plasmas have recently stimulated considerable interest due to possible implications for supersonic flow control. These may include drag reduction, varying lift-to-drag ratio, MHD energy extraction, and MHD boundary layer control. This Tphenomenon has been extensively studied for the last 15 years, mostly in Russia at the A.F. Ioffe Physicotechnical Institute in St. Petersburg and the Radio Technical Institute in Moscow. More recently, similar experiments have been conducted at the U.S. Air Force Laboratories - Wright Patterson and the Arnold Engineering Development Center. The results demonstrate the following effects:

acceleration, weakening, and splitting of shock waves launched into glow discharge plasmas with ionization fraction of 100 ppb - 1 ppm

weakening of a bow shock formed ahead of a projectile moving through a discharge plasma at a supersonic velocity (shock stand-off distance increase)

dramatic wave drag reduction on the projectile (up to 50%)

These effects have been reported in discharges in various gases (Ar, He, N 2 , air, and CO 2 ) at pressures of 3- 30 Torr, and for Mach numbers in the range 1.5-4.5. They also persist for significant time durations after the discharge is turned off (~1 msec in air). The Nonequilibrium Thermodynamics Laboratory has been conducting an experimental and theoretical study of these phenomena over the last three years. The experiments are conducted in (A) a long run time nonequilibrium plasma wind tunnel, and (B) a diffuse nonequilibrium plasma sustained in a shock tube by a glow discharge or by a CO laser.

A. Plasma Wind Tunnel Experiment

In this experiment, we study oblique shocks attached to a small wedge model placed in a steady state cold plasma flow in a two-dimensional supersonic nozzle. The stable diffuse plasma is created either by a high- pressure aerodynamically stabilized DC discharge sustained in the nozzle plenum or by a transverse RF discharge sustained in the supersonic test section. Compared to the DC discharge, the RF discharge creates a much higher ionization in the test section. In both cases, the gas temperature in the test section, made of transparent acrylic plastic, is low (below room temperature). This is verified by accurate flow temperature measurements using infrared emission spectroscopy. The whole supersonic flow in the test section, including shocks, boundary layers, wakes, and expansion waves, is visualized by the plasma (see the photographs). This makes shock angle measurements quite straightforward.

Photograph of the M=2 plasma wind tunnel

The experiments show that the shock angle in the RF discharge plasma increases. This corresponds to the flow Mach number reduction, i.e. shock weakening. However, the shock weakening occurs very slowly, within a few seconds after the discharge is turned on. Also, the shock angle increase (i.e. the apparent Mach number reduction) turns out to be consistent with the flow temperature increase in the RF discharge. All this suggests that the shock weakens simply because the flow is heated by the RF discharge.

Shock weakening by the RF plasma in a M=2 N 2 -He flow. The larger angle shock (117°) corresponds to the "RF on" frame, and the smaller angle shock (105°) corresponds to the "RF off" frame.

The flow is primarily heated in the boundary layers near the RF discharge electrodes. This effect has been reduced by replacing the wedge model by a cone model and increasing the distance between the nozzle walls at the same time, so that the entire model was placed in the supersonic core flow. In this case, no effect of the RF discharge plasma on the conical shock wave angle has been detected. This results also suggests that the previously observed shock weakening was due to the heating of the flow by plasmas.

B. Plasma Shock Tube Experiment

In this experiment, shock waves generated by a spark discharge are propagating into a nonequilibrium diffuse glow discharge plasma sustained in a small-scale glass shock tube. Shock acceleration, attenuation, and splitting are measured using a photoacoustic deflection (PAD) technique. Gas temperature is measured by infrared emission spectroscopy. The results of these experiments are compared with computer calculations, which simulate shock propagation through the plasma. The model is based on two-dimensional compressible Navier-Stokes equations. The results suggest that most of previously observed anomalous plasma shock effects can be explained by the effect of radial and axial temperature gradients in the plasma. These temperature gradients produce distortion and splitting of the shock front.

In an additional series of plasma shock tube experiments, the spark-generated shock is propagating into a nonequilibrium optically pumped CO-Ar plasma sustained by a CO laser. Unlike the glow discharge, in this type of plasma electron density can be varied and controlled independently of the temperature distribution. This is done by adding trace amounts of oxygen or nitric oxide to the baseline gas mixture, which reduces the electron removal rate in the plasma. The results suggest that the shock wave weakening is independent of the electron density in the plasma. No separate non-thermal effect of ionization on the shock strength and structure in nonequilibrium optically pumped plasmas is detected.

Movie Clip: DC/RF Supersonic Plasma Flow

Movie Clip: RF Supersonic Plasma Flow