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

Abstract

DOI: 10.2514/1.27197 An experimental study of a direct-current, nonequilibrium glow plasma discharge in the presence of a Mach 2.85 supersonic flow is presented. The discharge is generated with pinlike electrodes flush-mounted on a plane surface with sustaining currents between 25 to 300 mA. In the presence of a supersonic flow, two distinct discharge modes (diffuse and constricted) are observed depending on the flow and discharge operating conditions. The effect of the dischargeonthe flow(“plasmaactuation”)ischaracterizedbytheappearanceofaweakshockwaveinthevicinityof the discharge. The shock is observed at low powers (10 W) for the diffuse discharge mode but is absent for the higher power (100 W) constricted mode. High-speed laser schlieren imaging suggests that plasma actuation is rapid as it occurs on a time scale that is less than 220 s. Rotational (gas) and vibrational temperature within the dischargeareestimatedbyemissionspectralline fitsofN2 andN 2 rovibronicbandsnear365–395nm.Theelectronic temperatures are estimated by using the Boltzmann plot method for Fe(I) atomic lines. Rotational temperatures are found to be high (1500 K) in the absence of a flow but drop sharply (500 K) in the presence of a supersonic flow for both the diffuse and constricted discharge modes. The vibrational and electronic temperatures are measured to be about 3000 K and 1.25 eV, respectively, and these temperatures are the same with and without flow. The gas temperature spatial profiles above the cathode surface are similar for the diffuse and constricted modes indicating that dilatational effects due to gas heating are similar. However, complete absence of flow actuation as indicated visually by the shock indicates that electrostatic forces may also play an important role in high-speed plasma-flow actuation phenomena. Analytical estimates using cathode sheath theory indicate thation pressure within sheath can besignificant,resulting ingascompressionwithin sheathandacorrespondingexpansionaboveit. Theexpansion,in turn, may fully negate the dilatational effect in the constricted case resulting in an apparent absence of forcing in the constricted case.