A study of direct-current surface discharge plasma for a Mach 3 supersonic flow control
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A direct-current, non-equilibrium surface glow discharge plasma in the presence of a Mach 2.85 flow is studied experimentally for flow control applications. The discharge is generated with pin-like electrodes flush mounted on a ceramic plate with sustaining currents from 25 mA 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. In cathode upstream location, both diffuse and constricted discharges are observed while in cathode downstream location, the discharge mostly exhibits either constricted mode or bistable mixed mode. The effect of the discharge on the flow ("plasma actuation") is characterized by the appearance of a weak shock wave in the vicinity of 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 the diffuse mode plasma actuation is rapid as it occurs on a time scale that is less than 100 [mu]sec. Rotational (gas) and vibrational temperatures within the discharge are estimated by emission spectral line fits of N₂ and N⁺₂ rovibronic bands near 365-395 nm. The electronic 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 (14500 K), respectively, and these temperatures are the same with and without flow. The gas (rotational) temperature spatial profiles above the cathode surface are found to be similar for the diffuse and constricted modes indicating that dilatational effects due to gas heating are similar. However, complete absence of flow actuation for the constricted mode suggests that electrostatic forces may also play an important role in supersonic plasma-flow actuation phenomena. Analytical estimates using cathode sheath theory indicates that ion pressure within the cathode sheath can be significant resulting in gas compression in the sheath and a corresponding expansion above it. The expansion in turn may fully negate the dilatational effect in the constricted case resulting in an apparent absence of forcing in the constricted case. Plasma-induced flow velocity reaches about 1 m/s in stagnant air at the discharge current of order tens of milliamps. This electrostatic forcing in the direction from anode to cathode can play an important role in the boundary layer of supersonic flow.