Electrode Polarity Effects in Surface Plasma Discharges for Supersonic Flow Control Applications: a Computational Study

Abstract

Computational simulations of air surface plasma discharges in the presence of supersonic flow are presented. The glow discharge model with finite-rate air plasma chemistry is coupled with a compressible Navier-Stokes solver and used to study a surface plasma discharge on M=3 supersonic flow at freestream pressure 18 Torr. The effect of the electrode polarity relative to the supersonic flow direction on the discharge structure is studied by investigating the charged species concentrations, gas temperature and pressure rise. Qualitative and quantitative differences between the surface discharges with the cathode located upstream and downstream with respect to the supersonic flow are discussed. Results indicate that O - has comparable concentration to electrons in both cathode upstream and downstream cases. For fixed cathode source voltage, the cathode downstream discharge is found to be more electronegative than the cathode upstream case. The peak gas temperature with the cathode located upstream is 830 K with a corresponding peak pressure rise of about 400 Pa. With the cathode located downstream, the peak gas temperature is 620 K with a corresponding peak pressure rise of about 300 Pa. I. Introduction LOW discharge actuation of a supersonic flow occurs through volumetric Joule heating, plasma heating of adjacent solid surfaces, electrostatic forcing and Lorentz forces (in the presence of magnetic fields). For plasma actuator applications, the role of multiple positive ions, negative ions and radicals in air plasma discharge electrodynamics, finite-rate chemistry effects, non-linear phenomena associated with multiple modes in air plasma discharges, etc., are currently of much interest. Non-equilibrium glow discharges are particularly attractive for flow actuation since they have relatively low power requirements compared to other surface discharges such as thermal arcs. Shin et. al 1 performed a detailed characterization of pin-electrode direct-current glow discharges in the presence of Mach 3 supersonic flow. At a given pressure, three discharge modes were observed: a diffuse mode at low currents (~10-100 mA), a constricted mode at relatively higher current (~200 mA), and a bistable mode at intermediate currents where the discharge alternated between the diffuse and constricted modes. The discharge modes were observed both in the absence and presence of supersonic flow. The relatively low-power diffuse mode was found to produce a weak shockwave, but no shock was observed in the case of the high-power constricted mode. Peak rotational temperatures were estimated to be around 1200-1800 K in the absence of flow, and 500-800 K in the presence of supersonic flow. Different actuation characteristics were observed by varying the relative position of the cathode: with the cathode upstream with respect to the flow, a weak shock was recorded in the diffuse mode but at the same power level a much weaker actuation was observed with the cathode downstream. A difference in stagnation pressure profiles and drag measurements when the cathode was located upstream as opposed to when the cathode was located downstream was also observed by Menier et. al 2 , who performed experiments of