Forcing mechanisms in supersonic flow actuation achieved by direct-current surface glow discharge plasma

Forcing mechanisms in aerodynamic flow actuation in Mach 3 supersonic flow provided by a direct-current surface discharge are investigated experimentally and computationally. High-speed flow actuation is achieved by creating a near-wall ionization region produced by striking a discharge between two bare round electrodes which are flush mounted on a ceramic actuator plate. Flow actuation which is signatured by the occurrence of a weak oblique shock originating from the ionization region is observed with a lower power (∼10's W∼10's W) diffuse discharge which exhibits a volumetric ionization region above the cathode. This actuation effect is achieved when the cathode is placed upstream of the anode while no actuation is detected for the opposite case albeit with the same power input. Gas heating effect on flow actuation is evaluated by estimating gas (rotational) temperature using optical emission spectroscopy and numerical simulation. Significant gas heating as high as 400 K (free stream temperature is about 110 K) is observed near the cathode and numerical simulation confirms that gas heating is effective in supersonic flow actuation. However, the fact that the peak magnitude and spatial profile of gas temperature is similar for both cathode-upstream and cathode-downstream cases implies similar gas heating effect on supersonic flow actuation even though the actuation effect is different. Electrohydrodynamic (EHD) effect on flow actuation is emphasized by experimental results whose effect has been ignored in supersonic flow actuation and is supported by phenomenological modeling of electrostatic force. Analytic estimate of electrostatic forcing and corresponding computational result propose the potential role of EHD effect on supersonic flow actuation explaining the absence of flow actuation in the cathode downstream case.