Investigation of an Oscillating Surface Plasma for Turbulent Drag Reduction

An oscillating, weakly ionized surface plasma has been investigated for use in turbulent boundary layer viscous drag reduction. The study was based on reports showing that mechanical spanwise oscillations of a wall can reduce viscous drag due to a turbulent boundary layer by up to 40%. It was hypothesized that the plasma induced body force in high electric field gradients of a surface plasma along strip electrodes could also be configured to oscillate the flow. Thin dielectric panels with millimeter-scale, flushmounted, triad electrode arrays with one and two-phase high voltage excitation were tested. Results showed that while a small oscillation could be obtained, the effect was lost at a low frequency (< 100Hz). Furthermore, a mean flow was generated during the oscillation that complicates the effect. Hot-wire and pitot probe diagnostics are presented along with phase-averaged images revealing plasma structure. Introduction Dielectric-controlled, weakly ionized AC surface plasmas operating at ______________________________ * Senior Research Engineer , Flow Physics and Control Branch, Senior Member AIAA "This material is declared a work of the U.S. Government and is not subject to copyright protection in the United States" atmospheric pressure have recently become a topic of interest for flow control applications. The interest stems from the plasma’s ability to accelerate ionized air in regions of steep electric field gradients along strip electrode edges. The combination of steep edge gradients and charged particles within the plasma lead to a body force on the air through an electrodynamic collisional process. The existence of the body force has led to the search for flow control opportunities that can utilize such an effect. The plasma is typically generated between adjacent, thin, surface, strip electrodes with an intervening dielectric to prevent avalanche breakdown of the air. The most common configuration is thin foil strip electrodes bonded to opposite sides of a thin dielectric panel. The plasma is typically operated in the range of 1-10kHz, 1-10kV rms for the majority of low speed experiments and applications studied thus far. Some of the physical principles on which the plasma and body force generation operate are discussed by Roth and Massines. Kunhardt reviews this and similar plasmas and cites additional references. The present flow control investigation considers the possibility of reducing viscous drag due to boundary layer