Flow Control with Electrohydrodynamic Actuators

Abstract

Theabilityofanelectrohydrodynamicactuatortomodifythecharacteristicsofa e owovera e atplateisanalyzed. Thedeviceconsidered usese ush-mounted electrodes and a dcpowersupply to createa plasma sheeton thesurface of the plate. We analyze the mechanism of formation of this plasma sheet, which has some similarities with the phenomenon of streamer formation. Experimental results are presented concerning e ow visualizations obtained at low e ow velocities (o 1 m/s) and velocity e elds obtained with a particle image velocimetry technique for higher e ow velocities (range 11.0-17.5 m/s). These results show that the discharge can induce an important acceleration of the e ow close to the surface. ORONAS are self-sustaining discharges characterized by a strong inhomogeneity of the electric e eld cone guration and electrodes having a low curvature radius. This cone guration cone nes the ionization process to regions close to the high-e eld electrodes. Thus, in this phenomenon, there are active electrodes, surrounded by ionization regions where free charges are created, a low-e eld drift region where charged particles drift and react, and low-e eld passive electrodes. Coronas can be unipolar or bipolar if oneorbothelectrodesareactiveelectrodes.Bipolarcoronascanlead to the formation of streamers, weakly conducting plasma e laments extending from one electrode and carrying their own ionization re- gion ahead of themselves. Positive streamers are cathode directed and negative streamers anode directed. The physics of corona discharge occurring in a gas close to an insulating surfacehasnotbeen as widely studied as coronaswithout anyextraneousbodiesinthevicinityofthedischarge. 1;2 Becausethe discharge involves the movement of ions as well as a large amount of neutral particles, this situation becomes of special interest in aerodynamics for e ow and instabilities control. The induced e uid motion is usually called electroconvection or sometimeselectricwind.Coulombianelectroconvectiontakesplace ifthecoulombianforcesactingonthee uidparticlesarepredominant in relation to the polarization ones. This is usually the case when the e uid medium is air. The way the electric forces act on e uid particles may be explained by considering that ions in their drift motion from one electrode to the other will exchange momentum with the neutral e uid particles and induce their movement. Because currents involved in the process are so lowthat magnetic effects can be disregarded the phenomenon is described by the set of equations used in electrohydrodynamics (EHD) problems.