Radio-frequency surface discharges and their application to active flow control

Abstract

A 13.56 MHz, radio-frequency (RF) surface discharge is studied experimentally and developed as a means to increase the performance of plasma actuators foractive flow control. Research is undertaken at The Australian National University, GREMI Institute, CNRS-Universite d'Orleans, France and the Stanford University Plasma Physics Laboratory, USA. Experiments are first undertaken in atmospheric-pressure argon to study the fundamental physics of the discharge in an environment that has simpler chemistry than ambient air. The generation of a diffuse mode is achieved through the regulation of the peak pulse voltage subsequent to breakdown for continuous RF power. The presence of a "streamer-free" discharge is confirmed using fast imaging and measurements of the optical emission are applied in conjunction with a power balance to estimate its density. For increased power efficiency, the generation of the diffuse mode is also achieved with pulsed power and the transition between it and two distinct filamentary modes is investigated through the regulation of the peak pulse voltage and the time between pulses. It is found that there exists a critical pulse off-time for which the generation of the diffuse mode is attainable and potential physical mechanisms are discussed. The propagation of electrons with energy greater than 13.48 eV, which are of elevated temperature compared to the bulk discharge and hence key to ionization, are studied using phase-resolved emission spectroscopy for pulsed RF power. The formation of spatially periodic electron-impact excitation bands over the surface of the dielectric layer is investigated. These highly repeatable structures, which are not typically observed in lower frequency surface dielectric-barrier-discharges (DBDs), are explained through the deposition of electrons on the dielectric surface during the extension phase of the propagation cycle. In preparation for aerodynamic testing, the propagation of the discharge is characterised in atmospheric-pressure air with pulsed RF power. Particular interest is paid to its behaviour with respect to the formation of highly repeatable filamentary structures, its behaviour during breakdown and the "collective" formation of two distinct streamer modes throughout the phase of the voltage cycle. By correlating the phase-resolved emission of the discharge with perturbations in the RF current, the charge transferred per microdischarge is estimated to be in close agreement with previous findings. The performance of the RF surface discharge as a plasma actuator is investigated using 2D particle image velocimetry in ambient air under static flow conditions. Two actuator configurations are trialled: A pulsed RF discharge and a combined{u00AD}waveform prototype comprising RF pulses (5 {u03BC}s pulses at 5 kHz) and a 5 kHz sinusoidal bias voltage that is commonly applied to DBD plasma actuators. It is found that RF pulses alone may not be useful in generating energy-efficient flow actuation. However, the combined-waveform actuator is measured to increase the maximum stream-wise velocity compared to that powered by the 5 kHz bias voltage alone. Best results are achieved when the RF pulse is positioned at the bias minimum (one RF pulse per bias cycle) and the corresponding increase in the induced flow velocity is from 0.46 m/s to 0.62 m/s (35% increase). This suggests that a combined{u00AD}waveform actuator comprising a sinusoidal bias voltage and RF pulses may be useful in the further development of surface discharge plasma actuators for active flow control. Show more