Environmental Impacts on Dielectric Barrier Discharge Plasma Actuators

Plasma actuators based on dielectric barrier discharge promise a bright future in aerodynamical applications. By creating a body force in the surrounding gas through plasma – gas interaction, plasma actuators, operated in quiescent air, induce a weak flow above their surface with a velocity of typically 5-8 m/s, the so called ionic wind. The ionic wind can influence the boundary-layer of any externally applied flow. Thus, plasma actuators are used for flow-control applications with a wide range of different flow velocities, temperatures and pressures. In the present work, the impact of these environmental conditions on the power consumption, plasma length and resonance behavior of the plasma actuators is studied. It is shown that an increase of the temperature or decrease of the pressure favor the discharge, as they increase the plasma length and the power consumption for a constant voltage amplitude. During the experiments temperatures from ambient temperature up to 600°C were tested and the pressure ranges from ambient pressure down to p = 0.11 bar. Scaling numbers and power laws were found to describe the impact of these parameters on the power consumption and plasma length. A hypothesis that the mean free path of the ions and electrons could be the dominant impact factor on the discharge was derived. Thus, the temperature and pressure dependence on the mean free path leads to the observed effects. Also, it is shown that the impedance of the plasma actuator, which is a product of an active and passive component of the actuators capacitance, is increased by an increasing temperature or decreasing pressure. This effects lead to a decrease of the resonance frequency of the circuit. External flows with small velocities between 0 m/s and 21 m/s have no effect on the power consumption and resonance behavior of an actuator. High velocities decrease the power consumption and the impedance, as they reduce the discharge volume and relative discharge duration. These effects are shown in different experiments of a plasma actuator exposed to flow Mach numbers between 0.42 and 0.7. The second part of this thesis focuses on the investigation of the environmental and voltage impact on filaments in the discharge that are caused by an instability in the discharge process. It is shown that increasing the pressure or flow velocity stabilizes the discharge and thus delays the onset of the formation of filaments towards higher voltages. The temperature seems to have no impact on the onset voltage. These effects are in good agreement with the predications by a theoretical model of the filaments. With the help of streak-camera images it is shown that the filaments only occur during the negative half-cycle. Increasing the voltage amplitude or temperature increases the spacing between the filaments, which typically ranges in between 2 mm and 4 mm. The same effect is achieved by decreasing the pressure. Measurements with a Pitot tube and particle image velocimetry show that the filaments increase the local velocity of the ionic wind by a factor of roughly 1.1 - 1.6. Thus, the ionic wind of a discharge with filaments looses its two-dimensional character and becomes three-dimensional. This effect may allow advanced flow-control strategies, for example active vortex generation using the discharge filaments