Experimental and numerical investigations of the effect of electric discharges on the high speed gas flows are of considerable interest, which is related to a number of practical applications, in particular, as the plasma actuators [1]. Mechanism of action of the discharge on the flow can be thermal or plasma (ion wind). Nevertheless, shock waves formed by pulsed high-current discharges can significantly affect both the environment and the very region of the discharge [2]. This work presents the results of analysis of the dynamics of shock waves that arise at initiation of pulsed surface discharges in quiescent gases and in airflow.
High-current pulsed surface discharge (plasma sheet) is a system of plasma channels sliding over a dielectric surface [2]. Using this discharge it is possible to act upon the flow by means of shock waves formed at quick energy deposition, and fast heating of the near-surface gas layer with a thickness of ~0.5 mm [2]. The shock waves arise as a result of the development of a system of microchannels of discharge. The dynamics of shock waves originating from plasma sheet in quiescent air, nitrogen, helium and in airflow at velocities up to 1600 m/s at gas densities within 0.04–0.45 kg/m3 was investigated experimentally. The experiments were carried out in a shock tube with a discharge chamber, in which plasma sheets with an area of 30 cm2 were initiated on the opposite walls spaced by 24 mm. Voltage pulse amplitude was 25 kV. The discharge electric current time was about 300 ns. The specific deposited electric energy was of ~0.012 J/cm2. The evolution of the flow field formed within 100 microseconds after the discharge initiation was investigated by the shadow technique, single frame, streak, multiframe (high speed camera) modes. CFD 2D simulation was based on a solution of Navier–Stokes equations.
A multichannel structure of the pulse discharge resulted in the formation of semicylindrical shock waves from thin plasma channels on the surface. The interference of these waves during 1–3 μs leads to the formation of a quasi-plane shock-wave envelope front, moving from the wall (Fig.1).
Shock waves from plasma sheet velocities were measured in air, nitrogen, helium and also in transversal air flow behind the plane shock wave. Shock front velocity strongly depends on pressure and the deposited electric energy. The Mach numbers of shock waves are up to 1.5, their intensities are much higher than those of shock waves (compression waves) from DBD discharges (1.01-1.1 Mach numbers) [3].
 Fig.1.jpg)
Fig.1 Shadow images of shock waves from plasma sheet in transversal supersonic airflow at 3.2 μs and 7.5 μs after discharge initiation. Arrows indicate flow direction.
REFERENCES
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