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Jichul Shin, Venkat Narayanaswamy, L. L. Raja, N. T. Clemens
Plasma actuators possess several favorable characteristics like high bandwidth actuation, absence of moving parts etc., which the conventional mechanical actuators lack.Several researchers have reported flow actuation using different types of discharges in both low speed and supersonic flows. The DC glow discharge plasma actuators are particularly useful in supersonic flows as they require relatively low power per electrode pair and offer scalability over large actuation area. Hence we use these actuators for our flow control application.
The primary objective of the present study is to develop the DC glow discharge actuators to produce appreciable flow actuation. We scale the actuation area using an array of plasma discharge in order to produce a large-area actuation. We also attempt to understand the underlying mechanism of actuation using optical emission spectroscopic studies and schlieren imaging.
The following figure shows the schematic of the experimental setup. The input power was set between 50 W and 250 W. The discharge current was set between 50-200 mA. The static pressure of the incoming Mach 3 flow was approximately 25 torr. Figure below is the photograph of an array of discharge exposed to the incoming supersonic flow.
A typical frame of the schlieren video sequence of the flow field (flow from left to right) with a plasma discharge is shown below. The presence of the discharge produces of a weak oblique shock (flow actuation). It was observed using 10 kHz schlieren that actuation takes place within 100 microsec.
Shown below is the schlieren image taken by employing single (a) and an array of three discharges (b). Using an array of discharges the shock strength increases compared to that with a single discharge. This shows that with increasing the number of discharge, the disturbance produced by the discharge tends to become two-dimensional. The above claim is also supported by wall pressure measurements.
(a) (b)
The glow discharge exposed to an incoming flow is found to exist typically in two different discharge modes � mode D and mode C (shown below) � depending on the discharge and flow conditions. Mode D discharge, which is formed at a low discharge currents and low pressure, is characterized by a diffuse volumetric glow over cathode, which extends over a boundary layer thickness. Mode C, formed at elevated current and pressure, is seen as a constricted streamer that extends from cathode to anode. It spans over a smaller extent in the wall normal direction when compared to the Mode D discharge.
(a) (b)
Our schlieren images have shown that the flow actuation takes place only with a mode D discharge and not with mode C discharge. In order to explain the disparity in interaction of the plasma and also to understand the plasma flow interaction dynamics the knowledge of underlying mechanism is mandatory. It is shown by many previous researchers that the dominant mechanism of flow actuation of glow discharge is the electrothermal heating of bulk flow. An optical emission spectroscopic study was made to estimate the bulk gas temperature. The rovibronic spectrum emission band of C(3\sigma+) -> X(3\sigma-) (0,0) transition of first negative N2+ and the emission band of C(3\Pi+) -> B(3\Pi+) (0,2) transition of the second positive N2 are employed for the study.
Following figure shows the variation of the rotational and vibrational temperature at different setpoint discharge currents. It is seen that, in general, the discharge heats gas to about twice its adiabatic wall temperature ( ~ 300K).
This shows that there is significant contribution from joule heating to the plasma-flow interaction. However, as stated earlier, there is a patent difference in flow actuation between mode D and mode C discharge albeit their temperature profile is approximately same. This shows that there should be a non-trivial contribution from another mechanism towards the plasma-flowfield interaction. Studying the effect of switching the electrodes further motivates this point. The schlieren images with cathode upstream configuration and cathode downstream configuration are shown in the figure below.
The discharge current and flow conditions are fixed to identical values. The temperature profile measured for both the configurations are same within the uncertainties as shown in the figure below. However there is a significant disparity in the plasma flow interaction as observed from the schlieren images. This implies a possible role of another mechanism in the interaction.
Preliminary calculations using reacting sheath theory shows that there may be an appreciable contribution from the electrostatic force generated by the discharge. The electrostatic forcing by a glow discharge is demonstrated in the following video . A typical electrostatically induced velocity contour is shown below.
The sheath thickness of mode D discharge is estimated to be about 10 times that of the mode C discharge. However the ion pressure exerted by the discharge is approximately the same. Hence there is a considerably larger electrostatic force in a mode D discharge compared with a mode C discharge. This may explain the disparity in flow actuation between the modes that we earlier observed (link). Also the force direction reverses with switching the polarity of the electrode. This can be reflected as the difference in flow actuation as the polarity of the electrodes is switched.
Ongoing work
Till now the pressure increase even by employing an array of discharge is only about 10 %. Efforts are underway to increase the flow actuation by bringing the electrodes closer so that the interaction between the individual discharges can be enhanced. A pulsing circuit is also developed for periodic switching of plasma for better coupling with the incoming flow. The explanation of plasma flow interaction using electrostatic forcing is still incipient. Efforts are being made to study this mechanism in further detail in order to establish its role more strongly.
Publications:
- Shin, J., Narayanaswamy, V., Raja, L.L., Clemens, N.T., "Characterization of a Direct-Current Glow Discharge Plasma Actuator in Low-Pressure Supersonic Flow," AIAA Journal, Vol. 45, No. 7, pp. 1596-1605, 2007.
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