Venkat Narayanaswamy

Characterization of a Direct-Current Glow Discharge Plasma Actuator in Low-Pressure Supersonic Flow

DOI: 10.2514/1.27197 An experimental study of a direct-current, nonequilibrium glow plasma discharge in the presence of a Mach 2.85 supersonic flow is presented. The discharge is generated with pinlike electrodes flush-mounted on a plane surface with sustaining currents between 25 to 300 mA. In the presence of a supersonic flow, two distinct discharge modes (diffuse and constricted) are observed depending on the flow and discharge operating conditions. The effect of the dischargeonthe flow(“plasmaactuation”)ischaracterizedbytheappearanceofaweakshockwaveinthevicinityof the discharge. The shock is observed at low powers (10 W) for the diffuse discharge mode but is absent for the higher power (100 W) constricted mode. High-speed laser schlieren imaging suggests that plasma actuation is rapid as it occurs on a time scale that is less than 220 s. Rotational (gas) and vibrational temperature within the dischargeareestimatedbyemissionspectralline fitsofN2 andN 2 rovibronicbandsnear365–395nm.Theelectronic temperatures are estimated by using the Boltzmann plot method for Fe(I) atomic lines. Rotational temperatures are found to be high (1500 K) in the absence of a flow but drop sharply (500 K) in the presence of a supersonic flow for both the diffuse and constricted discharge modes. The vibrational and electronic temperatures are measured to be about 3000 K and 1.25 eV, respectively, and these temperatures are the same with and without flow. The gas temperature spatial profiles above the cathode surface are similar for the diffuse and constricted modes indicating that dilatational effects due to gas heating are similar. However, complete absence of flow actuation as indicated visually by the shock indicates that electrostatic forces may also play an important role in high-speed plasma-flow actuation phenomena. Analytical estimates using cathode sheath theory indicate thation pressure within sheath can besignificant,resulting ingascompressionwithin sheathandacorrespondingexpansionaboveit. Theexpansion,in turn, may fully negate the dilatational effect in the constricted case resulting in an apparent absence of forcing in the constricted case.

Control of a Shock/Boundary-Layer Interaction by Using a Pulsed-Plasma Jet Actuator

S HOCK wave/boundary-layer interactions (SWBLIs) are a common feature of supersonic/hypersonic flight, and the unsteadiness of strongly separated interactions can lead to rapid fatigue of structural panels as well as inlet instability and unstart. To mitigate these problems, there is interest in developing techniques for controlling the separatedflowunsteadiness by using both passive and active control techniques. Previous SWBLI control work has focused on reducing the size of the separated flow and/or shifting the frequency of the interaction unsteadiness to a band that does not coincide with the resonant frequency of structural panels. A detailed survey of the variousmeans used for controlling SWBLI until the late 1980s is given in [1]. Recently, plasma-based actuators have been used by researchers for active control of SWBLI, since these actuators have several inherent desirable features such as high bandwidth and no moving parts. For example, previous researchers have used surface-mounted arc discharges [2] and glow discharges with external magnetic fields [3,4] to achieve control of reflected SWBLI. Wang et al. [5] used surface-mounted arc discharges with external magnetic fields and demonstrated the weakening of the separation shock strength in front of a compression ramp. Recently, arc discharges have been employed by Grossman et al. [6] and subsequent researchers [7–11] to generate a pulsed synthetic jet, which they termed a spark jet. The spark-jet design was modified by Narayanaswamy et al. [12] to extend the pulsing frequency to the kilohertz range. They termed the actuator a pulsed-plasma jet since the term spark implies a thermal discharge, which was not the case at the pressures used in their study (and in the current work). Narayanaswamy et al. [12] performed a detailed parametric study of the velocity and temperature characteristics of the pulsed-plasma jets. They reported a jet-exit velocity of about 300 m=s and a bulk gas temperature in the range 600–1000 K for the range of discharge currents tested. The same pulsed-plasma-jet array actuator is used in the present work to control the separation shock of a SWBLI generated by a compression ramp in a Mach 3 flow.

Investigation of a pulsed-plasma jet for shock / boundary layer control

Pulsed jets with peak exit velocities as high as 250 m/s are generated by rapidly heating the air inside a chamber with an electrical discharge. The heated pressurized gas issues from a small orifice to form the pulsed plasma jet or ‘spark jet’. Pulsing frequencies as high as 5 kHz are obtained. An array of these jets, in a pitched and skewed configuration, is used to force the unsteady motion of the interaction formed by a 24° compression ramp in a Mach 3 flow. The Reynolds number of the incoming boundary layer is Re=3300. The effect of the plasma jet array on the separation shock motion is studied by using 10 kHz Schlieren imaging and fast-response wall pressure measurements. Results show that when the pulsed jet array is placed upstream of the interaction, the jets cause the separation shock to move in a quasi-periodic manner, i.e., nearly in sync with the pulsing cycle. As the jet fluid convects across the separation shock, the shock responds by moving upstream, which is primarily due to the presence of hot gas and hence the lower effective Mach number of the incoming flow. Once the hot gases pass through the interaction, t he separation shock recovers by moving downstream, and this recovery velocity is approximately 1% to 3% of the free stream velocity. With forcing, the low-frequency energy content of the pressure fluctuations at a given location under the intermittent region decreases significantly. This is believed to be a result of an increase in the mean scale of the interaction under forced conditions. Pulsed-jet injection was also employed within the separation bubble, but negligible changes to the separation shock motion were observed . These results indicate that influencing the dynamics of this compression ramp interaction is much more effective by placing the actuator in the upstream boundary layer.