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

Abstract

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 discharge on the flow ("plasma actuation") is characterized by the appearance of a weak shock wave in the vicinity of 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 discharge are estimated by emission spectral line fits of N 2 and N 2 rovibronic bands near 365-395 nm. The electronic 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 that ion pressure within sheath can be significant, resulting in gas compression within sheath and a corresponding expansion above it. The expansion, in turn, may fully negate the dilatational effect in the constricted case resulting in an apparent absence of forcing in the constricted case.

Simulation of Direct-Current Surface Plasma Discharges in Air for Supersonic Flow Control

“…4.3(a) show that the discharge is formed primarily above the cathode, with a spot attachment at the leading edge of the anode. Based on a depth of 1 mm, the current is ∼ 10 mA, which is similar to the low-power diffuse mode observed in experiments [14]. From Fig.…”

“…In the case of a stationary discharge (no bulk flow), it is reasonable to assume that Dirichlet boundary conditions are used for all boundaries with the temperature fixed at 300 K. An ion Joule heating thermalization factor η Th = 0.25 was used to compute the ion Joule heating source term in (2.7). Significant gas heating is observed above the cathode with a peak gas temperature of ∼ 1180 K, which is comparable to rotational temperatures of 1200-1800 K recorded in experiments [14] under similar discharge operating conditions.…”

“…The coupled plasma-compressible flow model is therefore parallelized using domain decomposition techniques (MPI), that enables the investigation of 3-D plasma actuators such as pin electrode actuators used in experiments [14].…”

“…Shin et al [14] performed a detailed characterization of pin-electrode direct-current plasma discharges in the presence of Mach 3 supersonic flow. At a given pressure, three discharge modes were observed: a diffuse mode at low currents (∼10-100 mA), a constricted mode at relatively higher current (∼200 mA), and a bistable mode at intermediate currents where the discharge alternated between the diffuse and constricted modes.…”

“…The symmetry condition was instead found to provide zero current in the far-field, with essentially similar results for the plasma species densities and discharge currents while allowing a larger time step for the electron energy and density equations, and hence was used in the calculations presented here. [14]. At the wall surface, viscous wall boundary conditions (no-slip) with fixed wall temperature = 300 K are applied.…”