Keisuke Takashima

Characterization of a surface dielectric barrier discharge plasma sustained by repetitive nanosecond pulses

The present work discusses experimental characterization of a surface Dielectric Barrier Discharge (DBD) plasma sustained by repetitive, high-voltage, nanosecond duration pulses. The measurements have been conducted in quiescent room air. Current, voltage, instantaneous power, and coupled pulse energy in the surface DBD actuator powered by high voltage nanosecond pulses have been measured for different pulse peak voltages, pulse repetition rates, and actuator lengths. Pulse energy per unit length is controlled primarily by the pulse peak voltage and is not affected by the actuator length. The results show that the actuator can be scaled to a length of at least 1.5 m. Images of the plasma generated during the nanosecond pulse discharge development have been taken by an ICCD camera with nanosecond gate. The results show that the plasma remains fairly uniform in the initial phase of discharge development and becomes highly filamentary at a later stage. Although the negative polarity nanosecond pulse discharge generates uniform plasma at low pulse repetition rates (~100 Hz), the plasma becomes strongly filamentary as the pulse repetition rate is increased beyond ~1 kHz. Phase-locked schlieren images have been used to visualize compression waves generated by the repetitively pulsed plasma and to measure the compression wave propagation speed. Density gradient in the compression waves generated by the nanosecond pulse discharge has been inferred from the schlieren images using calibration by a pair of wedged mirrors. The results demonstrate that compression waves generated by discharge filaments have higher amplitude and higher speed, compared to those produced in a diffuse discharge. Purely rotational CARS thermometry has been used to measure the temperature in a repetitive nanosecond pulse discharge filament, stabilized by using a sharp point floating electrode. The temperature rise in the filament, inferred from the CARS measurements, approximately ΔT=40 K, is significantly lower compared to the temperature rise in the filament inferred from the UV/visible emission spectroscopy measurements at the same conditions, ΔT=350 K. Comparison of the experimental density gradient in a compression wave generated by a nanosecond pulse discharge filament with modeling calculations suggests that the temperature inferred from the emission spectroscopy is more accurate.

Mach 5 bow shock control by a nanosecond pulse surface dielectric barrier discharge

Bow shock perturbations in a Mach 5 air flow, produced by low-temperature, nanosecond pulse, and surface dielectric barrier discharge (DBD), are detected by phase-locked schlieren imaging. A diffuse nanosecond pulse discharge is generated in a DBD plasma actuator on a surface of a cylinder model placed in air flow in a small scale blow-down supersonic wind tunnel. Discharge energy coupled to the actuator is 7.3–7.8 mJ/pulse. Plasma temperature inferred from nitrogen emission spectra is a few tens of degrees higher than flow stagnation temperature, T = 340 ± 30 K. Phase-locked Schlieren images are used to detect compression waves generated by individual nanosecond discharge pulses near the actuator surface. The compression wave propagates upstream toward the baseline bow shock standing in front of the cylinder model. Interaction of the compression wave and the bow shock causes its displacement in the upstream direction, increasing shock stand-off distance by up to 25%. The compression wave speed behind the...

Flow Separation Control over an Airfoil with Nanosecond Pulse Driven DBD Plasma Actuators

This work continues an ongoing development and use of dielectric barrier discharge (DBD) plasma actuators driven by repetitive nanosecond pulses for high Reynolds number aerodynamic flow control. These actuators are believed to influence the flow via a thermal mechanism which is fundamentally different from the more commonly studied AC-DBD plasmas. Leading edge separation control on an 8-inch chord NACA 0015 airfoil is demonstrated at various post-stall angles of attack (α) for Reynolds numbers (Re) and Mach numbers (M) up to 1.15x10 6 and 0.26 respectively (free stream velocity, U∞ = 93 m/s). The nanosecond pulse driven DBD can extend the stall angle at low Re by functioning as an active trip. At poststall α, the device generates coherent spanwise vortices that transfer momentum from the freestream to the separated region, thus reattaching the flow. This is observed for all Re and M spanning the speed range of the subsonic tunnel used in this work. The actuator is also integrated into a feedback control system with a stagnation-line-sensing hot film on the airfoil pressure side. A simple on/off type controller that operates based on a threshold of the mean value of the power dissipated by the hot film is developed for this system. A preliminary extremum seeking controller is also investigated for dynamically varying Re. Several challenges typically associated with integration of DBD plasma actuators into a feedback control system have been overcome. The most important of these is the demonstration of control authority at realistic takeoff and landing Re and M.

Experimental and modeling analysis of fast ionization wave discharge propagation in a rectangular geometry

Fast ionization wave (FIW), nanosecond pulse discharge propagation in nitrogen and helium in a rectangular geometry channel/waveguide is studied experimentally using calibrated capacitive probe measurements. The repetitive nanosecond pulse discharge in the channel was generated using a custom designed pulsed plasma generator (peak voltage 10–40 kV, pulse duration 30–100 ns, and voltage rise time ∼1 kV/ns), generating a sequence of alternating polarity high-voltage pulses at a pulse repetition rate of 20 Hz. Both negative polarity and positive polarity ionization waves have been studied. Ionization wave speed, as well as time-resolved potential distributions and axial electric field distributions in the propagating discharge are inferred from the capacitive probe data. ICCD images show that at the present conditions the FIW discharge in helium is diffuse and volume-filling, while in nitrogen the discharge propagates along the walls of the channel. FIW discharge propagation has been analyzed numerically usi...

Flow Separation Control Using Nanosecond Pulse Driven DBD Plasma Actuators

This work continues an ongoing effort aimed at development and use of dielectric barrier discharge (DBD) plasma actuators driven by repetitive nanosecond pulses for high Reynolds number aerodynamic flow control. These actuators are believed to influence the flow via a thermal mechanism which is fundamentally different from more commonly studied AC-DBD actuators. Leading edge separation control on an 8-inch chord NACA 0015 airfoil is demonstrated at various post-stall angles of attack for Mach numbers up to 0.26 (free stream velocity up to 93 m/s) and Reynolds numbers up to 1.15 X 106. The nanosecond (NS) pulse driven DBD is shown to extend the stall angle at low Reynolds numbers by functioning as an active trip. At post-stall angles of attack, the device is shown to excite shear layer instabilities and generate coherent spanwise vortices that transfer momentum from the freestream to the separated region, thus reattaching the flow. This is observed for all high Reynolds numbers and Mach numbers spanning th...

High Lift Airfoil Leading Edge Separation Control with Nanosecond Pulse Driven DBD Plasma Actuators

The efficacy of dielectric barrier discharge (DBD) plasmas driven by repetitive nanosecond (NS) pulses for flow separation control is investigated experimentally on an airfoil leading edge up to Re=1x10 (62 m/s). The NS pulse driven DBD plasma actuator (NSDBD hereafter) transfers very little momentum to the neutral air, but generates compression waves similar to localized arc filament plasma actuators. Experimental results indicate that NS-DBD plasma performs as an active trip at pre-stall angles of attack and provides high amplitude perturbations that manipulate flow instabilities and generate coherent spanwise vortices at post-stall angles. These coherent structures entrain freestream momentum thereby reattaching the normally separated flow to the suction surface of the airfoil. Such devices which are believed to function through thermal effects could result in a significant improvement over AC-DBD plasmas that rely on momentum addition which limits their performance at high speeds.

Ignition Time Measurements in Repetitive Nanosecond Pulse Hydrogen–Air Plasmas at Elevated Initial Temperatures

Ignition time is measured in premixed preheated hydrogen-air flows excited by a repetitive nanosecond pulse discharge in a plane-to-plane geometry. ICCD images of the plasma and the flame demonstrate that mild preheating of the fuel-air flow greatly improves plasma stability and precludes filament formation. At the initial temperatures of T0 = 100-200°C, hydrogen-air plasmas remain stable and uniform up to at least P = 150 torr, and ignition occurs in a large volume. In contrast, ignition in less uniform preheated ethylene-air plasmas occurs locally, near the electrode edges, with flame propagating toward the center of the plasma. Ignition time in hydrogen-air mixtures is measured at initial temperatures of T0 = 100-200°C, pressures of P = 40-150 torr, equivalence ratios of φ = 0.5-1.2, and pulse repetition rates of ν = 10-40 kHz. The results of ignition time measurements are compared with the predictions of the hydrogen-air plasma chemistry model, showing good agreement. Nitrogen emission spectra are used to measure time-resolved temperature in air and hydrogen-air plasmas. The results show that ignition begins at the plasma temperature of T ≈ 700 K and results in a rapid temperature rise. By turning off dominant plasma chemical radical generation processes in kinetic modeling calculations, while keeping discharge energy loading the same, it is demonstrated that ignition is driven by additional energy release in reactions of plasma-generated radicals with hydrogen. To determine if plasma-generated radicals may reduce ignition temperature, discharge pulse burst was terminated before the onset of ignition, and ignition delay time was measured versus plasma temperature at the end of the burst. Experimental ignition delay time is in reasonably good agreement with kinetic modeling calculations. The kinetic model predicts significant plasma-assisted ignition threshold temperature reduction at the present conditions compared to thermal ignition, up to ΔT = 180 K.

Nanosecond pulse surface discharges for high-speed flow control

The paper provides an overview of recent progress in the use of surface dielectric barrier discharges sustained by repetitive, high-voltage, nanosecond duration pulses for high-speed flow control. Experimental studies of diffuse and filamentary surface nanosecond pulse discharges in quiescent air demonstrate that they generate compression waves, due to rapid localized heating produced in the plasma. Compression waves produced by individual discharge filaments have higher amplitude and higher speed compared with waves produced in a diffuse discharge. Unlike surface dielectric barrier discharges sustained by AC voltage waveforms, nanosecond pulse discharges transfer little momentum to quiescent air, suggesting that localized heating and subsequent compression wave formation is the dominant flow control mechanism. Flow separation control using a nanosecond pulse surface discharge plasma actuator on an airfoil leading edge is studied up to M=0.26, Re=1.15·10 6 (free stream flow velocity 93 m/s), over a wide range of angles of attack. At pre-stall angles of attack, the actuator acts as an active boundary layer trip. At poststall angles of attack, strong flow perturbations generated by the actuator excite shear layer instabilities and generate coherent spanwise vortices. These coherent structures entrain freestream momentum, thereby reattaching the separated flow to the suction surface of the airfoil. Feasibility of supersonic flow control by low-temperature nanosecond pulse plasma actuators is demonstrated in Mach 5 air flow over a cylinder model. Strong perturbations of a bow shock standing in front of the model are produced by compression waves generated in the plasma. Interaction of the compression waves and the bow shock causes its displacement in the upstream direction, increasing shock stand-off distance by up to 25%. The effect of compression waves generated by nanosecond discharge pulses on shock stand-off distance is demonstrated for single-pulse and quasi-continuous actuator operation. A self-similar kinetic model is developed to analyze energy coupling to the plasma in a surface ionization wave discharge produced by a nanosecond voltage pulse. The model predicts key discharge parameters such as ionization wave speed and propagation distance, electric field, electron density, plasma layer thickness, and pulse energy coupled to the plasma, demonstrating good agreement with available experimental data and two-dimensional kinetic modeling calculations. The model allows an analytic solution and lends itself to incorporating into existing compressible flow codes, for in-depth analysis of the nanosecond discharge plasma flow control mechanism.

Development of a Mach 5 Nonequilibrium-Flow Wind Tunnel

A small-scale Mach 5 blowdown wind tunnel has been developed to generate steady-state nonequilibrium flows. The wind tunnel uses transverse nanosecond pulse discharge, overlapped with transverse dc discharge, to load internal energymodes ofN2 andO2 in plenum. The stable discharge is operated at high plenum pressures, at energy loadings of up to 0:1 eV=molecule in nitrogen, generating nonequilibrium nitrogen and airflows with run time of 5–10 s, translational/rotational temperature of T0 300–400 K, and N2 vibrational temperature of up to TV

Measurements and Kinetic Modeling Analysis of Energy Coupling in Nanosecond Pulse Dielectric Barrier Discharges 1

Nsec pulse discharge plasma imaging, coupled pulse energy measurements, and kinetic modeling are used to analyze the mechanism of energy coupling in high repetition rate, spatially uniform, nanosecond pulse discharges in air in plane-to-plane geometry. At these conditions, coupled pulse energy scales nearly linearly with pressure (number density), with energy coupled per molecule being nearly constant, in good agreement with the kinetic model predictions. In spite of high peak reduced electric field reached before breakdown, E/N ~ 500-700 Td, the reduced electric field in the plasma after breakdown is much lower, E/N~50-100 Td, predicting that a significant fraction of energy coupled to the air plasma, up to 30-40%, is loaded into nitrogen vibrational mode. A self-similar, local ionization kinetic model predicting energy coupling to the plasma in a surface ionization wave discharge produced by a nanosecond voltage pulse, has been developed. The model predicts key discharge parameters such as ionization wave speed and propagation distance, electric field, electron density, plasma layer thickness, and pulse energy coupled to the plasma, demonstrating good qualitative agreement with experimental data and two-dimensional kinetic modeling calculations. The model allows an analytic solution and lends itself to incorporating into existing compressible flow codes, at very little computational cost, for in-depth analysis of the nanosecond discharge plasma flow control mechanism. The use of the model would place the main emphasis on coupling of localized thermal perturbations produced by the discharge with the flow via compression waves, as well as on instability development and coherent structures formation, and would provide quantitative insight into the flow control mechanism on a long time scale.