High-speed Flow Control Research Articles

Experimental study on the characteristics of a two-electrode plasma synthetic jet actuator

Performance of a two-electrode plasma synthetic jet actuator has been experimentally studied by discharge measurements with high-speed shadowgraphy technology. Results show that the breakdown voltage and the peak discharge current of the actuator may be decreased by decreasing the ambient pressure and increasing the discharge frequency. The discharge developed in the actuator cavity is a spark-arc discharge. In the actuator could be created a strong “precursor shock” and a high kinetic energy jet. During the development of the plasma synthetic jet, the speed of the “precursor shock” is invariable and the jet propagates with an approximately local sonic velocity (350 m/s). But with decreasing jet exit diameter and ambient pressure, the increase of the cavity volume and the discharge frequency could lead to decreasing strength of the “precursor shock”. Heating efficiency of the gas in the cavity will decrease with the increase of the cavity volume and discharge frequency, and the jet velocity is decreased as well. The jet exit diameter and the ambient pressure can have their optimal values for affecting the velocity of the jet. Under all the working conditions we have studied, the plasma actuator may create a strong “precursor shock” and a high-speed jet, and also may have the potential to be used in high-speed active flow control.

Flow Physics of a Pulsed Microjet Actuator for High-Speed Flow Control

Flow control actuators based on a small-diameter source jet and a cylindrical cavity structure take advantage of the flow resonance within the cylindrical cavity to generate a variable-frequency, pulsed high-momentum microjet issuing through the cavity orifice. The flow-acoustic coupling, which leads to resonance within the cavity of the actuator, is the main driving mechanism behind the pulsed microjet. In the present study, a computational methodology based on high-order numerical techniques is used to simulate a highly unsteady and compressible pulsed actuator flowfield. Simulation generated flowfield results are analyzed to further understand the complex flow physics governing the pulsed actuator operation. The simulation provides significant details about the highly unsteady and complex microscale actuator flowfield, which are not observable from the experiments. Qualitative comparisons made between the simulated flowfield visualizations and the experimental microschlieren images show a reasonable level of agreement. We perform a dynamic mode decomposition to identify the dynamically important modes of the actuator flowfield. The additional insight gained into the flow physics through the simulation is useful in the design of more efficient and geometrically complex pulsed actuators for a range of high-speed flow and noise control applications.

Experimental Characterization of the Plasma Synthetic Jet Actuator

The plasma synthetic jet is a novel active flow control method because of advantages such as fast response, high frequency and non-moving parts, and it has received more attention recently, especially regarding its application to high-speed flow control. In this paper, the experimental characterization of the plasma synthetic jet actuator is investigated. The actuator consists of a copper anode, a tungsten cathode and a ceramic shell, and with these three parts a cavity can be formed inside the actuator. A pulsed-DC power supply was adopted to generate the arc plasma between the electrodes, through which the gas inside was heated and expanded from the orifice. Discharge parameters such as voltage and current were recorded, respectively, by voltage and current probes. The schlieren system was used for flow visualization, and jet velocities with different discharge parameters were measured. The schlieren images showed that the strength of plasma jets in a series of pulses varies from each other. Through velocity measurement, it is found that at a fixed frequency, the jet velocity hardly increases when the discharge voltage ranges from 16 kV to 20 kV. However, with the discharge voltage fixed, the jet velocity suddenly decreases when the pulse frequency rises above 500 Hz, whereas at other testing frequencies no such decrease was observed. The maximum jet velocity measured in the experiment was up to 110 m/s, which is believed to be effective for high-speed flow control.

Numerical characterization of high-temperature filament interaction with blunt cylinder at Mach 3

Energy deposition is a robust technique for various high-speed flow control applications including drag reduction. A numerical study of energy deposition is performed for perfect gas flow approaching a blunt cylinder at Mach 3. The energy deposition is simulated by a high-temperature filament injected instantaneously in front of the cylinder. The effect of important dimensionless parameters is studied to characterize the drag modification. The results indicate a saturation effect on maximum drag reduction at higher magnitudes of energy deposition. The computations reveal that the discharge location of the filament does not significantly impact the drag. A phenomenological examination of the interaction is performed. The effectiveness and efficiency of the filament on drag reduction are investigated. A one-dimensional analytical approach is studied to describe the numerical results.

Dynamics of Rapid Localized Heating in Nanosecond Pulse Discharges for High Speed Flow Control

Picosecond CARS spectroscopy and phased-locked schlieren imaging are used to measure time-resolved temperature and visualize compression waves in a diffuse filament, nanosecond pulsed discharge sustained between a pair of spherical electrodes in nitrogen and air at P=100 torr. The discharge generates stable plasma with high specific energy loading (up to ~0.5 eV/molecule), and with spatial dimensions sufficiently large to enable laser diagnostic studies. The results demonstrate that significant temperature rise, up to ∆T~200 K, occurs both in nitrogen and in air, on the time scale shorter than the acoustic time scale. The characteristic time for the rapid temperature rise in air, ~100 ns, is significantly shorter compared to that in nitrogen, ~1 µs. In air, a second significant temperature rise, up to ∆T~350 K, occurs on a time scale of ~100-500 µs. This “slow” temperature rise is almost entirely missing in nitrogen. Phase-locked schlieren images demonstrate a near cylindrical shape compression wave formed around the discharge filament, both in nitrogen and in air. An additional, near spherical shape compression wave is formed near the cathode, due to significant energy release in the cathode layer of the discharge. The compression waves, caused by rapid localized heating quantified by the present measurements, are similar to the ones produced by a surface nanosecond pulse discharge in atmospheric air used for high-speed flow control, where comparable temperature rise was detected previously.

Review of actuators for high speed active flow control

Actuators are one of the key points for the development of active flow control technology. Efficient methods of high speed flow control can provide enhanced propulsive efficiency and at the same time enable safe and maneuverable high speed flight. The development of high speed flight technology promotes the emergence of novel and robust actuators. This review introduces the state of the art in the development of actuators that can be used in high speed active flow control. The classification and different operation criteria of the actuators are discussed. The specifications, mechanisms and applications of various popular actuator types including fluidic, mechanical, and plasma actuators are described. Based on the realistic need of high speed flow control and the existing results of actuators, a new actuator design method is proposed. At last, the merits and drawbacks of the actuators are summarized and some suggestions on the development of active flow control technology are put forward.

High-Speed and High-Reynolds-Number Jet Control Using Localized Arc Filament Plasma Actuators

A class of plasma actuators called localized arc filament plasma actuators for high-speed and high-Reynoldsnumber flow and acoustic control has been developed at the Gas Dynamics and Turbulence Laboratory. Over the past several years, these high-bandwidth (0 to 200 kHz) and individually controlled actuators have been used successfully to excite the jet shear layer, jet column, and azimuthal instabilities in high subsonic and supersonic jets. The focus of this paper is to provide detailed information and sample results highlighting the capabilities and potential of the actuators and the control technique formixing enhancement, noise mitigation, and flow and acoustic diagnostics. The jet, using three different nozzles, is operated over a large range of jet Mach numbers (0.9 to 1.65), stagnation temperature ratios (up to 2.5), and Reynolds numbers (0:2 10 to 1:65 10). Over this space of operating conditions, the jet is found to respond to control with a large range of forcing Strouhal numbers and azimuthalmodes. The results reveal that the jet flowfield and acoustic farfield can be dramatically altered, providing a powerful control tool in these practical high-speed and high-Reynolds-number jets.

Influence of the energy dissipation rate in the discharge of a plasma synthetic jet actuator

A promising actuator for high-speed flow control, referred to as a plasma synthetic jet (PSJ), is being studied by the DMAE department of the ONERA, and the Laplace laboratory of the CNRS, in France. This actuator was inspired by the ‘sparkjet’ device developed by the Johns Hopkins University Applied Physics Laboratory. The PSJ, which produces a synthetic jet with high exhaust velocities, no active mechanical components and no mass flow admission, holds the promise of enabling high-speed flows to be manipulated. With this high-velocity jet it is possible to reduce fluid phenomena such as transition and turbulence, thus making it possible to increase an aircraft's performance whilst at the same time reducing its environmental impact. A thermal plasma discharge was created in a micro-cavity, causing the gas to be expelled. It is relevant that the velocity and momentum depend on the energy dispersed by the electric discharge. To control the frequency and energy dispersed in the plasma, the Laplace laboratory has developed two high-voltage power supply systems. These allow two different types of discharge to be produced, with energy being supplied to the discharge in two different manners. In this paper, we focus on the impact of the power supply on the plasma synthetic jet, and in particular on the role of the rate of energy dissipation in the discharge. In order to estimate the influence of the power supply on the machinery of the actuator, specific experimental techniques were used to investigate the electrical (voltage, current), thermal (Infra-red camera) and aerodynamic (jet duration, isentropic pressure, jet velocity) characteristics. These data sets were used to determine which of the two power supplies was more effective, thus allowing us to reach several conclusions concerning the importance of the energy dissipation rate on the PSJ actuator.

High-Bandwidth Pulsed Microactuators for High-Speed Flow Control

A systematic study on the design, development, and characterization of high-momentum, high-bandwidth microactuators for high-speed flow control is described in this paper. Beginning with building-block experiments, multiple resonant flow phenomena are used in the actuator design to arrive at an actuator configuration that provides the desired flow properties. The first-generation actuator design consists of an underexpanded source jet incident upon a cavity. The lower surface of this cavity contains micronozzles through which the unsteady microjets (400 μm) issue. Results show that microjets produced by this actuator have a high mean momentum (300―400 m/s) and a significant unsteady component (20―30% of the mean). Experiments were conducted over a large range of parameters in terms of cavity length, source jet nozzle pressure ratio, and impingement distance. The results unequivocally demonstrate the ability to vary the frequency and the amplitude of the mean and unsteady momentum of microjets issuing from this actuator. By varying the dimensions of the actuator by few hundred microns and/or source jet pressure by roughly 1 atm, one is able to vary the frequency rather precisely over a range of 5―20 kHz. A correlation based on Strouhal number and jet column length is suggested for the design of actuators. Actuators in the frequency range of a few to well over 50 kHz have been designed and characterized. It is believed that the frequency range may be extended down to O(100 Hz) and up to O(100 kHz) using this actuator approach.

Analysis of flow distribution from high-speed flow actuator using particle image velocimetry and digital speckle tomography

A variety of active flow control (AFC) methods are typically used in low-speed applications; however, the AFC techniques that are available for high-speed, supersonic applications are very limited. Under AFOSR (Air Force Research Laboratory) sponsorship, The Johns Hopkins University Applied Physics Laboratory (JHU/APL) is investigating a device that is intended for high-speed flow control; it is called the SparkJet actuator, which manipulates high-speed flows without active mechanical components. To date, actuator characterization has included computational and experimental techniques including parametric studies and flow visualization techniques to investigate the operation of the SparkJet device under various conditions. This paper focuses on the experimental flow measurement techniques that have been implemented. The results will be used for validating prospective computational studies that investigate the detailed characteristics of the SparkJet’s discharge and cooling stages after an energy deposition pulse. Current efforts include the use of high- resolution particle image velocimetry (PIV) to quantify the quiescent air operation of a single SparkJet pulse. However, the proper seeding of the SparkJet cavity continues to be challenging and has led to the use of digital speckle tomography (DST) to measure the temperature distribution in the core of the SparkJet plume. In this study, improved PIV techniques were used to acquire a higher-resolution image of the SparkJet-entrained flow. These PIV results show that the peak velocity in the entrained flow is around 53 m/s and the plume is sustained for 75–100 μs. Additionally, the DST data show a peak temperature of 1616.3 K at 75 μs and provide supporting information for interpreting the PIV data. These results are intended to calibrate and build confidence in a computational model.

Simulations of direct-current air glow discharge at pressures ∼1 Torr: Discharge model validation

Computational simulations of air glow discharge phenomena in the pressure range typical of plasma actuator applications for high speed flow control are presented. The model is based on a self-consistent, multispecies, and multitemperature continuum description of the plasma. A reduced air plasma model suitable for multidimensional simulations with 11 species and 21 gas phase chemical reactions is validated against experimental results in the literature. The discharge model predicts experimentally observed glow mode discharge operation, the current-voltage characteristics of the discharge, and spatial profiles of the electron temperature and positive ion number densities. For pressures of order 1 Torr, O2+ and N2+ are the dominant positive ion species in the discharge, and the concentration of O− negative ion is comparable to electron concentration. The two-dimensional structure of the discharge is predicted by the model is found to be in agreement with qualitative observations from the experiments.

Development of localized arc filament RF plasma actuators for high-speed and high Reynolds number flow control

Recently developed localized arc filament plasma actuators (LAFPAs) have shown tremendous control authority in high-speed and high Reynolds number flow for mixing enhancement and noise mitigation. Previously, these actuators were powered by a high-voltage pulsed DC plasma generator with low energy coupling efficiency of 5–10%. In the present work, a new custom-designed 8-channel pulsed radio frequency (RF) plasma generator has been developed to power up to 8 plasma actuators operated over a wide range of forcing frequencies (up to 50 kHz) and duty cycles (1–50%), and at high energy coupling efficiency (up to 80–85%). This reduces input electrical power requirements by approximately an order of magnitude, down to 12 W per actuator operating at 10% duty cycle. The new pulsed RF plasma generator is scalable to a system with a large number of channels. Performance of pulsed RF plasma actuators used for flow control was studied in a Mach 0.9 circular jet with a Reynolds number of about 623,000 and compared with that of pulsed DC actuators. Eight actuators were distributed uniformly on the perimeter of a 2.54-cm diameter circular nozzle extension. Both types of actuators coupled approximately the same amount of power to the flow, but with drastically different electrical inputs to the power supplies. Particle image velocimetry measurements showed that jet centerline Mach number decay produced by DC and RF actuators operating at the same forcing frequencies and duty cycles is very similar. At a forcing Strouhal number near 0.3, close to the jet column instability frequency, well-organized periodic structures, with similar patterns and dimensions, were generated in the jets forced by both DC and RF actuators. Far-field acoustic measurements demonstrated similar trends in the overall sound pressure level (OASPL) change produced by both types of actuators, resulting in OASPL reduction up to 1.2–1.5 dB in both cases. We conclude that pulsed RF actuators demonstrate flow control authority similar to pulsed DC actuators, with a significantly reduced power budget.

Plasma assisted ignition and high-speed flow control: non-thermal and thermal effects

The paper reviews recent progress in two rapidly developing engineering applications of plasmas, plasma assisted combustion and plasma assisted high-speed flow control. Experimental and kinetic modeling results demonstrate the key role of non-thermal plasma chemistry in hydrocarbon ignition by uniform, repetitively pulsed, nanosecond pulse duration, low-temperature plasmas. Ignition delay time in premixed ethylene–air flows excited by the plasma has been measured in a wide range of pulse repetition rates and equivalence ratios and compared with kinetic modeling calculations, showing good agreement. Comparing ignition delay time predicted by the model for plasma assisted ignition and for ignition by equilibrium heating demonstrated that chain reactions of radicals generated by the plasma reduce ignition time by up to two orders of magnitude and ignition temperature by up to 300 K. These results provide additional evidence of the non-thermal nature of low-temperature plasma assisted ignition.Experiments and flow modeling show that the dominant mechanism of high-speed plasma flow control is thermal, due to heating of the flow by the plasma. Development and characterization of pulsed dc and pulsed RF localized arc filament plasma actuator arrays for control of high-speed atmospheric pressure jet flows are discussed. Actuator power is quite low, ∼10 W at 10% duty cycle. Plasma emission spectra show that a greater fraction of the pulsed RF discharge power goes to heat the flow (up to 2500 °C), while a significant fraction of the pulsed dc discharge power is spent on electrode and wall heating, resulting in their erosion. Rapid localized heating of the flow by the pulsed arc filaments, at a rate of ∼1000 K/10 µs, results in the formation of strong compression/shock waves, detected by schlieren imaging. Effect of flow forcing by repetitively pulsed RF actuators is demonstrated in a M = 1.3 axisymmetric jet. These two case studies provide illustrative examples of isolating non-thermal (non-equilibrium plasma chemistry) and thermal (Joule heating) effects in plasmas and adapting them to develop efficient large-volume plasma igniters and high-speed flow actuators.

Repetitively Pulsed Nonequilibrium Plasmas for Magnetohydrodynamic Flow Control and Plasma-Assisted Combustion

This paper demonstrates significant potential of the use of high-voltage, nanosecond pulse duration, high pulse repetition rate discharges for aerospace applications. The present results demonstrate key advantages of these discharges: 1) stability at high pressures, high flow Mach numbers, and high-energy loadings by the sustainer discharge, 2) high-energy fractions going to ionization and molecular dissociation, and 3) targeted energy addition capability provided by independent control of the reduced electric field of the direct current sustainer discharge. These unique capabilities make possible the generation of stable, volume-filling, low-temperature plasmas and their use for high-speed flow control, nonthermal flow ignition, and gasdynamic lasers. In particular, the crossed pulsersustainerdischargewasusedformagnetohydrodynamic flowcontrolincoldM � 3 flows,providing firstevidenceof cold supersonic flow deceleration by Lorentz force. The pulsed discharge (without sustainer) was used to produce plasma chemical fuel oxidation, ignition, and flameholding in premixed hydrocarbon–air flows, in a wide range of equivalence ratios and flow velocities and at low plasma temperatures, 150–300 � C. Finally, the pulser-sustainer discharge was used to generate singlet oxygen in an electric discharge excited oxygen–iodine laser. Laser gain and output power are measured in the M � 3 supersonic cavity.

High-Speed Magnetohydrodynamic Flow Control Analyses with Three-Dimensional Simulations

Magnetohydrodynamic studies of high-speed flow control are described with emphasis on understanding fluid response to specific plasma-based perturbations. The theoretical model consists of a verified blend of first principles and empirical components. Detailed analysis is presented of the effect of magnitudes and gradients of magnetic and electric fields, their orientation relative to the velocity vector, ionized region location and extent, and various nondimensional parameters. The balance between ponderomotive force and heating is a major determinant of the effectiveness through competition between work and ohmic dissipation and viscous/inviscid interactions play a crucial role by distorting the velocity field. The interaction with an external circuit through electrodes is relatively efficient when fluid is slowed and energy is extracted, but yields high boundary-layer heating and loss of control performance when fluid is accelerated. These observations are employed to unify results focused on a broad range of objectives. Specific flowfields examined include heat transfer reduction in an Edney type-IV interaction at Mach 8, three-dimensional separation suppression at Mach 5 with magnetic-field-facilitated momentum transfer, inviscid instability-growth-rate modulation in an entropy layer at Mach 6, and energy management in simulated tip-to-tail scramjet designs of both axisymmetric and rectangular cross sections.

Development and use of localized arc filament plasma actuators for high-speed flow control

The paper discusses recent results on the development of localized arc filament plasma actuators and their use in controlling high-speed and high Reynolds number jet flows. Multiple plasma actuators (up to 8) are controlled using a custom-built 8-channel high-voltage pulsed plasma generator. The plasma generator independently controls pulse repetition rate (0–200 kHz), duty cycle and phase for each individual actuator. Current and voltage measurements demonstrated the power consumption of each actuator to be quite low (20 W at 20% duty cycle). Emission spectroscopy temperature measurements in the pulsed arc filament showed rapid temperature increase over the first 10–20 µs of arc operation, from below 1000 °C to up to about 2000 °C. At longer discharge pulse durations, 20–100 µs, the plasma temperature levels off at approximately 2000 °C.Modelling calculations using an unsteady, quasi-one-dimensional arc filament model showed that rapid localized heating in the arc filament on a microsecond time scale generates strong compression waves. The results of the calculations also suggest that flow forcing is most efficient at low actuator duty cycles, with short heating periods and sufficiently long delays between the pulses to allow for convective cooling of high-temperature filaments. The model predictions are consistent with laser sheet scattering flow visualization results and particle imaging velocimetry measurements. These measurements show large-scale coherent structure formation and considerable mixing enhancement in an ideally expanded Mach 1.3 jet forced by eight repetitively pulsed plasma actuators. The effects of forcing are most significant near the jet preferred mode frequency (ν = 5 kHz). The results also show a substantial reduction in the jet potential core length and a significant increase in the jet Mach number decay rate beyond the end of potential core, especially at low actuator duty cycles.

Magnetohydrodynamic and Electrohydrodynamic Control of Hypersonic Flows of Weakly Ionized Plasmas

The focus of this work is on theoretical analysis of fundamental aspects of high-speed flow control using electric and magnetic fields. The principal challenge is that the relatively cold gas is weakly ionized in electric discharges or by electron beams, with ionization fraction ranging from 10 −8 to 10 −5 . The low ionization fraction means that, although electrons and ions can interact with electromagnetic fields, transfer of momentum and energy to or from the bulk neutral gas can be quite inefficient. Analytical estimates show that, even at the highest values of the electric field that can exist in cathode sheaths of electric discharges, electrohydrodynamic, or ion wind, effects in a single discharge can be of significance only in low-speed core flows or in laminar sublayers of high-speed flows. Use of multi-element discharges would amplify the single-sheath effect, so that the cumulative action on the flow can conceivably be made significant. However, Joule heating can overshadow the cathode sheath ion wind effects. Theoretical analysis of magnetohydrodynamic (MHD) flow control with electron beam ionization of hypersonic flow shows that the MHD interaction parameter is a steeply increasing function of magnetic field strength and the flow velocity. However, constraints imposed by arcing between electrode segments can reduce the performance and make the maximum interaction parameter virtually independent of Mach number. Estimates also show that the MHD interaction parameter is much higher near the wall (in the boundary layer) than in the core flow, which may have implications for MHD boundary layer and transition control. The paper also considers “electrodeless” MHD turning and compression of high-speed flows. Computations of a sample case demonstrate that the turning and compression of hypersonic flow ionized by electron beams can be achieved; however, the effect is relatively modest due to low ionization level.

Development and Characterization of Hartmann Tube Fluidic Actuators for High-Speed Flow Control

An in-depth investigation of the Hartmann tube (HT) was carried out to gain a better understanding of and the effects of various parameters on its output flow characteristics. The HT used in this work consisted of an underexpanded jet directed into a close-ended cylindrical tube of the same diameter. The effects of tube depth, separation distance between the tube and the nozzle, and the jet Mach number were explored. Experiments were also performed on an HT fluidic actuator (HTFA). The HTFA operates under the same principles as the HT, except a major portion of the area between the nozzle and tube is shielded off, so that it can be used as a pulsating injector for flow control. Dynamic pressure measurements in the near field and microphone measurements in the far field provided temporal and spectral data. Instantaneous and phase-averaged images of the flow were obtained to explore the nature of turbulence structures within the flow. Limited hot-wire measurements were performed to characterize the velocity fluctuations at the exit of the HTFA

Development and characterization of Hartmann tube fluidic actuators for high-speed flow control

Development and characterization of Hartmann tube fluidic actuators for high-speed flow control