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.

Influence of high-voltage pulse parameters on the propagation of a plasma synthetic jet

In this work, a typical pin-to-pin plasma synthetic jet in static air is excited by a pulsed DC power supply. The influences of the pulse rising time, the amplitude and the repetition frequency of the pulse voltage on the jet flow have been investigated. First, using a high-speed Schlieren imaging technique, the induced shock waves and the fast jet flow generated by the plasma synthetic jet are characterized. With a deposited energy of 44 m J per pulse, the velocity of the shock wave and the maximum velocity of the jet flow reach 320 m s-1 and 100 m s-1,respectively. Second, when the applied voltage increases from 12.8 kV to 16 kV, the maximum jet velocity increases from 66 m s-1 to 93 m s-1. On the other hand, as the pulse rising time varies from 50 ns to 500 ns, or the pulse repetition frequency increases from 5 Hz to 40 Hz, the jet velocity induced by the plasma synthetic jet is weakly dependent. In addition, a comparative study of the plasma synthetic jets using three commercial pulsed power supplies(XJ-15, NPG-18, and PG-30) is implemented. It reveals that the maximum jet velocity of 120 m s-1 is obtained in the case of PG-30, with the longest pulse rising time and the lowest breakdown voltage, while the maximum velocity of 33 m s-1 is detected in the case of NPG-18, even though it has the shortest pulse rising time and the highest breakdown voltage.

Parametric study of high-frequency characteristics of plasma synthetic jet actuator

A major issue of plasma synthetic jet actuator(PSJA)is the severe performance deterioration at high working frequency.In this study,experiments and numerical simulation are combined together to investigate the influence of thermal conductivity,throat length(Lth)and discharge duration(Td)on the high-frequency characteristics of PSJA.Results show that the variation of the actuator thermal conductivity and discharge duration will not alter the saturation frequency of the actuator,whereas decreasing the throat length results in an increase of the saturation frequency.For a short-duration capacitive discharge of 1.7μs,a clear shock wave is issued from the orifice,followed by a weak jet.As a comparison,when the discharge duration is increased up to 202.6μs,a strong jet column is formed and no obvious shock wave can be visualized.Based on numerical simulation results,it becomes clear that the long-duration pulse-DC discharge is able to heat the cavity gas to a much higher temperature(3141 K)than capacitive discharge,greatly improving the conversion efficiency of the arc discharge energy to the internal energy of the cavity gas.In addition,high-speed Schlieren imaging is deployed to study the performance degradation mechanism of PSJA at high working frequency.Monitor of the exit jet grayscale indicates that as long as the saturation frequency is exceeded,the actuator becomes unstable due to insufficient refresh time.The higher the discharge frequency,the more frequently the phenomenon of‘misfires’will occur,which explains well the decaying jet total pressure at above saturation frequency.

Control of flow separation over a wing model with plasma synthetic jets

An array of 30 plasma synthetic jet actuators(PSJAs)is deployed using a modified multichannel discharge circuit to suppress the flow separation over a straight-wing model.The lift and drag of the wing model are measured by a force balance,and the velocity fields over the suction surface are captured by a particle imaging velocimetry system.Results show that the flow separation of the straight wing originates from the middle of the model and expands towards the wingtips as the angle of attack increases.The flow separation can be suppressed effectively by the PSJAs array.The best flow control effect is achieved at a dimensionless discharge frequency of F^+=1,with the peak lift coefficient increased by 10.5%and the stall angle postponed by 2°.To further optimize the power consumption of the PSJAs,the influence of the density of PSJAs on the flow control effect is investigated.A threshold of the density exits(with the spanwise spacing of PSJAs being 0.2 times of the chord length in the current research),below which the flow control effect starts to deteriorate remarkably.In addition,for comparison purposes,a dielectric barrier discharge(DBD)plasma actuator is installed at the same location of the PSJAs.At the same power consumption,4.9%increase of the peak lift coefficient is achieved by DBD,while that achieved by PSJAs reaches 5.6%.

Influence of air pressure on the performance of plasma synthetic jet actuator

Plasma synthetic jet actuator（PSJA） has a wide application prospect in the high-speed flow control field for its high jet velocity.In this paper,the influence of the air pressure on the performance of a two-electrode PSJA is investigated by the schlieren method in a large range from 7 k Pa to 100 k Pa.The energy consumed by the PSJA is roughly the same for all the pressure levels.Traces of the precursor shock wave velocity and the jet front velocity vary a lot for different pressures.The precursor shock wave velocity first decreases gradually and then remains at 345 m/s as the air pressure increases.The peak jet front velocity always appears at the first appearance of a jet,and it decreases gradually with the increase of the air pressure.A maximum precursor shock wave velocity of 520 m/s and a maximum jet front velocity of 440 m/s are observed at the pressure of 7 k Pa.The averaged jet velocity in one period ranges from 44 m/s to 54 m/s for all air pressures,and it drops with the rising of the air pressure.High velocities of the precursor shock wave and the jet front indicate that this type of PSJA can still be used to influence the high-speed flow field at 7 k Pa.

Experimental and numerical investigation of a self-supplementing dual-cavity plasma synthetic jet actuator

The primary issue regarding the plasma synthetic jet actuator(PSJA)is its performance attenuation at high frequencies.To solve this issue,a self-supplementing,dual-cavity,plasma synthetic jet actuator(SD-PSJA)is designed,and the static properties of the SD-PSJA are investigated through experiments and numerical simulations.The pressure measurement shows that the SD-PSJA has two saturation frequencies(1200 Hz and 2100 Hz),and the experimental results show that both the saturation frequencies decrease as the volume of the bottom cavity of the SD-PSJA increases.As the size of the supplement hole increases,the first saturation frequency increases continuously,while the second saturation frequency shows a trend of first decreasing and then increasing.Numerical simulations show that the working process of the SD-PSJA is similar to that of the PSJA,but the volume of the cavity in the SD-PSJA is smaller than that of the PSJA;the SD-PSJA can supplement air to the top cavity through two holes,thus reducing the refresh time and effectively improving the jet intensity of the actuator at high frequencies.

等离子体合成射流改善S弯进气道气动性能的实验研究

为了对狭缝式S弯进气道中的流动分离进行有效控制,本文基于等离子体合成射流开展了激励特性和相关流动控制实验研究。结果表明:在激励器能量沉积比确定的条件下,等离子体合成射流激励器的放电能量与激励频率无关,然而不同能量沉积比都存在一个最佳激励频率,使得射流速度达到最大。在激励特性诊断的基础上通过风洞实验验证了等离子体合成射流改善狭缝式S弯进气道气动性能的有效性,最佳激励效果为在出口速度为Ma=0.25时使得总压畸变指数降低22.59%。

Experimental study on shock interaction control of double wedge in high-enthalpy hypersonic flow subject to plasma synthetic jet

The hypersonic shock-shock interaction flow field at double-wedge geometries controlled by plasma synthetic jet actuator is experimentally studied in a Ma = 8 high-enthalpy shock tunnel with the purpose of exploring a novel technique for reducing surface heat flux in a real flight environment. The results demonstrate that increasing the discharge energy is advantageous in eliminating the shock wave, shifting the shock wave interaction point, and shortening the control response time. The oblique shock wave can be completely removed when the actuator's discharge energy grows from 0.4 J to 11.5 J, and the displacement of the shock wave interaction point increases by 124.56%, while the controlled response time is shortened by 30 μs. Besides, the reduction in diameter of the jet exit is firstly proved to have a negative impact on energy deposition in a working environment with incoming flow, which reduces the discharge energy and hence decreases the control effect. The shock wave control response time lengthens when the jet exits away from the second wedge. Along with comparing the change in wall heat flux at the second wedge over time, the control effect of plasma synthetic jet actuator with and without inflation is also analyzed. When plasma synthetic jet works in inflatable mode, both the ability to eliminate shock waves and the shifting effect of the shock wave interaction point are increased significantly, and the wall heat flux is also reduced.

Large eddy simulation study on drag reduction performance of array-based plasma synthetic jet actuators

Large Eddy Simulation(LES)is first used to investigate the drag reduction effect of an array-based configuration of Plasma Synthetic Jet Actuators(PSJAs)on a hemisphere in supersonic inflow,and analyze the effect of energy allocation and array angle on the drag reduction performance of opposing Plasma Synthetic Jet(PSJ)in this paper.Numerical simulation results have been compared with experimental data,confirming the validity of the simulation method.The results show that different energy allocations have a significant effect on the drag of the hemisphere.However,the effect of the change in array angle on the drag of the hemisphere is not as noticeable as the effect caused by energy allocation.Interference regions between the two PSJAs occur,which undermine the effectiveness of drag reduction.High Turbulent Kinetic Energy(TKE)regions primarily concentrate on the core region of the jet and downstream of the bow shock.The influence of the array angle on TKE is most evident in the downstream region of the exits of the PSJs on both sides.Temporal evolution of the coherent structures reveals that as the PSJ intensity decreases,the largescale vortices progressively break up into smaller-scale vortices,and energy is also transferred from large-scale structures to small-scale structures.

两电极/三电极等离子体合成射流激励器工作特性对比研究

为了实现等离子体合成射流激励器性能设计优化,采用放电测量和高速阴影技术对两电极和三电极等离子体合成射流激励器工作特性进行了实验研究,开展了不同环境压强下两种激励器放电特性、流场特性和能量沉积特性的对比分析,获得了两种激励器工作特性差异的物理机制。研究结果表明,两电极和三电极等离子体合成射流激励器具有相似的放电电压-电流变化过程,相同的击穿电压和峰值电流随环境压强的变化趋势,相近的射流建立和流场响应速度。但在相同电容能量、腔体尺寸和环境压强下,两电极激励器产生的射流和前驱激波速度峰值分别为300 m/s和350 m/s,而三电极激励器则高达480 m/s和520 m/s,并且三电极激励器具有更小的放电电磁干扰。三电极激励器更强的射流形成能力是由于其可以以较小的击穿电压,配合大放电电容和大电极间距,将电源电容到激励器电弧的能量转化效率从两电极激励器的20%提升至90%,实现对激励器腔体更充分的加热。

基于等离子体合成射流的高超声速飞行器标模激波控制实验

基于等离子体合成射流(plasma synthetic jet,PSJ)的新型主动流动控制技术由于具有无需气源、控制力强及激励频带宽等优势,在激波控制领域极具应用潜力。在高超声速风洞中,通过实验研究了单脉冲PSJ对高超声速飞行器标模头部弓形激波及侧翼激波的控制效果及其对飞行器的减阻作用。结果表明,逆向PSJ可使高超声速飞行器标模头部弓形激波脱体距离显著增大,横向PSJ可使侧翼激波基本消除。动态力传感器测得飞行器最大瞬时减阻率约为15.5%,但传感器测得的阻力变化存在大约250μs的延迟。还研究了放电能量、来流总压、出口直径和腔体体积对头部弓形激波控制效果的影响。

Experimental and numerical investigation on opposing plasma synthetic jet for drag reduction

Experimental and numerical studies are carried out to validate the potential of opposing Plasma Synthetic Jet(PSJ)for drag reduction for a hemisphere.Firstly,flow field changes of opposing PSJ are analyzed by comparing the experimental schlieren images and simulation results in a supersonic free stream of Mach number 3.As PSJ is a kind of unsteady pulsed jet,the shock standoff distance increases initially and then decreases under the control of PSJ,which corresponds to the change of the strength of PSJ.Accordingly,the amount of drag reduction of the hemisphere increases initially and then decreases.It is found that there is a short period of“drag rise”during the formation of PSJ before the drag reduction,which is induced by the generation of normal shock waves and the area difference of the cavity wall of PSJ Actuator(PSJA).Secondly,the effects of five parameters,including exit diameter,discharge energy of PSJA,Mach number,static pressure of incoming flow and angle of attack,on drag reduction of opposing PSJ were studied in detail by using numerical method.It is found that the Maximum Pressure Ratio(MPR)has a significant impact on the average drag reduction for a configuration-determined PSJA.For the configuration selected in this study,the flow field of opposing PSJ shows typical Short Penetration Mode(SPM)in a control cycle of PSJ when the MPR is less than 0.89.However,the flow field shows typical Long Penetration Mode(LPM)at some time when the MPR is bigger than 0.89.Relatively better drag reduction is achieved in this case.

Experimental characteristics of a two-electrode plasma synthetic jet actuator array in serial

Plasma Synthetic Jet(PSJ) actuators have shown wide and promising application prospects in high-speed flow control, due to their advantages including high exhaust speed, wide frequency band, rapid response, and non-moving components. Although previous studies on PSJ actuators are abundant, most of them have focused on the performance of a single actuator. However, in practice, an actuator array is very necessary for large-scale aerodynamic actuation on account of the small affected area of a single PSJ. In this paper, the characteristics of a twoelectrode plasma synthetic jet actuator array in serial are investigated experimentally. Compared to a parallel actuator array, the serial actuator array requires simpler power supply design and is much easier to realize. High-speed photography of the discharge evolution, voltage-current measurement, and shadowgraphy visualization are used in the investigation. Experimental results show that, for the serial actuator array, weak discharges happen firstly between energized and suspending electrodes, and then a strong pulse arc discharge is triggered. The breakdown voltage in serial is irrelevant to such factors as the number of actuators, the maximum or minimum gap in serial,the connection sequence, etc. It is mainly determined by the sum of gaps. For serial actuators with the same anode-to-cathode spacing, the energy deposition is the same, and the jet is synchronous and similar. Because of the entrainment and merging of adjacent jet vortices, the jet front speed of an aligned synchronous jet array increases as the orifice distance decreases. To achieve the highest jet front velocity, the orifice of the actuator has an optimal diameter.

Effect of three-electrode plasma synthetic jet actuator on shock wave control

A three-electrode high-energy plasma synthetic jet(PSJ) actuator was used for shock wave control. This actuator is an enhanced version of the two-electrode actuator as a high-voltage trigger electrode is added to increase the cavity volume and the input energy while retaining a relatively low disruptive voltage. The electrical properties were studied using current-voltage measurements, and the energy consumption was calculated. To assess the jet strength, the penetration of PSJ was compared with empirical values, and the results show that the momentum flux ratio of PSJ for a capacitance of 0.96, 1.6, and 3 μF was approximately equal to 0.6, 1.0, and 1.3, respectively. The interaction of PSJ with shock waves was acquired using high-speed shadowgraph imaging. The shock was generated by a 25° compression ramp in Mach 2 flow, and PSJ actuator was placed up-stream of the compression ramp. Under the action of PSJ, the strength of the shock was notably weakened, and the near-wall part of the shock was entirely eliminated. The results show the good control effect of the three-electrode high-energy PSJ in high-speed flow.

Aerodynamic control of NACA 0021 airfoil model with spark discharge plasma synthetic jets

Spark discharge plasma synthetic jets(SPJs) have been used for the active flow control study on an NACA 0021 straight-wing model in a wind tunnel. The model forces and moments were measured using a six-component sting balance at a 20 m/s wind speed. The aim was to explore the SPJ's effect on airfoil aerodynamic by examining SPJ generators' position along the chordwise and the jet flow direction about the chord. Near the wing leading edge, two SPJ generators raised the stall angle by 2° and increased the maximum lift coefficient by 9%. The drag coefficient was decreased by 33.1%, and the lift-drag ratio was increased by 104.2% at an angle of attack above 16°. The rolling-moment coefficient was modified by 0.002, and the yawing-moment coefficient was changed by 0.0007 at angles of attack in the range of 0°–16°. The results showed that SPJs can control wing aerodynamic forces at a high angle of attack and moments at a low angle of attack.

Effect of pressure on the performance of plasma synthetic jet actuator

The effects of the ambient air pressure level on the performance of plasma synthetic jet actuator have been investigated through electrical and optical diagnostics.Pressures from 1 atm down to 0.1 atm were tested with a 10 Hz excitation.The discharge measurement demonstrates that there is a voltage range to make the actuator work reliably.Higher pressure level needs a higher breakdown voltage,and a higher discharge current and energy deposition are produced.But when the actuator works with the maximum breakdown voltage,the fraction of the initial capacitor energy delivered to the arc is almost invariable.This preliminary study also confirms the effectiveness of the plasma synthetic jet at low pressure.Indeed,the maximum velocities of the precursor shock and the plasma jet induced by the actuator with maximum breakdown voltage are independent of the ambient pressure level;reach about 530 and 460 m/s respectively.The mass flux of the plasma jet increases with ambient pressure increasing,but the strength of the precursor shock presents a local maximum at 0.6 atm.

Suppressing unsteady motion of shock wave by high-frequency plasma synthetic jet

Three Plasma Synthetic Jet Actuators(PSJA)under the high-frequency actuation are used to control the Shock Wave Boundary Layer Interaction(SWBLI),a high-speed schlieren image processing method based on spatial Fourier transform as well as snapshot proper orthogonal decomposition were used to study the control effect of high-frequency plasma synthetic jet on lowfrequency unsteadiness of SWBLI.The analysis of the base flow shows that the separated shock wave actually has both large-and small-amplitude vibrations at low frequency.And the results revealed that the PSJA with an operating frequency of 2 k Hz has the ability to reduce the energy of low-frequency component of shock wave motion,indicating that the 2 k Hz actuation can effectively suppress low-frequency unsteadiness of the separated wave.Compared with the actuation frequency of 2 k Hz,the energy of low-frequency component of the shock wave is enhanced under the8 k Hz actuation,which aggravates the low-frequency unsteady motion of the shock wave.It is likely that the actuation frequency is too high,thus the intensity of the precursor shock wave induced by PSJA becomes weaker.Additionally,as the 4 k Hz actuation is applied,the pulsation of the separation region was enhanced,it is speculated that the actuation frequency is coupled with the oscillation frequency of the separation region.

Active flow control of S-duct by plasma synthetic jet

Flow separation and secondary flow in the S-duct of an aircraft engine cause severe pressure loss and airflow distortion at the outlet,lowering engine performance.Herein,a serial two-electrode plasma synthetic jet(PSJ)actuator array is used to actively control the flow field in the duct and improve its characteristics.The results show that the PSJ significantly increases the wall pressure recovery coefficient,suppresses flow separation,and improves the outlet pressure distortion.The primary and secondary orders of the influencing factors are as follows:control position>jet momentum coefficient>excitation frequency>jet configuration.The best jet control position is near the separation location,and the best jet configuration is the‘Λ’configuration.The higher the jet momentum coefficient and excitation frequency,the better the flow control.The wall pressure coefficient increases by up to 127.8%,and the outlet steady pressure distortion index decreases by 9.15%.The control mechanism is the direct energy injection into the flow boundary layer through a high-speed jet and the indirect control effect of the induced streamwise vortex.On the one hand,the PSJ suppresses flow separation by improving the ability of the boundary layer to resist the inverse pressure gradient.On the other hand,it reduces pressure distortion by decreasing the intensity of the secondary flow and weakening the backflow.This study thus provides a new technology for the active control of the flow-field characteristics in an S-duct and has significance for guiding the application of synthetic jet technology in S-ducts.

The effect of actuation frequency on the plasma synthetic jet

The flow induced by plasma synthetic jet actuator was simulated through solving the Reynolds-averaged Navier-Stokes equations augmented by body force phenomenological plasma model.The effect of actuation frequency on the plasma synthetic jet was examined by case study.The numerical results present that with the actuation frequency increasing,the stream-wise distance of the adjacent vortex pairs induced by the actuator decreases monotonically,which is the same as the situation of the velocity fluctuations field caused by the vortex pairs.When the actuation frequency is 60 Hz,the vortex pairs formed during the adjacent actuation periods merge together quickly,and the flow structure in the downstream region is more close to that of the steady case.The actuation frequency has no visible influence on the time-averaged flow field of plasma synthetic jet.However,when the actuation frequency is relatively low(f<40 Hz),the momentum flux close to the actuator increases with the actuation frequency increasing,which is contrary to the situation in the far field from the wall.

等离子体流动控制技术原理及典型应用

等离子体流动控制激励器由于其具有响应时间短、激励频带宽、器件小、能耗低的技术优势,是流动控制领域最活跃的控制方式之一。等离子体激励器在飞行器的增升减阻、性能增强、力矩控制等方面具有重要的应用前景。文中通过实验和数值模拟方法研究了介质阻挡放电等离子体激励器(DBD)和等离子体合成射流激励器(PSJ)的基本气动特性。AC-DBD激励器主要产生射流扰动,NS-DBD等离子体激励器产生的扰动包括压力波、射流和旋涡结构,PSJ激励器能形成压力波和速度更高的射流扰动。采用AC-DBD激励器可以在低速流动下,推迟平板转捩;采用NS-DBD激励器可以在来流速度为25 m/s时,抑制翼型的流动分离,同时NSDBD在Ma=4.6时,可以控制圆柱脱体激波的激波强度。