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%.     

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 1.7 μs,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 202.6 μs,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(3141K)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.     

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 frequen...     

Numerical Study of Fluid Dynamics and Heat Transfer Induced by Plasma Discharges

A numerical investigation is conducted to explore the evolution of a plasma discharge and its interaction with the fluid flow based on a self-consistent fluid model which couples the discharge dynamics with the fluid dynamics.The effects of the applied voltage on the distribution of velocity and temperature in initially static air are parainetrically studied.Furthermore,the spatial structure of plasma discharge and the resulting force contours in streamwise and normal directions are discussed in detail.The result shows that the plasma actuator produces a net force that should always be directed away from the exposed electrode,which results in an ionic wind pushing particles into a jet downstream of the actuator.When the energy added by the plasma is taken into account,the ambient air temperature is increased slightly around the electrode,but the velocity is almost not affected.Therefore it is unlikely that the induced flow is buoyancy driven.For the operating voltages considered in this paper,the maximum induced velocity is found to follow a power law,i.e.,it is proportional to the applied voltage to the 3.5 power.This promises an efficient application in the flow control with plasma actuators.     

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 to110 m/s,which is believed to be effective for high-speed flow control.     

Dynamics of plasma bullets by nanosecond pulsed micro-hollow cathode discharge within air

In this paper, the air plasma jet produced by micro-hollow cathode discharge(MHCD) is investigated. The discharge is powered by a positive nanosecond pulse high voltage supply. The waveforms of the discharge, the images of the jet, the evolution of the plasma bullet and the reactive species are obtained to analyze the characteristics of the MHCD plasma jet. It is found that the length of the plasma jet is almost proportional to the air flow rate of 2–6 slm. Two plasma bullets appear one after another during a single period of the voltage waveform, and both of the two plasma bullets are formed during the positive pulse voltage off. The propagation velocity of the two plasma bullets is on the order of several hundred m/s, which is approximate to that of the air flow. These results indicate that the gas flow has an important influence on the formation of this MHCD plasma jet.     

Analysis on discharge process of a plasma-jet trigered gas spark switch

The plasma-jet triggered gas switch (PJTGS) could operate at a low working coefficient with a low jitter. We observed and analyzed the discharge process of the PJTGS at the lowest working coefficient of 47% with the trigger voltage of 40 kV and the pulse energy of 2 J to evaluate the effect of the plasma jet. The temporal and spatial evolution and the optical emission spectrum of the plasma jet were captured. And the spraying delay time and outlet velocity under different gas pressures were investigated. In addition, the particle in cell with Monte Carlo collision was employed to obtain the particle distribution of the plasma jet varying with time. The results show that, the plasma jet generated by spark discharge is sprayed into a spark gap within tens of nanoseconds, and its outlet velocity could reach 104ms−1. The plasma jet plays a non-penetrating inducing role in the triggered discharge process of the PJTGS. On the one hand, the plasma jet provides the initial electrons needed by the discharge; on the other hand, a large number of electrons focusing on the head of the plasma jet distort the electric field between the head of the plasma jet and the opposite electrode. Therefore, a fast discharge originated from the plasma jet is induced and quickly bridges two electrodes.     

Study of generation characteristics of glow-type atmospheric-pressure plasma jet based on DC discharge in air

In order to form an atmospheric-pressure plasma jet without airflow, a needle–ring electrode structure is proposed in this paper. When heteropolar potentials are applied to a needle and a ring, a marked electric field strength enhancement around the needle's pointed end has been found. When the same potential is applied to both the needle and the ring, the lateral electric field strength for the needle can be weakened. By using the above two methods, an increase of the difference between the pointed end electric field strength and the lateral one is achieved and stable plasma jets are formed. A symmetrical space electric field distribution is established at the pointed end of the needles when several sets of heteropolar needle–ring electrodes are uniformly arranged, which is conducive to forming a uniform array plasma jet. Under DC discharge conditions, a safe and stable plasma jet of high density and an array plasma jet are successfully achieved.     

Electrical and optical characteristics of atmospheric helium jet array plasma

In this paper, a honeycomb structure jet array with seven jet units was adopted to generate plasmas. Both the average discharge power and the emission intensity of the main excited species increase with increasing applied voltage. There are three stages of discharge evolution at different applied voltages: initial discharge, uniform discharge and strong coupling discharge.The spatial distribution of the emission intensity of the excited species can be divided into three categories: growth class, weakening class and variation class. The gas temperature along the whole plasma plume at different applied voltages is maintained at around 320K and can be widely used in heat-labile applications.     

Electrical and aerodynamic characteristics of sliding discharge based on a microsecond pulsed plasma supply

Plasma flow control technology has broad prospects for application. Compared with conventional dielectric barrier discharge plasma actuators (DBD-PA), the sliding discharge plasma actuator (SD-PA) has the advantages of a large discharge area and a deflectable induced jet. To achieve the basic performance requirements of light weight, low cost, and high reliability required for UAV (Unmanned Aerial Vehicle) plasma flight experiments, this work designed a microsecond pulse plasma supply that can be used for sliding discharge plasma actuators. In this study, the topology of the primary circuit of the microsecond pulse supply is determined, the waveform of the output terminal of the microsecond pulse plasma supply is detected using the Simulink simulation platform, and the design of the actuation voltage, the pulse frequency modulation function and the construction of the hardware circuit are achieved. Using electrical diagnosis and flow field analysis, the actuation characteristics and flow characteristics of sliding discharge plasma under microsecond pulse actuation are studied, the optimal electrical actuation parameters and flow field characteristics are described.     

Design,Construction and Characterization of AC Atmospheric Pressure Air Non-thermal Plasma Jet

This paper presents the design and construction of non-thermal plasma jet device which was built in plasma phys. Dept., NRC, AEA, Egypt with a plasma application group. This design will be useful to initiate research in different fields such as low temperature plasma, polymer and biomedical applications. The experimental operation of this device is conducted with power supply of (10 kV, 30 mA, and 20 kHz). The discharge process takes place by using Air as input gas with different flow rates. The experimental results showed that the maximum plasma jet length of 7 mm is detected at air flow rate of 12 L/min. The electrical characteristics of discharge at different flow rates of Air such as discharge voltage, current, mean power, power efficiency, and energy have been studied by using potential dividers and Lissajous figure techniques. The results of plasma jet temperature along the jet length showed that the jet plasma has approximately a room temperature at the end of jet column.     

A saw-tooth plasma actuator for film cooling efficiency enhancement of a shaped hole

This paper reports the large eddy simulations of the effects of a saw-tooth plasma actuator and the laidback fan-shaped hole on the film cooling flow characteristics,and the numerical results are compared with a corresponding standard configuration (cylindrical hole without the sawtooth plasma actuator).For this numerical research,the saw-tooth plasma actuator is installed just downstream of the cooling hole and a phenomenological plasma model is employed to provide the 3D plasma force vectors.The results show that thanks to the downward force and the momentum injection effect of the saw-tooth plasma actuator,the cold jet comes closer to the wall surface and extends further downstream.The saw-tooth plasma actuator also induces a new pair of vortex which weakens the strength of the counter-rotating vortex pair (CRVP) and entrains the coolant towards the wall,and thus the diffusion of the cold jet in the crossflow is suppressed.Furthermore,the laidback fan-shaped hole reduces the vertical jet velocity causing the disappearance of downstream spiral separation node vortices,this compensates for the deficiency of the saw-tooth plasma actuator.Both effects of the laidback fan-shaped hole and the saw-tooth plasma actuator effectively control the development of the CRVP whose size and strength are smaller than those of the anti-counter rotating vortex pair in the far field,thus the centerline and the spanwise-averaged film cooling efficiency are enhanced.The average film cooling efficiency is the biggest in the Fan-Dc =1 case,which is 80％ bigger than that in the Fan-Dc =0 case and 288％ bigger than that in the Cyl-Dc =0 case.     

Yield of Ozone,Nitrite Nitrogen and Hydrogen Peroxide Versus Discharge Parameter Using APPJ Under Water

Discharge plasma in and in contact with water can be accompanied with ultraviolet radiation and electron impact,thus can generate hydroxyl radicals,ozone,nitrite nitrogen and hydrogen peroxide.In this paper,a non-equilibrium plasma processing system was established by means of an atmospheric pressure plasma jet immersed in water.The hydroxyl intensities and discharge energy waveforms were tested.The results show that the positive and negative discharge energy peaks were asymmetric,where the positive discharge energy peak was greater than the negative one.Meanwhile,the yield of ozone and nitrite nitrogen was enhanced with the increase of both the treatment time and the discharge energy.Moreover,the p H value of treated water was reduced rapidly and maintained at a lower level.The residual concentration of hydrogen peroxide in APPJ treated water was kept at a low level.Additionally,both the efficiency energy ratio of the yield of ozone and nitrite nitrogen and that of the removal of p-nitrophenol increased as a function of discharge energy and discharge voltage.The experimental results were fully analyzed and the chemical reaction equations and the physical processes of discharges in water were given.     

Measurement ofHe2* density with an auxiliary measuring electrode in atmospheric pressure plasma jet

In this study, the density of metastable He₂* in an atmospheric-pressure plasma jet operating in helium with 0.001% nitrogen has been measured using an auxiliary measuring electrode technique. In the glow discharge mode, waveforms from two grounding electrodes, including one main discharge electrode and one auxiliary electrode, are captured. The isolated current peak formed by Penning ionization in waveforms from the auxiliary measuring electrode is identified to calculate the density of metastable He₂*. In our discharge environment, the helium metastable densities along the jet axis direction are between 2.26× 10¹³ and 1.74× 10¹³ cm^-3, which is in good agreement with the results measured by other techniques. This measurement technique can be conveniently applied to the diagnosis of metastableHe₂* in an atmospheric-pressure plasma jet array.     

Experimental investigation of lift enhancement for flying wing aircraft using nanosecond DBD plasma actuators

The effects of the arrangement position and control parameters of nanosecond dielectric barrier discharge(NS-DBD)plasma actuators on lift enhancement for flying wing aircraft were investigated through wind tunnel experiments at a flow speed of 25 m s~(-1).The aerodynamic forces and moments were obtained by a six-component balance at angles of attack ranging from-4°to 28°.The lift,drag and pitching moment coefficients were compared for the cases with and without plasma control.The results revealed that the maximum control effect was achieved by placing the actuator at the leading edge of the inner and middle wing,for which the maximum lift coefficient increased by 37.8%and the stall angle of attack was postponed by 8°compared with the plasma-off case.The effects of modulation frequency and discharge voltage were also investigated.The results revealed that the lift enhancement effect of the NS-DBD plasma actuators was strongly influenced by the modulation frequency.Significant control effects were obtained at/=70 Hz,corresponding to F~+≈1.The result for the pitching moment coefficient demonstrated that the plasma actuator can induce the reattachment of the separation flows when it is actuated.However,the results indicated that the discharge voltage had a negligible influence on the lift enhancement effect.     

Evaluation and Optimization of Electrode Configuration of Multi-Channel Corona Discharge Plasma for Dye-Containing Wastewater Treatment

A discharge plasma reactor with a point-to-plane structure was widely studied experimentally in wastewater treatment.In order to improve the utilization efficiency of active species and the energy efficiency of this kind of discharge plasma reactor during wastewater treatment,the electrode configuration of the point-to-plane corona discharge reactor was studied by evaluating the effects of discharge spacing and adjacent point distance on discharge power and discharge energy density,and then dye-containing wastewater decoloration experiments were conducted on the basis of the optimum electrode configuration.The experimental results of the discharge characteristics showed that high discharge power and discharge energy density were achieved when the ratio of discharge spacing to adjacent point distance(d/s) was 0.5.Reactive Brilliant Blue(RBB) wastewater treatment experiments presented that the highest RBB decoloration efficiency was observed at d/s of 0.5,which was consistent with the result obtained in the discharge characteristics experiments.In addition,the biodegradability of RBB wastewater was enhanced greatly after discharge plasma treatment under the optimum electrode configuration.RBB degradation processes were analyzed by GC-MS and IC,and the possible mechanism for RBB decoloration was also discussed.     

Numerical study of the influence of dielectric tube on propagation of atmospheric pressure plasma jet based on coplanar dielectric barrier discharge

A 2D fluid model was employed to simulate the influence of dielectric on the propagation of atmospheric pressure helium plasma jet based on coplanar dielectric barrier discharge (DBD). The spatio-temporal distributions of electron density, ionization rate, electrical field, spatial charge and the spatial structure were obtained for different dielectric tubes that limit the helium flow. The results show that the change of the relative permittivity of the dielectric tube where the plasma jet travels inside has no influence on the formation of DBD itself, but has great impact on the jet propagation. The velocity of the plasma jet changes drastically when the jet passes from a tube of higher permittivity to one of lower permittivity, resulting in an increase in jet length,ionization rate and electric field, as well as a change in the distribution of space charges and discharge states. The radius of the dielectric tube has a great influence on the ring-shaped or solid bullet structure. These results can well explain the behavior of the plasma jet from the dielectric tube into the ambient air and the hollow bullet in experiments.     

Numerical and Experimental Investigation on the Attenuation of Electromagnetic Waves in Unmagnetized Plasmas Using Inductively Coupled Plasma Actuator

The attenuation of electromagnetic(EM) waves in unmagnetized plasma generated by an inductively coupled plasma(ICP) actuator has been investigated both theoretically and experimentally. A numerical study is conducted to investigate the propagation of EM waves in multilayer plasma structures which cover a square flat plate. Experimentally, an ICP actuator with dimensions of 20 cm×20 cm×4 cm is designed to produce a steady plasma slab. The attenuation of EM waves in the plasma generated by the ICP actuator is measured by a reflectivity arch test method at incident waves of 2.3 GHz and 10.1 GHz, respectively. A contrastive analysis of calculated and measured results of these incident wave frequencies is presented, which suggests that the experiment accords well with our theory. As expected, the plasma slab generated by the ICP actuator can effectively attenuate the EM waves, which may have great potential application prospects in aircraft stealth.     

Atmospheric Pressure Plasma Jet in Ar and O2/Ar Mixtures： Properties and High Performance for Surface Cleaning＊

An atmospheric pressure plasma jet generated in Ar and O2/Ar mixtures has been investigated by specially designed equipment with double power electrodes at 20~32 kHz, and their effects on the cleaning of surfaces have been studied. Properties of the jet discharge are studied by electrical diagnostics, including the waveform of discharge voltage, discharge current and the Q-V Lissajous figures. The optical emission spectroscopy is used to measure the plasma parameters, such as the excitation temperature and the gas temperature. It is found that the consumed power and the excitation temperature increase with increase of the discharge frequency. On the other hand, at the same discharge frequency, these parameters in O2/Ar mixture plasma are found to be much larger. The effect on surface cleaning is studied from the changes in the contact angle. For Ar plasma jet, the contact angle decreases with increase of the discharge frequency. For O2/Ar mixture plasma jet, the contact angle decreases with increase of discharge frequency up to 26 kHz, however, further increase of discharge frequency does not show further decrease in the contact angle. At the same discharge frequency, the contact angle after O2/Ar mixture plasma cleaning is found to be much lower compared to the case of pure Ar. From the results of quadrupole mass-spectrum analysis, we can identify more fragment molecules of CO and H2O in the emitted gases after O2/Ar plasma jet treatment compared with Ar plasma jet treatment, which are produced by the decomposition of surface organic contaminants during the cleaning process.     

Study on an Atmospheric Pressure Plasma Jet and its Application in Etching Photo-Resistant Materials

An atmospheric pressure radio-frequency plasma jet that can eject cold plasma has been developed. In this paper, the configuration of this type of plasma jet is illustrated and its discharge characteristics curves are studied with a current and a voltage probe. A thermal couple is used to measure the temperature distribution along the axis of the jet stream. The temperature distribution curve is generated for the He/O2 jet stream at the discharge power of 150 W. This jet can etch the photo-resistant material at an average rate of 100 nm/min on the surface of silicon wafers at a right angle.