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ARTICLE IN PRESS Journal of Electrostatics 65 (2007) 655–659 A DC corona discharge on a flat plate to induce air movement Pierre Magniera,Ã, Dunpin Hongb, Annie Leroy-Chesneaua, Jean-Michel Pouvesleb, Jacques Hureaua a b ´canique et d’Energe ´tique, 8, Rue Leonard de Vinci, 45072 Orle ´ans Cedex 2, France Laboratoire de Me ´ ´ans, 14, Rue d’Issoudun, 45072 Orle ´ans Cedex 2, France GREMI, UMR 6606 CNRS/Universite d’Orle Received 3 January 2007; received in revise
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  Journal of Electrostatics 65 (2007) 655–659 A DC corona discharge on a flat plate to induce air movement Pierre Magnier a, à , Dunpin Hong b , Annie Leroy-Chesneau a , Jean-Michel Pouvesle b ,Jacques Hureau a a Laboratoire de Me´ canique et d’Energe´ tique, 8, Rue Leonard de Vinci, 45072 Orle´ ans Cedex 2, France b GREMI, UMR 6606 CNRS/Universite´ d’Orle´ ans, 14, Rue d’Issoudun, 45072 Orle´ ans Cedex 2, France Received 3 January 2007; received in revised form 2 March 2007; accepted 16 April 2007Available online 11 May 2007 Abstract This paper describes a DC surface corona discharge designed to modify the airflow around a flat plate. The electrode configurationconsisted of two thin copper layers placed on each side of the plate’s attack edge. Discharge optical measurements with a photomultipliertube indicated that the light emitted by the plasma is pulsating at a frequency that increases with applied voltage. Moreover, with voltagehigher than a threshold value, the electric discharge changes regime with brighter pulses. This discharge also induced an ‘‘ionic wind’’ whosevelocity was measured with a pressure sensing probe (up to 1m/s). Experiments with the particle image velocimetry system in a subsonic windtunnel showed that this discharge can reduce the separated airflow on the flat plate for a flow of 14m/s (Reynolds number of 187,000). r 2007 Elsevier B.V. All rights reserved. Keywords: Corona discharge; Electroaerodynamics; Ionic wind; Plasma actuator 1. Introduction The use of so-called ionic wind induced by a high voltagedischarge[1]has been studied in electrostatic precipitators[2,3]and, more recently, in aerodynamics[4]. In this latter case, the ionic wind is used to modify the airflow around anobstacle in order to control the airflow in the boundarylayer and reduce drag. For this purpose, different electricdischarges have recently been developed, such as the ‘‘OneAtmosphere Uniform Glow Discharge Plasma’’[5,6]andDC surface corona discharges[7–9]. Labergue et al.[7] used the latter discharge on an inclined wall to detachthe flow; Le ´ger et al.[8]and Artana et al.[9]re-attached the flow on a flat plate. Published results indicate that thedischarge can be used for active control of low-velocityairflow, but the mechanism of interaction between dis-charge and flow has not yet been clarified.In this paper, we present an investigation of a DCcorona discharge and measurements of the induced flow.The electrode configuration consisted of two metallic tapesplaced on each side of a circular leading edge (seeFig. 1).This configuration slightly differs from that in reportedworks[8,9], where both wire electrodes were placed on thesame side of the attack edge. 2. Experimental setup  2.1. Plasma actuator and power supply The plasma actuator consisted of a DC surface coronadischarge established between two electrodes (copper,170mm long, 25mm wide and 35 m m thickness) mountedon both sides of the circular leading edge of a flat plate(polyvinyl chloride (PVC), 213mm  200mm  15mm), asshown inFig. 1. The anode was placed 7.5mm downstreamof the leading edge and connected to a positive high-voltagesource (SPELLMAN SL300, 0–60kV, 5mA); the cathode,placed 37mm from the edge, was connected to ground. A15-M O series resistor prevented the transition to an arc regime.  2.2. Optical setup As the light emitted by the discharge was very weak, ahighly sensitive photomultiplier tube (Hamamatsu R928having a spectral domain from 185 to 900nm) was used to ARTICLE IN PRESS$-see front matter r 2007 Elsevier B.V. All rights reserved.doi:10.1016/j.elstat.2007.04.002 à Corresponding author. E-mail address: (P. Magnier).  detect the plasma fluorescence. A load resistance of 10k O was placed between the anode of the PMT and ground tomeasure the photocurrent. The rise time of the detectionsystem was about 200ns. The PMT signal was measuredwith an oscilloscope. The setup shown inFig. 2used aquartz lens to capture the ultraviolet (UV) light from thedischarge.  2.3. Wind tunnel  Experiments with external airflow were performed in asubsonic wind tunnel (Fig. 3) of 50cm  50cm cross-section in a 2-m long test section (mean turbulence ratio of 0.5%), where a flow with maximum velocity of 50m/s wasgenerated by a 30-kW electric fan. The test flat plate wasplaced between two transparent rotating disks, shown inthe test section inFig. 3, which allow for obtaining thedesired attack angle.  2.4. Particle image velocimetry system Measurements of the flow velocity fields were performedusing the particle image velocimetry (PIV) system shown inFig. 4. A laser beam of wavelength 532nm (Nd:Yag laser,Spectra Physics 400) was transformed into a laser lightsheet using mirrors and lenses. The laser sheet illuminatedsmoke particles (generated by incense sticks) that seededthe flow. Images of illuminated smoke particles werecaptured using a charge coupled device (CCD) cameracalled PIVCAM. The vector displacement of each particlecould be determined using two images recorded for twosuccessive laser pulses having a time delay of 10 m s. Thevelocity fields presented in this paper are the mean vectorfields of 500 pairs of such images recorded over a durationof 50s at repetition rate of 10Hz.  2.5. Pressure sensing probe Velocity measurements were made with a pressuresensing probe consisting of a tube made of glass, showninFig. 5, thus avoiding conductive materials near thedischarge that could cause unwanted arcs. The tubewas connected to a differential low-pressure transducerDruck TM LPM 9481 (0–20Pa, output voltage 0–5V)and measurements were acquired on a PC using a 16-bitacquisition card, over a 2-s interval at a 2-kHz sampl-ing rate. The flow velocity V  was determined fromthe differential pressure values D P  using the Bernoullirelation D P  ¼ 1 = 2 r V  2 , where r is the air density. Thissensor was calibrated with a classical Pitot tube in a ARTICLE IN PRESS Fig. 1. DC surface corona discharge actuator on a flat plate.Fig. 2. Optical setup for measurements using photomultiplier tube.Fig. 3. Schematic side view of the wind tunnel.Fig. 4. Experimental setup of the particle image velocimetry system.Fig. 5. Pressure sensing probe made of glass. P. Magnier et al. / Journal of Electrostatics 65 (2007) 655–659 656  calibration wind tunnel (DANTEC StreamLine 90H02flow unit). 3. Results and discussion 3.1. Discharge characteristics Using the configuration presented inFig. 1, a stabledischarge was obtained using a positive DC high voltage of 44kV as shown by the photography inFig. 6. The room’srelative humidity was 44%. This photo was taken using anintensified CCD camera with a gating time of 1s. The meandischarge current was 0.20mA. The current per unitlength of the electrode perpendicular to the edge was thus1.2mA/m. This value is approximately the same as thatused by other authors[7–10].This apparent homogeneity in fact hides a much morecomplex time structure. Indeed, time resolved measure-ments using the PMT, shown inFig. 7a, reveal that thedischarge is composed of a multitude of streamers at arepetition rate that depends on the working voltage. Noticethat each negative peak of the PMT signal corresponds to alight pulse emitted by a streamer (called a ‘‘micro-discharge’’ in this paper in consideration of the low valueof the corresponding measured intensity) mainly in UVrange (because the light was strongly attenuated by glassnon-transparent to UV). The frequency as a function of working voltage, up to 38kV, is shown inFig. 7b. Despitethe constancy of voltage, the corona discharge can be anintermittent one as shown by PMT measurement. Thisintermittence was discovered in Loeb’s laboratory in 1938[11]. In case of a negative corona, the pulses are calledTrichel pulses and its frequency is much higher than theone of positive corona used in this experiment[12].In our specific setup, the discharge regime changed whenthe working voltage was higher than 38.5kV. Indeed, theappearance of the discharge became much more luminousand visible to the naked eye. The PMT signal, inFig. 8,showed the appearance of some stronger discharges (calledmacro-discharges in this paper). Each macro-dischargecaused a voltage drop as shown by the second curve inFig. 8. 3.2. Induced flow velocity In an initially still atmosphere, the DC surface coronadischarge induces airflow (called ionic wind) due to themovement of positive ions from anode to cathode. Usually,plasma is composed mainly of electron and positive ions. ARTICLE IN PRESS Fig. 7. (a) Time evolution of corona discharges at 35kV and (b) frequency of the corona micro-discharges versus high voltage.Fig. 6. Photography of the discharge established on the flat plate.Fig. 8. Time evolution of corona discharges and high-voltage variations at39kV. P. Magnier et al. / Journal of Electrostatics 65 (2007) 655–659 657  However, due to the electron attachment, plasma createdin air also contains negative ions of oxygen. But the effectof these negative ions is less important than the effect of positive ions because the induced airflow is always fromanode to cathode in the case of a positive working voltage[10]. This weak contribution of negative ions is probablydue to a lower concentration of these ions.The induced flow velocity profiles at two positions weremeasured and results are given inFig. 9for a workingvoltage of 44kV. The maximum of the time-averagedvelocity values, obtained above the cathode, wasabout 1m/s. 3.3. Airflow modification by plasma The flat plate equipped with the DC plasma actuator wasinstalled in the wind tunnel (seeFig. 3). Velocity fieldmeasurements were performed using the PIV system, withand without plasma. Results with streamlines deducedfrom velocity fields are shown inFig. 10. The separationbubble size decreased with the electric discharge (angle of attack of 2.5 1 , inlet airflow velocity in the wind tunnel of 14m/s, a Reynolds number of 187,000;Fig. 10a), and thefully separated flow was re-attached beyond a bubble(5 1 , 8m/s, Re ¼ 107,000;Fig. 10b). This clearly visibleeffect was obtained at rather high Reynolds numberscompared to those previously reported[6](up to 68,600 inthis mentioned work) also with a DC corona discharge ona flat plate. 4. Conclusion A DC surface corona discharge established between twothin copper layer electrodes placed on each side of the plateattack edge is presented in this paper. Although the plasmaappears as a homogeneous layer, measurements with aphotomultiplier tube showed that it is, in fact, composed of a multitude of small, pulsed discharges, whose repetitionfrequency increases with applied high voltage.The positive ion movement in the plasma inducedairflow around the flat plate. Measurements with a pressuresensing probe indicate that the maximum mean velocity of ionic wind in the configuration studied was about 1m/s.This value is quite low compared to the airflow in the windtunnel, but even in these conditions, the presence of theplasma was efficient in reducing the separated flow on the ARTICLE IN PRESS Fig. 9. Ionic wind velocity profiles at different positions (44kV).Fig. 10. Time averaged velocity fields with and without discharge (40kV, 0.8mA/m) (a) for an angle of attack of 2.5 1 , and an inlet flow velocity of 14m/s,(b) for 5 1 and 8m/s. P. Magnier et al. / Journal of Electrostatics 65 (2007) 655–659 658
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