Fig. 1. TED actuator operating in different mode: DBD mode (a), SD mode (b) and DBDE mode (c).

I. INTRODUCTION

Flow control is a topic of crucial research for aircraft manufacturers, both economically and environmentally. Among the many industrial problems related to aircraft, control of airflow above an airfoil is primordial because it conditions its aerodynamic performance. To achieve this, the aeronautic industry is considering to use active control systems enabling a control in real time, with a short response time. Different systems are being studied, among which we find, for example: the fluid type actuators (synthetic jet, etc.), the M.E.M.S. or the plasma based devices [1-3]. Recently, a new type of plasma-based device (or plasma actuator) has been developed and studied [4, 5]. This new actuator based on three-electrode, also called three-electrode discharge (TED), is supplied by an AC high voltage plus a DC component. On one hand, when the DC component is switched off, the TED actuator can operate as a conventional surface DBD actuator as shown in Fig. 1a. On other hand, depending on the polarity of the DC voltage, one can produce different discharges: 1) a wide luminescent plasma sheet, named sliding discharge (SD), with negative component (see Fig. 1b). 2) a discharge visually equivalent to a surface DBD, named dielectric barrier discharge enhanced (DBDE), with positive component (see Fig. 1c). Both discharge regimes are as stable as the surface DBD actuator and have the advantage that they allow large-scale applications because the discharge extension may be increased up to the gap between two electrodes located on the upper side of the insulating wall [5-10].

In these studies, the performance improvement of the three-electrode discharge is obtained by modifying two parameters: 1) the shape of the actuator and 2) the input electric parameters. In fact, these enhancements consist to increase the injected power within the plasma. Here, a new manner to modify the properties of a TED actuator is proposed. This method consists to integrate passive components between the power supply and the discharge. This paper is organized as follows: in a first part, an electromechanical comparison is performed between a classical discharge and one containing coil. The second part highlights the effects obtained on the discharge behavior when an inductor and HV capacitor are associated. Firstly the system has been characterized electrically by voltage and current measurements, and secondly the electric wind velocity has been measured, resulting in time-averaged velocity profiles. The experiments were performed without external flow so as to focus on the discharge properties.

Influence of Coil and Capacitor on the Three-electrode Discharge Behavior

J. Jolibois, K. Takashima, and A. Mizuno

Department of Environmental and Life Sciences, Toyohashi University of Technology, Japan

Abstract—Over the past ten years, research on the surface discharges has been particularly intense because the

associated industrial applications are immense, especially for aeronautic industry. Among the many industrial problematics related to aircraft, active flow control is crucial because it conditions the aerodynamic performances of aircraft and especially those of airfoils. In case of the surface plasma devices, the improvement of the aerodynamic performances depends to the velocity of the induced electric wind. Generally, the ways used to enhance the electric wind consist to modify the shape of the plasma actuator or vary the input electrical parameters. This paper deals with a new way to increase the electromechanical properties of a surface discharge and more specifically the three-electrode plasma actuator. It consists of integrating passive components, i.e. an inductor and HV capacitor, between the power supply and the discharge. The experimental measurements of current-discharge, power consumed and velocity generated show that the properties of the three-electrode discharge can be significantly modified and more particularly the injected power within the plasma. In addition, the results highlight the ability to set in resonance the discharge and power supply through the coil, resulting a reduction of the electrical power consumption and a velocity gain. Consequently, the discharge effectiveness is increased.

Keywords—Three-Electrode Discharge (TED), surface discharge, electric wind, plasma actuator

Corresponding author: Akira Mizuno e-mail address: mizuno@ens.tut.ac.jp

Received; October 6, 2010

Jolibois et al. 93

Fig. 2. Sketch of the TED actuator.

Fig. 3. Cross-view of the experimental setup with an electrical circuit and coordinate system.

Fig. 4. Sketch of the experimental setup.

II. EXPERIMENTAL APPARATUS

The discharge used in this study consists of two-

electrode surface barrier discharge (electrodes # 1 and # 2) flush mounted on each side of a flat plate, plus a counter-electrode (electrode # 3) placed on the upper side of the insulating wall (see Fig. 2). This counter-electrode is separated relatively to the right edge of the electrode # 1 by an air gap of 40 mm, called SD gap. Each electrode is made of aluminum strip whose ends are oval in order to reduce the edge effects. Their dimensions are 50 μm-thick, 100 mm-long, 20 mm-width for electrodes # 1 and # 3 and 15 mm for electrode # 2. The dielectric used in this study is an alumina plate (150 mm × 150 mm) of 1.5 mm-thick. The inter-gap between electrodes # 1 and # 2, named DBD gap, is 5 mm.

A variable power supply (AC sine voltage) is applied to electrode # 1 as shown in Fig. 3. The AC high voltage VAC is obtained with the help of a high voltage transformer supplied by a power amplifier (NF Corporation, model 4510, 1.25 kVA). In this case, the AC power supply can deliver several hundreds of watts. While, the electrodes # 2 and # 3 are grounded. Consequently the TED actuator operates in DBD mode. Furthermore, passive components such as an inductor and a HV capacitor, are placed between the power supply and the discharge.

The electric current, i1, flowing the system was measured via a non-inductive resistors r1 as shown in Fig. 3. The value of the resistor r1 was equal to 100 Ω. The AC voltage applied to electrode # 1 was measured with an HV probe (Tektronix, model P6015A, 3 pF, 100 MΩ). Each electrical waveform was recorded using a digital oscilloscope (Tektronix, model TDS 2014B, 100 MHz, 1 GS/s).

From both voltage and currents curves, the total power consumed can be calculated as following:

d (1)

where v(t) and i1(t) are the measured voltage and current versus time, respectively, and T is the waveform period. Measurements of the induced velocity above the TED actuator are realized with a total pressure probe made in glass connected to a low differential pressure manometer (Manostar, model W081, 0 to 50 Pa). The probe can be moved precisely along 2 axes, x (horizontal) and y (vertical) above the actuator. The origin of coordinates corresponds to the right edge of the electrode # 1 as shown in Fig. 3. This setup allows to determine the time-averaged velocity component U along the x axis by using the following expression:

∆ (2)

where ∆p is the pressure difference between the measured pressure p and the atmospheric pressure patm, and ρ is the air density.

III. RESULTS

A. Influence of a coil in serie with the TED In this section, we have analyzed the influence of a

coil placed in series between the power supply and the discharge. The idea is to reduce current peaks of micro-discharges occur during the operating cycle of the DBD, since the coil generally opposes to the abrupt variations of the current. For this experiment, we integrated a coil, whose the inductance values are ranging between 1 ≤ L ≤ 3 H, in the electrical circuit (see Fig. 4). In addition, the frequency range tested is included between 0.5 to 4.5 kHz, with the voltage amplitude VAC-p fixed at 11 kV.

To highlight the impact of an inductor on the electromechanical behavior of the discharge, the induced airflow by the discharge has been measured at the location x equal to 10 mm and y = 0.2 mm. Likewise the discharge current and the applied voltage have been recorded.

First, we study the variation of the electrical power consumption as a function of the frequency (see Fig. 5). From this figure, it clearly appears that the power evolution differs totally that one obtained with the classic case, i.e. without coil. Indeed, it may be noted that the power consumed can be divided into several distinct parts. For the frequencies ranging from 0.5 to 2 kHz, the inductors do not seem to influence the discharge because the power increases linearly as we can be seen in the literature [11]. While at f = 3 kHz, a power gap occurs whatever the value of the inductor used. The effect of the coils on the discharge-current thus appears at this frequency. In addition, we observe that the gain of power increases with the value of the inductor.

Contrary to what we thought obtain, i.e. a continuous increasing of electric power, we note that at frequency of

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Fig. 5. Electrical power consumption versus frequency without and with coil. Applied voltage: sine wave, 11 kV. Fig. 6. Discharge current and voltage waveform versus time with L = 2H. Applied voltage: sine wave, 11 kV, 3 kHz. Fig. 7. Discharge current and voltage waveform versus time with L = 2H. Applied voltage: sine wave, 11 kV, 4 kHz.

Fig. 8. Evolution of the electric wind versus applied voltage without and with coil at x = 10 mm. Applied voltage: sine wave, 11 kV. Fig. 9. Evolution of the electric wind versus electrical power consumption without and with coil at x = 10 mm. Applied voltage: sine wave, 11 kV.

4 kHz the power consumed drop. The drop of power seems to vary depending on the inductance value. In this experiment, it is more consequent with L = 2 H. This result obtained at 4 kHz is very interesting because it highlights a particular operating point of the discharge. It means that the discharge current is reduced probably due to a tuning frequency of the transformer in resonance with the whole coil and TED actuator. Beyond 4 kHz, the power consumed increases again. And the power reaches significant values up to about 150 W.

To understand what happens at this particular frequency, we present the temporal evolution of discharge current and the applied voltage for the frequencies of 3 and 4 kHz (see Figs. 6 and 7, respectively). The observation of the current curves is not sufficient to determine some differences between the discharge currents. A work based on a statistical analysis of the current pulse part is required. Moreover, one can also see that the HV transformer provided a slightly different driving voltage waveform, which may alter the discharge behavior. However, it seems unlikely that this slight difference in the voltage curves can give a difference of 20 W between the two cases.

Fig. 8 displays the electric wind velocity versus frequency without and with inductor. It shows that the addition of inductor in serie modifies the induced airflow by the discharge. Indeed, it seems that the electric wind velocity is more important with coil, except at f = 0.5 kHz. This can be explained by the fact that the inductors are resistive, of the order of 4 kΩ/H. Under certain conditions, a resistive system may lead to a reduction of the electric wind velocity and probably a shifting of the position of the maximum electric wind velocity. This may explain the difference in measured velocity at f = 0.5 kHz and likewise the case where L = 3 H.

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Fig. 10. Sketch of the experimental setup.

Fig. 11. Electrical power consumption versus frequency without and with the L filter. Applied voltage: sine wave, 12 kV.

Fig. 12. Discharge current and voltage waveform versus time with the L filter (2H, 2.6 nF). Applied voltage: sine wave, 12 kV, 3 kHz.

If we compare the evolution of the electric wind as a function of the electrical power consumption without and with inductor (see Fig. 9), we observe a singular behavior. Indeed, the induced velocity by the generated airflow is enhanced asymptotically when the power consumed increases. This increasing is regular up to the measuring point corresponding to f = 4 kHz.

From the Fig. 5, we have identified a particular operating point of the actuator TED, i.e. a drop in power consumed. With reference to the literature [12], a reduction of the electric wind velocity should be measured in agreement with the evolution of electric power. It appears that this is not the case in our experiment. Instead, we get a velocity gain. Consequently, this measurement is in disagreement with previous work.

The cusp highlights a coupling between the power supply and the actuator. This coupling appears to be related to specific values of voltage and frequency. This suggests an impedance matching of electrical circuit of the discharge or a tuning frequency of the transformer in resonance with the whole coil and TED actuator. Furthermore, it is remarkable that this particular point occurs at the same frequency whatever the inductor used. This is probably due to the frequency range tested which is rather narrow, i.e. 0.5 ≤ f ≤ 4.5 kHz.

Beyond 4 kHz, we find again a classical evolution of the velocity as a function on the electrical power consumption. B. Effect of an L filter on the TED properties In this experiment, the effect of an L filter is examined. To achieve this, an inductor (1 ≤ L ≤ 3 H) is integrated in serie between the power supply and the discharge while a HV capacitor (C = 2.6 nF) is placed in parallel as shown in Fig. 10. Moreover, all the measurements were carried out with the same experimental protocol as above. Fig. 11 presents the electrical power consumption as a function of the frequency without and with the L filter. It appears that the overall behavior is dominated by the presence of the coil because the evolution of the power consumed is similar to that obtained in Fig. 5. Nevertheless, one can see some differences such as: 1) at the frequency of 4.5 kHz, impossibility to perform measurements with an inductance of 3 H. In this case, the electrical losses are important to the point that the power supply does not provide the required power. 2) the power drop is more important with L = 2 H. Indeed, we determined a power consumed of about 60 W at 4 kHz while this one is approximately 105 W with f = 3 kHz. For both other values of inductance, the decrease of power consumed is less pronounced. This one represents only about ten watts. This relative decline can be explained by the fact that the voltage applied here is equal to 12 kV rather than the effect induced by the HV capacitor. This result clearly highlights the importance of input parameters, voltage and frequency, coupled to the

inductance on the modification of electrical properties of the discharge. These parameters may enhance the effect obtained or smoothed it. In order to confirm the resultant of L filter at this particular frequency, the temporal evolution of discharge current and the applied voltage for two frequencies (3 and 4 kHz) is presented in Figs. 12 and 13, respectively. Excepted two high peaks current (see Fig. 12), it is not easy to highlight a significant difference between both discharge current. It means that a work based on a statistical analysis of the current pulse part is required to determine the modifications occurring in the discharge current thereby explaining the evolution of the power.

96 International Journal of Plasma Environmental Science & Technology, Vol.5, No.1, MARCH 2011

Fig. 13. Discharge current and voltage waveform versus time with the L filter (2H, 2.6 nF). Applied voltage: sine wave, 12 kV, 4 kHz. Fig. 14. Evolution of the electric wind versus frequency without and with L filter at x = 10 mm. Applied voltage: sine wave, 12 kV. Fig. 15. Evolution of the electric wind versus electrical power consumption without and with L filter at x = 10 mm. Applied voltage: sine wave, 12 kV.

In addition, we can observe the same variation of the applied voltage that these obtained in Figs. 6 and 7. It confirms that the HV capacitor does not significantly affect the discharge properties in this experiment. The evolution of the electric wind velocity as a function of frequency without and with the L filter is showed in Fig. 14. It appears that a slight gain of velocity

occurs when the L filter is integrated into the discharge circuit, except where the inductance is 1 H. In this experiment the velocity measurements are performed at a fixed position. But the position of the maximum electric wind velocity, related to the plasma extension, changes as a function of the input electric parameters [12, 13]. Consequently, it is likely that the velocity increase is more substantial. Fig. 15 displays the electric wind velocity versus the electrical power consumption without and with L filter. In the present case, the evolution of induced velocity is similar to that obtained in the Fig. 9. However, some differences appear. These are related to the addition of a HV capacitor but mainly by applying a voltage amplitude of 12 kV instead of 11 kV, which greatly alters the power consumed (see Fig. 11). But comparisons between the results presented by these both figures are difficult because the experiments were performed with different electrical parameters. Nevertheless, we can notice that the voltage-frequency couple used enables to obtain a gain in term of electromechanical efficiency with the coils at the specific frequency of 4 kHz. For example, with the inductance equal to 2 H, the induced velocity is about 4 m/s with a power consumed of 105 W, corresponding to a frequency of 3 kHz. While we measured a velocity about 4.1 m/s for only 60 watts at frequency of 4 kHz. Consequently the electromechanical effectiveness of the TED actuator is increased. This point exhibits clearly that the frequency of the applied HV affects significantly the plasma topology when inductor is integrated in the electrical circuit, and then the electric wind characteristic.

IV. CONCLUSION

This study deals with a new way to enhance the

properties of plasma actuators, specifically their electromechanical performances, and/or to extend the surface discharge area. For this, a three-electrode discharge (TED) actuator has been made.

As observed from the literature, the TED actuator enables to increase the plasma length up to 40 mm. But this type of discharge is not systematically more effective than a single DBD in term of pure velocity generated. Indeed, it appears that the electric wind velocity is the same order of magnitude, i.e. a few meters per second.

This paper proposes a new method to modify the intrinsic characteristics of a three-electrode discharge operating in DBD mode by adding passive components in serie with the electrical circuit of the discharge.

In a first set of experiments, inductors are placed in series between the power supply and the TED actuator. To investigate and show the effects of a coil, three different values of inductance are used (L = 1, 2 and 3 H). The results reveal that the inductors impact significantly the discharge current, resulting an injected power more consequent into the plasma. This increasing of power

Jolibois et al. 97

consumed enhances the electric wind velocity produced by the TED actuator. These results also denote a coupling between the power supply and the three-electrode discharge principally due to the applied frequency value (here f = 4 kHz). In this case, a reduction of the electrical power consumption occurs whereas the electric wind velocity continuous to accelerate. The electromechanical efficiency thus obtained is improved.

The second set of experiments consists to integrate an L filter, which is composed by a coil and HV capacitor, in the electrical circuit of the discharge. It appears that the influence is mainly due to the presence of the inductor. Indeed, the global evolution of the properties of the discharge is similar than obtained without HV capacitor.

REFERENCES [1] E. Moreau, "Airflow control by non-thermal plasma actuators,"

Journal of Physics D: Applied Physics, vol. 40, pp. 605-636, 2007.

[2] T. C. Corke, M. L. Post, and D. M. Orlov, "SDBD plasma enhanced aerodynamics: concepts, optimization and applications," Progress in Aerospace Sciences, vol. 43, pp. 193-217, 2007.

[3] T. C. Corke, C. L. Enloe, and S. P. Wilkinson, "Dielectric Barrier Discharge Plasma Actuators for Flow Control," Annual Review of Fluid Mechanics, vol. 42, pp. 505-529, 2010.

[4] C. Louste, G. Artana, E. Moreau, and G. Touchard, "Sliding discharge in air at atmospheric pressure: electrical properties," Journal of Electrostatics, vol. 63, pp. 615-620, 2005.

[5] C. Louste, E. Moreau, and G.Touchard, "Sliding discharge in air at atmospheric pressure: electrical behavior and measurements of the induced ionic wind," in Proc. of the ESA/IEJ/IEEE/SFE Conference on Electrostatics, Berkley, CA, pp. 85-94, 2006.

[6] K. Takashima, N. Zouzou, E. Moreau, A. Mizuno, and G. Touchard, "Generation of extended surface barrier discharge on dielectric surface – electrical properties, International Journal of Plasma Environment Science & Technology, vol. 1, pp. 14-20, 2007.

[7] R. Sosa, H. Kelly, D. Grondona, A. Marquez, V. Lago, and G. Artana, "Electrical and plasma characteristics of a quasi-steady sliding discharge," Journal of Physics D: Applied Physics, vol. 41, pp. 5202-5210, 2008.

[8] E. Moreau, R. Sosa, and G. Artana, "Electric wind produced by surface plasma actuators: a new dielectric barrier discharge based on a three-electrode geometry," Journal of Physics D: Applied Physics, vol. 41, pp. 5204-5216, 2008.

[9] R. Sosa, E. Arnaud, E. Memin, and G. Artana, "Study of the flow induced by a sliding discharge," IEEE Transactions on Dielectrics and Electrical Insulation, vol. 16, pp. 305-311, 2009.

[10] R. Sosa, J. D'Adamo, and G. Artana, "Circular cylinder drag reduction by three-electrode plasma actuators," Journal of Physics: Conference Series, vol. 166, pp. 2015-2029, 2009.

[11] B. Dong, J. M. Bauchire, J. M. Pouvesle, P. Magnier, and D. Hong, "Experimental study of a DBD surface discharge for the active control of subsonic airflow," Journal of Physics D: Applied Physics, vol. 41, pp. 5201.1-5201.9, 2008.

[12] J. Jolibois and E. Moreau, "Enhancement of the Electromechanical Performances of a Single Dielectric Barrier Discharge Actuator," IEEE Transactions on Dielectrics and Electrical Insulation, vol. 16, pp. 758-767, 2009.

[13] V. I. Gibalov and G. J. Pietsch, "The development of dielectric barrier discharges in gas gaps and on surfaces," Journal of Physics D: Applied Physics, vol. 33, pp. 2618-2636, 2000.

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