# Nanosecond-Pulsed Dielectric Barrier Discharge Plasma Actuator for Airflow Control Along an NACA0015 Airfoil at High Reynolds Number

Éric Moreau, Antoine Debien, Nicolas Benard, Noureddine Zouzou

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Éric Moreau, Antoine Debien, Nicolas Benard, Noureddine Zouzou. Nanosecond-Pulsed Dielectric Barrier Discharge Plasma Actuator for Airflow Control Along an NACA0015 Airfoil at High Reynolds Number. IEEE Transactions on Plasma Science, 2016, 44 (11), pp.2803-2811. 10.1109/TPS.2016.2603226. [hal-03641370](https://hal.science/hal-03641370v1)

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# Nanosecond-Pulsed Dielectric Barrier Discharge Plasma Actuator for Airflow Control Along an NACA0015 Airfoil at High Reynolds Number

Eric Moreau, Antoine Debien, Nicolas Benard, and Noureddine Zouzou

**Abstract**—The goal of the present experimental study is to investigate the ability of a surface nanosecond pulsed dielectric barrier discharge plasma actuator, located at the leading edge of the airfoil, to delay flow separation along the suction side of an NACA0015 airfoil inclined at 13.5°, using a particle image velocimetry system. The free-stream velocity is equal to $U_0 = 40$ m/s, giving a chord-based Reynolds number of $Re_c = 1.33 \times 10^6$. At such conditions, the baseline flow separation occurs at about 50% of chord. On the one hand, the results show that the separation location moves progressively toward the trailing edge when the high-voltage frequency $F_{pulse}$ is increased, up to 85% of chord at 1630 Hz (reduced frequency $F_{pulse}^+ = 10.6$), which is due to the increase of the injected electrical power with $F_{pulse}$ (from 8 mW/cm at 40 Hz to 290 mW/cm at 1630 Hz). On the other hand, the use of a burst-modulated actuation (with a constant electrical power consumption equal to 145 mW/cm) does not produce the same behavior. At the lowest and highest frequencies (16 and 408 Hz, corresponding to the reduced frequencies of 0.1 and 2.65, respectively), the separation is delayed to about 60% of chord, while the most effective actuation is observed at $F_{burst} = 80$ Hz ($F_{burst}^+ = 0.53$) with a separation delaying to 83% of chord.

**Index Terms**—Airflow control, electrohydrodynamics (EHDs), plasma actuators, pulsed dielectric barrier discharge (DBD).

## I. INTRODUCTION

ACTIVE airflow control with plasma actuators is widely studied all over the world, because it could be an alternative to traditional techniques, such as the use of mechanical slats and flaps on the wings of airplane. The reduction of flow separation over curved or inclined surfaces, the drag reduction in view of fuel consumption improvement, or the manipulation of laminar-to-turbulent transition are some of the practical applications where plasma discharges demonstrated a good effectiveness, at least for in-lab conditions [1]–[4].

On the one hand, most of researches are focused on the surface single dielectric barrier discharges (DBDs) based on

two electrodes mounted on both the sides of a dielectric layer (AC-DBD) driven by an ac sine high voltage [1]. The physical mechanism behind the control effectiveness is the electrohydrodynamic (EHD) force produced inside the discharge, which can interact with the boundary layer of the incoming flow [1]–[4]. In fact, the produced ions are accelerated by the high electric field, resulting in an electric (or ionic) wind of a few meters per second very close to the wall. Consequently, this momentum transfer can dynamically affect the flow characteristics inside the boundary layer. Typically, single DBD based on two electrodes can produce mean force and electric wind velocity up to 1 mN/W and 6.5 m/s, respectively [3], [5]. In the case of an actuator array composed of several DBDs [6]–[9], velocity higher than 10 m/s has been reached [8], [9].

On the other hand, if the high voltage has a nanosecond repetitively pulsed waveform (NS-DBD), a new mechanism of control, which is based on a fast gas heating, is observed. Roupassov et al. [10] investigated the ability of a pulsed discharge to reattach a fully detached airflow along a small size airfoil for a velocity up to 100 m/s ($Re_c = 9.6 \times 10^5$). Little et al. [11] used a nanosecond plasma actuator positioned in counterflow direction to effectively manipulate the flow over a profile at $Re_c = 1 \times 10^6$. Recently, Kato and Breitsamter [12] investigated the effect of such an actuator for Reynolds numbers up to $2.3 \times 10^6$. In the different works on flow control by NS-DBD [10]–[15], the authors assume that the main mechanism is not the EHD force but a rapid variation of the surrounding gas temperature. This abrupt change of gas temperature, which is mainly caused by vibrational excitation of air molecules, induces a localized pressure wave propagating from the air-exposed electrode edge at sound speed [16]–[27]. Moreover, the pressure wave magnitude is strongly linked to the electrical deposited energy within the discharge. For instance, Bayoda et al. [25] measured a pressure gradient of 500 Pa for a discharge energy of 0.3 mJ/cm. However, values greater than 10 000 Pa for an input energy of about 10 mJ/cm of electrode length have been reported in [21].

The present contribution aims at completing the recent literature with the specific case of flow separation at mid-chord of an NACA0015 model, at an angle of attack of 13.5° and $Re_c = 1.3 \times 10^6$. The goal of the plasma actuation, which is located at the leading edge of the airfoil, is to delay a turbulent

Manuscript received January 29, 2016; revised May 6, 2016 and July 25, 2016; accepted August 23, 2016. This work was supported in part by the 7th Framework Program of European Commission PlasmAero under Grant 234201 and in part by the French Government Program Investissements d'Avenir, LABEX INTERACTIFS, under Grant ANR-11-LABX-0017-01.

The authors are with the Centre National de la Recherche Scientifique, Institut Pprime, ISAE/ENSMA, University of Poitiers, 86962 Chasseneuil-du-Poitou, France (e-mail: eric.moreau@univ-poitiers.fr).

Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/TPS.2016.2603226

AQ:1

AQ:2
<figure><img src="image_2.png" /><figcaption>Fig. 1. (a) Schematics of the plasma actuator and (b) its location at the leading edge of the airfoil.</figcaption></figure>

flow separation at the mid-chord. This paper is divided into four main sections. In Section II, the experimental setup is presented. In Section III, the electrical characteristics of the surface nanosecond DBD are discussed. In Section IV, the baseline flow is studied by particle image velocimetry (PIV) measurements. In Section V, the effects of the pulsed actuation on the flow are investigated and presented. Finally, Section VI provides the conclusion.

## II. EXPERIMENTAL SETUP

The present experiments have been conducted in a subsonic closed wind tunnel. This facility operates at ambient conditions and has a large test section (2.6 m high, 2.4 m in width, and 6 m in length). The maximum freestream velocity can reach 50 m/s with a turbulence level of about 0.5%.

A 2-D model with an NACA0015 profile is mounted horizontally in the test section between two profiled end plates that contribute to a thinning of the boundary layer developing on the side walls. These two end-plates are 5 m in length in the free stream direction and they cross the test section from bottom to top, reducing the section width to 1.2 m. The model has then a spanwise length of $L = 1.2$ m and a chord length equal to $c = 0.5$ m. The plasma actuator is located at the leading edge of the airfoil. It is composed of a 10-mm-wide air-exposed active electrode and a 20-mm-wide encapsulated grounded electrode placed below the dielectric. These electrodes are flush mounted on both the sides of a 180-µm-thick dielectric of Kapton, as shown in Fig. 1(a), and this actuator is stuck on the wall of the leading edge of the airfoil [Fig. 1(b)]. There is no gap between both electrodes, except the dielectric thickness. The edge of the air-exposed electrode is located at 1% of chord; the produced pressure wave will start from this location. The actuation in spanwise is equal to 57 cm, corresponding to about 50% of the airfoil span, this length being restricted by the electrical power that can be delivered by the power supply as it will be discussed later in this paper.

Time-averaged PIV measurements have been undertaken in order to access the whole velocity field on the suction side of the airfoil. Velocity measurements have been performed in two overlapping planes using two charge-coupled device (CCD) cameras (Pulnix, RM-4200CL, 2048 × 2048 pixels²), resulting in a wide flow field of view that starts 70 mm upstream of the leading edge of the profile and finishes at 120 mm downstream of the trailing edge (Fig. 2). The pixel size is equal to 172 µm

and the acquisition frequency is about 7 Hz. The Nd-YAG laser sheet (Quantel Big Sky Laser, 2 × 191 mJ) is located at the middle of the model spanwise, and both the cameras are equipped with a bandpass lens filter (Nikon Nikkor 50 mm, f/1: 1, 2, and bandwidth 532 ± 10 nm) in order to cancel the parasitic light emitted by the discharge. The PIV posttreatment consists of a multipass adaptative correlation (from 64 × 64 down to 16 × 16 pixels²) with an overlap of 50%, resulting in a vector every 1.4 mm. Moreover, 300 instantaneous flow fields are recorded to compute a time-averaged flow field with converged first-order flow quantities.

Two coordinate systems are used to characterize the flow field. The first one, {$O, x/c, y/c$}, is normalized by the chord length of the profile. Its origin is located at the leading edge of the profile, as shown in Fig. 2. The second system {$O, x_c/c, y_c/c$} is a curvilinear coordinate system that follows the suction side of the profile. Subsequently, the $x_c$-axis is always tangent to the profile wall when the $y_c$-axis is perpendicular to the wall. In this paper, the second system will be mainly used, more particularly for velocity profiles.

Using the first coordinate system, the presence of vortex structures beyond the instantaneous separation point is investigated by using a $\Gamma_1$ criterion [28]. This criterion is dedicated to the identification of vortex center superposed on a small-scale turbulent velocity field. To characterize this criterion at a point P, the sinus of the angle between the velocity vector at the point M and the radius vector PM is computed and averaged on an area $S$ surrounding P. Near a vortex center, $|\Gamma_1|$ is close to 1, while $|\Gamma_1|$ is close to zero in a freestream flow. In this paper, to overcome the multiple detection of a same vortex center, the local maximum of $|\Gamma_1|$ is identified in the areas satisfying $|\Gamma_1| > 0.8$.

<figure><img src="image_3.png" /><figcaption>Fig. 2. Flow field view using two cameras and coordinate systems.</figcaption></figure>

## III. ELECTRICAL CHARACTERISTICS OF THE PULSED DBD

As shown in Fig. 3, the surface discharge is energized with the help of an high-voltage solid-state pulser

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<figure><img src="image_4.png" /><figcaption>Fig. 3. Schematic of the electrical experimental setup.</figcaption></figure>

(DEI, model PVX-4110, ±10 kV, ±30 $A_{peak}$) supplied by a dc power supply (Matsuda, model 10P30, ±10 kV, ±30 mA) and controlled by a digital pulse generator (Stanford, model DG645). The system can provide nanosecond high voltage pulses with the following characteristics: high voltage magnitude up to ±10 kV and rise and decay times of about 50 ns with adjustable width (from 100 ns to dc) and frequency up to 10 kHz. The current and voltage waveforms are recorded using a digital oscilloscope (Lecroy, model WaveSurfer 424, 200 MHz bandwidth) connected to dedicated probes. The current probe is a fast current transformer (Bergoz, model CT-D1.0, 500 MHz bandwidth). The voltage between both electrodes is measured with a high voltage probe (Lecroy, model PPE20kV, 100 MHz bandwidth).

In this paper, the discharge is supplied by a high voltage that has a peak value equal to $V_p = 9$ kV, as shown in Fig. 4(a). Certainly because the electrode spanwise is high (57 cm), the power supply is slightly undersized and not well adapted to the actuator capacitance. Then, the voltage rise time is higher than 50 ns and is equal to about 100 ns. The voltage pulsewidth (50% of maximum voltage) is equal to 250 ns. The current is composed of two current peaks; the first one occurs when the voltage increases ($t \approx 60$ ns). Its magnitude is equal to about 0.6 A/cm (current value divided by the electrode spanwise length). This current peak is due to the deposition of positive electrical charges on the dielectric wall and the ignition of a streamerlike discharge, as illustrated by the iCCD view [positive discharge, *P* in Fig. 4(b)]. The discharge extension on the dielectric surface is limited to about 10 mm. Then, during the voltage plateau ($100 \le t \le 250$ ns), the current level decreases rapidly and the discharge is in a silent period, because there is no electric field across the dielectric surface. When the high voltage decreases ($t > 250$ ns), the potential difference between the air-exposed electrode and the positively charged surface is sufficient to ignite a new discharge responsible for a negative current peak occurring during the fall time of the voltage. Even if the shape of both current peaks is similar, the discharge regime associated with each one is completely different. The positive discharge is filamentary and can extend up to about 10 mm, while the negative discharge is a glowlike one [Fig. 4(b), view *N*]; this last one is composed of 3-mm-long ionized channels that start from individual spots and propagates with a plume shape.

<figure><img src="image_5.png" /><figcaption>Fig. 4. (a) Waveform of the applied high voltage and the associated discharge current. (b) Top iCCD views of the positive (*P*) and negative (*N*) discharge. Red dotted line: active electrode edge.</figcaption></figure>

<figure><img src="image_6.png" /><figcaption>Fig. 5. Electrical power consumption and energy versus time ($V_p = 9$ kV and $F_{pulse} = 1630$ Hz).</figcaption></figure>

The typical waveforms of the electric power and energy versus time are shown in Fig. 5. The signal of the electric power is composed of two peaks, during the rising and falling periods of the voltage. The positive peak is higher than 3.5 kW/cm during about 25 ns. During this first peak, the electrical power comes from the power supply and energizes the discharge. During the falling period of the voltage, the electrical power consumption is negative, meaning that the power is induced by the electric field due to the space charges remained at the dielectric surface since the positive discharge; the power

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<figure><img src="image_7.png" /><figcaption>Fig. 6. Electrical power consumption and energy per voltage pulse versus frequency $F_{\text{pulse}}$ ($V_p = 9$ kV).</figcaption></figure>

<figure><img src="image_8.png" /><figcaption>Fig. 7. Time-averaged flow field of the horizontal normalized velocity component $U/U_0$ for the baseline configuration ($\alpha = 13.5^\circ$ and $U_0 = 40$ m/s).</figcaption></figure>

supply is then a receptor. Subsequently, the difference of area between the areas of the both peaks is proportional to the time-averaged power dissipated by the actuator during a complete cycle of the voltage. By integrating the power curve, one can plot the energy curve. From Fig. 5, one can deduce that the energy consumed by one high voltage pulse is here equal to 0.16 mJ/cm. At such a condition, the gradient of the pressure wave induced by each voltage pulse is equal to about 300 Pa.

Fig. 6 shows the energy per voltage pulse and the power consumption as a function of the high voltage frequency $F_{\text{pulse}}$. It highlights that the energy per pulse is not quite constant; it decreases slightly with $F_{\text{pulse}}$, certainly due to the power supply limitations. The time-averaged electrical power consumption increases with $F_{\text{pulse}}$ from 16 mW/cm at 100 Hz to 450 mW/cm at 3 kHz. One can notice that these power consumption values are very small compared with the ones of others actuators used for airflow control.

## IV. BASELINE FLOW

In this paper, the angle of attack is set to $\alpha = 13.5^\circ$ and the free-stream velocity to $U_0 = 40$ m/s, resulting in a chord-based Reynolds number of $Re_c = 1.33 \times 10^6$. Fig. 7 shows the time-averaged velocity field of the baseline flow without any control (the air-exposed electrode is present but it is not actuated). It highlights the presence of a flow separation that occurs at about mid-chord, resulting in a wide recirculation zone downstream of the separation. To determine precisely the mean separation point, near-wall time-averaged

<figure><img src="image_9.png" /><figcaption>Fig. 8. Near-wall velocity profiles at different locations from 45% to 51% of chord.</figcaption></figure>

<figure><img src="image_10.png" /><figcaption>Fig. 9. Location of the separation point versus time, the time between two successive fields being equal to 0.14 s (1/$f_{\text{PIV}}$ with $f_{\text{PIV}} = 7$ Hz).</figcaption></figure>

<figure><img src="image_11.png" /><figcaption>Fig. 10. PIV results for the baseline configuration ($\alpha = 13.5^\circ$ and $U_0 = 40$ m/s). Detection of vortices by a $\Gamma_1$ criterion, computed with a 6 × 6 window and a threshold equal to 0.8.</figcaption></figure>

velocity profiles have been plotted in the coordinate system $\{O, x_c/c, y_c/c\}$ at different locations around 50% of chord, as shown in Fig. 8. Up to 47% of chord ($x_c/c \le 0.47$), the near-wall velocity is positive and it decreases toward zero at the wall; the flow is not detached. From $x_c/c = 0.49$, the near-wall velocity becomes negative, highlighting the presence of a recirculation zone. One can then assume that the mean separation point is located at $x_c/c = 0.48$, where the velocity value of the first point of the profile (at $y_c/c = 0.0028$) is equal to zero.

Moreover, the instantaneous separation point has also been determined for all the 300 velocity vector fields (Fig. 9). The results indicate that the location of the flow detachment fluctuates from $x_c/c = 0.22$ to $x_c/c = 0.84$ (in the extreme cases) with a mean separation occurring at $x_c/c = 0.48$ and an rms value equal to 0.13c. That highlights that the flow separation is strongly unsteady.
<figure><img src="image_12.png" /><figcaption>Fig. 11. Profiles of the longitudinal component of the velocity for the baseline flow and for different actuation frequency values $F_{\text{pulse}}^{+}$, at different locations along the airfoil chord $x_{c}/c$ ($\alpha = 13.5^{\circ}$ and $U_{0} = 40$ m/s). The velocity indicated for the profiles corresponds to $0.05 \times U/U_{0}$.</figcaption></figure>

<figure><img src="image_13.png" /><figcaption>Fig. 12. Profiles of the longitudinal component of the velocity for the baseline flow and for different actuation frequency values at $x_{c}/c = 1.15$ ($\alpha = 13.5^{\circ}$ and $U_{0} = 40$ m/s).</figcaption></figure>

<figure><img src="image_14.png" /><figcaption>Fig. 13. Position of the flow separation versus actuation frequency ($\alpha = 13.5^{\circ}$, $U_{0} = 40$ m/s, and $V_{p} = 9$ kV).</figcaption></figure>

they are convected by the flow. As shown in Fig. 10, the first vortices are located at about 25% of chord when the mean separation occurs at mid-chord, confirming one more time that the separation point moves versus time. The red stars represent the vortices with a counterclockwise rotation. They are primarily observed at the trailing edge due to the interaction with the flow incoming from the pressure side of the airfoil, but some are also identified in the recirculation zone due to small-scale flow structures. On the 300 flow field snapshots, 1398 clockwise vortices and 603 counterclockwise vortices have been detected, respectively. The flow separation is then at the origin of an intense production of vortices that are convected by the flow, resulting in a flapping of the vortex street downstream of the separation.

## V. FORCED FLOW

Two different actuation types are used. In the first set of experiments, the operating frequency $F_{\text{pulse}}$ is varied from 40 up to 1630 Hz, corresponding to a reduced frequency range of $0.26 \le F_{\text{pulse}}^{+} \le 10.6$. This reduced frequency is estimated with the following relation: $F_{\text{pulse}}^{+} = L_{\text{sep}} \times F_{\text{pulse}}/U_{0}$,

In Fig. 10, a $\Gamma_{1}$ criterion [28] has been used to detect the center of the vortices embedded in the turbulent separated shear layer and in the near-field wake of the profile. The black crosses correspond to the vortices with a clockwise rotation. They form usually at the separation location and

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<figure><img src="image_15.png" /><figcaption>Fig. 15. Profiles of the longitudinal component of the velocity for the baseline flow and for different actuation frequency values $F_{\text{burst}}^{+}$, at different locations along the airfoil chord $x_{c}/c$ ($\alpha = 13.5^{\circ}$ and $U_{0} = 40$ m/s). The velocity indicated for the profiles corresponds to $0.05 \times U/U_{0}$.</figcaption></figure>

where $L_{\text{sep}}$ is the mean separation length, i.e., 0.26 m in our case. This type of actuation is considered as a pulsed mode. In this mode, the power injected inside the boundary layer increases with the frequency $F_{\text{pulse}}$, from 8 mW/cm at 40 Hz to 290 mW/cm at 1630 Hz. In a second set of experiments, the operating frequency $F_{\text{pulse}}$ is set to a constant value as well as the voltage amplitude ($F_{\text{pulse}}$ = 1630 Hz and $V_{p}$ = 9 kV). This pulsed signal is then burst-modulated by a gate signal at a lower frequency $F_{\text{burst}}$ and a duty cycle of 50%, that is the burst-modulated mode. Here, $F_{\text{burst}}$ will vary from 16 to 408 Hz, corresponding to a reduced frequency range of $0.1 \le F_{\text{burst}}^{+} \le 2.65$, with $F_{\text{burst}}^{+} = L_{\text{sep}} \times F_{\text{burst}}/U_{0}$. This second mode of actuation has two advantages: the power injected in the flow is constant whatever $F_{\text{burst}}$ (it is equal to 145 mW/cm), and thus, perturbations at low frequency can be induced with a higher energy than the pulsed mode.

### A. Pulsed Mode Actuation

Fig. 11 shows the profiles of the velocity longitudinal component along the suction side, from 50% to 98% of chord, without actuation and with an actuation in the pulsed mode. First, in the absence of actuation [Fig. 11 (black curve)], the velocity profiles present negative values close to the wall, due to the flow separation. Second, it highlights that for $F_{\text{pulse}}^{+} = 1.06$, the negative value of the near-wall flow is increased, meaning that the recirculation zone downstream of the separation point has been enlarged. On the contrary, for $F_{\text{pulse}}^{+} \ge 2.12$, the velocity gain near the wall increases progressively with the $F_{\text{pulse}}^{+}$ value. This is also confirmed by the velocity profiles downstream of the trailing edge of the airfoil ($x_{c}/c$ = 1.15) in Fig. 12. On one hand, for $F_{\text{pulse}}^{+} = 1.06$, the velocity deficit in the wake is lower than $0.1 \times U_{0}$ when it is equal to $0.15 \times U_{0}$ for the baseline flow. On the other hand, the smallest velocity at $F_{\text{pulse}}^{+} = 8.5$ is equal to $0.4 \times U_{0}$, confirming that the size of the recirculation zone has been strongly reduced.

Fig. 13 shows the separation point location versus actuation frequency. It indicates that in the present flow conditions, the discharge has a negative effect when it is supplied with a

voltage whose reduced frequency is close to one. On the contrary, the separation point moves progressively toward the trailing edge when the high voltage frequency $F_{\text{pulse}}$ is increased. In the best case ($F_{\text{pulse}}^{+} = 10.6$, i.e., $F_{\text{pulse}} = 1630$ Hz), the separation point is delayed up to 85%. This result shows that the actuation effect is enhanced when the deposited energy is increased. Here, the flow is more sensitive to the amount of deposited energy than to its frequency. This can explain why a reduced frequency equal to unity is not more effective, while such a unity value has been reported many times in the literature as the most effective for reattaching separated turbulent flows.

Finally, Fig. 14 shows the number of detected vortices versus actuation frequency. It confirms the previous results: on one hand, when $F_{\text{pulse}}^{+} \approx 1$, the discharge does not allow us to delay the separation point location and the vortex numbers rise up. On the other hand, when the frequency actuation is increased, the flow is partially reattached and the vortex number decreases, meaning that the flow is stabilized by the actuation.

### B. Burst-Modulation Actuation

In the second set of experiments, the operating frequency $F_{\text{pulse}}$ is set to 1630 Hz ($F_{\text{pulse}}^{+} = 10.6$ and $V_{p}$ = 9 kV), and this pulsed signal is burst-modulated at a frequency $F_{\text{burst}}$. With this actuation mode, the electrical power is constant (145 mW/cm) whatever the burst frequency is; it is then possible to determine the frequency effect on the control authority, at a constant power consumption. Such a control method has already been implemented in context of nanosecond pulsed DBD. In [14], the train of pulses at 14 kHz with a variable frequency modulation has been applied to suppress a leading-edge stall at $Re_{c}$ = $1.4 \times 10^{6}$. It was shown that the lift can be increased by the DBD in all the cases and a frequency dependence was highlighted; the lift was increased for an increasing reduced frequency but it was saturated for reduced frequency larger than one. Fig. 15 shows the velocity profiles of the baseline flow and the forced flow for several $F_{\text{burst}}^{+}$ values. It highlights that the actuation results
<figure><img src="image_16.png" /><figcaption>Fig. 16. Profiles of the longitudinal component of the velocity for the baseline flow and for different actuation frequency values at $x_c/c = 1.15$ ($\alpha = 13.5^\circ$ and $U_0 = 40$ m/s).</figcaption></figure>

<figure><img src="image_17.png" /><figcaption>Fig. 17. Position of the flow separation versus the burst-modulation frequency $F_{\text{burst}}^{+}$ ($\alpha = 13.5^\circ$, $U_0 = 40$ m/s, $V_p = 9$ kV, $f_{\text{pulse}} = 1630$ Hz, and $F_{\text{burst}}^{+}$ varying from 0.1 to 2.65).</figcaption></figure>

in a velocity gain whatever the frequency value, and that the maximum velocity gain in the boundary layer is obtained at $F_{\text{burst}}^{+} = 0.84$. That is confirmed by Fig. 16 that shows velocity profiles in the wake, at $x_c/c = 1.15$. As previously (Fig. 12), the minimum velocity of the baseline flow is equal to $0.15 \times U_0$. With actuation at $F_{\text{burst}}^{+} = 0.1$ and $2.65$, the minimum velocity increases up to $0.28 \times U_0$ and the maximum effect is obtained for $F_{\text{burst}}^{+} = 0.84$ for which the minimum velocity is equal to $0.41 \times U_0$.

The location of the separation point $X_{\text{sep}}$ versus $F_{\text{burst}}^{+}$ is shown in Fig. 17, in order to determine accurately the most effective frequency. It shows that the influence of the frequency on the separation point delay is not monotonous; at $F_{\text{burst}}^{+} = 0.1$, the separation is delayed at $x_c/c = 0.57$. By increasing the frequency up to $F_{\text{burst}}^{+} = 0.53$, the flow separation is progressively delayed up to $x_c/c = 0.83$, that is the most effective case. Finally, when the frequency is increased above $0.53$, the control authority decreases; at $F_{\text{burst}}^{+} = 2.65$, the flow separation occurs at $x_c/c = 0.62$. These results show that there is a strong frequency effect, because the power injected in the flow is constant, whatever the $F_{\text{burst}}^{+}$ value is, and that the most effective actuation is observed for a frequency range $0.5 \le F_{\text{burst}}^{+} \le 0.8$. That is confirmed by Fig. 18 in which the number of vortices is minimum in this

frequency range, meaning that the separation is delayed and that the flow is stabilized.

## VI. CONCLUSION

The goal of the present experimental study was to investigate the ability of surface nanosecond pulsed DBD plasma actuator to delay flow separation along the suction side of an NACA0015 airfoil. A 2-D profile with an NS-DBD actuator located at 1% of the chord length has been mounted horizontally in a large subsonic wind tunnel. The angle of attack was set to $13.5^\circ$ and the free-stream velocity to $U_0 = 40$ m/s, resulting in a chord-based Reynolds number of $Re_c = 1.33 \times 10^6$. In such conditions, the baseline flow separation occurs at 48% of chord, resulting in a large recirculation zone downstream of the separation. However, it has been demonstrated that the location of the flow detachment widely fluctuates from $x_c/c = 0.22$ to $x_c/c = 0.85$, with a mean separation point located at $x_c/c = 0.48$.

Two different actuation types have been used. In a first set of experiments, the frequency $F_{\text{pulse}}^{+}$ was varied from 0.26 to 10.6, corresponding to electrical power consumption from 8 to 290 mW/cm. With such an actuation, the separation location moves progressively toward the trailing edge when the high voltage frequency $F_{\text{pulse}}$ is increased. In the best case ($F_{\text{pulse}}^{+} = 10.6$, i.e., $F_{\text{pulse}} = 1630$ Hz), the separation point is delayed up to 85%. This result shows that the actuation is more effective when the electrical power consumption increases, but no frequency effect can be really highlighted, because one cannot separate the effect of the frequency with the one of the injected power.

In a second set of experiments, the operating frequency $F_{\text{pulse}}^{+}$ was set to a constant value and the pulsed high voltage was burst-modulated at a lower frequency $F_{\text{burst}}^{+}$ with $0.1 \le F_{\text{burst}}^{+} \le 2.65$. With this actuation mode, the power injected in the flow is constant whatever $F_{\text{burst}}^{+}$ (145 mW/cm); it is then possible to determine the frequency effect on the control authority. Indeed, in this case, the influence of the frequency is clearly observable and its effect on the separation point delay is not monotonous. On one hand, at $F_{\text{burst}}^{+} = 0.1$ and $2.65$ (smallest and highest frequencies), the separation was

<figure><img src="image_18.png" /><figcaption>Fig. 18. Number of detected clockwise and anticlockwise vortices as a function of the actuation frequency $F_{\text{burst}}^{+}$ ($\alpha = 13.5^\circ$ and $U_0 = 40$ m/s).</figcaption></figure>

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delayed at about 60% of chord. On the other hand, in the frequency range $0.5 \le F_{\text{burst}}^{+} \le 0.8$, the flow separation occurred downstream of $x_c/c = 0.80$ with a most effective effect at $F_{\text{burst}}^{+} = 0.53$ with a delay of the separation location up to $x_c/c = 0.83$.

To conclude, this paper showed that NS-DBD plasma actuators can compete with other types of active control devices to reattach separated flow at high Reynolds number, with a very low power consumption (145 mW/cm, corresponding to a total consumption of 8.26 W for a 57-cm-long electrode) and a weak induced pressure wave (300 Pa). However, the present investigation in large-scale wind tunnel was not suitable to identify the control mechanism behind the flow separation mitigation. Indeed, it is not possible to assume that the flow manipulation is due to an increase of turbulence inside the boundary layer or to an interaction between the induced pressure waves and natural flow structures. Moreover, one cannot explain why the maximum effect is obtained for $0.5 \le F_{\text{burst}}^{+} \le 0.8$. Meanwhile, a last remark can be done. If $F_{\text{burst}}^{+}$ had been based on the airfoil chord $c = 0.5$ m instead of the separation length $L_{\text{sep}} = 0.26$ m, then the optimum $F_{\text{burst}}^{+}$ would have been equal to one, this value being the usual most effective reduced frequency for the control of leading edge separation. That could signify that the most effective physical actuation frequency is always the same, whatever the separation location is.

Thus, future experiments will focus on this aspect by a close inspection of the flow conditions in the vicinity of the boundary wall flow.

REFERENCES

[1] E. Moreau, "Airflow control by non-thermal plasma actuators," *J. Phys. D, Appl. Phys.*, vol. 40, no. 3, pp. 605-636, 2007.

[2] T. C. Corke, C. L. Enloe, and S. P. Wilkinson, "Dielectric barrier discharge plasma actuators for flow control," *Annu. Rev. Fluid Mech.*, vol. 42, no. 1, pp. 505-529, 2009.

[3] N. Benard and E. Moreau, "Electrical and mechanical characteristics of surface AC dielectric barrier discharge plasma actuators applied to airflow control," *Experim. Fluid*, vol. 55, Nov. 2014, Art. no. 1846.

[4] J.-J. Wang, K.-S. Choi, L.-H. Feng, T. N. Jukes, and R. D. Whalley, "Recent developments in DBD plasma flow control," *Prog. Aerosp. Sci.*, vol. 62, pp. 52-78, Oct. 2013.

[5] A. Debien, N. Benard, L. David, and E. Moreau, "Unsteady aspect of the electrohydrodynamic force produced by surface dielectric barrier discharge actuators," *Appl. Phys. Lett.*, vol. 100, no. 1, p. 013901, 2012.

[6] G. Neretti, A. Cristofolini, and C. A. Borghi, "Experimental investigation on a vectorized aerodynamic dielectric barrier discharge plasma actuator array," *J. Appl. Phys.*, vol. 115, no. 16, p. 163304, 2014.

[7] G. Neretti, A. Cristofolini, C. A. Borghi, A. Gurioli, and R. Pertile, "Experimental results in DBD plasma actuators for air flow control," *IEEE Trans. Plasma Sci.*, vol. 40, no. 6, pp. 1678-1687, Jun. 2012.

[8] A. Debien, N. Benard, and E. Moreau, "Streamer inhibition for improving force and electric wind produced by DBD actuators," *J. Phys. D, Appl. Phys.*, vol. 45, no. 21, p. 215201, 2012.

[9] A. Berendt, J. Podliński, and J. Mizeraczyk, "Comparison of airflow patterns produced by DBD actuators with smooth or saw-like discharge electrode," *J. Phys., Conf. Ser.*, vol. 301, no. 1, p. 012018, 2011.

[10] D. V. Roupassov, A. A. Nikipelov, M. M. Nudnova, and A. Y. Starikovskii, "Flow separation control by plasma actuator with nanosecond pulsed-periodic discharge," *AIAA J.*, vol. 47, no. 1, pp. 168-185, 2009.

[11] J. Little, K. Takashima, M. Nishihara, I. Adamovich, and M. Samimy, "Separation control with nanosecond-pulse-driven dielectric barrier discharge plasma actuators," *AIAA J.*, vol. 50, no. 2, pp. 350-365, 2012.

[12] K. Kato, C. Breitsamter, and S. Obi, "Flow separation control over a Gö 387 airfoil by nanosecond pulse-periodic discharge," *Experim. Fluids*, vol. 55, p. 1795, Aug. 2014.

[13] P. Bletzinger, B. N. Ganguly, D. Van Wie, and A. Garscadden, "Plasmas in high speed aerodynamics," *J. Phys. D, Appl. Phys.*, vol. 38, no. 4, pp. 33-57, 2005.

[14] C. L. Kelley *et al.*, "Leading-edge separation control using alternating-current and nanosecond-pulse plasma actuators," *AIAA J.*, vol. 52, no. 9, pp. 1871-1884, 2014.

[15] T. Michelis *et al.*, "Disturbance introduced into a laminar boundary layer by a NS-DBD plasma actuator," in *Proc. 51st AIAA Aeros. Sci. Meeting Including New Horizons Forum Aerosp. Expo.*, 2013, paper #2013-0752.

[16] T. Unfer and J. P. Boeuf, "Modelling of a nanosecond surface discharge actuator," *J. Phys. D, Appl. Phys.*, vol. 42, no. 19, p. 194017, 2009.

[17] K. Takashima, Y. Zuzeek, W. R. Lempert, and I. V. Adamovich, "Characterization of a surface dielectric barrier discharge plasma sustained by repetitive nanosecond pulses," *Plasma Sour. Sci. Technol.*, vol. 20, no. 5, p. 055509, 2011.

[18] N. Benard, N. Zouzou, A. Claverie, J. Sotton, and E. Moreau, "Optical visualization and electrical characterization of fast-rising pulsed dielectric barrier discharge for airflow control applications," *J. Appl. Phys.*, vol. 111, no. 3, p. 033303, 2012.

[19] R. Dawson and J. Little, "Characterization of nanosecond pulse driven dielectric barrier discharge plasma actuators for aerodynamic flow control," *J. Appl. Phys.*, vol. 113, no. 10, p. 103302, 2013.

[20] K. Takashima, Z. Yin, and I. Adamovich, "Measurements and kinetic modeling analysis of energy coupling in nanosecond pulse dielectric barrier discharge," *Plasma Sour. Sci. Technol.*, vol. 22, p. 015013, 2013.

[21] Z. Zhao, J.-M. Li, J. Zheng, Y. D. Cui, and B. C. Khoo, "Study of shock and induced flow dynamics by nanosecond dielectric-barrier-discharge plasma actuators," *AIAA J.*, vol. 53, no. 5, pp. 1336-1348, 2015.

[22] Y. Zhu, Y. Wu, W. Cui, Y. Li, and M. Jia, "Modelling of plasma aerodynamic actuation driven by nanosecond DBSD discharge," *J. Phys. D, Appl. Phys.*, vol. 46, no. 35, p. 355205, 2013.

[23] J. G. Zheng, Z. J. Zhao, J. Li, Y. D. Cui, and B. C. Khoo, "Numerical simulation of nanosecond pulsed dielectric barrier discharge actuator in a quiescent flow," *Phys. Fluids*, vol. 26, no. 3, p. 036102, 2014.

[24] J. Little, K. Takashima, M. Nishihara, I. Adamovich, and M. Samimy, "High lift airfoil leading edge separation control with nanosecond pulse DBD plasma actuators," in *Proc. 5th Flow Control Conf.*, 2010, paper #2010-4256.

[25] K. D. Bayoda, N. Benard, and E. Moreau, "Nanosecond pulsed sliding dielectric barrier discharge plasma actuator for airflow control: Electrical, optical, and mechanical characteristics," *J. Appl. Phys.*, vol. 118, no. 6, p. 063301, 2015.

[26] K. D. Bayoda, N. Benard, and E. Moreau, "Elongating the area of plasma/fluid interaction of surface nanosecond pulsed discharges," *J. Electrostatics*, vol. 74, pp. 79-84, Apr. 2015.

[27] K. D. Bayoda, N. Benard, and E. Moreau, "Electrical and mechanical characteristics of nanosecond pulsed sliding dielectric barrier discharges with different electrode gaps," *J. Phys.*, vol. 646, no. 1, p. 012054, 2015.

[28] L. Graftieaux, M. Michard, and N. Grosjean, "Combining PIV, POD and vortex identification algorithms for the study of unsteady turbulent swirling flows," *Meas. Sci. Technol.*, vol. 12, no. 9, pp. 1422-1429, 2001.

<figure><img src="image_19.png" /></figure>

Eric Moreau was born in France in 1969. He received the Ph.D. degree in electrical engineering from the University of Poitiers, Poitiers, France, in 1997.

He became an Assistant Professor in 1998. He is currently a Full Professor in electrical engineering with the Centre national de la recherche scientifique, Institut Pprime, University of Poitiers. He has authored or co-authored over 100 peer-reviewed articles published in international journals and about 150 papers in conference proceedings.

His current research interests include electrohydrodynamics, mechanical phenomena in atmospheric discharges, EHD force, ionic wind, and particle precipitation, and plasma actuators for airflow control, but a part of his work is dedicated to ion-gas pumps and electrostatic precipitation of particles.

Dr. Moreau is a member of the Advisory Board of LABEX INTERACTIFS National Grant. He has been involved in several European projects CAFEDA, AVERT, OPENAIR, PLASMAERO, MARS, ORINOCO, and DRAGY and national projects COS, INCA, SINAPS, and INOPLAS.

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<figure><img src="image_20.png" /></figure>

**Antoine Debien** received the M.Sc. degree in electrical engineering and fluid mechanics and the Ph.D. degree from the University of Poitiers, Poitiers, France, in 2009 and 2013, respectively.

He held a post-doctoral position with the Prisme Laboratory, University of Orléans, Orléans, France, for one year 1/2, where he studied experimentally the effect of several types of actuators on flow separation. He is currently a Post-Doctoral Researcher with ONERA, Toulouse, France. He is also involved in airflow control by sparkjet actuators.

<figure><img src="image_21.png" /></figure>

**Nicolas Benard** was born in France in 1975. He received the Ph.D. degree in fluid dynamics in 2005.

He is currently an Associate Professor with the Centre national de la recherche scientifique, Institut Pprime, University of Poitiers, Poitiers, France, and the École nationale supérieure de mécanique et d'aérotechnique, Chasseneuil-du-Poitou, France. His main technical expertise is the development of innovative plasma actuators with particular emphasis on their definition, enhancement, and use as flow

control devices. He has particular involvement in control of separated flow by open- and closed-loop approaches, and control of flows with periodic turbulent components. He has authored over 40 full papers in the field as author or co-author and participated in about 60 international conferences related to control of laminar and turbulent flows by plasma actuation.

Dr. Benard has been involved in a number of EU projects AVERT, OPENAIR, PLASMAERO, ORINOCO, MARS, and DRAGY, and he co-ordinate the National Granted Project INOPLAS under DGA supervision.

<figure><img src="image_22.png" /></figure>

**Noureddine Zouzou** was born in Biskra, Algeria, in 1975. He received the M.Eng. degree in electrical engineering from the University of Biskra, Biskra, in 1998, the Ph.D. degree from Paul Sabatier University, Toulouse, France, in 2002, and the Habilitation degree in electrical engineering from the University of Poitiers, Poitiers, France.

He was an Assistant Professor with the Department of Electrical Engineering, National Polytechnic Institute of Toulouse, Toulouse. He is currently an Associate Professor with the Department of Elec-

tronic, Electrical, and Automatic Engineering, University of Poitiers. He has authored or co-authored over 35 peer-reviewed articles published in international journals and 70 papers in conference proceedings. His current research interests include the application of electrostatics, nonthermal plasma technology, air pollution control, space charge in solid dielectrics, and physical and chemical characterization of dielectric materials.

Dr. Zouzou is a member of the IEEE Dielectrics and Electrical Insulation Society, the IEEE Industry Applications Society, and the French Society of Electrostatics.

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## AUTHOR QUERIES

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# Nanosecond-Pulsed Dielectric Barrier Discharge Plasma Actuator for Airflow Control Along an NACA0015 Airfoil at High Reynolds Number

Eric Moreau, Antoine Debien, Nicolas Benard, and Noureddine Zouzou

**Abstract**—The goal of the present experimental study is to investigate the ability of a surface nanosecond pulsed dielectric barrier discharge plasma actuator, located at the leading edge of the airfoil, to delay flow separation along the suction side of an NACA0015 airfoil inclined at 13.5°, using a particle image velocimetry system. The free-stream velocity is equal to $U_0 = 40$ m/s, giving a chord-based Reynolds number of $Re_c = 1.33 \times 10^6$. At such conditions, the baseline flow separation occurs at about 50% of chord. On the one hand, the results show that the separation location moves progressively toward the trailing edge when the high-voltage frequency $F_{pulse}$ is increased, up to 85% of chord at 1630 Hz (reduced frequency $F_{pulse}^+ = 10.6$), which is due to the increase of the injected electrical power with $F_{pulse}$ (from 8 mW/cm at 40 Hz to 290 mW/cm at 1630 Hz). On the other hand, the use of a burst-modulated actuation (with a constant electrical power consumption equal to 145 mW/cm) does not produce the same behavior. At the lowest and highest frequencies (16 and 408 Hz, corresponding to the reduced frequencies of 0.1 and 2.65, respectively), the separation is delayed to about 60% of chord, while the most effective actuation is observed at $F_{burst} = 80$ Hz ($F_{burst}^+ = 0.53$) with a separation delaying to 83% of chord.

**Index Terms**—Airflow control, electrohydrodynamics (EHDs), plasma actuators, pulsed dielectric barrier discharge (DBD).

## I. INTRODUCTION

ACTIVE airflow control with plasma actuators is widely studied all over the world, because it could be an alternative to traditional techniques, such as the use of mechanical slats and flaps on the wings of airplane. The reduction of flow separation over curved or inclined surfaces, the drag reduction in view of fuel consumption improvement, or the manipulation of laminar-to-turbulent transition are some of the practical applications where plasma discharges demonstrated a good effectiveness, at least for in-lab conditions [1]–[4].

On the one hand, most of researches are focused on the surface single dielectric barrier discharges (DBDs) based on

two electrodes mounted on both the sides of a dielectric layer (AC-DBD) driven by an ac sine high voltage [1]. The physical mechanism behind the control effectiveness is the electrohydrodynamic (EHD) force produced inside the discharge, which can interact with the boundary layer of the incoming flow [1]–[4]. In fact, the produced ions are accelerated by the high electric field, resulting in an electric (or ionic) wind of a few meters per second very close to the wall. Consequently, this momentum transfer can dynamically affect the flow characteristics inside the boundary layer. Typically, single DBD based on two electrodes can produce mean force and electric wind velocity up to 1 mN/W and 6.5 m/s, respectively [3], [5]. In the case of an actuator array composed of several DBDs [6]–[9], velocity higher than 10 m/s has been reached [8], [9].

On the other hand, if the high voltage has a nanosecond repetitively pulsed waveform (NS-DBD), a new mechanism of control, which is based on a fast gas heating, is observed. Roupassov et al. [10] investigated the ability of a pulsed discharge to reattach a fully detached airflow along a small size airfoil for a velocity up to 100 m/s ($Re_c = 9.6 \times 10^5$). Little et al. [11] used a nanosecond plasma actuator positioned in counterflow direction to effectively manipulate the flow over a profile at $Re_c = 1 \times 10^6$. Recently, Kato and Breitsamter [12] investigated the effect of such an actuator for Reynolds numbers up to $2.3 \times 10^6$. In the different works on flow control by NS-DBD [10]–[15], the authors assume that the main mechanism is not the EHD force but a rapid variation of the surrounding gas temperature. This abrupt change of gas temperature, which is mainly caused by vibrational excitation of air molecules, induces a localized pressure wave propagating from the air-exposed electrode edge at sound speed [16]–[27]. Moreover, the pressure wave magnitude is strongly linked to the electrical deposited energy within the discharge. For instance, Bayoda et al. [25] measured a pressure gradient of 500 Pa for a discharge energy of 0.3 mJ/cm. However, values greater than 10000 Pa for an input energy of about 10 mJ/cm of electrode length have been reported in [21].

The present contribution aims at completing the recent literature with the specific case of flow separation at mid-chord of an NACA0015 model, at an angle of attack of 13.5° and $Re_c = 1.3 \times 10^6$. The goal of the plasma actuation, which is located at the leading edge of the airfoil, is to delay a turbulent

Manuscript received January 29, 2016; revised May 6, 2016 and July 25, 2016; accepted August 23, 2016. This work was supported in part by the 7th Framework Program of European Commission PlasmAero under Grant 234201 and in part by the French Government Program Investissements d'Avenir, LABEX INTERACTIFS, under Grant ANR-11-LABX-0017-01.

The authors are with the Centre National de la Recherche Scientifique, Institut Pprime, ISAE/ENSMA, University of Poitiers, 86962 Chasseneuil-du-Poitou, France (e-mail: eric.moreau@univ-poitiers.fr).

Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/TPS.2016.2603226

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<figure><img src="image_23.png" /><figcaption>Fig. 1. (a) Schematics of the plasma actuator and (b) its location at the leading edge of the airfoil.</figcaption></figure>

flow separation at the mid-chord. This paper is divided into four main sections. In Section II, the experimental setup is presented. In Section III, the electrical characteristics of the surface nanosecond DBD are discussed. In Section IV, the baseline flow is studied by particle image velocimetry (PIV) measurements. In Section V, the effects of the pulsed actuation on the flow are investigated and presented. Finally, Section VI provides the conclusion.

## II. EXPERIMENTAL SETUP

The present experiments have been conducted in a subsonic closed wind tunnel. This facility operates at ambient conditions and has a large test section (2.6 m high, 2.4 m in width, and 6 m in length). The maximum freestream velocity can reach 50 m/s with a turbulence level of about 0.5%.

A 2-D model with an NACA0015 profile is mounted horizontally in the test section between two profiled end plates that contribute to a thinning of the boundary layer developing on the side walls. These two end-plates are 5 m in length in the free stream direction and they cross the test section from bottom to top, reducing the section width to 1.2 m. The model has then a spanwise length of $L = 1.2$ m and a chord length equal to $c = 0.5$ m. The plasma actuator is located at the leading edge of the airfoil. It is composed of a 10-mm-wide air-exposed active electrode and a 20-mm-wide encapsulated grounded electrode placed below the dielectric. These electrodes are flush mounted on both the sides of a 180-µm-thick dielectric of Kapton, as shown in Fig. 1(a), and this actuator is stuck on the wall of the leading edge of the airfoil [Fig. 1(b)]. There is no gap between both electrodes, except the dielectric thickness. The edge of the air-exposed electrode is located at 1% of chord; the produced pressure wave will start from this location. The actuation in spanwise is equal to 57 cm, corresponding to about 50% of the airfoil span, this length being restricted by the electrical power that can be delivered by the power supply as it will be discussed later in this paper.

Time-averaged PIV measurements have been undertaken in order to access the whole velocity field on the suction side of the airfoil. Velocity measurements have been performed in two overlapping planes using two charge-coupled device (CCD) cameras (Pulnix, RM-4200CL, 2048 × 2048 pixels²), resulting in a wide flow field of view that starts 70 mm upstream of the leading edge of the profile and finishes at 120 mm downstream of the trailing edge (Fig. 2). The pixel size is equal to 172 µm

and the acquisition frequency is about 7 Hz. The Nd-YAG laser sheet (Quantel Big Sky Laser, 2 × 191 mJ) is located at the middle of the model spanwise, and both the cameras are equipped with a bandpass lens filter (Nikon Nikkor 50 mm, f/1: 1, 2, and bandwidth 532 ± 10 nm) in order to cancel the parasitic light emitted by the discharge. The PIV posttreatment consists of a multipass adaptative correlation (from 64 × 64 down to 16 × 16 pixels²) with an overlap of 50%, resulting in a vector every 1.4 mm. Moreover, 300 instantaneous flow fields are recorded to compute a time-averaged flow field with converged first-order flow quantities.

Two coordinate systems are used to characterize the flow field. The first one, {$O, x/c, y/c$}, is normalized by the chord length of the profile. Its origin is located at the leading edge of the profile, as shown in Fig. 2. The second system {$O, x_c/c, y_c/c$} is a curvilinear coordinate system that follows the suction side of the profile. Subsequently, the $x_c$-axis is always tangent to the profile wall when the $y_c$-axis is perpendicular to the wall. In this paper, the second system will be mainly used, more particularly for velocity profiles.

Using the first coordinate system, the presence of vortex structures beyond the instantaneous separation point is investigated by using a $\Gamma_1$ criterion [28]. This criterion is dedicated to the identification of vortex center superposed on a small-scale turbulent velocity field. To characterize this criterion at a point P, the sinus of the angle between the velocity vector at the point M and the radius vector PM is computed and averaged on an area $S$ surrounding P. Near a vortex center, $|\Gamma_1|$ is close to 1, while $|\Gamma_1|$ is close to zero in a freestream flow. In this paper, to overcome the multiple detection of a same vortex center, the local maximum of $|\Gamma_1|$ is identified in the areas satisfying $|\Gamma_1| > 0.8$.

<figure><img src="image_24.png" /><figcaption>Fig. 2. Flow field view using two cameras and coordinate systems.</figcaption></figure>

## III. ELECTRICAL CHARACTERISTICS OF THE PULSED DBD

As shown in Fig. 3, the surface discharge is energized with the help of an high-voltage solid-state pulser

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<figure><img src="image_25.png" /><figcaption>Fig. 3. Schematic of the electrical experimental setup.</figcaption></figure>

(DEI, model PVX-4110, ±10 kV, ±30 $A_{peak}$) supplied by a dc power supply (Matsuda, model 10P30, ±10 kV, ±30 mA) and controlled by a digital pulse generator (Stanford, model DG645). The system can provide nanosecond high voltage pulses with the following characteristics: high voltage magnitude up to ±10 kV and rise and decay times of about 50 ns with adjustable width (from 100 ns to dc) and frequency up to 10 kHz. The current and voltage waveforms are recorded using a digital oscilloscope (Lecroy, model WaveSurfer 424, 200 MHz bandwidth) connected to dedicated probes. The current probe is a fast current transformer (Bergoz, model CT-D1.0, 500 MHz bandwidth). The voltage between both electrodes is measured with a high voltage probe (Lecroy, model PPE20kV, 100 MHz bandwidth).

In this paper, the discharge is supplied by a high voltage that has a peak value equal to $V_p = 9$ kV, as shown in Fig. 4(a). Certainly because the electrode spanwise is high (57 cm), the power supply is slightly undersized and not well adapted to the actuator capacitance. Then, the voltage rise time is higher than 50 ns and is equal to about 100 ns. The voltage pulse width (50% of maximum voltage) is equal to 250 ns. The current is composed of two current peaks; the first one occurs when the voltage increases ($t \approx 60$ ns). Its magnitude is equal to about 0.6 A/cm (current value divided by the electrode spanwise length). This current peak is due to the deposition of positive electrical charges on the dielectric wall and the ignition of a streamerlike discharge, as illustrated by the iCCD view [positive discharge, *P* in Fig. 4(b)]. The discharge extension on the dielectric surface is limited to about 10 mm. Then, during the voltage plateau ($100 \le t \le 250$ ns), the current level decreases rapidly and the discharge is in a silent period, because there is no electric field across the dielectric surface. When the high voltage decreases ($t > 250$ ns), the potential difference between the air-exposed electrode and the positively charged surface is sufficient to ignite a new discharge responsible for a negative current peak occurring during the fall time of the voltage. Even if the shape of both current peaks is similar, the discharge regime associated with each one is completely different. The positive discharge is filamentary and can extend up to about 10 mm, while the negative discharge is a glowlike one [Fig. 4(b), view *N*]; this last one is composed of 3-mm-long ionized channels that start from individual spots and propagates with a plume shape.

<figure><img src="image_26.png" /><figcaption>Fig. 4. (a) Waveform of the applied high voltage and the associated discharge current. (b) Top iCCD views of the positive (*P*) and negative (*N*) discharge. Red dotted line: active electrode edge.</figcaption></figure>

<figure><img src="image_27.png" /><figcaption>Fig. 5. Electrical power consumption and energy versus time ($V_p = 9$ kV and $F_{pulse} = 1630$ Hz).</figcaption></figure>

The typical waveforms of the electric power and energy versus time are shown in Fig. 5. The signal of the electric power is composed of two peaks, during the rising and falling periods of the voltage. The positive peak is higher than 3.5 kW/cm during about 25 ns. During this first peak, the electrical power comes from the power supply and energizes the discharge. During the falling period of the voltage, the electrical power consumption is negative, meaning that the power is induced by the electric field due to the space charges remained at the dielectric surface since the positive discharge; the power

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<figure><img src="image_28.png" /><figcaption>Fig. 6. Electrical power consumption and energy per voltage pulse versus frequency $F_{\text{pulse}}$ ($V_p = 9$ kV).</figcaption></figure>

<figure><img src="image_29.png" /><figcaption>Fig. 7. Time-averaged flow field of the horizontal normalized velocity component $U/U_0$ for the baseline configuration ($\alpha = 13.5^\circ$ and $U_0 = 40$ m/s).</figcaption></figure>

supply is then a receptor. Subsequently, the difference of area between the areas of the both peaks is proportional to the time-averaged power dissipated by the actuator during a complete cycle of the voltage. By integrating the power curve, one can plot the energy curve. From Fig. 5, one can deduce that the energy consumed by one high voltage pulse is here equal to 0.16 mJ/cm. At such a condition, the gradient of the pressure wave induced by each voltage pulse is equal to about 300 Pa.

Fig. 6 shows the energy per voltage pulse and the power consumption as a function of the high voltage frequency $F_{\text{pulse}}$. It highlights that the energy per pulse is not quite constant; it decreases slightly with $F_{\text{pulse}}$, certainly due to the power supply limitations. The time-averaged electrical power consumption increases with $F_{\text{pulse}}$ from 16 mW/cm at 100 Hz to 450 mW/cm at 3 kHz. One can notice that these power consumption values are very small compared with the ones of others actuators used for airflow control.

## IV. BASELINE FLOW

In this paper, the angle of attack is set to $\alpha = 13.5^\circ$ and the free-stream velocity to $U_0 = 40$ m/s, resulting in a chord-based Reynolds number of $Re_c = 1.33 \times 10^6$. Fig. 7 shows the time-averaged velocity field of the baseline flow without any control (the air-exposed electrode is present but it is not actuated). It highlights the presence of a flow separation that occurs at about mid-chord, resulting in a wide recirculation zone downstream of the separation. To determine precisely the mean separation point, near-wall time-averaged

<figure><img src="image_30.png" /><figcaption>Fig. 8. Near-wall velocity profiles at different locations from 45% to 51% of chord.</figcaption></figure>

<figure><img src="image_31.png" /><figcaption>Fig. 9. Location of the separation point versus time, the time between two successive fields being equal to 0.14 s ($1/f_{\text{PIV}}$ with $f_{\text{PIV}} = 7$ Hz).</figcaption></figure>

<figure><img src="image_32.png" /><figcaption>Fig. 10. PIV results for the baseline configuration ($\alpha = 13.5^\circ$ and $U_0 = 40$ m/s). Detection of vortices by a $\Gamma_1$ criterion, computed with a $6 \times 6$ window and a threshold equal to 0.8.</figcaption></figure>

velocity profiles have been plotted in the coordinate system $\{O, x_c/c, y_c/c\}$ at different locations around 50% of chord, as shown in Fig. 8. Up to 47% of chord ($x_c/c \le 0.47$), the near-wall velocity is positive and it decreases toward zero at the wall; the flow is not detached. From $x_c/c = 0.49$, the near-wall velocity becomes negative, highlighting the presence of a recirculation zone. One can then assume that the mean separation point is located at $x_c/c = 0.48$, where the velocity value of the first point of the profile (at $y_c/c = 0.0028$) is equal to zero.

Moreover, the instantaneous separation point has also been determined for all the 300 velocity vector fields (Fig. 9). The results indicate that the location of the flow detachment fluctuates from $x_c/c = 0.22$ to $x_c/c = 0.84$ (in the extreme cases) with a mean separation occurring at $x_c/c = 0.48$ and an rms value equal to 0.13c. That highlights that the flow separation is strongly unsteady.
<figure><img src="image_33.png" /><figcaption>Fig. 11. Profiles of the longitudinal component of the velocity for the baseline flow and for different actuation frequency values $F_{\text{pulse}}^{+}$, at different locations along the airfoil chord $x_{c}/c$ ($\alpha = 13.5^{\circ}$ and $U_{0} = 40$ m/s). The velocity indicated for the profiles corresponds to $0.05 \times U/U_{0}$.</figcaption></figure>

<figure><img src="image_34.png" /><figcaption>Fig. 12. Profiles of the longitudinal component of the velocity for the baseline flow and for different actuation frequency values at $x_{c}/c = 1.15$ ($\alpha = 13.5^{\circ}$ and $U_{0} = 40$ m/s).</figcaption></figure>

<figure><img src="image_35.png" /><figcaption>Fig. 14. Number of detected clockwise and anticlockwise vortices as a function of the actuation frequency ($\alpha = 13.5^{\circ}$ and $U_{0} = 40$ m/s).</figcaption></figure>

<figure><img src="image_36.png" /><figcaption>Fig. 13. Position of the flow separation versus actuation frequency ($\alpha = 13.5^{\circ}$, $U_{0} = 40$ m/s, and $V_{p} = 9$ kV).</figcaption></figure>

In Fig. 10, a $\Gamma_{1}$ criterion [28] has been used to detect the center of the vortices embedded in the turbulent separated shear layer and in the near-field wake of the profile. The black crosses correspond to the vortices with a clockwise rotation. They form usually at the separation location and

they are convected by the flow. As shown in Fig. 10, the first vortices are located at about 25% of chord when the mean separation occurs at mid-chord, confirming one more time that the separation point moves versus time. The red stars represent the vortices with a counterclockwise rotation. They are primarily observed at the trailing edge due to the interaction with the flow incoming from the pressure side of the airfoil, but some are also identified in the recirculation zone due to small-scale flow structures. On the 300 flow field snapshots, 1398 clockwise vortices and 603 counterclockwise vortices have been detected, respectively. The flow separation is then at the origin of an intense production of vortices that are convected by the flow, resulting in a flapping of the vortex street downstream of the separation.

## V. FORCED FLOW

Two different actuation types are used. In the first set of experiments, the operating frequency $F_{\text{pulse}}$ is varied from 40 up to 1630 Hz, corresponding to a reduced frequency range of $0.26 \le F_{\text{pulse}}^{+} \le 10.6$. This reduced frequency is estimated with the following relation: $F_{\text{pulse}}^{+} = L_{\text{sep}} \times F_{\text{pulse}}/U_{0}$,

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<figure><img src="image_37.png" /><figcaption>Fig. 15. Profiles of the longitudinal component of the velocity for the baseline flow and for different actuation frequency values $F_{\text{burst}}^{+}$, at different locations along the airfoil chord $x_{c}/c$ ($\alpha = 13.5^{\circ}$ and $U_{0} = 40$ m/s). The velocity indicated for the profiles corresponds to $0.05 \times U/U_{0}$.</figcaption></figure>

where $L_{\text{sep}}$ is the mean separation length, i.e., 0.26 m in our case. This type of actuation is considered as a pulsed mode. In this mode, the power injected inside the boundary layer increases with the frequency $F_{\text{pulse}}$, from 8 mW/cm at 40 Hz to 290 mW/cm at 1630 Hz. In a second set of experiments, the operating frequency $F_{\text{pulse}}$ is set to a constant value as well as the voltage amplitude ($F_{\text{pulse}}$ = 1630 Hz and $V_{p}$ = 9 kV). This pulsed signal is then burst-modulated by a gate signal at a lower frequency $F_{\text{burst}}$ and a duty cycle of 50%, that is the burst-modulated mode. Here, $F_{\text{burst}}$ will vary from 16 to 408 Hz, corresponding to a reduced frequency range of $0.1 \le F_{\text{burst}}^{+} \le 2.65$, with $F_{\text{burst}}^{+} = L_{\text{sep}} \times F_{\text{burst}}/U_{0}$. This second mode of actuation has two advantages: the power injected in the flow is constant whatever $F_{\text{burst}}$ (it is equal to 145 mW/cm), and thus, perturbations at low frequency can be induced with a higher energy than the pulsed mode.

### A. Pulsed Mode Actuation

Fig. 11 shows the profiles of the velocity longitudinal component along the suction side, from 50% to 98% of chord, without actuation and with an actuation in the pulsed mode. First, in the absence of actuation [Fig. 11 (black curve)], the velocity profiles present negative values close to the wall, due to the flow separation. Second, it highlights that for $F_{\text{pulse}}^{+} = 1.06$, the negative value of the near-wall flow is increased, meaning that the recirculation zone downstream of the separation point has been enlarged. On the contrary, for $F_{\text{pulse}}^{+} \ge 2.12$, the velocity gain near the wall increases progressively with the $F_{\text{pulse}}^{+}$ value. This is also confirmed by the velocity profiles downstream of the trailing edge of the airfoil ($x_{c}/c$ = 1.15) in Fig. 12. On one hand, for $F_{\text{pulse}}^{+} = 1.06$, the velocity deficit in the wake is lower than $0.1 \times U_{0}$ when it is equal to $0.15 \times U_{0}$ for the baseline flow. On the other hand, the smallest velocity at $F_{\text{pulse}}^{+} = 8.5$ is equal to $0.4 \times U_{0}$, confirming that the size of the recirculation zone has been strongly reduced.

Fig. 13 shows the separation point location versus actuation frequency. It indicates that in the present flow conditions, the discharge has a negative effect when it is supplied with a

voltage whose reduced frequency is close to one. On the contrary, the separation point moves progressively toward the trailing edge when the high voltage frequency $F_{\text{pulse}}$ is increased. In the best case ($F_{\text{pulse}}^{+} = 10.6$, i.e., $F_{\text{pulse}} = 1630$ Hz), the separation point is delayed up to 85%. This result shows that the actuation effect is enhanced when the deposited energy is increased. Here, the flow is more sensitive to the amount of deposited energy than to its frequency. This can explain why a reduced frequency equal to unity is not more effective, while such a unity value has been reported many times in the literature as the most effective for reattaching separated turbulent flows.

Finally, Fig. 14 shows the number of detected vortices versus actuation frequency. It confirms the previous results: on one hand, when $F_{\text{pulse}}^{+} \approx 1$, the discharge does not allow us to delay the separation point location and the vortex numbers rise up. On the other hand, when the frequency actuation is increased, the flow is partially reattached and the vortex number decreases, meaning that the flow is stabilized by the actuation.

### B. Burst-Modulation Actuation

In the second set of experiments, the operating frequency $F_{\text{pulse}}$ is set to 1630 Hz ($F_{\text{pulse}}^{+} = 10.6$ and $V_{p}$ = 9 kV), and this pulsed signal is burst-modulated at a frequency $F_{\text{burst}}$. With this actuation mode, the electrical power is constant (145 mW/cm) whatever the burst frequency is; it is then possible to determine the frequency effect on the control authority, at a constant power consumption. Such a control method has already been implemented in context of nanosecond pulsed DBD. In [14], the train of pulses at 14 kHz with a variable frequency modulation has been applied to suppress a leading-edge stall at $Re_{c}$ = $1.4 \times 10^{6}$. It was shown that the lift can be increased by the DBD in all the cases and a frequency dependence was highlighted; the lift was increased for an increasing reduced frequency but it was saturated for reduced frequency larger than one. Fig. 15 shows the velocity profiles of the baseline flow and the forced flow for several $F_{\text{burst}}^{+}$ values. It highlights that the actuation results
<figure><img src="image_38.png" /><figcaption>Fig. 16. Profiles of the longitudinal component of the velocity for the baseline flow and for different actuation frequency values at $x_c/c = 1.15$ ($\alpha = 13.5^\circ$ and $U_0 = 40$ m/s).</figcaption></figure>

<figure><img src="image_39.png" /><figcaption>Fig. 17. Position of the flow separation versus the burst-modulation frequency $F_{\text{burst}}^{+}$ ($\alpha = 13.5^\circ$, $U_0 = 40$ m/s, $V_p = 9$ kV, $f_{\text{pulse}} = 1630$ Hz, and $F_{\text{burst}}^{+}$ varying from 0.1 to 2.65).</figcaption></figure>

in a velocity gain whatever the frequency value, and that the maximum velocity gain in the boundary layer is obtained at $F_{\text{burst}}^{+} = 0.84$. That is confirmed by Fig. 16 that shows velocity profiles in the wake, at $x_c/c = 1.15$. As previously (Fig. 12), the minimum velocity of the baseline flow is equal to $0.15 \times U_0$. With actuation at $F_{\text{burst}}^{+} = 0.1$ and $2.65$, the minimum velocity increases up to $0.28 \times U_0$ and the maximum effect is obtained for $F_{\text{burst}}^{+} = 0.84$ for which the minimum velocity is equal to $0.41 \times U_0$.

The location of the separation point $X_{\text{sep}}$ versus $F_{\text{burst}}^{+}$ is shown in Fig. 17, in order to determine accurately the most effective frequency. It shows that the influence of the frequency on the separation point delay is not monotonous; at $F_{\text{burst}}^{+} = 0.1$, the separation is delayed at $x_c/c = 0.57$. By increasing the frequency up to $F_{\text{burst}}^{+} = 0.53$, the flow separation is progressively delayed up to $x_c/c = 0.83$, that is the most effective case. Finally, when the frequency is increased above $0.53$, the control authority decreases; at $F_{\text{burst}}^{+} = 2.65$, the flow separation occurs at $x_c/c = 0.62$. These results show that there is a strong frequency effect, because the power injected in the flow is constant, whatever the $F_{\text{burst}}^{+}$ value is, and that the most effective actuation is observed for a frequency range $0.5 \le F_{\text{burst}}^{+} \le 0.8$. That is confirmed by Fig. 18 in which the number of vortices is minimum in this

frequency range, meaning that the separation is delayed and that the flow is stabilized.

## VI. CONCLUSION

The goal of the present experimental study was to investigate the ability of surface nanosecond pulsed DBD plasma actuator to delay flow separation along the suction side of an NACA0015 airfoil. A 2-D profile with an NS-DBD actuator located at 1% of the chord length has been mounted horizontally in a large subsonic wind tunnel. The angle of attack was set to $13.5^\circ$ and the free-stream velocity to $U_0 = 40$ m/s, resulting in a chord-based Reynolds number of $Re_c = 1.33 \times 10^6$. In such conditions, the baseline flow separation occurs at 48% of chord, resulting in a large recirculation zone downstream of the separation. However, it has been demonstrated that the location of the flow detachment widely fluctuates from $x_c/c = 0.22$ to $x_c/c = 0.85$, with a mean separation point located at $x_c/c = 0.48$.

Two different actuation types have been used. In a first set of experiments, the frequency $F_{\text{pulse}}^{+}$ was varied from 0.26 to 10.6, corresponding to electrical power consumption from 8 to 290 mW/cm. With such an actuation, the separation location moves progressively toward the trailing edge when the high voltage frequency $F_{\text{pulse}}$ is increased. In the best case ($F_{\text{pulse}}^{+} = 10.6$, i.e., $F_{\text{pulse}} = 1630$ Hz), the separation point is delayed up to 85%. This result shows that the actuation is more effective when the electrical power consumption increases, but no frequency effect can be really highlighted, because one cannot separate the effect of the frequency with the one of the injected power.

In a second set of experiments, the operating frequency $F_{\text{pulse}}^{+}$ was set to a constant value and the pulsed high voltage was burst-modulated at a lower frequency $F_{\text{burst}}^{+}$ with $0.1 \le F_{\text{burst}}^{+} \le 2.65$. With this actuation mode, the power injected in the flow is constant whatever $F_{\text{burst}}^{+}$ (145 mW/cm); it is then possible to determine the frequency effect on the control authority. Indeed, in this case, the influence of the frequency is clearly observable and its effect on the separation point delay is not monotonous. On one hand, at $F_{\text{burst}}^{+} = 0.1$ and $2.65$ (smallest and highest frequencies), the separation was

<figure><img src="image_40.png" /><figcaption>Fig. 18. Number of detected clockwise and anticlockwise vortices as a function of the actuation frequency $F_{\text{burst}}^{+}$ ($\alpha = 13.5^\circ$ and $U_0 = 40$ m/s).</figcaption></figure>

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delayed at about 60% of chord. On the other hand, in the frequency range $0.5 \le F_{\text{burst}}^{+} \le 0.8$, the flow separation occurred downstream of $x_c/c = 0.80$ with a most effective effect at $F_{\text{burst}}^{+} = 0.53$ with a delay of the separation location up to $x_c/c = 0.83$.

To conclude, this paper showed that NS-DBD plasma actuators can compete with other types of active control devices to reattach separated flow at high Reynolds number, with a very low power consumption (145 mW/cm, corresponding to a total consumption of 8.26 W for a 57-cm-long electrode) and a weak induced pressure wave (300 Pa). However, the present investigation in large-scale wind tunnel was not suitable to identify the control mechanism behind the flow separation mitigation. Indeed, it is not possible to assume that the flow manipulation is due to an increase of turbulence inside the boundary layer or to an interaction between the induced pressure waves and natural flow structures. Moreover, one cannot explain why the maximum effect is obtained for $0.5 \le F_{\text{burst}}^{+} \le 0.8$. Meanwhile, a last remark can be done. If $F_{\text{burst}}^{+}$ had been based on the airfoil chord $c = 0.5$ m instead of the separation length $L_{\text{sep}} = 0.26$ m, then the optimum $F_{\text{burst}}^{+}$ would have been equal to one, this value being the usual most effective reduced frequency for the control of leading edge separation. That could signify that the most effective physical actuation frequency is always the same, whatever the separation location is.

Thus, future experiments will focus on this aspect by a close inspection of the flow conditions in the vicinity of the boundary wall flow.

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<figure><img src="image_41.png" /></figure>

Eric Moreau was born in France in 1969. He received the Ph.D. degree in electrical engineering from the University of Poitiers, Poitiers, France, in 1997.

He became an Assistant Professor in 1998. He is currently a Full Professor in electrical engineering with the Centre national de la recherche scientifique, Institut Pprime, University of Poitiers. He has authored or co-authored over 100 peer-reviewed articles published in international journals and about 150 papers in conference proceedings.

His current research interests include electrohydrodynamics, mechanical phenomena in atmospheric discharges, EHD force, ionic wind, and particle precipitation, and plasma actuators for airflow control, but a part of his work is dedicated to ion-gas pumps and electrostatic precipitation of particles.

Dr. Moreau is a member of the Advisory Board of LABEX INTERACTIFS National Grant. He has been involved in several European projects CAFEDA, AVERT, OPENAIR, PLASMAERO, MARS, ORINOCO, and DRAGY and national projects COS, INCA, SINAPS, and INOPLAS.

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<figure><img src="image_42.png" /></figure>

**Antoine Debien** received the M.Sc. degree in electrical engineering and fluid mechanics and the Ph.D. degree from the University of Poitiers, Poitiers, France, in 2009 and 2013, respectively.

He held a post-doctoral position with the Prisme Laboratory, University of Orléans, Orléans, France, for one year 1/2, where he studied experimentally the effect of several types of actuators on flow separation. He is currently a Post-Doctoral Researcher with ONERA, Toulouse, France. He is also involved in airflow control by sparkjet actuators.

<figure><img src="image_43.png" /></figure>

**Nicolas Benard** was born in France in 1975. He received the Ph.D. degree in fluid dynamics in 2005.

He is currently an Associate Professor with the Centre national de la recherche scientifique, Institut Pprime, University of Poitiers, Poitiers, France, and the École nationale supérieure de mécanique et d'aérotechnique, Chasseneuil-du-Poitou, France. His main technical expertise is the development of innovative plasma actuators with particular emphasis on their definition, enhancement, and use as flow

control devices. He has particular involvement in control of separated flow by open- and closed-loop approaches, and control of flows with periodic turbulent components. He has authored over 40 full papers in the field as author or co-author and participated in about 60 international conferences related to control of laminar and turbulent flows by plasma actuation.

Dr. Benard has been involved in a number of EU projects AVERT, OPENAIR, PLASMAERO, ORINOCO, MARS, and DRAGY, and he co-ordinate the National Granted Project INOPLAS under DGA supervision.

<figure><img src="image_44.png" /></figure>

**Noureddine Zouzou** was born in Biskra, Algeria, in 1975. He received the M.Eng. degree in electrical engineering from the University of Biskra, Biskra, in 1998, the Ph.D. degree from Paul Sabatier University, Toulouse, France, in 2002, and the Habilitation degree in electrical engineering from the University of Poitiers, Poitiers, France.

He was an Assistant Professor with the Department of Electrical Engineering, National Polytechnic Institute of Toulouse, Toulouse. He is currently an Associate Professor with the Department of Elec-

tronic, Electrical, and Automatic Engineering, University of Poitiers. He has authored or co-authored over 35 peer-reviewed articles published in international journals and 70 papers in conference proceedings. His current research interests include the application of electrostatics, nonthermal plasma technology, air pollution control, space charge in solid dielectrics, and physical and chemical characterization of dielectric materials.

Dr. Zouzou is a member of the IEEE Dielectrics and Electrical Insulation Society, the IEEE Industry Applications Society, and the French Society of Electrostatics.

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