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Airflow control by non-thermal plasma actuators

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INSTITUTE OFPHYSICSPUBLISHING JOURNAL OFPHYSICSD: APPLIEDPHYSICS

J. Phys. D: Appl. Phys.40(2007) 605636 doi:10.1088/0022-3727/40/3/S01

Airflow control by non-thermal plasma

actuatorsEric Moreau

Laboratoire dEtudes Aerodynamiques, Groupe Electrofluidodynamique, CNRS 6609,

University of Poitiers, Bd Curie, 86962 Futuroscope, France

E-mail:[email protected]

Received 24 July 2006, in final form 23 October 2006Published 19 January 2007Online atstacks.iop.org/JPhysD/40/605

AbstractActive flow control is a topic in full expansion due to associated industrialapplications of huge importance, particularly for aeronautics. Among allflow control methods, such as the use of mechanical flaps, wall synthetic jetsor MEMS, plasma-based devices are very promising. The main advantagesof such systems are their robustness, simplicity, low power consumption andability for real-time control at high frequency. This paper is a review of theworldwide works on this topic, from its origin to the present. It is dividedinto two main parts. The first one is dedicated to the recent knowledgeconcerning the electric wind induced by surface non-thermal plasmaactuators, acting in air at atmospheric pressure. Typically, it can reach8 m s1 at a distance of 0.5mm from the wall. In the second part, worksconcerning active airflow control by these plasma actuators are presented.

Very efficient results have been obtained for low-velocity subsonic airflows(typicallyU 30ms1 and Reynolds number of a few 105), and

promising results at higher velocities indicate that plasma actuators could beused in aeronautics.

(Some figures in this article are in colour only in the electronic version)

1. Introduction

Active airflow control consists of manipulating a flow to affecta desired change [1]. For example, efficient flow controlsystems could modify the laminarturbulent transition insidetheboundarylayer, toprevent or to induceseparation, toreduce

the drag and to enhance the lift of airfoils, to stabilize or tomix airflow in order to avoid unsteadiness which generatesunwanted vibrations, noise and energy losses. This is of largetechnological importance for industries where internal andexternal airflows occur, and more specifically for aeronautics.

In order to manipulate a free airflow, three mainphenomena may be modified: the laminar-to-turbulenttransition, the separation and the turbulence. Delayinglaminar-to-turbulent transition of a boundary layer has a lotof advantages. For instance, skin-friction drag of a laminarboundary layer may be in certain conditions one order ofmagnitude lower than turbulent drag. For an aircraft, reduceddrag means reduced fuel cost, longer range and higher speed

[1]. To delay this transition, wall suction devices and MEMSmay be effective. Flow separation may be illustrated by theexample of a plane wing [2]. The maximum lift and stall

characteristics of a wing affect many performance aspects ofaircraft, including take-off and landing distance for instance.Thelift at a givenangleof attackcan be increasedby increasingcamber, but the maximum achievable lift is limited by theability of a flow to follow the curvature of the airfoil. When itcannot, the flow separates. One solution to prevent separation

is touse a leading edgeslat, trailingedgeflaps orwall syntheticjets. The last aerodynamic phenomenon is turbulence. Anincrease in turbulence can lead to better flow mixing, in thecase of mixing layers for example. A decrease in turbulencecan play a fundamental role in aerodynamic noise reduction.

For a few years, the topic of active flow control has beengrowing constantly. For instance, in 2002 the well-knownAmerican Institute of Aeronautics and Astronautics (AIAA)held a conference especially dedicated to this subject calledFlow Control Conference. The third and last one was heldat San Francisco in June 2006. The book of Gad-El-Hak [1],intended for engineers and researchers, is a very clear reviewof flow control.

Among all the active methods, a new and originaltechnology using non-thermal surface plasmas is in fullexpansion. Indeed, although mechanical devices may be

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Figure 1.2D visualization of a manipulated airflow along a flatplate.

effective, they have some drawbacks. In particular, they arecomplicated, addweight, havevolumeandaresources of noise

and vibration. Moreover, they are composed of mechanicalparts that wear away and that may break down. Consequently,plasmaactuatorsarenow in full expansion because they do notexhibit all these drawbacks. These plasma actuators consist of

using the discharge-induced electricwind within the boundary

layer to modify its properties and then to actively manipulatethe airflow. In most of the cases, the actuator is composed of

at least two electrodes flush-mounted at the wall of the profilebetween which a high voltage is applied, resulting in a coldplasma sheet. In the case of a dc discharge for example, theactuator is extremely simple: it consists of two low-diameter

wires flush-mounted on the surface between which a dc highvoltage is applied. In ambient air, a corona is formed aroundthe lowest diameter electrode (usually the positive one) and an

electric wind is created, tangentially to the wall. Under certainconditions, the plasma may extend to the second electrode.The aim of using electric wind is in most cases to acceleratethe airflow tangentially and very close to the wall in order

to modify the airflow profile within the boundary layer. Asan example, figure1shows a manipulated airflow along a flatplate. In theabsenceofdischarge, thesmokewire ishorizontal,

as the streamlines. When the discharge is established, theelectric wind induces a depression at the anode, resulting ina deviation of the smoke wire and the airflow is acceleratedinside the plasma area.

The main advantage of this process is that it directly

converts electric energy into kinetic energy without involvingmoving mechanical parts. Thus it may be considered tobe a very simple MEMS. Secondly, its response time isvery short and enables a real-time control at high frequency.

Its disadvantage is the low efficiency of energy conversion.

Surface discharges may also modify the gas properties at thewall, such as density or viscosity. However it seems that this

effect is negligible in the case of velocities below 30m s1.Indeed, all the authors consider only the effect of the electricwind, exceptedfor high velocities. This pointwill bediscussedat the end of this paper.

Before 2000, very few works were published on this

subject. In fact, preliminary research was started in the 1950sand a few patents were developed, in Europe [3]and the USA[4, 5]. However, the first scientific papers were not published

before the 1968 by Velkoff and Ketchman [6]and by Yabeet alin 1978 [7]. In the 1980s, a few papers were published([8,9]for instance) but the topic really emerged in the middle

of the 1990s. During that period, several research groupsworked on airflow control by dc surface corona discharges,experimentally [10, 11]and theoretically [1215].

At the end of the 1990s, two groups started to work morecontinuously on this subject. The first group included peoplefrom the University of Poitiers (France) and the University ofBuenos Aires (Argentina). At this period, this group focusedon the study of dc coronas for airflow applications. They

published a dozen papers between 1999 and 2002 ([1628]for instance). The second group was the group directed byJ R Roth, from the University of Tennessee, who worked withthe NASA Langley Research Center, USA. In 1992, J R Rothsgroup perfected and developed a new kind of surface plasma,

primarily dedicated to decontamination. Realizing that thisdischarge could induce a secondary airflow of several m s1,theyproposedtouseitforairflowcontrolby1994. In1998theypublished their first results [29,30]. This new discharge was a

surfacedielectricbarrier discharge(DBD), calledOAUGDPTM

and was protected by a US patent since 1995 [31]. It is clearthat this surface plasma has considerably influenced researchon airflow control by plasmas because the simplicity of its

use allowed many researchers in aerodynamics to work onthis subject, without necessarily being a specialist in plasmageneration.

Therefore, one can say that airflow control by a plasmaactuator was really born in 2000. Several papers were writtenfor the general audience, such as in Air and Cosmos in

France[32,33]. Asanexample, ifoneconsiders only theworkspresented at the AIAA meetings, three papers were publishedin 2000 while about fifteen papers were published in 2003 andas many in 2004. At that time, several major groups wereknown for their works on this topic: University of Tennesseewith NASA Langley (USA), University of Poitiers (France)

with University of Buenos Aires (Argentina), University of

Notre-Dame with US Air Force Academy (USA), Universityof Moscow (Russia). On the one hand, since 2005, the subjecthasbeen growing considerably, andabout 30 groupsnow workon plasmas for aerodynamic applications in Europe and the

USA, and a few others worldwide. As a result, about 100150papers have been published to date. On the other hand, thereis one particularity: in most cases US groups use actuatorsbased on surface DBDs, following Roths technique, whereasothers works sometimes use the DBD actuator or sometimes

the corona one.In this paper, the goal is to present a review covering the

worldwide development of plasma actuators, from their originto the present. It is divided into two main parts.

The first part deals with the electrical and mechanicalcharacterization of plasmaactuators in theabsence of a free airstream. Indeed, although many researchers study the effects

of plasma actuators on airflow, only a few study the discharge-induced electric wind in the absence of free airflow. However,that part seems crucial to be able to optimize and to well knowthe mechanical effects of such actuators in order to use themefficientlyfor flow control. Consequently, in this part, the goalis to describe briefly the physics and electrical parameters of

atmospheric plasmas, and then to focus more particularly onthe mechanical effects of the discharge, such as electric wind,induced body force and induced kinetic power.

The second part deals with the application of plasma

actuators forairflow control. A review of theworldwideworksis presented. Regarding the large number of publications,plasma actuators have shown their good efficiency for airflow

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control atvelocitiesup to30 m s1, andsomesignificant results

have been obtained up to 110 m s1 and more. A great numberof well-known aerodynamic applications have been studied.One can mention the case of airflow around cylinders, flatplates and airfoils, and the case of free jets and mixing layers.

However, it is not possible to report precisely all these worksin the present paper. Most papers will be referenced, but onewill focus more precisely on typical examples, covering themost-known aerodynamic cases and industrial applications.

2. Plasma actuators

As previously indicated, this part deals with the secondaryairflow, usually called electricwind or ionicwind, inducedby atmospheric pressure non-thermal plasma discharges

established in air. Indeed, although it seems crucial to knowwell theelectro-mechanical effectsof plasmaactuatorsin orderto use them efficiently in flow control, a few researchers did

this work. Here, one is going to focus on these works.This part is divided into four sections. First, a brief

introduction concerning atmospheric cold plasmas deals withelectric wind. The next two sections concern the two most-

used discharge actuators, i.e. the surface corona dischargeactuator and the single dielectric barrier discharge actuator,respectively. Although the DBD-based devices are now themost used, thecorona-baseddevicesarepresentedfirst becausethey were historically the first ones. Then the last section

deals with other types of plasma actuators. Indeed, a fewauthors have proposed other non-thermal discharge actuators,by either modifying originally the electrode geometry or theHV excitation, or by using another type of discharge.

2.1. Atmospheric cold plasmas

2.1.1. Discharge mechanisms. For a few decades, non-thermal atmospheric pressure plasmas have been studied for

numerous industrial applications such as ozone generation,pollutant removal and surface treatment. Several papers andbooks givea complete review of this type of discharge [3438].These non-thermal plasmas may be produced by a variety ofelectrical discharges andarevery low energy cost because they

have the particularity that the majority of the electrical energyprimarily goes into the production of energetic electrons,instead of heating the surrounding gas.

Briefly, the formation of such a discharge is basedon the Townsend mechanism, or electron avalanche whichcorresponds to the multiplication of some primary electrons incascade ionization. Letus consider thesimplecase of a dc high

voltage applied between two plane electrodes in atmosphericpressure air. In the gap, electrons are usually formed byphoto ionization. Under the electric field, these electrons areaccelerated towards the anode and ionize the gas by collisionswith neutral moleculessuch as A+e A++2e whereAisa

neutral particle and A+ a positive ion. An avalanche developsbecause the multiplication of electrons proceeds along theirdrift from the cathode to the anode. A discharge current isthen created. Different current behaviours may be obtained

when the high voltage increases. The voltagecurrent curveusually allows one to determine the discharge regime. Moredetails may be found in [3538].

HV point

Ionization zone

Ion drift zone

Figure 2.Schematic of a point-to-plate corona.

Thedischarge plasmas used forairflow control areusuallyatmospheric pressure corona discharges and dielectric barrierdischarges. Their characteristics are typically as follows: high

voltage of a few kV to several tens of kV with dc or acexcitation, with a frequency from 50 Hz to 50 kHz, electricalcurrent from a fewA to a few mA. In these conditions, the

density of charged species is between 109 and 1013 cm3 andthe electron temperature is a few eV. More accurate values willbe given in the next two parts.

2.1.2. Dc corona discharge. Corona discharge is a weakly

luminous discharge, which usually appears at atmosphericpressure near sharp pointsor thin wires,where theelectric field

is sufficiently large. Corona may be considered as a Townsenddischarge or a negative glow discharge, depending on field andpotential distribution [36].

Figure2shows a schematic of the point-to-plane corona,inducing a strong electric field, ionization and luminosityaround the point where the high voltage is applied. Several

authors gave an expression of this electric field ( [39,40]for instance). The mechanism of sustaining the continuousionization near the high voltage electrode depends on its

polarity. If the high electric field is located at the cathode,it is a negative corona. If the strong electric field is near theanode, it is a positive corona.

Inthe caseofa positivecorona, it is assumed thatelectronsare produced by photo ionization in the electrode gap. Theseelectrons are accelerated towards the anode, resulting in an

avalanche. An ionization zone is created around the point,

where the number of positive ions and electrons is equal. Thiscorresponds to a plasma zone of a few tenths of a millimetre.

Then ions are repulsed towards the cathode by Coulombianforces, and constitute the drift region, where one can assumethat there is no recombination because the electric field is

not sufficiently large. In the case of the negative corona,Goldman and Sigmond [41]assume that positive ions createdby detachment in the ionization zone go rapidly towards the

cathode. Then negative ions created by attachment drifttowards the grounded plane electrode.

In the case of positive corona, one can assume that

the discharge current is composed of two components: analternative streamer current due to the streamer propagation,

and a continuous unipolar current due to the ion drift thatare collected by the grounded plane. These streamers extendbetween the electrodes. They have a typical frequency of

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E Moreau

10kHz, a diameter of 100 m and a displacement velocityof a few 105 m s1. This value exceeds by a factor of 10 thetypical electron drift velocity in an avalanche. In the plasmanear thepoint, the density of charged species rapidly decreaseswith distance from about 1013 to 109 cm3 [41]. In the case of

a negative corona, there are Trichel pulses at a frequency thatdepends on the applied high voltage. The electrical power ofthe continuous coronas is very low because when the voltageandcurrent increase above a given threshold, there is a corona-to-arc transition.

When the applied high voltage is not dc (ac excitation),the discharge mechanisms are similar when the frequency issmall and when the residual charges have time to be collectedbetween two successive half-cycles. Then the current iscomposed of three components: a capacitive current due tothe gas in the electrode gap, a synchrone current that is inphase with the applied voltage and a pulsed current due tothe streamer pulses during the positive half period and Trichel

pulses during the negative one. An increase in corona voltageand power without a spark becomes possible by using a pulseHV.

The corona discharge generates lots of charged species.More details may be found in [36]and the large number ofpapers referenced in this paper.

2.1.3. Electric wind. As explained by Robinson in hisfamous paper [42], the phenomenon variously known as theelectric wind, corona wind and electric aura refers to themovement of gas induced by the repulsion of ions from thevicinity of a high voltage electrode. This phenomenon wasreported for the first time in 1709 by Hauksbee and the first

explanation was given by Faraday in 1838. A history of theelectric wind is given in [43]. Many current works are stilldealing with electric wind.

Indeed, the electric wind is due to the collisions betweenthe ions that drift and the neutral particles in the electrode gapregion. However, although theelectronvelocity is muchhigherthan the ion one, one can neglect the role of electrons becausetheir mass is very low compared with the ion one.

Thefirst expressionof theelectric wind velocity wasgivenin 1961 by Robinson [42]:

vG = k

i

, (1)

where k is a constant that depends mainly on the electrodegeometry, i the time-averaged discharge current, the gasdensity and the ion mobility. This shows that the electricwind velocity is proportional to the square root of the current.One must distinguish the gas velocity and the ion velocity thatmay reach a few thousands of ms1 and which is given by:

vi = E, (2)

where E is the electric field. From 1970 to 2000, the groupof Goldman (Supelec, Paris, France) worked especially oncoronas, andthe induced electricwind. In1993, theexpressionof Robinson was completed by Sigmond and Lagstadt [44]:

vG =

id

AG(3)

withdthe electrode gap and AGthe discharge cross-section.In practice, the plasma researchers do not agree on the

phenomena that induce the electric wind. A part of themconsiders that it is only due to ion drift while the other partassumes that it is due to the streamer propagation. In fact, and

this point have been recently confirmed, it seems that electricwind is due to both phenomena [45].

Several studies showed that the maximum achievablevelocity is about 10 m s1 in the case of positive coronas ([4648]for instance).

2.2. Surface corona discharge actuator

In this section, weare going to focus onplasmaactuators basedon corona discharge. In the next one, we will see the dielectricbarrier discharge-based actuator.

2.2.1. History. Thefirstworksonairflowcontrolbyelectrical

discharges dealt with dc coronas. For example, Velkofet al[6,49] demonstrated that the transition point on a flat platecould be affected by the application of an electric field. Inthis application, the plasma actuator consisted in four HVwire electrodes placed above the wall surface of a flat plate(figure3(a)). There was no grounded electrode. Bushnel[8]and Malik et al[9]reported an electric wind of severalm s1 contributing to drag reduction. In 1992, Soetomo [10]experimentally observed a drag reduction effect induced by acand dc corona discharges along a flat plate in the case of flow

velocities up to 2m s1. In this case, the corona discharge was

established between two razor blades flush-mounted on thewall of a glass flat plate (figure 3(e)). In 1997, Nogeret al[11]

used a point-to-plane configuration in order to manipulate anairflow around a cylinder. More recently, new geometrieshave been perfected. All these geometrical configurations aresummarized in figure3. However, only one group has beenworking continuously (from 1998 until now) on the surface-corona-based-actuator, and characterizing its electrical andmechanical properties in the absence of a free air stream. That

group is composed of people from the University of Poitiers(France) and the University of Buenos Aires (Argentina). Inthis section, we will focus on their works.

2.2.2. Electrical properties. Here, we are going to considerthe geometry presented in figure3(g). The plasma actuator

consists of two wire electrodes placed inside a groove at thewall surface. The electrode diameter is nearly 1mm. In order

to induce a stronger electric field at the anode compared withthe cathode, the anode diameter is lower than the cathode one.One can then assume that the anode plays the role of the pointand the cathode corresponds to the plane, in a point-to-planeconfiguration. For instance, the anode diameter may be equalto 0.6 mm when the cathode one is 2 mm. The electrode gaphere is equal to 40mm. To establish the discharge, a negativeHV of10 kV is applied at the cathode instead of ground, andthe anode HV is progressively increased.

In these conditions, above the onsetvoltage, the dischargecurrent increases with the applied potential difference.

Figure 4 shows a typical example of the time-averageddischarge current per unit length I (current idivided by theelectrode length in mA m1, and called current density) as a

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HV

(a)

HV

(a)

HV

(b)

HV

(b)

HV

(c)

HV

(c)

HV

(d)

HV

(d)

HV

(e)

HV

(e)

HV(f)

HV

Y

X

(g)

Figure 3.Different electrode geometrical configurations found in the literature as dc plasma actuators: (a) volume multiple wire [6], (b)volume wire-to-plate in plane configuration [7], (c) volume wire-to-plate in cylindrical configuration [50], (d) surface wire-to-plate orpoint-to-plate in cylindrical configuration [11,21], (e) surface plate-to-plate [10], (f) surface wire-to-plate [19], (g) surface wire-to-wire [20].

0.0

0.5

1.0

1.5

2.0

2.5

Experiments

Fitting

Curren

tdensi

tyI(mA/m)

Electric field E (kV/cm)

Figure 4.Current densityIversus reduced electric fieldEin thecase of wire-to-wire dc surface discharge (the gap is 40 mm and thedielectric is Plexiglas).

function of the reduced electric field E(potential differenceV divided by the electrode gap in kV cm1). Usually, inpoint-to-plane configuration, i.e. for volume coronas, the Vicharacteristic may be fitted by this expression:

i = CV(V V0), (4)

where iis the time-averaged discharge current, V the potentialdifference,V0the onset voltage and C a constant depending

on the electrode gap (typically between 0.1 and 1 A (kV)1

in air [47]). In figure4,the thick line corresponds to the bestfit obtained with equation (4), when the symbols represent the

behaviour of the surface corona experimental measurements.

This shows that the surface corona and the volume one arestrongly different, because from 7 kV cm1, thesurface corona

current increases suddenly. This important difference is

certainly due to the gassolid interface, where the corona acts.Indeed, in this case, the discharge is located just above the

dielectricwall, wherethe surroundingconditions areparticular.

In the case of surface coronas, five corona dischargeregimes may be observed when the voltage between both

electrodes is increased. Briefly, above the corona-starting

voltage V0, the first regime is the spotregime. The discharge isconcentratedwithinsome visible spots on thesmaller diameter

wire. The current density Iis smaller than0.2 mAm1

and theelectric wind is negligible. As the electric field is increased, a

thin sheet of blue ionized air between both electrodes may beobserved. This mode is called the streamerdischarge. In this

regime, the current density values vary from0.2 to0.5 mAm1

and the electric power consumption Pelecis about 50 mW persquare centimetre of plasma sheet. For higher voltage/gap

ratios (current density from 0.5 to several mA m1 and 50

airflowcontrolbynonthermalplasmaactuators0022 3727-40-3 s01

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