RESEARCH ARTICLE

High-lift airfoil trailing edge separation controlusing a single dielectric barrier discharge plasma actuator

Jesse Little • Munetake Nishihara • Igor Adamovich •

Mo Samimy

Received: 4 March 2009 / Revised: 15 September 2009 / Accepted: 22 September 2009 / Published online: 13 October 2009

� Springer-Verlag 2009

Abstract Control of flow separation from the deflected

flap of a high-lift airfoil up to Reynolds numbers of 240,000

(15 m/s) is explored using a single dielectric barrier dis-

charge (DBD) plasma actuator near the flap shoulder.

Results show that the plasma discharge can increase or

reduce the size of the time-averaged separated region over

the flap depending on the frequency of actuation. High-

frequency actuation, referred to here as quasi-steady forcing,

slightly delays separation while lengthening and flattening

the separated region without drastically increasing the

measured lift. The actuator is found to be most effective for

increasing lift when operated in an unsteady fashion at the

natural oscillation frequency of the trailing edge flow field.

Results indicate that the primary control mechanism in this

configuration is an enhancement of the natural vortex

shedding that promotes further momentum transfer between

the freestream and separated region. Based on these results,

different modulation waveforms for creating unsteady DBD

plasma-induced flows are investigated in an effort to

improve control authority. Subsequent measurements show

that modulation using duty cycles of 50–70% generates

stronger velocity perturbations than sinusoidal modulation

in quiescent conditions at the expense of an increased power

requirement. Investigation of these modulation waveforms

for trailing edge separation control similarly shows that

additional increases in lift can be obtained. The dependence

of these results on the actuator carrier and modulation

frequencies is discussed in detail.

1 Introduction

High-lift airfoils typically employ trailing edge flaps that

can be deflected during takeoff or landing and stowed

during cruise. Such devices enhance the lift curve of con-

ventional airfoils, but can impose a penalty due to flow

separation that occurs when the momentum of fluid in the

boundary layer is not sufficient to overcome wall friction

and the adverse pressure gradient encountered as it travels

over the deflected flap surface. Traditional methods of

eliminating flow separation on high-lift airfoils utilize

multi-element flaps that allow mixing of fluid between

the pressure and suction sides. These systems, while effec-

tive for augmenting lift, create significant increases in

mechanical complexity and weight of the aircraft. In addi-

tion, the external hinges and positioning actuators required

for such devices generate parasitic drag when stowed in the

cruise configuration. The replacement of conventional

multi-element flap systems with a simple flap utilizing

active flow control technology is a viable alternative if the

necessary performance criteria can be enhanced or at least

maintained.

Control of flow separation over simple flaps on high-lift

airfoils has been investigated in both open-loop (Kiedaisch

et al. 2006; Melton et al. 2006) and closed-loop (Becker

et al. 2007) configurations using synthetic jets and more

recently with plasma actuators (Mabe et al. 2009). Syn-

thetic jets possess good control authority, but are limited to

actuation from specific locations that must be chosen and

integrated into the design of the airfoil model. This places

restrictions on the control effectiveness of such systems

due to the variable separation location and corresponding

receptivity region that is inherent for airfoil applications

where both incidence and flap deflection angles can vary.

DBD plasma actuators produce induced flows at least an

J. Little � M. Nishihara � I. Adamovich � M. Samimy (&)

Gas Dynamics and Turbulence Laboratory,

Department of Mechanical Engineering,

The Ohio State University, 2300 West Case Road,

Columbus, OH 43235, USA

e-mail: Samimy.1@osu.edu

123

Exp Fluids (2010) 48:521–537

DOI 10.1007/s00348-009-0755-x

order of magnitude lower than most synthetic jets, but in

their simplest laboratory form (thin adhesive tape) can be

located anywhere on the airfoil surface and allow the

flexibility to conform to surface curvature. These devices

also present the possibility of placing multiple actuators in

various locations and orientations on the airfoil to allow

vectored momentum addition over 180� referenced to the

model surface (Porter et al. 2009).

While the potential of plasma-based actuation for sep-

aration control is apparent, these devices possess some

drawbacks that should be mentioned. They have primarily

been limited to relatively low speed (U?\ 30 m/s) low

Reynolds number (*105) applications such as those

associated with micro air vehicles due to the weak induced

flows generated by the plasma discharge (Moreau 2007;

Mabe et al. 2009). More recently, claims of control

authority for freestream velocities as high as 60 m/s with

Re = 106 have been presented in the literature (Patel et al.

2008). However, significant work still remains to make

these viable devices for realistic flight applications.

Namely, the performance of DBD plasma in harsh envi-

ronments has not been fully explored, and the penalties

associated with transporting power supplies capable of

producing the high voltages necessary for plasma genera-

tion may prove costly. Nevertheless, the possible gains of

an actuator that is simple to construction, lacks moving

parts, exhibits no parasitic drag and is capable of high-

bandwidth excitation are too tempting to resist at this point

in its maturation.

The advantages and disparities of DBD plasma actuators

with respect to the synthetic jets as well as the variety of

parameters that can be explored (size, location, number,

etc.) make them amenable devices for studies of both

canonical and more complex separation control problems

in laboratory settings (Greenblatt et al. 2008b). The

developmental nature of such actuators coupled with the

complexity of flow separation and the possibility of feed-

back control makes this a very rich problem spanning

multiple disciplines. In this work, we demonstrate the use

of a single DBD plasma actuator for controlling flow

separation on the deflected flap (30�) of a high-lift airfoil

for Reynolds numbers up to 240k (15 m/s) at zero inci-

dence. The high-lift airfoil is a simplified version of the

NASA Energy Efficient Transport (EET). A single DBD

actuator is placed near the flap shoulder (x/c = 0.775,

where c is the airfoil chord) and used to force the flow with

both quasi-steady and unsteady plasma-induced flows. The

unsteady nature of the DBD plasma actuator is also

examined by comparing sinusoidal and duty cycle modu-

lation waveforms.

In the following portions of this work, some background

information on separation control in high-lift applications

is given with an additional focus on the current state of

DBD plasma actuators. This is followed by an explanation

of the experimental techniques and results of the control

effectiveness of a single DBD plasma actuator for reducing

separation on the deflected flap of a high-lift airfoil. Based

on these results, the unsteady nature of a low-frequency

modulated DBD plasma actuator is examined, and possible

future DBD actuator design criteria are discussed.

2 Background

2.1 Separation control with synthetic jets

Separation control is a broad and widely studied topic, and

thorough reviews on its various applications have been

published (Gad-el-Hak and Bushnell 1991; Greenblatt and

Wygnanski 2000). Passive separation control techniques

that generally constitute geometric changes such as vortex

generators and slotted flaps/slats are employed on many

operational aircraft. These control elements are effective if

the aircraft is operating in a flight regime that is in their

design envelope. However, in off-design conditions, pas-

sive control elements can have detrimental effects that are

often manifested in the form of increased drag. Despite this

drawback, the benefits of passive control techniques often

outweigh the incurred cost created by their application to

the aerodynamic surface.

Active separation control has gained popularity in

recent years due to its potential for maintaining or

enhancing the benefits of passive control techniques

without the penalty associated with operation in off-

design conditions. The main difference here is that active

control can be turned on and off by command allowing

additional flexibility. Active control strategies also have

the potential to be implemented in a feedback system that

coupled with adequate sensors and controller could create

even greater benefits in flight efficiency and maneuver-

ability. A complete review of active separation control is

a subject in itself. Rather, the following background

information focuses on separation control studies that

examine the effect of two-dimensional actuation on two-

dimensional airfoil models.

Technological advances over the last few decades have

allowed researchers to more fully explore the wide

parameter space associated with this research topic.

Accordingly, significant advances in the understanding of

separated flow phenomena in response to actuation have

followed. Among the most widely accepted is that

unsteady actuation via pulsed blowing, pulsed suction or

both (zero net mass flux) is more effective than steady

forcing (Seifert et al. 1996). The range of effective

dimensionless frequencies associated with this is on the

order of unity for which the dimensionless frequency

522 Exp Fluids (2010) 48:521–537

123

(reduced frequency or Strouhal number), F?, is defined as

F? = fxsp/U? where f is the forcing frequency, xsp is the

length of the separated region and U? is the freestream

velocity (Darabi and Wygnanski 2004; Glezer et al.

2005). This parameter underscores the importance of the

characteristic length scale of separated flow phenomena,

xsp, which is the length of the separation zone over the

body in question (Seifert et al. 1996). Physically, the

reduced frequency of unity requires that a perturbation

must be introduced during the time that the freestream

flow propagates over the separated region. The importance

of actuator location is closely related to this expression

since the shear layer created between the freestream and

the low-speed separated region by nature selectively

amplifies small perturbations if these are introduced near

its receptivity region. The optimum choice of this location

for unsteady actuation is generally at or slightly upstream

of the separation point. This ensures that the shear layer is

excited by the control perturbations near its receptivity

point. Successful introduction of such forcing creates

large spanwise vortices that develop via the Kelvin–

Helmholtz instability. These vortices encourage momen-

tum transport between the freestream and the separated

region thus reattaching the flow (Darabi and Wygnanski

2004; Melton et al. 2005). Forcing at higher frequencies

(F? [ 10) has been classified as a different regime and is

characterized by enhanced energy dissipation associated

with spatial scales in the boundary layer (Amitay and

Glezer 2002). Studies on control of trailing edge separa-

tion have shown that significantly more momentum input

is required in comparison with leading edge control

(Melton et al. 2006). This is commensurate with the

existence of a thicker, likely turbulent boundary layer that

develops along the main element of the airfoil. Such

results also support leading edge separation control find-

ings that show greater centripetal acceleration created by

airfoil surface curvature requires additional momentum

for realizing similar control authority (Greenblatt and

Wygnanski 2003). Because of these challenges, it is dif-

ficult to fully attach the flow over the trailing edge flap.

Consequently, both experimental and numerical studies

show that lift gains associated with this are often mani-

fested from upstream effects such as an increase in overall

circulation (Kiedaisch et al. 2006; Melton et al. 2006).

Additional work has shown that the simultaneous use of

multiple actuators distributed along the airfoil chord has

been more successful than the contribution from each

actuator alone (Greenblatt 2007). Not surprisingly, the

relative phase between actuator input signals is an

important parameter that is highly dependent on the

spacing of the actuators, the excitation frequency and the

velocity just external to the boundary layer (Greenblatt

2007; Melton et al. 2007).

2.2 Separation control with DBD plasma actuators

The recent interest in plasma actuators for aerodynamic

flow control is motivated by their simple construction,

lack of moving parts, high bandwidth and ease of imple-

mentation. Because of these amenable characteristics,

researchers have investigated their application in a variety

of flow control problems particularly those associated with

flow separation. These studies and the current state of the

DBD plasma knowledge base are summarized in various

review articles (Moreau 2007; Corke et al. 2009). While

such devices are relatively new to the aerodynamic com-

munity, DBD plasma actuators have long been used in a

variety of industrial applications such as ozone generation

(Kogelschatz 2003).

The DBD plasma actuator for aerodynamic flow control

is usually composed of two electrodes separated by a

dielectric material arranged in an asymmetric fashion

(Moreau 2007; Corke et al. 2009). Application of a suffi-

ciently high-voltage AC signal between the electrodes

weakly ionizes the air over the dielectric covering the

encapsulated electrode. The dielectric barrier allows

the generation of a large volume of plasma by preventing

the discharge from collapsing into an arc. The DBD plasma

actuator is a self-limiting device in that the accumulation of

charged particles onto the dielectric surface opposes the

electric field requiring consistently higher voltages to sus-

tain the discharge. This is circumvented using an AC

waveform that, because of a change in polarity, creates

movement of charged species back and forth between the

exposed electrode and the dielectric surface at the AC

driving frequency. The movement of these charged parti-

cles transfers momentum to the flow via ion–neutral col-

lisions. In quiescent conditions, the asymmetric plasma

actuator creates suction above the exposed electrode and a

pseudo wall jet over and downstream of the covered

electrode. The velocity of a DBD plasma-induced wall jet

varies with dielectric properties, voltage and frequency.

Maximum velocities generated by a single actuator can

range from 1 to 6 m/s a few millimeters from the wall

(Moreau 2007). The induced flow is predominantly direc-

ted away from the exposed electrode due to the asymmetry

of the actuator geometry and behavior of the discharge over

the two waveform half cycles (Enloe et al. 2004a, 2008).

Early examples of plasma actuators as flow control

devices demonstrated their potential for boundary layer and

leading edge airfoil separation control applications (Roth

et al. 2000; Post and Corke 2004). More recently, experi-

mental studies using such devices have broadened to

include jet mixing, cavity tone attenuation, noise control

and aero-optics (Benard et al. 2007; Chan et al. 2007;

Freeman and Catrakis 2008; Thomas et al. 2008). While

these new applications have gained popularity in recent

Exp Fluids (2010) 48:521–537 523

123

years, the majority of DBD plasma work is still applied to

separation control. These actuators are particularly

appealing for this application due to the nature of their

induced flow when arranged asymmetrically. In this con-

figuration, the induced flow produces a jet with maximum

velocity less than 5 mm from the wall that is often ame-

nable for influencing boundary layers. DBD plasma actu-

ators are also widely investigated using modeling and

computations (Jayaraman and Shyy 2008; Rizzetta and

Visbal 2009).

The use of DBD plasma actuators for airfoil separation

control at locations other than the leading edge has been

limited. Studies have reported that actuators placed near

the trailing edge of airfoils can produce effects similar to

plain flaps with deflections of a few degrees (Vorobiev

et al. 2008). This results in a uniform increase in lift

coefficient across all angles of attack and a slight reduction

in minimum drag coefficient, CD, at Reynolds numbers on

the order of 105 corresponding to velocities of a few tens of

meters per second (He et al. 2009). To date, DBD plasma

actuators have not produced sufficient momentum to

eliminate separation for flows over simple deflected flaps at

Re [ 105 unless the freestream velocity is quite low (Mabe

et al. 2009).

The mechanism responsible for separation control by

DBD plasma is most often associated with the wall jet

generation described earlier, but whether this results in

boundary layer tripping, energizing or amplification of

instabilities is still in debate and depends on the flow

system under consideration. For separation control

explicitly, the state of the boundary layer (laminar or

turbulent) just upstream of the actuator will also play a

role. Unlike traditional unsteady jets created with voice

coils or piezo-ceramic disks, the exact location at which

the plasma actuator accomplishes control is not immedi-

ately obvious, but actuators placed at or slightly upstream

of the separation location give favorable results (Huang

et al. 2006; Sosa et al. 2007; Jolibois et al. 2008). This

appears consistent with modeling results that show the

highest force density associated with such devices is near

the edge of the exposed electrode (Enloe et al. 2004b;

Corke et al. 2007).

Like synthetics jets, AC driven DBD plasma actuators

are often most effective for separation control and lift

enhancement when excitation is created with reduced fre-

quency (F?) on the order of unity (Huang et al. 2006;

Greenblatt et al. 2008a; Patel et al. 2008; Benard et al.

2009b). To operate in this fashion, the actuator must be

excited with a sufficiently high carrier frequency to pro-

duce the plasma (1–10 kHz) and modulated at a lower

frequency to excite the long wavelength instabilities asso-

ciated with most separated flow dynamics. This behavior is

analogous to synthetic jets created by piezoelectric

diaphragms that produce the highest intensity fluctuations

when excited near the resonant frequency of the disk and/or

cavity that is often on the order of a few kHz. Many studies

of separation control with DBD plasma actuators assume

that the flow does not feel perturbations created by the

high-frequency carrier signal. For the majority of low-

speed applications, this is true because the instabilities

involved are not receptive to high-frequency perturbations

and instead feel their effect as a quasi-steady phenomenon.

However, it has been confirmed that the movement of

charged species in the plasma does in fact create a per-

turbation at the frequency of plasma generation and thus

suggests the possibility of using of DBD plasma actuators

for high-frequency forcing applications if sufficient

amplitude can be produced (Takeuchi et al. 2007; Bouc-

inha et al. 2008).

It has been shown that the force production of AC

driven DBD plasma actuators is dependent on the oxygen

content, ambient pressure and humidity of the environ-

ment (Kim et al. 2007; Abe et al. 2008; Benard et al.

2009a). This makes the application of such devices at

cruising altitudes in working flight environments ques-

tionable at this time. Nevertheless, strides continue to be

made for DBD plasma implementation as they have been

used with varying degrees of success in feedback control

and flight testing (Patel et al. 2007; Sidorenko et al.

2008). Even more promising is ongoing research for

improved methods of generating DBD plasma actuation

that rely on nanosecond pulses (Likhanskii et al. 2007;

Opaits et al. 2007). These waveforms seem to accomplish

control based on thermal effects alone similar to arc fil-

ament-based plasma actuators (Samimy et al. 2007) and

have demonstrated leading edge airfoil separation control

authority up to Mach 0.85 (Roupassov et al. 2009).

Because of the amenable characteristics outlined earlier

and the variety of parameters that can be explored,

plasma actuators continue to be an emphasized point of

research in aerospace applications.

3 Experimental techniques and data processing

3.1 Wind tunnel

A Gottingen-type, closed, recirculating wind tunnel with an

optically accessible 61 9 61 9 122 cm3 (2 9 2 9 4 ft3)

test section serves as the test bed for this study. Test section

walls are constructed of 25.4 mm (1 in) thick super abra-

sion-resistant acrylic. Each side wall is fitted with a

30.5 cm (12 in) diameter port that is located 30.5 cm (12

in) from the test section floor and 61 cm (24 in) down-

stream of the test section entrance. Air flow in the tunnel is

continuously variable from 3 to 90 m/s (10–300 ft/s). Flow

524 Exp Fluids (2010) 48:521–537

123

conditioning upstream of the test section includes a hex-

agonal cell aluminum honeycomb while high-porosity

stainless steel screens are mounted downstream of the test

section as a safety catch. Four high-efficiency turning

cascades fabricated of galvanized steel are installed in each

of the four tunnel elbows. This assembly results in free-

stream turbulence levels on the order of 0.25% with ±1%

variation in mean freestream velocity measured 152 mm

(6 in) from the test section inlet. The tunnel is also

equipped with a commercial aluminum fin/copper tube,

double row heat exchanger with set point controller and

electronic modulating valve. This arrangement allows the

tunnel freestream operating temperature to be maintained

at ±1�C from the ambient when supplied a sufficient

source of cooling water (max 189 lpm (50 gpm)). Overall

dimensions of the tunnel are 9.8 9 2.2 9 4.1 m3 (32.2 9

7.2 9 13.5 ft3) with a test section centerline height of

1.4 m (4.6 ft).

3.2 Airfoil

A simplified high-lift version of the NASA Energy Effi-

cient Transport (EET) airfoil has been chosen as the test

model. The 2D EET airfoil was thoroughly examined by

(Lin and Dominik 1997) but more recently, significant

studies on active separation control with synthetic jets have

been completed for a similar simplified version (Melton

et al. 2007). The OSU version has a chord of 25.4 cm (10

in) and fully spans the 61 cm (24 in) test section in a

horizontal configuration. The model is equipped with a

deflectable (0–60�) trailing edge flap that is 25% of the

airfoil chord, but for simplicity, lacks the leading edge slat

used by NASA. It is constructed of a nylon compound

(Duraform GF) and has been fabricated using selective

laser sintering (SLS) technology. Independent settings for

the incidence and flap deflection angles are done manually

using separate wall plugs. A digital photograph showing

the airfoil with trailing edge flap deflected, and flap wall

plug is shown in Fig. 1. Instrumentation in the model

includes 45 staggered static pressure taps located near the

test section centerline and 15 static pressure taps at � and

� spans. The model is also instrumented with seven high-

bandwidth Kulite pressure transducers flush mounted near

the centerline. Figure 2 shows the airfoil profile and the

location of static pressure taps and transducers near the

centerline. The focus of this study is takeoff and landing

applications. To date, all separation control experiments

have been performed at zero incidence with flap deflections

greater than 10�. Future work is intended to examine the

effect of non-zero incidence angles. Aerodynamic charac-

teristics of this simplified version of the NASA EET at pre-

stall conditions have been verified previously (Little et al.

2008).

3.3 Experimental measurements

Measurements of static pressure from taps on the model

surface are acquired using Scanivalve digital pressure

sensor arrays (DSA-3217). Values of dimensionless pres-

sure (CP) and lift coefficient (CL) are averaged over 50

samples acquired at 10 Hz. Kulite pressure transducers

installed in the model are powered using an in-house

constructed signal conditioner that amplifies each sensor

output by 1,000 and low-pass filters at 10 kHz. The

resulting pressure traces are sampled simultaneously at

50 kHz using a National Instruments PCI-6143 data

acquisition board. Average pressure spectra are calculated

from 50 blocks of 8,192 pressure samples that result in a

frequency resolution of approximately 6 Hz.

Two-component particle image velocimetry (PIV) is

used to obtain quantitative measurements of the velocity

fields for both the airfoil and the actuator on a flat plate

in still air. Images are acquired and processed using a

LaVision PIV system operating software version DaVis

7.2. Nominally submicron olive oil seed particles are

introduced upstream of the test section contraction using a

Fig. 1 OSU version of the simplified high-lift EET airfoil with

trailing edge flap deflected

Fig. 2 2D profile of the airfoil in cruise configuration showing the

approximate location of static pressure taps and high-bandwidth

pressure transducers near the airfoil centerline (not to scale)

Exp Fluids (2010) 48:521–537 525

123

6-jet atomizer. A dual-head Spectra Physics PIV-400

Nd:YAG laser is used in conjunction with spherical and

cylindrical lenses to form a thin light sheet that allows

particle visualization. The time separation between laser

pulses used for particle scattering is tuned according to the

flow velocity, camera magnification and correlation win-

dow size. Two images corresponding to the pulses from

each laser head were acquired by a LaVision 14 bit 2,048

by 2,048 pixel Imager Pro-X CCD camera equipped with a

Nikon Nikkor 50 mm f/1.2 lens. The wall plug arrange-

ment requires that the camera views the laser from a

downstream angle of approximately 14� in the airfoil case.

Data for the actuator in still air are acquired with the

camera perpendicular to the light sheet. For each image

pair, subregions are cross-correlated using decreasing

window size (642–322 pixel2) multi-pass processing with

50% overlap. An image correction algorithm is applied to

the data set due to the non-orthogonal viewing angle in the

airfoil case. The resulting velocity fields are post-processed

to remove any remaining spurious vectors using an

allowable vector range and median filter. Removed vectors

are replaced using an interpolation scheme, and a

smoothing filter is also applied to the calculated velocity

fields. The PIV data are sampled at 10 Hz.

Assuming negligible laser timing errors, full-scale

accuracy for instantaneous velocity measurements is con-

servatively estimated at less than 1% based on correlation

peak estimation error of 0.1 pixels and maximum particle

displacement of 12 pixels. The error in measurements of

time-averaged velocity is dependent on the sample stan-

dard deviation, s, and sample size, N, of the data set by

zcs=ffiffiffiffi

Np

where zc is 1.96 for a 95% confidence interval

(Bendat and Piersol 2000). For the airfoil flow field, this

estimation results in relative error of at worst 5% based on

freestream velocity (15 m/s) using 500 samples. The flat

plate time-averaged data have relative error of at worst

10% based on the maximum wall jet velocity (1.2 m/s)

using 100 samples. The spatial resolution of PIV data for

the flat plate and airfoil data sets are *0.5 and *1.5 mm,

respectively. Near surface measurements for the flat plate

and airfoil data sets are obtained within *1.5 and *3 mm

of the substrate, respectively.

Conditional sampling of PIV data (phase-locking) is

accomplished using the programmable timing unit of the

LaVision system. In this case, the acquisition is synced

with the modulation frequency of the actuation signal. The

baseline pressure signal is not sufficiently periodic to allow

this acquisition in the airfoil case. Velocity fields at various

phases of the actuator modulation frequency are investi-

gated by stepping through the actuator period using time

delays. The resulting phase locked data sets are averaged

over 50 images for each phase which is found to be

sufficient for resolving the primary features (velocity and

vorticity) of the flow fields. Conditionally sampled (phase-

locked) PIV data are acquired at 5 Hz.

For all airfoil measurements, data are acquired by

establishing a separated flow baseline condition then

energizing the actuator. For repeated samples of different

forcing cases, the baseline separated condition is reestab-

lished between consecutive control cases to eliminate

hysteresis effects.

3.4 DBD plasma actuators

Input signals for the DBD plasma actuators are generated

using a dSpace DSP 1103 board. Signals generated by

dSpace are used as inputs to a Powertron Model 1500S AC

power supply and step-up high-voltage transformer.

Amplified signals are sent to a low-power (200 W) high-

voltage (0–20 kVrms) transformer designed to operate in the

frequency range of 1–5 kHz. Voltage measurements are

acquired and monitored at the secondary side of the high-

voltage transformer with a Tektronix P6015A high-voltage

probe. The power dissipated by the actuator is calculated

with the Lissajous figure using charge–voltage measure-

ments (Falkenstein and Coogan 1997). A 47 nF capacitor is

connected in series with the covered ground electrode in

each case. The voltage across the capacitor is measured

using a Tektronix P6111B voltage probe. The corre-

sponding signals are monitored on a LeCroy Waverunner

6050A oscilloscope, but the actual power calculation is

performed offline. For this work, the actuator is operated

using a 2 kHz sinusoidal carrier frequency at a voltage of

20 kVpp with variable modulation waveform.

3.5 Actuator design

It is well known that DBD plasma actuators that integrate

relatively thick dielectrics (on the order of a few mm) into

the model geometry create the greatest induced flows due

to their ability to withstand higher voltage inputs without

entering the Corona or streamer mode (Corke et al. 2009).

These types of designs are ideal for situations in which the

separation location and model geometry are relatively

invariant (e.g., cylinder in cross-flow). For the purposes of

this work, it is essential to maintain both the flexibility and

the modular nature of the actuators due to the variable

separation location and number of parameters (actuator

location, geometry, orientation and number) that are

intended for investigation. More importantly, we wish to be

able to move these devices without modifying the airfoil.

For these reasons, DBD plasma actuators whose con-

struction is based on thin flexible adhesive materials (i.e.,

tapes) are chosen. Such devices are cheap, readily available

and easily removable which allows the actuator location

and orientation to be varied. The thin profile and ability to

526 Exp Fluids (2010) 48:521–537

123

conform to surface curvature are also appealing since this

allows the application of the device to the surface with

minimal alteration of the basic features of the flow field.

It is widely believed that the electrode material is much

less important than the dielectric when inducing flows

based on DBD plasma discharges (Hoskinson et al. 2008).

Accordingly, the most common electrode used in the lit-

erature, copper, is selected for this study, and no attempt is

made to optimize this choice. It has a total thickness of

0.09 mm (0.0035 in) and is bonded with an acrylic adhe-

sive that is 0.05 mm (0.0021 in) thick. The exposed and

covered electrodes have widths of 6.35 mm (0.25 in) and

12.7 mm (0.50 in), respectively. The covered electrode

width of 12.7 mm (0.50 in) allows the use of standard

25.4 mm (1 in) wide dielectric tapes.

Various studies recommend the use of a slight gap

(1–5 mm (0.08–0.20 in)) between exposed and covered

electrodes (Roth and Dai 2006; Forte et al. 2007). The

latter work showed a modest velocity increase (50 cm/s)

from the 0 to 5 mm (0.20 in) gap case. Because of the

difficulty repeating this exact gap size for multiple itera-

tions, the use of no gap or slight overlap between exposed

and covered electrodes is preferable (Corke et al. 2007). It

is generally accepted that DBD plasma body forces are

voltage driven phenomenon as both the thrust and the

dissipated power are proportional to Vac7/2 when the device is

operating in the normal glow regime (Corke et al. 2009).

Beyond this region, the dissipated power tends to increase

while the induced flow reaches some maximum value

determined by the excitation waveform and actuator con-

struction (Forte et al. 2007). Note that at higher voltages,

the discharge eventually collapses into an arc filament that

eliminates velocity generation.

There is an optimal frequency for DBD plasma-induced

thrust that is dependent on the bulk capacitance of the

dielectric (e/t, where e and t are the dielectric constant and

thickness, respectively). In addition, for a given dielectric

operating at its optimized frequency, the thrust created from

the actuator is dependent on the bulk capacitance of the

dielectric (Corke et al. 2009). For a given voltage input,

dielectrics with larger bulk capacitance tend to produce

greater thrust when operated in the Vac7/2 region due to an

enhanced electric field. Consequently, an idealized dielec-

tric will have a large dielectric constant and a small thick-

ness to create a larger bulk capacitance. The caveat here is

that dielectrics with smaller thickness generally have lower

dielectric strength and cannot withstand high voltages

without entering the Corona/streamer or arc regime. The

use of relatively thick materials increases dielectric strength

at the expense of requiring higher voltages to initially ionize

the gas and sustain the discharge in the Vac7/2 region. This

results in high values of thrust, but an increased actuator

thickness and power requirement (Corke et al. 2009).

The self-imposed mandate that the actuator materials

(both electrode and dielectric) be composed of thin

adhesive tapes in this work limits the dielectric material

selection to a few basic choices. Referring to Table 1 in

Roth and Dai (2006), which summarizes various properties

of dielectric materials, it can be seen that Kapton has

medium to low dielectric constant (e * 3.5) compared to

other dielectrics surveyed, but its dielectric strength is

approximately an order of magnitude greater (154 kV/

mm). Assuming that the dielectric strength is an indicator

of the ability for a dielectric to maintain operation in the

normal glow regime makes this a superior choice, espe-

cially for the purposes of this work in which very thin

adhesive materials are necessary. Also note that in prac-

tice, the adhesive and layering abilities of Kapton tape

have been found to be superior to others explored. This

layering decreases the bulk capacitance, but the increased

dielectric strength allows application of higher voltages

without entering the Corona/streamer regime. The Kapton

tape in this study has thickness of 0.09 mm (0.0035 in)

and dielectric strength of 10 kV. Each tape has a silicon

adhesive layer that is 0.04 mm (0.0015 in) thick. The

effect of the silicon adhesive on the dielectric performance

is not examined here. The total thicknesses of the

dielectric and the device as a whole are 0.44 mm (0.0175

in) and 0.62 mm (0.025 in), respectively, unless otherwise

noted.

4 Results

4.1 DBD plasma actuator

The induced flow created by the actuator is characterized

on a flat plate substrate constructed of the same material as

the airfoil model (Duraform GF resin). The dielectric

strength and dielectric constant of this material are 15 kV/

mm and 3.7, respectively. The generated plasma is limited

in the streamwise direction by the extent of the covered

electrode (12.7 mm) and spans approximately 16 cm of the

flat plate substrate. The data are acquired near the mid-

plane of the device. Reliable data are only obtained starting

*1.5 mm from the surface due to image contamination

from laser reflections. The actuator is operated using a

2 kHz sinusoidal carrier frequency at a voltage of 20 kVpp.

The recommended carrier frequency for the Kapton

dielectric is 5 kHz (Corke et al. 2009). This has not been

optimized for the dielectric presented here, but results

suggest that this should have little effect on the maximum

body force generated due to the relatively broad peaks

associated with thrust optimized excitation frequency. A

typical actuator lasts roughly 1 h of non-continuous actual

run time. Note that in practical flight applications, more

Exp Fluids (2010) 48:521–537 527

123

robust dielectrics that are embedded in the substrate will be

required.

Time-averaged streamwise velocity (U) profiles created

by a typical asymmetric DBD plasma actuator operating in

still air are given in Fig. 3. Profiles are shown at three

streamwise locations. The average velocity profiles are

created from an ensemble set of 100 instantaneous velocity

fields. The origin corresponds to the downstream edge of

the exposed electrode (see Fig. 11). The maximum velocity

created by the actuator (not shown) is 1.2 m/s at

x = 35 mm approximately 2 mm from the surface. The

profile at x = 30 mm in Fig. 3 has been fitted with a

skewed Gaussian curve (Hoskinson et al. 2008). The ana-

lytical fit is required to obey the no-slip condition at the

wall. In this work, we choose the profile containing max-

imum velocity (x = 35 mm) fitted with a skewed Gaussian

analytical function to quantify the time-mean momentum

addition, J=q ¼R1

0U2dy, using a constant density

assumption expected to be accurate within 2% of the

background (Enloe et al. 2006; Greenblatt et al. 2008a). It

should be noted that other methods for this calculation have

been suggested, but a widely accepted standard for plasma

actuators does not exist to the authors’ knowledge (Pons

et al. 2005; Porter et al. 2007; Greenblatt et al. 2008a;

Hoskinson et al. 2008). Thus, any reported values should

be taken as order of magnitude approximation only. With

these assumptions, the time-mean momentum J/q is cal-

culated as 5 9 10-3 N/m. This value is reported in terms

of time-mean momentum coefficient, Cl = 2J/qU?2 c, in

the results that follow where applicable (Greenblatt et al.

2008a). The oscillatory component of momentum has not

been calculated due to reasons discussed in Sect. 4.3.

Figure 4 gives an example of the typical power per unit

length dissipated by the actuator as a function of applied

voltage. Recall the discharge is generated by a 2 kHz

carrier frequency. Like the studies previously mentioned,

the dissipated power is found to increase proportionally

with voltage to the power 3.51 where the constant of pro-

portionality, a, is approximately 2 9 10-5. The data have

been plotted on a log–log scale to emphasize the validity of

the fit at higher voltages while highlighting considerable

scatter below 5 kVpp. This scatter occurs over a voltage

region where no discharge exists, and the Vac7/2 power law is

not valid. Roth and Dai (2006) refer to this region as being

characterized by dielectric heating. The curve fit is used to

estimate dissipated power for both zero (quiescent) and

non-zero freestream flow experiments. The electrical

power per unit length, P/l, dissipated by the discharge is

0.74 W/cm at 20 kVpp unless otherwise stated. This value

is reported in terms of electrical power coefficient,

CE = 2P/qU?3 c, in the results that follow (Greenblatt et al.

2008a).

4.2 Airfoil

As previously mentioned, DBD plasma actuators have been

successful for controlling flow separation primarily at the

leading edge of airfoils. The focus of this work is to

investigate the possibility of using such actuators to control

flow separation over deflected trailing edge flaps. The

choice of an initial baseline case for this study has been

made with the consideration of various factors. The first

and perhaps most important is to choose a sufficiently low

flow velocity such that control using DBD plasma is

Fig. 3 Time-averaged DBD plasma-induced streamwise velocity (U)

in quiescent air for 2 kHz carrier frequency at 20 kVpp. The profile at

x = 30 mm has been fitted with a skewed Gaussian function

Fig. 4 Dissipated power per unit length as a function of applied

voltage calculated using charge–voltage measurements and curve fit

of the experimental data. The constant of proportionality, a, is

*2 9 10-5

528 Exp Fluids (2010) 48:521–537

123

feasible. The current state of the work suggests that free-

stream velocities of less than 30 m/s are appropriate

(Moreau 2007). This, coupled with our focus on takeoff

and landing applications, deems that non-zero flap deflec-

tions are a priority. To study specifically the flow over a

deflected flap, separation must be limited to this region so a

low angle of attack is desired. In addition, it is important to

maintain some possibility of comparison with NASA

results for a similar airfoil using synthetic jet actuation.

Lastly, we hope to eventually develop a low-dimensional

model-based feedback controller for this flow. Conse-

quently, the presence of a dynamically rich flow field of

sufficiently low order is desired. With these factors in

mind, various baseline candidates were surveyed (Little

et al. 2008). Based on this information and additional

unpublished data, the baseline flow associated with zero

incidence, flap deflection of 30� and Re = 240k

(U? = 15 m/s) is chosen. This condition represents a case

for which flow separates near the flap shoulder with rec-

ognizable dynamic signature measured by pressure trans-

ducers on the flap while also possessing sufficiently low

velocity such that DBD plasma actuators should exhibit

some control authority.

This case is initially investigated using a single DBD

plasma actuator as described in the previous section placed

near the flap shoulder (Fig. 5). This location is an accept-

able estimate of the separation point where actuators are

generally found to be effective for controlling flow sepa-

ration. The dimensionless static pressure (CP) on the airfoil

surface for the baseline and two open-loop control cases at

zero incidence for Re = 240k (15 m/s) with a flap deflec-

tion of 30� is plotted in Fig. 6. It should be noted that each

time an actuator is placed on the model, the baseline flow

changes slightly due to the presence of the actuator alone,

most notably from the surface discontinuity created at the

actuator leading and trailing edge. This changes the Cp

profiles somewhat, but the dynamic signature of the shed-

ding vortices remains consistent. For the remainder of the

work, any reference to the baseline configuration pertains

to the case where an actuator is present on the model

surface without plasma.

A single DBD plasma actuator is located at x/c = 0.775

referenced to the downstream edge of the exposed elec-

trode (Fig. 5) and used to force the flow with quasi-steady

and unsteady plasma-induced flows. In practice, the flap is

deflected, and the actuator is then adhered to the model.

Thus, the actuator covers the flap joint, conforms to sur-

face curvature and does not allow flap rotation when

installed. The placement of an actuator on the surface

obstructs static pressure taps and does not allow mea-

surements at these points. This can be seen in Fig. 6 by

the lack of measurements between x/c = 0.70 and

x/c = 0.825. Plasma is formed over the middle � span of

the model (46 cm or 18 in) to promote two-dimensional

actuation near the model centerline. The dimensionless

carrier frequency and voltage for these cases are F? = 8.5

(2 kHz) and 20 kVpp, respectively. The momentum and

power coefficient for this quasi-steady actuation case are

estimated at *0.02 and *6.9%, respectively. Unsteady

actuation is created using a low-frequency sine wave to

modulate the carrier frequency (see Fig. 10a). Control

results are reported in dimensionless form based on the

modulation frequency that is approximately 85 Hz. For

simplicity, the characteristic length scale, xsp, is assumed

to be equivalent to the flap length, 6.35 cm (2.5 in)

although the dependability of this length scale depends on

the flow response to control (Wygnanski 2004). Thus, the

reduced frequency for the modulation becomes approxi-

mately F? = 0.4.Fig. 5 Example of a DBD plasma actuator applied at the flap

shoulder (x/c = 0.775) of the simplified NASA EET airfoil

Fig. 6 Dimensionless pressure (Cp) on the airfoil surface at zero

incidence with 30� flap deflection and Reynolds number of 240k

(15 m/s) for baseline and controlled cases. DBD plasma actuator is

located at x/c = 0.775 with sinusoidal excitation of 20 kVpp at

F? = 8.5 (2 kHz) and sinusoidal amplitude modulation at Fm? = 0.4

(85 Hz)

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123

The difference between the baseline and both actuation

cases is clearly visible on the flap region of Fig. 6. The

quasi-steady actuation case at F? = 8.5 generates less

suction on the flap than the baseline case. The unsteady

forcing near Fm? = 0.4 increases the suction on the flap and

enhances circulation around the model. This circulation

increase is indicated by stronger suction at the leading edge

and over the main body of the model. This behavior is

typical of trailing edge airfoil separation control in that a

significant portion of the lift enhancement is due to

upstream effects rather than full reattachment of flow to the

flap (Kiedaisch et al. 2006; Melton et al. 2006). It should

also be noted that the model scale and obstruction of static

pressure taps by the actuator do not allow the resolution of

any suction peak near the flap shoulder as exhibited in

NASA results for both quasi-steady and unsteady actuation

(Melton et al. 2006).

Figure 7 provides a more clear explanation for the

behavior exhibited in the Cp curves. The streamlines are

calculated from an ensemble average of 500 instantaneous

planar velocity fields measured using two-component PIV.

These results more clearly indicate the effect of actuation

on the time-averaged flow field. Figure 7a is the baseline

flow that is characterized by separation near the shoulder

and a large recirculation region over the flap. The vertical

lines indicate the separation location and the extent of the

recirculation region for the baseline flow. Figure 7b shows

the effect of quasi-steady actuation at F? = 8.5 with no

amplitude modulation. In this case, the separation has been

moved slightly downstream, and the recirculation region

has been flattened and lengthened causing higher pressures

on the main flap element. This likely results in a slight

suction peak near the shoulder as exhibited in Melton et al.

(2006); however, as previously mentioned, the actuator

precludes measurements in this region. Figure 7c shows

the effect of using sinusoidal amplitude modulation

(Fm? = 0.4) along with the high-frequency carrier wave

(F? = 8.5). The separation point is similar to the baseline

case, but the recirculation region becomes significantly

smaller in comparison with both of the previous two cases

resulting in greater suction on the flap and enhanced cir-

culation as seen in Fig. 6. This value for dimensionless

modulation frequency (Fm?) of approximately 0.4 is lower

than the previously discussed optimized value of F? = 1;

however, it is consistent with previous studies of trailing

edge separation control (Melton et al. 2006). This likely

arises from the simplified assumption of the flap length as

the length of the separated region, xsp.

It is important to note that the frequency associated with

forcing at Fm? = 0.4 is the natural oscillation frequency of

the trailing edge flow field. This can be seen by examina-

tion of Fig. 8, which shows the pressure spectrum at

x/c = 0.90 (see Fig. 2) for the baseline and controlled flow

at F? = 8.5 with modulation near F? = 0.4. It is obvious

that actuation at this unsteady frequency has significantly

increased both the coherent (F? = 0.4) and broadband

pressure fluctuations of the shedding wake in comparison

with the baseline. These results are consistent with similar

Fig. 7 Streamlines for the airfoil at zero incidence with 30� flap

deflection and Reynolds number of 240k (15 m/s) for baseline (a) and

controlled cases. DBD plasma actuator is located at x/c = 0.775 with

sinusoidal excitation of (b) 20 kVpp at F? = 8.5 (2 kHz) and

sinusoidal amplitude modulation at Fm? = 0.4 (85 Hz) (c). The

vertical lines indicate the separation location and extent of the

recirculation region for the baseline case

530 Exp Fluids (2010) 48:521–537

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studies using synthetic jets (Melton et al. 2006). It is

worthwhile to note that signal processing theory predicts

fundamental spectral peaks at F?± Fm? for sinusoidal

modulation; however, the velocity perturbation created by

the plasma actuator clearly produces vortex shedding at the

modulation frequency (see Fig. 8) which corresponds to

the frequency of high-voltage bursts in the actuator signal

(see Fig. 10a). The spectrum associated with F? = 8.5 is

not presented due to contamination from electromagnetic

interference (EMI); however, the previously cited work

suggests that high-frequency forcing should decrease the

broadband pressure and shift the coherent oscillation to a

slightly higher frequency (Melton et al. 2006). Further

work is needed to confirm this for a DBD plasma actuator.

Two phases of phase-averaged vorticity data calculated

from 2D PIV in Fig. 9 give an indication of the temporal

nature of the actuated flow field. The phase-averaged fields

have been calculated from 50 conditionally sampled

(phase-locked) frames. The vorticity field has been chosen

to illustrate the periodic vortex shedding that alternates

between the pressure and suction sides. The flow has

become locked to the actuation signal which is evident

from the clear organization of the vortices in the figure as

well as the remaining frames that are not shown. As sug-

gested by the pressure spectrum in Fig. 8, forcing at the

characteristic frequency of the wake locks the flow to the

actuator and thus provides a relatively periodic oscillation

during which conditional sampling is possible. In the

baseline case, the pressure signature from transducers on

the flap is not strong enough to render itself for conditional

sampling. Similarly, the quasi-steady actuation case

(F? = 8.5) does not permit this measurement due to the

lack of a robust trigger signal at F? = 0.4. However,

previous analysis using proper orthogonal decomposition

(POD) suggests the natural, but weak oscillation near

F? = 0.4 persists for quasi-steady forcing case (Little et al.

2008). This is also consistent with synthetic jet results

(Melton et al. 2006).

Forcing at the natural oscillation frequency of the wake

as measured by pressure transducers on the flap is consis-

tently the most successful for enhancing lift in this system.

To date, this has been independent of Reynolds number,

flap deflection angle and actuator location with the caveat

that the actuator is placed near the separation point (Little

et al. 2009). Since separation is not significantly delayed,

this presents strong evidence that amplification of the

natural instability is the primary mechanism by which the

asymmetric DBD plasma actuator accomplishes control in

this scenario. Forcing with quasi-steady plasma-induced

flow at F? = 8.5 (2 kHz) in an attempt to energize the

boundary layer profile has been moderately successful in

moving the separation point further down the flap, but the

measured lift differs little from the baseline. The very

different behavior created by the two actuation methods

implies that the control mechanism is not the result of a

Fig. 8 Pressure spectra at x/c = 0.90 for airfoil at zero incidence

with 30� flap deflection and Reynolds number of 240k (15 m/s) for

baseline and controlled flow. DBD plasma actuator is located at x/c = 0.775 with sinusoidal excitation of 20 kVpp at F? = 8.5 (2 kHz)

and sinusoidal amplitude modulation at Fm? = 0.4 (85 Hz)

Fig. 9 Two phases of phase-averaged normalized vorticity (Xc/U?)

based on 50 samples for the airfoil at zero incidence with 30� flap

deflection and Reynolds number of 240k (15 m/s). DBD plasma

actuator is located at x/c = 0.775 with sinusoidal excitation of

20 kVpp at F? = 8.5 (2 kHz) and sinusoidal amplitude modulation at

Fm? = 0.4 (85 Hz). Phase difference, DU, between the two frames is p

Exp Fluids (2010) 48:521–537 531

123

laminar to turbulent transition. To fully eliminate this

possibility, detailed measurements of the boundary layer

upstream of the plasma actuator are intended.

The presented results indicate that unsteady actuation is

preferable to quasi-steady actuation for reducing the size of

the recirculation region for the cases surveyed. This is

consistent with studies of trailing edge separation control in

that it is difficult to reattach such massively separated flows

with a single actuator, especially one that produces a fairly

weak momentum addition like a plasma discharge. The

excitation of natural instabilities is a more efficient choice

in this case because these phenomena are very receptive to

disturbances introduced at the proper frequency. The

instability for this flow is the well-known Kelvin–Helm-

holtz instability that arises when the streamwise velocity

profile contains an inflection point due to the existence of

a shear layer. Introduction of periodic forcing near the

separation location has been effective for reducing the time-

averaged separation in a variety of flow systems (Greenblatt

and Wygnanski 2000). Physically, the amplification of this

natural instability promotes greater mixing between the

high- and low-speed fluids by creating larger and more

energetic flow structures. Such mixing increases the

entrainment of freestream momentum into the separated

region thus reducing the size of the time-averaged separa-

tion. It should be noted that the increased pressure fluctu-

ations (see Fig. 8) may have detrimental effects on the

structural lifetime of the flap element, and this must be

considered in the future.

4.3 Unsteady DBD plasma actuation

The previous results show that actuation at the natural

oscillation frequency of the trailing edge flow field is most

effective for reducing separation. With this knowledge,

the unsteady flow generated by a DBD plasma actuator is

investigated further. Previously, unsteady plasma forcing

at frequencies that correspond to those observed in air-

foil dynamics has been accomplished using sinusoidal

modulation. Recall that these frequencies are approximately

an order of magnitude lower than the carrier frequency used

for plasma generation. A typical high-voltage input signal of

this type is shown in Fig. 10a. However, unsteady flow fields

can also be created using a variation of the duty cycle. An

example of this type of high-voltage input for the same

carrier and modulation frequency is shown in Fig. 10b. The

carrier frequency of the waveforms in Fig. 10 is 2 kHz, and

the modulation frequency is 100 Hz.

The ability of these waveforms to produce fluctuations

in quiescent conditions is examined using phase-averaged

PIV and the derived vorticity field for sinusoidal modula-

tion (Fig. 11 top) and duty cycles of 10, 30, 50, 70 and

90%. For brevity, only one phase of the oscillation is

shown, but animations of four phases of the excitation

period confirm that the structures generated by plasma

convect in still air. As before, the plasma is created using a

carrier frequency of 2 kHz at 20 kVpp. The modulation

frequency is 100 Hz. It should be noted that more quanti-

tative methods for characterizing the oscillatory momen-

tum introduced by DBD plasma have been proposed, but

the small number of phases acquired (4) would likely

introduce significant error in our calculations (Greenblatt

et al. 2008a). Instead, the behavior of the oscillatory

plasma-induced flow is presented in a less quantitative, but

more global fashion in Fig. 11.

The velocity fields are averaged over 50 instantaneous

conditional (phase-locked) samples. Only the vertical

component of velocity (V) is presented to better emphasis

the pulsating nature of the DBD plasma-induced flow. Note

that the dominant feature is the pulsed suction initialized

near the electrode interface that is released over the cov-

ered electrode before creating a vortex train further

downstream indicated in the vorticity data. The sinusoidal

modulation displays a well-organized vortex train com-

mensurate with the modulation frequency that is sustained

for approximately 40 mm before dissipating. At the lowest

duty cycle (10%), the pulsed suction is quite weak and no

vortex is visible because not enough carrier cycles are

Fig. 10 Sample input

waveforms for sinusoidal a and

50% duty cycle b amplitude

modulation of 2 kHz frequency

using 100 Hz

532 Exp Fluids (2010) 48:521–537

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produced to create significant plasma-induced flows. The

30% duty cycle case shows greater suction and some sus-

tained vortex behavior. As the duty cycle is further

increased, pulsed suction near the electrode interface is

stronger and covers a much larger region while the gen-

erated vortices persist downstream. In the 70% duty cycle

case, the suction near the electrode interface also generates

a secondary flow in the form of a stationary clockwise

rotating vortex that is visible just upstream of the exposed

electrode. At 90%, the pulsing nature of the actuation is

essentially lost with little primary vortex generation. While

the behavior of actuators in quiescent flows is not neces-

sarily indicative of their behavior in a flow control envi-

ronment, these results suggest that greater control authority

may be possible by changing the way low frequency

modulation is produced. Other methods of creating low-

frequency perturbations include providing separate excita-

tion signals to each electrode (Post 2004) or more simply

using a very low carrier frequency that is not optimal for

plasma generation.

The postulation that more control authority can be

obtained using a different modulation waveform is con-

firmed by examining the increase in lift coefficient as a

function of duty cycle for an actuator operating at an

optimized modulation frequency (Fm? = 0.4) in Fig. 12.

The dielectric for this test is constructed of only three

layers of the Kapton tape previously described. Initially,

the five layers of tape were used to protect the airfoil model

from damage resulting from a dielectric breakdown.

Experience with the device as well as knowledge that a

thinner dielectric with greater bulk capacitance may pro-

duce more flow for the input signals used in this work

motivated the reduction in thickness (Corke et al. 2009).

For example, the sinusoidal modulation (solid line) now

produces DCL = 0.079 compared to the DCL = 0.05

reported in Fig. 6. The total thickness of the dielectric and

actuator is now 0.27 mm (0.011 in) and 0.44 mm (0.018

in), respectively. Note that in either case, the reported

values of DCL represent a worst case scenario since any

increase in lift from a local suction peak at the flap

Fig. 11 Phase-averaged DBD plasma-induced vertical velocity fields

(V, left) and vorticity fields (X, right) based on 50 samples in

quiescent air for modulation using a sine wave (top) as well as 10, 30,

50, 70 and 90% duty cycles (2nd from top to bottom). Carrier

frequency is 2 kHz 20 kVpp modulated at 100 Hz. Actuator size is notto scale in y and has been shifted down for clarity. Color scale is m/s

(left) and s-1 (right)

Exp Fluids (2010) 48:521–537 533

123

shoulder is not resolvable due to the obstruction of static

pressure taps. The corresponding power coefficient that is

proportional to duty cycle is also presented for this modi-

fied actuator. The power coefficient for sinusoidal modu-

lation is indicated by the dashed line at CE = 4.4% that,

due to the thinner dielectric, is increased from the 2.6%

presented in Fig. 6. As expected from the results of Fig. 11,

the 10% duty cycle case is less effective for increasing lift.

Duty cycle modulation at 20, 30 and 40% is only slightly

more effective than sinusoidal modulation. Raising the

duty cycle to 50 and 60% results in a measurable increase

in lift coefficient at the expense of increased power

requirements. The lift increase peaks at 60% and falls off

linearly for the remaining data. These characteristics

although for a slightly different actuator are qualitatively

similar to those observed in Fig. 11 especially if one

focuses specifically on the behavior near the electrode

interface. Recall that numerical results show this region

contains the highest force density, and thus is likely the

most important region for affecting the flow (Enloe et al.

2004b; Corke et al. 2007). As expected, the control

authority is not directly dependent on the power coefficient.

Rather, for a given actuator, actuator location and flow

system it is a complex function of F?, CE and duty cycle,

the latter of which can likely be expressed as an oscillatory

momentum coefficient. Future work is intended to acquire

a more exhaustive set of actuator characterization data like

that of Fig. 11 in hopes of confirming this assumption

using the methods previously discussed (Greenblatt et al.

2008a).

The results of Figs. 11 and 12 show that the effect of

DBD plasma actuators can be increased by changing the

manner in which unsteady actuation (i.e., pulsing) is

employed. It should be noted that many leading edge airfoil

separation control studies report full reattachment for duty

cycles as low as 6% (Benard et al. 2009b). The consider-

ably larger duty cycle requirement for trailing edge sepa-

ration control is certainly due to the system in question,

which has been shown to be significantly more difficult to

control due to the thicker and likely turbulent boundary

layer that forms along the chord of the airfoil. This low

momentum boundary layer encountering the adverse

pressure gradient imposed by the deflected flap is not

surprisingly very difficult to reattach to the surface.

Lastly, the effect of modulation and carrier frequency

must be considered. It is well known that DBD plasmas

create most of the momentum transfer during the forward

stroke negative half cycle of the carrier frequency (Forte

et al. 2007; Enloe et al. 2008). For example, 100 Hz

modulation of a 2 kHz carrier frequency with 50% duty

cycle corresponds to 10 high-frequency cycles per modu-

lation cycle (see Fig. 10b). A 4 kHz carrier frequency with

the same modulation frequency (100 Hz) and duty cycle

(50%) would contain 20 high-frequency cycles per modu-

lation cycle. This implies that for a given carrier and

modulation frequency, there may exist an optimum number

of high-frequency cycles and subsequent relaxation time

for creating the strongest perturbations. This optimum

would obviously be governed by the system in question,

the time response of the induced flow to the pulsed signal

and how receptive a particular flow field is to excitation. In

the quiescent and low-speed results presented here, the

relationship appears qualitatively similar as *60% duty

cycle shows the greatest effect. This also implies that for a

given dielectric with optimum carrier frequency, there

exists an upper limit for low-frequency modulation. As

before, this is governed by both a minimum number of

high-frequency cycles to affect the flow and the necessary

relaxation time to create the perturbation effect.

These findings indicate that dielectrics with high-opti-

mized frequencies give the most flexibility for exciting

high-bandwidth low-frequency modulations. In practical

applications, this point may be moot since scaling by reduced

frequency (F?) dictates that as length scales increase

frequency scales decrease thus easing requirements for high-

frequency excitation. However, for the sake of argument,

a future AC DBD plasma actuator design criteria could

center around selecting a suitably robust dielectric that has a

dimensionless optimum carrier frequency on the order

of F?*10 to allow the high-frequency forcing benefits

suggested by Amitay and Glezer (2002) while retaining

significant bandwidth in the low-frequency regime more

traditionally investigated. The success of such a device

would hinge on the selection or design of dielectric materials

specifically optimized for DBD plasma actuation.

Fig. 12 DCL and power coefficient, CE, as a function of modulation

waveform for the airfoil at zero incidence with 30� flap deflection and

Reynolds number of 240k (15 m/s). DBD plasma actuator is located

at x/c = 0.775 with sinusoidal excitation of 20 kVpp at F? = 8.5

(2 kHz) and modulation at Fm? = 0.4 (85 Hz). The standard deviation

for values of DCL is approximately 0.005

534 Exp Fluids (2010) 48:521–537

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5 Conclusions and future work

Results of the effectiveness of a single DBD plasma

actuator for controlling trailing edge separation on the flap

of a high-lift airfoil have been presented and discussed.

The airfoil model is a simplified version of the NASA EET.

The baseline configuration chosen for the current investi-

gation is at zero incidence with flap deflection of 30� for a

Reynolds number of 240k (15 m/s). Actuators are fabri-

cated using copper tape electrodes and Kapton tape

dielectric, which conform to model surface curvature.

Actuators are mounted across the span of the airfoil to

produce two-dimensional perturbation/momentum addition

in the streamwise direction. The induced flow and power

characteristics of the actuator are documented and are in

agreement with existing literature.

A single DBD plasma actuator placed at the airfoil flap

shoulder (x/c = 0.775) is effective for increasing lift and

reducing the time-averaged recirculation region over the

flap when operated in an unsteady fashion via amplitude

modulation at the natural oscillation frequency of the

trailing edge flow field (Fm? = 0.4). This occurs without a

drastic change in separation location. Quasi-steady DBD

plasma-induced flows (F? = 8.5) in this configuration

have been found to slightly delay separation while

lengthening and flattening the recirculation region. This has

little effect on the measured lift coefficient. Control results

with DBD plasma actuators show qualitative agreement

with similar studies using synthetic jet actuators (Melton

et al. 2006). Further analysis of the unsteady actuated flow

field (Fm? = 0.4) with conditionally sampled (phase-

locked) PIV shows that the flow locks to the actuation

signal. Actuation increases the pressure fluctuations on the

flap and indicates that the primary control mechanism for

this particular configuration is an amplification of the nat-

ural instabilities that entrains freestream momentum into

the separated region thereby reducing the size of the time-

averaged separation. The effect of the increased pressure

fluctuations on the structural fidelity of the flap element has

not been examined, but should be considered in the future.

The reported control results are in agreement with previous

work showing the lift enhancement gained from trailing

edge separation control is often a result of increased cir-

culation (Kiedaisch et al. 2006; Melton et al. 2006).

Because of these findings, the effect of modulation

waveform and the unsteady flow field generated by these

devices is further examined. Sinusoidal and duty cycle

modulation waveforms are compared using conditionally

sampled (phase-locked) particle image velocimetry (PIV)

for a single actuator operating in quiescent conditions. For

the results presented, the dominant flow structure is the

pulsed suction generated at the electrode interface and over

the encapsulated electrode, which convects and turns into a

vortex train further downstream. Results show that ampli-

tude modulation at low frequency using a sine wave creates

weaker pulsed suction compared to duty cycle modulation

at 30, 50 and 70%. The pulsing nature of the actuator

begins to subside at 70% and is nearly absent at 90% due to

lack of sufficient flow relaxation time. Duty cycles in the

range 50–70% appear to create the greatest velocity fluc-

tuations in quiescent conditions at the expense of an

increased power requirement.

In practice, optimized characteristics of actuators in

quiescent flow will not necessarily lead to their optimized

performance in a flow control environment. However, an

examination of different modulation waveforms and their

effect on DCL shows that qualitatively similar behavior is

obtained between control results, and the amplitude of

velocity perturbations in still air as duty cycle modulation at

60% is most effective for the flow and actuation parameters

studied. This is likely due to the low-speed freestream

(*15 m/s) and the control mechanism involved, which is

the amplification of the Kelvin–Helmholtz instability. In

addition to its superiority for reducing time-averaged sep-

aration, the optimized unsteady actuation requires approx-

imately 60% of the power budget of the quasi-steady

plasma generation. As expected, control efficacy is not

directly related to the power coefficient of the device, but

for a given actuator, actuator location and flow system are a

more complex function of F?, CE and duty cycle.

Additional results showing that trailing edge separation

control requires more energy in comparison with leading

edge separation control point to the necessity for further

optimization and characterization of the actuator (Melton

et al. 2006). Future work is intended to examine the effects

of both single and multiple DBD plasma actuators placed

upstream and on the airfoil flap over a wider aerodynamic

parameter space. This portion of the work will be done

with an emphasis on optimizing the relative phase between

actuators and its effect on the separated region. Previous

work on the same profile with synthetic jets suggests that

optimization of this relative phase can have further benefits

(Melton et al. 2004). Additional actuator geometries and

layouts are also intended for study. Because of the flexi-

bility of DBD plasma actuators, such work can be used to

examine the effect of vectoring the actuator-induced flow

and the possible production of streamwise vorticity.

Acknowledgments This work is supported by the Air Force

Research Laboratory (AFRL), Dayton Area Graduate Studies Institute

(DAGSI) Student-Faculty Graduate Fellowship and the Howard D.

Winbigler Professorship at The Ohio State University. The help of

LaTunia Melton, James Myatt, Jamey Jacob, Jolanta Janiszewska and

John Lee at the inception of this project was vital. The authors would

like to thank Jim Gregory, Kihwan Kim, Jin-Hwa Kim, Edgar

Caraballo, Annirudha Sinha, Martin Kearney-Fischer and Kristine

McElligott for help and fruitful discussions. The comments provided

by the reviewers of this paper were thorough and appreciated.

Exp Fluids (2010) 48:521–537 535

123

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