high-lift airfoil trailing edge separation control using a
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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)
Exp Fluids (2010) 48:521–537 529
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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