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Plasma Science and Technology, Vol.15, No.4, Apr. 2013
Flow Control over a Conical Forebody by Periodic Pulsed Plasma
Actuation∗
ZHENG Borui (郑博睿)1, GAO Chao (高超)1, LI Yibin (李一滨)1, LIU Feng (刘锋)2,
LUO Shijun (罗时钧)2
1Northwestern Polytechnical University, Xi’an 710072, China
2University of California, Irvine, CA 92697-3975, USA
Abstract The flow control mechanism of plasma actuators with periodic pulsed discharge to
control the bi-stable vortices over a cone-cylinder is investigated. The actuators are installed on
the leeward surface near the apex of a cone which has a semi-apex angle of 10o. The effectiveness
of the plasma actuation under different free-stream velocities and angles of attack is analyzed. The
pressure distributions over the conical forebody are measured by both steady and dynamic pressure
transducers. The transient dynamic pressure distribution tends to gradually become steady as
the free-stream velocity increases, that is, the pulsed actuation approximates a continuous one.
Furthermore, the flow control effectiveness becomes less noticeable as the free-stream velocity or
the angle of attack increases under certain controlling electrical parameters.
Keywords: flow control, conical forebody, periodic pulsed actuation, plasma actuator
PACS: 47.85.L-
DOI: 10.1088/1009-0630/15/4/08
1 Introduction
As the angle of attack is increased beyond a certain
value, the initially symmetric separation vortices over
slender wings or bodies become asymmetric, causing a
large lateral aerodynamic load. In addition, conven-
tional aerodynamic control surfaces become ineffective
in such situations because of the vortex wakes gener-
ated by the forebody. Proportional lateral control on
slender forebodies at high angles of attack is highly
needed in aerodynamic design of aircraft. The fact that
the separation vortices over pointed forebodies generate
large air loads and are very sensitive to small perturba-
tions near the body apex offers an exceptional opportu-
nity for manipulating them with little energy input to
achieve active lateral control of the vehicle in place of
conventional control surfaces. It has been found exper-
imentally that unsteady control techniques are needed
to achieve this goal [1∼3].
Recently, LIU [4] reported the nearly linear propor-
tional control of lateral forces and moments over a slen-
der conical forebody at high angles of attack by employ-
ing a novel design of a pair of surface dielectric barrier
discharge (SDBD) plasma actuators near the cone apex
combined with a kind of double-sided alternate pulsed
discharge.
PATEL [5] showed that periodic pulsed discharge
yields a greater impact on flow separation than contin-
uous steady discharge. ZHENG [6] studied the mecha-
nism of periodic pulsed discharge, and confirmed that
the main mechanism of the periodic pulsed actuation
in momentum transfer is the formation of strong vor-
ticity, rather than the gas acceleration. The optimum
pulsed discharge frequency had been investigated by
ZHENG [7] for plasma flow control of a cone-cylinder
model, in which the reduced pulse-repetition frequency
based on the local diameter at the plasma actuator
equal to one yielded the highest effectiveness among
the cases considered.
To the authors’ knowledge, there have been few in-
vestigations on the flow control mechanism of peri-
odic pulsed actuation on bodies of revolution at dif-
ferent free-stream velocities and angles of attack. In
the present paper, wind tunnel experiments have been
conducted to study the flow control mechanism under
different free-stream velocities and angles of attack by
analysis of both the steady and unsteady pressure dis-
tributions over the conical forebody.
2 Experimental setup
The model and plasma actuators are the same as
those described in Ref. [6], except for the pressure in-
strumentations. The model consists of two separate
pieces. The frontal portion of the cone is made of plas-
tic and is 150 mm in length. The rest of the model is
made of metal. The total length of the cone is 463.8 mm
with a base diameter of 163.6 mm, as shown in Fig. 1.
Two long strips of SDBD plasma actuators are installed
on the plastic frontal cone near the apex, as shown in
Fig. 2(a). The frontal piece of the cone is interchange-
∗supported by the Foundation for Fundamental Research of the Northwestern Polytechnical University (NPU-FFR-W018102 and
JC201103)
ZHENG Borui et al.: Flow Control over a Conical Forebody by Periodic Pulsed Plasma Actuation
able so that cones with different plasma actuator de-
signs can be tested. Care is taken in the manufacture
and mounting of the frontal cone to the rear portion of
the model to make sure that they are well aligned.
Fig.1 The model
Relatively small SDBD plasma actuators are made
so that they can be placed as close to the cone apex
as possible. The plasma actuator consists of two asym-
metric copper electrodes, each of 0.03 mm thickness. A
thin Kapton dielectric film wraps around the cone sur-
face and separates the encapsulated electrode from the
exposed electrode, as shown in Fig. 2(b). The length of
the electrodes is 20 mm along the cone meridian with
the leading edge located 9 mm from the cone apex. The
widths of the exposed and encapsulated electrode are
1 mm and 2 mm, respectively. The two electrodes are
separated by a gap of 1.5 mm, where the plasma is cre-
ated and emits a blue glow in darkness.
Fig.2 Sketches of the plasma actuators
Three actuator operation modes are defined. The
plasma-off mode corresponds to the case when nei-
ther of the two actuators is activated. The plasma-
on mode refers to the conditions when either the port
or starboard actuator is activated while the other is
kept off during the test. These are called the port-on
and starboard-on modes, respectively. Each of the two
actuators on the cone model is separately driven by
a multi-channel plasma generator, which was designed
and made by Y. B. LI, one author of the present pa-
per. The waveform of the AC source is a sine wave,
the peak-to-peak voltage Vp−pis set to 12.7∼14.2 kV
and the carrier frequency is fixed at F= 30 kHz. The
pulse frequency can be set by the controlling software
attached to the plasma generator. Two periodic pulse
frequencies, 50 Hz and 500 Hz, are applied during the
experiments. The input power for the plasma pulsed
discharge is 224 W at τp= 50%.
The tests were conducted in an open-circuit low-
speed wind tunnel at Northwestern Polytechnical
University. The wind tunnel’s test section has a
3.0 m×1.6 m cross section and the model is rigidly
mounted on a support from the port side of the model
aft-cylinder, as shown in Fig. 3. The support is fixed
onto the turning plate of the angle of attack imbed-
ded into the bottom wall of the test section. The
Reynolds number based on the cone base diameter is
about 1.0×105∼2.5×105. The model is carefully
cleaned before each run.
Fig.3 Model in the wind tunnel
In order to capture the most detailed flow infor-
mation possible, nine stations (cross sections) of the
conical forebody x/L= 0.360 to 0.964 (Lrepresents
the length of the cone forebody) are chosen to in-
stall pressure transducers. At stations 1 to 7 and
station 9, 36 pressure transducers are uniformly dis-
tributed circumferentially with an interval of 10 deg az-
imuthal angle and time-averaged pressure transducers
are used in these stations to measure the static pres-
sure. At station 8, 24 dynamic pressure transducers
are mounted to measure the transient static pressure
around the circumference, as shown in Fig. 4. The
time-averaged pressure transducers are PSI-9816 which
samples at a frequency of 100 Hz, and the dynamic
pressure transducers are Kulite XCQ-093 with a sam-
pling frequency of 5000 Hz. The input pressure range
is 0.35 bar and the perpendicular acceleration sensitiv-
ity is 1.5×10−3%FS/g. The pressure data are acquired
after consecutive 10 second periods for both the steady
and dynamic pressure transducers.
Fig.4 Kulite distributions on station 8
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Plasma Science and Technology, Vol.15, No.4, Apr. 2013
3 The base measurements
3.1 Plasma-off flow at zero angle of at-
tack
In order to check the accuracy of the model setup in
the wind tunnel, a test is run at a zero angle of attack
with plasma-off. Fig. 5 presents the ensemble-averaged
pressure distributions round the circumference of sta-
tions 1∼7 and station 9 at α= 0o,U∞= 20 m/s. Aside
from some slight irregularities, the measured pressure
distributions exhibit an essentially axisymmetric flow
around the cone. In the present study, the plasma ac-
tuators are made by hand and then attached to the cone
tip surface with glue. The dielectric film wraps around
the entire circumference. No allowance is made on the
cone surface for the attachment, which could have been
the cause for the mentioned irregularities of the pres-
sure distributions. Nevertheless, the disturbances were
tolerably small.
Fig.5 U∞= 20 m/s, α= 0o,Cpvs. θ, plasma-off
3.2 Convergence of ensemble averaged
pressures with sampling time
Fig. 6 presents the convergence of the ensemble-
averaged pressure distribution at station 8 under
U∞= 20 m/s, α= 45ofor starboard periodic pulsed ac-
tuation with F= 30 kHz, pulse frequency fp= 50 Hz,
duty cycle τp= 50%, and Vp−p= 12.7∼14.2 kV. The
pressure distributions around the circumference of sta-
tions 1∼9 where the PSI-9816 and Kulite transduc-
ers are mounted with sampling frequencies of 100 Hz
and 5000 Hz, respectively. Comparisons of the data ac-
quired at different sampling instants reveal that there
is little difference between the pressures acquired at the
three instants, that is, the pressure distribution is stable
and the data acquisition is reliable. The same is true
for plasma-off and port-on (not shown here for brevity).
We will present the averaged data within 10 seconds
in the subsequent sections. It is seen that the suction
peaks of pressure distributions seem to be well captured
by the 24 unsteady pressure transducers at station 8.
Fig.6 Comparison of pressures ensemble-averaged over
1∼10 s for periodic pulse starboard-on, U∞= 20 m/s,
α= 45o,fp= 50 Hz, τp= 70%, Vp−p= 12.7 kV, station 8
4 Different free-stream velocities
The pressure distributions in Fig. 7 show that the
flow control of plasma is effective at U∞= 10 m/s,
15 m/s, 20 m/s and 25 m/s, under α=45o. The typ-
ical bi-stable mode may be affected by free-stream con-
ditions and slight geometric imperfections of the cone
near the apex. By taking advantage of the sensitivity of
the flow near the apex of the cone, we can control the
vortex structure and thus the side force and moment
by activating one of the installed plasma actuators. It
should be noted that the mode of the bi-stable vortices
at the leeward side of the conical model might be differ-
ent even under the same free-stream condition, which
is due to model imperfection or the free-stream condi-
tion. The pressure distributions of plasma-off should
be measured every time before the plasma actuators
are activated.
Fig. 7(a) indicates that the starboard-on mode raises
the port suction peak and lowers the starboard suction
peak. The pressure distributions under the plasma-off
case and the port-on case nearly coincide, which means
that the strength and position of the two vortices do
not vary with the actuation intensity within the range
in the present experiment, or in other words, bi-stable
vortices are stable. The result of the starboard-on case
almost overlaps that of the plasma-off case, as shown
in Fig. 7(b). This is because the asymmetric perturba-
tions produced by the port-side plasma actuator merely
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ZHENG Borui et al.: Flow Control over a Conical Forebody by Periodic Pulsed Plasma Actuation
Fig.7 Cpvs. θ,α= 45o,fp= 50 Hz, τp= 70%, Vp−p= 12.7 kV, F= 30 kHz, station 1
reassure the preexisting plasma-off asymmetry of the
flow. Fig. 7(c) illustrates that when the port plasma
actuator is activated, the pressure distributions exhibit
stronger suction on the starboard side and weaker suc-
tion on the port side, indicating that the starboard-
side vortex moves closer to the cone while the port
vortex moves farther away from the cone. It leads to
the change of positions of the two vortices. The lo-
cation of the boundary layer separation point can be
inferred as the end point of the pressure recovery as
demonstrated by HALL [8] . The plasma blowing edge
is located at θ=±120o. The plasma blows in the
circumferential and downward direction tangent to the
cross section surface of the cone. The plasma jet tends
to stay attached to the surface in circumferential di-
rection due to the Coanda effect. In comparison with
the plasma-off case, when plasma is port-on, the port-
side boundary-layer separation point moves downward
from θ= 120oto θ= 110owhile the starboard-side
boundary-layer separation point moves upward from
θ=−120oto θ=−110o. Although the differences
between the pressure distributions of the three modes
are small at U∞= 25 m/s, as shown in Fig. 7(d), sig-
nificant effects of the plasma-on flow are still observed.
It is noted that the changes produced by port-on and
starboard-on are opposite in direction but not equal in
magnitude.
Among other factors, imperfections in the model,
particularly those due to the installation of the plasma
actuators mentioned earlier, are believed to prevent
the results from being exactly bi-stable. It is known
that the flow asymmetry depends on the body roll an-
gle or the micro surface imperfections of the model for
plasma-off [6]. Actuating the actuators on both sides of
the conical model with the same electrical parameter,
the two suction peaks should be axisymmetric theoreti-
cally, but the pressure distributions from the wind tun-
nel experiments show different results, whose suction
peaks just change a little in magnitude. It is suggested
that the momentum induced by the actuators is still not
large enough to alter the suction peaks of the bi-stable
vortices.
Table 1 compares the local side force coefficient cal-
culated from the pressure distributions of plasma-off,
port-on and starboard-on modes. The CYd is the local
Table 1. Flow control effectiveness at different free-stream velocities
(α= 45o,fp= 50 Hz, τp= 70%, Vp−p= 12.7 kV, F= 30 kHz, station 1)
U∞10 m/s 15 m/s 20 m/s 25 m/s
CYd 4CYd CYd 4CYd CYd 4CYd CYd 4CYd
Plasma-off 1.181 0 −0.894 0 −0.712 0 −0.931 0
Port-on 1.177 −0.004 −0.552 0.342 0.538 1.250 −0.691 0.240
Starboard-on 0.769 −0.412 −0.994 −0.100 −0.427 0.285 −0.908 0.0228
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Plasma Science and Technology, Vol.15, No.4, Apr. 2013
side force coefficient and the 4CYd is the deviation of
the local side force coefficient under actuator port-on
case or starboard-on case from plasma-off case. The
deviation 4CYd at U∞= 20 m/s is larger than in other
cases, which is agreement with the suction peak changes
shown in Fig. 7. This indicates that the flow control ef-
fectiveness is dramatic at U∞= 20 m/s, which may
be caused by the coupling of the actuator controlling
electrical parameters and the free-stream velocity.
The transient characteristics of the flow induced by
the periodic pulsed plasma actuations are investigated
with the phase-locked method, which has been dis-
cussed in Ref. [6]. As the free-stream velocity increases,
the difference of pressure distributions between differ-
ent phase angles becomes small, and the pressure dis-
tributions under all phase angles approach the ensem-
ble average one as shown in Fig. 8. This is probably
because of the unchanged plasma actuation which can
not match the increased free-stream velocity, thus fails
to exert influence on the flow. Fig. 9 shows that the
flow control effectiveness diminishes as the angle of at-
tack increases and that the pressure distributions un-
der three different conditions nearly coincide at U∞=
25 m/s. The pressure distributions at U∞= 10 m/s in
Fig. 9(a) are greatly different from those at other three
free-stream velocities for some unknown reasons.
Fig.8 Cpvs. θ,α= 45o,fp= 50 Hz, τp= 70%, Vp−p= 12.7 kV, F= 30 kHz, port on, station 8
5 Different angles of attack
The flow control effectiveness at different angles of
attack has been investigated by analyzing the pressure
distributions measured by both the steady and the dy-
namic pressure transducers. The flow control is effec-
tive at α= 45oand 50o, but less effective at 55o, as
shown in Fig. 10. From Table 2, it can be found that
∆CYd becomes smaller as αincreases, and nearly equals
zero at 55o. Fig. 11 reveals that the unsteady pressure
distributions under different phase angles coincide as
a whole although the actuation on the apex of conical
model is unsteady. As the angle of attack increases,
the flow control effectiveness becomes less noticeable
as shown in Fig. 12. This might be attributed to the
boundary layer separation line, which moves upstream
and widens the separation area when the angle of at-
tack increases. The momentum induced by plasma ac-
tuators cannot stand up to the increased adverse pres-
sure gradient, so the position of the separation line of
the boundary layer remains unchanged under plasma
actuations, and the two vortices remain unchanged. A
larger intensity of actuation should probably be used to
function for larger separation areas and larger vortices.
In fact, the effectiveness of the actuator configuration
for flow control has been proved for the same cone-
cylinder model under the free-stream velocity U∞=
5 m/s, and angles of attack α= 35oto 50oin Ref. [4].
Thus it can be concluded that the actuator is effective
when the free-stream velocity U∞is less than 25 m/s
at angles of attack α= 45oto 50o.
354
ZHENG Borui et al.: Flow Control over a Conical Forebody by Periodic Pulsed Plasma Actuation
Fig.9 Cpvs. θ,α= 45o,fp= 50 Hz, τp= 70%, Vp−p= 12.7 kV, F= 30 kHz, station 8
Fig.10 Cpvs. θ,U∞= 20 m/s, fp= 500 Hz, τp= 80%, Vp−p= 14.2 kV, F= 30 kHz, station 1
Fig.11 Cpvs. θ,U∞= 20 m/s, fp= 500 Hz, τp= 80%, Vp−p= 14.2 kV, F= 30 kHz, port on, station 8
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Plasma Science and Technology, Vol.15, No.4, Apr. 2013
Table 2. Flow control effectiveness at different angles of attack
(U∞=20 m/s, fp=500 Hz, τp=80%, Vp−p=14.2 kV, F=30 kHz, station 1)
α α = 45oα= 50oα= 55o
CYd 4CYd CYd 4CYd CYd 4CYd
Plasma-off 0.150 0 0.963 0 −0.148 0
Port-on 0.757 0.606 1.051 0.089 −0.116 0.032
Starboard-0n −0.270 −0.420 0.486 −0.476 −0.130 0.018
Fig.12 Cpvs. θ,U∞= 20 m/s, fp= 500 Hz, τp= 80%, Vp−p= 14.2 kV, F= 30 kHz, station 8
6 Conclusion
The effectiveness of plasma flow control under dif-
ferent free-stream velocities and angles of attack has
been studied. A pair of surface dielectric barrier dis-
charge plasma actuators has been installed near the
cone apex to alter the bi-stable vortices. From the mea-
sured pressure distributions, the following conclusions
can be drawn.
a. Starting from plasma-off pressure distribution,
the port-on mode raises the starboard suction peak and
lowers the port suction peak, while the starboard-on
mode raises the port suction peak and lowers the star-
board suction peak.
b. Measurement of the unsteady pressure distribu-
tions reveals that the effectiveness of the pulsed actua-
tion tends to be equivalent to a continuous steady ac-
tuation as the free-stream velocity increases.
c. The flow control effectiveness is dramatic at U∞
= 20 m/s under certain electrical parameters, which is
due to the coupling of controlling electrical parameters
and free-stream velocity, and the plasma control is ef-
fective when the free-stream velocity U∞is less than
25 m/s at angles of attack α= 45oto 50o. As the free-
stream velocity increases or the angle of attack is equal
to or greater than 55o, the flow control effectiveness be-
comes weak. As for angles of attack below 45o, further
experiments might be needed.
Further investigations should be conducted to study
the corresponding correlation between the cone surface
pressure distribution and the structure and strength
of separation vortices by visualization techniques such
as PIV to improve the flow control effectiveness of the
plasma actuators.
Acknowledgments
The 1st author would like to thank Professor
Zhengke ZHANG of Northwestern Polytechnical Uni-
versity for helpful discussions during the wind-tunnel
experiments, and also thank to Yinzhe LI, Weimin
GENG, Yu YANG, and Jiapei SI of Northwestern Poly-
technical University (NPU) for providing support with
the data acquisition system, electronics, and experi-
mental setup.
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(Manuscript received 17 January 2012)
(Manuscript accepted 23 February 2012)
E-mail address of ZHENG Borui: narcker@hotmail.com
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