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Mechanics and Mechanical Engineering
Vol. 22, No. 1 (2018) 239–251
c
⃝Lodz University of Technology
Experimental Study of Flow Control Over an Ahmed Body Using
Plasma Actuator
S. Shadmani
S. M. Mousavi Nainiyan
R. Ghasemiasl
Department of Mechanical Engineering
West Tehran Branch
Islamic Azad University, Tehran
Iranmousavi.mojtaba@wtiau.ac.ir
M. Mirzaei
S. G. Pouryoussefi
Department of Aerospace Engineering
K. N. Toosi University of Technology, Tehran, Iran
Received (18 September 2017)
Revised (14 October 2017)
Accepted (18 November 2017)
Ahmed Body is a standard and simplified shape of a road vehicle that’s rear part has an
important role in flow structure and it’s drag force. In this paper flow control around
the Ahmed body with the rear slant angle of 25◦studied by using the plasma actuator
system situated in middle of the rear slant surface. Experiments conducted in a wind
tunnel in two free stream velocities of U = 10 m/s and U = 20 m/s using steady and
unsteady excitations. Pressure distribution and total drag force was measured and smoke
flow visualization carried out in this study. The results showed that at U = 10 m/s using
plasma actuator suppress the separated flow over the rear slant slightly and be effective
on pressure distribution. Also total drag force reduces in steady and unsteady excitations
for 3.65% and 2.44%, respectively. At U = 20 m/s, using plasma actuator had no serious
effect on the pressure distribution and total drag force.
Keywords: automotive, aerodynamics, wind tunnel, plasma actuator, Ahmed body.
1. Introduction
Increasing growth of the number of road transportation vehicles and hence fossil
fuel consumption have raised air pollution level to alert status in many cities [1]. In
this regard, considering international policies for reducing the consumption of fuels
and greenhouse gases generated by motor vehicles, researchers in the field of car
aerodynamics have tried to minimize the resisting force generated as air flow hits
the vehicles. The lower amount of this force (known as drag force), the lower will be
240 Shadmani, S., Mousavi Nainiyan, S. M., Ghasemiasl, R., Mirzaei, M. ...
its fuel consumption and pollutant emission [2]. This force can only be controlled
by accurate characterization of the flow pattern.
Since experimental study of flow control accurately and comprehensively is an
expensive and time-intensive practice due to differences in their apparent structures,
and the fact that, it is not reasonable to generalize the results of such studies to
other cases, it will be a useful practice to present a simple and standard model of
a vehicle with similar flow pattern to that of an actual car [3]. Therefore, a model
known as Ahmed body model [3] is referred to which resembles three-dimensional
flow pattern of an actual car due to the rear slant angle behind that (Fig. 1).
Figure 1 Flow pattern behind the Ahmed body model at rear slant angle of 25◦[3]
In research works, active and passive flow controls were adopted in the tailing
section of the model in an attempt to modify the flow in this region and reduce the
drag force applied to the model. Installation of various types of deflectors [4, 5] is
the passive flow control method which require no external energy [4]. Uncontrollable
performance is a disadvantage of this method [6]. Active control methods such as
mixed jets [6], pulse jets [7] and fluidic oscillators [8] are paid attention because of
their controllable nature which allows them to be turned on/off on demand. Results
of these methods in terms of reducing the drag force applied to Ahmed model are
given in Table 1.
Experimental Study of Flow Control Over an Ahmed Body ... 241
Table 1 Results of the research on the reduction of drag force
Drag Reduction (%) Reynolds Number Studies
9.3-10.7 8.7×105Hanfeng et al. [4]
9 3.1-7.7×105Fourrie et al. [5]
6.5-8.5 1.2×106Kourtaa et al. [6]
6-8 1.4×106Joseph et al. [7]
7.5 1.4×106Metka et al. [8]
Dielectric barrier discharge (DBD) plasma actuator is another modern tool for
active flow control which is used in aviation applications. As can be observed in
Figure 2, a plasma actuator is composed of two electrodes separated by a dielectric
barrier, whereby air molecules above the insulated electrode is ionized by establish-
ing a strong electric field. Known as plasma, this ionized gas extends from the edge
of the upper exposed electrode to the tailing edge of the lower insulated electrode,
and adds up local momentum to the flow passing above the region by colliding
moving charged particles to other neutral particles of the background gas [9].
Figure 2 DBD plasma actuator mechanism
These actuators have found many applications in separation control on airfoils
[10-12], and flow control on bluff bodies [13-17].
In automobile aerodynamics, Khalighi et al. 2016 [18] installed plasma actuators
on the four edges of an Ahmed body at rear slant angle of 0◦to analyze passing
flow from the tailing part of a model and showed that drag force decreases by 21.4%
at 10m/s velocity, while the reduction at 20m/s velocity is 2.8%.
In the present research, flow control around an Ahmed body is experimentally
studied by using plasma actuator system that located on the middle of rear slant
surface. For this aim, investigating pressure distribution, drag force and observing
flow pattern was performed for the first time. Experiments carried out at Reynolds
numbers of Re = 4.5×105and Re = 9×105. In order to have the most complete
three-dimensional flow pattern and highest drag force [3], angle of rear slant selected
25◦(Fig. 3).
242 Shadmani, S., Mousavi Nainiyan, S. M., Ghasemiasl, R., Mirzaei, M. ...
Figure 3 Plot of drag coefficient vs. slant angle (Ahmed et al. 1984)
2. Experimental details
Experiments were performed in the open circuit wind tunnel at K. N. Toosi Uni-
versity (Iran) with a cross sectional area of 1×1.2 m2and length of 3 m (Fig. 4).
Turbulence intensity of the flow was below 0.2% and non-uniformity of the flow
along the test section was about ±5%.
Figure 4 Used wind tunnel in this research
3. Model description
The model was constructed from 6mm-thick Plexiglas at 0.64 scale with respect to
the original model. As was mentioned before, in order to establish three-dimensional
flow behind the model and maximize the drag force obtained from flow separation,
the rear slant angle was considered to be 25◦. The model dimension was 671 ×250
×18.5 mm3(length ×width ×height), based on which one could calculate wind
tunnel section blockage ratio as 3.8%.
In order to eliminate the effects of boundary conditions on the bottom of the
wind tunnel, according to Fig. 5, the model was placed on a rectangular surface at
given spacing. The distance between surface and tunnel floor was 16×10−2m, with
the model connected to the surface via four cylindrical supports of the diameter
2×10−2m and height 32×10−2m. Moreover, leading edge of the surface was de-
signed sharply to minimize the thickness of the developed boundary layer on that.
Thickness of the boundary layer formed on the surface at flow velocities of 10 to
50 m/s was about 1.5×10−2m [6], so that the boundary layer on the surface has no
effect on the model.
Experimental Study of Flow Control Over an Ahmed Body ... 243
Figure 5 Position of the model on the surface
4. Instruments and measurement techniques
Pressure distribution was obtained using 52 pressure sensors (pressure transducers)
installed on half of the slant and vertical surfaces of the model due to symmetry.
Arrangement of the sensors is demonstrated on Fig. 6. Each sensor was composed of
a tube of inner diameters of 2 mm, connected to a 28-channel pressure transducer
(Honeywell-DC005NDC4). The results were monitored on a computer using an
A/D (PCI6224) board, Lab view software, and Farasanjesh pressure distribution
software. Sampling interval was set to 5s and pressure coefficients were calculated
with the help of the Eq. 1:
Cp=P−P∞
0.5×ρ×U2(1)
where Pis static pressure at any point on the model, P∞is static pressure of
free flow, U∞is the velocity of free flow, and ρis air density. Uncertainty of the
measurement device is about ±0.01.
The applied drag force to the model, which is exerted to the model along the
air flow, was determined using Load Cell.
Input voltage to the plasma actuator was supplied by a High-Voltage Altering
Current (A.C.) with sinusoidal carrier wave and maximum output electrical power
of 1000 W. Two digital multimeter sets were used to calculate actuation and carrier
frequencies, with the duty cycle and applied voltage been monitored by a (GW
INSTEK GDS-1072-U) oscilloscope. Furthermore, a digital multimeter (True RMS
MS8226T) was used to measure mean electrical current.
According to Fig. 7, the plasma actuator position is in such a way that, the
exposed and covered electrode are installed on the middle of slant, in an edge-to-
edge arrangement. The electrodes were made of copper, with their thickness being
50 microns. In terms of length, the electrodes were equal to the model width 25 cm,
with the widths of the open and covered electrodes being 5 and 15 mm, respectively.
In all arrangements, plasma actuator dielectric was composed of six layers of Capton
adhesive at breakdown voltage of 7 kV/mm and dielectric constant of 3.4.
244 Shadmani, S., Mousavi Nainiyan, S. M., Ghasemiasl, R., Mirzaei, M. ...
Figure 6 Position of pressure sensors
Figure 7 Position of plasma electrodes
In static flow, installing a plasma actuator on flat surface, a digital manometer
(Testo 0560 5126) along with a silicon micro-tube were applied to determine maxi-
mum induced velocity based on the applied voltage, with the corresponding voltage
applied in all experiments identically. In Table 2, input electrical parameters of the
plasma actuator, which are used in all experiments on the model are presented.
Experimental Study of Flow Control Over an Ahmed Body ... 245
Table 2 Input electrical parameters of plasma actuator
Duty cycle
(%)
Carrier fre-
quency (kHz)
Excitation fre-
quency (Hz)
Voltage
(kV)
Type Velocity
(m/s)
No.
100 10 - 6 Steady 10 1
50 10 20 6 Unsteady 10 2
100 10 - 6 Steady 20 3
50 10 40 6 Unsteady 20 4
5. Results and discussion
5.1. Pressure distribution
Three dimensional flow pattern and pressure difference between the regions in front
of and behind the model develop some compressive drag force on the model. Kra-
jnovic and Davidson [19] showed that, contribution of compressive drag force (Form
drag) into total drag is about 80%. This necessitates obtaining the pattern of pres-
sure variations across two end surfaces of the model to investigate the effect of flow
across the region.
Figure 8 Contours of pressure distribution on the rear slant of an Ahmed body in plasma off
mode at velocities of a) 10 m/s and b) 20 m/s
Fig. 8 shows contours of pressure coefficients distribution on the slant surface
behind the model prior to the installation of the plasma actuator at U = 10 m/s
(Re = 4.5×105) and U = 20 m/s (Re = 9×105), exhibiting agreement with the
results reported by Joseph et al. [7], Lienhart et al. [20] and Keogh et al. 2016 [21]
(Fig. 9).
According to Fig. 8, low-pressure regions of the flow extend along leading edge
of the slant, so that one can stipulate that, separation bubble and flow jump over
246 Shadmani, S., Mousavi Nainiyan, S. M., Ghasemiasl, R., Mirzaei, M. ...
the leading edge of slant have resulted in the development of such regions. As one
moves toward central areas, the low-pressure zones are gradually replaced by zones
of more pressure, i.e. the pressure is recovered. Of the other regions where low-
pressure zones are developed on the surface, one may refer to lateral edges of the
surface, indicating the isolation of longitudinal vortices from the top corner of the
surface along the surface length (see Fig. 1).
Figure 9 Contours of pressure distribution on the rear slant of the Ahmed model in velocity of
20m/s and no actuation condition in the studies of: a) Keogh et al. [21]; b) Lienhart et al. [20];
c) Joseph et al. [7] and; d) Present study
Figure 10 Contours of pressure coefficient distribution on the rear slant of Ahmed body at the
velocity of 10 m/s in: a) Plasma Off, b) Steady actuation, and c) Unsteady actuation
Experimental Study of Flow Control Over an Ahmed Body ... 247
As can be observed on Fig. 10, applying plasma actuation at the velocity of
U = 10 m/s, volume of the low-pressure zones on the leading edge of the surface
reduced, so that the region was much disappeared in steady state. This indicates
that, actuating the plasma, the developed vortex in this zone is degraded and the
separated flow tends to stick to the surface. This will be further observed in vi-
sualization tests. Moreover, a little reduction of low-pressure zones in the lateral
edges of the surface is indicative of reduced effect of longitudinal vortexes behind
the model. The effect of steady actuation was seen to be more significant than that
of unsteady actuation. Based on Fig. 11, at the velocity of U = 20 m/s, plasma
actuation imposed no significant effect on the reduction of low-pressure regions.
Figure 11 Contours of pressure coefficient distribution on the rear slant of Ahmed body at the
velocity of 20 m/s in a) Plasma Off, b) Steady actuation, and c) Unsteady actuation
Figure 12 Plot of pressure coefficient distribution on the centerline of the model at U = 10 m/s
and U = 20 m/s
248 Shadmani, S., Mousavi Nainiyan, S. M., Ghasemiasl, R., Mirzaei, M. ...
Figs. 12 and 13 illustrate the plot of pressure distribution in no-actuation, steady
actuation, and unsteady actuation cases on the centerline and lateral edge of the
model at velocities 10 m/s and 20 m/s, respectively. Fig. 12 shows at the velocity
of 10 m/s, plasma actuation increased pressure coefficients, particularly across the
regions related to slant. This is while the effect of plasma actuator is insignificant at
the velocity of 20 m/s. Fig. 13 indicate that at velocity of 10 m/s, plasma actuator
modified and enhanced pressure coefficient diagram considerably. But as expected
at velocity of 20 m/s no significant effect occurred in diagram.
Figure 13 Plot of pressure coefficient distribution on the lateral edge of the model at velocities
of U = 10 m/s and U = 20 m/s
5.2. Drag measurement
Total drag force applied to the model is made up of compressive and frictional drag
forces applied to different regions of the model. Based on the pressure distribution
pattern on the tailing surfaces of the model (described in Section 3.3), negative
pressure gradient behind the model results in the formation of pressure difference
between the leading and tailing zones, ending up exerting compressive drag force
onto the model. The frictional force is a result of the friction and direct collision of
the air flow with lateral, lower, and upper surfaces of the model. Proposing some
relationships, Thacker et al. [11] specified the contribution from each of the two
forces into the overall force applied to model.
In the present research, total drag force applied to the model was measured
using Load cell. Prior to applying plasma actuation, the drag force applied to the
model was found to be equal to 0.8 N at Re = 4.5×105(U=10m/s) and 3.22 N at
Re = 9×105(U = 20m/s). Since drag force is directly proportional to the velocity
squared, it can be observed that, doubling the velocity applies a force of four times
as large to the model. Considering the relationship for drag force base on Eq. 2,
drag coefficient of the model was found to be 0.29, which is in agreement with the
Experimental Study of Flow Control Over an Ahmed Body ... 249
results reported in Ahmed et al. [4] (see Fig. 3).
Cd=FD
0.5×ρ×A×U2(2)
where U is free flow velocity, Ais effective cross-sectional area in front view, and ρ
is air density in experimental conditions
Base on Fig. 14, applying plasma actuation at the velocity of 10 m/s, the values
of drag force reduced by about 3.65% and 2.44% in steady and unsteady actuations,
respectively. This is while, at the velocity of 20 m/s, the reduction of drag force in
steady and unsteady actuations reached 0.91%. The results indicate that, at lower
velocities, particularly in steady actuation, the plasma actuator can reduce overall
drag force applied to the model more considerably. Considering the fact that the
dominant drag force in the Ahmed model is pressure drag, the results obtained from
drag force measurement using Load cell exhibit good agreement with the pressure
measurement results.
Figure 14 Plot of drag reduction
5.3. Flow visualization
Considering the results obtained in previous sections indicating proper performance
of plasma actuator at lower velocities, in this section, the smoke flow visualization at
velocity of U = 10 m/s across tailing section of the model was visualized. According
to Fig. 15a, in Plasma off mode, the air flow separates from the surface as it passes
over the leading edge of the rear slant, developing a vortex on the slant surface.
Moreover, with the help of the flow passing through the lower part of the model,
a larger vortex is formed behind the vertical surface of the model. As can be
observed in Figs 15b and 15c, the plasma actuator somewhat suppresses the flow
passing over the surface edge by actuating shear layer, and the separated flow from
the slant surface gets closer to the surface. It is more sensible in steady actuation.
250 Shadmani, S., Mousavi Nainiyan, S. M., Ghasemiasl, R., Mirzaei, M. ...
Figure 15 Pattern of the flow passing through the region behind the model at U = 10 m/s in
different actuations: a) Plasma off, b) Steady actuation, and c) Unsteady actuation
6. Conclusion
In the present research, a plasma actuator was used to study flow control around
an Ahmed body with a rear slant angle of 25◦. Plasma actuator electrodes were
installed on the middle of the rear slant surface and actuations were performed in
steady and unsteady actuations at different Reynolds numbers and flow velocities
of U = 10 m/s and U = 20 m/s.
Visualization results indicate that, at U = 10 m/s the plasma actuator is capable
of suppressing the flow separated from the leading edge of rear slant and sticking it
back to the surface by actuating the shear layer. In this case, volume of longitudinal
vortexes behind the model is reduced while increasing pressure coefficient on the
slant at the same time. With reducing the pressure difference between leading and
tailing sections of the model, compressive drag force and hence total drag force
decrease, so that the reduction of drag force in steady and unsteady actuations was
3.65% and 2.44%, respectively. At the velocity of U = 20 m/s, the results indicated
that, the plasma actuator may not significantly affect pressure enhancement on
ending surfaces of the model, with the reduction of drag force in both steady and
unsteady state actuations being about 1%. Therefore, application of the plasma
actuator at low velocities, particularly in steady actuation, can serve as an effective
tool for flow control and suppression of the drag force applied to the model.
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