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7/28/2019 Art-1. Exp Fluids-2010, Vol.48 (Pages 116)
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RE S E A RCH A RT I CL E
Aerodynamic drag reduction by vertical splitter plates
Patrick Gillieron
Azeddine Kourta
Received: 8 June 2008/ Revised: 13 June 2009/ Accepted: 15 June 2009 / Published online: 2 July 2009
Springer-Verlag 2009
Abstract The capacity of vertical splitter plates placed at
the front or the rear of a simplified car geometry to reducedrag, with and without skew angle, is investigated for
Reynolds numbers between 1.0 9 106 and 1.6 9 10
6. The
geometry used is a simplified geometry to represent estate-
type vehicles, for the rear section, and MPV-type vehicle.
Drag reductions of nearly 28% were obtained for a zero
skew angle with splitter plates placed at the front of models
of MPV or utility vehicles. The results demonstrate the
advantage of adapting the position and orientation of the
splitter plates in the presence of a lateral wind. All these
results confirm the advantage of this type of solution, and
suggest that this expertise should be used in the automotive
field to reduce consumption and improve dynamic stability
of road vehicles.
List of symbols
LA Length of the Ahmed body
lA Rear window length
wA Width of the Ahmed body
HA Total height of the Ahmed body
h Height of the geometry front partw Width of the geometry front part
Re Reynolds number based on the geometry
length
sl Viscous shear stress tensor
st Turbulent shear stress tensor
Pio Farfield total pressure
P Static pressure
dr Surface element
n~ Normal vector unit
x~ Vector unit in the longitudinal plane
R Surface of the outlet boundaries around
Ahmed body (R RL Se Ss Sc)RL Lateral surface
Se Inlet section (engine compartment)
Ss Outlet section (engine compartment)
Sc Body surface
V~0 Upstream velocity vector
V~ Local velocity vector
Vx, Vy, Vz Velocity components
x~; y~; z~ Vector unit system related to the model
S Transversal section immediately downstream
of the bluff body
Cp Static pressure coefficient
q Density
a Angle between the rear window and the
upstream flow direction
b Skew angle
k Orientation angle of the splitter plate related to
the body
h Angle between the splitter plate and the
velocity VoCd Aerodynamic drag coefficient
Cdref Reference aerodynamic drag coefficient
P. Gillieron
Research Division, Fluid Mechanics & Aerodynamics,
Renault Group, 1, avenue du Golf (TCR AVA 058),
78288 Guyancourt, France
e-mail: [email protected]
A. Kourta (&)
Institut PRISME, ESA, PolytechOrleans, 8 rue Leonard de
Vinci, 45072 Orleans Cedex 2, France
e-mail: [email protected]
123
Exp Fluids (2010) 48:116
DOI 10.1007/s00348-009-0705-7
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1 Introduction
The growing request on fossil energy transforms today the
world energetic system becoming incompatible with the
available resources and the necessity to reduce gas emis-
sion with greenhouse effects. As an example, the number
of Chinese road vehicles in 20 years from now will be
approximately 270 millions30 times more than that in2002 (Passenger Cars 2006). East Europe and Africa
follow the same procedure at more or less equivalent
proportion and at more or less long term. In this context,
the gas with greenhouse effects emission will increase of
about 57% in 2030 with environmental and climate strong
repercussion (IEA 2007). Solutions have hence to be
searched in different physical domains, by and for transport
industry, to reduce significantly the CO2 emission. In this
situation and independently of the energy used (fossil
combustible, electric, hydrogen, etc.), the aerodynamic
flow control for the road vehicles becomes necessary to
optimise the loaded energy. Scientific researchers work toreduce the vehicle drag of about 30% and to reduce the
CO2 emission of about 12 (New European Driving Cycle as
NEDC) to 24 g/km (customer real cycle). All solutions and
results obtained previously and currently need to be actu-
alised, adapted and improved in the perspective use for the
road vehicle.
Amongst the solutions retained in previous studies
(Gad-el-Hak 1996), the use of splitter plates can be a good
and an interesting solution because it constitutes a simple
device without any electronic (no sophisticated actuator).
The use of these splitter plates has been performed before
without testing their effects on specific shape related to
automotive vehicle (front right side for a bus, inclined for
monospace (Fastback) type, rear right side for utility
vehicle (square back), inclined rear window for estate car,
and so on) for Reynolds number higher than 106. Their use
for real car depends on the results on simplified car
geometry in the presence of lateral wind. This study is
related to this subject and hence concerned by the evalu-
ation of splitter plate effect on simplified car geometry with
or without side wind.
Roshko and Koenig (1978) demonstrated that it is
possible to reduce by 97% the drag on a cylinder with
its axis parallel to the incident flow direction V0 using
circular discs placed perpendicularly upstream to the
velocity direction of V0. The result is obtained for a
Reynolds number based on the cylinder diameter equal
to 5 9 105. Mair (1965) analysed the effect of splitter
discs set downstream of the bases and perpendicular to
the incident flow. Experiments performed on a torpedo-
type obstacle equipped with a splitter disc downstream
of the base for a Reynolds number equal to 6 9 105
demonstrated that aerodynamic drag may be reduced by
35%. A second splitter disc placed downstream of the
first disc achieved drag reductions of nearly 55%.
However, these geometries remain very far from the
road vehicle geometries and, the ground effects have never
been considered. Significant results have to be obtained on
representative and simplified geometry representing better
the automotives to convince the industry for the splitter
plates use. The work proposed here is conducted on theAhmed body, geometry commonly used as one represen-
tative of automotive aerodynamics. Some studies have
been conducted on this geometry with longitudinal splitter
plates (Baudoin and Aider 2008) and vertical splitter plates
(Levallois and Gillieron 2005).
This work completes previous studies and aims to
characterise the influence of splitter plates on aerodynamic
drag with simplified geometry hatchback and MPV- or
utility-type vehicles. Swept angle effect on the efficiency
of the splitter plates placed front or behind the model (with
inclined front and square base) is analyzed. The experi-
ments were performed in a wind tunnel, around Ahmedbody, varying the position, orientation and dimensions of
the splitter plates. The splitter plate, with different skew
angle, placed at front or rear of the geometry is also
examined.
2 Theoretical bases
Aerodynamic drag is defined as an integral over the
surface vehicle of the static pressure, friction and tur-
bulence stresses. It can be also obtained by a simplified
analytical model based on the momentum equation
applied to the air inside a stream tube enclosing the
vehicle (see Fig. 1).
From the pressure, viscosity and turbulent stress distri-
bution over the vehicles boundary surface Sc (Fig. 1), the
aerodynamic drag is given by
Fx
ZSc
PIn~dr
ZSc
sl stn~dr
264
375x~ 1
Fig. 1 Integral momentum balance
2 Exp Fluids (2010) 48:116
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where sl;, st and PI represent the viscous, turbulent and
pressure stress tensors, respectively. The vector n~ is a unit
vector moving towards the outside of the fluid domain, and
x~ is a unit vector with the same direction, collinear to the
far field velocity V0!
.
Viscous and turbulent stresses are connected to the
formation and development of boundary layers on the
vehicle surface. The aerodynamic pressure forces dependon the vehicle geometry and also on a distribution and
evolution of the pressure related to the wake vortices. The
aerodynamic drag given by Eq. 1, and averaged over a
duration Dt, was measured in a wind tunnel using an
aerodynamic balance. For an estate car, the aerodynamic
pressure and friction contributions represented, respec-
tively, 90 and 10% of the total aerodynamic drag (classical
result).
The aerodynamic pressure forces deduced from Eq. 1
and expressed as a function of the static pressure coeffi-
cient Cp is defined by
Cp P P0q2
V202
where P0 is the static pressure, V0 is the upstream farfield
velocity and q is the density of air, and the aerodynamic
drag is given by
Fx q
2V20
ZSc
Cpn~x~dr 3
With a rounded front face-like Rankines half-oval without
flow separation, aerodynamic drag is directly controlled by
a static pressure distribution on the base. In general, on an
automotive vehicle, the static pressure coefficient Cp on the
base is between -0.05 and -0.20. Controlling the flow
therefore consists of obtaining a zero static pressure coef-
ficient on the base.
Aerodynamic drag can also be analysed using pressure,
viscous and turbulent information distributed over the
boundary surface R (Fig. 1), and contained essentially in
the wake (classical momentum equation, see Eq. 1). In this
case, its expression is given by
Fx ZR
PIn~dr ZR
sl stn~dr24 35x~
ZR
qV~x~V~n~dr 4
where V!
is the local velocity vector.Onorato et al. (1984)
proposed a simplified analytical model based on the
momentum equation applied to the flow inside a stream
tube enclosing the vehicle (see Fig. 1). The mean flow is
assumed to be steady and incompressible, and gravity and
turbulence effects are considered negligible compared to
the pressure effect (Cousteix 1989). These simplifications
lead to the Onorato expression of the aerodynamic drag,
given in Eq. 5.
Fx qV20
2
ZS
1 Vx
V0
!2dr
qV20
2
ZS
Vy
V0
2
Vz
V0
2" #dr
ZS
Pi0 Pidr 5
where Pi0 is the reference total pressure, V0 is the external
flow velocity, q is the density, Pi is the total pressure and
Vx, Vy, Vz are the components of the velocity vector. The
expression (5) is then used to define the aerodynamic drag
of a motor vehicle according to velocity and total pressure
fields measured in the wake cross section S, downstreamfrom the base (Ardonceau and Amani 1992).
The first term of the expression (5) represents the drag
associated with the longitudinal velocity deficit measured
inside the near-wake zone. Far downstream, in the wake,
the longitudinal velocity component Vx becomes approxi-
mately equal to Vo and hence this term is equal to zero.
This term is related to the development of transversal
vortices at the base (Onorato et al. 1984). The second term
corresponds to the vortex drag, associated with the devel-
opment of longitudinal vortices on the geometry. Finally,
the third term expresses the drag induced by the total
pressure loss between the upstream and the downstream of
the motor vehicle, associated with the formation and
maintenance of separated swirling structures in the wake.
According to the Onorato expression (5), the aerody-
namic drag of a motor vehicle is mainly due to the for-
mation of separated flow on the geometry, and on the
formation of transversal and longitudinal swirling struc-
tures in the wake. Therefore, the drag reduction can be
obtained by reducing, or even eliminating, the longitudinal
vortices (second term, 20% of a total drag), by reducing the
wake cross section Sor by limiting the total pressure loss in
the wake (third term, 80% of a total drag). The separation
locations depend on the local wall curvature, the longitu-
dinal static pressure gradient (Cousteix 1989), the inflow
turbulence intensity (Arnal et al. 1976) or the roughness
(Granville 1985).
3 Experiment conditions
The experiments were performed in the wind tunnel at the
Paris ENSAM [Ecole Nationale Superieure dArts &
Exp Fluids (2010) 48:116 3
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Metiers] Aerodynamics Laboratory. This Prandtl-type
closed wind tunnel has a working section of 2.00 m long,
1.65 m wide and 1.35 m in height. It is an open work
section wind tunnel (Fig. 2). The maximum turbulence
level of this wind tunnel is less than 1%, and the maximum
inflow velocity Vo may reach 40 m/s. This wind tunnel is
equipped with a three-component aerodynamic balance
allowing the measurement of the drag, the side force and
the yawing moment. The experimental model is placed on
a circular cylinder raised from the wind tunnel floor
(Fig. 3). This element allows to reduce the boundary layer
thickness up to 80% at the wind tunnel roof and to model
the lateral wind (side wind).
The experiments were performed on a simplified auto-
motive geometry (Fig. 4), namely, Ahmed body at scale
0.75 (Ahmed et al. 1984; Gillieron and Chometon 1999).
The Ahmed body lengths LA and lA, width wA and height
HA used are, respectively, equal to 783, 158, 216 and
292 9 10-3 m. In these conditions, the blockage ratio is
less than 3%. The model is positioned on the aerodynamic
balance with the help of three circular cylinders (diameter
2 9 10-3 m), and the distance between the model and the
plateau representing the road is 7 9 10-3 m. For this
geometry, the angle a represents the rear window inclina-
tion relatively to the horizontal plane. Tests were per-
formed at two angles a = 0 and 25 (Fig. 4). With the
angle a equal to 0, the flow is separated at the periphery of
the base to form a tore vortex (Gillieron and Chometon
1999; Spohn and Gillieron 2002). The Ahmed body is also
representative of a square-back vehicle. When the angle a
equals 25, the separation at the end of the roof interacts
with the two contra-rotating longitudinal vortices issued
from both sides of the rear window. The separation on the
rear window reaches the wake. Inside this separation,
Fig. 2 Sketch of wind tunnel
Fig. 3 Circular cylinder to control roof boundary layer thickness
(indicated by an arrow)
Fig. 4 Geometry, scale 0.75 compared to the Ahmed body reference
(Ahmed et al. 1984): the line A represents the end of the roof and the
start of the rear window
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appear two flow rotations centred on two focuses which
contribute to the base separation. Detailed description of
these phenomena was presented by Spohn and Gillieron
(2002). The Ahmed body can be also likened to a simpli-
fied vehicle with tailgate-type rear section.
The splitter plates are attached to the experimental
model with circular cylinder having 4 9 10-1 m length
and 10-2 m diameter. To fix correctly these connectionsand stop translation and rotation motions, sharp pressure
screws are used. The error on the measurement of the
distance between the splitter plate and the Ahmed body is
less than 10-3 m.
4 Results
Experiments are performed with splitter plates placed at the
rear of the base and in the front of the Ahmed body.
Analysis is performed only by comparing the drag forces.
In the case where the splitter plates are placed behind thebase, analysis is performed only for the rear window angle
equal to 0 (a = 0). When the splitter plates are positioned
in front of the model, three different shapes of the model
front are examined by varying or not the model orientation
(rotation of 180). The skew angle effect on the standard
Ahmed body with square back and by using splitter plates
in the front and in the back of the experimental model is
then examined.
For all these configurations, the goal consists on
reducing the surface of the wake that contributes to the
drag force (see Eq. 5). To limit the side force on the splitter
plates, the working velocity does not exceed 30 m s-1.
Taking into account the flow velocity (20 and 30 m s-1)
and the reduced size of the used model, the aerodynamic
coefficient cannot be directly compared to the Ahmed
study performed at 60 m s-1 and using one model at size 1.
In this study, for the square-back geometry without adding
a splitter plate, the aerodynamic drag coefficient is equal to
0.305 at 30 m s-1. For the configuration with a rear win-
dow inclined at 25 (used in the front side), it is equal to
0.448. In the following, only the percentages of the devi-
ation from the aerodynamic coefficients obtained on the
reference geometry are specified.
The height and width of splitter plates were between
0.6 and 0.9 times the height and width of the Ahmed
body. The analysis was performed by comparing the
aerodynamic drag coefficient values measured with and
without a splitter plate. If Cd and Cdref are the aerody-
namic drag coefficients measured, respectively, with and
without a splitter plate, the relative drag reduction
100[Cdref - Cd/Cdref] were plotted and analysed below for
various skew angles b and various splitter plate orienta-
tions k, as defined in Fig. 5.
4.1 Vertical splitter plates effects
The experiments were performed with splitter plates posi-
tioned downstream and upstream of the Ahmed body, and
with a zero skew angle (b = 0) (see Fig. 6). The analysis
is performed by varying the distance between the base andsplitter plate (Mair 1965). The Reynolds number used in all
the experiments is based on the model length L.
4.1.1 Downstream vertical splitter plates
The vertical splitter plates were positioned on a square-
back-type model (a = 0). The origin of the coordinates in
this case is in the plane of the base, and the position of the
plate in the x direction is kept non-dimensional by dividing
x by the base height HA. The results plotted in Fig. 7 are
obtained for three different plate sizes and for an upstream
velocity V0 = 30 m s-1. With each splitter plate size, the
results show that the drag reduction increases with the
distance from the base x/HA, reaches a minimum and then
decreases. It can be observed that the maximum aerody-
namic drag reduction, close to 12%, is obtained with
a splitter plate measuring 0.9HA 9 0.9wA, placed at
x/HA = 0.5. An enhancement of the drag is observed for
the splitter plate 0.6 when the non-dimensional distance
x/HA becomes higher than 1.3. This result suggests an
amplification of the shear layer instabilities (issued from
V0
Skew angle
x/H
< 0
Reduced abscissa
Plate orientation
angle
+-
Fig. 5 Definitions of skew angle b and orientation angle k
Fig. 6 Vertical splitter plate downstream the rear of the square-back
Ahmed body (a = 0)
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the separation at the end of the roof) that increases the
pressure force on the front side of the splitter plate. This
phenomenon is like the one observed in a deep cavity. For
the cavity flow, when the aspect ratio (width over deep) is
small, the upstream-separated shear layer from the corner
does not interact with the downstream corner and hence
does not affect the farfield flow. When this aspect ratio
increases, the instability of the shear layer amplifies and
interacts with the downstream vertical wall of the cavity
inducing retroactive pressure wave. These pressure oscil-
lations amplify the shear layer instabilities, increase the
parietal pressure distribution along the downstream cavity
vertical wall (Rossiter 1964; Kourta and Vitale 2008) and
contribute to increase the splitter plate drag. So the drag of
the global geometry (body ? plate) increases. The results
show clearly that the efficiency of the plate increases when
the relative distance x/HA is reduced. Considering h the
angle between the upper part of the plate and the rear plane
of the model, the higher drag reduction is obtained for
h = 5.7, 7.1 and 11.3 (Fig. 8). These angles are lower
than critical angle a = 12 corresponding to the appear-
ance of rear window separation (Ahmed et al. 1984).
Figures 9 and 10 show, respectively, the vertical and the
horizontal planes of the flowfield for different longitudinal
plate positions. The formation and the evolution of two
vortical tore structures between the base of the model and
the splitter plate are clearly observed in the vertical planes
(Levallois and Gillieron 2005). When the longitudinal
position x/HA increases, the upper vortex centre moves
downstream, near to the splitter plate. At the same time, the
lower vortex moves from the model base to the bottom side
towards the wind tunnel wall. When the maximum drag
reduction is reached, the upper vortex centre is near the
cavity centre. Moving the splitter plate downstream, this
vortex centre moves towards the splitter plate and at the
same time the aerodynamic drag increases. In all cases, the
drag reduction is correlated with the transversal wake
section diminution (the surface S in Eq. 5).
From the horizontal planes (Fig. 10), for x/HA\ 0.6,
between the base and the splitter plate, no vortex appears.
For x/HA higher or equal to 0.6, two vortices are observed.
When x/HA increases these vortices move from the splitter
plate towards the base and the drag increases. For
x/HA = 0.7, the vortex centres are located at the mid dis-
tance between the base and the splitter plate. Behind the
splitter plate, the wake exhibits the classical tore structure.
The Reynolds number effect was also analysed at two
inflow velocities, V0 = 20 m s-1 and V0 = 30 m s
-1,
corresponding, respectively, to a Reynolds numbers of
1.0 9 106 and 1.6 9 106. The obtained results are plotted
in Fig. 11. It can be observed that the Reynolds number has
small effect on the drag reduction obtained by using ver-
tical splitter plates. The value and the maximum of drag
reduction position are not affected by the Reynolds num-
ber. This result confirms the weak influence of the Rey-
nolds number on the aerodynamic drag beyond 25 m s-1.
The influence of a second splitter plate positioned
downstream of the first one measuring 0.9HA 9 0.9wA at
x/HA = 0.5 was also analysed. This configuration allows to
reduce the surface S of Eq. 5 and to decrease the aerody-
namic drag for the same wake topology. The fact that this
result is not obtained suggests the existence of cavity
instability producing additional aerodynamic drag due to
the front side of the last or of the both splitter plates.
-1%
1%
3%
5%
7%
9%
11%
0,0 0,3 0,5 0,8 1,0 1,3 1,5
Reduced Abscissa x/HA
DragReduction%
splitter plate 0.6
splitter plate 0.8
splitter plate 0.9
Fig. 7 Aerodynamic drag coefficient reduction versus x/HA with
splitter plates of 0.9HA 9 0.9wA, 0.8HA 9 0.8wA and 0.6HA 90.6wA (V0 = 30 m s
-1)
x
y
z
Fig. 8 Vertical splitter plate downstream the rear of the square-back
Ahmed body (a = 0)
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Fig. 9 Mean velocity
distribution and streamlines for
different longitudinal splitter
plate positions: a x/HA = 0.4.
b x/HA = 0.5. c x/HA = 0.6.
d x/HA = 0.7 (splitter plates of
0.9HA 9 0.9wA, vertical plane
at y = -27.5 mm)
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Fig. 10 Mean velocity
distribution and streamlines for
different longitudinal splitter
plate positions: a x/HA = 0.4.
b x/HA = 0.5. c x/HA = 0.6.
d x/HA = 0.7 (splitter plates of
0.9HA 9 0.9wA, horizontal
plane at z = 27.5 mm)
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4.1.2 Upstream vertical splitter plates
Significant drag reductions could also be achieved by
positioning vertical splitter plates upstream of the model
(Roshko and Koenig 1978). The experiments were per-
formed with various front section forms and various splitter
plate sizes and positions. In each test, the reference height h
and width w were related to the front section geometry (see
Fig. 12).
The first test series were performed on the Ahmed body,
for which the height h and width w are defined in Fig. 12,
configuration (a) where h = HA and w = wA. When the
rear window inclination a is equal to zero (see Fig. 4), the
geometry is a square back. When this angle equals 25, it
corresponds to a rear window for a simplified vehicle with
tailgate rear window. The second and third test series were
obtained by swinging round the Ahmed body by 180 on itsown yaw axis (vertical). For the second configuration, the
rear window angle of inclination from horizontal was 25
(Fig. 12, configuration (b)). The reference height h and
width w can therefore be likened to the height and width of
the straight base underneath the rear window. This con-
figuration is representative of an MPV (Espace) or utility
vehicle front (h = HA and w = wA). For the third test
series, the base was still facing the wind, but with a rear
window angle of inclination of zero (a = 0, Figs. 4 and
12, configuration (c)). This configuration corresponds to a
front part of truck or bus. In this case, the reference height
h and width w can be likened to the Ahmed body height HAand width wA.
For these three configurations, the experiments were
performed using two splitter plates with a height and width
values of 0.8 and 0.9 times the reference height h and width
w. The origin of the abscissas in this case belonged to the
splitter plate plane. As above, the effect of each splitter
plate is analysed when the reduced distance x/h between
the plate and the front of the model increased. The aero-
dynamic drag coefficient reduction percentages obtained
with these three configurations are plotted in Figs. 13, 14
and 15, respectively.
0%
2%
4%
6%
8%
10%
12%
0,00 0,25 0,50 0,75 1,00 1,25 1,50
Reduced Abscissa x/HA
DragReduc
tion%
splitter plate 0.6 - 20m/s
splitter plate 0.8 - 20m/s
splitter plate 0.9 - 20m/s
splitter plate 0.6 - 30m/s
splitter plate 0.8 - 30m/s
splitter plate 0.9 - 30m/s
Fig. 11 Reynolds number effect on aerodynamic drag coefficient
reduction: Inflow velocity of 20 and 30 m s
-1
, Re = 1.044 9 10
6
and 1.566 9 106
Fig. 12 The various
configurations and definitions
of the Ahmed body (height h
and width w)
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With vertical splitter plates measuring 0.9h 9 0.9w and
0.8h 9 0.8w positioned upstream of the rounded part of the
Ahmed body (Fig. 12, configuration (a)), the experimental
results show that aerodynamic drag decreases, reaches a
minimum and then increases rapidly as the reduced dis-
tance x/h increases (Fig. 13). The results obtained with a
farfield velocity of 20 and 30 m s-1 demonstrate that the
drag reduction decreases as the Reynolds number increa-
ses. At these two velocities, the maximum drag reduction
values observed in the vicinity of x/h = 0.15 are 2.00 and
0.15%, respectively. These reductions are insignificant, but
the influence of the relative position of the splitter plate
appears to be important.
The experiments performed on MPV- and/or utility-type
fronts (front end inclined 25/horizontal, Fig. 12 configu-
ration (b)) are more interesting. With splitter plates mea-
suring 0.8h 9 0.8wA, drag reductions of nearly 28% were
obtained with the reduced position x/H equal to 0.3
(Fig. 14). Drag reduction remains greater than 25% over
the reduced position interval [0.30.6]. The Reynolds
number has very little influence on the results. Also,
through a geometry adaptation designed to recover the flow
for engine cooling, this type of solution could be a mean of
progress for reducing automotive drag.
With a straight front end (a = 0, Figs. 4, 12 configura-
tion (c)), drag reduction is particularly significant (Fig. 15).
This reduction reaches 45% with a vertical splitter plate
measuring 0.8HA 9 0.8wA placed at the reduced position
x/HA = 0.3. It remains greater than 40% as x/HA increases
from 0.3 to 1.0. The sensitivity to the x/HA relative position
in this case appears to be less significant than above. By
enabling a reduction in the surface area of the transverse
section separated from the upstream end of the model, the
results obtained in this case confirm the advantage of
rounding vehicle front end connection surfaces (transversal
wake surface reduction).
-8%
-6%
-4%
-2%
0%
2%
0,0 0,2 0,4 0,6
Reduced Abscissa x/h
DragReduction%
splitter plate 0.8 - 20m/s
splitter plate 0.9 - 20m/s
splitter plate 0.8 - 30m/s
splitter plate 0.9 - 30m/s
Fig. 13 Percentage of aerodynamic drag coefficient reduction with
splitter plates measuring 0.9h9
0.9w and 0.8h9
0.8w as a functionof relative x/h positions. Splitter plates upstream of Ahmed body
(Fig. 12, configuration (a))
0%
5%
10%
15%
20%
25%
30%
0,00 0,25 0,50 0,75 1,00 1,25 1,50
Reduced Abscissa x/h
DragRe
duction%
splitter plate 0.6 - 20m/s
splitter plate 0.8 - 20m/s
splitter plate 0.9 - 20m/s
splitter plate 0.6 - 30m/s
splitter plate 0.8 - 30m/s
splitter plate 0.9 - 30m/s
Fig. 14 Percentage of aerodynamic drag coefficient reduction
with splitter plates measuring 0.9h 9 0.9wA, 0.8h 9 0.8wA and 0.6h
90.6wA upstream, as a function of reduced abscissas x/h. Upper frontpart inclined 25, estate or MPV configuration (Fig. 12, configuration
(b))
0%
10%
20%
30%
40%
50%
0,00 0,50 1,00 1,50
Reduced Abscissa x/HA
DragReduc
tion%
splitter plate 0.6 - 20m/s
splitter plate 0.8 - 20m/s
splitter plate 0.9 - 20m/s
splitter plate 0.6 - 30m/s
splitter plate 0.8 - 30m/s
splitter plate 0.9 - 30m/s
Fig. 15 Percentage of aerodynamic drag coefficient reduction withsplitter plates measuring 0.9HA 9 0.9wA and 0.8HA 9 0.8wA as a
function of reduced position x/HA. Straight front geometry (Fig. 12,
configuration (c))
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For the configuration (b) with front splitter plate, PIV
measurement can provide explanation of the drag reduc-
tion. Figures 16 and 17 show the vertical and horizontal
planes, respectively, for three different plate positions. In
Fig. 16, for the vertical plane, the separation zone is at the
beginning near to the body, and when the plate moves
upstream, this separation zone grows and moves far from
the body so it does not influence the aerodynamic
Fig. 16 Mean velocity distribution and streamlines (configuration (a)) for different longitudinal front splitter plate positions:a x/h = 0.24.
b x/h = 0.4. c x/h = 0.56 (splitter plate of 0.9h 9 0.9wA, vertical plane at y = 26.8 mm)
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Fig. 17 Mean velocity distribution and streamlines (configuration (b)) for different longitudinal front splitter plate positions:a x/h = 0.24.
b x/h = 0.4. c x/h = 0.56 (horizontal plane at z = 26 mm)
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performance. The bubble is somewhere between the splitter
plate and the front body. The flow is no more separated on
the body. For the horizontal plane, the flow configuration
near the front body does not change when the splitter plate
moves upstream. The velocity flowfields show that the
local evolution at the bottom (Fig. 16a, c) and lateral sides
(Fig. 17a, c) of the body changes the upstream velocity
direction. The transversal size decreases with the increaseof the relative distance x/h. Hence, the drag related to the
transversal wake surface S (Eq. 5) decreases.
4.2 Splitter plates with non-zero skew angle
The influences of skew angle b and the effects of orien-
tation angle k on aerodynamic drag evolution were ana-
lysed using splitter plates applied on type (b) and (c)
geometries; see Figs. 12, 18 and 19. All the results,
expressed as percentages, were determined relatively to
values measured without a splitter plate for the same skew
angles b.
4.2.1 Downstream vertical splitter plates
The experiments were performed on a straight base model
(configuration (a) representative of the rear of a Renault
Espace-type vehicle, with a = 0), fitted with a vertical
splitter plate, with height and width values of 0.90HA and
0.86wA, respectively. The aerodynamic drag measured
with a splitter plate was always less than its value observed
without a splitter plate, but the splitter plates influence
decreases as the skew angle b increases (Fig. 20). Drag
reductions greater than those obtained with splitter plates
positioned parallel to the base demonstrate the advantage
of adapting the splitter plate orientation k (orientation/
vertical) to the skew angle b. Finally, the changes observed
as a function of the orientation angle with a skew angle
b = 5 suggest the existence of strong interactions between
the splitter plate and the base.
The influence of longitudinal spacing between the ver-
tical splitter plate and the base was analysed with a skew
angle b = -15, varying the reduced position x/HA at
various orientation angle k values (Figs. 21, 22). With
orientations angle k greater than -15, the aerodynamic
drag observed with a splitter plate at skew angle b = -15
was always less than its value observed without a splitter
plate, and the optimum aerodynamic drag reduction posi-
tion was poorly influenced by the orientation angle k on the
angular domain [-20, ?20]. This optimum position is
closer to the base (from x/HA = 0.3 to x/HA = 0.4) at non-
zero, when positive value of the orientation angle k is
increased (leeward part nearer the base than the windward
part), and is further from the base when its negative absolute
value is increased (from x/HA = 0.4 to x/HA = 0.5). At
skew angle b = -15, the maximum drag reduction
(10.6%) is observed at x/HA = 0.4 with orientation angle of
k = 5 (Figs. 21, 22).
In addition, at all x/HA positions less than 0.4 (maximum
drag reduction with k = 0, Fig. 21), the drag observed at
positive orientation angle k values was less than its value
observed with vertical splitter plate (k = 0). The maxi-
mum drag reduction was therefore obtained by moving the
windward part (the leeward part respectively) of the splitter
plate away from (respectively closer to) the base. All these
V0
Skew angle
x/H = 0.6
< 0
Reduced abscissa
Plate orientation
angle
+-
Fig. 18 Skew angle b and orientation angle k of downstream splitter
plate, configuration (a) with a = 0 (Fig. 2)
V0
Skew
anglex/h = 0.4
< 0
Reduced abscissa
-
+
Plate orientation
angle
Fig. 19 Skew angle b and orientation angle k of upstream splitter
plate, type (b) geometry
0%
2%
4%
6%
8%
10%
12%
14%
-20 -10 0 10 20
Orientation angle ()
Dragre
duction(%)
-20 skew -15 skew -10 skew
-5 skew 0 skew
Fig. 20 Rear splitter plate 0.90HA 9 0.86wA with x/HA = 0.6: Per-
centages of aerodynamic drag coefficient reduction as a function of
orientation angle k with various skew anglesb
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results demonstrate the advantage of adapting splitter plate
orientation to external conditions.
4.2.2 Upstream vertical splitter plates
The vertical splitter plate was positioned on the front of a
type (b) model, front configuration of a RenaultEspace
(Square-back)-type vehicle (Fig. 19). Its height and width
values were 0.8 times the height h and width w = wA defined
in Fig. 12. Theset-up geometry restricted thex positioning of
the splitter plate, and the lowest possible value for the x/h
ratio increased with the orientation angle a. The measure-
ments were performed at a skew angle b = -15.
At a skew angle b = -15, and beyond a maximum
positive orientation angle k, the value of which increases
with reduced longitudinal spacing x/h, the aerodynamic
drag rapidly decreases, and its value may fall below its
value observed without a splitter plate (Fig. 23). In this
configuration, unlike the results observed with the down-
stream splitter plate, the maximum drag reduction was
obtained by moving the leeward part (the windward part
respectively) of the splitter plate away from (respectively
closer to) the vehicle, see Figs. 19 and 23. Drag reductions
of approximately 7.1% were obtained with the orientation
angles k = 15 and 20.
At a skew angle b = -15 and zero orientation
(k = 0), the splitter plate always has an adverse effect on
the aerodynamic drag coefficient value (Figs. 23, 24),
orientation curve k = 0. Analysis of the results demon-
strated that the reductions are greater when the splitter
plate was placed as close as possible to the model (Fig. 24).
At a given reduced position less than 0.35, the maximum
reductions were observed at maximum positive orientation
angle values (with negative b).
5 Conclusion
In the worldwide context strongly constrained by the
climatic consequence of CO2 emission and the fossil
-4%
-2%
0%
2%
4%
6%
8%
10%
12%
0,2 0,3 0,4 0,5 0,6
Reduced abscissa x/HA
Dragreduction(%)
-20 orientation
-15 orientation
-10 orientation
-5 orientation
0 orientation
5 orientation
10 orientation
15 orientation
20 orientation
Fig. 21 Rear splitter plate 0.90HA 9 0.86wA: Percentage aerody-
namic drag coefficient reduction as a function of reduced positionx/HA with various orientation angles k and skew angle b = -15
-4%
-2%
0%
2%
4%
6%
8%
10%
12%
-20 -10 0 10 20
Orientation angle ()
Dragred
uction(%)
x/HA = 0,2
x/HA = 0,3
x/HA = 0,4
x/HA = 0,5
x/HA = 0,6
Fig. 22 Rear splitter plate 0.90HA 9 0.86wA: Percentage of aerody-
namic drag coefficient reduction as a function of orientation anglek
with various reduced abscissas x/HA, skew angle b = -15
-8%
-6%
-4%
-2%
0%
2%
4%
6%
8%
-20 -10 0 10 20
Orientation angle ()
Dragreduction(%)
x/h = 0,23
x/h = 0,3
x/h = 0,4
x/h = 0,5
x/h = 0,6
HA = 0,6
Fig. 23 Front splitter plate 0.8 h 9 0.8wA: Percentage of aerody-
namic drag coefficient reduction versus the orientation angle k atvarious reduced abscissas x/h, and skew angle b = -15
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combustible rarefaction, automotive industry has to search
for a new solution to increase the loading energy efficiency
(fossil energies, hydrogen or electric). In this situation,
many works on passive and active separation control have
been initiated both in industry and academic research
laboratories.
The obtained results confirm the interest of using splitter
plates to reduce the aerodynamic drag for the road vehicle.
By choosing adapted positions and orientations with respect
to the flow conditions, drag reduction can be obtained. By
using the splitter plate alone or with another control solution
(vortex generators or active control systems), these solu-
tions allow to reduce for at least 10% the drag of the road
vehicles. For vehicles with square back, the gas consump-
tion will be reduced by 0.8 L for 100 km at stabilized
vehicle velocity of 130 km h-1. If the added mass related to
the splitter plates is compensated by the reduction of engine
mass due to the engine size reduction (downsizing), at this
velocity, the CO2 will be diminished by 20 g/km and by
3.5 g/km on the NEDC (New European Driving Cycle)
reference. The final goal is to integrate these solutions on
road vehicle at the horizon of 2015. The solutions obtained
on different models studied here show the complexity of
physical phenomena and the necessity to continue the
development of knowledge in this case. Noise, instabilities
or at least interactions with vortex structures coming from
front part of the model or from the rear window have to be
carefully analysed to improve the control performances.
With vertical splitter plates positioned downstream of a
straight base, drag reductions of nearly 12% were obtained.
The reductions increased with increasing splitter plate
transverse dimensions and decreasing distance between the
splitter plate and the base. The influence of the Reynolds
number remained low, with apparently no influence on the
physical phenomena, whereas drag reduction increased
with increasing Reynolds numbers.
The influence of vertical splitter plates positioned
upstream was also analysed. Drag reductions of nearly 27
and 45% were obtained, respectively, using splitter platespositioned upstream of an angled front face with or without
an inclination from vertical. The Reynolds number influ-
ence appears to be less significant than for splitter plates
positioned on the rear of the models.
For splitter plates mounted downstream, the optimum
longitudinal position of the splitter plates was poorly
influenced by the skew angle and the angular variations of
the splitter plate from vertical. For orientation angle k less
than -15, the aerodynamic drag observed with a splitter
plate at a skew angle b = -15 was always less than its
value observed without a splitter plate. The greatest
reductions were obtained with optimum x/H positionobserved with zero skew angle by moving the windward
part of the splitter plate away from the base. Drag reduc-
tions of nearly 10.6% were observed. With splitter plates
positioned upstream in parallel to the front face of a
straight base-type model, the skew effect had an adverse
effect on aerodynamic drag, and the results demonstrated
the need to adapt the splitter plate orientation to the skew
angle. Drag reductions of nearly 7% were observed, and
the maximum drag reductions were obtained by moving the
leeward part of the splitter plate away from the vehicle.
Finally, the results presented herein confirm the advan-
tage of splitter plates in reducing aerodynamic drag, and
demonstrate the need to develop systems capable of
adapting their position and angular orientation to external
conditions.
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