Art-1. Exp Fluids-2010, Vol.48 (Pages 1–16)

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

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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)

Exp Fluids (2010) 48:116 5

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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


DragReduc

tion%

splitter plate 0.6 - 20m/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%





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%







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


DragReduc

tion%







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


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


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.

References

Ahmed SR, Ramm R, Faltin G (1984) Some salient features of the

time averaged ground vehicle wake. SAE technical Paper Series

840300

Ardonceau P, Amani G (1992) Remarks on the relation between liftinduced drag and vortex drag. Eur J Mech B 11(4):455460

Arnald D, Cousteix J, Michel R (1976) Couche limite se developpant

avec gradient de pression positif dans un ecoulement exterieur

turbulent, La Recherche Aerospatiale, n1976-1

Baudoin JF, Aider JL (2008) Drag and lift reduction of a 3D bluff

body using flaps. Exp Fluids 44:491501. doi:10.1007/

s00348-007-0392-1

Cousteix J (1989) Aerodynamique, Turbulence et Couche Limite.

Cepadues-Editions, Toulouse, pp 1114

Gad-el-Hak M (1996) Compliant coatings: a decade of progress.

In: Workshop on flow controlfundamentals and practices,

Cargese, Corse, 1996

-8%

-6%

-4%

-2%

0%

2%

4%

6%

8%

0,2 0,3 0,4 0,5 0,6

Reduced abscisse x/h

Dragreduction(%)

-20 orientation

-15 orientation

-10 orientation

- 5 orientation

0 orientation

5 orientation

10 orientation

15 orientation

20 orientation

Fig. 24 Front splitter plate 0.8: Percentage of aerodynamic drag

coefficient reduction versus x/h at various orientation angles a andskew angle b = -15

Exp Fluids (2010) 48:116 15

123
http://dx.doi.org/10.1007/s00348-007-0392-1http://dx.doi.org/10.1007/s00348-007-0392-1http://dx.doi.org/10.1007/s00348-007-0392-1http://dx.doi.org/10.1007/s00348-007-0392-1


16/16

Gillieron P, Chometon F (1999) Modelling of stationary three

dimensional detached airflows around an Ahmed reference body.

In: Proceedings of the third international workshop on vortex,

ESAIM, vol 7, pp 173182

Granville PS (1985) Mixing length formulation for turbulent boundary

layers over arbitrarily rough surfaces. J Ship Res 29(4):223233

International Energy Agency (IEA) (2007) World energy outlook

2007China and India insights (executive summary). Interna-

tional Energy Agency, ISBN: 978-92-64-02730-5

Kourta A, Vitale E (2008) Analysis and control of cavity flow. Phys

Fluids 20:0771041 (10 pp)

Levallois E, Gillieron P (2005) Controle des ecoulements en

aerodynamique automobile & reduction de tranee par elements

separateurs - Analyse par PIV, Colloque National de Visualisa-

tion et de Traitement dImages en Mecanique des Fluides,

Fluvisu11, Lyon, 69 juin

Mair WA (1965) The effect of a rear mounted disc on the drag of a

blunt based body of revolution. Aeronaut Q 16:350360

Onorato M, Costelli A, Garonne A (1984) Drag measurement through

wake analysis, SAE, SP-569. In: International congress &

exposition, Detroit, MI, 27 February2 March 1984, pp 8593

Passenger Cars, University of Sheffield and University of Michigan,

Produced by SASI Group (Sheffield) and Mark Newmann

(Michigan), Map031, 2006

Roshko A, Koenig K (1978) Interaction effects on the drag of bluff

bodies in tandem. In: Sovran G, Morel T, Mason WT (eds)

Aerodynamic drag mechanisms of bluff bodies and road

vehicles. Plenum, New York, pp 253286

Rossiter JE (1964) Wind tunnel experiments on the flow over

rectangular cavities at subsonic and transonic speeds. Aeronau-

tical Research Council Reports and Memo n 3438

Spohn A, Gillieron P (2002) Flow separations generated by a

simplified geometry of an automotive vehicle. In: IUTAM

symposium on unsteady separated flows, Toulouse, France, 812

April 2002

16 Exp Fluids (2010) 48:116

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