Cranfield University
Robert Cousseau
Rear Mounted Wings on Saloon
Race Cars
School of Engineering
MSc
Cranfield University
School of Engineering
MSc
2007
Robert Cousseau
Rear Mounted Wings on Saloon Race
Cars
Supervisor: Prof KP Garry
29 August 2007
This thesis is submitted in partial fulfilment of the requirements foe the
Degree of Aerospace Dynamics.
Cranfield University, 2007. All rights reserved. No part of this publication may be reproduced without
the written permission of the copyright holder
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Abstract
An experimental investigation into the impact of a mounting rear wing within the flow
structure in the near wake of a saloon race car has been carried out using a scale glass
fibre model in the Cranfield University Aerodynamics Laboratories wind tunnels.
Surface flow visualisation involving the oil-dot technique on the car with and without
the wing has been performed in order to gain an understanding of why the near wake
has a particular structure involving trailing edge vortices and contra-rotating vortices
and how these features are affected by the presence of the wing. The effect of sideslip
has also been investigated. Force measurements were also carried out using an internal
balance in order to support the results obtained.
The flow visualisation results showed that the presence of sideslip or the wing has a
significant effect on the flow over the backlight whereas the flow over the trunk was
virtually unaffected. Sideslip had an effect on the balance of the contra-rotating vortex
wake structure and determined which one of the pair is dominant. The presence of the
wing and its location had an effect on the whole contra-rotating vortices structure,
making it smaller as the wing was close to the backlight. These results showed some
differences with what has been noted in previous studies.
The force measurements results showed that some wing/body interactions were
involved and produced some favourable and unfavourable effects which significantly
influenced the lift and drag experienced by the model.
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Acknowledgements
I would like to thank Prof KP Garry for his help and support during thesis project.
I would also like to thank John, Jenny and Linton for their help during the experiments
in wind tunnels.
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Contents
ABSTRACT ............................................................................................................................................... I
ACKNOWLEDGEMENTS ......................................................................................................................... III
TABLE OF CONTENTS ............................................................................................................................. V
LIST OF FIGURES ................................................................................................................................. VIII
LIST OF TABLES ......................................................................................................................................X
NOMENCLATURE ..................................................................................................................................XI
1 INTRODUCTION ............................................................................................................................ 1
1.1 AIMS ................................................................................................................................................ 1
1.2 OBJECTIVES ........................................................................................................................................ 2
2 LITERATURE REVIEW ..................................................................................................................... 3
2.1 FLOW DEVELOPMENT IN THE WAKE OF A NOTCHBACK CAR ........................................................................... 3
2.1.1 The rear-body ....................................................................................................................... 3
2.1.2 Flow separation - Vortices .................................................................................................... 4
2.1.3 Sideslip .................................................................................................................................. 7
2.2 LIFT-REDUCING SURFACES ..................................................................................................................... 8
2.2.1 Rear lip spoilers ..................................................................................................................... 8
2.2.2 Wings .................................................................................................................................... 9
2.3 WIND TUNNEL TESTING ...................................................................................................................... 14
2.3.1 Wind tunnel testing issues .................................................................................................. 14
2.3.2 Flow visualisation ............................................................................................................... 19
2.4 PREVIOUS WORK ............................................................................................................................... 22
3 EXPERIMENTAL SET UP ............................................................................................................... 25
3.1 MODEL ........................................................................................................................................... 25
3.1.1 Car ...................................................................................................................................... 25
3.1.2 Wing ................................................................................................................................... 25
3.1.3 Wing support ...................................................................................................................... 26
3.2 WIND TUNNELS ................................................................................................................................ 27
3.2.1 Sideslip tests ....................................................................................................................... 27
3.2.2 Effects of wing position on the structure of the wake ........................................................ 27
3.2.3 Force measurements .......................................................................................................... 27
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4 TEST METHOD ............................................................................................................................. 29
4.1 OIL-DOT TECHNIQUE .......................................................................................................................... 29
4.1.1 Mixtures tested ................................................................................................................... 29
4.1.2 Repeatability tests .............................................................................................................. 32
4.2 SIDESLIP TESTS .................................................................................................................................. 33
4.3 EFFECTS OF THE WING ON THE STRUCTURE OF THE WAKE ........................................................................... 34
4.3.1 Without sideslip .................................................................................................................. 35
4.3.2 With sideslip ....................................................................................................................... 35
4.4 FORCE MEASUREMENTS ...................................................................................................................... 35
4.4.1 Experimental method ......................................................................................................... 35
4.4.2 Force measurements repeatability errors ........................................................................... 36
5 RESULTS ...................................................................................................................................... 39
5.1 SIDESLIP TESTS .................................................................................................................................. 39
5.1.1 Structure of the near wake ................................................................................................. 39
5.1.2 Effects of sideslip ................................................................................................................ 42
5.2 EFFECT OF WING POSITION ON THE STRUCTURE OF THE WAKE ..................................................................... 49
5.2.1 Effects of wing axial location .............................................................................................. 50
5.2.2 Effects of wing height ......................................................................................................... 53
5.2.3 Effect of wing axial position with sideslip ........................................................................... 57
5.3 LIFT AND DRAG MEASUREMENTS .......................................................................................................... 61
5.3.1 Effect of wing axial location ............................................................................................... 61
5.3.2 Effect of wing vertical location ........................................................................................... 65
5.4 EFFECTS OF THE MODEL MOUNTING STRUT ............................................................................................. 69
6 DISCUSSION OF RESULTS ............................................................................................................ 71
6.1 STRUCTURE OF THE WAKE ................................................................................................................... 71
6.1.1 Formation of the contra-rotating vortices .......................................................................... 71
6.1.2 Flow over the trunk ............................................................................................................. 73
6.2 EFFECTS OF SIDESLIP .......................................................................................................................... 75
6.3 EFFECT OF A STRUT ............................................................................................................................ 77
6.4 EFFECTS OF WING AXIAL POSITION ........................................................................................................ 78
6.4.1 Effects on the structure of the wake ................................................................................... 78
6.4.2 Effect on downforce ............................................................................................................ 79
6.4.3 Effect on drag ..................................................................................................................... 80
6.5 EFFECTS OF WING VERTICAL POSITION ................................................................................................... 81
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6.5.1 Effect on the structure of the wake .................................................................................... 81
6.5.2 Effect on downforce ............................................................................................................ 82
6.5.3 Effect on drag ..................................................................................................................... 84
6.6 LIFT OVER DRAG RATIO ....................................................................................................................... 85
6.7 EFFECT OF WING AXIAL POSITION WITH A 2 SIDESLIP ANGLE ...................................................................... 85
7 CONCLUSION & FURTHER WORK ................................................................................................ 87
7.1 WAKE STRUCTURE ............................................................................................................................. 87
7.2 INFLUENCE OF SIDESLIP ....................................................................................................................... 87
7.3 EFFECT OF THE WING ON THE WAKE STRUCTURE ...................................................................................... 88
7.4 EFFECT OF THE WING ON THE OVERALL VEHICLE LIFT AND DRAG CHARACTERISTICS .......................................... 89
7.5 EFFECT OF THE MODEL SUPPORT STRUT ON THE WAKE STRUCTURE .............................................................. 89
7.6 COMPARISON WITH PREVIOUS WORK .................................................................................................... 89
7.7 FURTHER WORK ................................................................................................................................ 90
REFERENCES ......................................................................................................................................... 91
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Figures
FIGURE 2-1: REAR-END FORMS: NOTCHBACK (LEFT); HATCHBACK (CENTRE); SQUAREBACK (RIGHT) ................................... 4
FIGURE 2-2: RAKE ANGLE OF A HATCHBACK CAR ...................................................................................................... 4
FIGURE 2-3: NOTCHBACK REAR END PARAMETERS ................................................................................................... 5
FIGURE 2-4: TRANSVERSE VORTEX ........................................................................................................................ 6
FIGURE 2-5: C-PILLAR VORTICES ........................................................................................................................... 6
FIGURE 2-6: THE ARCH VORTEX ........................................................................................................................... 7
FIGURE 2-7: THE EFFECT OF A REAR SPOILER ........................................................................................................... 9
FIGURE 2-8: EFFECT OF A REAR WING ON THE STREAM LINES NEARBY A GENERIC BODY .................................................. 10
FIGURE 2-9: EFFECT OF DISTANCE TO THE BODY AND ASPECT RATIO ON THE WING EFFECTIVENESS ................................... 11
FIGURE 2-10: EFFECT OF WING PROXIMITY TO THE GROUND ON THE DOWNFORCE ...................................................... 11
FIGURE 2-11: COMPARISON OF THE CP DISTRIBUTIONS OF A MOUNTED WING WITH THAT OF THE WING ALONE ................ 12
FIGURE 2-12: THE EFFECT OF END PLATES ........................................................................................................... 13
FIGURE 2-13: BOUNDARY LAYER AND SCALE EFFECT .............................................................................................. 15
FIGURE 2-14: MOVING ROAD PROBLEM .............................................................................................................. 16
FIGURE 2-15: VARIOUS METHODS FOR SIMULATING A MOVING GROUND IN A WIND TUNNEL ......................................... 17
FIGURE 2-16: BLOCKAGE EFFECT ........................................................................................................................ 18
FIGURE 2-17: RESULTS OBTAINED WITH THE OIL DOT TECHNIQUE ............................................................................. 21
FIGURE 3-1: GLASS FIBRE MODEL ....................................................................................................................... 25
FIGURE 3-2: MODEL MOUNTED IN THE 86 WIND TUNNEL WITH THE WING ............................................................. 26
FIGURE 3-3: WING SUPPORT ............................................................................................................................. 27
FIGURE 4-1: COMPARISON BETWEEN THE TWO MIXTURES USED (POSTER PAINT ON THE TOP PICTURE, PARAFFIN ON THE
BOTTOM PICTURE) .................................................................................................................................. 31
FIGURE 4-2: REPEATABILITY TESTS WITH DOTS OF MEDIUM SIZE ON THE TOP RIGHT CORNER........................................... 32
FIGURE 4-3: REPEATABILITY TESTS WITH BIG DOTS PLACED ON THE TOP LEFT CORNER.................................................... 33
FIGURE 4-4: ORIENTATION OF THE MODEL IN THE WEYBRIDGE WIND TUNNEL ............................................................ 34
FIGURE 4-5: AXES USED TO LOCATE THE WING ...................................................................................................... 34
FIGURE 4-6: WING DRAG COEFFICIENT VARIATION WITH WING VERTICAL LOCATION WITH ERROR BARS............................. 37
FIGURE 5-1: FLOW COMING FROM THE SIDES ....................................................................................................... 39
FIGURE 5-2: PATTERNS OBTAINED ON THE BACKLIGHT FOR = 0 ............................................................................. 40
FIGURE 5-3: PATTERNS OBTAINED ON THE BACKLIGHT AND THE TRUNK FOR = 0 ....................................................... 41
FIGURE 5-4: REATTACHMENT LINE ...................................................................................................................... 41
FIGURE 5-5: DOTS PLACED ON THE TRUNK ........................................................................................................... 42
FIGURE 5-6: FEATURES OF THE PATTERNS ............................................................................................................ 43
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FIGURE 5-7: AXES USED ................................................................................................................................... 44
FIGURE 5-8: PATTERNS OBTAINED WITH THE OIL-DOT TECHNIQUE FOR DIFFERENT SIDESLIP ANGLE ................................... 45
FIGURE 5-9: EFFECT OF SIDESLIP ON THE CONTRA-ROTATING VORTICES SIZES .............................................................. 46
FIGURE 5-10: EFFECT OF SIDESLIP ON THE CONTRA-ROTATING VORTICES POSITIONS ..................................................... 47
FIGURE 5-11: BEHAVIOUR OF THE NON DOMINANT CONTRA-ROTATING VORTEX FOR EXTREME SIDESLIP ANGLES (LEFT PICTURE:
=-3; RIGHT PICTURE: =4) .................................................................................................................. 48
FIGURE 5-12: EFFECT OF SIDESLIP ON THE REATTACHMENT LINE POSITION .................................................................. 48
FIGURE 5-13: EFFECT OF SIDESLIP ON THE TRAILING EDGE VORTICES AND THE CENTRE LIMIT ........................................... 49
FIGURE 5-14: EFFECT OF WING AXIAL POSITION ON THE RIGHT CONTRA-ROTATING VORTEXS SIZE ................................... 50
FIGURE 5-15: EFFECT OF WING AXIAL POSITION ON THE RIGHT CONTRA-ROTATING VORTEX POSITION .............................. 51
FIGURE 5-16: EFFECT OF WING AXIAL POSITION ON THE REATTACHMENT LINE POSITION ................................................ 52
FIGURE 5-17: EFFECT OF WING AXIAL POSITION ON THE TRAILING EDGE VORTICES AND THE CENTRE LIMIT ......................... 53
FIGURE 5-18: EFFECT OF WING HEIGHT ON THE RIGHT CONTRA-ROTATING VORTEXS SIZE .............................................. 54
FIGURE 5-19: EFFECT OF WING HEIGHT ON THE RIGHT CONTRA-ROTATING VORTEX POSITION ......................................... 55
FIGURE 5-20: EFFECT OF WING AXIAL POSITION ON THE REATTACHMENT LINE POSITION ................................................ 56
FIGURE 5-21: EFFECT OF WING HEIGHT ON THE TRAILING EDGE VORTICES AND THE CENTRE LIMIT ................................... 57
FIGURE 5-22: EFFECT OF WING AXIAL POSITION ON THE RIGHT CONTRA-ROTATING VORTEXS SIZE WITH SIDESLIP ............... 58
FIGURE 5-23: EFFECT OF WING AXIAL POSITION ON THE RIGHT CONTRA-ROTATING VORTEX POSITION WITH SIDESLIP ........... 59
FIGURE 5-24: EFFECT OF WING AXIAL POSITION ON THE REATTACHMENT LINE POSITION WITH SIDESLIP ............................ 60
FIGURE 5-25: EFFECT OF WING AXIAL POSITION ON THE RIGHT CONTRA-ROTATING VORTEX POSITION WITH SIDESLIP ........... 61
FIGURE 5-26: WING LIFT COEFFICIENT VARIATION WITH AXIAL POSITION .................................................................... 62
FIGURE 5-27: WING DRAG COEFFICIENT VARIATION WITH AXIAL LOCATION ................................................................. 63
FIGURE 5-28: CAR LIFT COEFFICIENT INCREMENT VARIATION WITH WING AXIAL LOCATION ............................................. 64
FIGURE 5-29: CAR DRAG COEFFICIENT INCREMENT VARIATION WITH WING AXIAL LOCATION .......................................... 64
FIGURE 5-30: CAR LIFT OVER DRAG RATIO VARIATION WITH AXIAL LOCATION .............................................................. 65
FIGURE 5-31: WING LIFT COEFFICIENT VARIATION WITH WING VERTICAL LOCATION ...................................................... 66
FIGURE 5-32: WING DRAG COEFFICIENT VARIATION WITH WING VERTICAL LOCATION ................................................... 66
FIGURE 5-33: CAR LIFT COEFFICIENT INCREMENT VARIATION WITH WING VERTICAL LOCATION ........................................ 67
FIGURE 5-34: CAR DRAG COEFFICIENT INCREMENT VARIATION WITH WING VERTICAL LOCATION ...................................... 68
FIGURE 5-35: CAR LIFT OVER DRAG RATIO VARIATION WITH WING VERTICAL LOCATION ................................................. 69
FIGURE 5-36: OIL FLOW OVER THE REAR OF THE MODEL WITH THE STRUT................................................................... 70
FIGURE 6-1: STRUCTURE OF THE NEAR WAKE ........................................................................................................ 71
FIGURE 6-2: DOTS PLACED NEAR THE ROOFS SIDE EDGE ......................................................................................... 72
FIGURE 6-3: CONTRA-ROTATING VORTICES FORMATION ......................................................................................... 73
FIGURE 6-4: FLOW OVER THE TRUNK ................................................................................................................... 74
FIGURE 6-5: FLOW OVER THE REAR PART OF THE MODEL ......................................................................................... 74
FIGURE 6-6: FLOW COMING FROM THE SIDES ....................................................................................................... 74
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FIGURE 6-7: EFFECTS OF SIDESLIP ON THE CONTRA-ROTATING VORTICES .................................................................... 75
FIGURE 6-8: FORMATION OF THE CONTRA-ROTATING VORTICES WITH A POSITIVE SIDESLIP ANGLE ................................... 76
FIGURE 6-9: FLOW OVER THE TRUNK WITH A POSITIVE SIDESLIP ANGLE....................................................................... 76
FIGURE 6-10: STRUCTURE OF THE WAKE WHEN THE STRUT IS USED ........................................................................... 78
FIGURE 6-11: CORNER FLOW WHEN THE WING IS CLOSE TO THE BACKLIGHT ................................................................ 79
FIGURE 6-12: WING/TRUNK STRUCTURE ACTING LIKE A DIFFUSER ............................................................................. 80
FIGURE 6-13: EFFECT OF WING AXIAL LOCATION ON WING DOWNFORCE .................................................................... 80
FIGURE 6-14: EFFECT OF WING AXIAL LOCATION ON TOTAL CAR DRAG ....................................................................... 81
FIGURE 6-15: EFFECT OF WING VERTICAL LOCATION ON WING DOWNFORCE ............................................................... 82
FIGURE 6-16: EFFECT OF WING HEIGHT ON CAR DOWNFORCE DUE TO WING-BODY INTERACTION ................................... 83
FIGURE 6-17: EFFECT OF WING AXIAL LOCATION ON TOTAL CAR DRAG ....................................................................... 85
Tables
TABLE 4-1: MIXES OF POSTER PAINT AND WATER USED ........................................................................................... 29
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Nomenclature
Roman
Drag coefficient
Lift coefficient
Backlight length
Trunk length
Reynolds number
Backlight's top edge width
Trunk width
/ Normalised wing axial location
/ Normalised axial location on the backlight
/ Normalised axial location on the trunk
/ Normalised lateral location on the backlight
/ Normalised lateral location on the trunk
/ Normalised wing vertical location
Greek
Sideslip angle
Car drag coefficient increment due to the wing
Car lift coefficient increment due to the wing
Subscribe
Backlight
Trunk
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1 - Introduction
1
1 Introduction
Increasingly designers and engineers work together. When a new project is being
developed, two main requirements must be fulfilled; the mechanical aspect, dealt with
by the engineers, must obviously be conceived with great care since it gives the product
its main function; and the aesthetical aspect, dealt with by the designers, which must not
be neglected either as the exterior look is often a very important parameter for
customers.
The automotive industry is not an exception. One very important feature to take into
account when conceiving an automobile is its drag coefficient since it influences engine
requirements, fuel consumption and the overall aerodynamic performance. The major
component of the drag force is called the form drag and mainly arises from the flow
separation. However, some appealing designs happen to increase flow separation.
In addition, race cars must generate some downforce to improve tire adhesion and as a
result, vehicle acceleration and turning rate. The addition of spoilers such as a rear
mounted wing is a very good way to increase the downforce. But to understand properly
the effects of such a wing and its shape, location or inclination on a race cars
aerodynamic properties, it is necessary to have a good understanding of the features of
the wake of the car without spoilers and of how the flow separates.
Two contra-rotating vortices are located on in the near wake of a notchback car. Fisher
(1) found that as the wing is moved forward, one of these vortices becomes dominant
and eventually, when the wing is at its foremost position, only one bigger vortex exists.
1.1 Aims
The aim of this study is to investigate the effect that adding a rear mounted wing to a
saloon car has on the structure of the wake and understand why one of the contra-
rotating vortices becomes dominant as the wing is moved forward.
1 - Introduction
2
It will be important to get an understanding of what gives this particular structure to the
wake and how its structure is affected by external parameters. The influence of side
winds on the structure on the wake will be investigated.
Finally, the interaction of the wing with the two contra-rotating vortices located near the
backlight of the car will be investigated. More particularly, the influence of axial and
vertical position will be of interest.
1.2 Objectives
To achieve this, wind tunnel testing will be carried out using a quarter scale model of
the car. A particular technique, surface oil-dot flow visualisation, will be used to
precisely visualise the structure in the wake. The technique will first have to be adjusted
for this particular case. The analysis of the effect of wing location on the wing lift and
drag and on the car total lift and drag will also enable a better understanding of the flow
structure.
2 - Literature review
3
2 Literature review
As mentioned earlier, it is very important to understand the structure of the wake behind
a car when no spoilers are mounted on it. In the first part of this section, the basic
feature of the wake will be described. The second part of this section will consist of a
short description of rear spoilers and wings and the effect they have on the main
aerodynamic characteristics. This will be a good starting point for further investigation
which will be carried out in the frame of this thesis project. Finally, the last part of this
section will describe some issues encountered in wind tunnel testing as well as some
flow visualisation techniques useful for this thesis.
2.1 Flow development in the wake of a notchback car
The flow development behind a car is very complicated. Moreover it is very likely to
vary depending on the shape of the rear part of the vehicle.
2.1.1 The rear-body
Three main types of rear- body form exist for passenger cars as shown in Figure 2-1.
They are usually named the hatchback (or fastback), the notchback and the squareback.
Even though the hatchback seems to have the most efficient rear- body of the three
aerodynamically speaking, the drag coefficient of such a car can be higher than
expected. They can produce strong trailing edge vortices which help the flow over the
backlight to remain attached. By remaining attached over the rear screen, the flow is
strongly pulled down at the rear. The consequential effect is that the change of
momentum results in the production of both lift and drag. It was also noted that the rake
angle (Figure 2-2) has a strong influence on the drag experienced by the car. Barnard (2)
stated that for a rake angle smaller than 10, which could be assimilated to a squareback
configuration, there are no trailing edge vortices therefore the drag decreases with
increasing angle as the normal pressure decreases with more taper. However, from 10,
strong trailing edge vortices start to form and get stronger with increasing rake angle.
2 - Literature review
4
As a consequence, the drag also starts to increase from 10 and a peak in drag
coefficient has been revealed for rake angles around 30 (2).
Figure 2-1: Rear-end forms: notchback (left); hatchback (centre); squareback (right)
Figure 2-2: Rake angle of a hatchback car
2.1.2 Flow separation - Vortices
In this thesis, the case of a car with a notchback (Figure 2-3) will be of interest since the
test model is of this type. Two types of separation characterise the flow over this type of
car: quasi-two-dimensional and three-dimensional separations (3).
2 - Literature review
5
Figure 2-3: Notchback rear end parameters (4)
2.1.2.1 Quasi-two-dimensional separation
This type of separation occurs when the flow at the roof trailing edge undergoes an
adverse pressure gradient which causes the boundary layer to detach. The state of the
boundary layer determines where the flow will separate, however if the trailing edge is
sharp, the separation will inevitably occur at the rooftop trailing edge. In 1974, Carr (5)
showed that for 35, a transverse vortex is formed as shown in Figure 2-4. As a
result, the circulation involved in the transverse vortex has the effect of reducing the
pressure over the decklid (cover of the trunk) and in this way creating some downwash.
The consequence of this is an increase in pressure at the end of the decklid, therefore
generating some downforce in this area.
Due to separation, a shear layer is created which may reattach on the decklid or not
depending on the backlight angle (), the height (d), the decklid length (t) (Figure 2-3)
and the downwash created by the transverse vortex. In 1990, Nouzawa, et al. (6)
determined that = 25 is the critical angle above which the flow does not reattach and
bellow which it does reattach. If the flow reattaches, a separation bubble is created as a
part of the flow is engulfed in the recirculation area.
Quasi-two-dimensional separation also occurs at the decklid trailing edge and another
separation bubble behind the base is formed when the flows from below and above the
car merge.
2 - Literature review
6
Figure 2-4: Transverse vortex (5)
2.1.2.2 Three-dimensional separation
Three-dimensional separation occurs at the C-pillar. The flow coming from the sides of
the car is sucked towards the centre line by the lower pressure of the flow over the
decklid. As a result, C-pillar vortices are formed and propagate downstream as show in
Figure 2-5. The formation of these vortices is dependent on the backlight angle and the
aspect ratio (7). Indeed, the C-pillar vortices do not appear when the backlight angle is
more than 43 degrees (8) and become weaker with higher aspect ratios (7).
Figure 2-5: C-pillar vortices (6)
2 - Literature review
7
One major effect of these vortices is the downwash they produce in the centre plane
since it pushes the shear layer created by the rooftop separation towards the decklid and
therefore helps the flow to reattach and delays the decklid trailing edge separation (5).
However this effect is not so pronounced on notchback cars as their aspect ratios are
larger and consequently the produced downwash is weaker.
2.1.2.3 Arch vortex
If the angle defined in Figure 2-3 is below 25 but close or equal to this value, the
recirculation within the separation bubble forms an arch vortex as show in Figure 2-6.
The base of the arch vortex forms two contra-rotating vortices on the surface of the
trunk between which some reverse flow goes upwards.
Figure 2-6: The Arch vortex (6)
2.1.3 Sideslip
The effect of crosswinds can have a strong influence on the vehicles behaviour. If the
effective yaw angle is high, the rear positive lift coefficient can be dramatically
increased, inducing a decrease in tyre grip and in this way a decrease in acceleration and
turning rate capabilities. The drag coefficient undergoes a similar change with sideslip
(9).
2 - Literature review
8
However, the effect of sideslip on saloon cars and the efficiency of spoilers in the case
of a cross wind has yet to be investigated since the wake structure may be affected and
as a consequence, the loading of the wing would be significantly different. This is all
the more important as in a real situation it is very unlikely that there will be a fully
streamwise air flow.
2.2 Lift-reducing surfaces
During the 1960s, the top speed reached by race cars was such that the aerodynamic
forces had a strong influence on performance. Indeed at these speeds, the drag was
becoming very important and the car shapes designed to reduce it were producing some
positive lift. This effect was very unsettling for drivers as the positive lift reduces tyre
grip therefore making the vehicle very unstable (10). To counter these effects, lift-
reducing devices such as spoilers were developed. In this section, devices which are
continuous with the body of the car will be referred to as rear lip spoilers in order to
avoid the confusion with rear wings which are fundamentally different in the way they
affect the air flow around the body.
2.2.1 Rear lip spoilers
Both front spoilers and rear spoilers exist but only rear spoilers will be described here as
only the rear part of the car is of interest in this thesis. The main purpose of using rear
lip spoilers is to reduce or cancel the positive drag produced by the fast and smooth
flow. This is done by causing the flow to separate, or to separate sooner if it has already
separated over the rear end of the car as shown in Figure 2-7. By doing so, the flow
velocity over the rear end of the car is decreased and the pressure increased, therefore
reducing the positive lift.
2 - Literature review
9
Figure 2-7: The effect of a rear spoiler (10)
It was also noted that the use of rear lip spoilers does not necessarily come with an
increase in drag. In some cases, fitting a car with a rear spoiler can actually decrease the
drag. But even though in the majority of cases rear lip spoilers increase the drag and
decrease the top speed, the overall performance such as acceleration and turning rate are
increased and as a consequence, the lap times decrease (10).
Another way to reduce the overall lift coefficient is to produce some downforce using
an inverted wing at the rear of the car. Wings are lifting surfaces usually used to lift
aircraft off the ground, so by using wings upside down, the force generated is logically
pointed towards the ground.
2.2.2 Wings
2.2.2.1 Wing/body interactions
It is interesting to note that the flow over a saloon car rear-mounted wing can be very
different from that of the wing alone. Indeed, there are strong interactions between the
wing and the car body. The effects of adding a rear wing are described bellow.
2.2.2.1.1 Effect on the cars own downforce
The addition of a wing can increase the downforce of the body itself. Indeed, the flow
going between the wing and the body is deflected upwards. This means that the flow
going under the body is also deflected upwards and its velocity is increased. And as a
2 - Literature review
10
higher velocity implies a lower pressure, more downforce is produced by the body itself,
independently of the wings own downforce, as shown in Figure 2-8.
Figure 2-8: Effect of a rear wing on the stream lines nearby a generic body (11)
2.2.2.1.2 Effect on reattachment
Another noticeable effect is that as the flow velocity increases between the wing and the
body, it partially reattaches on the body. Consequently, the drag is reduced due to a
reduction of the separation area.
2.2.2.1.3 Ground effect
The effectiveness of the wing is strongly dependent on its position relative to the body
and on parameters related to its shape such as its thickness or chord length. As can be
seen in Figure 2-9 and Figure 2-10, as the wing is fixed closer to the body, the
downforce produced drastically increases. This is called Ground effect and is due to
the fact that the flow velocity between the wing and the body increases with the wing
proximity. However, if the wing proximity becomes less than
= 0.5, the boundary
layer generated on the trunk blocks the flow under the wing. Therefore, the flow mainly
goes over the wing and the downforce produced is highly reduced. Note that this
2 - Literature review
11
minimum distance depends on the vehicle configuration. The downforce generated is
also more important for a large chord wing than for a small chord wing. It can also be
noted that neither the wing proximity nor the chord length has a strong effect on the
vehicles drag.
Figure 2-9: Effect of distance to the body and aspect ratio on the wing effectiveness (12)
Figure 2-10: Effect of wing proximity to the ground on the downforce (11)
2.2.2.1.4 Angle of attack
The angle of attack also strongly influences the effectiveness of the wing. With higher
angle of attack, a wing produces more lift or downforce in the case of an inverted rear
mounted wing. Figure 2-11 shows that the lower surface distributions of the
2 - Literature review
12
mounted wing and the wing alone are very different in shape and magnitude. The
mounted wings lower surface experiences a much larger suction than the wing alone
while the upper surface distribution is roughly the same in both cases. Consequently,
the wing produces more downforce when it is mounted on the car. This is explained
firstly by the fact that, as mentioned previously, the flow velocity is increased over the
lower surface when the wing is close to the car. The second reason is the fact that the
upstream flow changes direction because of the body. The flow is deflected in the
downward direction which effectively increases the angle of attack and, as stated above,
the lift is increased.
Figure 2-11: Comparison of the CP distributions of a mounted wing with that of the wing alone (11)
2.2.2.2 Wings mounting
The way the wing is fixed to the body can also have an effect on its efficiency. The aim
is to have the maximum plane area given the maximum width permitted to have the
maximum downforce. Therefore, the manner of attachment must use as little of this
plane area as possible. There are two way of attachment for rear wings which can be
combined: the centre post and end plates. If a centre post is used, it should be shaped in
such way that it does not interfere too much with the airflow. If end plates are used,
2 - Literature review
13
their thickness should be chosen carefully as too thick end plates will eat too much
plane area for a fixed span and too thin end plates will not be rigid enough (10).
2.2.2.3 End plates
End plates are not just used to mount the wing on the car, they are also used to increase
the downforce generated by the wing and reduce the drag. Without end plates, the
difference of pressure between the upper and the lower surface of the wing makes the
air from the high pressure surface move to the low pressure surface. This has the effect
of decreasing the difference of pressure between the two sides and as a result the
downforce is reduced. By adding end plates to the wing, the difference of pressure is
maintained and no loss of downforce is experienced. The migration of the air from one
side to the other also generates tip vortices and therefore a large amount of drag. The
addition of end plates prevents the formation of tip vortices and in this way the drag is
decreased (13).
Figure 2-12: The effect of end plates (10)
Due to the small amount of published data, it is difficult to predict with precision how
the addition of a rear wing may affect the flow structure. This issue will be investigated
within the framework of this thesis project.
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14
2.3 Wind tunnel testing
It is necessary to assess the aerodynamic performance of the vehicle during the design
process for several reasons. One is the necessity to have an acceptable compliance with
official requirements; another reason is the desire to check the efficiency of the design.
Two test methods are available: road testing and wind tunnel testing. Even though road
testing seems to be the most natural and accurate method since in this way the car is
tested in real conditions, there are many drawbacks in this method. First, it is really
expensive to perform such tests as it is necessary to have a full scale real car equipped
with all the measuring instrumentation. Moreover, model changes such as different car
fore-bodies are not easy to do because of the instrumentation and therefore, the
repeatability of test conditions are not easy. The uncontrollability of the environment is
also a problem.
In contrast, wind tunnel testing makes the tests a lot easier. The most obvious advantage
is that the car stays stationary while the air is moving. This also implies that it is
possible to use full scale or even reduced scale models. The aerodynamic loads can be
measured by a stationary scale, or balance. The test conditions can be controlled.
However there are, here as well, some issues in the use of wind tunnels such as scale or
Reynolds number effect, simulation of the moving road problem and errors due to
blockage (2).
2.3.1 Wind tunnel testing issues
2.3.1.1 Reynolds number effect
Scale effects can be expected due to the difference in model and full-scale Reynolds
number. Figure 2-13 shows a thin plate placed in a in a stream of air and its scale model
placed in the same stream of air and in the same conditions. It can be seen that the
transition point is located at the same distance from the leading edge. However the
boundary layer transition occurs at about 25% of the length from the leading edge for
the full scale plate but it occurs at more than 50% of the length from the leading edge
for the scale model. The drag per unit area is therefore lower for the scale model and is
not representative of the full scale phenomenon.
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15
Figure 2-13: Boundary layer and scale effect (2)
To prevent Reynolds number effects occurring, it is necessary to run the tests at the
same Reynolds number. To do so, the easiest way is often to increase the air velocity
( =
), however if the scale ratio is too high the velocity could have to be
increased so much that it would go supersonic and the flow over the model would be
completely different from the full size. To tackle this problem, the density can be
increased using a pressurized wind tunnel or the viscosity can be decreased by using
cryogenic cooling but even though these methods exist, they are extremely expensive.
For automotive wing tunnel testing, the solution is often to perform full scale tests, the
price of which is not excessive for major manufacturers (2). If full scale testing is
impossible, to avoid compressibility effects, the Mach number must not exceed 0.4.
This implies that, as the density and the viscosity are unlikely to be changed, the scaling
factor must be kept under a certain value depending on the full scale velocity that must
be tested (14).
2.3.1.2 Ground simulation
A simulation issue arises from the relative velocity of the wind and the ground in wind
tunnel testing. Indeed, in real conditions shown in Figure 2-14A, the vehicle is moving
relatively to the ground and the wind but the wind is not significantly moving relatively
to the ground. Therefore there is no road boundary layer. However in a wind tunnel
Figure 2-14B, the air is moving relatively to the vehicle and the ground, thus developing
a boundary layer. As a consequence, the ground plane boundary layer velocity profile is
not the same and the results will be significantly affected (2).
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Figure 2-14: Moving road problem (11)
There are many way to tackle this problem as shown in Figure 2-15.
Figure 2-15A shows the ground board method. By mounting the model above an
elevated board, the boundary layer developed will be a lot thinner and the errors will be
minimized. This is a very simple method that can be used in small wind tunnels.
The method shown in Figure 2-15B consists in boundary layer suction ahead of the
model. The boundary layer developed under the model is much thinner. This method is
also very simple. This method can be improved by applying the suction under the whole
model (see Figure 2-15C). This method is complex and expensive (11).
A similar method is to blow air into the boundary layer to reenergize it and make it
thinner (see Figure 2-15D). This is a quite efficient method but also expensive (11).
Another method is to use a mirror image underneath the model (see Figure 2-15E).
Since there is symmetry, the symmetry line between the two models is a stream line.
Therefore, no boundary layer effects are experienced. However, the models must be
exactly identical and every changes made on one model during the tests must be done
on the other one. Moreover, the test section size must be increased to contain the two
2 - Literature review
17
models. The costs and complexity of this method are thus very high which makes it
unused nowadays (2) & (11).
The last method is the moving belt technique, popular among race car designers, shown
in Figure 2-15F. It consists in removing the relative motion between the ground and the
model. Despite the good results this method gives, it is not simple to perform. The
model has to be mounted by above using a sting, which can interfere with the flow.
Suction must be applied before the belt and under the whole model and air must be
reintroduced behind it. The sting may also interfere with the measurements of lift and
drag. The last issue is the limited speed of the belt which is usually less than the
maximum wind tunnel speed (11) & (2).
Figure 2-15: Various methods for simulating a moving ground in a wind tunnel (11)
2.3.1.3 Test-section blockage
Blockage effects come from the fact that the model and the flow are constrained in the
walls of the facility. There is a distorting effect on the stream lines around a body
constrained in rigid walls (see Figure 2-16B) that does not exist in an open free stream
(see Figure 2-16A). Moreover, as the body partially blocks the working section, the air
speed is increased around the model. Then it is needed to apply some correction,
otherwise the lift, drag and other coefficients will be overestimated (2). A dilemma also
2 - Literature review
18
appears concerning the model size; it is always preferable to perform full scale testing in
order to keep the same Reynolds number as explained in Section 2.3.1.1 and also to
include some small details of the design whereas the model should be kept as small as
possible so that the blockage effect can be minimized. So there must be a compromise
the wind tunnel facility and design requirements since the wall interferences are not
negligible (11).
Figure 2-16: Blockage effect
The correction that must be applied mainly concerns the velocity, the dynamic pressure
and the Reynolds number. The real values of these parameters are greater than the ones
calculated with the wind tunnel instrumentation. They can be obtained as follows:
= 1 +
= 1 + 2
= 1 +
Where V, q and are the air velocity, the dynamic pressure and the Reynolds number
of the test section respectively and is the blockage correction factor.
Several ways to obtain the blockage correction factor exist. However, for unusual or
complicated shapes it can be very difficult to obtain it. It those cases, Barlow, Rae and
Pope (15) defined the following approximate blockage correction factor:
2 - Literature review
19
=1
4
2.3.2 Flow visualisation
There are many ways to assess the flow characteristics during wind tunnel testing. Some
are direct measurements and make it possible to obtain values of the main parameters
such as the lift, the drag and the moments. The use of balances is often a good way to
proceed since it allows accurate measurements. Other methods provide information
about the structure of the flow development and are called flow visualisation. They do
not provide any quantitative data but show the direction of stream lines and can reveal
the presence of vortices or point to the location of separation. There are many different
types of flow visualisation but only those that are most commonly used will be
described in this section.
2.3.2.1 Tuft flow visualisation
Tufts are often used to show the flow patterns. They can be used in several ways.
One way is called wool tufts flow visualisation. It consists in a surface flow
description using wool tufts. Wool tufts are taped to the model surface with adequate
spacing to prevent adjacent strands becoming tangled, then air speed is increased as to
the desired value and the tufts are monitored. This method gives information about the
flow direction and directional stability. The main advantage of this method is that it
provides a quick evaluation of flow direction on the surface; however the tufts can cause
flow disturbances and slightly modify the flow pattern. If the tufts are placed on a non
horizontal surface, the gravity can affect their direction. Another limitation of this
technique is that it is very time consuming to apply all the wool tufts on large areas (16).
Another way is the tuft wand technique. It consists in a flow field description using a
tuft wand. The direction of the flow field is revealed by introducing a wool tuft attached
to a rod into the airstream. The rod must be long enough to allow the use of this
technique from a sufficient distance so that the aerodynamic interferences are
minimized. The wool tuft used must also be long enough to examine the flow
phenomenon in question. This technique is easy and quick to deploy. Large scale or
2 - Literature review
20
small scale phenomena can be observed by changing the tuft length. However, in low
speed flow, the weight of the tuft modifies the indication of the flow and if the tuft
length is not properly adjusted, it may be difficult to observe small scale phenomena
(16).
2.3.2.2 Oil flow visualisation
This technique consists in applying a film of oil on the surface of the model. The oil
must be mixed with a pigment. The mixture can either be kerosene and a fluorescent
powder or liquid paraffin, titanium dioxide and oleic acid.
The airflow over the surface will create shear stresses which will move the pigment
particles and eventually show the flow lines. If the first mixture is used, then ultraviolet
lamps must be used to visualise the flow lines. This technique gives some detailed
qualitative information about the flow lines direction and speed of attached flow on the
surface of the model. The zone of separation as well as the vortices close to the surface
can also be observed.
The main advantages of this technique are that it is very simple and inexpensive. The
fact that the flow lines can still be observed after the wind tunnel has stopped is also an
advantage. However, it is quite messy to use as the fluorescent particles can deposit on
the floor downstream of the model and also on the clothes and hand of the tunnel staff
when handling the model. It is often difficult to differentiate forward from reverse flow
in the zones where the flow has separated. Another disadvantage is the time it takes for
the flow patterns to fully develop and gravity can affect the results on non horizontal
surfaces (16).
2.3.2.3 Oil dot flow visualisation
Oil dot flow visualisation involves the use of small dots of oil (or ink) to visualise the
surface stream lines (see Figure 2-17). Some droplets are placed on the surface of the
model so that the air flow blows them and in this way the stream lines appear. The oil
contains a pigment so as to have a good contrast with the surface colour. To help the oil
dots to move more easily, it is often necessary to cover the surface with a thin layer of
clear oil before applying the dots (17).
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21
Figure 2-17: Results obtained with the oil dot technique (16)
This technique gives indications about the surface flow direction, vortex patterns and
the location of flow separation.
This technique is widely used due to its numerous advantages. As the oil flow
visualisation technique described in Section 2.3.2.2, the flow patterns can be more
closely observed and studied after the wind tunnel has stopped. It is also convenient to
be able to develop the full scale patterns by successively applying the droplets and
blowing the tunnel. This technique also provides a precise indication of flow separation
points and enables to establish flow directions in the turbulent wake, which is usually
difficult with other flow visualisation methods.
This technique unfortunately comes with its limitations. It is difficult to use it on quasi-
vertical surfaces. The droplets can not flow past model joint lines if they are not taped.
The model must be cleaned after using this technique. The flow condition can not be
changed during the test; if several test conditions are to be tested such as several angles
of incidence or several wind tunnel speeds, then several tests have to be run otherwise
2 - Literature review
22
the patterns would be confused. Finally, a high freestream flow velocity is required to
observe the stream line in low surface velocity areas.
2.4 Previous work
In the frame of a previous thesis project by Fisher (1), the aerodynamic characteristics
of a rear mounted wing and how it interacts with the three dimensional wake of a
notchback car have been investigated. This has been done by studying the affect of wing
location and angle of attack in terms of flow structure, wing forces and overall effect on
the model.
This study confirmed the general wake structure suggested by Nouzawa et al (6), more
particularly the two contra-rotating vortices on the backlight and the trailing edge
vortices.
It was noted that the lift (downforce) increased as the wing was moved rearwards. The
lift also increased with the height of the wing.
The results obtained for the drag were quite surprising since for all wing locations in the
wake, the drag was negative. This result was not in accordance with the flow
visualisation and more detailed flow visualisation around the wing would have been
necessary to get a better understanding of how the flow is circulating around it.
The addition of the wing had the effect of increasing the general model downforce for
all locations and decreasing its drag for low and forward locations. For high and
rearward locations of the wing, the wake size and therefore the drag increased.
An increase of angle of attack caused the drag and lift to increase. An optimum angle of
5, leading to an increase in lift but a small increase in drag, was determined. The
downward flow direction of the wake increased the angle of attack and caused the wing
to stall at 10.
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The experiments also showed that the addition of the wing makes one of the two contra-
rotating vortices dominant on the other one and that this effect is more apparent as the
wing is moved forwards. At the wings foremost position, only one single region of
recirculation was observed. However the cause of this phenomenon is still not well
understood and more detailed flow visualisation is required. This will be done as a part
of this thesis project.
The effects of yaw and side winds were not studied by Fisher (1), even though these
factors would affect the wake structure and consequently the aerodynamic forces on the
car. This will also be studied in the frame of this thesis project.
Cranfield University Aerospace Dynamics
24
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3 - Experimental set up
25
3 Experimental set up
3.1 Model
3.1.1 Car
The model used was a scale glass fibre model based on the MIRA Variable Geometry
model for a notchback vehicle and created for the purpose of a previous MSc project by
Fisher (1) (Figure 3-1). This model was chosen due to its geometric similarities with
several saloon race cars. Its main glass fibre body is connected to four non rotating
wheels. The scale was chosen to limit the blockage effect by having a sufficiently
small frontal area.
Figure 3-1: Glass fibre model
3.1.2 Wing
The rear spoiler used for the experiments was an inverted wing with a Clark-Y aerofoil
section (Figure 3-2). The Clark-Y section was chosen for its good performance at low
Reynolds number and for its flat bottom making easier the manual settings of angle of
attack.
3 - Experimental set up
26
The wing is 415 long and 40 wide which make it no longer than the model is
wide as specified by the Touring Car Championship regulations. Thus, it has an aspect
ratio of 10.4 and a planform area of 0.0166 2.
Figure 3-2: Model mounted in the 86 wind tunnel with the wing
3.1.3 Wing support
The wing is mounted on the car using three struts linking it to an additional balance
located on the rear inside the model. A small horizontal strut first links the balance to
the second strut which is vertical. This one is linked to the third strut which is horizontal
and on which is fixed the wing. On the second and the third struts, several holes have
been drilled such that the wings axial location and height can be changed (Figure 3-3).
The wing is attached to the third strut by a unique screw. This enables to set an angle to
the wing before tightening the screw.
3 - Experimental set up
27
Figure 3-3: Wing support
3.2 Wind tunnels
The different experiments were conducted in different wind tunnels due to their
respective availability. However the same model was used for all experiments.
3.2.1 Sideslip tests
The adjustment of the oil-dot technique as well as the sideslip experiments were carried
out in the Weybridge wind tunnel in Cranfield University. This tunnel has a circular
open section with a diameter of 1070 . This section is rather small considering the
dimensions of the model and there is likely to be an effect but the programme is
primarily concerned with trends.
3.2.2 Effects of wing position on the structure of the wake
The tests were carried out in the Cranfield University G13 open working section, closed
return wind tunnel. The facility has an elliptic nozzle, 1.120 0.872 .
Again, this section is rather small considering the dimensions of the model and there is
likely to be an effect but the programme is primarily concerned with trends
3.2.3 Force measurements
The force measurements have been carried out in the Cranfield University 8 6
Automotive Wind Tunnel. This wind tunnel features a moving ground and boundary
3 - Experimental set up
28
layer suction. The model can be accurately located at a specified height with an active
driven servo strut system.
Two balances were used to measure aerodynamic forces. The Aerotech 6 component
Internal Balance was used to measure the total lift and drag of the car with the wing
mounted on it. It contains one strain gauge for each aerodynamic component.
The calibration, which was carried out by the manufacturer, takes into account all
interaction between the 6 components measured.
An additional balance was used to measure the lift and drag of the wing and its support
only. The balance used for the wing was composed of two 50 load cells connected
together in such way that one would measure lift and the other rotated through 90 to
measure drag without interacting with each other.
The calibration was carried out by Fisher (1) during her MSc project.
4 - Test Method
29
4 Test Method
4.1 Oil-dot technique
The oil-dot technique was used to visualise the surface flow in the near wake. This
technique had to be optimised for the case of low flow velocity on an inclined surface
since the optimum mixture to use is really dependent the flow conditions.
4.1.1 Mixtures tested
Two mixtures have been tested. The tests were carried out in the Weybridge wind
tunnel in Cranfield University. The wing was not mounted on the model and no sideslip
angle was set.
4.1.1.1 Poster paint
The first mixture tested was a mix of poster paint and water. Several were tested (Table
4-1).
Test number volume of poster paint volume of water
1 1 1
2 1 2
3 1 3
4 1 4
5 1 5
Table 4-1: Mixes of poster paint and water used
The mixtures used in tests 1 and 2 were too thick and the dots would not flow. The
mixture used in test 3 was still too thick and the dots just moved a few centimetres. The
mixture used in test 4 was thin enough to draw the patterns but the dots would not move
in the areas of very low velocity such as the reverse flow or within the contra-rotating
vortices. Moreover the water would not evaporate fast enough and the dots would flow
without leaving clear traces. The mixture used in test 5 was too thin and the dots could
not be placed on the inclined backlight as they would flow with gravity.
4 - Test Method
30
The mix used in test 4 gave the best results was still not suitable to visualise properly
the surface flow on the backlight.
4.1.1.2 Paraffin and invisible blue
The second mixture tested was a mix of paraffin and a fluorescent pigment, invisible
blue. Again, several proportions were tested. Since a very high precision balance would
have been needed to measure the quantity of invisible blue added to the paraffin and
considering that such a balance was not available, the quantity of invisible blue was
measured using the number of full teaspoons of powder added.
With high viscosity (one full teaspoon for 2 of paraffin) the dots would not flow.
With low viscosity (one full teaspoon for 7 of paraffin) the dots were difficult to
place on the inclined backlight and started to flow with gravity as soon as they were
placed so the wind tunnel had to be run as quickly as possible. However the results were
very good: the dots would flow even in the low speed area, enabling to visualise the
reverse flow and a good part of the contra-rotating vortices. The patterns were also very
clear due to the fluorescent pigment (Figure 4-1).
4 - Test Method
31
Figure 4-1: comparison between the two mixtures used (poster paint
on the top picture, paraffin on the bottom picture)
As said before, the limitation of this mixture is that to visualise the reverse flow, the
mixture must not be too viscous, therefore it starts to flow with gravity as soon as the
dot is placed and the tunnel must be run quickly. Therefore the first centimetres of the
patterns are not due to the flow but to gravity.
4 - Test Method
32
This mixture was chosen for the experiments since in spite of its limitations, it gave
good results for the visualisation of the contra-rotating vortices and the surface flow on
the trunk.
4.1.2 Repeatability tests
To assess the reliability of this technique, repeatability tests have been performed. They
involved placing one dot, running the wind tunnel, taking a picture and starting again
placing another dot at the exact same position. It was also important to wind up the
tunnel the same way so that the wind accelerates the same way every time.
Figure 4-2 shows the patterns left during three different runs with dots of medium size
placed on the top right corner. The patterns are very similar.
Figure 4-3 shows the patterns left during three different runs with big dots placed on the
top left corner. Again the patterns are very similar.
This shows that the oil-dots technique is reliable.
Figure 4-2: Repeatability tests with dots of medium size on the top right corner
4 - Test Method
33
Figure 4-3: Repeatability tests with big dots placed on the top left corner
4.2 Sideslip tests
The freestream flow velocity was 30 . 1 which corresponds to a Reynolds number
based on model length of 1.95 106.
The sideslip angle, , was set by turning the model as show in Figure 4-4. Positive
sideslip angles were defined as sideslip coming from the right when looking in the
upwind direction.
The model was tested at sideslip angles of 4 to +4 with an increment of 1. For
every sideslip angle tested, several runs were done with one dot each time in order to
have the complete picture of the flow structure.
4 - Test Method
34
Figure 4-4: Orientation of the model in the Weybridge Wind Tunnel
4.3 Effects of the wing on the structure of the wake
The interactions between the wing and the models near wake were investigated in order
to understand the results obtained by Fisher (1) (Section 2.4).
The freestream flow velocity was 30 . 1 which corresponds to a Reynolds number
based on model length of 1.95 106.
The wing was set to 5 angle of attack for all the wing locations tested. This angle was
chosen as it gave the best results in Fishers study (1).
The axial position was measured from the lower edge of the backlight to the leading
edge of the wing for a zero angle of attack. The height was measured from the trunk to
the underside of the wing (corresponding to the centre of the higher strut) for a zero
angle of attack (Figure 4-5).
Figure 4-5: Axes used to locate the wing
4 - Test Method
35
4.3.1 Without sideslip
The majority of the tests were done without sideslip. The exact wing locations tested
were
= 0.15, 0.13, 0.55 & 0.97 and
= 0.20, 0.41, 0.63 & 0.85. An extra run was
done without the wing in order to compare and study the effects of adding the wing,
regardless of its position.
4.3.2 With sideslip
A few runs were done with a sideslip angle of 2. The freestream flow velocity was
30 . 1 .Only the effect of axial position was assessed. The vertical position was
= 0.52 and the axial position tested were the same as before,
= 0.15, 0.13, 0.55 & 0.97 . A first run was done to visualise the flow with a 2
sideslip angle and without the wing on since the results may be different from the one
obtained previously as the tests were not carried out in the same wind tunnel.
4.4 Force measurements
4.4.1 Experimental method
Wing and body force measurements were carried out to get an understanding of the
effects of the wing location on the aerodynamic loads of both the wing and the car with
its wing mounted on it.
The model was suspended 3 above the moving belt (distance between the wheels
and the belt) using the strut connected to the six-component balance. The wing was set
to a 5 angle of attack for all the wing locations tested. The axial positions and vertical
positions were measured in the same way as described in Section 4.3. The exact
locations tested were
= 0.15, 0.13, 0.41, 0.69 & 0.97 and
= 0.30, 0.41, 0.52, 0.74, 0.95 & 1.17 . However the position corresponding to
= 0.15 and
= 0.41 could not be tested due to lack of time.
To be able to work out the lift and drag on the wing alone, two runs were done for each
wing set up: one with the struts and the wing and one with just the struts but without the
wing attached on it.
4 - Test Method
36
For all the tests the wind tunnel speed was set to 40 . 1 which corresponds to a
Reynolds number based on model length of 2.60 106.
4.4.2 Force measurements repeatability errors
The force measurements in the Cranfield University 8 6 Automotive Wind Tunnel
were all repeated several times in order to assess the repeatability.
The maximum repeatability errors as a percentage coefficient and are the following:
= 1.7%
= 125%
= 0.6%
= 0.2%
The maximum repeatability errors for the Car Lift Coefficient and the Car Drag
Coefficient are very good and the maximum repeatability error for the Wing Lift
Coefficient is quite good as well. The maximum repeatability error for the Wing Drag
Coefficient seems extremely high, however it is due to the values of the Wing Drag
Coefficient which are very close to zero for some wing locations. Figure 4-6 shows the
error bars on the curve of wing drag coefficient variation with wing vertical location. It
is now clear that the errors are very low for the majority of the points. Even for the
points with the highest errors, the trend of the curve is not affected.
4 - Test Method
37
Figure 4-6: Wing drag coefficient variation with wing vertical location with error bars
Cranfield University Aerospace Dynamics
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5 - Results
39
5 Results
5.1 Sideslip tests
5.1.1 Structure of the near wake
Figure 5-1 shows that the dots placed on the sides of the backlight are blown away from
the centre of the backlight (a) and the droplets placed on the front of the car flow
towards the rear and are sucked onto the backlight (b) and then blown away from the
centre and towards the sides (c). This suggests the presence of the two trailing edge
vortices.
Figure 5-1: Flow coming from the sides
5 - Results
40
The dots placed closer to the centre start to flow down, turn towards the centre of the
backlight and then go upwards (Figure 5-2). This indicates the presence two contra
rotating vortices inducing some reverse flow around the centre of the backlight. It was
also observed that the right vortex is the dominant of the pair. It is rounder and bigger
and its centre is close to the backlights centre whereas the left vortex is not round but
oval and it is difficult to locate its centre. The separation lines between the trailing edge
vortices and the contra-rotating vortices will be referred to as backlight trailing edge
vortex limits. The two trailing edge vortices seem to have the same size since the
backlight trailing edge vortex limits are at the same distance from the side edge of the
backlight.
Figure 5-2: Patterns obtained on the backlight for = 0
Figure 5-3 shows the patterns left by the dots placed on the trunk. It can be seen that the
dots placed near the centre and close to the backlight, flow towards the centre and then
upwards whereas then other dots flow downwards. This indicates that near the backlight
the flow is still separated the lower parts of the contra-rotating vortices are on the trunk
(Figure 5-4). The dots going downwards reveal the presence of attached flow, implying
that the flow reattaches on the trunk. The line separating the dots going upwards and the
dots going downwards will be referred to as reattachment line.
5 - Results
41
Figure 5-3: Patterns obtained on the backlight and the trunk for = 0
Figure 5-4: Reattachment line
Figure 5-3 also shows that the dots placed on the sides of the trunk flow away from the
centre whereas the dots placed closer to the centre flow towards the centre. This
indicates again the presence of the trailing edge vortices (Figure 5-5). The limits
between the trailing edge vortices and the reattached flow on the trunk can be found and
prolongs the backlight trailing edge vortex limits. These limits seem to be straight lines
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parallel to the axis of symmetry of the model. They will be referred to as trunk trailing
edge vortex limits
The droplets flowing towards the centre and downwards on the trunk meet on a line
which seems to continue the line separating the two contra-rotating vortices (Figure 5-3).
This line will be called centre limit.
Figure 5-5: Dots placed on the trunk
5.1.2 Effects of sideslip
To investigate the effects of sideslip, several specific features of the flow patterns were
considered (Figure 5-6):
- the size of the contra-rotating vortices. (A) shows the size of the right vortex. It
was defined as the distance between the first dot on the side going inwards and
the lateral centre of the reverse flow region.
- the positions of their centres. (B) shows the position of the right vortexs centre.
It was defined as the centre of the smallest circle drawn by the dots.
- the backlight trailing edge vortex limits (C). They were obtained by tracing
vertical lines on the backlight between the dots turning inwards and the dots
turning outwards.
- the trunk trailing edge vortex limits (D). They were obtained by tracing vertical
lines on the trunk between the dots turning inwards and the dots turning
outwards. These lines always prolonged the trailing edge vortex limits.
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- the centre limit (E). It was obtained by tracing a vertical line on the trunk where
the dots going inwards meet.
- the reattachment line (F). It was obtained by tracing a line on the trunk between
the dots going upwards and the dots going downwards.
The location of these features could not be determined precisely, the error on the
locations of these lines is estimated to 1 on the model which is not excessive
compared to the model width (40.1 ). Moreover, only the trends were of interest in
this thesis.
Note that since the backlight trailing edge vortex limits are always prolonged by the
trunk trailing edge vortex limits, it is not useful to study both their locations. Therefore
only the trunk trailing edge vortex limits will be looked at.
Figure 5-6: Features of the patterns
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The position of the trunk trailing edge vortex limits, the limit between the contra
rotating vortices on the backlight's lower edge and the centre limit will be given as the
ratio of the distance from the centre line on the trunk () over the trunks width ();
the reattachment lines position will be given as the ratio of the distance from the
trunks rear edge () over the trunks length (); the size of the contra-rotating vortices
and the position of their centres will be given as the ratio of the distance from centre
line on the backlight () over the roofs width () or/and the ratio of the distance
from the backlights lower edge ( ) over the backlights length () (Figure 5-7).
Figure 5-7: Axes used
5.1.2.1 Results obtained with the oil-dot technique
Figure 5-8 shows the patterns left using the oil-dot technique for several sideslip angles.
It must be noted that the angle for which the patterns are symmetric is not zero (as could
be expected) but is very close to two degrees. For a zero angle of sideslip, the right
vortex is clearly already dominant. The angle for which the patterns are symmetric will
be referred as the critical angle since it is the angle at which the dominant side changes.
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Figure 5-8: Patterns obtained with the oil-dot technique for different sideslip angle
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5.1.2.2 Effects of sideslip on the contra-rotating vortices
Figure 5-9 shows the effect of changes in sideslip angle on the size of the contra-
rotating vortices. It must be noted that the diameters could not be measured when the
vortex was not dominant as in that case, it was not round.
The first thing to notice is that the sideslip angle for which the two vortices are the same
size is not zero as could be expected but is between two and three degrees. For a zero
angle of sideslip, the right vortex is clearly already dominant. Approaching = 2, the
right vortexs size starts to decrease and the left vortexs size increases. At
approximately = 2.5, the left vortex becomes bigger than the right one. It is also
notable that for angles below zero, the size of the right vortex does not change
significantly. The size of the vortices mainly changes at angles close to the critical angle.
Figure 5-9: Effect of sideslip on the contra-rotating vortices sizes
Figure 5-10 shows the effect of changes in sideslip angle on the positions of the contra-
rotating vortices. The axial position seems more affected than the lateral position. Again,
as the non-dominant vortex dos not have a round shape, is was not possible to determine
the position of both vortices for all sideslip angles.
For sideslip angles below zero, the right vortex was centred near the centre of the
backlight. Approaching the critical angle, the right vortex starts to move up to the top
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right corner of the backlight. After the critical angle, the left vortex moves towards the
centre line with increasing sideslip angle but stays at the same axial position.
Even when the left vortex is fully dominant, it is not centred on the centre of the
backlight.
Figure 5-11 shows the position of the non-dominant vortex for extreme angles. It can be
seen that for extreme angles the non-dominant vortex, either left or right, eventually
moves up. It is also notable that the non-dominant right vortex is significantly bigger
than the non-dominant left vortex.
Figure 5-10: Effect of sideslip on the contra-rotating vortices positions
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Figure 5-11: Behaviour of the non dominant contra-rotating vortex
for extreme sideslip angles (left picture: =-3; right picture: =4)
5.1.2.3 Effects on the flow over the trunk
Figure 5-12 shows the effect of changes in sideslip angle on the reattachment line
position. These results are approximate since the exact location of the reattachment line
is difficult to identify. However it can be seen that sideslip angle has very little effect on
the reattachment line location.
Figure 5-12: Effect of sideslip on the reattachment line position
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Figure 5-13 shows effect of sideslip on the trailing edge vortices and the centre limit.
The change of sideslip angle seems to have very little effect on the trailing edge vortices.
The right trailing edge vortex limit does not undergo any significant changes with .
The left trailing edge vortex limit moves slightly towards the centre to reach
= 0.27 of the trunk for positive angles whereas for negative angles it stays close to
= 0.3. This can be another indication that the flow structures for the two extreme
sideslip angles may not be mirror images.
The centre limit position moves from the left to the right as the sideslip angle increases
showing once again that the left side is dominant for angles higher than the critical
angle and the right side is dominant for angles lower than critical. It can also be noted
that for negative angles, even though the vortexs sizes stay the same, the centre limit
keeps moving to the left as the angle is decreased.
Figure 5-13: Effect of sideslip on the trailing edge vortices and the centre limit
5.2 Effect of wing position on the structure of the wake
The same flow features as for sideslip tests (Section 5.1.2) were studied. However the
flow velocity in the near wake was so low because of the presence of the wing that the
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left vortex of the contra-rotating pair was often not observable precisely enough to study
its size and position. The right one was always observable since it was often bigger than
the other one.
5.2.1 Effects of wing axial location
5.2.1.1 Effects on the Contra-rotating vortices
Figure 5-14 shows the effect of changing the wing axial position on the size of the right
one of the pair of contra-rotating vortices. The general trend is an increase in size as the
wing is moved rearwards until
= 0.5 and then a stabilisation. For
= 0.20 & 0.41, at
the wings rearmost positions, the vortex is even bigger than without the wing. At
= 0.85, the vortexs size remains significantly bellow the value obtained without the
wing.
Figure 5-14: Effect of wing axial position on the right contra-rotating vortexs size
Figure 5-15 shows the effect of changing the wing axial position on the position of the
centre of the right sided vortex within the contra-rotating pair. The vortex seems to
move down slightly and go away from the centre line as the wing is moved rearwards
until
= 0.5. From
= 0.5 , the vortex seems to stabilise. However, at the wings
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lowest location, the vortexs centres lateral position does not follow this trend and
see