Aerodinamica Autos Parte Posterior

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

Cranfield University Aerospace Dynamics

i

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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iii

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.


iv



v

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


vi

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


vii

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


viii

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


ix

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


xi

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


xii


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.


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).


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.


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)


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).


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.


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


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


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


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,


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.


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.


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).


16

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


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


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:


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


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).


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


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.


23

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.


24


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.


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.


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


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


38


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

5 - Results

42

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

Aerodinamica Autos Parte Posterior

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