CFD STUDY OF DRAG REDUCTION OF A GENERIC SPORT UTILITY VEHICLE

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CFD
STUDY OF
DRAG REDUCTION OF A GENERIC SPORT UTILITY VEHICLE




Pramod Nari Krishnani

B.S., Mumbai University, Mumbai,
India

2006







THESIS




Submitted in partial satisfaction of

the requirements for the degree of





MASTER OF SCIENCE



in



MECHANICAL ENGINEERING





at



CALIFORNIA STATE UNIVERSITY, SACRAMENTO



FALL

2009







ii






CFD DRAG REDUCTION OF A GENERIC SPORT UTILITY VEHICLE




A Thesis



by



Pramod Nari Krishnani













Approved by:


__________________________________
, Committee Chair

Dr. Dongmei Zhou


____
______________________________, Second Reader

Dr. Ilhan Tuzcu




____________________________

Date






iii













Student:

Pramod Nari

Krishnani




I certify that this student has met the requirements for format contained in the University
format manual, and that this thesis is suitable for shelving in the Library and credit is to
be awarded for the thesis.





__________________________
, Graduate Coordinator

___________________

Dr. Kenneth S. Sprott







Date

Department of
Mechanical Engineering








iv


Abstract


of


CFD DRAG REDUCTION OF A GENERIC SPORT UTILITY VEHICLE


by


Pramod Nari Krishnani




In the
history of aerodynamic research around the bluff bodies like SUV, trucks and
trailers, it has always been observed that their shapes with square base have served as an
obstacle

improving vehicle fuel economy
. When the air passes over the vehicle surface, i
t
makes the air over the surface change its behavior,
resulting in a
low pressure
region
and
a high pressure
region.
This pressure difference along with the vortex shedding
causes drag thereby increases the fuel consumption.


Researchers around the world
have tried to

reduce
the drag of trailers

or truck

by
using external devices like spoilers, vortex strake device (VCD) and under
-
carriage flow
device (UFD). The purpose of this thesis is to check the feasibility of using such external
devices for reducing
drag on the large size SUV. The
generic SUV

model
created by
previous research experiments

along with the Ahmed's reference model is considered as
a benchmark for validating all the simulation results.


Simple external devices like
b
oat tail plate and the

foot step are simulated and
optimized using commercial software packages like FLUENT, GAMBIT, T
-
grid and
Solidworks. Fifteen different angular combinations of upper and lower boat tail plates
are simulated and optimized to get the best angle suitable for
drag reduction of Generic


v


SUV model. The second external device commonly known as the foot step is simulated
with five configurations by vary
ing the width of the foot step. The foot step bypasses the
air around the rear wheel and thereby reduces drag coeff
icient of the complete SUV
model compared to the original SUV model. The optimal
sizes of the width favorable for
the drag reduction are

discussed in this thesis.




_______________________
, Committee Chair

Dr. Dongmei Zhou



_______________________

Date


























vi






ACKNOWLEDGMENTS


I would like to express my
heartfelt

thanks to Dr. Dongmei Zhou for her guidance and
support in the completion of my thesis. My thesis would have never been completed
without her.
I would like to thank Mr. Bahram Khalighi (Aerodynamic Manager, GM)
for sharing the results of the experiments.
Also, I will like to thank Dr. C.P. “CASE” Van
Dam for his valuable advice at hard times of my Thesis. Lastly, I will like to thank the
Departme
nt of Mechanical Engineering at California State University, Sacramento for the
encouragement and help to complete my Master in Mechanical Engineering.


Pramod Nari Krishnani

B.S. Mechanical Engineering

Mumbai University, 2006

INDIA



















vii


TABLE

OF CONTENTS











Page


Acknowledgments….
................................
................................
................................
...............


vi

List of Tables

................................
................................
................................
............................


x

List of Figures

................................
................................
................................
..........................


xi


Chapter

1.
AUTOMOBILE AERODYNAM
ICS
...

................................
................................
...............


1


1.1 What is Aerodynamics?


................................
................................
...................
1


1.2 Scope of Automobile
A
erodynamics

................................
................................
....

2


1.3 External flow phenomena of Automobile

................................
............................

4


1.4 Factors contributing to flow field around vehicle
................................
.................

6


1.5 Forces and moment on vehicle

................................
................................
................

8


2.
IMPLEMENTATION OF CFD IN VEHICLE AERODYNAMICS

................................
.


12


2.1 What is CFD?

................................
................................
................................
............

12


2.2 Outline of Computational Fluid Dynamic Process

................................
...................

12


2.3 Meshing or pre
-
proces
sing

................................
................................
........................

13


2.4 Numerical Solver

................................
................................
................................
......

14


2.5 Post processor

................................
................................
................................
...........

15


2.6 Summary

................................
................................
................................
....................

15


3.
SPORTS UTILITY VEHICLE

................................
................................
..........................


16


3.1 Introduction to SUV

................................
................................
................................
..

16


3.2 Historical Drag and Frontal Area trends

................................
................................
...

17


3.3 Shape changes affecting Drag

................................
................................
...................

19


3.4 Effect of Accessories

on Drag

................................
................................
....................

21





viii


4.
VALIDATION OR BENCHMARK OF CFD MODEL

................................
....................


23


4.1 Introduction to Benchmarking procedure

................................
................................
.

23


4.2 Wind tunnel Experiment

................................
................................
............................

24



4.2.1 Wind tunnel facility

................................
................................
...........................

24



4.2.2 SUV Generic Model

................................
................................
..........................

26



4.2.3 Experiment results

................................
................................
............................

28


4.3 Numerical
Simulation

................................
................................
................................

31



4.3.1 CAD SUV Generic Model

................................
................................
.................

31



4.3.2 Virtual Wind Tunnel and Vehicle Orientation

................................
...................

32



4.3.3 Discretisation (or Meshing) setup

................................
................................
......

34



4.3.4 Solver Setting

................................
................................
................................
.....

38



4.3.5 Simulation results

................................
................................
..............................

40


4.4 Benchmark Conclusion

................................
................................
..............................

47


5.
EFECTS OF BOA
T TAIL PLATE ON OVERALL DRAG
.......
............................

..........


4
8


5.1 Introduction

................................
................................
................................
................

4
8


5.2 CAD Model of Boat tail plate

................................
................................
....................

4
8


5.3 Optimization Process details

................................
................................
......................

50


5.4 Virtual Wind Tunnel and Vehicle Orientation

................................
...........................

5
2


5.5 Discretisation (or Meshing)
setup and Solver Setting

................................
................

5
2


5.6 Simulations results and discussion

................................
................................
.............

5
3


5.7 Summary

................................
................................
................................
....................

60


6.
EFFECT OF FOOT STEP ON OVERALL DRAG
.......
............................

........................


61


6.1 Introduction

................................
................................
................................
...............

61


6.2 CAD Model of foot step

................................
................................
............................

62


6.3 Optimization Process deta
ils

................................
................................
......................

6
3


6.4 Virtual Wind Tunnel and Vehicle Orientation

................................
...........................

6
3


6.5 Discretisation (or Meshing) setup and Solver Setting

................................
................

6
4


6.6 Simulations results and discussion

................................
................................
.............

6
6


6.7 Summary

................................
................................
................................
....................

71



ix


7.
CONCLUSION AND FUTURE WORK
.......
............................
................................
........


72


7.1 Conclusion

................................
................................
................................
.................

72


7.2 Future work

................................
................................
................................
...............

7
3



References

................................
................................
................................
...............................


75










x


LIST OF TABLES




Page


1.

Table 3.1 Typical increase in drag for the various accessories

[1] ……
………
.
..22

2.

Table 4.1 Solver setting
……………………………………………………….…3
9

3.

Table 4.2 Viscous mo
del and Turbulence model settings
…………………….…39

4.

Table 4.3 Boundary condition settings
………………………………………..…39

5.

Table 5.1 Simulation details of Boat tail plates
………………………………….5
1

6.

Table 5.2 Drag and Lift results of all the simulations

[26]…………
……………5
9

7.

Table 6.1
Dimensional details of all the simulation on SUV foot step
………..…6
3

8.

Table 6.2 Details drag value of each surfaces of SUV for all simulations
………6
9








xi


LIST OF FIGURES


Page


1.

Figure 1.1 Spectrum of Task for vehicle Aerodynamics

[24]
………
….
……
...
….3

2.

Figure 1.2
Typical Fuel Energy usage at urban and highway driving

[24]

.

...
.3

3.

Figure 1.3 Streamline external flows around a stationary vehicle

[25]
…………...5

4.

Figure 1.4 Sketch views of the various forces and moment on

vehicle body

[
24]……………………………………………………………
..
…...
8

5.

Figure 3.1 Twenty year trend of drag coefficient in SUV

[1]
………………
..
.…17

6.

Figure 3.2 Twenty year trend of Frontal Area in SUV

[1]
………………
...
..…
...
19

7.

Figure 3.3 Shape changes to reduce the Drag Coefficient

[1]
……………


..
20

8.

Figure 3.4 Loaded off
-
road SUV with all
the accessories

[1]
……………
...
...
.…21

9.

Figure 4.1 Wind tunnel experimental configuration at University of
Michigan [
22]


...
.........................
.
..................................................
.
................................................
24

10.

Figure
4.2 Schematic diagram of Experimental setup

[2]

………………
...

...
..25

11.

Figure 4.3 Dimensions of the Generic SUV model
[2]
…………………………..2
7

12.

Figure 4.4 Mean pressure coefficient plot on the symmetry plane of

SUV
[2]
……
……………………………………………………………………...
28

13.

Figure 4.5 Downstream mean velocity profiles in the horizontal center

plane
[2]

………………………………………………………………………..
30

14.

Figure 4.6 Surfaces of the SUV CAD model
…………………………………….31

15.

Figure 4.7 SUV orientations in the first benchmarking simulation
……………...33

16.

Figure 4.8
SUV orientations in the second benchmarking simulation
…………..3
4

17.

Figure 4.9 Details view of unstructured cells near the
wheels
[26]
………
……………………………………………………
…………..35




xii


18.

Figure 4.10 Hex core refinement regions of first benchmark

simulation
[26]
………
…………………………………………………………...
.36

19.

Figure 4.11 Unstructured mesh refinement region view of second benchmark

simulation
………………………………………………………………………...38

20.

Figure 4.12 Cab ‘Cp’ plot compa
rison with experiment for first

benchmark
[26]

……………………………………………………………….
..40

21.

Fig
ure 4.13 Underbody ‘Cp’ plot comparison with experiment for first

benchmark
[26]
………………………………………………………………
.
…..41

22.

Figure 4.14 Cab ‘Cp’ plot comparison with experiment for second

benchmark
……………………………………………………………………..…42

23.

Figure 4.15 Underbody ‘Cp’ plot compari
son with experiment for second

benchmark
………………………………………………………………………..42

24.

Figure 4.16 Mean downstream velocity profiles of the flow at Z = 0.830184m

and X = 5.484m

…………………………………………………………
….
…4
4

25.

Figure 4.17 Mean downstream velocity profiles of the flow at Z = 0.830184m

and X = 5.784m
………………………………………………
……
………
.
……4
5

26.

Figure 4.18 Mean downstream velocity profiles of the flow at Z = 0.830184m

and X = 6.384m
…………………
……………………………………
……
.
……4
5

27.

Figure 4.19 Mean downstream velocity profiles of the flow at Z = 0.830184m

and X = 6.984m
………………………
……………………………………
.
……4
6

28.

Figure 4.20 Mean downstream velocity profiles of the flow at Z = 0.830184m

and X = 7.584m
……………………………
....................
…………………
.
……4
6

29.

Fig
ure 4.21 Mean downstream velocity profiles of the flow at Z = 0.830184m

and X = 8.184m
…………………
……………
…………………………
.
………4
7

30.

Figure 5.1 Orientation of Boat tail plates
………………………………………...4
9

31.

Figure 5.2 Side view of Boat tail plate along with dimensions
……………….…
50

32.

Figur
e 5.3 SUV orientation
…………………………………………………....…5
2



xiii


33.

Figure 5.4 Hybrid Mesh near the upper boat tail plate
………………………..…5
3

34.

Figure 5.5 Total Pressure Contour of the Benchmark Simulation at symmetry


plane
………………
……………………………………………………………..
54

35.

Figure 5.6 Total Pressure
Contour of the A10B10 Simulation at symmetry


plane
……
………………………………………………………………………..
55

36.

Figure 5.7 Total Pressure Contour of the A20B20 Simulation at symmetry

plane
……
………………………………………………………………………...
55

37.

Figure 5.
8

Magnitude velocity path lines for simulation 1 to 10
[26]
……
……...

57

38.

Figure 5.
9

Magnitude velocity path lines for simulation 11 to 15
[26]
…………..
58

39.

Figure 5.
10

Drag and lift coefficient variation for all the simulations

.…….

59

40.

Figure 6.1 Foot Step of SUV veh
icle
[5]
……………………………………...


61

41.

Figure 6.2 Surfaces of the Foot step installed on SUV
…………………………..
62

42.

Figure 6.3 SUV orientations for foot step simulations
…………………………..
64

43.

Figure 6.4 Mesh view at the plane near the foot step
……………………………
65

44.

Figure 6.5 Drag coefficie
nt plot for all the simulations on foot step
………….…
66

45.

Figure 6.6 Total Pressure contour for all the simulations at Y= 0.79m
……….…
67

46.

Figure 6.7 Pressure coefficient variations for all foot step simulations
……..…..
68

47.

Figure 7.1 Vortex strake device from DOE 20
04 Paper
[14]

…………………

74


1



C
hapter

1

AUTOMOBILE AERODYNAM
ICS


1.1

WHAT IS AERODYNAMICS
?


Aerodynamics


is a branch of
fluid
dynamics concerned with studying the motion of
air, particularly when it interacts
with a moving object. Aerodynamics is

also

a subfield
gas dynamics, with much theory shared
with
fluid dynamics
.

Aerodynamics is often used
synonymously with gas dynamics, with the difference being that gas dynamics applies to
all gases. Understanding the
motion of air (often called a flow field) around an object
enables the calculation of forces and moments acting on the object. Typical properties
calculated for a flow field include velocity, pressure, density and temperature as a
function of position and
time. By defining a control volume around the flow field,
equations for the conservation of mass, momentum, and energy can be defined and used
to solve for the properties. The use of aerodynamics through mathematical analysis,
empirical approximation and w
ind tunnel experimentation form the scientific basis.

Aerodynamics and its analysis are basically divided into two major sub
-
categories,
namely the external and internal aerodynamics.
External aerodynamics is the study of
flow around solid objects of vario
us shapes. Evaluating the lift and drag on an airplane,
the shock waves that form in front of the nose of a rocket
,

or the flow of air over a
wind
turbine blade
are examples of external aerodynamics.

On the other hand, i
nternal
aerodynamics is the study of flow through passages in solid objects. For instance, internal
2



aerodynamics encompasses the study of the airflow through a jet engine or through an air
conditioning pipe.


Apparently, this thesis work concentrates more
on the external category of the
aerodynamics related to Sport Utility vehicle.

1.2

SCOPE OF AUTOMOBILE
AERODYNAMICS

The rapidly increasing fuel prices and the regulation of green house gasses to control
global warming have given tremendous pressure on the des
ign engineers to enhance the
current designs of the automobile using minimal changes in the shapes. To full fill the
above requirements, design engineers have been using the concepts of aerodynamics to
enhance the efficiency of automobiles.

The
Figure 1.1

shows the spectrum of task for vehicle aerodynamics. The figure
illustrates the various problems which can be solved using the aerodynamics of the
vehicle. Aerodynamics is used by design engineers for cooling the engines, improving the
performance of the v
ehicle, enhancing the comfort of the rider, stabilizing the car in
external wind conditions and also increas
ing

the visibility of the rider.

Although aerodynamics has so many tasks in its basket, this thesis concentrates on
external devices which affect th
e flow around the automobile body to reduce the
resistance of the vehicle in normal working conditions.


3




Figure 1.1

Spectrum of Task for vehicle Aerodynamics

[24]


Figure 1.2 Typical Fuel Energy usage at urban and highway driving

[24]

4



The
Figure 1.2

s
hows the description of the fuel energy used in a modern vehicle at
urban driving and highway driving. The shape of the vehicle uses about 3 % of fuel to
overcome the resistance in urban driving, while it takes 11% of fuel for the highway
driving. This con
siderable high value of fuel usage in
highway
driving attracts several
design engineers to enhance the aerodynamics of the vehicle using minimal design
changes. This brings the idea of using external devices which could be attached to the
present vehicle without changing the body. This thesis is based

on the design and
developments

of external devices which will let the manufacturers of Sport Utility
Vehicle (especially known as ‘SUV’) make the present vehicles more aerodynamically
attractive
.


1.3

EXTERNAL FLOW PHENOM
ENA OF AUTOMOBILE

The
Figure 1.3

sho
ws the
streamline

of an

external flow around a
stationary vehicle.
When the vehicle is moving at an undistributed velocity, the viscous effects in the fluid
are restricted to a thin layer called boundary layer.

Outside th
e boundary layer is
the
inviscid
fl
ow
.

. This fluid flow imposes pressure force on the boundary layer. When the
air reaches the rear part of the vehicle, the fluid gets
detached. Within the boundary layer,
the movement of the fluid is totally governed by the viscous effects of the fluid.

5




Figure 1.3 Streamline
of
external flows around a stationary vehicle

[
25]


The boundary
does not exist

for the Reynolds Number which is lower than 10
4
. The
Reynolds number is dependent on the characteristic length of the vehicle, the kinematic
viscosity a
nd the speed of the vehicle. Apparently, the fluid moving around the vehicle is
dependent on the shape of the vehicle and the Reynolds number. There is another
important phenomenon which affects the flow of the car and the performance of the
vehicle. This
phenomenon is commonly known as ‘Wake’ of the vehicle. When the air
moving over the vehicle is separated at the rear end, it leaves a large low pressure
turbulent region behind the vehicle known as the wake. This wake
contributes to the
formation of
pressu
re drag, which is eventually reduces the vehicle performance.




6



1.4

FACTORS CONTRIBUTING

TO FLOW FIELD AROUND

VEHICLE

The major factors which affect the flow field around the vehicle are the boundary
layers, separation of flow field, friction drag and lastly the pressure drag.


BOUNDARY LAYER
: The Aerodynamics boundary layer was first defined by the
Aerodynamic engineer


Ludwig Prandtl
’ in the conference at Germany. This allows
aerodynamicists to simplify the equations of fluid flow by dividing the flow field into two
areas: one inside the boundary layer and the one outside the boundary layer. In this
boundary layer aroun
d the vehicle, the viscosity is dominant and it
plays a major role in
drag of the vehicle.
T
he viscosity is neglected in the fluid
regions
outside this boundary
layer since it does not have significant effect on the solution. In the design of the body
shap
e, the boundary layer is given high attention to reduce drag. There are two reasons
why designers consider
the boundary layer as a
major factor in aerodynamic drag. The
first is that the boundary layer adds to the effective thickness of the body, through t
he
displacement thickness, hence increasing the pressure drag. The second reason is that the
shear forces at the surface of the vehicle causes skin friction drag.


SEPARATION
: During the flow over the surface of the vehicle, there is a point when the
chan
ge in velocity comes to sta
ll
and the fluid starts flowing in reverse direction.
This
phenomenon is called ‘Separation’ of the fluid flow.
This is usually occurred at the rear
part of the vehicle. This separation is highly dependent on the pressure distri
bution
which is imposed by the outer layer of the flow. The turbulent boundary layer can
7



withstand much higher pressure without separating as compared to laminar flow. This
separation causes the flow to change its behavior behind the vehicle and thereby af
fect
the flow field around the vehicle. This phenomenon is the major factor to be considered
while studying the wake of the vehicle.


FRICTION DRAG
: Each wall surface or material has a distinct friction which resists the
flow of fluids. Due to molecular f
riction, a stress acts on every surface of the vehicle. The
integration of the corresponding force component in the free stream direction leads to a
friction drag. If the separation does not occur, then friction drag is one of the main
reasons to cause ove
rall drag.


PRESSURE DRAG
: The blunt bodies like large size vehicle show different drag
characteristics. On the rear part of such vehicles, there is an extremely steep pressure
gradient which leads to the separation of the flow separation in viscous flow.

The front
part of the flow field show
s

high pressure value
, whereas on the rear part flow separates
leading to a high suction in the area. As we integrate the force component created by such
high change in pressure, the resultant is called as ‘Pressure Dr
ag’. This factor is affected
by the
height

of the vehicle as well as the separation of the flow field.





8



1.5
FORCES AND MOMENT ON

VEHICLE


When the vehicle is moving at a considerable speed, the air passing over it
imposes various forces and moment on t
he vehicle. The
Figure 1.4

shows the details
sketch view of the various forces and moment acting on the vehicle body.



Figure 1.4 Sketch views of the various forces and moment on vehicle body

[24]


The vertical force acting on the body indicated by the letter “L” is known as Lift
force. This force causes the vehicle to get lifted in air as applied in the positive direction,
whereas it can result in excessive wheel down force if it is applied in negat
ive direction.
Engineers try to keep this value to a required limit to avoid excess down force
or
lift.


9



The formula usually used to define this force is written
as:














------------------------------------------

(1)

where:






=

Lift
Force





=

Lift Coefficient




=

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䅥牯ry湡浩c
d
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c潭灬e瑥⁢潤y⸠周
e⁁e牯dy湡浩c
d
牡g⁦ 牣e⁩猠calc畬慴u搠dy⁴桥⁦潬o潷楮g 景f浵ma㨠












------------------------------------------

(
2
)

where





=

䅥牯ry湡浩c 䑲ag
c
潥ff楣楥湴






=

F牯湴r氠䅲ea映瑨f⁖ 桩c汥

䅩爠Ae湳楴y


=

周T 獩摥s潲ce 楳i
c牯獳ri湤nac瑩湧n潮o瑨攠癥桩c汥la湤n畮摥爠瑨e 獴敡dy 獴慴e 睩湤w
c潮摩o楯湳⁴桥⁥煵q瑩潮⁦潲⁳楤o景fce⁣a汣畬l瑩潮⁩猠ex灲p獳敤⁡猺

10













------------------------------------------

(3)

where:





=

S楤敦潲oe




=

呯瑡氠l楮搠噥汯捩ly





=

S楤敦潲oe⁃潥晦楣楥湴
Fu湣瑩潮o⁴桥⁒ 污瑩癥⁗楮搠䅮i汥)




=

F牯湴r氠䅲ea映瑨f⁖ 桩c汥

周T 灩pc桩湧 浯me湴Ⱐ丬N瑲t湳晥牳r睥ig桴h扥瑷te渠瑨攠晲潮琠a湤n牥a爠ax汥猠a湤n楳
牥灲pse湴敤⁢y⁴桥⁦潬o潷楮i⁥煵a瑩潮o














------------------------------------------

(4)

where:





=

P楴c桩湧⁍潭e湴





=

P楴c桩湧⁍潭e湴⁃潥晦楣楥湴




=

F牯湴r氠䅲ea映瑨f⁖ 桩c汥




=

W桥e汢慳l

C牯獳r楮摳i 灲潤pce a 獩摥s潲oe 潮o a 癥桩捬e 瑨慴t ac瑳t a琠 瑨攠 浩摤汥 潦o 瑨攠
睨we汢慳l,
a湤n睨w渠瑨t c牯獳r楮摳i摯d湯琠ac琠a琠瑨攠浩摤汥d潦o瑨t 睨wel扡獥 a ya睩w朠
浯me湴⁩猠灲潤畣e搮†周e ya睩wg潭e湴⁩猠牥灲e獥湴敤⁢y⁴桥⁦潬o潷楮g e煵q瑩潮o

11















------------------------------------------

(5)

where:





=

奡睩wg⁍潭e湴





=

奡睩wg⁍潭e湴⁃潥晦楣楥湴
噡物r猠睩瑨⁗楮搠䑩牥c瑩潮o




=

F牯湴r氠䅲ea映瑨f⁖ 桩c汥




=

W桥e汢慳l

W桥渠瑨攠c牯獳r楮搠 灲潤pce猠 a 獩摥s潲oe a琠a渠e汥癡瑥搠 灯楮p 潮o a ve桩捬hⰠa
牯汬楮g潭e湴⁩猠灲潤畣e搠d湤⁩猠牥灲e獥湴敤⁢y⁴桥⁦潬汯睩湧

e煵q瑩潮o










------------------------------------------

(6)

where:





=

R潬o楮i⁍潭e湴





=

R潬o楮i⁍潭e湴⁃潥晦楣ie湴
na物e猠睩瑨⁗楮搠䑩牥c瑩潮o




=

F牯湴r氠䅲ea映瑨f⁖ 桩c汥




=

W桥e汢慳l



12



C
hapter

2

I
MPLEMENTATION

O
F

CFD

S
IMULATION

I
N

V
EHICLE

A
ERODYNAMICS


2.1 WHAT IS CFD?

Computational fluid dynamics

(CFD)

is one of the branches of fluid mechanics
that uses numerical methods and algorithms to solve and analyze problems that involve
fluid flows. Computers are used to

simulate the interaction of liquids and gases with
surfaces defined by boundary conditions. Even with high
-
speed supercomputers only
approximate solutions can be achieved in many cases.


2.2 OUTLINE OF COMPU
TATIONAL FLUID DYNAM
IC PROCESS

Computational Fl
uid Dynamic codes are structured around the numerical
algorithms that can tackle fluid flow problems. All the CFD codes available in the market
have three basic elements which divide the complete analysis of the
numerical
experiment to be performed on the
specific domain or geometry.

The three basic elements are

(i) Pre
-
processor

(ii) Solver and

(iii) Post
-
Processor




13



2.3 MESHING
AND

PRE
-
PROCESSING

The pre
-
processing of the CFD process consists of the input of a flow problem by
means of user
-
friendly programs or software and the subsequent transformation of this
input into a form
is made
suitable to use by the solver.
T
he pre
-
processor is the link
be
tween the user and the solver. The
u
ser activity at the pre
-
processing stage of the CFD
process involves the following:

1)

Definition of Geometry or region of Interest: This process involves several
computer aided design (CAD) software like CATIA, Solidworks
, Pro
-
E and
much more. By the help of CAD software, the topology of the fluid
flow
region of interest is defined. This software plays a major part of the design
and optimization process in research analysis.

2)

Grid Generation or Meshing: Since the CFD proce
ss is a numerical
approximation method using finite volume method, the given domain or
region of interest needs to be divided into several structured elements. All the
elements or cells are connected to each other through nodes to form the
required region
of flow. For this purpose, special meshing or grid generation
software like GAMBIT and T
-
grid are used. This stage is the key element in
the CFD finite volume numerical simulation and it also contributes to the
accuracy of the final results.

3)

Definition of
Fluid properties: Every fluid domain or surface has its own
distinct property. The properties of the fluid used in the CFD domain or region
14



of interest are defined at this stage of the CFD Process. Usually the CFD code
software has this facility.

4)

Boundary

Conditions: Every different setup of the CFD domain needs to have
an initialization, which is fulfilled by the boundary conditions input. The CFD
code usually has this facility to define the boundary conditions of the CFD
problem, where each cells at spec
ific boundary are given finite values.


2.4 NUMERICAL SOLVER


The numerical solver is the key elements of the CFD process and covers the major
part of the CFD process. In the current market, the solvers usually use three distinct ways
of calculating the s
olutions
,
namely
,
the finite difference method, finite element method
and the finite volume method.


The finite difference and element method are usually suitable for stress and
structure analysis and does not suite the requirements of the CFD process. Th
e finite
volume method is the most suitable method for the CFD process. As the name implies,
finite volume method is the numerical algorithm calculation process involving the use of
finite volume cell
s.

The steps involved in this solving process are usuall
y carried out in
the following
sequence:


1.

Formal integration of the governing equations of fluid flow over all the control
volumes or finite volumes of the solution domain.

2.

The conversion of the integral forms of the equations into a system of algebraic
equations.

15



3.

Calculations of the algebraic equations by an iterative method.


2.5 POST PROCESSOR

The post processor is the last phase of the CFD process which involves data v
isualization
and results analysis of the CFD process. This phase uses the versatile data visualization
tools of the CFD solver to observe the following results of the simulation:

1.

Domain geometry and Grid display

2.

Vector plots

3.

Line and shaded contour plot
s

4.

2D and 3D surface plots

5.

Particle tracking

6.

XY plots and graphs of results


2.6 SUMMARY

In this thesis, th
e

outline
described in Chapter 2
is followed to complete the
analysis of each CFD simulation performed. The CFD process starts with defining the
geometry using the CAD software ‘Solidworks’ and then it is followed by the meshing
software GAMBIT, which is used to create the surface mesh accordingly. After the
surface mesh is created, the resulting surface mesh is imported to special unstructured
vol
ume meshing software, known as ‘T
-
grid’. This software creates a hybrid mesh in the
required domain of interest with the help of progressive boundary layer, tetra and Hex
core cells.

16



C
hapter

3

S
PORTS

U
TILITY

V
EHICLE


3.1 INTRODUCTION TO
SUV

A sport utilit
y vehicle (SUV) is a generic marketing term for a vehicle similar to a
station wagon, but built on a light
-
truck chassis.
SUVs are u
sually equipped with four
-
wheel drive for on
-

or off
-
road ability, and with some pretension or ability to be used as
an off
-
road vehicle,
and
some SUVs include the towing capacity of a pickup truck with
the passenger
-
carrying space of a minivan. Since SUVs are considered

as

light trucks and
often share the same platforms of pick
-
ups, they are regulated less strictly than passen
ger
cars under the two laws in the United States, the Energy Policy and Conservation Act for
fuel economy, and the Clean Air Act for emissions.

In recent years there has been a phenomenal growth in the market for Sport Utility
Vehicles. The first vehicle,
which would today be recognizable as a SUV, is generally
considered to have been the original Range Rover from Land Rover, which was launched
in 1970, an at the time was a unique concept. The sizes of the SUVs are available in a
wide range from small size
weighing 1 ton to large sizes covering up to 3 tons. From the
mid 70’s the growth in this sector was an almost exclusively North American trend, such
that today 25% of the total passenger cars sold in USA are SUVs

[1]
. This growing
popularity of the SUVs h
as pulled the attention of environmental organization due to the
amount of CO
2

emission and the global warming concerns. The SUV, because of its size
and shape, is not fulfilling efficient body and engine requirement. Today designers have
17



been trying to bu
ild a good image of the SUVs by making them more efficient in shape
and engine. In this thesis, the external devices which reduce drag and make the body
designs more efficient are discussed. The other aspects of the aerodynamics like cooling
airflow, heat
management and aero
-
acoustics along with engine performance
will not be
included.


3.2 HISTORICAL DRAG
AND FRONTAL AREA TRE
NDS

The
Figure 3.1

shows
the
trend of drag coefficient in SUV vehicles

during
the
last twenty year
. The open red spots indicate the SUV vehicles with high accessories
attachments and thereby are termed as the fully loaded SUVs which are often used for
off
-
road driving requirements. The open red spot indicates the general SUVs have made
the SUV market s
ignificant and competitive. These SUVs are not fitted with extra heavy
duty accessories and they have been kept in production for keeping the market
requirement for large size luxury passenger vehicles.


Figure 3.1 Twenty year trend of drag coefficient i
n SUV [1]

18



This data set, collected from Land Rover dataset and Motor Industry Research
Association (MIRA) aerodynamic surveys
[1]
, shows a significant drop in the drag
coefficient in the last twenty years. This downward trend has surely proven the transiti
on
to
the
softer shapes of SUV
, which is
help
ful t
o increase the efficiency of the vehicle
aerodynamics. The lowering
d
rag coefficient shows the taste of competitive environment
in the field of designing SUV, where every possible year there is a decrease i
n drag
coefficient due to improved techniques.

The drag coefficient of the early 1994 Range rover was about 0.40 and was
considered as the best design till then. Later the drag coefficient reduced and today the
new designs like BMW X5, X3 and Lexus RX 300

claim to have a drag coefficient of
0.35. The latest design of the Mercedes ML is
the
current class
-
leading vehicle with a
drag coefficient of 0.34. This lower trend in the aerodynamic drag coefficient show
s

that
in the future years, it is possible to ach
ieve a much lower drag coefficient by
implementing new designs or external devices.

On the other hand, t
he
Figure 3.2

shows the trend of frontal area for the same set
of vehicle described in the
Figure 3.1
,
indicat
ing
that as the vehicles are showing a
decrease in vehicle drag, the size of the vehicles
is i
ncreasing. This reflects that the
vehicle weight is also increasing along with the frontal area. This could be considered as
the disadvantage of the transition to
the

soft body of the SUVs for acquiring lower drag
coefficient.

19




Figure 3.2 Twenty year trend of Frontal Area in SUV [1]


3.3
SHAPE CHANGES AFFECT
ING DRAG


The Shape of the vehicle plays an important role in the drag reduction. Low drag
is achieved by a shape which avoids sudden changes in the cross sectional area and has a
degree of tapering towards the base of the vehicle. In practical design environment, d
rag
reduction comes from attention to detail and it results from the accumulation of small
incremental benefits in the development process. The shape changes which can affect the
performance of the vehicle body are shown in the
Figure 3.3
.

The arrows show
the
required direction to morph the surface to create drag reduction, although this is totally
dependent on the initial shape. So the direction is susceptible to change as the
original
shape of the SUV could be
aerodynamically friendly or filled with blunt

edges.

20




Figure 3.3 Shape changes to reduce the Drag Coefficient [1]


The
drag at the SUV base
can be reduced by increasing the pressure

in the

base

area

and reducing the base area. Tapering the body sides and roof has a significant effect,
but
this
will
compromise the loading area at the tailgate and reduce rear passenger
headroom. If steps are made to make small chamfers at the rear end of the roof and the
side body, there
will be
a significant change in the drag. The foot step of the vehicle is
moved do
wnward to decrease the ground clearance near the wheels

and this
makes major
changes in the drag of the vehicle. Lowering the front bumper and bonnet, inclining the
front windshield, rounding off the corners and sharp edges and lastly extending the front
b
umper are some of the ways contribut
ing

to reduce drag. The aerodynamicist usually
works closely with the designers to use these ways with high level of compromises
to
make the vehicle more comfortable for the customers. This thesis takes this point of
21



com
promise in view and makes available several external devices which will fulfill the
requirement of the designs as well as the aerodynamicist.


3.4
EFFECT OF ACCESSORIE
S ON DRAG


The
Figure 3.4

shows the general accessories which are attached on SUVs. All
the accessories used for an SUV have the tendencies to increase the drag

and thus drag
coefficient. The
Table 3.1

shows the typical increase in drag for the various accessories
used in SUVs. The Roof box
covering
about 0.3 m
2

frontal area shows a drag incr
ement
of about 0.075. The headlamp protectors show a drag increment of about 0.006. Even the
smallest accessories like the mud flaps show an increment in drag of about 0.011. The
drag penalties for a particular accessory will depend on the detail design an
d the vehicle
to which they are fitted.



Figure 3.4 Loaded off
-
road SUV with all the
accessories

[1]

22




Table 3.1 Typical increase in drag for the various accessories

[1]


In this thesis, the accessories like the
S
ide steps and the rear spoilers
will be
optimized to get the lowest drag or reduction in drag from the original.
















23



C
hapter

4

V
ALIDATION
O
F

CFD M
ODEL


4.1
INTRODUCTION TO
VALIDATING

PROCEDURE


Good engineering practice suggests that prior to using an analysis technique on a
new configuration,
one

should benchmark (validate) the technique against a known
(respected) test case similar to the new configuration.
If no suitable test case exists, then

cross referencing with another analysis technique, such as a wind tunnel, is essential. The
benchmark test process is the process of numerical analysis performed on a case which is
replica of the real time testing or previous results of numerical simulati
ons. While
performing the benchmark testing, the results of the test

will be further
compared with
other
available results.

For CFD, the benchmarking process should result in guidelines for a specific class
of problems. The guidelines would
describe
the p
referred boundary conditions
,
turbulence model and meshing strategy (clustering and growth rate) required to achieve a
desired level of confidence
and
accuracy in the results.

For
current benchmarking

process, the SUV generic model

will be simulated
. The
Generic model is fabricated by Michigan State University with the help of General
Motors. They performed a wind tunnel test on this model using advance PIV techniques.
The results of this wind tunnel test are compared with the results of the numerical
simu
lations using advance solvers like Fluent. The model us
ed by the wind tunnel test
was 3D
printed
and it will be
used for
all the

numerical simulations.

24



4.2
WIND TUNNEL EXPERIME
NT


This section describes the details of the
w
ind tunnel experiments performed at the
Wind Tunnel facility located at University of Michigan.

4.2.1 WIND TUNNEL FA
CILITY

The experiments over the proto
-
type scaled model of SUV Generic model were
conducted in the 2’ x 2’ wind tunnel at the Aerospace En
gineering Department of the
University of Michigan. The wind tunnel was an open


return suction wind tunnel
equipped with glass test section so that the optical measurements were possible.



Figure 4.1 Wind tunnel experimental
configuration

at University

of Michigan

[22]


The
Figure 4.1

above demonstrates the wind tunnel facility and the setup of the
different components used inside the wind tunnel. The 1/12 scaled model is kept inside
the glass
-
walled wind tunnel with the wheels facing towards the top. T
he top wall of the
25



wind tunnel is holding the ground board which is about 0.1m below the upper wall. This
ground wall is used to create the ground effects of the road over the SUV model and it is
also used to equip the pressure sensor tubes to pass over it

without affecting the flow. The
schematic diagram of the experimental setup is displayed in the
Figure 4.2

below.


Figure 4.2 Schematic diagram of Experimental setup
[2]


The test section cross section area is approximately 0.60 x 0.60 m
2
. The tip of th
e
SUV model front bumper is considered as the origin of the co
-
ordinate measurements of
x
-
direction, y
-
direction and the z
-
direction. The inlet or test section entrance is about
383.79 mm in front of the SUV model and the length of the wind tunnel is appro
ximately
2.1m long. The inlet of the wind tunnel is approximately 2.5 times base size of the SUV
model ahead of the front bumper and the exit is about 8 times base size behind the SUV
26



model. These empirical relations are used in the numerical simulations t
o define the wind
tunnel dimensions.


4.2.2 SUV GENERIC MO
DEL

The Generic model of the SUV is shown in the
Figure 4.3

below with relevant
dimensions. The length of the model is 432 mm, the width of the model is 152 mm, and
the height of the model is 148
mm. The maximum cross section of this model is about
approximately 0.020 m
2

giving the blockage area ratio of 5.2 %. The figure also shows
the origin of the coordinate system used. The x
-
axis is in the flow direction with its origin
at the front bumper. Th
e y
-
axis is in the horizontal direction across the flow with its
origin at the symmetry plane of the model. The z
-
axis is in the vertical direction with its
origin at the underbody of the model. The model was fitted with 70 pressure taps which is
shown in
Figure 4.3

as bubbles. These pressure taps measured the surface pressure
coefficient at the symmetry plane and the base. These measurements are discussed in
Section 4.2.3.

This Generic model of the SUV has a strong front shape which resembles the
typical
shape of a modern sport utility vehicle or a pick
-
up truck. Apparently, the base
part of the vehicle has lower shape definition to make the analysis simple and easy to
simulate in numerical analysis. The base of the SUV is kept flat, while there is smooth
shape bending at the pillar A of the vehicle. The wheels of the SUV model are not given
many details. The wheel edges are given sharp angles and only half wheel is defined. The
underbody of the vehicle is also given flat faces with no details of the transm
ission line
27



and fuel tank. The geometry is defined in such a way that most of the aerodynamic
characteristics of the SUV are maintained, thereby minimizing the use of heavy
computational resource in the numerical analysis.




Figure 4.3 Dimensions of the Generic SUV model
[2]



28



4.2.3 EXPERIMENT RES
ULTS

The pressure taps located at the symmetry plane of the SUV model measured the
mean pressures at the top and bottom surface of the SUV model. The wind tunnel tests
were conducted

at a free stream velocity of 30 m/s which corresponds to a Reynolds
number of 2.88 x 10
5
(based on the height of the SUV model).



Figure 4.4 Mean pressure coefficient plot on the symmetry plane of SUV
[2]



The

Pressure coefficient (
C
p
)

is a

dimension
less number

which describes the relative
pressures throughout a flow field in

fluid dynamics. The pressure coefficient is used
in

aerodynamics

and hydrodynamics. Every point in a fluid flow field has its own
unique pressure coefficient,

C
p
.





29




-----------------------------------
(7)

where:

p

is the

pressure

at the point for which pressure coefficient is being evaluated


is the pressure in the free

stream (i.e. remote from any disturbance)


is the free

stream

fluid density

(Air at

sea level

and
15 °C is 1.225

kg

/

m
3
)


is the free

stream velocity of the fluid, or the velocity of the body through
the fluid


The experimental results presented by Mr. Khalighi from the reference paper
[2]

are shown in the following context. The
Figure 4.4

shows the mean pressure coefficient
on the symmetry plane of the SUV model. As shown in the
Figure 4.4
,
the mean pressure
of the SUV engine hood and the passenger roof is marked by the term ‘Cab’ while the
underbody is indicated by term ‘Bottom’. As the wi
nd speed accelerates, the Cp over the
front bumper of the vehicle reflects a stagnation value of 1.0. There is a sudden drop to a
negative value of Cp as the air slips from the front bumper to the radiator. This negative
value of Cp changes gradually to a
positive value as the air reaches the intersection of the
hood and windshield. This intersection is one of the two high pressure areas. Usually this
place is used for the inlet of the air conditioning of the vehicle. The mean pressure
coefficient of the ai
r near the edge of the passenger roof entrance shows a significant
drop in pressure causing high velocity over the roof surface. Finally, the negative
30



pressure on the roof increases to a value near to the negative base pressure before
separation of air tak
es place at the end of the roof.

Although several experimental data is collected during the experiments, only the
pressure coefficient data at the symmetry plane and the horizontal center plane
downstream mean velocity in the wake region is considered for

the validation process.
This decision was made to support the decision of using symmetry plane in CFD
analysis. The use of symmetry plane complies with the limited computational resources.
The second data used for the benchmark analysis is the downstream
mean velocity
profiles of the flow in the horizontal plane (z = 69.2 mm) of the wake of the SUV.

The
Figure 4.5

plots the downstream mean velocity profiles collected at 450mm,
500mm, 550mm, 600mm and 700mm measured by the PIV method in the wake region of
the SUV model at the horizontal plane. The distance of the measurements above are
measured from the origin of the SUV model.


Figure 4.5 Downstream mean velocity profiles in the horizontal center plane

[2]

31



4.3 NUMERICAL SIMULA
TION


This section describe
s about the numerical simulations performed on the SUV
model using state of the art CFD techniques. The software used for the numerical
analysis was ANSYS® FLUENT 6.3, GAMBIT, and T
-
grid 5.0 ®. Since the process of
CFD meshing is a learning process, severa
l meshing were applied and tested. Due to
limited time and space, only two major benchmarking simulations will be described.


4.3.1 CAD SUV GENERI
C MODEL

The SUV model used in the Wind tunnel experiment at University of Michigan is
3D printed using advance techniques to CAD format for numerical analysis.



Figure 4.6 Surfaces of the SUV CAD model

32



The
Figure 4.6

above shows the details named to each distin
ct surfaces of the
SUV in the CFD simulations discussed in the later part of this thesis. This geometry was
shared from Mr. Khalighi who worked on the experimental analysis in wind tunnel. The
CAD model of the SUV was refined using GAMBIT, since the model
had too many
intersecting surfaces and free edges. After the model was completely refined, Solidworks
was used to add the external devices for testing them. As seen from the figure above, the
radiator, the hood, the windshield and the passenger roof are in
dicated by a term ‘Cab’.
The term ‘side’ includes the complete side surface of the SUV model including the doors
and the windows. The back surface of the SUV model is indicated by ‘Base’. The term
‘underbody’ is given to all the surfaces below the car exce
pt the wheels. The front wheels
are indicated by the term ‘fwheel’. The rear wheel surfaces are divided into two terms.
The front surface of the rear wheel is named as ‘rwheel1’ while the other surfaces are
named as ‘rwheel2’.


4.3.2 VIRTUAL WIND T
UNNEL AN
D VEHICLE ORIENTATIO
N

The SUV model shown above used in the wind tunnel is used for the simulation.
The location of the SUV model in the wind tunnel differentiates the two major
benchmarking described in this thesis. As discussed in Section 4.2.1, the wind

tunnel
used in the experiments had the cross section of 0.60 x 0.60 m
2
. If noticed, the ground
plate used actually separated the 0.60 x 0.10 m
2

area from the complete cross section.
Due to this reason, the cross section of the virtual wind tunnel is appro
ximately 0.60 x
0.50 m
2
.

33



In the first benchmarking simulation, which was used for the boat tail plate
simulation illustrated in Chapter 5, the inlet of the wind tunnel was placed two times the
length of the SUV model ahead of the SUV with origin defined in

S
ection 4.2.2
. The
pressure outlet of the wind tunnel was placed five times the length of the SUV model
behind the SUV base. The
Figure 4.7

below shows a preview of the location of the SUV
model in the first benchmarking simulation.



Figure 4.7 SUV orientations in the first benchmarking simulation


In the second benchmarking simulation, which was used for the foot step
simulation illustrated in Chapter 5, the inlet of the wind Tunnel was placed four times the
length of the SUV model ahead of the SUV with origin defined in
S
ection 4.2.2
. The
pressure
outlet of the wind tunnel was placed four times the length of the SUV model
behind the SUV base. The
Figure 4.8

below shows a preview of the location of the SUV
model in the first benchmarking simulation.



5L L 2L

34




Figure 4.8 SUV orientations in the second benchmarking simulation


The inlet of the Wind tunnel in the simulation is given maximum velocity of
about 30m/s, while the outlet pressure (gage) is defined as zero.


4.3.3
DISCRETISATION (OR M
ESHING) SETUP

Due to the complexity of the simulation with limited computer resources and
time, the complete domain was divided to half using a symmetry plane at Y = 0. Similar
to the strategy in
Section 4.3.2
, different meshing charact
eristics were used for the
different wind tunnel domains. Although the methods were different, the same mixed
configuration of triangular and hex core cells were used for both the domains. The
triangular shape surface mesh was used due to its proximity to
changing curves and
bends. These elements easily adjust to the complex bodies used in automobile and
aerospace bodies. In both the benchmarking simulations, the vehicle component surfaces
were discretized with triangular mesh elements. A typical surface me
sh size did not
exceed the value of 36mm on the first benchmarking simulation, while surface mesh
element size of 12mm was used for the second benchmarking simulation. The original
1/12
th

scale model of the SUV used in the experiment
[2]

was scaled up by t
he factor of
4L L

4L

35



12. The surfaces of the virtual wind tunnel were discretized with a larger triangular mesh
to define course meshes near the surface of the wind tunnel surfaces away from the SUV
model. Cells of mixed cell type were used in the computational do
main. Soon after the
surface meshes on the vehicle surfaces, seven layer prismatic layers were defined over
the vehicle surface and the floor to resolve the boundary layers over the surface of the
vehicle and the floor. The first layer of the boundary laye
r was set to approximately
0.2241684

mm using the NASA y+ online calculator
[
3
]
. The growth rate of these
prismatic layers was set to be1.1.



Figure 4.9 Details view of unstructured cells
near the wheels

[26]


HEX CORE CELLS

PRISMATIC
BOUNDARY
LAYER

TETRA CELLS

36



As seen in the
Figure 4.9
,
next

to these prismatic cells, tetra cells were generated
to connect the prismatic layers to the Hex core cells. The unstructured grid included the
Hex
-
core cells to fill the remaining computational domain a
s well as to accelerate the
simulation process. To increase the reliability and accuracy of the simulations, two
HEXCORE refinement regions were defined in the computational domain (as shown in
Figure 4.10
). Hex core cells of size 2mm were used constantly
over these entire
refinement regions.



Figure 4.10 Hex core refinement regio
ns of first benchmark simulation [26]


Now it is time to describe the refinement region. In the first benchmarking
simulation, there were two refinement regions defined. The firs
t refinement region was
37



placed right before the front bumper of the vehicle and had the purpose to solve the
stagnation pressure created at the first contact part of the SUV model i.e. the front
bumper. The second refinement region was placed at a location

just behind the vehicle
which is most frequently termed as the ‘wake’ of the vehicle. This space behind the SUV
model plays an important role in the aerodynamics of the vehicles. As shown in the
Figure 4.10

above, the refinement region used for resolving
the stagnation pressure is
indicated by ‘HEX A’, while the second refinement region used for the wake region is
indicated by ‘HEX B’. The refinement regions are measured in terms of base height “H”.
The ‘HEX A’ region is approximately 1H ahead of the SUV m
odel, while the ‘HEX B’ is
approximately 2H behind the Base of the SUV. The y+ value in this benchmarking exists
in the range of 60 to 150 over the surface of the vehicle.

For the second benchmarking simulation, the following details concentrate on the
un
ique changes made in the second benchmarking simulation. In this simulation, the two
refinement regions were combined to a single refinement region. The vehicle body
surfaces were more refined to a value of 1 mm all over the SUV model. The value of y+
over

the body surfaces were maintained in the range of 1 to 8 for all the simulations as
shown in Chapter 6 and in the second benchmark simulation. The Cartesian box, which
included the refined and constant hex
-
core cells around the vehicle body,
was measured
with 11.184m in the length, 2.64m in width, and

1.992m in height
. The
Figure 4.11

showed below gives the view of the domain mesh at the wheel center. For the Generic
SUV model simulation in the second benchmark simulation, the cell count was slightly
above

the value of 5.8 million cells. After the completion of first convergent, the mesh
38



adaptation module in FLUENT was used to adapt the cells over the boundary of the SUV
model. Initially the y+ value was in the range of 150 to 200, but later the boundary ce
lls
had values of y+ within 8. This value of y+ is really good for solving the boundary layer
near the surfaces and thereby solving the pressure near the surface of the SUV model.
Also, the value of the cells increased from 5.8 million cells to 7.2 million

cells after the
adaptation.


Figure 4.11 Unstructured mesh refinement region view of second benchmark




4.3.4 SOLVER SETTING



The problem of SUV numerical analysis requires the solver settings to be
completed before starting the simulations. The solver setting includes type of solver (3D
or 2D), the viscous model, boundary condition and solution controls. The inlet of the
wind tu
nnel is indicated by the term ‘Velocity inlet’, while the outlet of the wind tunnel is
termed as ‘Pressure outlet’. The fluid properties were calculated taking into account the
temperature and density of the average ambience condition of the area near the
lab of
University of Michigan. The solver settings and boundary condition for both the
benchmark simulations are shown in the
Table 4.1, Table 4.2
and

Table 4.3
.



Hex Core
Refinement
region

39



CFD Simulation

3ddp (3
-
D Double Precision)

Solver

Solver

Segregated

Space

3D

Formulation

Implicit

Time

Steady

Velocity Formulation

Absolute

Gradient Option

Cell
-
Based

Porous Formulation

Superficial Velocity

Table 4.1 Solver setting

Viscous Model

Benchmark #

Benchmark 1

Benchmark 2

Turbulence
Model

k
-
ε (2 eqn)

k
-
ε (2 eqn)

k
-
epsilon Model

Standard

Realizable

Near
-
Wall Treatment

Standard Wall Functions

Enhanced Wall Functions

Operating Conditions

Ambient

Ambient

Table 4.2 Viscous model and Turbulence model settings


Boundary Conditions

Velocity
Inlet

Magnitude (Measured
normal to
Boundary)

30 m⁄s (constant)



Turbulence Specification Method

Intensity and Viscosity Ratio



Turbulence Intensity

1.00%



Turbulence Viscosity Ratio

20

Pressure
Outlet

Gauge Pressure magnitude

0 pascal



Gauge Pressure direction

normal to boundary



Turbulence Specification Method

Intensity and Viscosity Ratio



Backflow Turbulence Intensity

10%



Backflow Turbulent Viscosity Ratio

10

Wall Zones

No Slip

Fluid
Properties

Fluid Type

Air



Density

ρ = 1.175 (kg⁄m^3 )



Kinematic viscosity

v = 1.8247×10^(
-
5) (kg⁄(m∙s))

Table 4.3 Boundary condition settings

40



4.3.5 SIMULATION RES
ULTS

The results of the simulations are described in this section. First the simulation
results of the first benchmark will be illustrated. A discussed earlier, only the Cp plots
over the cab and underbody was calculated for the first benchmark analysis.
Figure

4.12

and
Figure 4.13

show the plot of Cp over the cab and underbody respectively for the first
benchmark.

As seen in the figures below, the blue symbol indicates the experimental results
while the red symbol indicates the simulations results of the first

benchmark. Although
the results of the pressure coefficient on the cab surface closely match the experimental
results, the underbody pressure coefficient shows a slight variation but the pattern of the
plot is similar. The maximum pressure coefficient is
found near the front bumper of the
SUV model. The result of maximum coefficient on the front bumper was 0.97 for the
simulation which is quite close to the value for the experiments, i.e., 0.98.


Figure 4.12 Cab ‘Cp’ plot comparison with experiment for fi
rst benchmark

[26]

41




Figure 4.13 Underbody ‘Cp’ plot comparison with experiment for first benchmark

[26]



Next, the second benchmarking simulations will be discussed. In this simulation
model, some changes were made in the parameters of the turbulence mo
del as well as
some finer surface mesh was used all over the SUV model
. The
Figure 4.14

and
Figure
4.15

shown below illustrates the
pressure coefficient plots extracted from the second
benchmark simulations.

42




Figure 4.14 Cab ‘Cp’ plot comparison with exp
eriment for second benchmark



Figure 4.15 Underbody ‘Cp’ plot comparison with experiment for second benchmark

43




As shown in the figures above, the pattern and value of the results are much closer
to those of experiments than the previous benchmark simulat
ions. Since the results of the
coefficient pressure plots are not sufficient enough to conclude the benchmark results,
mean velocity profiles in the downstream direction were plotted in the central horizontal
plane i.e. z=69.2 mm (1:12 scale) or Z = 0.830
4 m (12:1 scale).

The figures from
Figure 4.16

to
Figure 4.21

shown below illustrate the
comparison of the mean velocity downstream profiles with the experimental results in the
wake region at distinct locations. The
Figure 4.16
shows the comparison of the Mean
downstream velocity at 0.3 m behind the base of the SUV model. The value of the mean
velocities are very close at Y= 30, 67 and 77mm. The pattern of the plot is closely
matching the results of the experiments near the symme
try plane, but it shows a drastic
deviation near the outer edges of the SUV model (i.e. Y= 74mm). The
Figure 4.17
shows
the comparison of the mean velocities at 0.6 m behind the SUV base. The results of the
benchmark at this wake spot moves closer to the e
xperimental results at the symmetry
plane and overlaps at several point with the experimental results. This shows that the
wake profile is captured correctly at this distance from the base of the SUV model.
Similar to
Figure 4.17
,
the
Figure 4.18
illustrat
es that the values of the benchmark
results, the profile of the plot and the wake profiles are nearly matching the experimental
results at 1.2 m behind the base of the SUV model. The
Figure 4.19, Figure 4.20 and
Figure 4.21
shows a similarity between them.

The values of the mean velocities from the
benchmark simulations are very close to the experimental results near the symmetry
plane. At the same time they show huge deviations near Y= 74 mm. The
comparison of
44



the mean velocities concludes

that the profile
s of the mean velocity profile are closely
matching the results of the experiments. Since the experiments are performed using the
symmetry plane, the downstream velocity are compared and benchmarked and the lateral
velocity component are neglected.




Figure 4.16 Mean downstream velocity profiles of the flow at Z = 0.830184m and X = 5.484m

45




Figure 4.1
7

Mean downstream velocity profiles of the flow at Z = 0.830184m and X = 5.
7
84m




Figure 4.1
8

Mean downstream velocity profiles of the flow at Z = 0.830
184m and X =
6.3
84m

46




Figure 4.1
9

Mean downstream velocity profiles of the flow at Z = 0.830184m and X =
6.9
84m




Figure 4.
20

Mean downstream velocity profiles of the flow at Z = 0.830184m and X =
7.5
84m

47





Figure 4.
21

Mean downstream velocity profiles of the flow at Z = 0.830184m and X =
8.1
84m




4.4 BENCHMARK CONCLU
SION


From the simulation results and the setting shown above, the conclusion can be
drawn from the benchmarking process for the SUV model used. The resu
lts give us the
suitable turbulence model and the mesh settings to be used for the optimization process of
the boat tail plate and the side steps. The results of the simulation agree well with
experimental results for downstream components considered. If t
he results are to be made
completely reliable, the re
-
simulation is needed without using any symmetry plane. For
complete domain simulation, very competitive computer configuration and power will be
required, which is beyond the present thesis work.

48



C
hapt
er

5

E
FFECTS

O
F

B
OAT

T
AIL

P
LATE

O
N

O
VERALL

D
RAG


5.1 INTRODUCTION

In this section, first external device is tested and optimized to reduce the drag of
the complete SUV model benchmarked in the previous section. The external device used
is termed as ‘
b
oat
tail plate’. This external device is

a

simple device in a shape of flat
plate connected to the base portion of the SUV. The idea of using this device had
emerged from numerical and experimental simulations completed by
P. Gilliéron and F.
Chometon over the

Ahmed Reference model at different angles of

the rear windshield
[4]
.

The Ahmed Reference Model, also termed as ‘ARM Model’, showed large variation
in drag when the value of the angle of rear windshield
changed from 5


to 40

. When the
angle
reached 12.5˚
, t
his value was expected to show the lowest drag and needed to be
simulated for the boat tail plate to
investigate

the effects on the flow behind the vehicle
and drag of the overall SUV model.



5.2 CAD MODEL OF BOA
T TAIL PLATE


The CAD model of SUV mod
el from the first benchmarking is modified using
CAD software Solidworks®. The
Figure 5.1

shown below gives the view of the boat tail
plate installed on the base of the SUV model. There are basically two plates installed on
the base of the SUV model. The t
op plate is termed as the ‘
Upper plate
’ and the bottom
plate is termed as the ‘
Lower plate
’.

This location of the boat tail plate is inspired from
49



the imaginary sketch created by Gaylard published in his published paper ‘
IMPROVING
SUV AERODYNAMICS’ at the
Motor Industry Research Association (MIRA)
conference

[4]
. The imaginary drag improving sketch of SUV is shown in
Figure 3.3

in
Chapter 3
.



Figure 5.1 Orientation of Boat tail plates



As seen in the
Figure 3.3
, the rear base of the SUV has a spoiler ext
ension at a
particular angle. This spoiler angle is expected to play a major role in reducing drag for
the
c
omplete SUV. The base of the SUV is directly connected to the low pressure wake
region of the SUV. If the size of the SUV wake region is reduced, th
en the low pressure
at the wake region will have the tendency to increase. This will cause the pressure
50



difference of the vehicle in the flow direction to decrease. Eventually the pressure drag
on the SUV model will
be
reduce
d, g
iv
ing

advantage to the SUV
model in propulsion.


5.3 OPTIMIZATION PRO
CESS DETAILS


Optimization is a process of getting the right solution or dimension required for
the particular problem. Every problem can achieve highly effective results if
optimizations are performed on it
s

desi
gn variables. The design variables are the
variables which contribute to the performance of the device to be tested.


The design variables for the boat tail plate are horizontal length of the plate,
thickness of the plate and the angle of the plates. Sinc
e the angle of the plates were
considered as the prime design variable for the testing of this device on the SUV, all the
other design variables except the angle are kept at constant value throughout all the
simulations.



Figure 5.2 Side view of Boat ta
il plate along with dimensions

[26]

51




The
Figure 5.2

above shows the detail sketch view of the
b
oat tails plates
installed on the base of the SUV. The horizontal length of the upper plate was kept at a
value of 18.75 mm (1/12 scale value) while the horizontal length of the lower plate was
kept at a value of 14.06 mm. These values were extracted after

live measurements of the
five different SUV vehicle using spoilers in the current marker of year 2008. The
thickness of the plates were randomly selected as 2mm (1/12 scale value). The angle of
upper boat tail plate is termed as ‘angle A’, while the angle

of the lower boat tail plate is
termed as ‘angle B’. These angles are incremented at a value of 5 degrees and simulated
at different combinations to determine the effects of it on the overall drag of the SUV as
well as to find the best angle for optimizat
ion. The
Table 5.1

below shows the details
about the values of angle A and angle B for each simulation performed.

Simulation #

Simulation Name

Angle A

Angle B

1

A
-
B
-

OR Benchmark Model

-

-

2

A0B0

0

0

3

A1B1

1

1

4

A5B1

5

1

5

A5B5

5

5

6

A10B5

10

5

7

A10B10

10

10

8

A15B15

15

15

9

A20B15

20

15

10

A20B20

20

20

11

A25B25

25

25

12

A30B25

30

25

13

A30B30

30

30

14

A35B30

35

30

15

A35B35

35

35

Table 5.1 Simulation details of Boat tail plates

52



5.4 VIRTUAL WIND TUN
NEL AND VEHICLE ORIE
NTATION


The wind tunnel size and dimensions are adapted in the same way as used
for the first benchmark simulation discussed in
Section 4.3.2
. The inlet section of the
wind tunnel is approximately 0.60 x 0.50 m
2
. The inlet of the
w
ind tunnel is placed at two
times

the vehicle body length ahead of the SUV model and the velocity of the air at the
inlet is 30 m/s. On the other hand
,

the outlet of the wind tunnel is placed five times the
length of the SUV model measured from base of the SUV model. The pressure outlet i
s
set to
be
atmospheric value. The
Figure 5.3

shows a preview of the orientation of the
vehicle.




Figure 5.3 SUV orientation


5.5
DISCRETISATION (OR M
ESHING) SETUP AND SO
LVER SETTING

The first benchmark, as discussed in Chapter 4, is considered as a reference of all
the mesh setting used in the all the simulations performed for the boat tail plates. As
described in section 4.3.3, two

refinement regions were used in the simulation. The first
refinement box was implemented to resolve the stagnation pressure near the first contact
element of the SUV model i.e. the front bumper. Similarly, the second refinement box

5L L 2L

53



was used to resolve the

wake profiles of the SUV model. The second refinement in these
simulations helps to better understand the dynamics of the air flow over the SUV boat tail
plates. The
Figure 5.4

below gives the view of the mesh generated at the symmetry plane
near the boat

tail plate.


Figure 5.4 Hybrid Mesh near the upper boat tail plate


5.6 SIMULATIONS RESU
LTS AND DISCUSSION


The
Figure 5.5
shows the Total pressure contour at the symmetry plane of the
Benchmark simulation. There is very high pressure on the surrounding air which is
represented by dark red. As the air passes over the vehicle, a low pressure wake region is
formed. This wake reg
ion, along with the pressure inside it, plays a major role in
controlling the drag coefficient of the SUV model.
If the size of the SUV wake region is
54



reduced, then the low pressure at the wake region will have the tendency to increase. This
will cause the

pressure difference of the vehicle in the flow direction to decrease.
Eventually the pressure drag on the SUV model will
be
reduce
d, g
iv
ing

advantage to the
SUV model in propulsion
.


Figure
5.5 Total Pressu
re Contour of the Benchmark Simulation at symmetry plane


The
Figure 5.6
shows the total pressure contour of the A10B10 simulation. The
drag coefficient of this simulation is the lowest among all the simulations performed on
the SUV model. The angle value of 10˚ is the optimum angle of upper and lower boat tail
plate for reduci
ng drag of the complete SUV model. After comparing the
F
igure
5.
5

and
F
igure
5.
6
, it is seen that the pressure of the air above the SUV model has reduced. At
the same time, the pressure inside the wake region has increase drastically. The pressure
difference in the flow direction has decreased, thereby causing a decrease in the drag o
f
the SUV model.

The
Figure 5.7
illustrates the Pressure contour of the simulations which has the
least lift coefficient. In this simulation, the SUV model gives the optimum lift coefficient.
The wake profile of this simulation is compressed, but the pres
sure around the top surface
ORIGINAL SIMULATION

Benchmark Simulation