DRAG REDUCTION OF PICKUP TRUCK USING ADD-ON DEVICES ...
DRAG REDUCTION OF PICKUP TRUCK USING ADD-ON DEVICES ...
DRAG REDUCTION OF PICKUP TRUCK USING ADD-ON DEVICES ...
Create successful ePaper yourself
Turn your PDF publications into a flip-book with our unique Google optimized e-Paper software.
<strong>DRAG</strong> <strong>REDUCTI<strong>ON</strong></strong> <strong>OF</strong> <strong>PICKUP</strong> <strong>TRUCK</strong> <strong>USING</strong> <strong>ADD</strong>-<strong>ON</strong> <strong>DEVICES</strong><br />
Feysal Ahmed Adem<br />
B.S., Addis Ababa University, Ethiopia, 1999<br />
THESIS<br />
Submitted in partial satisfaction of<br />
the requirements for the degree of<br />
MASTER <strong>OF</strong> SCIENCE<br />
in<br />
MECHANICAL ENGINEERING<br />
at<br />
CALIFORNIA STATE UNIVERSITY, SACRAMENTO<br />
FALL<br />
2009
© 2009<br />
Feysal Ahmed Adem<br />
ALL RIGHTS RESERVED<br />
ii
Approved by:<br />
<strong>DRAG</strong> <strong>REDUCTI<strong>ON</strong></strong> <strong>OF</strong> <strong>PICKUP</strong> <strong>TRUCK</strong> <strong>USING</strong> <strong>ADD</strong>-<strong>ON</strong> <strong>DEVICES</strong><br />
A Thesis<br />
by<br />
Feysal Ahmed Adem<br />
__________________________________, Committee Chair<br />
Dr. Dongmei Zhou<br />
__________________________________, Second Reader<br />
Dr. Akihiko Kumagai<br />
____________________________<br />
Date<br />
iii
Student: Feysal Ahmed Adem<br />
I certify that this student has met the requirements for format contained in the University<br />
format manual, and that this thesis is suitable for shelving in the Library and credit is to<br />
be awarded for the thesis.<br />
__________________________, Graduate Coordinator ___________________<br />
Dr. Kenneth S. Sprott Date<br />
Department of Mechanical Engineering<br />
iv
Abstract<br />
of<br />
<strong>DRAG</strong> <strong>REDUCTI<strong>ON</strong></strong> <strong>OF</strong> <strong>PICKUP</strong> <strong>TRUCK</strong> <strong>USING</strong> <strong>ADD</strong>-<strong>ON</strong> <strong>DEVICES</strong><br />
by<br />
Feysal Ahmed Adem<br />
Nowadays the reduction of drag is becoming a very important challenge for all the car<br />
manufacturers as they are competing intensely to produce powerful pickup trucks with<br />
better gas mileage in the market regulated with law reinforcement on fuel emissions and<br />
consumers’ need for bigger size trucks with more horse power and cargo capacity. Lower<br />
drag provides better performances such as higher top speed and better stability. It also<br />
often lowers aerodynamic noise and greenhouse gas emission above all decreases in fuel<br />
consumption. However, modern designs of pickup trucks tend to go higher and wider and<br />
thus they have higher frontal areas due to the functional, economic and aesthetic<br />
requirements. Increasing frontal area of the vehicle tend to increase the drag force acting<br />
on the vehicle which is proportional to the dimensionless drag coefficient and the<br />
projected area of the vehicle. Consequently, to hold or even decrease the drag on a truck<br />
that has a larger frontal area, tremendous effort has to be made.<br />
The purpose of this research is to design various aerodynamic add-on devices that can be<br />
attached to the pickup truck and reduce aerodynamic drag of the vehicle without<br />
comprising on its main design features. The research approach is using computational<br />
v
fluid dynamics (CFD) technique. This thesis focuses on investigating the effects of add-<br />
on devises such as Tonneau cover, Rear Roof Garnish, Tail plates, Airdam, Traditional<br />
canopy, and Aerocap with 5 different rear inclination angles. After the effect of these<br />
add-on devise was quantified, Aerocap with rear inclination angle of 12⁰, was identified<br />
as the one that had the maximum drag reduction and it was further modified to increase<br />
the drag reduction by using the 3D curved Aerocap. The effect that drag reduction had on<br />
the fuel economy of the truck was also analyzed. Results from numerical simulations and<br />
analyses indicated that the 3D curved Aerocap, modified from the Aerocap with<br />
inclination angle α= 12°, had successfully reduced the rear width than the original one.<br />
As a result, it had reduced the drag coefficient by about 19.84%. It also reduced the<br />
lift coefficient by about 40.72%. At last the impact of 3D curved Aerocap on the fuel<br />
economy of the pickup truck was analyzed over the U.S. EPA driving schedules and<br />
conclusions were drawn.<br />
_______________________, Committee Chair<br />
Dr. Dongmei Zhou<br />
_______________________<br />
Date<br />
vi
ACKNOWLEDGMENTS<br />
First off, I would like to express my sincerest thanks to Dr. Dongmei Zhou for her<br />
guidance and support in the completion of my thesis. My thesis would have never been<br />
completed without her, I am so grateful to have had the opportunity to work under<br />
Professor Zhou.<br />
Secondly, I would like to thank Professor Akihiko Kumagai who spent his<br />
precious time reviewing and making suggestions in my thesis.<br />
Lastly, I would like to thank my family and friends for their help and support<br />
throughout my life.<br />
vii<br />
Feysal Ahmed Adem<br />
B.S. Mechanical Engineering<br />
July, 1999
TABLE <strong>OF</strong> C<strong>ON</strong>TENTS<br />
Acknowledgments...................................................................................................... vii<br />
List of Tables…………………………………………………………………………..x<br />
List of Figures ............................................................................................................. xi<br />
Chapter<br />
1. INTRODUCTI<strong>ON</strong> TO VEHICLE AERODYNAMICS ………………………. 1<br />
1.1 Introduction ………………………………………………………………. 1<br />
1.2 Flow around a Vehicle …………………………………………………. 3<br />
1.3 Boundary layer and separation of flow over a vehicle …………………….. 5<br />
viii<br />
Page<br />
1.4 Aerodynamic forces on vehicles ………………………………………….. 10<br />
1.5 Fuel economy ................................................................................................15<br />
2. BACKGROUND AND OBJECTIVE ..................................................................22<br />
2.1 Motivation ................................................................................................…...22<br />
2.2 Pickup truck history …………………………………………………………23<br />
2.3 Previously conducted research.........................................................................25<br />
2.4 Objective..........................................................................................................26<br />
2.5 Outlines ……………………………………………………………………...27<br />
3. PROBLEM FORMULATI<strong>ON</strong>............................................................................... 28<br />
3.1 Introduction................................... ................................................................. 28<br />
3.2 Aerodynamic drag on Vehicles. .................................................................... 29<br />
3.3 CFD problem formulation... .......................................................................... 31
3.4 Baseline Pickup truck CFD method and setup. ............................................. 34<br />
3.5 Baseline pickup truck results and discussion.................................... ............. 37<br />
3.6 Summary.................................... .................................................................... 46<br />
4. STUDY <strong>OF</strong> <strong>ADD</strong>-<strong>ON</strong> <strong>DEVICES</strong> ......................................................................... 48<br />
4.1 Pickup truck model with Tonneau cover............................................ ........... 48<br />
4.2 Pickup truck model with Rear Roof Garnish................................................. 55<br />
4.3 Pickup truck model with Tail Plates....................................................... ....... 60<br />
4.4 Pickup truck model with Airdam............................... .................................... 63<br />
4.5 Pickup truck model with Traditional canopy..................................................68<br />
4.6 Pickup truck model with Aerocap...................................................................71<br />
4.7 Pickup truck model with 3D curved Aerocap................................................ 85<br />
4.8 Impact of 3D curved Aerocap on fuel economy of pickup truck.................. 90<br />
5. C<strong>ON</strong>CLUSI<strong>ON</strong> AND FUTURE WORK ………………………………………. 93<br />
5.1 Conclusions.................................................................................................... 93<br />
5.2 Future work................................................. ................................................... 95<br />
Appendix......................................................................................................................96<br />
References…………………………….……………………………………………...97<br />
ix
LIST <strong>OF</strong> TABLES<br />
1. Table 3.1 Solver setting…………………………………………………………...35<br />
2. Table 3.2 Viscous model and Turbulence model settings………………………...35<br />
3. Table 3.3 Boundary condition settings……………………………………………36<br />
4. Table 3.4 Solution controls………………………………………………………..36<br />
5. Table 4.1.1 Comparison of drag and lift coefficient of baseline pickup truck model<br />
with a model fitted with Tonneau cover……………….……………………………..55<br />
6. Table 4.2.1 Comparison of drag and lift coefficient of baseline pickup truck model<br />
with a model attached with Rear Roof Garnish………………………………………59<br />
7. Table 4.3.1 Comparison of drag and lift coefficient of baseline pickup truck with a<br />
model attached with Tail plates………………………………………………………63<br />
8. Table 4.4.1 Comparison of drag and lift coefficient of baseline pickup truck with a<br />
model attached with Airdam-3in and Airdam-6in……………………………………68<br />
9. Table 4.5.1 Comparison of drag and lift coefficient of baseline truck model with a<br />
model attached with Traditional canopy……………………………………………..70<br />
10. Table 4.6.1 Comparison of drag and lift coefficient of pickup truck with Aerocap at<br />
different rear inclination angle α with the baseline truck…………………………….85<br />
11. Table 4.7.1 Comparison of drag and lift coefficient of baseline pickup truck with<br />
Aerocap α=12° and 3D curved Aerocap………………………………………...……90<br />
12. Table 4.8.1 Impact of 19.83% reduction in A on Composite Fuel Economy using<br />
G. Sovran [5] charts in Figure 1.12 …………………………………………………..91<br />
x<br />
Page
LIST <strong>OF</strong> FIGURES<br />
1. Figure 1.1 Typical energy uses and losses in a vehicle [9]………………………..2<br />
2. Figure 1.2 Flow over a cylinder at different Reynolds number [20]……………...3<br />
3. Figure 1.3 Streamline about passenger vehicle in the symmetry plane [8] ……….5<br />
4. Figure 1.4 Boundary layer velocity profiles [14]………………………………….6<br />
5. Figure 1.5 Areas of flow separation around a vehicle [5]………………………....8<br />
xi<br />
Page<br />
6. Figure 1.6 Flow separation on a bluff body (separation line perpendicular to the flow<br />
direction) [5]…………………………………………………………………………..10<br />
7. Figure 1.7 Flow separation on a bluff body with oblique blunt base (separation line at<br />
an angle to the flow direction) [5]………………………………………………….....10<br />
8. Figure 1.8 Aerodynamic force and moments acting on a vehicle………………..11<br />
9. Figure 1.9 versus road speed V for typical radial tires [5]……………………16<br />
10. Figure 1.10 Typical bsfc maps for a gasoline and a diesel engine [5]…………...18<br />
11. Figure 1.11 EPA driving cycle [5]……………………………………………….20<br />
12. Figure 1.12 G. Sovran charts for the impact of changes in aerodynamic drag on the<br />
fuel consumption for vehicles driving on the EPA schedules [5]………………….…21<br />
13. Figure 2.2.1 Ford Model-TT from 1916…………………………………………23<br />
14. Figure 2.2.2 Ford F-100 from 1951……………………………………………...24<br />
15. Figure 2.2.3 Ford F-100 from 1966……………………………………………...24<br />
16. Figure 2.2.4 Ford F-100 from 1997……………………………………………...25<br />
17. Figure 2.2.5 Ford F-100 from 2008-2009………………………………………..25
18. Figure 3.1 Flow past a circular cylinder: (a) laminar separation; (b) turbulent<br />
separation; (c) theoretical and actual surface-pressure distribution, [7] ……………..31<br />
19. Figure 3.2 Original 1/12th-scale generic pickup truck model used in [1], [2] …..33<br />
20. Figure 3.3 1/12th scale of flow domain used in present simulation, all dimensions are<br />
in mm………………………………………………………………………………….33<br />
21. Figure 3.4 (a) Pressure on pickup cab (b) Pressure on pickup floor ……..……...37<br />
22. Figure 3.5 (a) Pressure on pickup cab from [1]. (b) Pressure on pickup floor from<br />
[1]…………………………………………………………………………………......38<br />
23. Figure 3.6 (a) Pressure on tailgate (outside). (b) Pressure on the tailgate (outside)<br />
from [1]……………………………………………………………………………......39<br />
24. Figure 3.7 (a) Pressure on tailgate (inside). (b) Pressure on the tailgate (inside) from<br />
[1]…………………………………………………………………………………......39<br />
25. Figure 3.8 (a) u-velocity in y=0 plane (inside box). (b) u-velocity in y=0 plane<br />
(inside box) from [1]…………...…………………………………………………......40<br />
26. Figure 3.9 (a) u-velocity in y=0 plane (outside box). (b) u-velocity in y=0 plane<br />
(outside box) from [1]…………………………………………………………….......41<br />
27. Figure 3.10 (a) u-velocity for z=73mm and x=450mm (scaled down model) (b) u-<br />
velocity for z=73mm and x=450mm from [1]………………………….………….....41<br />
28. Figure 3.11 (a) u-velocity for z=15mm and x=450mm (scaled down model) (b) u-<br />
velocity for z=15mm and x=450mm from [1]……………………………………...…42<br />
29. Figure 3.12 Pressure distributions over the pickup………………………..…42, 67<br />
xii
30. Figure 3.13 Wake profile for baseline truck (velocity vector on iso-velocity surface at<br />
3m/s)……………………………………………………………………………....42, 54<br />
31. Figure 3.14 (a) Streamline on z=73 mm (scaled down model) plane. (b) Streamline<br />
on z=73mm plane from [1] …………………………………………………………...43<br />
32. Figure 3.15 (a) Streamline on symmetry plane (b) Streamline on symmetry plane<br />
from [1]……………………………………………………………………………43, 53<br />
33. Figure 3.16 Static pressure distributions over the baseline truck and symmetry<br />
plane………………………………………………………………………………44, 51<br />
34. Figure 3.17 Total pressure distributions over the baseline truck and symmetry<br />
plane………………………………………………………………...…………….44, 52<br />
35. Figure 3.18 Streamline flow over the baseline pickup truck…………………….45<br />
36. Figure 4.1.1 Pickup truck with Tonneau cover…………………………………..48<br />
37. Figure 4.1.2 (a) Pressure coefficient plot in the symmetry plane for truck with<br />
Tonneau cover (b) Pressure coefficient plot in the symmetry plane for baseline<br />
truck…………………………………………………………………………..…..49, 65<br />
38. Figure 4.1.3 Static pressure distribution over truck with Tonneau cover and<br />
symmetry plane…………………………………………………………………….....50<br />
39. Figure 4.1.4 Total pressure distribution over truck with Tonneau cover and symmetry<br />
plane………………………………………………………………………………......51<br />
40. Figure 4.1.5 Velocity magnitude vector over symmetry plane for pickup with<br />
Tonneau cover………………………………………………………………………...53<br />
xiii
41. Figure 4.1.6 Wake profile for pickup truck with Tonneau cover (velocity vector on<br />
iso-velocity surface at 3m/s)…………………………………………………………..54<br />
42. Figure 4.2.1 Pickup truck with attached Rear Roof Garnish…………………......55<br />
43. Figure 4.2.2 (a) Pressure coefficient plot on the symmetry plane for flow over a<br />
pickup truck with Rear Roof Garnish (b) Pressure coefficient plot on top and floor<br />
surface of the base line truck in the symmetry plane………………………..………..56<br />
44. Figure 4.2.3 Static pressure contour over the pickup with Rear Roof Garnish and<br />
symmetry plane………………………………………………………..……………...57<br />
45. Figure 4.2.4 Total pressure contour over the pickup with Rear Roof Garnish and<br />
symmetry plane………………………………………………………………….……58<br />
46. Figure 4.2.5 Velocity magnitude vector for a Pickup truck with Rear Roof Garnish<br />
on the symmetry plane ………………………………………………………….…....59<br />
47. Figure 4.2.6 Wake profile over a pickup truck with Rear Roof Garnish (velocity<br />
vector on iso-velocity surface at 3m/s)………………..…………………....………....59<br />
48. Figure 4.3.1 Pickup truck with attached Tail plates………………………...……60<br />
49. Figure 4.3.2 Static pressure distribution over model with tail plates and symmetry<br />
plane…………………………………………………………………………………..61<br />
50. Figure 4.3.3 Total pressure distribution over model with tail plates and symmetry<br />
plane………………………………………………………………………………..…61<br />
51. Figure 4.3.4 Velocity magnitude vector on the symmetry plane for model with tail<br />
plates……………………………………………………………………………….....62<br />
xiv
52. Figure 4.3.5 Wake profile over pickup truck with tail plates (velocity vector on iso-<br />
velocity surface at 3m/s)……………………….……………………………………...63<br />
53. Figure 4.4.1 Pickup truck with Airdam ………………………………………....64<br />
54. Figure 4.4.2 (a) Pressure coefficient plot over a model with Airdam (3in clearance<br />
from the ground). (b) Pressure coefficient plot over a model with Airdam (6in clearance<br />
from the ground) ………………………………………………………………….…65<br />
55. Figure 4.4.3 (a) Pressure contour over pickup with Airdam (3in clearance from the<br />
ground). (b) Pressure contour over pickup with Airdam (6in clearance from the<br />
ground)..........................................................................................................................66<br />
56. Figure 4.5.1 Pickup truck with traditional canopy………………………….........68<br />
57. Figure 4.5.2 Pressure coefficient plot on symmetry plane for model with Traditional<br />
Canopy…………………………………………………………………………...…...69<br />
58. Figure 4.5.3 Static pressure distribution over model with Traditional<br />
Canopy………………………………………………………………………………..69<br />
59. Figure 4.5.4 Wake profile behind the pickup truck with traditional canopy (velocity<br />
vector on iso-velocity surface at 3m/s)………………………………………………..70<br />
60. Figure 4.6.1 Pickup truck model with Aerocap of a rear inclination angle<br />
α=10°………………………………………………………………………………….71<br />
61. Figure 4.6.2 Pressure coefficient plot in symmetry plane over model with Aerocap at<br />
different α……………………………………………………….…………………….72<br />
62. Figure 4.6.3 Total pressure on symmetry plane when rear inclination angle<br />
α=5°…………………………………………………………………………………...73<br />
xv
63. Figure 4.6.4 Total pressure on symmetry plane when rear inclination angle<br />
α=10°…………………………………………………………………………….........73<br />
64. Figure 4.6.5 Total pressure on symmetry plane when rear inclination angle<br />
α=12°………………………………………………………………………………….74<br />
65. Figure 4.6.6 Total pressure on symmetry plane when rear inclination angle<br />
α=15°………………………………………………………………………………….74<br />
66. Figure 4.6.7 Total pressure on symmetry plane when rear inclination angle<br />
α=18.77°………………………………………………………………………………75<br />
68. Figure 4.6.8 Pressure on symmetry plane when rear inclination angle<br />
α=5°…………………………………………………………………………………...76<br />
69. Figure 4.6.9 Pressure on symmetry plane when rear inclination angle<br />
α=10°………………………………………………………………………………….76<br />
70. Figure 4.6.10 Pressure on symmetry plane when rear inclination angle<br />
α=12°………………………………………………………………………………….77<br />
71. Figure 4.6.11 Pressure on symmetry plane when rear inclination angle<br />
α=15°………………………………………………………………………………….77<br />
72. Figure 4.6.12 Pressure on symmetry plane when rear inclination angle<br />
α=18.77°………………………………………………………………………………78<br />
73. Figure 4.6.13 Velocity magnitude path line on symmetry plane when rear inclination<br />
angle α=5°……………………………………………………………………………..79<br />
74. Figure 4.6.14 Velocity magnitude path line on symmetry plane when rear inclination<br />
angle α=10°…………………………………………………………………………....79<br />
xvi
75. Figure 4.6.15 Velocity magnitude path line on symmetry plane when rear inclination<br />
angle α=12°…………………………………………………………………………...80<br />
76. Figure 4.6.16 Velocity magnitude path line on symmetry plane when rear inclination<br />
angle α=15°…………………………………………………………………………...80<br />
77. Figure 4.6.17 Velocity magnitude path line on symmetry plane when rear inclination<br />
angle α=18.77°………………………………………………………………………..81<br />
78. Figure 4.6.18 Wake profile behind the pickup truck with Aerocap when rear<br />
inclination angle α=5° (velocity vector on iso-velocity surface at 3m/s)……………..82<br />
79. Figure 4.6.19 Wake profile behind the pickup truck with Aerocap when rear<br />
inclination angle α=10° (velocity vector on iso-velocity surface at 3m/s)…………....82<br />
80. Figure 4.6.20 Wake profile behind the pickup truck with Aerocap when rear<br />
inclination angle α=12° (velocity vector on iso-velocity surface at 3m/s)……………83<br />
81. Figure 4.6.21 Wake profile behind the pickup truck with Aerocap when rear<br />
inclination angle α=15° (velocity vector on iso-velocity surface at 3m/s)…………....83<br />
82. Figure 4.6.22 Wake profile behind the pickup truck with Aerocap when rear<br />
inclination angle α=18.77° (velocity vector on iso-velocity surface at 3m/s)………...84<br />
83. Figure 4.6.23 (a) Drag Coefficient ( ) versus rear inclination angle α. (b) Lift<br />
Coefficient ( ) versus rear inclination angle α……………………………………....85<br />
84. Figure 4.7.1 Shape changes to reduce drag of SUV [19]………………………...86<br />
85. Figure 4.7.2 Pickup truck with 3D curved Aerocap……………………………..86<br />
86. Figure 4.7.3 Pressure distribution over pickup with 3D curved Aerocap in the<br />
symmetry plane………………………………………………………………………..87<br />
xvii
87. Figure 4.7.4 Total pressure distribution over pickup with 3D curved Aerocap in the<br />
symmetry plane……………………………………………………………………….88<br />
88. Figure 4.7.5 Velocity magnitude path line on symmetry plane for flow over model<br />
with 3D curved Aerocap…………………………………………………….………..89<br />
89. Figure 4.7.6 Wake profile behind the pickup truck with 3D curved Aerocap (velocity<br />
vector on iso-velocity surface at 3m/s)………………………………………………..89<br />
xviii
1.1 Introduction<br />
Chapter 1<br />
INTRODUCTI<strong>ON</strong> TO VEHICLE AERODYNAMICS<br />
The continuing increase in fuel price coupled with uncertainty of future supply<br />
has created widespread interest in vehicles with high efficiency including pickup trucks.<br />
Pickup trucks, vans and SUVs account for 48% of sales fraction of light duty vehicle in<br />
United States while light duty vehicles account for approximately 40% of all US oil<br />
consumption [9]. Therefore improving the fuel economy of pickup trucks will have<br />
tremendous impact on energy security, emission of green house gas and cost of fueling<br />
when gasoline price rises.<br />
Today auto manufacturers are competing intensely to produce a powerful pickup<br />
truck with better gas mileage in the market regulated with law reinforcement on fuel<br />
emissions and consumers’ need for bigger size truck with more horse powers and cargo<br />
capacity. Energy efficiency of vehicles can be improved by reducing the total structural<br />
mass, using engine with higher thermally efficiency, or altering the exterior body shape<br />
to reduce the aerodynamic drag. According to US department of energy [10], in urban<br />
driving aerodynamic drag accounts for 2.6% of the 12.6% of fuel energy being used to<br />
propel the car as shown in Figure 1.1. Since the aerodynamic drag increases at higher<br />
speeds, the aerodynamic drag on a highway driving accounts for 11% of 20% fuel energy<br />
needed to propel the vehicle. Therefore improving vehicle aerodynamics is one of the<br />
1
factors that play crucial role for getting better mileage and better performance including<br />
the handling of the vehicle especially at high speeds.<br />
The body shapes of pickup trucks are primarily designed to meet the functional,<br />
economic and aesthetic requirements. Aerodynamic drag is often the consequence of the<br />
body shape designed to meet the functional, economic and aesthetic design constraints.<br />
The use of add-on devise enables us to reduce the aerodynamic drag of the vehicle<br />
without compromising on its main design features. Studying flow over a pickup truck<br />
with add-on devices is costly in wind tunnel due to cost for the setup as well as number<br />
of runs required for successful drag reduction and optimization of the add on devises.<br />
With the use of CFD these costs are avoided and multiple runs can be set up at the same<br />
time for comparison and optimization. It is motivated for this thesis by using a CFD<br />
approach to analyze the flow over pickup truck with add-on device such as Aerocap,<br />
Tonneau cover, Tail plates and Rear Roof Garnish for drag reduction.<br />
Figure 1.1 Typical energy uses and losses in a vehicle [9].<br />
2
1.2 Flow around a vehicle<br />
External flow past objects encompass an extremely wide variety of fluid<br />
mechanics phenomena and the characteristic of the flow fields is a function of shape of<br />
the body. For a subsonic flow past a given shaped object, the characteristic of the flow<br />
typical depends on the Reynolds number Re. Figure 1.2 shows flow over a cylinder at<br />
Re=0.1, 50 and 10 5 . For low Reynolds number, Re= 0.1, the flow is laminar and the<br />
viscous effect plays important role throughout the flow. As Reynolds number is increased<br />
to Re= , the flow separates and the viscous effect is limited in boundary layer and the<br />
wake region is formed behind the cylinder. The separation point is where the flow starts<br />
to separate as shown in Figure 1.2.<br />
Figure1.2 Flow over a cylinder at different Reynolds number [20]<br />
3
As opposed to streamlined bodies such as airfoils, road vehicle exist as blunt<br />
bodies in close proximity to ground. The complex geometries of the vehicle associated<br />
with the rotating wheels, engine compartment and cooling vents add to complexity of the<br />
flow over the vehicle, which makes the flow over ground vehicle fully turbulent and three<br />
dimensional with steep pressure gradient. Road vehicles also operate in the surrounding<br />
ambient turbulent wind that is almost constantly present. Furthermore, road vehicles<br />
travel at various yaw angles depending on the nature of the cross wind which increase the<br />
chance for the flow to separate on the leeward side of the vehicle and thus adding more<br />
complexity to the flow field. Clearly, flow fields from a flow past vehicles are much<br />
more complex compared to the flow past a simple geometry cylinder or more streamlined<br />
body-shape of aircraft and ships.<br />
Figure 1.3 shows flow streamlines over a passenger vehicle in the symmetry<br />
plane. As air flow approaches the stagnation point A, where the static pressure equals the<br />
total pressure, the flow divides into two, above and below the vehicle. At point B, the<br />
pressure lowers than the total pressure, even lower than the ambient pressure, as the<br />
velocity of the flow increases. After point C, the flow detaches from the vehicle surface<br />
and then attach again at point D which is located on the windscreen. On the roof the<br />
pressure between points E and F is again low but the pressure distribution will depend on<br />
the roof shape and curvature. At the end of the roof the flow must slow down and<br />
pressure should rise. After point F, the flow gets easily detached and the separation point<br />
is located at the rear edge of the roof as shown in Figure 1.3. Actually any sharp surface<br />
irregularity can trigger the separation to form a wake.<br />
4
Figure 1.3 Streamline about passenger vehicle in the symmetry plane [8]<br />
1.3 Boundary layer and separation of flow over a vehicle<br />
The air flow movement causes boundary layer to develop on the surface of the<br />
vehicle and it thickness as flow over the vehicle progress. In this relatively small region<br />
adjacent to the vehicle, the effect of viscosity must be taken in to account. This concept<br />
was introduced by Ludwig Prandtl in 1904. Outside this region the boundary layer is<br />
assumed to be inviscid or frictionless.<br />
As shown in Figure 1.4, during the initial stage, the boundary layer flow near the<br />
front edge of the vehicle exists in a laminar manner. Friction drag formed between the<br />
layers of the airflow and the surface of the vehicle will create a velocity gradient and as<br />
the result outer layer moves faster the inner one. This slowing-down effect spreads<br />
outwards and the boundary later gradually become thicker. According to Bernard [6], on<br />
most ground vehicles the laminar boundary layer does not extend for much more than<br />
about 30mm from the front of the vehicle. Further down the flow transition to turbulent<br />
flow take place after passing the critical distance. In the turbulent boundary layer, eddies<br />
are formed resulting in rapid mixing of fast and slow moving masses of air (i.e. turbulent<br />
5
diffusion). The turbulent mixing will then move further outwards from the surface.<br />
However, very close to the surface with in a turbulent boundary layer flow, a thin sub<br />
layer of laminar flow still exists. The two distinct differences between the flow<br />
mechanisms in the laminar and turbulent flow is that in laminar flow, the influence of the<br />
surface is transmitted outward mainly by a process of molecular impacts, whereas in the<br />
turbulent flow the influence is spread by turbulent mixing.<br />
Figure 1.4 Boundary layer velocity profiles [14]<br />
In the turbulent boundary layer, some of the energy is dissipated in friction,<br />
slowing airflow velocity, resulting in a pressure increase. If the increase in pressure is<br />
gradual, the process of turbulent mixing will cause a transfer of energy from the fast<br />
moving eddies to slower ones in the turbulent boundary layer. If the rate of change in<br />
pressure is too great, for example in sharp corners, the mixing process will be too slow to<br />
push the slower air molecules moving. When this happens, the boundary layer flow stops<br />
following the contours of the surface, resulting in separation. Air particles downstream of<br />
6
the separation region will then move towards the lower pressure region in the reverse<br />
direction to the main flow, the separation region will reattach. In the region between<br />
separation and reattachment points, air flow is circulating and this is called the<br />
‘separation bubble’. Separation will normally occur if the resultant flow encounters a<br />
sharp edge and that is why it is always important for ground vehicles to have smoothly<br />
rounded edges everywhere. Each type of separation can form a separation-bubble zone<br />
either by reattaching itself downstream to the flow or being transmitted into a wake,<br />
where the separation bubble re-circulates frequently. Hucho [5] named this frequent<br />
circulation as “dead water” zone. Separation bubble zone happens normally on the<br />
surface area in front of the windshield and on the side of the fenders while “dead water”<br />
zone normally happens on the rear surface of the ground vehicles.<br />
Vehicle aerodynamics operates mainly in the Reynolds number region in excess<br />
of according to Ahmed [11], and the effect of separation and reattachment dominates<br />
most of the ground vehicles surface region. As shown in the Figure 1.5, typical areas<br />
around the vehicle that exhibit small region of separation are the body appendages such<br />
as the mirrors, headlights, windshield wipers, door handles and windshield junction.<br />
Larger flow separation regions around the vehicle include the A-pillar, body under side,<br />
rear body of the vehicle and in the wheel wells [5]. In a similar prospective, Ahmed [11]<br />
defined the airflow as three dimensional with steep pressure gradients and having regions<br />
of separated flow. Regions of separated flow are categorized into small and large regions.<br />
Small regions of separated flow occur normally around attached component on a vehicle<br />
body such as headlights, mirror, door handles and windshield wipers. Large regions of<br />
7
separated flow occur on the A-pillar, at the rear of the vehicle, underneath the vehicle and<br />
around the wheel region. In present study, the focus will be on the wake near the rear of<br />
the vehicle<br />
.<br />
Figure 1.5 Areas of flow separation around a vehicle [5]<br />
8
Flow separations that lead to a pressure drag can be divided in two different<br />
groups, according to Hucho [5]. If the separation line is located perpendicular to the flow<br />
direction as shown in Figure 1.6, the vortices generated will have the axis perpendicular<br />
to the outer flow and parallel to the line of separation. Figure 1.6 shows that a<br />
symmetrical flow exists only for low Reynolds number. For larger Reynolds number,<br />
periodic vortex shedding occurs, and the flow in the separated region is unsteady. The<br />
kinetic energy of the vortex field is rapidly dissipated by the turbulent mixing and<br />
irreversibly converted into frictional heat [5], and it leads to considerable total pressure<br />
loss in the region behind the body and the corresponding deficit in kinetic energy is equal<br />
to the work needed to overcome the pressure drag. Behind the body a wake is formed in<br />
which, time averaged, relatively uniform suction and very low flow velocities are present.<br />
The second type of flow separation is characterized by separation line inclined<br />
with respect to the flow as shown in Figure 1.7, the vortex generated have axis nearly<br />
parallel to the line of separation with vortex shedding [5]. In this case a well-ordered<br />
steady three dimensional flow separation is found and on the rearward surface of the<br />
body and the separated flow induces suction which leads to pressure drag. On the<br />
inclined surface the flow is attached and behind the body only relatively small total<br />
pressure losses are observed. The flow field of the concentrated vortices, however,<br />
contains a lot of kinetic energy which corresponds to the work necessary to overcome<br />
pressure drag.<br />
9
Figure 1.6 Flow separations on a<br />
bluff body (separation line<br />
perpendicular to the flow<br />
direction) [5]<br />
1.4 Aerodynamic forces on vehicles<br />
The air flow over a vehicle transmits an aerodynamic force to the vehicle through<br />
pressure and shear stress distribution acting on the surface of the vehicle. Pressure and<br />
shear stress act at every point on the body with pressure normal to the surface of the<br />
vehicle, the shear stress tangential to the surface. The net effect of the aerodynamic force<br />
includes drag D, lift L, side force component S, and various moments PM, RM, YM as<br />
shown in Figure 1.8 acting on a principal axis of a vehicle. Each one is described as<br />
follows.<br />
Figure 1.7 Flow separation on a<br />
bluff body with oblique blunt base<br />
(separation line at an angle to the<br />
flow direction) [5]<br />
10
Drag<br />
Figure 1.8 Aerodynamic force and moments acting on a vehicle [17]<br />
Drag is force acting on the surface of the vehicle by the flow in direction<br />
opposing the motion of the vehicle. The drag is the integral of local stream-wise<br />
component of normal (pressure) and tangential (skin friction) surface forces over all<br />
surface exposed to the stream. Direct evaluation of drag requires knowledge of the<br />
detailed stress distribution and also integrating the pressure distribution over the complex<br />
surface of the vehicle which is extremely difficult to obtain. But with the help of CFD<br />
detailed surface pressure distribution for a flow over an object can be easily obtained<br />
after the CFD set up is adequately validated.<br />
During the analysis of aerodynamics performance of two vehicles, comparing the<br />
drag and lift forces do not yield much. One vehicle can generate less drag or lift than<br />
other depending on test speed, density of air and projected frontal area of the vehicle.<br />
11
Thus the non-dimensional coefficient is introduced to compare aerodynamic<br />
performances of a vehicle. The non-dimensional drag coefficient is defined as<br />
Where:<br />
= Aerodynamic Drag Coefficient<br />
= Frontal Area of the Vehicle<br />
= Air Density<br />
= Total Wind Velocity<br />
According to Hucho [5], the contribution of the front body to drag is usually<br />
small, the rear shape of the vehicle contribute greatly to the aerodynamic drag because of<br />
the low pressure turbulent wake region is formed at the rear creating large pressure<br />
difference between the front and rear ends of the vehicle.<br />
Lift<br />
Aerodynamic lift is the component of aerodynamic force perpendicular to the free<br />
stream velocity. It is mainly created by the pressure difference on the top and bottom<br />
surface of a vehicle. Aerodynamic lift has a strong influence on driving stability and it is<br />
very important not to negatively affect it so that the vehicle remains stable. If<br />
aerodynamic lift increases too much then it will cause the vehicle wheels to have less<br />
traction force with the road, and this will cause the vehicle to become very unstable and<br />
risk rollover. The following equation represents aerodynamic lift Coefficient:<br />
12
where:<br />
Sideforce<br />
L = Lift Force<br />
= Lift Coefficient<br />
Sideforce is produced by the crosswind acting on the vehicle and under steady<br />
state wind conditions and the non dimensional side force coefficient is given by:<br />
Where:<br />
S = Sideforce acting on the vehicle<br />
= Sideforce Coefficient (Function of the Relative Wind Angle)<br />
Pitching moment<br />
Pitching moment affects the weight distribution between the front and the non<br />
dimensional pitching moment coefficient is:<br />
Where:<br />
= Pitching Moment Coefficient<br />
PM = Pitching Moment<br />
L = Wheelbase<br />
13
Yawing moment<br />
Crosswinds produce a side force on a vehicle that acts at the middle of the<br />
wheelbase. When the crosswinds do not act at the middle of the wheelbase a yawing<br />
moment is produced. The yawing moment coefficient is represented by the following<br />
equation:<br />
Where:<br />
= Yawing Moment Coefficient (Varies with Wind Direction)<br />
YM = Yawing Moment<br />
A = Frontal Area of the Vehicle<br />
L = Wheelbase<br />
Rolling moment<br />
When the crosswind produces a side force at an elevated point on a vehicle, a<br />
rolling moment is produced and the rolling moment coefficients varies with wind<br />
direction and it is represented by the following equation:<br />
Where:<br />
= Rolling Moment Coefficient<br />
RM = Rolling Moment<br />
A = Frontal Area of the Vehicle<br />
L = Wheelbase<br />
14
1.5 Fuel economy<br />
Fuel economy is the measure of how many miles a vehicle can travel in certain<br />
amount of fuel. In United States it is measured in mile per gallon. Fuel economy and<br />
increasing global warming are the current key arguments to reduce aerodynamic drag of<br />
vehicles.<br />
Vehicle fuel consumption is a matter of demand and supply [5]. On the demand<br />
side is the mechanical energy to propel the vehicle forward and on supply side is the<br />
efficiency with which the energy can be generated and transmitted through the power<br />
train to the point of application. Vehicle aerodynamics have a role on the demand side of<br />
the equation and lowering the aerodynamic drag lowers the Road load part of the tractive<br />
force needed to drive the car. The tractive force required at the tire/road interface of<br />
a car's driving wheels is defined as (Sovran and Bohn, [12]<br />
Where: is tractive force, R the tire rolling resistance, D the aerodynamic drag, M the<br />
vehicle effective mass, g the acceleration of gravity, θ is the inclination angle of the road.<br />
The rolling resistance of the vehicle, R is given by:<br />
(1.7)<br />
R= G (1.8)<br />
15
Where: G= mg the gravitational force the vehicle exerts on the road, is the coefficient<br />
of rolling resistance of the vehicle which needs to be determined experimentally and it<br />
depends on the speed of the vehicle as shown in Figure 1.9.<br />
Figure 1.9 versus road speed V for typical radial tires [5]<br />
The effective mass of the vehicle, M, is given by<br />
M= m (1+ ) (1.9)<br />
Where is the equivalent translational mass of the rotating parts of the power train of<br />
the vehicle? The mass fraction depends on the gear engaged and the suffix i denotes<br />
the gear engaged.<br />
The corresponding tractive power is:<br />
= *V (1.10)<br />
16
Where V= velocity of the car. And the tractive energy required for propulsion during any<br />
given driving period is:<br />
From the above equations, equation 1.8 to 1.12, if the drag force acting on the<br />
(1 .11)<br />
vehicle increase, the amount of energy needed to propel the vehicle through the air will<br />
also increase. This means that burning more fuel is needed.<br />
Fuel consumption of a road vehicle is a measure of volume of fuel consumed to<br />
travel a specific unit of distance. In Europe, fuel consumption of the vehicles is specified<br />
as liter of fuel consumed to travel 100 Km. However in USA different method is used to<br />
measure fuel economy; it is measured by the amount of miles a vehicle can travel with a<br />
gallon of fuel. These two methods can be related using Equation 1.12 as<br />
MPG=235.2/ (L/100KM)<br />
L/100Km =235.2/mpg (1 .12)<br />
Fuel consumption of a vehicle B [L/100km] can be evaluated analytically by<br />
integration the instantaneous fuel consumed [L/s] over a period of time T [s] and then<br />
averaging the integral over the distance travelled during the period of T[s].<br />
Where: V is the velocity of the vehicle.<br />
17<br />
(1 .13)
Since driving a vehicle on the road, involves acceleration, deceleration and idle,<br />
fuel consumption of the vehicle should be determined based on these three different<br />
modes of vehicle operations: power drive, Braking and Idle.<br />
During Powered drive, >0, the amount of fuel consumed is<br />
18<br />
(1 .14)<br />
where is the engine power required to drive vehicle accessories like air conditioning,<br />
is the density of fuel, is the specific fuel consumption also known as bsfc brake<br />
specific fuel consumption and typical bsfc maps for gasoline and diesel engine is shown<br />
in Figure [1.10] as below, is the efficiency of the drive train between the transmission<br />
input and the tire patch of the drive wheels.<br />
Figure 1.10 Typical bsfc maps for a gasoline and a diesel engine [5]
During breaking the < 0 and the total volume of fuel consumed is given by:<br />
Where: brake volume fuel rate.<br />
During idles the velocity of the vehicle V=0 and the amount of fuel consumed is:<br />
Where: is idle volume flow rate.<br />
By adding equations 1.15, 1.16 and 1.17the total fuel consumed B:<br />
To maintain uniformity in the process of determining the fuel consumption of<br />
19<br />
(1 .15)<br />
vehicles, a standard driving cycles has to be used. In U.S., fuel economy is determined<br />
based the EPA driving schedule which consists of Urban and highway driving cycles<br />
(1 .16)<br />
(1 .17)<br />
shown in Figure 1.11. Vehicles fuel economy is tested in a US EPA laboratory by placing<br />
the vehicle drive wheels on a dynamometer which simulate the EPA's driving schedule<br />
and measure the carbon content in the vehicles exhaust pipe to calculate the amount of<br />
fuel consumed during the test.
Figure 1.11 EPA driving cycle [5]<br />
To determine numerically the effects of improved aerodynamics on fuel economy<br />
by using Equation 1.17 is a complex task. Sovran and Bohn [12] developed a method to<br />
determine tractive energy equation for EPA urban and highway driving schedules. Later<br />
Sovran [13] used Equation 1.17 and tractive energy Equation 1.11 to developed charts<br />
that show the impact of changes in aerodynamic drag on composite fuel consumption for<br />
the EPA schedules. The composite fuel consumption for EPA driving schedules is given<br />
by equation 1.18.<br />
= (1 .18)<br />
20
Figure 1.12 shows G. Sovran [13] charts to determine the impacts of changes in<br />
aerodynamic drag on fuel consumption for EPA schedules given the change in the<br />
product of aerodynamic drag coefficient and frontal area of the vehicle ( A).<br />
Figure 1.12 G. Sovran charts for the impact of changes in aerodynamic drag on the fuel consumption<br />
for vehicles driving on the EPA schedules [5]<br />
21
2.1 Motivation<br />
Chapter 2<br />
BACKGROUND AND OBJECTIVE<br />
Most ground vehicle research has been performed on passenger automobiles, race<br />
cars and commercial truck tractor assembly. Research conducted on a pickup truck by<br />
large automakers was mainly for commercial use and the results are not accessible for<br />
researchers. However with advancement in computer and CFD tools institutional<br />
researchers are able to study the complex three-dimensional (3-D) turbulent flow<br />
structure around blunt bodies like pickup trucks.<br />
The pickup truck segment now accounts for about 15 percent of annual vehicle<br />
sales in the U.S. [9] and this indicates that pickup trucks have a larger weighting on the<br />
national oil consumption. Current pickup truck design has higher aerodynamic drag and<br />
exhibit suboptimal fuel economy. The pickup trucks in the market today have higher<br />
aerodynamic drag than other type of light vehicle with the same projected frontal area.<br />
For example, current production pickup trucks have aerodynamic drag coefficient in the<br />
range of 0.463-0.491 and in comparison the aerodynamic coefficient for typical SUV<br />
would be in the range of 0.414-0.44 [1].<br />
Previous research [19] suggests that drag coefficient for light trucks can be<br />
reduced. Reduction in drag has been shown to improve fuel economy by several miles<br />
per gallon on average. If all trucks were to improve their drag coefficients by this margin,<br />
billions of barrel of oil would be saved and also reduce carbon emission to the<br />
environment.<br />
22
2.2 Pickup truck history<br />
Pickup trucks have been around almost since the advent of the automobile. There<br />
was a Ford model TT that was sold in 1916. It has just been in recent years that the light<br />
duty truck, pickup trucks, SUV and vans, has gained a large market share in US. In 1990,<br />
47.5 million light trucks were registered in US and by year 2000 the number of light<br />
trucks registered were increased by 63.8% to 77.8 million [18]. Since 1975 pickup trucks<br />
account for a stable 13% vehicle sales fraction in US [16] and in 2005 there were 40<br />
million registered pickup trucks. The sales of pickup trucks are expected to be stable<br />
despite the current rise in fuel price. However this trend has not been translated to the<br />
level of effort placed on improving light truck aerodynamics although many<br />
improvements have been made from the initial Model-TT in 1916 shown in Figure 2.2.1.<br />
Figure 2.2.1 Ford Model-TT from 1916<br />
After 30 years of development, covered wheels and curved front appear in the<br />
ford trucks as shown in Figure 2.2.2, the Ford F-100.<br />
23
Figure 2.2.2 Ford F-100 from 1951<br />
The F-100 of 1966 was boxier and less aerodynamic but it provided the consumer<br />
with greater capacity in terms of payload and towing.<br />
Figure 2.2.3 Ford F-100 from 1966<br />
The 1997 Ford F-150 from was proclaimed (by all automotive journalists) to be<br />
the most aerodynamic light truck form to date. This may be obvious to the casual<br />
observer based upon its almost car-like curves. Ironically, the curved shape was cited as<br />
one of the reasons that Ford’s newest design lost market share, due to consumer<br />
preference for “tough” looking trucks.<br />
24
Figure 2.2.4 Ford F-100 from 1997<br />
The newer ford F100 2008-2009 model had improved aerodynamic design with<br />
better engines and better fuel management electronic systems. However, aesthetic feature<br />
gave a sturdy look to it.<br />
2.3 Previously conducted research<br />
Figure 2.2.5 Ford F-100 from 2008-2009<br />
Unlike researches on sedan and SUVs, only fewer publications of flow over<br />
pickup trucks are available to the public. Al-Garni, Bernal, and Khalighi conducted<br />
experiment to investigate the flow in the near wake of a generic pickup truck [2]. The<br />
experiment was conducted in a 2X2 wind tunnel at Aerospace Engineering Department at<br />
University of Michigan. They used PIV velocity measurement method to measure the<br />
25
turbulent flow in the near wake of a generic truck. The objective of their experiment was<br />
to provide qualitative data for CFD validation. Later Yang and Khalighi [1] conducted<br />
CFD simulations using the same vehicle models as those of Al-Garni, Bernal and<br />
Khalighi [2] to address the issue if the two-equation k-ε turbulence model could capture<br />
steady flow around the pickup truck. They compared the data from CFD simulations with<br />
excremental data collected form Al-Garni, Bernal and Khalighi‘s experiment [2] and<br />
stated that the steady state formulation was good enough to study vehicle aerodynamics.<br />
Cooper [3] investigated the effect of tail gate position at different yaw angles as<br />
well as the effect of different box configurations on aerodynamic drag of a pickup truck.<br />
He conducted a full scale test in National Research Council of Canada (NRC) wind<br />
tunnel and presented the results with CFD analysis to visualize the flow structure of<br />
tailgate up and tail gate off configuration at zero degree yaw angle.<br />
Recently, Mukhtar, Britcher and Camp [4] conducted experimental investigation<br />
and CFD simulation to analyze the flow around pickup truck with several configurations.<br />
Their objective was to determine the influence of these configurations on aerodynamic<br />
drag of the vehicle. They simulated the airflows at different yaw angles and the CFD<br />
results from the simulation were compared with the experimental data they obtained from<br />
a full scale experiment conducted at Langley full scale wind tunnel.<br />
2.4 Objective<br />
The objective of this thesis is to investigate the effect of add-on devices on a flow<br />
over a pickup truck. The primary tool that will be used to accomplish this will be<br />
26
computational fluid dynamics (CFD). In effort to reduce the aerodynamic drag of pickup<br />
trucks, aerodynamic add-on devices such as canopies, Rear Roof Garnish, Tail plates,<br />
Airdam and Aerocap will be mounted on the baseline pickup truck and the air flow will<br />
be simulated. This paper will quantify the effect of the aerodynamic accessories on the<br />
pickup truck aerodynamics through CFD modeling. Once general effects of the<br />
accessories have been quantified, the accessory that yields the best drag reduction will be<br />
optimized.<br />
2.5 Outlines<br />
The rest of chapters will be arranged as follows. The next chapter discusses CFD<br />
problem formulation and results from a flow over the baseline truck. The CFD result<br />
from present simulation was compared and validated against those from Yang and<br />
Khalighi [1]. In Chapter 4, the problem formulation developed in Chapter 3 were used to<br />
study flow over a pickup trucks with add-on devises: Tonneau cover, Rear Roof Garnish,<br />
tail plates, Airdam, Traditional canopy, Aerocap with rear inclination angel of<br />
5⁰,10⁰,12⁰,15⁰ and 18.77⁰. Also In Chapter 4, flow over 3D curved Aerocap was<br />
investigated to quantify the impact of drag reduction achieved by the 3D curved Aerocap<br />
on fuel economy. Chapter 5 presents conclusion and offers some recommendations for<br />
future research<br />
27
3.1 Introduction<br />
Chapter 3<br />
PROBLEM FORMULATI<strong>ON</strong><br />
Traditionally, wind tunnel and road tests are required to investigate the<br />
aerodynamics performance of the vehicles. However, Full-scale wind tunnel and road<br />
tests are time consuming and expensive to operate as multiple tests are usually required in<br />
achieving the desired aerodynamic shape or characteristic during the design process of<br />
vehicles.<br />
Aerodynamic evaluation of air flow over an object can be performed using<br />
analytical method or CFD approach. On one hand, analytical method of solving air flow<br />
over an object can be done only for simple flows over simple geometries like laminar<br />
flow over a flat plate. If air flow gets complex as in flows over a bluff body, the flow<br />
becomes turbulent and it is impossible to solve Navier- Stokes and continuity equations<br />
analytically. On the other hand, obtaining direct numerical solution of Navier-stoke<br />
equation is not yet possible even with modern day computers. In order to come up with<br />
reasonable solution, a time averaged Navier-Stokes equation was being used (Reynolds<br />
Averaged Navier-Stokes Equations – RANS equations) together with turbulent models to<br />
resolve the issue involving Reynolds Stress resulting from the time averaging process.<br />
With the reduction on computational cost today, aerodynamic simulation using<br />
CFD have a faster turnaround time and will only be at a small fraction of the cost of the<br />
wind tunnel or road tests. One can analyze the flow over vehicles by solving RANS<br />
28
equations and turbulence modeling equations and yet get a near realistic result. In present<br />
work the k-ε turbulence model with non-equilibrium wall function was selected to<br />
analyze the flow over the generic pickup truck model. This k-ε turbulence model is very<br />
robust, having reasonable computational turnaround time, and widely used by the auto<br />
industry. Since the main aerodynamics force acting on road vehicle is aerodynamic drag,<br />
this thesis project focuses on studying aerodynamic drag along with generated lift due to<br />
air flow over the vehicle at zero degree yaw angle.<br />
3.2 Aerodynamic drag on vehicles<br />
Aerodynamic drag is generated by the interaction of a solid body with a fluid<br />
which results in the difference in velocity between the solid object and the fluid. It can be<br />
regarded as aerodynamic resistance to motion of the object through the fluid medium. To<br />
reduce Aerodynamic drag of ground vehicle, it is very important to understand the source<br />
of aerodynamic drag for a flow over a vehicle which is described as follows:<br />
1. Skin friction: the interaction between the flowing air molecule and the solid<br />
object causes friction drag on the object. Skin friction is dominant on streamlined<br />
objects like airplane wing while pressure drag is dominant on bluff bodies.<br />
2. Boundary layer pressure loss: as the air flows over the body, boundary layer<br />
develops. The boundary layer is a thin layer over the body where the velocity of<br />
the flow varies from zero on the surface of the object to free flow velocity at the<br />
edge of the boundary layer. The viscous effect within the boundary layer is very<br />
important. Boundary layer gets thicker as it progress from the front to rear of the<br />
29
vehicle. The thicker boundary layer at the rear of the vehicle makes the rear<br />
stagnation pressure of the flow less than the front stagnation pressure, so there is<br />
effective pressure drop along the length of the body, which causes flow<br />
separation. For non-streamlined bluff bodies such as pickup trucks immersed in a<br />
flow, the flow separates from the body near sharp edges and creates a wake region<br />
of turbulence. Pressure will drop in the turbulence region, resulting in the pressure<br />
difference between the front and rear of the vehicle – the pressure the drag.<br />
Since blunt bodies have a larger rear area, they have larger pressure drag. For<br />
streamlined body, this term is less significant.<br />
3. Induced drag: when a body such as a vehicle spoiler is immersed in a flow it<br />
generates a lift which also induces drag. The drag on a body increases as lift<br />
increases. Thus minimum drag occurs when the lift on the body is zero. As road<br />
vehicle are bluff bodies in close proximity to the ground and the pressure<br />
difference between the under body and upper surface of the vehicle create lift<br />
which could induce drag.<br />
4. Interference drag: it is caused by imperfection on the body of the vehicle<br />
surfaces as windshield wipers, door handles.<br />
As mentioned previously, separation of the boundary layer and the ensuing<br />
turbulence complicates the problem dramatically. In White [7], it is demonstrated that a<br />
cylinder with a laminar separation oriented 82 degree relative to the free stream had a<br />
coefficient of drag of 1.2. The same cylinder has a coefficient of drag of 0.3 when the<br />
30
Reynolds number increased to allow the turbulent flow separation to occur at 120 degree,<br />
resulting in smaller wake and higher pressure at the rear, and thus reduced drag. The<br />
same premises of reducing the wake region and also increasing the pressure at the rear<br />
were used in this paper to improve the aerodynamic drag of the vehicle.<br />
Figure 3.1 Flow past a circular cylinder: (a) laminar separation; (b) turbulent separation; (c)<br />
theoretical and actual surface-pressure distribution, [7]<br />
3.3. CFD problem formulation<br />
The greatest benefit from computational fluid dynamics is to gain insight into a<br />
particular phenomenon by establishing the trends in the aerodynamic characteristics. It is<br />
valuable in understanding and exploiting the trends of shape change that will affect the<br />
31
flow field and improve the aerodynamic of the model. However, before the CFD model<br />
with add on devise can be designed and simulated, CFD method for flow over a generic<br />
pickup truck needs to be validated against CFD simulation of flow over the same generic<br />
model [1]. Yang and Khaligi’s [1] CFD simulation of flow over a pickup truck was<br />
reproduced and used as bench mark for the present CFD method, given that the results<br />
from CFD simulation [1] agreed with experimental data [2],.<br />
Figure 3.2 shows the generic pickup truck used by Yang and Khalighi [1] and the<br />
present CFD simulation. The full size generic pick up is 5.184m long, 1.824m wide,<br />
1.786 m high and with a projected frontal area of 2.809m 2 . The origin of the coordinate<br />
axis used in present simulation was attached to the bumper of the vehicle. The pickup<br />
truck box floor lies on Z-zero axis as the X axis lies along the length of the vehicle as<br />
seen on Figure 3.2. Figure 3.3 shows the 1/12 scale of the flow domain used in the<br />
present simulation. The virtual wind tunnel has dimension of 10.4m wide, 5.4m high and<br />
58m long. The virtual wind tunnel used by Yang and Khalighi [1] and by present CFD<br />
simulation had the same cross sectional area with area blockage ratio of 5%. However the<br />
length of the wind tunnel used in the study of Yang and Khalighi [1] is 23 m which is<br />
about 4.6L, where L is the length of the full size generic pick up. That leaves only 3.6L of<br />
the flow domain to be ahead and back of the generic pickup truck. These make the flow<br />
over the vehicle to be highly affected by inlet and outlet boundary condition set for the<br />
CFD simulation. Thus it is a good CFD practice to increase the length of the virtual wind<br />
tunnel. In present simulation, the length of the virtual wind tunnel was increased to 58m<br />
instead of 23m used in the study of Yang and Khalighi [1], which leaves 3.6 times the<br />
32
length of the vehicle (L) ahead of the model and 6.6L behind the model from the base of<br />
the vehicle.<br />
Figure 3.2 Original 1/12 th -scale generic pickup truck model used in [1] and [2].<br />
Figure 3.3 1/12th scale of flow domain used in present simulation, all dimensions are in mm.<br />
The virtual wind tunnel was scaled down by 12 and imported in to Gambit to<br />
create surface meshes on the vehicle and the virtual wind tunnel surfaces. A surface mesh<br />
of 1.5 mm size was created on the vehicle surface. On the ground face, a size function<br />
was used to vary the mesh size on the face from 1.5 mm to 30mm with a growth rate<br />
1.05. On the inlet, outlet, top and side faces of the virtual wind tunnel a uniform mesh<br />
33
size of 30 mm was used. The flow domain with the generated surface mesh was imported<br />
into the commercial volumetric meshing software TGrid to descretize the domain with a<br />
hybrid meshes. Prismatic layer was created over the vehicle surface to capture the<br />
boundary layer characteristics and a layer of tetra cell was created to connect the prism<br />
layer with hex core domain. The hex core cells were refined in a 1m long, 0.25m wide<br />
and 0.22 high box enclosing the scaled down pickup model. Further hex refinement was<br />
created between the floor of pickup truck and the ground face of the virtual wind tunnel.<br />
In present simulation, the flow domain was descretized with about 9 to 10 million hybrid<br />
cells.<br />
3.4 Baseline pickup truck CFD method and setup<br />
The CFD simulation by Yang and Khalighi [1] was reproduced in the present<br />
simulation. Table 3.1, Table 3.2, Table 3.3 and Table 3.4 shows the solver setup, viscous<br />
model and Turbulence model settings, boundary condition settings and solution controls<br />
for present simulation respectively. The Reynolds number of the air flow was Re=<br />
7.8* based the vehicle length L =5.184m. According to Yang and Khalighi [1], if the<br />
Reynolds number of the flow is above the critical Re= 8.56* ,based on the length of<br />
the model, the flow properties will be similar and one will be able to compare results<br />
from CFD simulation[1] with any Reynolds number above the critical Reynolds number.<br />
The assumptions made in present simulation were the air flow was steady state with<br />
constant velocity at inlet and with zero degree yaw angle, constant pressure outlet, no slip<br />
34
wall boundary conditions at the vehicle surfaces, and inviscid flow wall boundary<br />
condition on the top, sidewalls and ground face of the virtual wind tunnel.<br />
CFD Simulation 3ddp (3-D Double Precision)<br />
Solver<br />
Solver Segregated<br />
Space 3D<br />
Formulation Implicit<br />
Time Steady<br />
Velocity Formulation Absolute<br />
Gradient Option Cell-Based<br />
Porous Formulation Superficial Velocity<br />
Table 3.1 Solver setting<br />
Turbulence Model k-ε (2 eqn)<br />
k-epsilon Model Standard<br />
Near-Wall Treatment Enhanced wall Function<br />
Operating Conditions Ambient<br />
Table 3.2 Viscous model and Turbulence model settings<br />
35
Velocity<br />
Inlet<br />
Pressure<br />
Outlet<br />
Wall Zones<br />
Fluid<br />
Properties<br />
Boundary Conditions<br />
Magnitude (Measured normal to<br />
Boundary)<br />
22 m⁄s (constant)<br />
Turbulence Specification Method Intensity and Viscosity Ratio<br />
Turbulence Intensity 1.00%<br />
Turbulence Viscosity Ratio 20<br />
Gauge Pressure magnitude<br />
0 pascal<br />
Gauge Pressure direction normal to boundary<br />
Turbulence Specification Method Intensity and Viscosity Ratio<br />
Backflow Turbulence Intensity 10%<br />
Backflow Turbulent Viscosity Ratio 10<br />
- vehicle surface-noslip wall B/c<br />
- Ground face- invicisd wall B/C<br />
-Side faces -inviscid wall B/C<br />
Fluid Type<br />
Air<br />
Density ρ = 1.175 (kg⁄m^3 )<br />
Kinematic viscosity v = 1.7894×10^(-5) (kg⁄(m∙s))<br />
Table 3.3 Boundary condition settings<br />
Equations Flow and Turbulence<br />
• Pressure: Standard<br />
• Momentum: Second Order Upwind<br />
• Turbulence Kinetic Energy: Second Order Upwind<br />
Discretization • Turbulence Dissipation Rate: Second Order Upwind<br />
Monitor Residuals & Drag Coefficient<br />
Convergence<br />
Criterion<br />
- Continuity = 0.001<br />
- X-Velocity = 0.001<br />
- Y-Velocity = 0.001<br />
- k = 0.001<br />
- epsilon = 0.001<br />
Table 3.4 Solution Controls<br />
36
3.5 Baseline pickup truck results and discussion<br />
Figure 3.4a and Figure 3.5a shows the pressure coefficient plot on the symmetry<br />
plane from present simulation and that of Yang and Khalighi [1] respectively. The<br />
pressure coefficient plot shows that the stagnation point was created on the front surface<br />
of the pickup truck. The pressure coefficient also indicates that CFD simulations have a<br />
tendency to overshoot the Cp value at stagnation point. The Maximum Cp value obtained<br />
in present simulation was Cp= 1.01 and from Yang and Khalighi [1] the maximum<br />
pressure coefficient value was approximately Cp= 1.15 as shown in Figure 3.5a. These<br />
indicate that the present simulation was reasonably accurate in predicting the pressure<br />
distribution over the top surface of the vehicle.<br />
Figure 3.4b and Figure 3.5b show the pressure coefficient plot of the vehicle<br />
underbody on the symmetry from present simulation and the study of Yang and Khalighi<br />
[1]. Near the front end of the vehicle the pressure coefficient plots vary slightly but it was<br />
within acceptable error margin of less than 10%.<br />
1.5<br />
1<br />
0.5<br />
0<br />
-0.5<br />
-1<br />
-1.5<br />
-2<br />
-2.5<br />
0 100 200 300<br />
Figure 3.4 (a) Pressure on pickup cab (b) Pressure on pickup floor<br />
0<br />
-0.5<br />
-1<br />
-1.5<br />
-2<br />
-2.5<br />
0 100 200 300 400<br />
37
Figure 3.5 (a) Pressure on pickup cab from [1]. (b) Pressure on pickup floor from [1].<br />
Figure 3.6 and Figure 3.7 show pressure coefficient distribution on the tail-out<br />
and tail-in surface of the vehicle on symmetry plane, respectively. Both the pressure plots<br />
from the present simulation and that of Yang and Khalighi [1] were close to the<br />
experimental data obtained by Al-Garni, Bernal, and Khalighi [2] with an acceptable<br />
error margin. As seen in these figures, the pressure coefficient distribution on the outer<br />
tail gate surface was relatively higher than the pressure coefficient on the inside of the<br />
tailgate, which indicates if leaving the tail gate up it increases the pressure at the rear of<br />
the vehicle than the case of leaving the tailgate open. This findings confirms the<br />
conclusion made by Cooper “Pickup truck aerodynamics-keep your tailgate up” [3].<br />
38
60<br />
50<br />
40<br />
30<br />
20<br />
10<br />
0<br />
-0.09 -0.11<br />
-10<br />
Cp-Tailgate-O<br />
uter<br />
simulated<br />
expermental<br />
-0.13<br />
-0.15<br />
-0.17<br />
-0.19<br />
Figure 3.6 (a) Pressure on tailgate (outside). (b) Pressure on the tailgate (outside) from [1]<br />
Figure 3.7 (a) Pressure on tailgate (inside). (b) Pressure on the tailgate (inside) from [1]<br />
Figures 3.8, 3.9, 3.10, and 3.11 show the u-velocity plots at points inside and<br />
outside of the pickup box from present simulation and that of Yang and Khalighi [1]. ,<br />
they match very well with nearly identical plots. Figure 3.12 shows static pressure<br />
distribution over the pickup truck surfaces, indicating that pressure was very high on the<br />
grill of the vehicle where the velocity of the flow becomes zero and stagnation point was<br />
created. Figure 3.12 also shows relatively high static pressure created at the junction of<br />
39
the windshield with the hood of the vehicle. Both front and rear tires also experience high<br />
static pressure but the front wheels were subjected to slightly higher static pressure than<br />
the rear. On the sharp edges of the vehicle with the A-pillar, the edges of the hood, grill<br />
junctions with side-frame and edges of the wind shield, flow separation was expected to<br />
occur and the static pressure was low. The pressure difference created between the front<br />
and rear end of the vehicle causes the net aerodynamic force acting on the vehicle to<br />
generate a drag against the motion of the vehicle. Figure 3.13 shows the wake profile for<br />
baseline truck (velocity vector on iso-velocity surface at 3m/s), indicating that turbulent<br />
wake was formed inside the box and also behind the truck.<br />
140<br />
120<br />
100<br />
80<br />
60<br />
40<br />
20<br />
0<br />
U, x=400mm ,y=0<br />
-0.6 -0.1 0.4 0.9 1.4<br />
Figure 3.8 (a) u-velocity in y=0 plane (inside box). (b) u-velocity in y=0 plane (inside box) from [1].<br />
40
140<br />
90<br />
40<br />
-10<br />
-60<br />
0 0.5 1<br />
Figure 3.9 (a) u-velocity in y=0 plane (outside box). (b) u-velocity in y=0 plane (outside box) from [1].<br />
Figure 3.10 (a) u-velocity for z=73mm and x=450mm (scaled down model) (b) u-velocity for z=73mm<br />
and x=450mm from [1].<br />
U,x=500mm,y=0<br />
U,x=450mm,z=73mm<br />
1.2<br />
1<br />
0.8<br />
0.6<br />
0.4<br />
0.2<br />
0<br />
-120 -80 -40 0 40 80 120<br />
41
Figure 3.11 (a) u-velocity for z=15mmand x=450mm (scaled down model) (b) u-velocity for z=15mm<br />
and x=450mm from [1].<br />
U,x=450mm,z=15mm<br />
1.2<br />
-120 -70<br />
0<br />
-0.2 -20 30 80<br />
1<br />
0.8<br />
0.6<br />
0.4<br />
0.2<br />
Figure 3.12 Pressure distributions over the pickup<br />
Figure 3.13 Wake profile for baseline<br />
truck (velocity vector on iso-velocity<br />
surface at 3m/s)<br />
Figures 3.14(a) and (b) compare the velocity magnitude vectors at z =73mm for a<br />
1/12-scale vehicle mode from present simulation with that of Yang and Khalighi [1]. The<br />
stream lines appear to be identical with the wake created in the pickup box. Figures<br />
3.15(a) and (b) compare the velocity magnitude vectors in the symmetry plane from<br />
42
present simulation with that of Yang and Khalighi [1]. The vectors indicate the flow<br />
separation occurring at the rear edge of the cab and the vortex created in the box of the<br />
truck. It also indicates the downwash created at the outer edge of the tailgate behind the<br />
truck.<br />
Figure 3.14 (a) Streamline on z=73 mm (scaled down model) plane. (b) Streamline on z=73mm plane<br />
from [1].<br />
Figure 3.15 (a) Streamline on symmetry plane. (b) Streamline on symmetry plane from [1]<br />
Figure 3.16 shows the static pressure distribution on the symmetry plane and on<br />
the surface of the pickup truck, indicating that pressure dooms were created in front of<br />
the vehicle and the maximum pressure was created on the front vehicle surface near the<br />
bumper. The figure also shows that the low pressure was created in the pickup box and<br />
43
also over the cab of the vehicle, which tends to increase the drag and lift coefficient of the<br />
baseline truck. Figure3.17 shows the total pressure distribution in the symmetry plane and<br />
over the surface of the truck, indicating a high total pressure gradient region where the<br />
flow separates with the flow recirculation created.<br />
Figure 3.16 Static pressure distributions over the baseline truck and symmetry plane<br />
Figure 3.17 Total pressure distributions over the baseline truck and symmetry plane<br />
44
Figure 3.18 shows the velocity streamline around the pickup truck. The<br />
streamlines are generated using a horizontal rack line located upstream the vehicle. Due<br />
to interaction between the shear layer surrounding the separation region and the flow<br />
around the vehicle, a strong recirculation region was generated and two contra rotating<br />
voices were formed behind the vehicle.<br />
Figure 3.18 Streamline flow over the baseline pickup truck<br />
The aerodynamic drag and lift coefficients computed from the simulation were<br />
= 0.345 and = 0.28 respectively. However, in the real world pickup trucks<br />
manufactured today have a drag coefficient at = 0.463 ~ 0.491 [1]. The drag<br />
coefficient from CFD simulation was predicted less than the real life drag coefficient of<br />
pickup trucks. This phenomenon was also observed by Mukhtar, Britcher and Camp [4],<br />
when they conducted CFD simulation on generic model of the pickup truck used in their<br />
experimental investigation. These might be due to the fact that the generic pickup model<br />
lacks accessories such as side mirror and windshield wipers. Also in the case of the<br />
45
generic pickup model there were no exposed axles, underbody, radiator cooling vents and<br />
many cavities on the surface of the vehicle that connects the inside of the vehicle to the<br />
flow.<br />
3.6 Summary<br />
CFD Simulation for Flow over Pickup Trucks conducted by Yang and Khalighi<br />
[1] was reproduced in present study. The same generic pickup truck model was used in<br />
present simulations by using a virtual wind tunnel that had the same cross section area as<br />
the one used in [1]. The length of the virtual wind tunnel used in [1] is only 23m, which<br />
is about 4.4 times the length of the full-size pickup truck. However the virtual wind<br />
tunnel used in present simulation is 58m long about 11 times the length of the full size<br />
truck which is 5.184m. The reason to increase the length of the wind tunnel was to make<br />
sure the flow over the pickup model would not be affected by inlet and outlet boundary<br />
conditions imposed on the inlet and outlet of the flow domain.<br />
After surface meshes were generated and boundary zones were defined on the<br />
surfaces of the flow domain in GAMBIT, the flow domain were imported in to<br />
volumetric meshing software TGRID to generate hybrid mesh. The meshed file was<br />
imported into FLUENT for simulations. A realizable k-ε turbulence model with non-<br />
equilibrium wall function was selected to solve the Reynolds averaged Navier-stokes<br />
equation in Fluent. The flow was assumed to be steady and incompressible with uniform<br />
inlet velocity of 22m/s and turbulence intensity of 1%. The results from present CFD<br />
simulation of flow over the generic pickup truck were compared with those of Yang and<br />
46
Khalighi [1]. Results were presented as pressure coefficient plot, u-velocity plots and<br />
velocity magnitude vector streamlines. The pressure coefficient and u-velocity plots<br />
shown in Figure 3.4 to Figure 3.11 indicate the present CFD simulation of flow over<br />
pickup truck was in good agreement with that of Yang and Khalighi [1]. The velocity<br />
magnitude vectors shown in Figure 3.14 and Figure 3.15 also confirm the present<br />
simulation was properly validated.<br />
47
4.1 Pickup truck model with Tonneau cover<br />
Chapter 4<br />
STUDY <strong>OF</strong> <strong>ADD</strong>-<strong>ON</strong> <strong>DEVICES</strong><br />
The cargo box of the base line truck was covered with flat wall under a boundary<br />
condition similar to Tonneau cover as shown in Figure 4.1.1. This truck with Tonneau<br />
cover was simulated using CFD. By comparing the pressure distribution plot on the<br />
symmetry plane of the pickup truck with Tonneau cover, shown in Figure 4.1.2(a), with<br />
that of the baseline model in Figure 4.1.2(b) it indicates that the pressure distribution plot<br />
over the rear end of the Tonneau cover is larger than the pressure distribution plot over<br />
the under body of the vehicle. This causes a reduction on lift force in the case of the<br />
model with Tonneau cover. Pressure distributions in symmetry plane over the under body<br />
of the vehicle are similar for both cases.<br />
Figure 4.1.1 Pickup truck with Tonneau cover<br />
Tonneau<br />
Cover<br />
48
The Cp plot over the rear end of<br />
the Tonneau cover is higher than<br />
that of the under body<br />
Figure 4.1.2 (a) Pressure coefficient plot in the symmetry plane for pickup truck with Tonneau cover<br />
Figure 4.1.2 (b) Pressure coefficient plot in the symmetry plane for baseline truck<br />
49
Figure 4.1.3 shows the static pressure distribution over the truck with Tonneau<br />
cover on the symmetry plane. By comparing Figure 4.1.3 with Figure3.16, it shows that<br />
the static pressure at cab rear of the vehicle with Tonneau cover is about -1.02*<br />
Pascal and it is higher than the static pressure of -1.32* Pascal for the baseline truck.<br />
This contributes to reduce the lift and drag coefficient of the model with Tonneau cover.<br />
Similarly, the total pressure behind the cab of the truck with Tonneau cover is about -<br />
6.39* Pascal as shown in Figure 4.1.4 and it is higher than that of the baseline truck,<br />
which is -8.71* Pascal as shown in Figure 3.17, signifying a reduced aerodynamic<br />
drag and lift in the case of the pickup model mounted with Tonneau cover.<br />
Figure 4.1.3 Static pressure distribution over pickup truck with Tonneau cover and symmetry plane<br />
50
Figure 3.16 Static pressure distributions over the baseline truck and symmetry plane<br />
Figure 4.1.4 Total pressure distribution over pickup truck with Tonneau cover and symmetry plane<br />
51
Figure 3.17 Total pressure distributions over the baseline truck and symmetry plane<br />
Figure 4.1.5 shows the velocity magnitude vector on the symmetry plane for air<br />
flow over the pickup truck with Tonneau cover, indicating a small three dimensional flow<br />
circulation formed over the Tonneau cover and behind the truck. By comparing the<br />
velocity magnitude vectors in Figure 4.1.5 with that of the baseline truck shown in Figure<br />
3.15, it indicates that the size of the circulation area behind the cab decreased for the<br />
model with Tonneau cover.<br />
52
Figure 4.1.5 Velocity magnitude vector over symmetry plane for pickup with Tonneau cover<br />
Figure 3.15 (a) Velocity magnitude vectors over the symmetry plane for the base line truck<br />
Figure 4.1.6 shows the wake profile for the vehicle with the Tonneau cover. By<br />
comparing the wake profile of the truck with Tonneau cover in Figure 4.1.6 with that of<br />
baseline truck in Figure 3.13, the wake region appears to be smaller for the pickup truck<br />
with Tonneau cover on both locations, where, one location is right behind the cab and the<br />
other is right behind the truck.<br />
53
Figure 4.1.6 Wake profile over a pickup truck with Tonneau cover (velocity vector on iso-velocity<br />
surface at 3m/s).<br />
Figure 3.13 Wake profile for baseline truck (velocity vector on iso-velocity surface at 3m/s)<br />
Overall effect of Tonneau cover on drag and lift is summarized in Table 4.1.1. It<br />
indicates that the pickup truck fitted with Tonneau cover has reduction of aerodynamic<br />
drag coefficient by 1.16% and of lift coefficient by 6.64% when compared with<br />
the baseline truck model.<br />
54
Configurations<br />
Drag<br />
Coefficient<br />
% diff. from<br />
baseline<br />
Lift<br />
Coefficient<br />
% diff. from<br />
Baseline<br />
Baseline 0.3453 0 0.2193 0<br />
Tonneau Cover 0.3413 -1.158412974 0.1828 -16.64386685<br />
Table 4.1.1 Comparison of drag and lift coefficient of baseline pickup truck model with a model<br />
fitted with Tonneau cover.<br />
4.2 Pickup truck model with Rear Roof Garnish<br />
A Rear Roof Garnish, 15cm long, was attached to the rear of the cab at an<br />
inclination angle of 12° as shown in Figure 4.2.1. It was expected that the Rear Roof<br />
Garnish will delay the separation of flow that normally occurs at the rear edge of the roof.<br />
It will also direct the air flow over the box to the edge of the tailgate.<br />
Rear Roof<br />
Garnish<br />
Figure 4.2.1 Pickup truck with the attached Rear Roof Garnish.<br />
Figure 4.2.2(a) shows the pressure coefficient distribution over the pickup model<br />
with Rear roof garnish. By comparing Figure 4.2.2(a) with Figure 4.2.2(b), the plots of<br />
pressure coefficient Cp indicate that the pressure on the top surface of the Rear Roof<br />
Garnish suddenly decreases. This tends to increase the pressure difference between the<br />
55
pickup underbody and top surfaces, which causes more lift in the case of pickup model<br />
with Rear Roof Garnish.<br />
Top surface<br />
Floor surface<br />
Figure 4.2.2(a) Pressure coefficient plot on symmetry plane for flow over a pickup truck with Rear<br />
Roof garnish<br />
Top surface Floor surface<br />
Figure 4.2.2(b) Pressure coefficient plot on top and floor surfaces of the base line truck in the<br />
symmetry plane<br />
Figure 4.2.3 shows the pressure distribution on the surface of the truck with Rear<br />
Roof Garnish in the symmetry plane. On the top surface of the Rear Roof Garnish, the<br />
pressure is about -1.24* Pascal and from the pressure distribution over baseline truck<br />
56
in Figure 3.16, the pressure at the rear of the cab is about -8.36* Pascal, which is<br />
higher than that of the pick truck mounted with Rear Roof Garnish. This indicates that the<br />
Rear Roof Garnish tends to increase the lift force on the vehicle. Figure 4.2.4 shows the<br />
total pressure distribution over the model with Rear Roof Garnish in the symmetry plane,<br />
indicating that the total pressure drop occurs in the box of the truck as well as in the<br />
region behind the truck.<br />
Figure 4.2.3 Static pressure contour over the pickup with Rear Roof Garnish and symmetry plane<br />
57
Figure 4.2.4 Total pressure contour over the pickup with Rear Roof Garnish and symmetry plane<br />
Figure 4.2.5 shows the velocity magnitude vector in the symmetry plane for<br />
airflow over the pickup truck attached with Rear Roof Garnish. By comparing velocity<br />
magnitude vector shown in Figure 4.2.5 with that of the baseline model in Figure 3.15, it<br />
appears that the flow circulation in the box is similar, except for the region very near to<br />
the Rear Roof Garnish. Comparison between the wake profiles in Figure 4.2.6 and Figure<br />
3.13 indicates that the wake region in the box of pickup model with Rear Roof Garnish is<br />
relatively smaller in size than that of the baseline truck. Table 4.2.1 presents the overall<br />
effect of using read roof garnish. It shows that by attaching Rear Roof Garnish to the<br />
baseline truck model, aerodynamic drag coefficient was reduced by about 2.4%;<br />
howeve,r the lift coefficient was increased by about 33%.<br />
58
Figure 4.2.5 Velocity magnitude vector for a pickup truck with Rear Roof Garnish on the symmetry<br />
plane<br />
Figure 4.2.6 Wake profile over a pickup truck with Rear Roof Garnish (velocity vector on isovelocity<br />
surface at 3m/s).<br />
Configurations<br />
Drag<br />
Coefficient<br />
% diff. from<br />
baseline<br />
Lift<br />
Coefficient<br />
% diff.<br />
from Baseline<br />
Baseline 0.3453 0 0.2193 0<br />
Rear Roof<br />
Garnish 0.337 -2.403706922 0.2916 32.96853625<br />
Table 4.2.1 Comparison of drag and lift coefficient of baseline pickup truck model with a model<br />
attached with Rear Roof Garnish.<br />
59
4.3 Pickup truck model with Tail plates<br />
In order to decrease the velocity of air flow from the underbody to the rear of the<br />
vehicle, a diffuser type tail plate was mounted at the rear of the vehicle as shown in<br />
Figure 4.3.1. A half foot long plate was attached to the floor of the vehicle and a 5cm<br />
long plate was attached to the top outer edge of the tailgate, both at 12 degree angle<br />
inclination.<br />
Figure 4.3.1 Pickup truck with attached Tail plates<br />
By comparing the static pressure in Figure 4.3.2 with that in Figure 3.16, it is seen<br />
that the static pressure acting on the tail gate of the base line truck is about -3.55*<br />
Pascal which have a suction effect at the rear of the vehicle. However, the static pressure<br />
on the tail gate of the pickup truck with tail plates is about 4.52 Pascal. This indicates<br />
that, in the case of model with the tail plates, the pressure difference between the front<br />
and rear end of the truck is smaller than that of the baseline truck, contributing to the<br />
reduction of drag force acting on the vehicle. Figure 4.3.3 shows the total pressure<br />
Tail<br />
Plates<br />
contour over the model with the tail plates and it can be read that the total pressure on the<br />
60
tail gate is -1.28* Pascal and from the total pressure contour of baseline truck in<br />
Figure 3.17 the pressure on the tail gate of the baseline truck is about -4.58* Pascal.<br />
This indicates the rise of total pressure behind the tailgate of the model with tail plates.<br />
Figure 4.3.2 Static pressure distribution over model with tail plates and symmetry plane<br />
Figure 4.3.3 Total pressure distribution over model with tail plates and symmetry plane<br />
61
Figure 4.3.4 shows the velocity magnitude vector in the symmetry plane for air<br />
flow over the truck with tail plates, indicating that the underbody flow was deflected<br />
upwards and the velocity of the downwash flowing over the edge of the tailgate was<br />
reduced. This tends to increase the static pressure behind the tailgate which contributes<br />
positively to the reduction of drag force acting on the vehicle. By comparing the wake<br />
profile over the truck with tail plates in Figure 4.3.5 with wake profile of the baseline<br />
truck in Figure 3.13, the wake profile seems to be similar in the box but behind the truck<br />
the wake profile in case of the model with tail plates become longer and flatter. The<br />
overall effect of tail plated is summarized in Table 4.3.1. It indicates that, by attaching a<br />
tail plate to the baseline model, a reduction of aerodynamic drag coefficient by 3.48%<br />
and lift coefficient by 40.54% was achieved.<br />
Figure 4.3.4 Velocity magnitude vector on the symmetry plane for model with tail plates<br />
62
Figure 4.3.5 Wake profile over pickup truck with tail plates (velocity vector on iso-velocity surface at<br />
3m/s)<br />
Configurations<br />
Drag<br />
Coefficient<br />
% diff. from<br />
baseline<br />
Lift<br />
Coefficient<br />
% diff.<br />
from Baseline<br />
Baseline 0.3453 0 0.2193 0<br />
Tail Plates 0.3333 -3.475238923 0.1304 -40.5380757<br />
Table 4.3.1 Comparison of drag and lift coefficient of baseline pickup truck with a model attached<br />
with Tail plates.<br />
4.4 Pickup truck model with Airdam<br />
Figure 4.4.1 shows the pickup truck mounted with airdam that has 6 in clearance<br />
from the ground. Two airdam configurations, one with 6 in clearance from the ground<br />
and the other with 3 in clearance, were used to investigate the effect of airdam on the<br />
aerodynamic drag of the vehicle. The aim to attach an airdam was to reduce the drag<br />
coming from the underside of the vehicle with the premise that by reducing the air speed<br />
under the vehicle it is likely to minimize the contribution of the underbody flow to the<br />
overall drag. However, the front projected area of the vehicle is increased with the airdam<br />
63
attached and this could increase the drag. Therefore, careful attention was required to<br />
achieve the desired net effect.<br />
Airdam with 6in<br />
clearance from the<br />
ground<br />
Figure4.4.1 Pickup truck with Airdam<br />
Figures 4.4.2 shows the pressure coefficient plots in the symmetry plane for the<br />
truck mounted with airdam; Figure 4.4.2(a) is for the case having a 3-in clearance from<br />
the ground and Figure 4.4.2(b) is for the case having 6in clearance from the ground<br />
respectively. Comparison between Figures 4.4.2 (a), Figure 4.4.2(b) and Figure 4.1.2 (b)<br />
indicates that the stagnation area in the model with airdams is longer along the X-axis.<br />
Also in the case of the airdam with 3 in clearance from the ground, the Cp plot in Figure<br />
4.4.2(a) shows that the Cp plot in the box has a higher value than the Cp plot over the<br />
underbody. This will lower the lift force acting on the model.<br />
64
The Cp plot in the box is higher<br />
than that of the under body<br />
Figure4.4.2 (a) Pressure coefficient plot over a model with Airdam (3in clearance from the ground)<br />
Figure 4.4.2(b) Pressure coefficient plot over model with Airdam (6in clearance from the ground)<br />
Figure 4.1.2(b) Pressure coefficient plot over baseline pickup in the symmetry plane<br />
65
Figures 4.4.3 shows the pressure contour over the vehicle with airdam. By<br />
comparing the model case of having airdams with that of baseline truck in Figure 3.12, it<br />
indicates that in the frontal area over which stagnation of flow occurs, the pressure is<br />
larger when the airdams are used. This tends to increase the drag force acting on the<br />
vehicle.<br />
Figure4.4.3 (a) Pressure contour over pickup with Airdam (3in clearance from the ground)<br />
Figure4.4.3 (b) Pressure contour over pickup with Airdam (6in clearance from the ground)<br />
66
Figure 3.12 Pressure distributions over the pickup<br />
Table 4.4.1 shows drag and lift coefficient reduction achieved by the Airdam.<br />
Airdam with 6 inch clearance from the ground increased drag of the model. But, airdam<br />
with 3 inch clearance from the ground have reduced drag by 0.35% and the drag<br />
reduction is very small to merit the cost and the risk of bumping onto objects on the road.<br />
On the other hand, the lift reduction coefficient achieved by employing airdam with 3in<br />
clearance from the ground is 326.45% and from airdam with 6in clearance is 36.48%.<br />
This indicates airdams are very effective in reducing lift force acting on a vehicle and<br />
should be employed on a race car to increase traction and handling while maneuvering<br />
curves or slippery roads.<br />
67
Configurations<br />
Drag<br />
Coefficient<br />
% diff. from<br />
baseline<br />
Lift<br />
Coefficient<br />
% diff. from<br />
Baseline<br />
Baseline 0.3453 0 0.2193 0<br />
Airdam -3in 0.3441 -0.35 -0.4966 -326.45<br />
Airdam-6in 0.3661 6.03 0.1393 -36.48<br />
Table 4.4.1 Comparison of drag and lift coefficient of baseline pickup truck with a model attached<br />
with Airdam-3in and Airdam-6in.<br />
4.5 Pickup truck model with Traditional Canopy<br />
The baseline pickup truck was mounted with a traditional canopy as shown in<br />
Figure 4.5.1. The air flow was then simulated to investigate the flow structure around the<br />
vehicle. The traditional canopy was also used as reference for the design of aerocap.<br />
Figure 4.5.1 Pickup truck with traditional canopy<br />
Figure 4.5.2 shows the pressure coefficient plots in the symmetry plane for the<br />
model with traditional canopy. By comparing with the case of baseline truck as shown in<br />
Figure3.4, it indicates that near the rear of the vehicle the pressure over the top surface of<br />
the canopy is higher than the underbody. Thus the truck with the traditional canopy will<br />
have lesser lift force than the baseline truck.<br />
68
Figure 4.5.2 Pressure coefficient plot on symmetry plane for model with Traditional Canopy<br />
Figure 4.5.3 shows the static pressure contour over model with canopy. By<br />
Comparing the static pressure at the base of the vehicle with canopy with the pressure at<br />
the rear of the cab and tail of the baseline truck in Figure 3.16, it indicates that the<br />
pressure at the base of the truck with the canopy is -3.44* Pascal and it is higher than<br />
that of the baseline truck which equals to -8.36* Pascal. This contributes to reducing<br />
the pressure difference between the front and base of the model with canopy and results<br />
in lesser drag.<br />
Figure 4.5.3 Static pressure distribution over model with Traditional Canopy<br />
69
Figure 4.5.4 shows the wake profile behind the truck with canopy, indicating that<br />
the wake region behind the base is larger than that of the baseline truck shown in<br />
Figure3.13. In case of the model with traditional canopy, flow separation occurs at the<br />
top edge of the base while for the baseline truck the flow separation occurs at the rear<br />
edge of the roof.<br />
Figure 4.5.4 Wake profile behind the pickup truck with traditional canopy (velocity vector on isovelocity<br />
surface at 3m/s)<br />
Table 4.5.1 summarizes the overall effect of traditional canopy on the drag and<br />
lift. It can be seen that the computed drag coefficient for pickup truck mounted with<br />
traditional canopy was = 0.3157, a reduction of 8.57% when compared to baseline truck.<br />
Configurations Drag Coefficient % diff. from baseline<br />
Baseline 0.3453 0<br />
Traditional canopy 0.3157 - 8.57<br />
Table 4.5.1 Comparison of drag and lift coefficient of baseline truck model with a model attached<br />
with Traditional canopy.<br />
70
4.6 Pickup truck model with Aerocap<br />
Figure 4.6.1 shows the pickup truck fitted with Aerocap attached to the box of the<br />
baseline truck. The aim of using Aerocap is to improve the flow structure around the<br />
vehicle so as to reduce aerodynamic drag ( ). CFD simulation of the air flow over the<br />
model with Aerocap was conducted under the setting that the rear inclination angle is<br />
varied for α=5⁰,10⁰,12⁰,15⁰ and 18.77⁰. The size of the wake region behind the vehicle is<br />
determined by the pressure and velocity relationship which depends on the rear<br />
inclination angle α. The optimum rear inclination angle should increase the static pressure<br />
at the rear end while the flow remain attached the vehicle surfaces.<br />
Figure 4.6.1 Pickup truck model with Aerocap of a rear inclination angle α= 10°<br />
Figure 4.6.2 only shows the pressure coefficient plot over the symmetry plane for<br />
rear inclination angle α=5⁰,12⁰ and 18.77⁰. The plots indicates as rear inclination angle α<br />
increase the pressure near the top edge of the inclined face of aerocap decreases while<br />
the pressure plots remain similar over the rest of the surfaces.<br />
71
Figure 4.6.2 Pressure coefficient plot in symmetry plane over model with Aerocap at different α<br />
Figures 4.6.3, 4.6.4, 4.6.5, 4.6.6, and Figure 4.6.7 shows the total pressure contour<br />
in the symmetry plane and over the surface of the pickup model with Aerocap when the<br />
rear inclination angle is specified as α=5⁰, 10⁰, 12⁰, 15⁰ and 18.77⁰ respectively. It<br />
indicates that a lower total pressure area was created at the base of the model when<br />
Aerocap was set with different inclination angles. The total pressure contours also<br />
indicate that as the rear inclination angle α increases the region with total pressure<br />
gradient at the base of the model decreases. This region is also associated with the size of<br />
the wake region.<br />
72
Figure 4.6.3 Total pressure on symmetry plane when rear inclination angle α=5°<br />
Figure 4.6.4 Total pressure on symmetry plane when rear inclination angle α=10°<br />
73
Figure 4.6.5 Total pressure on symmetry plane when rear inclination angle α=12°<br />
Figure 4.6.6 Total pressure on symmetry plane when rear inclination angle α=15°<br />
74
Figure 4.6.7 Total pressure on symmetry plane when rear inclination angle α=18.77°<br />
Figure 4.6.8 to Figure 4.6.12 shows the static pressure contour in the symmetry<br />
plane and over the surface of the model truck with Aerocap when the rear inclination<br />
angles α=5⁰, 10⁰, 12⁰, 15⁰ and 18.77⁰ are specified, respectively. The pressure contours<br />
indicate that as the rear inclination angle α increases the pressure on the cab roof<br />
decreases. This tends to increase lift as the rear inclination angle α increases.<br />
75
Figure 4.6.8 Pressure on symmetry plane when rear inclination angle α=5°<br />
Figure 4.6.9 Pressure on symmetry plane when rear inclination angle α=10°<br />
76
Figure 4.6.10 Pressure on symmetry plane when rear inclination angle α=12°<br />
Figure 4.6.11 Pressure on symmetry plane when rear inclination angle α=15°<br />
77
Figure 4.6.12 Pressure on symmetry plane when rear inclination angle α=18.77°<br />
Figure 4.6.13 to Figure 4.6.17 shows the velocity magnitude streamline for the<br />
rear inclination angle α=5⁰, 10⁰, 12⁰, 15⁰ and 18.77⁰, respectively. The streamlines<br />
indicate that turbulent wake region develops at the base of the vehicle have two vertices,<br />
one on top and the other in the wake region. When the rear inclination angle reaches<br />
α=10⁰ the two vertices are of similar size as shown in Figure 4.6.14. However, as the rear<br />
inclination angle increases the vortex on the top became larger than the one in the bottom<br />
and the center the bottom vortex slightly moves towards the tail of the vehicle. The<br />
stream lines also shows that as the rear inclination angle increases the height and the<br />
length of the wake region decrease.<br />
78
Figure 4.6.13 Velocity magnitude path line on symmetry plane when rear inclination angle α=5°<br />
Figure 4.6.14 Velocity magnitude path line on symmetry plane when rear inclination angle α=10°<br />
79
Figure 4.6.15 Velocity magnitude path line on symmetry plane when rear inclination angle α=12°<br />
Figure 4.6.16 Velocity magnitude path line on symmetry plane when rear inclination angle α=15°<br />
80
Figure 4.6.17 Velocity magnitude path line on symmetry plane when rear inclination angle α=18.77°<br />
Figure 4.6.18 to Figure 4.6.22 shows the wake profile for air flow over pick up<br />
with Aerocap at inclination angle α=5⁰, 10⁰, 12⁰, 15⁰ and18.77⁰, respectively. These<br />
figures indicate that as the rear inclination angle α increases the size of the wake region<br />
behind the vehicle decreases. For Aerocap with rear inclination angle α= 5⁰, 10⁰ and 12⁰,<br />
there is a formation of horse-shoe shaped vortices at the rear of the vehicle, however, as<br />
the rear inclination angle increases to the angle α=15⁰ and 18.77⁰, these horse-shoe<br />
shaped vertices are not present.<br />
81
Figure 4.6.18 Wake profile behind the pickup truck with Aerocap when rear inclination angle α=5°<br />
(velocity vector on iso-velocity surface at 3m/s)<br />
Figure 4.6.19 Wake profile behind the pickup truck with Aerocap when rear inclination angle α=10°<br />
(velocity vector on iso-velocity surface at 3m/s)<br />
82
Figure 4.6.20 Wake profile behind the pickup truck with Aerocap when rear inclination angle α=12°<br />
(velocity vector on iso-velocity surface at 3m/s)<br />
Figure 4.6.21 Wake profile behind the pickup truck with Aerocap when rear inclination angle α=15°<br />
(velocity vector on iso-velocity surface at 3m/s)<br />
83
Figure 4.6.22 Wake profile behind the pickup truck with Aerocap when rear inclination angle<br />
α=18.77° (velocity vector on iso-velocity surface at 3m/s)<br />
Table 4.6.1 and Figure 4.6.23 show the comparison of drag and lift coefficient for<br />
pickup truck model with Aerocap at rear inclination angle α= 5⁰, 10⁰, 12⁰, 15⁰ and<br />
18.77⁰. Compared to the baseline model, pickup truck with the entirely studied Aerocap<br />
configurations have a reduction in drag coefficient. The drag coefficient for model with<br />
aerocap decreases quickly for the rear inclination angle between 5⁰ and ⁰10 and it slightly<br />
decrease when the rear inclination angle is between 10⁰ and 12⁰. However, when the rear<br />
inclination angle is greater than 12⁰ the drag coefficient increases dramatically as shown<br />
in Figure 4.6.23(a). The minimum lift coefficient achieved is for aerocap with the rear<br />
inclination angle of 5⁰ and the lift coefficient increase with the increase of rear inclination<br />
angle as shown in Figure 4.6.23(b).<br />
84
Configurations<br />
Drag<br />
Coefficient<br />
% diff. From<br />
baseline<br />
Lift<br />
Coefficient<br />
% diff. from<br />
Baseline<br />
Baseline 0.3453 0 0.2193 0<br />
Aerocap α=5° 0.2957 -14.36432088 0.0497 -77.3369813<br />
Aerocap α=10° 0.2894 -16.18882131 0.1097 -49.97720018<br />
Aerocap α=12° 0.2892 -16.24674196 0.1579 -27.99817601<br />
Aerocap α=15° 0.2987 -13.49551115 0.2296 4.696762426<br />
Aerocap<br />
α=18.77° 0.3091 -10.48363742 0.3587 63.56589147<br />
Table 4.6.1 Comparison of drag and lift coefficient of pickup truck with Aerocap at different rear<br />
inclination angle α with the baseline truck.<br />
0.32<br />
0.3<br />
0.28<br />
0.26<br />
drag coefficient<br />
drag<br />
coefficient<br />
5 10 12 15 18.77<br />
Figure 4.6.23: (a) Drag Coefficient ( ) versus rear inclination angle α. (b) Lift Coefficient ( )<br />
versus rear inclination angle α.<br />
4.7 Pickup truck model with 3D curved Aerocap<br />
From the aerodynamic analysis of Aerocap with 5 different rear inclination<br />
angles, Aerocap with the rear inclination angle α= 12⁰ has the smallest drag coefficient<br />
= 0.2892 as shown in Table 4.6.1. By decreasing the rear width of the aerocap it is<br />
possible to further reduce the aerodynamic drag. It is motivated for a study of using 3D<br />
curved Aerocap. The aim of using 3D curved Aerocap is to make static base pressure at<br />
the end of the vehicles body as high as possible but at the base itself, this base pressure is<br />
0.4<br />
0.3<br />
0.2<br />
0.1<br />
0<br />
Lift coefficient Vs rear inclination angle<br />
0 5 10 15 20<br />
85
made as small as possible which would require tapering the rear end. In the present study,<br />
tapering the rear end of Aerocap was made with rear inclination angle α= 12⁰.<br />
Experimental investigation by Gaylard and Howell [19] from Jaguar Land Rover<br />
showed that possible combination of shape modification on SUV as shown in Figure4.7.1<br />
could improve aerodynamic drag. One of the recommended solutions was to decrease the<br />
width of the SUV side frames at the rear. Since the flow over a pickup truck with<br />
Aerocap is similar to the flow over SUV, this recommendation also holds for the pickup<br />
trucks fitted with Aerocap. Thus Aerocap with the rear inclination angle of α= 12⁰ is<br />
modified by narrowing the rear width and streamlining the Aerocap as shown in Figure<br />
4.7.2.<br />
Figure 4.7.1 Shape changes to reduce drag of SUV [19].<br />
Figure 4.7.2 Pickup truck with 3D<br />
curved Aerocap<br />
For a pickup with 3D curved aerocap, it was expected that its aerodynamics<br />
improvement over a model truck with Aerocap inclination angle α= 12⁰ is due to the<br />
86
improvement in the flow structure at the rear of the model as well as the increase of static<br />
pressure at the base of the model. Figure 4.7.3 shows the static pressure contour over a<br />
symmetry plane for pickup truck with 3D curved Aerocap. It can be seen that the pressure<br />
at the rear of the vehicle is about -6.54 Pascal. From Figure 4.6.10, the static pressure is<br />
about -8.34 Pascal at the base of the pickup truck with Aerocap α= 12⁰ and the pressure<br />
value is even lower in some areas on the tailgate. The negative pressures have more<br />
suction effect in the case of the vehicle mounted with Aerocap α= 12⁰ and this verifies<br />
aerodynamic drag improvement of model with 3D curved aerocap over the model with<br />
Aerocap α= 12⁰.<br />
Figure 4.7.3 Pressure distribution over pickup with 3D curved Aerocap in the symmetry plane<br />
Figure 4.7.4 shows the total pressure contour over the model with 3D curved<br />
aerocap and on symmetry plane. By comparing with Figure 3.17, it indicates that the total<br />
87
pressure at the rear of the vehicle is -2.16* Pascal which is higher than total pressure<br />
of the baseline truck at -8.71* Pascal.<br />
Figure 4.7.4 Total pressure distribution over pickup with 3D curved Aerocap in the symmetry plane<br />
Figure 4.7.5 shows the velocity magnitude path line on symmetry plane for flow<br />
over model with 3D curved Aerocap. By comparing with the streamline flow over a<br />
symmetry plane for airflow over a model with 2D Aerocap at α=12⁰ in Figure 4.6.15, it<br />
indicates that the vortexes behind the trucks are very similar. Figure 4.7.6 shows the<br />
wake profile behind the model with 3D curved aerocap and by comparing with the case<br />
of the mode with Aerocap α= 12⁰ in Figure 4.6.20 it indicates that the horse-shoe shaped<br />
vortices presented at the rear of the model with Aerocap α= 12⁰ is not present in the case<br />
of the model with 3D curved aerocap..<br />
88
Figure 4.7.5 Velocity magnitude path line on symmetry plane for flow over model with 3D curved<br />
Aerocap<br />
Figure 4.7.6 Wake profile behind the pickup truck with 3D curved Aerocap (velocity vector on isovelocity<br />
surface at 3m/s)<br />
Table 4.7.1 shows drag and lift coefficients for the truck mounted with Aerocap<br />
α=12⁰ and 3D curved Aerocap. It can be seen that aerodynamic drag reduction of 19.84%<br />
89
and lift reduction of 40.72 % were achieved by mounting 3D curved Aerocap on the<br />
truck. The table also shows that the 3D curved Aerocap has better aerodynamic<br />
characteristic than that of Aerocap α=12⁰.<br />
Configurations<br />
Drag<br />
Coefficient<br />
% diff. from<br />
baseline<br />
Lift<br />
Coefficient<br />
% diff.<br />
from Baseline<br />
Baseline 0.3453 0 0.2193 0<br />
Aerocap α=12° 0.2892 -16.24674196 0.1579 -27.99817601<br />
3D curved<br />
Aerocap 0.2768 -19.83782218 0.13 -40.72047424<br />
Table 4.7.1 Comparison of drag and lift coefficient of baseline pickup truck with Aerocap α=12⁰ and<br />
3D curved aerocap<br />
4.8 Impact of 3D curved Aerocap on fuel economy of pickup truck<br />
In order to analyze the effect of mounting a 3D curved aerocap on the truck on the<br />
fuel economy, G. Sovran's [13] method was used in present work. G. Sovran [13] used<br />
the tractive energy equation (1.11) to develop charts as plotted in Figure 1.12 to show the<br />
impact of changes in the product of drag coefficient and projected area ( A) on the fuel<br />
consumption based on EPA driving schedule. The charts can be used to determine the<br />
reduction in fuel consumption, the equivalent reduction in weight of the vehicle and the<br />
equivalent reduction in vehicle resistance coefficient for any given change in A. The<br />
composite fuel economy for EPA driving is given by equation (4.8.1) as<br />
Consider the 2007 Ford F-150 pickup 2WD and 4.2 liter engine. It has 16mpg<br />
urban and 20mpg EPA rating. Assuming the curb weight of the generic pickup truck M =<br />
90
2000 kg, tire rolling resistance coefficient = 0.009, using equation 4.8.1 and the EPA<br />
rating of the Ford Truck mentioned above, the composite fuel economy would be<br />
7.58 mpg. The A term for the baseline pickup truck is 0.97 and for pickup truck<br />
with the 3D curved aerocap, A is 0.778 m 2 . Therefore the percentage reduction in A<br />
is 19.83 between the pickup truck with 3D curved aerocap and the baseline truck. The<br />
term for the base line truck is 4.85E-04 . Using G. Sovran's [1] chart in<br />
Figure 1.12, the impact of a 19.83% reduction on the composite fuel economy is<br />
summarized in Table 4.8.1 and the reduction in fuel consumption is .<br />
Vehicle Variables Aerodynamic Drag Coefficient,<br />
Frontal Area,<br />
Curb Weight,<br />
Tire Rolling Resistance Coefficient,<br />
Composite Fuel Economy,<br />
City Fuel Economy, 16 mpg<br />
Highway Fuel Economy, 20 mpg<br />
Reduction in Fuel Consumption<br />
Increase in Fuel Economy<br />
Equivalent Reduction in Weight<br />
Equivalent Reduction in Tire Rolling<br />
Resistance Coefficient<br />
Table 4.8.1 Impact of 19.83% reduction in A on Composite Fuel Economy using G. Sovran [5]<br />
charts in Figure 1.12.<br />
Average driver in U.S. drive about 15,000 miles annually and according to<br />
Federal Highway Administration [18] in year 2005, 39,987,802 pickup trucks were<br />
registered in U.S. that was close to 40 million. Assuming all the 40 million pickup trucks<br />
in U.S. had installed 3D curved aerocap; 1,800,000,000 gallons of fuel will be saved.<br />
91
About 46% of each barrel of crude oil is refined into automobile gasoline and one barrel<br />
of crude oil yields 19.3 gallons of gasoline [22]. Thus the amount of crude oil which<br />
could have been saved equates to 92.26 million barrels. Inflation adjusted average price<br />
of a barrel of crude oil in 2005 was 55.21 dollars [21], and had all the pickup trucks<br />
registered in 2005 installed the 3D curved aerocap the U.S. had saved about 5.09 billion<br />
dollars every year. This is significant contribution.<br />
92
5.1 Conclusions<br />
Chapter 5<br />
C<strong>ON</strong>CLUSI<strong>ON</strong>S AND FUTURE WORK<br />
The effects of different aerodynamic add-on devices on flow and its structure over<br />
a generic pickup were analyzed using CFD approach. The objective is to reduce<br />
aerodynamic drag acting on the vehicle and thus improve the fuel efficiency as well as<br />
reduce the carbon print of pickup trucks.<br />
Flow over the generic pickup model was simulated using CFD and the results<br />
from the simulation were validated against CFD results of flow over the same generic<br />
model from Yang and Khalighi [1]. The results from present simulation was compared<br />
with results from Yang and khalighi[1] in chapter 3 and the results were found to be in<br />
complete agreement.<br />
The thesis studied the flows over a pickup truck with add-on devices: (1) Tonneau<br />
cover, (2) Rear Roof Garnish, (3) Tail plates, (4) Airdam with 3in and 6 in clearance<br />
from ground, (5) Traditional canopy, Aerocap at 5 different rear inclination angles, and<br />
(6) a 3D curved Aerocap. Table A1 in Appendix 1 shows the drag and lift coefficient of<br />
the entire studied add-on devices. Except for Airdam with 6in clearance from the ground,<br />
all the studied add-on devices reduced the drag coefficient when it was compared to the<br />
result of baseline truck.<br />
The maximum reduction of aerodynamic drag coefficient, , was 19.84% which<br />
was achieved by employing 3D curved Aerocap and it was followed by Aerocap α=12⁰ at<br />
93
CD=16.24%, the second maximum reduction. The impact of the 3D curved Aerocap on<br />
the fuel economy of the pickup truck was analyzed in section 4.8. It was concluded that<br />
installing 3D curved Aerocap on baseline truck will save 0.003 .<br />
In section 4.8 it was also tried to quantify the impact of drag reduction on the<br />
composite fuel economy and the amount of barrels of crude oil or dollars it could save in<br />
the U.S. Assuming all the 40 million pickup trucks registered in 2005 in U.S. had<br />
installed 3D curved Aerocap, 1.8 billion gallons of fuel will be saved based on average<br />
driving of 15,000 miles annually. If this was converted to the amount of crude oil<br />
consumption and the amount of money to spend on it, having all the pickup trucks<br />
installed the 3D curved Aerocap the U.S. would have saved about 5.09 billion dollars<br />
every year.<br />
The minimum reduction of aerodynamic drag coefficient, CD, was 0.35% which<br />
was obtained by employing Airdam with 3in clearance from the ground. It was flowed by<br />
Tonneau cover in drag reduction by 1.16% when it was compared with the results of<br />
baseline truck. However, Airdam with 3 in clearance from the ground is not practical to<br />
mount on pickup trucks unless a devise was coupled with a sensor which moves the<br />
Airdam up and down when the road condition permits. Apart from practicality, drag<br />
reduction achieved by the Airdam is very small to merit the cost and risk of bumping on<br />
to objects on the road. On the other hand, the lift coefficient reduction achieved by<br />
employing Airdam with 3in clearance from the ground was 326.45%. This indicates that<br />
Airdams are very effective in reducing lift force acting on a vehicle. It was recommended<br />
94
to install the airarm on race cars to increase the traction and handling, especially during<br />
the process of maneuvering curves or driving on slippery roads.<br />
5.2 Future work<br />
Although maximum reduction of aerodynamic drag coefficient, , was achieved<br />
as by using 3D curved Aerocap in the present study, improvement of 3D curved Aerocap<br />
to further reduce the aerodynamic drag can be made possible by using the optimization<br />
software or using synergetic effect of aerodynamic devices such as Tail plates with the<br />
3D curved Aerocap.<br />
The flow over the generic pickup truck in present CFD simulation was simplified<br />
due to hardware limitation, including that the side mirrors were removed and non-rotating<br />
wheals were used. It was also assumed a steady flow of air with zero degree yaw angle.<br />
However, in reality the flow over the vehicle is unsteady and very turbulent. The next<br />
step would be conducting unsteady flow over a realistic pickup truck with optimized 3D<br />
curved Aerocap. Besides combing with Tail plates, the research study could continue to<br />
analyze the synergetic effect of employing Side skirts, Vortex generators or other<br />
aerodynamic devices with the optimized 3D curved Aerocap. If it is possible, the<br />
experimental test will be conducted after the add-on devices are built and mounted on a<br />
pickup truck.<br />
95
APPENDIX<br />
Drag % diff. from Lift % diff. from<br />
Configurations Coefficient baseline<br />
Coefficient Baseline<br />
Baseline 0.3453 0 0.2193 0<br />
Aerocap α=5° 0.2957 -14.36432088 0.0497 -77.3369813<br />
Aerocap α=10° 0.2894 -16.18882131 0.1097 -49.97720018<br />
Aerocap α=12° 0.2892 -16.24674196 0.1579 -27.99817601<br />
Aerocap α=15°<br />
Aerocap<br />
0.2987 -13.49551115 0.2296 4.696762426<br />
α=18.77°<br />
Traditional<br />
0.3091 -10.48363742 0.3587 63.56589147<br />
canopy 0.3157 - 8.57<br />
Airdam -3in 0.3441 -0.35 -0.4966 -326.45<br />
Airdam-6in 0.3661 6.03 0.1393 -36.48<br />
Tail Plates<br />
Rear Roof<br />
0.3333 -3.475238923 0.1304 -40.5380757<br />
Garnish 0.337 -2.403706922 0.2916 32.96853625<br />
Tonneau Cover<br />
3D curved<br />
0.3413 -1.158412974 0.1828 -16.64386685<br />
Aerocap A0.2768 -19.83782218 0.13 -40.72047424<br />
Table A1 Drag and lift coefficient of all studied Add-on devises<br />
96
REFERENCES<br />
1. Yang, Z., and Khalighi, B., “CFD Simulation for Flow Over Pickup Trucks”,<br />
SAE Paper No. 2005-01-0547, 2005.<br />
2. Al-Garni, A., Bernal, L., and Khalighi, B., “Experimental investigation of the<br />
Near Wake of a Pick-up Truck”, SAE Paper No. 2003-01-0651, 2003.<br />
3. Cooper, K., “Pickup trucks Aerodynamics - Keep your tailgate Up”, SAE Paper<br />
No. 2004-01-1146, 2004.<br />
4. Mokhtar, W., Britcher, C., Camp, R.,”Further Analysis of Pickup trucks<br />
Aerodynamics”, SAE Paper No. 2009-01-1161, 2009.<br />
5. “Aerodynamics of Road Vehicles”, Edited by Wolf-Heinrich Hucho, SAE<br />
International, Warrendale, PA, 1998.<br />
6. Barnard, R.H., “Road Vehicle Aerodynamic Design-An Introduction”, 2nd<br />
Edition, Longman ISBN 0-582-24522-2, 1996.<br />
7. White, F.M.,”Fluid Mechanics 4th Ed.” Boston. MA. WCB McGraw-Hill, 1999<br />
8. Giancarlo Genta “Motor vehicle dynamics: modeling and simulation-Vol43”<br />
World scientific Publishing Co. ISBN: 978-981-02-2911-5<br />
9. U.S. Environmental Protection Agency<br />
http://www.epa.gov/oms/fetrends.htm#appendixes<br />
Accessed: October 20, 2009<br />
10. U.S. Department of Energy<br />
Energy Efficiency and Renewable Energy<br />
http://www.fueleconomy.gov/feg/atv.shtml<br />
11. Ahmed, S.R., “Computational Fluid Dynamics”, Chapter XV in Hucho, W.H<br />
(Ed.), Aerodynamics of Road Vehicles, 4th Edition, SAE International,<br />
Warrendale, PA, USA, 1998<br />
12. Sovran, G., Bohn, M.S., “Formula for Tractive Energy Requirements of Vehicles<br />
During the EPA-Schedules,” SAE Paper No. 810184, Society of Automotive<br />
Engineers, Warrendale, Pa., 1981<br />
97
13. Sovran, G., “Tractive Energy Based Formulae for the Impact of Aerodynamics on<br />
Fuel Economy over the EPA-Driving Schedules,” SAE Paper No. 830304,<br />
Society of Automotive Engineers, Warrendale, Pa., 1983<br />
14. Thomas R. Yechout “Introduction to Aircraft Flight Mechanics AIAA Education<br />
Series” American Institute of Aeronautics and Astronautics Inc, 2003<br />
ISBN 1-56347-577-4<br />
15. http://www.fueleconomy.gov/feg/FEG2007.pdf<br />
Accessed: October 20, 2009<br />
16. U.S. Department of Transportation<br />
Federal Highway Administration<br />
http://www.fhwa.dot.gov/policy/ohim/hs05/htm/mv9.htm<br />
Accessed: October 20, 2009<br />
17. Hucho, Wolf-Heinrich., Sovran, Gino.”Aerodynamics of Road Vehicles” Annual.<br />
Reviews Inc. Fluid Mech. 1993.25 :485-537<br />
18. http://www.fhwa.dot.gov/ohim/onh00/line2.htm.<br />
Accessed: October 20, 2009<br />
19. Jeff Howell, Adrian Gaylard," Improving SUV Aerodynamics", Motor Industry<br />
Research association (MIRA) Technical Paper.<br />
20. Munson, Young, Okiishi “Fundamentals of Fluid Mechanics 4th Edition” John<br />
Wiley and Sons, 2002<br />
21. http://www.inflationdata.com/inflation/Inflation_Rate/Historical_Oil_Prices_Tabl<br />
e.asp<br />
Accessed: October 20, 2009<br />
22. http://www.californiagasprices.com/crude_products.aspx<br />
Accessed: October 20, 2009<br />
98