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Procedia Engineering 194 (2017) 160 – 165
10th International Conference on Marine Technology, MARTEC 2016
CFD Analysis of Passenger Vehicleat Various Angle of Rear End
Spoiler
Rubel Chandra Dasa,∗, Mahmud Riyada
a Department
of Mechanical Engineering,Khulna University of Engineering & Technology,Fulbarigate,Khulna-9203,Bangladesh
Abstract
Automotive vehicle’s performance, safety, maneuverability be influenced by multi-disciplinary factors such as car engine, tires,
aerodynamics, and ergonomics of design. With the recent years, inflation in the fuel prices & the demand to have reduced greenhouse emissions has played a significant role in redefining the car aerodynamics. The shape of the vehicle uses about 3% of fuel
to overcome the resistance in urban driving, while it takes 11% of fuel for the highway driving. This considerable high value of
fuel usage in highway driving attracts several design engineers to enhance the aerodynamics of the vehicle using minimal design
changes. Besides, automotive vehicles have become so much faster experiencing uplift force which creates unexpected accidents.
This brings the idea of using external devices, which could be attached to the present vehicle without changing the body. This paper
is based on the design, developments and numeral calculation of the effects of external device, which will be spoiler that mounted
at the rear side of the vehicle to make the present vehicles more aerodynamically attractive. The influence of rear spoiler on the
generated lift, drag, and pressure distributions are investigated and reported using commercially available Autodesk Simulation
CFD software tool.
c 2017
2017Published
The Authors.
Published
by Elsevier
©
by Elsevier
Ltd. This
is an openLtd.
access article under the CC BY-NC-ND license
(http://creativecommons.org/licenses/by-nc-nd/4.0/).
Peer-review
under responsibility of the organizing committee of the 10th International Conference on Marine Technology.
Peer-review under responsibility of the organizing committee of the 10th International Conference on Marine Technology.
Keywords: Spoiler; lift; drag; CFD
1. Introduction
For better cruising conditions, greater stability of navigation, and for lower energy consumption, the vehicles body
& frame should be deigned in such a way that it reduces its total weight & improve the vehicles overall aerodynamic
characteristics. These subjects are also indirectly related to environmental protection and noise pollution. [1] In the
process of car design, the aerodynamics must be seriously considered. A car design can only be acceptable if its form
drag reduced. Many researchers have made use of CFD techniques [2–5] to perform numerical simulations related to
automobile.
At present, automotive vehicles runs so much fast which creates unexpected accidents for such excessive amount
of speed. This shows the necessity for inventing an aerodynamic wing, spoiler which creates a carefully controlled
stall over the wing portion behind the spoiler, basically by reducing the lift of that wing section. Spoilers are designed
to reduce lift also making considerable increase in drag [6]. The invention of the spoiler was inevitable, because of
having capability for reducing aerodynamic lift; in the view of safety aspect is more important than the aerodynamic
drag, which influences the car performance as well as its economical side.
∗
Corresponding author. Tel.: +8801751-589010
E-mail address: [email protected]
1877-7058 © 2017 Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license
(http://creativecommons.org/licenses/by-nc-nd/4.0/).
Peer-review under responsibility of the organizing committee of the 10th International Conference on Marine Technology.
doi:10.1016/j.proeng.2017.08.130
Rubel Chandra Das and Mahmud Riyad / Procedia Engineering 194 (2017) 160 – 165
Aerodynamic phenomena i.e. spoiler generated drag & down-force is significant aspect for the investigation of the
phenomena of car. It is estimated that the aerodynamic drag is the governing form of resistance when vehicles run at
speeds of 80 km/h or greater, especially considering the fact that 65% of the power required at 110 km/h is consumed
due to overcoming aerodynamic drag [7,8].
A spoiler is an automotive aerodynamic device whose intended design function is to ’spoil’ unfavorable air movement across a body of a vehicle in motion, usually described as turbulence or drag. Rear spoilers are provided to
increase the negative lift of the vehicle. An investigation is performed to study the effect of change of rear end
spoiler inclination angle over co-efficient of drag & lift. Numerical investigation is so much helpful for declaration of
spoiler’s capability of better traction, faster turning, proper controlling i.e. acceleration & brake as well as increasing
the vehicle safety.
2. Model & the Computational Domain
2.1. Vehicle generic model
This process involves computer aided design (CAD) software like Solidworks-2015. The help of CAD software
defines the topology of the fluid flow region of interest. This software plays a major part of the design and optimization
process in research analysis. A generic model of a passenger vehicle is shown in fig. 1 with its relevant dimensions.
The length of this model is 4.36 m, width 1.885m & the height is 1.424 m. There are six different types of variation
created by modifying spoiler’s angle with positive portion of X-axis. These modified angles are -2, 4, 6, 8, 10 & 12
degrees. The actual dimensions of the designed car with its spoiler as well as only spoiler part are shown in the Fig.
1 below:
b
a
Fig. 1: (a) 3D vehicle model with relevant dimensions (meter) ; (b) Spoiler model with relevant dimensions (meter)
2.2. The computational domain
A large air domain is used that avoids artificial acceleration due to squeezing air around the side and top of the car.
The velocity inlet surface, in front of the car is situated at the two times the vehicle length. But, outlet is four times
the vehicle length behind the car. There is a symmetry side shown in the mentioned fig. 2.
3. Formulations
3.1. Methodology & meshing
Generating air domain for modeled vehicle, boundary conditions are applied.Simulation & testing of the model
for various rear end spoiler angle.Simulation is performed with proper meshing & iterations required for obtaining
optimum results.Since, simulation results are more involved in the rear side of vehicle, which is where the ”wake of
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Rubel Chandra Das and Mahmud Riyad / Procedia Engineering 194 (2017) 160 – 165
Fig. 2: A virtual wind tunnel
vehicle” phenomenon occurs, enough space has been kept in the rear portion of the vehicle model to capture the flow
behavior mostly behind the vehicle. Mesh sizes are assigned to all volumes in the model, and then finer sizes are
applied to surfaces and edges where necessary in order to capture strong flow gradients or to represent complicated
geometric features. An analysis is performed on a coarse mesh to qualitatively assess the flow features present and
identify meshing needs in high gradient regions without a severe time penalty. Looking at the results on the coarse
mesh, the mesh is refined in the high gradient regions.
3.2. Boundary condition & governing equations
Boundary conditions for all of the modifications are constituted in Autodesk simulation CFD 2015 & the conditions
are mentioned in table 1. The value of density of air is 1.2041 kg/m3 at STP. Co-efficient of drag & lift are calculated
using the two equations. These are C D = (2 ∗ F D )/(ρ ∗ V 2 ∗ A) & C L = (2 ∗ F L )/(ρ ∗ V 2 ∗ A). F D = Drag Force (Force
in the direction of flow), F L =Lift force (N), ρ=Density of fluid (kg/m3 ), V =velocity relative to fluid (m/s), A=Cross
sectional area, (m2 )
Table 1: Boundary conditions for all cases & benchmarks
Inlet Boundary
Conditions
Type
Unit
Time
Method
Direction
Spatial variations
Velocity magnitude Velocity
Slip/Symmetry
Wall
Yes
(One wall surface)
Outlet Boundary
Conditions
Type
Unit
Time
Pressure
Gage/Absolute
Static/Total
Km/h
Static
Normal
Reverse normal
Constant
70, 90, 110, 130
Pressure
Pa
Steady state
0
Gage
Static
4. Results & Discussion
For all high speedy vehicles, produced lift force is most important concern in the case of stability feature. Lift is
the negative pressure build up at the top and rear surfaces of a car where velocity of flow is higher when compared to
that at the front and bottom of the car. Two important factors i.e. drag & lift should be looked into while designing
any vehicle. Numerical investigation software greatly reduces time-to-market by reducing the need for costly physical testing and prototyping. So, numerical investigation is executed as a practical tool for the analysis of vehicle
aerodynamics.
Rubel Chandra Das and Mahmud Riyad / Procedia Engineering 194 (2017) 160 – 165
4.1. Color contour analysis
The distribution of pressure on most of the surface of the vehicle is done by using Bernoulli’s equation. Static
pressure distribution result in fig. 3 indicates how much pressure is produced at different portions of the moving
vehicle. There is a scale provided which indicates the amount of pressure produced for different varieties of velocity.
Close observation of the pressure changes in the marked regions tells us the trend in pressure variation along the
length of the vehicle. At the front end the vehicle run into a high pressure as all the air flowing towards the vehicle
gets compressed due to obstruction. As the air finds its way towards the rear its pressure is observed to reduce until it
encounters the wind screen area. After the air crosses the wind screen it flows over the top surface. There is a higher
pressure concentration on the front part of the as well as near at the rear end spoiler part. Higher pressure generated
at the rear portion of the vehicle implies downward forces i.e. lift is reduced.
a
b
c
d
e
f
Fig. 3: Static Pressure Contours on the passenger vehicle with a rear spoiler at V = 90 Km/h ;(a) Spoiler inclination angle -2 degree; (b) Spoiler
inclination angle 4 degree;(c) Spoiler inclination angle 6 degree; (d) Spoiler inclination angle 8 degree; (e) Spoiler inclination angle 10 degree; (f)
Spoiler inclination angle 12 degree
Whenever the air flows over the body, it creates a velocity distribution resulting in the aerodynamic loads acting on
the body of the moving vehicle. It displays the amount of velocity present at different regions surrounding the vehicle
for applied velocity. Besides, rear end spoiler influences the flow of air to spoil the velocity direction to reduce lift.
Velocity is squarely proportional to lift & drag forces. Hence, the amount of velocity affects the total co-efficient of
drag & lift.
Flow separation shown in fig. 4 occurs when the boundary layer travels far enough against an adverse pressure
gradient that the speed of the boundary layer relative to the object falls almost to zero. The fluid flow becomes
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Rubel Chandra Das and Mahmud Riyad / Procedia Engineering 194 (2017) 160 – 165
detached from the surface of the object, and instead takes the forms of eddies and vortices. The flow of air becomes
turbulent and a low-pressure zone is created, increasing drag and instability. Adding a rear spoiler could be considered
to make the air ”see” a longer, gentler slope from the roof to the spoiler, which helps to delay flow separation and the
higher pressure in front of the spoiler can help reduce the lift on the car by creating down force. This may reduce drag
in certain instances and will generally increase high speed stability due to the reduced rear lift. From color contours,
it is clear that spoiler inclination angle has a great effect to delay flow separation.
a
b
c
d
e
f
Fig. 4: Particle traces on the moving vehicle & flow separation view at V = 90 Km/h on the rear of the passenger vehicle with a rear spoiler ;(a)
Spoiler inclination angle -2 degree; (b) Spoiler inclination angle 4 degree;(c) Spoiler inclination angle 6 degree; (d) Spoiler inclination angle 8
degree; (e) Spoiler inclination angle 10 degree; (f) Spoiler inclination angle 12 degree
4.2. Graphical representation
After simulation being completed, the numerical values are plotted in the graph (Fig. 5 & Fig. 6) to ease the
investigation for optimum inclination angle of spoiler. Inclination angle effects are clearly demonstrated in the graph
for fluctuating vehicle speed. From table 2, maximum amount of C D is generated at 6 degree inclination angle of
spoiler (V=70 Km/h), minimum C D is found for 8 degree inclination angle of spoiler (V=130 km/h). Here, 12 degree
inclination angle of spoiler is most convenient for less fluctuations of C D . Besides, highest & lowest value of C D
is close enough which creates fewer troubles.Maximum amount of C L is generated at -2 degree inclination angle of
spoiler (V=110 Km/h), minimum CL is found for 6 degree inclination angle of spoiler (V=90 km/h). At this point,
C L values fluctuations are closer for 8 degree inclination angle of spoiler. Besides, highest & lowest value of C L is
less than other modification model of spoiler.
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Fig. 5: C D vs. α graph for different velocity
Fig. 6: C L vs. α graph for different velocity
Table 2: Values of C D & C L for all modifications
Modification of the model,
(Inclination angle of spoiler)
-2 degree
4 degree
6 degree
8 degree
10 degree
12 degree
Co-efficient of drag(CD )
Co-efficient of lift(CL )
0.28185
0.255775
0.274975
0.2694
0.263675
0.25984
0.11115
0.09345
0.082475
0.074875
0.0865
0.0720575
5. conclusion
Observation can be concluded that at a particular spoiler height the spoiler that possess smaller angle of wind collision gives higher drag force. This is due to the fact that with smaller angle of wind collision, the spoiler would create
smaller recirculation zone behind the rear end of the running vehicle. This implies to higher pressure behind spoiler
but lower pressure behind the rear end of the vehicle. Rear spoilers redirect the airflow behind the vehicle & increase
the negative lift of the vehicle. In the investigation, six modifications are simulated & 12 degree spoiler inclination
angle model is the most optimum though it creates 1.56% extra CD than 4 degree inclination angle. Minimum CL is
maintained in the model which is basic concern for better stability of high speedy vehicle.
Acknowledgments
The authors like to thank Dr. Mohammad Mashud, Professor, Department of Mechanical Engineering, KUET for
his support, valuable suggestions, continuous encouragement and constructive instructions throughout this research
work. The authors also gratefully acknowledge the Department of Mechanical Engineering, KUET due to allow using
Computational Fluid Dynamics (CFD) lab.
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