The performance of a sports car is not only limited to its engine power but also to aerodynamic properties of the car. By decreasing the drag force it is possible to reduce the engine power required to achieve same top speed thus decreasing the fuel requirement. The stability of a sports car is considerably important at high speed. The provision of a rear wing increases the downforce thus reducing the rear axle lift and provides increased traction. In this study an optimum rear wing is designed for the high performance car so as to decrease drag and increase downforce. The CAD designed baseline model with or without rear wing is being analyzed in computational fluid dynamics software. The lift and drag coefficient are calculated for all the design thus an optimum rear wing is designed for the considered baseline model.

1. INTERNATIONAL JOURNAL FOR TRENDS IN ENGINEERING & TECHNOLOGY
VOLUME 5 ISSUE 2 – MAY 2015 - ISSN: 2349 - 9303
166
Design of Rear wing for high performance cars and
Simulation using Computational Fluid Dynamics
T.Maniraj1
1
Anna University, Automobile Engineering,
tamilnambimaniraj4@gmail.com
S.Sathishkumar2
2
Anna University, Automobile Engineering,
sssatzsathi07@gmail.com
Abstract— The performance of a sports car is not only limited to its engine power but also to aerodynamic properties of the car.
By decreasing the drag force it is possible to reduce the engine power required to achieve same top speed thus decreasing the
fuel requirement. The stability of a sports car is considerably important at high speed. The provision of a rear wing increases the
downforce thus reducing the rear axle lift and provides increased traction. In this study an optimum rear wing is designed for the
high performance car so as to decrease drag and increase downforce. The CAD designed baseline model with or without rear
wing is being analyzed in computational fluid dynamics software. The lift and drag coefficient are calculated for all the design
thus an optimum rear wing is designed for the considered baseline model.
Index Terms— CAD, CFD, downforce, drag, lift, rear wing, traction
——————————  ——————————
1 INTRODUCTION
Aerodynamics makes it major impact on modern vehicles through its
contribution to road load. Aerodynamic forces interact with the
vehicle causing drag, lift, lateral forces, and moments in roll, pitch,
yaw and noise. These impact fuel economy, handling and NVH.The
flow around the vehicle not only leads to drag but also causes
aerodynamic forces and moments which are components of
resulting wind force this affects driving stability[1].The airflow
pattern resulting from the forward motion of the vehicle produces lift
and pitching moment this affects traction. Due to the lateral side
force the yawing moment and rolling moment results. To control the
stability of the vehicle these aerodynamic forces can be used
positively by producing the needed negative lift that is the downforce
to give more traction to the driving wheels [1].
1.1 Aerodynamic Drag force
Aerodynamics drag force is the force which opposes the forward
motion of the vehicle when the vehicle is traveling. The
aerodynamics drag force acts externally on the body of a vehicle.
The aerodynamics drag affects the performance of a car in both
speed and fuel economy as it is the power required to overcome the
opposing force. Because air flow over a vehicle is so complex, it is
necessary to develop semi empirical models to represent the effect
[1]. Therefore, aerodynamic drag force is characterized by,
D = ½ ρ v2
CD A (1)
Coefficient of drag is defined as how the aerodynamic the
shape is to the incoming air. CD is determined empirically for the car
[2]. It is possible to have an aerodynamic drag coefficient greater
than 1 if the air is pushed outward such that the effective area of the
air movement is greater than the area of object facing the air.
1.2 Aerodynamic Lift force
The aerodynamic drag force is acted horizontally to the vehicle and
there is another component, directed vertically, called aerodynamic
lift. It reduces the frictional forces between the tires and the road,
thus changing dramatically the handling characteristics of the
vehicle. This will affect the handling and stability of the vehicle. The
pressure differential from the top to the bottom of vehicle causes a
lift force. These forces are significant concerns in aerodynamic
optimization of a vehicle because of their influence on driving
stability [2]. The force, L is quantified by the equation
L = ½ ρ v2
CL A (2)
The lift force dependent on the overall shape of the vehicle. At zero
wind angle, lift coefficient normally fall in the range of 0.3 to 0.5 for
modern passengers car [1], but under crosswinds conditions the
coefficient may increase dramatically reaching value of 1 or more.
1.3 Pressure distribution over the vehicle
Most of the lift comes from the surface pressure distribution. A
typical pressure distribution on a moving car is shown in figure 1.
The distribution for the most part with simple Bernoulli
equation analysis. Location with high speed flow (i.e. over the
roof and hood) has low pressure while location with low speed
flow (i.e. on the grill and windshield) has high pressure. It is easy
to believe that the integrated effect of this pressure distribution
would provide a net upward force [3]. This force is negative
force, meaning that the force that no need to enhance the stability
of a vehicle. The opposite force of upward force is downforce.
1.4 Rear wing

2. INTERNATIONAL JOURNAL FOR TRENDS IN ENGINEERING & TECHNOLOGY
VOLUME 5 ISSUE 2 – MAY 2015 - ISSN: 2349 - 9303
167
A Rear wing is an aerodynamic device attached to an automobile
rear boot whose intended design function is to modify
unfavorable air movement across a body of a vehicle[1].Rear
wing aids to produce down force by creating a "dam" at the rear
lip of the trunk. This can result in improved vehicle stability by
decreasing lift or decreasing drag that may cause unpredictable
handling in a car at high speed. Rear wing are often fitted to race
and high performance sports car, although they have become
common on passenger vehicles as well. Rear wing located on the
rear deck may serve several purposes. By deflecting the air
upward, the pressure on the rear deck is increase, hence creating
a down force at the most advantageous point on the vehicle to
reduce lift. If this modified pressure distribution is integrated in
the x and y direction, the result is lower drag and lift. A rear wing
can have three effects. It can reduce drag, reduce rear-axle lift
and reduce dirt on the rear surface. With rear wing also, attention
first focused on drag, but increasing emphasis is now placed on
negative lift [1][2].
The influence on the pressure distribution is shown in figure
below. The possibility of reducing drag is comparatively low. In
fact on sporty cars, and even more so on racing cars, even an
increase in drag is accepted in order to ensure that the rear-axle
lift gets low[2].
2 METHODOLOGY
2.1 Modelling in CAD software
CAE tools will be used for modeling and analyzing the models.
First, the models will build up in CAD (Computer-Aided Design)
software. Mostly people use CAD software to design and build
up the model. For this project, SolidWorks and CATIA will be
used to build up the model and the model will be designed
according the actual dimension. The design of the rear wing is
not just accurate in dimension, but also fixes perfectly with the
baseline model that will be use. This precaution step can avoid
any errors during analysis and also to make the model of rear
wing is easily mate with the baseline model. The baseline model
that will be use in this project also must build up according the
actual design. The baseline model used is PAGANI HUAYRA
and rear wings are NACA designated.
2.2 Dimensions of Rear wing and CAD model
The rear wing are modelled in the CATIA software by importing
the coordinates from the NACA airfoil generator to produce smooth
surface whose dimension are mentioned in the figure below.
2.3 Inputs and selection of Wind tunnel
1.Testing of baseline model
The selected wind tunnel is GM wind tunnel whose test section
dimensions are as follows:
Length: 20.3 m
Width: 10.4 m
Height: 5.4 m

3. INTERNATIONAL JOURNAL FOR TRENDS IN ENGINEERING & TECHNOLOGY
VOLUME 5 ISSUE 2 – MAY 2015 - ISSN: 2349 - 9303
168
2. Testing of aero foils
The selected wind tunnel is AUDI wind tunnel whose test section
dimensions and area are as follows:
Length: 7.5 m
Area: 3.01 m2
3. CFD process inputs
The various parameters to be set are mentioned as follows:
Material: Air with density 1.2754 kg/m3
at 200
C
Boundary condition:
Inlet: Velocity 80km/h,100km/h,150km/h
Outlet: Pressure 0 Pascal
Flow: Incompressible Laminar Flow.
2.3 Analyzing in CFD software
During this project, Simulation CFD will be use to analyze the
car model with its attachment, which is the rear wing. Simulation
CFD is the only fluid flow analysis tool for designers fully
embedded with Autodesk products. With this software, it can
analyze the solid model directly. The model that has been built up
in SolidWorks then will be export into Simulation CFD to
analyze the model. Through this software, it can analyze parts,
assemblies, subassemblies, and multibodies. Detail steps for use
this software is include in its tutorial. The design will analyze, the
data will interpret, the result will produce and analysis will
summarize and present in form of Flow simulation, table, graph,
chart or etc. During the analysis, some errors may come. Besides
that, some limitations must be considered during analyzing the
model to make sure that the results are acceptable. Ground line
must be 0.120 m. Ground line is a distance between road surface
and bottom part of the car. This distance is important to keep in
constant so that the CD and CL are acceptable for high
performance car. Another part that is important is the location of
installment of rear wing. The location of base of rear wing must
be same for all type of rear wing, so that the pressure acting at the
back is in same region [3].

4. INTERNATIONAL JOURNAL FOR TRENDS IN ENGINEERING & TECHNOLOGY
VOLUME 5 ISSUE 2 – MAY 2015 - ISSN: 2349 - 9303
169
3 CONCLUSION
The baseline model created using CAD software (solid works &
CATIA) has been imported into Autodesk CFD software. On the
baseline model flow analysis has been performed. The velocity
and pressure distribution has been simulated. The NACA 2412
and modified NACA 2412 has been analyzed in CFD for various
velocity. The baseline model with NACA 2412 and modified
NACA 2412 has been analyzed in CFD software. The velocity
and pressure distribution has been simulated. The corresponding
drag and lift force are calculated by the wall calculator function
of the CFD software.
The coefficient of drag and lift are calculated manually with
result data from the CFD software for the NACA 2412 and modified
NACA 2412. It is seen that NACA 2412 has less drag and more lift. The
modified NACA 2412 has more drag than NACA2412 but produces the
required downforce which is needed for the designed baseline model.
On comparison it is seen that the baseline model equipped
with NACA 2412 has reduced the lift by varying the flow at the
baseline models rear end. The baseline model with modified NACA
2412 has reduced the baseline models drag and increased the
downforce thereby achieving the objective of the study. The rear
wing modified NACA 2412 has altered the flow at the rear end of
baseline model considerably. The calculated lift and drag coefficients
show that the baseline model equipped with modified NACA 2412
has low drag and lift coefficient comparing to the other two.
ACKNOWLEDGMENT
We wish to thank Autodesk for providing licensed software for
carrying out our research work and Mr. J.Dhanabal ,Assistant
Professor,SVCE, for being our mentor, teaching us basics of
Automotive Aerodynamics and providing us SAE edition books
for reference.
REFERENCES
[1] Wolf Heinrich Hucho, Aerodynamics of road vehicles
Fourth SAE edition.
[2] Thomas D.Gillespsie, Fundamentals of Vehicle Dynamics
Third SAE edition.
[3] Dainel bell, Aerodynamic analysis and optimization of rear
wing of a WRC car, MSc project, Oxford Brookes
University.
Author Profile:
 T.Maniraj is currently pursuing bachelor’s degree program
in automobile engineering in Anna University, India
PH-9884268182.
E-mail: tamilnambimaniraj4@gmail.com
 S.Sathishkumar is currently pursuing bachelor degree
program in automobile engineering in Anna University,
India
.