The document discusses the aerodynamic properties of different car back shapes, specifically fastbacks, square backs, and variations in backlight angle. It summarizes that fastbacks have a sloped roofline that follows the curve of the rear, resulting in conical vortices but higher drag at extreme angles. Square backs have sharp edges that prevent smooth airflow, creating a large turbulent wake and higher drag. Computational fluid dynamics simulations were used to study how backlight angle affects drag coefficient, finding an optimal angle of around 15 degrees for fastbacks that minimizes drag.

1. Case study on shape optimization for fast
back,hatch back and square back cars
PROFESSIONAL ELECTIVE
(Automotive Aerodynamics)
Name : Vamsi.K
Reg.no : 15104047
Dept : Auto-7A

2. INTRODUCTION
A real-life automobile is very complex shape to model or to study experimentally.
The main developments with vehicle aerodynamics probably occurred during the
early 198Os, and the use of low-drag vehicles has now become common. The
development of low-drag vehicle shapes is now more rapid because of greater past
experience and better computational techniques.
Computational Fluid Dynamic simulation studies investigated the effect of geometric
form design, with a focus on rear vehicle backlight angle, to determine upondrag and
lift values.
The behavior of these backlight angles was studied, and compared to the results with
the different literature research papers. Finally the modification and the future
advancement have been proposed on the basis of the outcome results.
Low drag model was built in early 1980’s which is also known as the ‘jelly-mould’
shape. These type model cars usually have the less drag acting on the car. However,
it has the low stability in the crosswinds conditions. The following figure shows the
shape of the car. Those cars have the slop
. Figure 1: Jelly Mould shape

3. The standard way to study the flow over the car and the airplane surfaces are the
CFD flow simulation over the car 3d models. This way is way cheaper than the
wind tunnel because this involves the designing part and then CFD flow
simulation which saves lots of times and the effort.
The two figures are describing the wind tunnel and the figure 4 describes the CFD
flow simulation over the car
There are lots of research were taken on the aerodynamics of the vehicle body, to
analysis the drag and the lift coefficient acting on the car body. base so in these
kinds of slopes the flow doesn’t have the large-separations)
Aerodynamics Design
It is the design of the car which has the low drag and the low coefficient of
lift. Cars are like the bluff bodies which has the drag coefficient of raging from
0.3-0.4. (Watkins & Vino, 2008). The researches have trying to design which has
the low number of drag coefficient and the coefficient of lift and other side
moments of the cars. This increases the stability of the car while it’s moving and
also gives the driver the confidence in handling properly. The back shape has
more effect on reducing the drag and the lift.
* A moving object which is exposed to air always experience three main
forces and movements respectively as shown in the figure.
Figure 2: Wind tunnel testing (AudiAG,
1999)
Figure 3: CFD Simulation (Ringis,
2013)

4. 1. lift force
2. drag
3. side force
Vehicle axis frame xyz. Again this is the right angled frame but this time it is
assumed to be fixed with the vehicle and also it moves along with it. The z force
called the lift forcewhich is acting on upwards and x is the horizontal forcewhich
is moving opposite to the vehicle body called drag. At the last force on the side
horizontally called the side force which causes the momentum force.
Aerodynamics Forces
Drag Force
The bodyshapewhen it exposed to air experience the different pressureon
the surface. By breaking this into five constituent elements will gives a better
understanding of drag force acting on the vehicle body.
Figure 4: Forces and moments acting on the vehicle.

5. Pressure drag
This is the componentwhich is identified onthe external surface of the car.
As when the vehicle moves with the forward direction of the air then the surface
of the car experience the pressure which is vary over the different points of the
car as shown in the following figure. To have a look it very closely the small area
of the flat surface is considered then the force which is acting on the axis of the
car the drag force depends on the magnitude of the pressure.
Surface Drag
This type of drag is due to the stress and drag values which is from the
friction between air and the body surface for a small element. This type of drag
only happens due to the effect of viscosity at the surface of the car.
Lift
In the simple words lift in the vehicle is the pressure difference between
the upper and the down side of the car. Nowadays higher top speed modern cars
are manufactured which high stability need while on the road. Upper surface area
near the hood, wind shield and underbodysuchas suspension, exhaust system are
the main dependents oflift force. Studies shows that it is notthe commonproblem
at the low speed butwhen the vehicle goes to the high speed and then the pressure
difference is a lot then lift force is the problem.
Figure 5: Pressure distribution over a car body. Figure 6: Force acting on one surface
element.

6. Turbulent and the laminar boundary layers
Boundary layer flow has two distinctive types when the flow layer passes
over the surface of the vehicle. The following figure 7 shows the flow layers on
the top of the bus. Smooth air flow can be seen in the front edge. In this as it is
clear that the moment of the outer layer is faster than the inner layers becausethe
friction is effecting on the inside layer which has the direct contact to the surface.
This type of flow is known as the Laminar flow. Mostly the turbulent bodyflow
is streamlined.
Difference between the turbulent body flow and the separated
flow
The stream line flow is mostly knows as the turbulent body flow. Which
always follows with the outlines of the bodyon the other hand the separated flow
doesn’t follow the outline of the body.
Figure 7: Flow layers on the top of the bus.
Figure 8: Separated flow body VS Streamline
body
Figure 9: Small zone of turbulence

7. Different types of fluid over the vehicle body
Streamline flows
Streamline flow is a kind of flow which at any point over the car surface
remains constant with the same pattern. On the other hand if the flow follows
the outline of the vehicle body which is streamlined. In this casethe flow can be
say that it is attached. It is shown in the figure 6
Stagnation regions
This is the nature of the air which is strikes a vehicle body it divides into
the different flow lines over the body. The divided flow goes over and under the
body. The point where the air strikes and then stays stationary the position or the
part where this occurs is known as the stagnation region.
Separation bubbles
When the air touches the surface of the car at some points the air doesn’t
perform the streamline flows it detaches. So the separation bubbles are formed in
the area between the air flows separates and then reattaches. The figure 10 shows
the separation and the reattachment of the flow
Figure 10: Separation and reattachment of the flow.

8. Reynolds Number
Boundary layer and the thickness of the layer affect the friction on the
surface, flow separations etc. Theflow patterns depends onthelength of the body,
viscosity, speed and the density which can also be name as the one quantity as
knows as the Reynolds Number.
Turbulence
It can be defined as the unsteady or the unusual manner of the flow over
the car bodyis known as the turbulence flow. This can be termed as the swirling
or the eddies turbulent motions which varies of sizes.
Vortices
In the flow regions there are some parts which often knows as the vortices
which is mostly formed by the swirling flow structures, which mostly occurs with
the whirlwinds. This project is given based on the backlight angle of the car so
the vortices which formed at the backof the carare knows as the trailing vortices.
These vortices also decide the lift situation of the car which either is positive or
the negative.
These vortices do not exist on the very long length in space. It merges in the
surface or formed the closed loop shape for example like a smoking ring.
Karman Vortices
These types of vortices are formed mostly in the flow over the bluff bodies
like buses, trucks or the simple car body shapes like Ahmed model to study the
basic aerodynamics of the vehicle body. These types of vortices have the
alternative patterns at a regular frequency. The frequency of these vortices is
known as the Strouhal frequency.
Figure 11: Karman Vortices

9. New Model
To study the aerodynamics over the car bodyat the back angle by using the CFD
technology for this report it is proposed to study the air flow by using the Ahmed
model. For the designing of this
Body of Vehicle and its concepts
The saloon cars are described in different books in different theory books. Cars
are the bluff bodies with the drag coefficient of 0.3-0.4. (Watkins & Vino, 2008)
There are multiple theories behind the vehicle body and its concepts. When the
solid bodystartto experience any fluid either it is gas orthe liquid, the fluid resists
the motion of the object in the opposite direction. The large effect on the
aerodynamics of the body is dependent on the geometry of the object. Drag and
lift depends mainly on the size of the object which experience under the fluid.
Form drag is determined by the cross-sectional shape of the object. (National
Aeronautics and Space Administration, 2012) model solid works was used
Fluid (Air) Flow structure around the vehicle with different
backlight angle:
This project basically focused on the rear backlight angle of a car. The area
of this research will be based on the three common backlight angles which
can be classified as follows:
Figure 12: Ahmed model dimensionsin mm

10. • Fast back
• Square back
• Backlight angle
Fast back
A vehicle with the fast back is with the roof which has steady slant from the
front of the vehicle bodyto the back. On the other hand it is the roof which
proceeds to the rear and near the base of the car it gets closer. These fast back
mostly have the curved shapes vehicles from the back. The fast back is
rarely found in the today’s vehicle design. (Brennan, 2011)
It is clearly seen in the above figure. In these types of cars the upper surface has
relatively pressure which draws the air along sides of the car upwards and it will
leads to the creation of intense, conical vortices. On the other hand in olden days
teardrop-based cars with the extend tail these kind of cars were the old shape of
fast back cars. It was considered as the very useful shape because the size of this
shapewas huge and it produces thelong vehicles. This type ofvehicle has increase
in the chamber which used to increase the lift. In the figure.14 Lay from the
university of Michigan in 1930s explain the results of the rear-end
Figure 13: Fastback with the flow patterns

11. As it is clearly seen from the above figure and the values of the drag as tail
length is increase the drag coefficient goes in increasing.
Square back
These types of vehicle bodies mostly have the square back. Happian explains
the flow structure of this vehicle as it is characterized by large, low pressure
wake. In this case the airflow unable to follow the body surface around the
sharp edges, it is clearly shown in the figure.16. Square back air flow shows
the large turbulent wake. This will lead to the large number of drag because
the air flow is unable to attach the surface of the vehicle body.
Figure 14: Results from W.E Lay’s study with different tail lengths.
Figure 15: Mercedes-Benzes CLS-class (fastback)

12. Relationship between the fastback and the square back:
It is clearly seen from the above graph that the fastback cars mostly fall in the
category of cars with the backlight angle ranging from 0 to 30. The graph shows
that the in fast back vehicles at around 15o the minimum drag can be achieved.
On the other hand as the angle goes on increasing it will give the high number of
drag, this is in the case of square back vehicles.
Figure 17: Backlight angle against drag coefficient
Figure 16: square back with re-circulatory

13. Thank you