1. The document discusses vehicle aerodynamics and drag reduction techniques. It covers the different components of drag force, methods of drag reduction like streamlining body shapes and adding spoilers, and the use of wind tunnel testing to evaluate designs. 2. Key methods to reduce drag mentioned are tapering rear sections, adding rear spoilers, using air dams, and shaping the underbody for lower pressure and slower flow. 3. Wind tunnel testing allows collecting data on forces, pressures, and flows to analyze designs and improvements. Both model and full-scale testing are used.

1. ME5551-Vehicle Body Engineering
Automotive Body Design (Aerodynamics)

2. Introduction
The three primary influences upon fuel efficiency are
1. mass of the vehicle,
2. efficiency of the engine
3. the aerodynamic drag.
Objectives of improvement of flow past vehicle bodies:
• reduction of fuel consumption
• more favourable comfort characteristics (mud deposition on body, noise,
ventilating and cooling of passenger compartment)
• improvement of driving characteristics (stability, handling, traffic safety)

3. Introduction
Vehicle aerodynamics includes three interacting flow fields:
• flow past vehicle body
• flow past vehicle components (wheels, heat exchanger, brakes, windshield),
• flow in passenger compartment

4. Aerodynamic forces

5. Aerodynamic forces
The force and moment coefficients are defined respectively as
where F is force (lift, drag or side),M is the moment, ρ is air density, v is
velocity, A is reference area (usually frontal area) and l is a reference
length.
The product CfA is commonly used as the measure of aerodynamic
performance, particularly for drag.

6. Drag

7. Drag
Five constituent elements
1. Form drag (or) Pressure drag
• component that is most closely
identified with the external shape of the
vehicle.
• As a vehicle moves forward the motion
of the air around it gives rise to
pressures that vary over the entire body
surface

8. Drag
If a small element of the surface area is
considered then the force component acting
along the axis of the car, the drag force,
depends upon
1. the magnitude of the pressure,
2. the area of the element upon which it
acts
3. and the inclination of that surface
element
Thus it is possible for two different designs,
each having a similar frontal area, to have
very different values of form drag.

9. Drag
2. Skin friction drag (or) surface drag (or) viscous drag
• As air flows across the surface of the car frictional forces are generated
giving rise to this second drag component
• If viscosity of air considered constant, the frictional forces depend upon
the shear stresses generated in the boundary layer.

10. Drag
3. Induced drag
It is an aerodynamic drag force that
occurs whenever a moving object
redirects the airflow coming at it.
The lift created due to air-flow,
whether positive (upwards) or
negative, induces changes in the
character of the flow which
themselves create an induced drag
force.

11. Drag
4. Excrescence drag
• Consequence of all those components that disturb the otherwise
smooth surface of the vehicle and which generate energy absorbing
eddies and turbulence.
• Eg: wheels and wheel arches, wing mirrors, door handles, rain gutters
and windscreen wiper blades but hidden features such as the exhaust
system are also major drag sources.
• Sum of all can be as high as 50%

12. Drag
5. Internal drag
• Arising from the cooling of the engine, the cooling of other
mechanical components such as the brakes and from cabin
ventilation flows
• Contributes up to 10% of total drag

13. Drag Reduction
• Drag reduction leads to noise reduction as well
• But drag reduction can have adverse effects on the dynamic stability

14. Drag Reduction
• Point ‘a’ has the largest pressure
contributing to form drag. To address this
drag source the following may be done
• The lowering and rounding of the sharp,
front corner
• Reduction or elimination of the flat,
forward-facing surface at the very front of
the car

15. Drag Reduction
• A second separation zone is observed at
the base of the windscreen ‘c’
• To reduce this wind screen rake is
provided.
• But larger windscreen rake => reduced
cabin space, reduced headroom and poor
visibility due to internal reflection

16. Drag Reduction
Reduction of base drag - boat-tailing,
tapering the rear part of the body, rounding
up of trailing edges
• Tapering of rear part results is reduction of
the size of rear separation bubble and
increase of pressure
• Rear spoiler and increase of boot height
reduces drag and lift simultaneously
• Slanted trailing edges can cause
longitudinal vortices, as shown in figure,
increasing the drag and lift

17. Drag Reduction
Reduction of base drag - boat-tailing,
tapering the rear part of the body, rounding
up of trailing edges
• Tapering of rear part results is reduction of
the size of rear separation bubble and
increase of pressure
• Rear spoiler and increase of boot height
reduces drag and lift simultaneously
• Slanted trailing edges can cause
longitudinal vortices, as shown in figure,
increasing the drag and lift
High tail low drag design

18. Drag Reduction

19. Drag Reduction
Reduction of underbody drag - reduction of
roughness, decrease of the velocity in the
underbody gap
• Air dams - reduce the lift force acting on
the front axle by reducing the pressure
beneath the front of the car.
• This is achieved by restricting the flow
beneath the nose which accelerates with a
corresponding drop in pressure.
• Air dam reduces the overall drag despite
the generation of an additional pressure
drag component.

20. Drag Reduction
Reduction of underbody drag - reduction of
roughness, decrease of the velocity in the
underbody gap
• The shaping of the floor-pan at the rear of
the car also offers the potential for reduced
drag (Fig).
• As the flow diffuses (slows) along the
length of the angled rear underbody the
pressure rises, resulting in reduced form
drag and also a reduced base area

21. Wind Tunnel Testing
• A wind tunnel - comprises a test
section where a model or vehicle
can be mounted and viewed
whilst air is either blown or more
usually sucked over it by a fan or
number of fans.
• Visualization techniques such as
adding smoke trails to the airflow
can be used to gain an
understanding of how certain
geometric features affect its
aerodynamic performance.

22. Wind Tunnel Testing
• Wind tunnels enable the following
data to be acquired:
-aerodynamic forces; drag, lift, side
force and moments; pitch, yaw, roll;
variation of aerodynamic forces and
moments with yaw; surface pressure
distribution; the influence of
different vehicle details on the above;
vehicle cooling drag; assessment of
brake cooling flows; aero-acoustic
data; affect of aerodynamic features
and aids.

23. Wind Tunnel Testing
Model scale testing
• Model scale testing is ideal for rapid
evaluation of the influence of different body
styles and features on the vehicle's
aerodynamics.
• Uses smaller facilities with lower running
costs than their full scale counterparts.
• Sophisticated test methods such as
employing a moving ground plane can be
done without great expense.
• Typical model sizes range from 30% to 60% of
full scale.

24. Wind Tunnel Testing
Model scale testing
• It is necessary to achieve geometric and
dynamic similarity.
• Usually Re is considered for this purpose
• The forward motion of a vehicle results not
only in relative motion between the vehicle
and the surrounding air but also between the
vehicle and the ground. In the wind tunnel it
is therefore necessary to move the ground
plane at the same speed as the bulk air flow,
and this is usually achieved by the use of a
moving belt beneath the model.

25. Wind Tunnel Testing
Full scale testing
• It is increasingly common to eliminate the model scale stage of an
aerodynamic development program and proceed directly to full scale
testing.
• This can be on program where a large proportion of the styling
evaluations have been carried out virtually with CFD being used to
predict the vehicles aerodynamic performance or
• On program where full scale styling models have been produced. If full
scale styling models or prototypes are available then the cost of model
production is avoided.

26. Drag Force Calculation
• Drag force 𝐹𝑑 =
1
2
𝜌𝑣2𝐶𝑑𝐴
• 𝜌 - fluid density (through which the vehicle is moving)
• 𝑣 – vehicle velocity
• 𝐶𝑑 - Drag Coefficient
• 𝐴 – projected or cross sectional area of the vehicle

27. Drag Force Calculation
1. The mass of a car is 1500 kg. The shape of the body is such that its
aerodynamic drag coefficient is 𝐶𝑑 = 0.330 and the frontal area is
2.50m2. Assuming that the drag force is proportional to v2 and
neglecting other sources of friction, calculate the power required to
maintain a speed of 100km/h as
(i) the car moves on a flat road.
(ii) the car climbs along a hill with slope of 3.20o.

28. Drag Force Calculation
2. The following wind tunnel test data from a 1:16 scale model of a bus are
available:
Considering air density as 1.23 kg/m3, calculate and plot the dimensionless
aerodynamic drag coefficient, versus Reynolds number 𝑅𝑒 =
𝜌𝑣𝑤
𝜇
, where w
is model width and v is speed. Find the minimum test speed above which 𝐶𝑑
remains constant. Estimate the aerodynamic drag force and power
requirement for the prototype vehicle at 100 km/hr. (The width and frontal
area of the prototype are 8 ft and 84 ft2, respectively.)

29. Drag Force Calculation

30. Drag Force Calculation

31. Drag Force Calculation

32. Drag Force Calculation

33. Drag Force Calculation
3. Measurements of drag force are made on a model automobile in a
towing tank filled with fresh water. The model length scale is 1/5 that
of the prototype. Determine the fraction of the prototype speed in air
at which the model test should be made in water to ensure dynamically
similar conditions. Measurements made at various speeds show that
the dimensionless force ratio becomes constant at model test speeds
above V = 4 m/s. The drag force measured during a test at this speed is
F = 182 N. Calculate the drag force expected on the prototype vehicle
operating at 90 km/hr in air.

34. Drag Force Calculation
4. A 1/8-scale model of a tractor-trailer rig is tested in a pressurized
wind tunnel. The rig width, height, and length are W=0.305 m, H=0.476
m, and L=2.48 m, respectively. At wind speed V=75.0 m/s, the model
drag force is F=128 N. (Air density in the tunnel is ρ=3.23 kg/m3.)
Calculate the aerodynamic drag coefficient for the model. Compare the
Reynolds numbers for the model test and for the prototype vehicle at
55 mph. Calculate the aerodynamic drag force on the prototype vehicle
at a road speed of 55 mph into a headwind of 10 mph.