Vehicle Aerodynamics

1. Design Alternatives
• Requirements for high performance
• low drag coefficient and a small frontal area to achieve
minimum air drag.
• high negative lift-to-drag ratio. Priority is given to either
drag reduction or negative lift increase, depending on
intended purpose.
• higher negative lift at the rear than at the front axle. No lift
should occur from yawed air flow up to about 10 to 15
degrees yaw angle, depending on intended use.
• in the case of yawed air flow, generation of a stabilizing
yawing moment about the vertical axis.
• sufficient cooling and ventilation without significant
deterioration of the aerodynamic coefficients.

2. 1.Drag and lift
• The drag force is proportional to the product
of drag coefficient and frontal area.
• It is not always possible to lower the drag
coefficient because of regulations.
• The reduction of the frontal area is limited by
the wide track required for handling reasons,
and the large diameter wheels necessary for
racing brakes.

3. Contd…
Fig.1 Aerodynamic coefficients of different front end configurations

4. Contd…
Fig. 2 Cl/Cd graphs for different racing cars

5. Contd…
Fig. 3 Front axle lift as a function of driving speed and the use of aerodynamic aids

6. Contd…
Fig. 4 Variation of drag coefficient as a function of front end height

7. Contd…
Fig.6. Aerodynamic coefficients of different rear end configurations

8. Contd…
• There are three main ways to generate
negative lift:
1. variation of the basic vehicle configuration
2. mounting of negatively inclined wings
3. built-in ground effect.
• The basic vehicle configuration for minimum
lift or negative lift has a low concave nose, a
smooth upper surface, and an elevated tail
section.

9. Contd…
Fig.7 Rear axle lift as a function of driving speed and the use of aerodynamic
aids

10. Contd…
Fig.8 Negative lift coefficient of a ground effect car as a function of ground clearance

11. Contd…
• The ground effect is based on the theory that
the vehicle underbody and the track surface
constitute a venturi nozzle producing a low
pressure area below the car and thus creating
negative lift forces.
• The negative pressure is maintained by lateral
skirts fixed to the car and preferably touching
the ground.

12. 2. Handling
• Vehicles reaching very high speeds, such as racing
cars but in particular record vehicles, are required
to have inherent directional stability to enhance
safety and to reduce stress on the driver.
• The vehicle should have a relatively long
wheelbase and negative lift should be present at
all axles, being more pronounced at the rear than
at the front to produce more understeer as speed
increases.

13. Contd…
Fig.9 Influence of a rear vertical fin on yaw moment

14. 3.Cooling and ventilation
• Although there are general guidelines for the
optimum design of air inlet, cooler location or
air outlets, each vehicle requires its own
solutions according to the special conditions,
vehicle layout, and function.

15. Contd…
Fig.10 Movable wheel-travel controlled flaps for stabilizing purposes

16. High-efficiency radiator arrangement
• A large volume of air flow through a radiator is achieved by
creating the highest possible pressure difference between
the cooling air inlet and outlet, so that the air is conducted
through a closed channel.
• To minimize the increased drag due to cooling air flow, a
radiator with a small inlet combined with a diffuser for
relatively slow flow velocities through the radiator matrix,
The warmed air is then accelerated and expelled.
• This system could be used to produce thrust if the air were
to be sufficiently warmed. In practice, though, such
conditions are unlikely to be realized because of lack of
space.

17. Contd…
Fig.11 High-efficiency radiator arrangement

18. Contd…
• The most convenient position for the cooling unit
is at the front of the vehicle.
• Positioning the intake at the stagnation point and
the outlet on the negative pressure area on the
top surface of the nose produces a large airflow
due to the high pressure difference.
• A further advantage is that the negative pressure
under the vehicle underbody—and therefore
negative lift—is not affected.

19. Contd…
Fig. 12 Air inlets and outlets of the Porsche 956

20. Contd…
• One of the main drawbacks is the restricted amount of
space in the nose, which limits the possibility of optimizing
the air ducting.
• The air inlets required for front brake cooling are arranged
in the vehicle nose.
• The oil cooler, intercooler, engine air induction and water
radiator are located in lateral sills close to the engine.
• Engine cooling air is taken in through a NACA inlet in the
roof and carried away through louvres in the underbody.
• Three NACA ducts located on the rear upper side of the car
are air intakes for the rear brakes and the transmission
radiator. The cockpit is ventilated through a slot in front of
the windshield.

21. Development methods and simulation
• The development of high-performance vehicles in the wind tunnel poses
problems that are more acute than when developing road vehicles for the
following reasons:
1. Wind tunnels available for vehicle investigation are usually not designed for
speeds of 250-400 km/h (150-250 mile/h) for sports cars and racing cars or
over 1000 km/h (620 mile/h) for record vehicles.
2.So special effect such as the actual attitude of the freely suspended car
relative to the ground and possible distortion of the outer panels under
the influence of wind forces cannot be recorded under all driving
conditions.
3.The boundary layer, building up on the test section floor affects the flow
conditions below the vehicle underbody and cannot be neglected,
because of the significantly lower ground clearance of racing and record
cars as compared to more usual road vehicles. The boundary layer reduces
the air flow rate under the car and does not equate with conditions on the
road. So the forces involved, particularly the vertical forces, cannot be
determined with precision.

22. Simulation
• The simulation of wheel rotation in wind
tunnel testing is difficult due to severe
problems in measuring the forces transferred
between rotating wheel and ground.
• Correct reproduction of the flow conditions
around the wheels is of special importance for
evaluation of vehicles with uncovered wheels.
• The test section floor is equipped with a
moving belt running at air speed.

23. Contd…
• The wheels are not connected to the body but
supported from outside and driven by the
moving belt.
• The disadvantages of this solution are that the
forces acting on the wheels cannot be
measured, the data evaluated are affected by
the suspending gear connecting the model
with the balance, and yawed air flow can only
be simulated by complicated technical means.

24. Contd…
Fig.13 Examples of a panelled sports car body (above) and a
racing car body (below)

25. Trends in future high-performance vehicle
development
• Sports cars:
-To improve fuel economy, further efforts will be made
to decrease drag, which is attainable by reducing
frontal area as well as drag coefficient.
-For production cars, zero overall lift is desirable, which
basically is in compliance with low drag, yet rear axle
lift should always be somewhat lower than at the front
to help handling and stability at high speeds.
- The best possible solution of the conflicting aims of
minimum drag and low cross-wind sensitivity will be a
stringent design goal for the future.

26. Racing cars
• Obviously development will be strongly
affected by future racing regulations.
• The present trend places emphasis on fuel
economy. This factor might put an end to the
emphasis on negative lift observed during
recent years.

27. Record vehicles
• Reaching the sound barrier has been a goal long
sought after and finally achieved.
• the sound barrier has been a goal long sought
after and finally achieved. Further increase in
speed will be limited because sufficiently long
test tracks are not available to allow such vehicles
to accelerate, be measured and come to a stop.
• Emphasis might instead be placed on realizing
fuel economy records at high average speeds,
thereby providing incentives to production car
development.