Mechanical Engineering

1. Vehicle Aerodynamics – Effects of side
winds, Vehicle exposed to cross wind

2. Vehicle Body Aerodynamics?
 Is a branch of physics and is concerned when air flows around
a body
 Deals with a solid body moving through atmosphere and the
interaction which takes place
 Depends on varying wind speeds and wind direction
 The most important factor is reducing fuel economy in
aerodynamics

3. Importance of aerodynamic study?
• Drag force is reduced. So maximum speed and acceleration are
obtained for the same power output.
• Fuel consumption of the vehicle can be reduced to the
maximum (about 35% of fuel cost).
• Gives better appearance and styling.
• By reducing the various forces and moments, good stability
and safety can be achieved.
• Helps to provide proper ventilation system.
• Aerodynamic noise could be reduced which results in quite
running of the vehicle.

4. Features incorporated to aerodynamics?
• Large corner radii on the front section
• Low level front skirts
• Fairings above and behind the driver’s cab
• Aerodynamically shaped mirrors
• Enclosed steps

5. Aerodynamic Drag
Aerodynamic drag is usually insignificant at low vehicle speed
but the magnitude of air resistance becomes considerable with
rising speed.

6. Aerodynamic Drag
Drag force depends on the following factors:
• The size and shape of the vehicle (area of nose end, vehicle
super structures)
• Travel speed
• Air density
• Wind direction and strength

7. FL = ½ Cd Aρv2
FL : Drag Force
Cd : Vehicle’s drag co-efficient
A : Area of vehicle’s nose end
ρ : Density of air
v : Velocity of the vehicle
PL = FL v = ½ Cd Aρv3
PL = Power to overcome drag

8. Types of Drag
• Pressure drag (or) Form drag (or) Profile drag - 57%
• Induced drag - 8%
• Friction drag - 10%
• Inference drag - 15%
• Cooling and ventilation system drag – 10%

9. Pressure Drag

10. Typical static pressure coefficient distribution

11. Induced Drag (or) Lift Drag
• This lift force depends on the contour of the body
• Normal Speed - Not a serious problem
• High Speed – Serious problem
• Lift affects stability and braking performance
• To reduce the accelerated flow upper side
• To reduce the deceleration flow under side will reduce the
aerodynamics lift

12. Friction Drag (or) surface drag (or) Skin friction
• This is caused by friction force between the boundary layer
and the body surface
• Shear stress generated in the boundary layer
• Laminar boundary layer should be maintained
• Well – Polished surface is not only attractive but also makes
the vehicle more economical
• Body smoothness is of the order of 0.5 to 1.0 microns.

13. Interference drag
• The flow over many exterior components interact with the
flow over basic body shape and this leads to drag
• Exterior components includes door handles, mirrors, aerials
and badges which project out from normal surface
• Mechanical components Engine parts, suspension system,
exhaust system, frame rail
• Exterior ornaments must be placed where the velocity is
minimum
• Door closer must be placed in a close proximity and
longitudinally in line with each other

14. Internal Drag (or) Cooling and ventilation system Drag
• Arising from cooling of the engine
• Brakes, cabin ventilation flows
• Contributes 10% of the overall drag

15. Drag Co-efficient
The aerodynamic drag coefficient (Cd) is a measure of the
effectiveness of a streamline aerodynamic body shape in
reducing the air resistance to the forward motion of the vehicle.

16. Aerodynamic forces
• Force of air drag in the direction of motion with wind angle
along longitudinal axis (Px)
• Cross wind force (Py)
• Aerodynamic lift (Pz)

17. Longitudinal air drag (Px)
 The longitudinal component of the resultant of pressure
distribution
 Magnitude is represented by
Px = (Cx p A V2 ) / 2

18. Cross wind air drag (Py)
 Its formed by asymmetric flow of air around the vehicle
body when the wind angle is not equal to zero
 Magnitude is represented by
Py= (Cy p A V2 ) / 2

19. Aerodynamic lift (Pz)
 It’s the vertical component of the resultant of the pressure
distribution over the vehicle body due to flow of air around it
 Magnitude is represented by
Pz = (Cz p A V2 ) / 2

22. Effect of cross wind

23. Aerodynamic moments
• Rolling moment - Mx
• Pitching moment – My
• Yawing moment - Mz

24. Rolling moment - Mx
 This moment caused by the cross wind force Py about the
longitudinal axis
 Magnitude is given by
Mx = Py a = Cmx p A L V2 / 2
 This effect is dangerous for tall van, where side force acts
much above the C.G
 The only solution is increase the wheel track

25. Pitching moment - My
 This moment caused about y-axis by cross wind force Py or
the longitudinal force Px
 Magnitude is given by
My = Pz b = Cmy p A L V2 / 2

26. Yawing moment - Mz
 This moment caused about z axis by cross wind force Py
 Magnitude is given by
Mz = Py c = Cmz p A L V2 / 2

27. DRIVER’S SEAT
 Important factors in relaxed, stress free driving is
ergonomically optimized seating for the driver and co-
driver
 Seating position and convenient adjustment
 Fundamental importance too that all the control elements
easy to reach

28. Geometrical relations to driver’s seat
 Seat position in relation to the steering wheel, foot controls and
other secondary controls

29. Geometrical relations to driver’s seat

30. Geometrical relations to driver’s seat
 The driver’s seat should be adjustable 45mm horizontally and
30mm vertically
 Avoid slanted body position will cause abnormal tension in the
pelvis muscle leading to tiredness
 The back rest must support the trunk in a vertical position and
extend in vertical up to spine . The improper seating leads to
breathing problems
 A good back rest relieves the neck and shoulders
 The angle between seat and back rest less than the passenger
seat

31. Geometrical relations to driver’s seat
 avoid
The seat being tilted too much
Insufficient room for the legs
An unsuitable angle between the seat and back rest
 The seat cushion should be fairly rigid and must having
sufficient shock absorbing capacity