Vehicle Aerodynamics – Effects of side

winds, Vehicle exposed to cross wind

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

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.

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

Aerodynamic Drag

Aerodynamic drag is usually insignificant at low vehicle speed

but the magnitude of air resistance becomes considerable with

rising speed.

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

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

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%

Pressure Drag

Typical static pressure coefficient distribution

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

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.

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

Internal Drag (or) Cooling and ventilation system Drag

• Arising from cooling of the engine

• Brakes, cabin ventilation flows

• Contributes 10% of the overall drag

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.

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)

Longitudinal air drag (Px)

The longitudinal component of the resultant of pressure

distribution

Magnitude is represented by

Px = (Cx p A V2 ) / 2

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

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

Effect of cross wind

Aerodynamic moments

• Rolling moment - Mx

• Pitching moment – My

• Yawing moment - Mz

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

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

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

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

Geometrical relations to driver’s seat

Seat position in relation to the steering wheel, foot controls and

other secondary controls

Geometrical relations to driver’s seat

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

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