Vehicle Aerodynamics

ByT.Veeramahantesh SwamyProfessor & HOD, Automobile Engineering

Introduction

� The importance of good aerodynamic parameters in the design of vehicles is being increasingly recognized.

� Constant striving for improved economy dictates the importance of the study of vehicle drag and the importance of vehicle handlingemphasis is the need.

� It should be stressed that above the speed of 70 km/hr aerodynamics exceeds 50 % of the total resistance to motion above 100km/hr it is very important factor.

� Aerodynamic forces and moments acting on the vehicle are balanced by wheel reactions.

� For analysis it must be considered vehicle in otion as a mass having six degrees of freedom.

Aerodynamic Forces insert aero drag vs rolling resistance

� PX = force of air drag in the direction of motion with the wind along the longitudinal axis (t = 0) or

� PX = force of air drag in the direction of motion if the wind vector forms angle t with longitudinal axis.

� PZ = aerodynamic lift

� PY = cross wind force

Aerodynamic Moments

PY about x-axisRollingMX

PY about z-axisYawingMZ

PZ and PX about y-axisPitchingMY

Caused byMomentSuffix of moment

Aerodynamic drag

Aerodynamic drag of a vehicle includes many factors such as

� Profile drag � Induced drag� Skin friction drag� Interference drag (trim mouldings,handles)� Cooling and ventilation drag

Relative values of coefficient aerodynamic drag value components

Aerodynamic drag components 1985 car model

0

0.1

0.2

0.3

0.4

0.5

Drag components

Val

ue

of

dra

g

Series1 0.262 0.03 0.04 0.064 0.053 0.45

Cxp Cxi Cxf Cxz Cxc Cx

Px = (Cx r V2 A) / 2

Cx = Cxp + Cxi + Cxf + Cxz + Cxc

Profile drag plays a vital role

Aerodynamic Drag Force

Aerodynamic drag force = PX

PX = CX r V2 A / 2 whereCX = coefficient of aerodynamic dragr = air density kg.sec2 /m4

V = velocity of the vehicle m/secA = cross sectional area of the vehicle

viewed from the front

How to reduce aerodynamic drag?

Effective design of air ducts.Ducts of the vehicle10.0Cooling and ventilation drag

15.0

10.0

8.0

57.0

% of total drag

Items to minimised, shape optimsed, rounded off.

Projecting door handles, mirrors, aerials, etc. Also projections below the floor such as axles, tow bars etc.

Interference drag

Surface to be maintained smooth to have laminar flow of wind.

Friction force between the boundary layer and the body surface.

Friction drag

Aerodynamic lift to be reduced as much as possible.

Vortices formed at the side of the vehicle and travelling downwind from it, caused by the aerodynamic lift of the body.

Induced drag

streamlines should be continuous and separation fo boundary layer with its attendent vortices should avoided.

Longitudinal section of the vehicle body

Profile drag

Solution Caused byAerodynamic drag component

Achievements of aerodynamic studies. Insert picture

� The development of aerodynamic studies decreased the drag coefficient values for cars, buses and coaches initially, but however the effect of styles demanded by market studies and three box shapes between 1952 to 1960 made them to increase a bit.

� Only sports cars have resisted this tendeany.

Aerodynamic lift and pitching moment

� The vertical component of the resultant of the pressure distribution is called aerodynamic lift (Pz).

� Majority of the vehicles have a profile which has the same effect as an aerofoil, with the streamlines over the upperpart of the body having a higher velocity than the streamlines below the vehicle. Insert figure

� Aerodynamic lift is applied through the centre of pressure of the body profile and since this point does not correspond to the centre of gravity, it creates a pitching moment about the lateral axis.

� Aerodynamic lift force PZ = CZ r V2A /2� Pitching moment MY = PZ.e = Cmy rV2A l / 2

where e = distance between the c.p. and c.g. in metersCmy = pitching moment coefficient and

l = characteristic length in meters (e.g. wheel base)The influence of PX on the pitching moment is usally small, as the vertical

seperation between cp and cg is not great.

Effect of aerodynamic lift and pitching moment

� Both these have undesirable effects.� The lift will tend to reduce the pressure between the wheels and

ground, which causes the loss of steering on the front axle and loss of traction on the rear wheels.

� Pitching moment is usually negative, i.e., nose down and this makes the rear axle lift off the ground and further reduce the available traction.

� As the cross wind angle increases the lift coefficient CZ increases parabolically.

� CZ increases from 0.15 to 0.55 for a bus while it increases from 0.4 to 1.0 for a car with three compartment.

� Aerodynamic lift increases from 8 to 10% of total weight for carspeeds moving to around 150kmph and 15 to 25 % of total weight of sports car speeds moving to around 300kmph.

Side force, yawing moment and rolling moment

� Side force is caused because of the asymmetric flow of wind around the vehicle body. PY

� This side force acts at the centre of pressure and creats moments about the centre of gravity – yawing moment MZ about the z-axis and a rolling moment MX about the x-axis.

� Side force PY = CY r V2A /2� CY increases linearly with increase in wind incident angle t, less for race

cars and more for normal cars.� Yawing moment MZ = PY e = CMZ r V 2 l / 2 � CMZ Values increase with the incident angles of wing ‘t’, shall be more for

highly streamlined vehicle (low values of CX, CY,CZ)� Therefore a vehicle with poor aerodynamic properties will have a lower

yawing moment coefficient.� The use of stabiliser fins can improve the directional stability.

Side force, yawing moment and rolling moment

� The centre of aerodynamic forces is usually above the centre of gravity and hence the side force PY creates a rolling moment MX. about the longitudinal (X) axis.

� Rolling moment MX = PY Ze = CMX r V 2 l / 2 Where Ze is the height of centre of thrust above the centre of gravity.CMX is the rolling moment coefficient..

� The rolling moment generated has a considerable effect the weight distribution on the wheels. Wheel load on the same axle can varyupto 100 kg. this effect is very dangerous for coaches particularly for tall vans, where the side force acts a long way above the centre of gravity.

� The only solution shall be to increase the wheel track for such vehicles.

Location of intakes and outlets

� Intakes should be placed in areas of positive pressures and outlets at negative pressures.

� This is very important for rear engine vehicles. The air inlet should be facing the rear of car. Often side openings which would be expected to provide air intakes for the rear engine are actually outlets.

� A car designed with sufficient aerodynamic considerations is costly however the economy can be improved.

� Careful study of these forces and moments result in vehicles which are much easier and safer.