vd chap- road loads

7/31/2019 VD Chap- Road Loads

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Road Loads

Aerodynamics (Mechanics of air flow around a vehicle,

Pressure distribution on a Vehicle, Aerodynamic Forces,

Drag Components, Aerodynamics Aids, Drag, Side Force,Lift Force, Pitch Moment, Yawing moment, Rolling

Moment, Crosswind Sensitivity)

Rolling Resistance(Factors affecting rolling resistance,

Typical coefficient) Total road Loads and effect of Road loads on Fuel

economy.


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Aerodynamics Aerodynamics makes its major impact on modern cars and

trucks through its contribution to "road load." Aerodynamic

forces interact with the vehicle causing drag, lift (or down load),

lateral forces, moments in roll, pitch and yaw, and noise.

These impact fuel economy and handling .

The aerodynamic forces produced on a vehicle arise from two

sources form (or pressure) drag and viscous friction.

First, the mechanics of air flow will be examined to explain thenature of the flow around the body of the vehicle. Then, vehicle

design features will be examined to show the qualitative

influence on aerodynamic performance.


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Mechanics of Air Flow Around a Vehicle

The gross flow over the body of a car is governed by the

relationship between velocity and pressure expressed in

Bernoulli's Equation (Bernoulli's Equation assumesincompressible flow, which is reasonable for automotive

aerodynamics, whereas the equivalent relationship for

compressible flow is the Euler Equation.)

The equation is:Pstatic + Pdynamic = Ptotal

P s+ 1/2 V2 = Pt

where:

= Density of air

V = Velocity of air (relative to the car)

1


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Bernoulli's equation states that the static plus the dynamic pressure of the air

will be constant (Pt) as it approaches the vehicle. Visualizing the vehicle as

stationary and the air moving (as in a wind tunnel), the air streams along lines,

appropriately called "streamlines." A bundle of streamlines forms a stream

tube. The smoke streams used in a wind tunnel allow stream tubes to be

visualized as illustrated in Figure


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At a distance from the vehicle the static pressure is simply the ambient, orbarometric, pressure (Patm)

The dynamic pressure is produced by the relative

velocity, which is constant for all streamlines approaching the vehicle.

Thus the total pressure, Pt, is the same for all streamlines and is equal to Ps

+ V2. As the flow approaches the vehicle, the stream tubes split, some going

above the vehicle, and others below. By inference, one streamline must go

straight to the body and stagnate (the one shown impinging on the bumper

of the car). At that point the relative velocity has gone to zero.

With the velocity term zero, the static pressure observed at that point on thevehicle will be Pt. That is, if a pressure tap is placed on the vehicle at this

point, it will record the total pressure.


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Consider what must happen to the streamlines flowing above the hood. As they

first turn in the upward direction, the curvature is concave upward.

At a distance well above the vehicle where the streamlines are still straight, the

static pressure must be the same as the ambient.

In order for the air stream to be curved upward, the static pressure in that

region must be higher than ambient to provide the force necessary to turn the

air flow.

If the static pressure is higher, then the velocity must decrease in this region in

order to obey Bernoulli's Equation.

Conversely, as the flow turns to follow the hood (downward curvature at the lip

of the hood) the pressure must go below ambient in order to bend the flow, and

the velocity must increase. These points are illustrated in Fig. showing flow

over a cylinder.


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Pressure and velocity gradients in the airflow over a body


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In the absence of friction the air would simply flow up over theroof and down the back side of the vehicle, exchanging

pressure for velocity as it did at the front.

In that case, the pressure forces on the back side of the vehicle

would exactly balance those on the front, and there would be

no drag produced.

The drag is due in part to friction of the air on the surface of

the vehicle, and in part to the way the friction alters the main

flow down the back side of the vehicle. Its

explanation comes about from understanding the action of

boundary layers in the flow over an object. Consider a uniform

flow approaching a sharp-edged body as shown in Fig.


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Development of boundary layers


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Flow separation in an adverse gradient pressure


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Because of the low pressure, the flow along the sides of the car will

also attempt to feed air into this region and may add to the potential for

separation. The general air flow patterns over the top and sides of a carare shown in Figure


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Pressure Distribution on Vehicle

Pressure co-efficients plotted normal to surface

Pressure distribution along centre line of a car


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Note that a negative pressure is developed at the front edge of the

hood asthe flow rising over the front of the vehicle attempts to turn and

follow horizontally along the hood.

The adverse pressure gradient in this region has the potential to

stall the boundary layer flow creating drag in this area. Near the base of the windshield and cowl, the flow must he turned

upward, thus high pressure is experienced.

Over the roof line the pressure again goes negative as the air flow

tries to follow the roof contour. The pressure remains low down over the backlite and on to the

trunk because of the continuing curvature.


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Aerodynamic lift and drag on different vehicle styles


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Effect of separation point dirt deposition at the rear

The high degree of turbulence in the separation zone entrains moisture and dirt kickedup from the roadway by the tires. If the separation zone includes these items, dirt will

be deposited on these areas and vision will be obstructed. Figure illustrates this

phenomenon.


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Aerodynamic forces

As a result of the air stream interacting with the vehicle, forces and

moments are imposed. These may be defined systematically as the three forces

and three moments shown in table below, acting about the principal axes of the

car. The reactions are as follows:

Direction Force Moment

Longitudinal (x-axis, positive rearward) Drag Rolling Moment

Lateral (y-axis, positive to the right) Side force Pitching Moment

Vertical (z-axis, positive upward) Lift Yawing Moment


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Drag Components

Drag is the largest and most important aerodynamic force

encountered by passenger cars at normal highway speeds.

The overall drag on a vehicle derives from contributions of

many sources. Various aids may be used to reduce the effects of

specific factors. Table lists the main sources of drag and the

potential for drag reductions in these areas estimated for cars in

the 1970s.

D ffi i t C t T i l l


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Drag co-efficient Component Typical values

Forebody 0.05

Afterbody 0.14

Underbody 0.06

Skin Friction 0.025

Total Body Drag 0.275

Wheels and wheel wells 0.09

Drip rails 0.01

Window Recesses 0.01

External mirrors 0.01

Total protuberance Drag 0.12

Cooling system 0.025

Total internal drag 0.025Overall total drag 0.42

Vehicle of 1980s

Cars 0.30-0.35

Vans 0.33-0.35

Pick up tricks 0.42-0.46


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For the vehicle represented in the table, approximately 65% (.275/.42) of the

drag arises from the body (forebody, afterbody, underbody and skin friction)

The major contributor is the afterbody because of the drag produced by theseparation zone at the rear.

It is in this area that the maximum potential for drag reduction is possible.

Fig. below shows the influence of rear end inclination angle on the drag for

various lengths of rear extension (beyond the rear edge of the roof line)

Influence of rear end inclination on drag


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Forebody drag is influenced by design of the front end and windshield

angle.

Generally the "roundness" of the front end establishes the area overwhich the dynamic pressure can act to induce drag. Fig. shows the

influence of the height of the front edge of the vehicle

Influence of front end design on drag


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The location of this point determines the location of the

streamline flowing to the stagnation point.

This streamline is important as it establishes the separation of

flow above and below the body.

Minimum drag is obtained when the stagnation point is kept

low on the frontal profile of the vehicle.

A well-rounded shape, in contrast to the crisp lines traditionally

given to the frontal/grill treatment of passenger cars, is equally

important to aerodynamics.

A rounded low hood line can yield reductions of 5 to 15% inthe overall drag coefficient.


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The windshield establishes the flow direction as it approaches

the horizontal roof.

Thus its angle has a direct influence on drag, particularly on

trucks.

Shallow angles reduce drag, but complicate vehicle design by

allowing increased solar heating loads and placing more critical

demands on the manufacturer of the windshield to minimize

distortion at shallow angles.

Fig. shows the change in drag as the windshield angle is

increased from the nominal angle of 28 degrees.

With a steep angle, the air velocity approaching the windshield

is reduced by the high pressure in that region. With a shallow

angle, the wind speed will he higher, adding to the

aerodynamic loads on the windshield wipers.


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Influence of windshield angle on drag


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The underbody is a critical area generating body drag. Suspensions,

exhaust systems and other protruding components on the underbody are

responsible for the drag.

The air flow in this area is a shear plane controlled by zero air speed on the

road surface, and induced flow by drag of the underbody components.

The recognized fix for minimizing underbody drag is the use of a smooth

underbody panel.

Protuberances from the body represent a second area where careful designcan reduce drag.

The wheels and wheel wells are a major contributor in this

class. Significant drag develops at the wheels because of the turbulent,

re-circulating flow in the cavities. Fig. illustrates the complex flow patterns

that occur around a wheel. The sharp edges of the wheel cutout provide opportunities to induce flow in

the horizontal plane, while the rotating wheel tends to induce circulation in

the vertical plane.

These effects allow the wheel to influence more flow than simply that which

is seen because of its frontal area presented to the flow.


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Air flow re-circulation in a wheel well


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The cooling system is the last major contributor to drag.

Air flow passing through the radiator impacts on the engine and the firewall,

exerting its dynamic pressure as drag on the vehicle. The air flow pattern inside a typical engine compartment may be very

chaotic due to the lack of aerodynamic treatment in this area. Fig. below

illustrates this situation compartment before spilling out through the

underside openings.

The momentum exchange translates directly into increased drag.

Air flow pattern in a typical engine compartment


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Aerodynamic Aids

Bumper Spoilers : Front bumper spoilers are aerodynamic surfaces extending downward

from the bumper to block and redirect the shear flow that impacts on the

underbody components. While the spoiler contributes pressure drag, at

least with a shallow depth the reduction in underbody drag is more

significant. As spoiler depth is increased, eventually the increasing pressure drag

outweighs further reduction in underbody drag and the overall drag

increases. The low pressure produced also has the effect of reducing

front-end lift.

Air-Dams : Air dams are flow-blocking surfaces installed at the perimeter of the

radiator to improve flow through the radiator at lower vehicle speeds.

The improvement derives from the decreased pressure behind the

radiator/fan, and may reduce drag by reduction of pressure on the

firewall.


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Deck-lid Spoilers :

Spoilers and air foils on the rear deck may serve several purposes.

By deflecting the air upward, as shown in Figure 4.18, the pressure isincreased on the rear deck creating a down force at the most

advantageous point on the vehicle to reduce rear lift.

The spoilers may also serve to stabilize the vortices in the separation

flow, thus reducing aerodynamic buffeting. In general, they tend to

increase drag.


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Window and Pillar Treatments :

Drip rails and offsets between windows and pillars on acarbody are always

sources of drag. Disturbance to the air flow in these regions may cause small separation zones.

The disturbance to the air in the high-velocity air stream causes momentum

loss which creates drag.

Smooth contours are important not only for drag reduction, but also for

reduction of aerodynamic noise.

Optimization :

Adaptation of streamlined shapes from other disciplines (e.g., ship building)

at the turn of the century.

Application of the knowledge of fluid mechanics from aircraft aerodynamicsaround the 1930s.

Current efforts to optimize the numerous details of the design to obtain good

air flow characteristics.


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Drag :- Because air flow over a vehicle (or any other body for that matter)is so complex, it is necessary to develop semi-empirical models to represent

the effect. Therefore, aerodynamic drag is characterized by the equation:

The term 1/2 V2 in the above equation is the dynamic pressure of the air,

and is often referred to as the "q," typically expressed in units of pounds persquare foot. The drag coefficient, CD, is determined empirically for the car.

1


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Air Density : The air density is variable depending on

temperature, pressure, and humidity conditions. Density at other conditions can be estimated for the

prevailing pressure, Pr, and temperature, Tr, conditions by

the equation:

2


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Drag co-efficient : The drag coefficient is determined

experimentally from wind tunnel tests or coast down tests. Thedefinition of CD comes from Eq.

The drag coefficient varies over a broad range with different

shapes. Fig. shows the coefficients for a number of shapes. In

each case it is presumed that the air approaching the body has

no lateral component (i.e., it is straight along the longitudinalaxis of the vehicle).

Note that the simple flat plate has a drag coefficient of 1.95.

3


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Drag co-efficient of various bodies


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This coefficient means that the drag force is 1.95 times as large as the dynamic

pressure acting over the area of the plate.

The extreme drag produced by the plate results from the fact that the air spilling

around the plate creates a separation area much larger than the plate itself.

In practice, a vehicle driving along a road experiences atmospheric winds

in addition to the wind component arising from its speed.

The atmospheric wind will be random in direction with respect to the vehicle's

direction of travel. Thus the relative wind seen by the vehicle will consist of the large component due to

its speed, plus a smaller atmospheric wind component in any direction. Fig.

illustrates how the relative wind will vary randomly.

Relative wind seen by motor vehicle on the road


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When the atmospheric wind blows toward the vehicle a

"headwind" is present, and the total velocity used in is:

4


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Side Force The lateral wind components will also impose a side force on

the vehicle attempting to change its direction of travel.

The exact effect depends both on the vehicle and the nature of

the wind.

In strong crosswinds, the side force is typically greater than thedrag force, such that the angle of the overall wind force is

much greater than the relative wind angle.

The aerodynamic shape of the vehicle and even the steering

system characteristics affect performance in this sense. Crosswind behavior is an important enough aspect of

aerodynamics that it is discussed separately in a later section.


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Under steady-state wind conditions, the side force imposed on

a vehicle in a crosswind is given by:

5


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Fig. shows typical characteristics of CS as a function of wind angle.

The side force coefficient is zero at zero relative wind angle, and grows

nearly linearly with the angle for the first 20 to 40 degrees.

The slope of the gradient varies somewhat with vehicle type, but will

typically he in the range of 0.035/deg to 0.06/deg.

Side force co-efficient as a function of yaw angle for typical vehicles


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Lift Force The pressure differential from the top to the bottom of the vehicle causes

a lift force.

These forces are significant concerns in aerodynamic optimization of a

vehicle because of their influence on driving stability.

The lift force is measured at the centerline of the vehicle at the center of the

wheelbase. The force, LA, is quantified by the equation:

6


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As seen in Figure, the lift force is dependent on the overall shape

of the vehicle. At zero wind angle, lift coefficients normally fall in the range

of 0.3 to 0.5 for modern passenger cars but under crosswind conditions

the coefficient may increase dramatically reaching values of 1 or

more


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Lift can have a negative impact on handling through the

reduced control forces available at the tires.

Front lift, which reduces steering controllability, is reduced by

front bumper spoilers and by rearward inclination of front

surfaces.

Lift at the rear of the vehicle, which also reduces stability, is

the most variable with vehicle design.

In general, designs that cause the flow to depart with a

downward angle at the rear of the vehicle create rear lift. Lift

can be decreased by use of underbody pans, spoilers, and achange in the angle of attack of the body (a 3-degree cant on

the body can decrease lift force by 40 percent).

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