aero dynamics

7/28/2019 Aero Dynamics

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Aerodynamics

This study concerns about the airflow around the vehicle body.

At a speed of about 70km/hr, aerodynamic drag exceeds to 50% of total resistance to

motion & above 100km/hr it is the most important factor.

Important of aerodynamic study:

To reduce drag force & achieve maximum speed and acceleration for the samepower output.

If drag force is reduced, fuel consumption of the vehicle can be reduced to themaximum 35% of fuel cost could be reduced by proper stream timing.

Good aerodynamic design, gives better appearance and styling. By reducing the various forces and moments, good stability and safety can be

achieved.

This study helps to provide proper ventilation system. Helps to understand the dirt flow and exhaust gas flow patterns. With proper aerodynamic design, aerodynamic noise could be reduced, which

results in quite running of the vehicle.

Aerodynamic drag:

This includesForm drag - 57%

Lift drag - 8%

Surface drag - 10%

Interference drag - 15%

Cooling and ventilation

System drag - 10%

Form drag:

Body shape that minimize positive aerodynamic force and the first and minimize

negative aerodynamic force i.e. suction on the rear exhibit low form drag small cars,

constantly retard and last back bodies have reduced form drag.


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Lift drag

A car body provides accelerated air flow and the corresponding low pressure onits upper surface. Especially in leading edge of head wind shield and leading edge

of roof. This produces aerodynamic lift.

Air flow and pressure distribution around a car: Air velocity is increased over the roofpressure drop suction zone. At the base of the windscreen stream have separates - +ve pressure due to low

velocity.

At the stagnation point our moves around the nose speeds up +ve pressure. Liftis not a serious problem at normal speed. If affect stability and brakingperformance.


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Surface drag:

The viscosity of air produces at thin layer (boundary) next to vehicle body. The drag at small surface imperfections with this layer is considered as surface

drag.

Body smoothness should be the order of 0.5 to 1.0 microns.Interference drag:

The flows over many exterior components interact with the flow over basic bodyshape.

This leads to interference drag. Components hood arrangement, wind shield wipers, radio aerial, rear view

mirror, no plate, door handles, rain gutters, exposed door hinges and roof racks.

Cooling and ventilation system drag;

The ideal circuit for engine radiator cooling system requires a smooth air intake

opening leading to a diffusing duct permitting the velocity energy to be converted into

pressure energy with low losses, can factory requirements of grill style mechanical fan drive

do not practice this system, thus circuit has to be optimized for reduced drag.

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AERODYNAMIC LIFT AND PITCHING MOMENT

The vertical component of resultant of pressure distributionAerodynamic lift. General vehicle profilesurface effect as air force stream line body higher velocity

on the upper parts and lower velocity below the vehicle.

Influence of force Px on pitching moment is usually small, as vertical separationbetween CG & CP is not great.

Explanations

Both lift & pitching moment have undesirable effect Lift tend to reduce pressure between wheels and ground Loss of steering on the front wheel Loss of traction on the rear axle Pitching momentnegativenose down Rear axle lift off the ground Further less of traction

Fig 1 Effect of cross wind is shown.

Lift coefficient increase parable with wind angle up to two and three times its value

when there is no side wind.


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Fig 3 Importance of lift

For salon carreaches 100kg8 to 10% of total weight.

For sports carreaches 130kg1 to 2% of total weight.

Fig 4 Lift coefficient effect for various vehicle profiles

Threebox configuration greater

spread of lift coefficient ( from 0.4to

10)

Flat frontSmallest range(0.15 to

0.55 )


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SIDE FORCE, YAWING MOMENT & ROLLING MOMENT:

Yawing moment try to turn the

vehicle away from the direction

motion loss of direction control.

Stream line vehicle have poor Cy,

however they have better Cmax. Thus

research has be carries out for better

values of Cy& Cmax.

Side force is formed by

asymmetric flow round the vehiclebody. When the wind angle is not

equal to zero. This force acts at CP

& creates moment about CG

yawing moment about Z axis

rolling moment about X- axis.


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LOW DRAG CARS:

Optimization Add on device AB into for low drag

Optimization

Step by step modification of the of the body detail obtain a better relationship bet drag

& geometric parameter of the part. Modification is carried out up to saturation point.

Boat Tailing can be incorporated tapering the side inward from front to rear

slopping the roof towards the rear end.

OPTIMIZATION TECHNIQUE

Modification of fore body:

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Fig shows on example for a front end that was developed purely empirically the initial

shape in designed fore body and illustrated in each case for comparison. The bar graph

shows the % change in drag in comparison with the initial shape. A small correction of the

shape on the front edge above reduced the drag by 6%. The front end shapes 3, 4 & 5

represent equal variants; they provide an important of 10% shapes 6 & 7 already showsignificant stylistic deviation from the initial shape. They are intended to show the maximum

improvement possible. In the present example a drag reduction of 14% was achieved with the

particular detail

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MODIFICATION OF WIND SHIELD:

If the air flows cross the front edge of the hood without separation, then separation

they occur at the cowl, while further downstream the flow will reattach somewhere on the

wind shield (fig 1). Fig 1 shows clearly how the point of separation S is displaced towards the

front and the point of reattachment R towards the rear as the angle of the wind shield being

steeper.

As the wind shield becomes flatter the aerodynamic drag decrease. This has been confirmed

by several anthers. Fig 2 & fig 3 shows the measurement made on research automobiles. The

measurement values from the development of Audi 100III are included in fig4 from all these.


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It can be concluded that the direct influence of wind shield inclination on drag is only

moderate.

Wind shield inclination angle of more than 60 are not practical because of light

diffusion. I addition large highly inclined windshield lead to increased solar heating passenger

compartment.


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MODIFICATION OF ROOF:

Roofs are designed with a convex shape to ensure sufficient rigidity. For stylistic

reasons an attempt is made to keep the convexity as small as possible. If this convexity is

increased the drag coefficient can be reduced fig. 5 shows this for a medium sized Notch back

car. If the convex shape is designed so that the frontal area A of the vehicle increases which

inform increases the aerodynamic drag. On the other hand; the original roof height is kept

constant the front and rear windows must be curved into the roof contour to eliminate

obstruction of the roof. This leads to expensive windows but results in lower drag. The

measurements plated in fig 6 show the same tendency for a fast back configuration near, thechord length of the roof are was used as a reference variable for the curvature instead the

height h as in fig.


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FiGEFFECT OF CAMBER ON DRAG OF A CAR WITH HIGH FAST BACK

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MODIFICATION OF VEHICLE REAR END:

Recovery of pressure can be achieved with boat tailing can be also be obtained by the

tapering the bottom upwards. Fig. shows corresponding test data on the research car.


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With a long diffuser, a notable reduction in the drag can be achieved with a very small angle

.

However the effect is only assured with a smooth underside.

Measurements made on the unicorn research automobile gave similar results.

Here too the longer diffuser has the greater effect.

Also note is that the lift at the rear axle is reduced by the diffuser.

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aero dynamics

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