Aero Dynamics
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Transcript of 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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