1

In this chapter the modifications that were carried out to reduce the drag and lift forces are

explained. Before carrying out the modifications, a study of the modifications on different

vehicle parts suggested by different researchers to reduce drag and lift forces was carried

out.

Study of Modifications

Several modifications in vehicle shapes to reduce aerodynamic drag and lift forces

are suggested by researchers from time to time. Below here is an illustration of some basic

modifications carried out by researchers on some vehicles and the results obtained. The

modifications were carried out on different parts of the vehicle. From these illustrations,

some basic ideas about shape modifications can be developed and can be applied to modify

different vehicle shapes on a random basis and can be further tested for drag and lift

reductions.

Forebody: Fig 5.1 shows an example for a front end that was developed purely empirically

by Hucho and Janssen.

Fig 5.1 Drag reduction by front end shape modifications

Here the development of the longitudinal midsection is illustrated. The initial shape is

designated ‘forebody 1’ and illustrated in each case for comparison. The bar graph shows

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

shape on the front edge alone reduced the drag by 6 percent. The front end shapes 3, 4 and 5

represent equal variants; they provide an improvement of 10 percent. Shapes 6 and 7

already show significant stylistic deviation from the initial shape; they are intended to show

the maximum improvements possible. In the present example a drag reduction a drag

2

reduction up to 14 percent was achieved with this particular detail. This explains how drag

can be reduced by modifying forebody i.e. by deviating the curvature more downwards.

Chamfering of the front edge of the hood was applied in the development of the

front end of the 1978 model VW Passat. As shown in Fig 5.2, the previous model had a

separation bubble at the engine hood, which contributed significantly to its drag.

Fig 5.2 Front end optimization, Volkswagen Passat (Dasher), model year 1978 (facelift)

The optimum front end indicated the possibility of a 15 percent reduction in drag. This

could be realized approximately with the adapted hood. At the same time the lift at the front

axle was reduced considerably.

To achieve a low aerodynamic drag it is not sufficient to design the front end simply

so that the air flows along it without separation. The position of the stagnation point

determines which portion of the flow passes over the vehicle and how much air must flow

between the bottom of the car and the road. Fig 5.3, after Buchheim, Deutenbach and

Luckoff, shows that there is an optimum stagnation point location. This depends upon the

shape of the vehicle and the design of the underside. Generally it can be stated that a low

stagnation point is favourable for low drag.

3

Fig 5.3 Effect of the stagnation point position on drag

The inclination of the engine hood also has an effect upon the drag. Fig 5.4, after Carr,

gives an example of this. Once the slope is steep enough to keep the flow attached, further

sloping does not reduce drag any further. The ‘optimum’ slope angle �F depending upon the

leading edge radius and on the windscreen rake.

Fig 5.4 Effect of bonnet slope � and windscreen rake � on drag

4

Windshield, A-pillar: If the air flows across the front edge of the engine hood without

separation, separation may occur at the cowl, while further downstream the flow will

reattach somewhere on the windshield. This phenomenon has been investigated by Scibor-Rylski. Fig 5.5 shows clearly how the point of separation S is displaced toward the front

and the point of reattachment R toward the rear as the angle � of the wind shield becomes

steeper.

Fig 5.5 Flow separation on the bonnet and reattachment on the windscreen, as a function of windscreen rake �

Here the longitudinal midsection is shown. In planes outboard of this section, the separation

point and the point of reattachment should be close to each other. The location and shape of

the three dimensional separation bubble is highly dependent upon the lateral curvature of

the windshield.

As the windshield becomes flatter, the aerodynamic drag decreases. This has been

known since the investigation by Lay and has been confirmed by several others e.g. Carr.

Fig 5.6 shows this on the VW 2000 research automobile according to measurements made

by Buchheim et al.

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Fig 5.6 Effect of windshield angle � on drag

The measured values according to Buchheim et al from the development of the Audi 100

III, model year 1982, are included in fig 5.7. From all these data it can be concluded that the

direct influence of windshield inclination on the drag is only moderate. The effect is

assumed to be more pronounced the more the flow is routed over the vehicle.

Fig 5.7 Effect of windshield angle � on drag

6

Windshield inclination angles � of more than 60o are not practical because of light

diffusion. In addition, large highly inclined windshields lead to increased solar heating of

the passenger compartment. Two effects are responsible for the favorable, though moderate,

influence of a highly inclined windshield on drag. First, the excessive speed in the area of

the A-pillar is reduced so that the momentum loss occurring at the point is smaller. Second,

the deflection of flow at the transition from the windshield to the roof is smaller. The low

pressure peak occurring there is therefore smaller and the positive pressure gradient in the

remaining flow is less steep. Hence the momentum loss in the boundary layer is lower,

allowing greater pressure recovery in the area of rear end. Therefore even if a strongly

inclined windshield does not contribute to a local drag reduction, it helps to improve the

flow over the rear part of the car and thus to reduce the overall drag.

An effective measure for reducing the drag is to round off the A-pillar. One example of this

is given in Fig 5.8, after Buchheim et al.

Fig 5.8 Development of the A-pillar and C-pillar of the Audi 100 III

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The flow of air around the A-pillar is also improved when the side window is recessed as

little as possible.

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. Fig 5.9 shows this for

a medium sized notchback car. If the convex shape is designed so that the frontal area A of

the vehicle increases, the aerodynamic drag of the vehicle decreases.

Fig 5.9 Effect of roof camber on drag of notchback car

On the other hand, if the original roof height is kept constant the front and rear windows

must be curved into the roof contour to eliminate obstruction of the view. This leads to

expensive windows but results in lower drag.

The measurements plotted in Fig 5.10 (after Buchheim et al) show the same tendency for a

car with a fastback. Here the chord length of the roof arch was used as the reference

variable for the curvature.

8

Fig 5.10 Effect of roof camber on drag of a car with a ‘high’ fastback

Aerodynamic drag reduces with increased convexity for two reasons. First, the higher

convexity allows for a larger radius at the transition from the windshield to the roof. This

results in a less pronounced suction peak at this location. The momentum loss in the

boundary layer during the following less steep adverse pressure gradient is therefore smaller

and the boundary layer itself is less endangered by separation. Second, the convexity

provides for gentle deflection of flow at the rear and the pressure rise at the rear end is

therefore enhanced. The convexity of the roof and the rear end shape must be carefully

matched.

Vehicle Rear End: An example of drag reduction by tapering the rear end is illustrated in

Fig 5.11, after Hucho, Janseen and Emmelmann.

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Fig 5.11 Boat-tailing applied to a notchback car

On the notchback car shown in longitudinal section the lateral taper was increased in

increments while holding the height of the trunk fixed. Starting with parallel side walls, i.e.

y = 0, a generally monotonic reduction of the drag is achieved up to shape B with increasing

taper. Further tapering then no longer improves the drag. Apparently the flow remains

attached up to the taper corresponding to shape B. The pressure recovery, which is

accomplished in this manner, provides for a reduction of the drag. Recovery of the pressure

can also be obtained by tapering the bottom upwards. Fig 5.12, after Buchheim et al, gives

corresponding test data on the Research Car 2000 from Volkswagen. With a long diffuser, a

notable reduction in drag can be achieved with a very small angle �. However this effect is

only assured with a smooth underside.

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Fig 5.12 Effect of underbody diffuser length and angle on drag

Measurements made by Potthoff on the Unicar research automobile gave a similar result

Fig 5.13). Here too the longer diffuser has the greater effect. Also of note is that the lift at

the rear axle is reduced by the diffuser.

Fig 5.13 Effect of underbody diffuser geometry on drag and rear axle lift

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The fact that two completely different types of flow occur depending upon the inclination of

the rear end, which lead to different drag, was observed for the first time during the

development of VW Golf I. Janseen and Hucho varied the angle of inclination of the rear

end � (see Fig 5.14) in increments. The drag coefficient is shown versus the angle of

inclination � in Fig 5.14.

Fig 5.14 Influence of rear end slope angle � on drag coefficient, separation line and wake,

measured on VW Golf I (Rabbit)

The development work in the wind tunnel began with a high angle of inclination at the rear

end, � = 45o. At this angle, flow separated at the end of the roof. The drag coefficient was

0.40. As the angle of inclination was diminished, the drag suddenly increased by 10 percent

at � = 30o. The line of separation jumped down to the lower edge of the inclined rear end.

Two strong inward-rotating longitudinal vortices were observed which induced very low

pressure on the slanted part of the back. As the angle was reduced further, the drag dropped

again. At � = 15o, a very flat angle, a drag minimum resulted. At still smaller angles the

same flow forms were reached as with a square back: CD = 0.40. In the area 28o< � <32o

a bistable condition was observed. Depending upon the curvature of the rear edge of the

roof, separation occurred at the top or bottom of the inclined rear end.

Front Spoiler

The friction drag along the underside of the vehicle is reduced with the aid of a front

spoiler. This also reduces the lift at the front of the car and increases the volumetric flow

through the cooling air duct.

The effect of the front spoiler on the pressure distribution was worked out by Schenkel (Fig

5.15). The pressure at the front part of the underside is greatly reduced. High velocities at

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these points must not be deduced from these high negative pressures under any

circumstances. Separated flow is present behind the spoiler.

Fig 5.15 Effect of spoiler height on the pressure distribution around a car

The lift at the front of the car is reduced by the front spoiler. Moreover, the low pressure on

the underside in the area of the engine compartment supports the flow of cooling air. The

front spoiler causes a growth of the boundary layer on the underside of the vehicle. This can

have an effect that an underbody which tapers up behind the rear axle cannot act as a

diffuser, because the low energy flow cannot withstand a pressure increase and therefore

separates from the contour.

The front spoiler must be matched carefully to the shape of the front end. This is shown in

the next example. The drag of a specific passenger car was reduced by 4 percent by

mounting an attachment front end (Fig 5.16) with shape A or shape B. The two shapes are

only slightly different. A spoiler was optimized for both. Front end A attained an 11 percent

reduction in the drag with spoiler ‘a’. Front end B provided a reduction of 16 percent with

spoiler ‘b’.

Fig 5.16 Matching of front end shape and spoiler geometry

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Rear spoiler: The effect of rear spoiler is basically different from that of the front spoiler.

While negative pressure is present on almost the entire rear end without a spoiler, the

spoiler creates a positive pressure over a large area. Fig 5.17 shows the pressure

distributions obtained on the rear of a car with and without rear spoiler, by Ohtani et al ,

which confirm the above stated facts.

Fig 5.17 Static pressure on the slope of a fastback; pressure increase due to the spoiler

If the pressure is plotted against the vehicle height z/h (Fig 5.18), the effect of rear spoiler

upon the drag becomes even clearer. The pressure at the front end of the car remains

unaffected by the rear spoiler. However, the pressure at the rear end is increased even in the

area of the wake.

Fig 5.18 Effect of rear spoiler on the pressure at the front and rear of a fastback

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That the rear spoiler reduces the lift at the rear of the car can be concluded directly from Fig

5.16. Rear spoilers can be used on fastback as well as notchback vehicles to reduce both

drag and lift.

Modifications Carried Out to Reduce Drag

Taking an idea from the modifications suggested in section 5.1, a new model was

created, keeping in mind the minimum variations of overall height, length and width. The

modified model is shown in Figs 5.19, 5.20 and 5.21 below.

Fig 5.19 Modified model in ICEM CFD 5.1

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Fig 5.20 Modified model (wireframe) in ICEM CFD 5.1

Fig 5.21 Another view of modified model

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In Fig 5.22 below, a comparison of the original and modified model is shown.

Fig 5.22 Comparison of original and modified models

The main modifications carried out are summarized as followed:

• A steeper front windshield.

• Engine hood bent downward.

• A more convex shaped roof.

• Proper matching of roof and rear portion.

• Increased angle of rear windshield.

• More rounded corners.

• Curved undersides at the front and the rear ends.

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Test was carried out on the modified model using the same boundary conditions and mesh

sizes as for Test 3 (refer Table 4.3). The boundary conditions and the results obtained are

given in Table 5.1 below.

Test Number 15 (Improved Model) ( A=1.592 m2 ) Surface Mesh Size 25 mm Global Mesh Size 500 mm

Mesh Sizes

Local Mesh Densities None Turbulence Model Shear Stress Transport Tyres Stationary Straight Wind Speed 50 m/s Side Wind Speed 0 m/s Wind Tunnel Standard

Minimum -4966.56 Pa Pressure(Reference) Maximum 1614.90 Pa

Minimum 1.16536 Kg/m3 Maximum 1.24496 Kg/m3

Air Density

Average 1.225 Kg/m3 Drag Force 1163.47 N CD = 0.4773 Lift Force 315.348 N CL = 0.1294

Table 5.1: Test 15

The frontal area was higher i.e. 1.592 m2 as compared to the original value of 1.51 m2.

The frontal area was calculated in the manner similar to as explained in section 4.1, Chapter

4.

For the modified model;

b = 1416.57 mm

h = 1387.60 mm

A = 0.81 × b × h = 0.81 × 1416.57 × 1387.60 = 1.592 m2

The pressure distributions on and around the car body are shown in Figs 5.23 and 5.24.

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Fig 5.23 Pressure distribution on the modified car body (front)

Fig 5.24 Pressure distribution on the modified car body (back)

As evident from the pressure distribution shown above, less negative pressure is developed

over the roof and on the back. Fig 5.25 below shows the air flow around the body. From the

figure it is evident the turbulent eddy formation in the wake region is reduced quite much,

which is the main reason of reduction of drag coefficient from 0.5124 to 0.4773.

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Fig 5.25 Less turbulence in the wake region of modified model

Modifications Carried Out to Reduce Lift

To reduce lift force, a wing type rear spoiler was attached to the original model as

shown below.

Fig 5.26 Wing type rear spoiler attached to the original model

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Fig 5.27 Wing type rear spoiler attached to the original model

The test was carried out using the same mesh sizes and boundary conditions as those for

Test 3 (refer Table 4.3). The boundary conditions and results obtained are summarized as

shown below:

Test Number 16 (Winged Model) Surface Mesh Size 25 mm Global Mesh Size 500 mm

Mesh Sizes

Local Mesh Densities None Turbulence Model Shear Stress Transport Tyres Stationary Straight Wind Speed 50 m/s Side Wind Speed 0 m/s Wind Tunnel Standard

Minimum -3950.87 Pa Pressure(Reference) Maximum 1590.60 Pa

Minimum 1.17765 Kg/m3 Maximum 1.24467 Kg/m3

Air Density

Average 1.228 Kg/m3 Drag Force 1226.76 N Lift Force 20.9393 N

Table 5.2: Test 16

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The pressure distribution and air flow around the car body are shown in Figs 5.28 and 5.29

below.

Fig 5.28 Air flow around winged model

Fig 5.29 Pressure distribution on the winged model

Fig 5.29 clearly shows local regions of high pressure created on the rear of the vehicle due

to the wing type spoiler, which is the reason of reduction of lift force to a quite low value of

20.9393 N. There is not much effect on the drag force.

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The earlier chapter describes the modifications that were made to reduce drag and lift

forces. The drag coefficient was reduced from 0.5124 to 0.4773. The turbulence in the wake

region was sufficiently reduced due to creation of more positive pressures on the roof and

the back. The lift force was reduced to a sufficient low value of 20.9393 N due to creation

of local regions of high pressure on the back of the vehicle. Other conclusions are that fine

locally dense meshes require more number of iterations; lift coefficients are not predicted

accurately due to insufficient physics, side wind tests are also doubtful due to insufficient

physics. Pressure distributions and streamline plots confirm according to the theoretical

descriptions for straight wind tests. The reduction in drag coefficient and lift force confirms

the applicability of the modifications carried out. Table below shows the percentage

reduction in relevant quantities after modifications (same mesh sizes and boundary

conditions).

Drag Coefficient

Lift Force

Original Model 0.5124

217.732 N

Modified Model 0.4773

Winged Model 20.9393 N

Percentage Reduction

6.85 % 90.38 %

Table 6.1

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Milovan Peric, CD adapco Group, Nurnberg Office, Germany.