1452 Exposed wheel air flow pattern

When a wheel rotates some distance from the ground air due to its viscosity attaches itself to the tread and in turn induces some of the surrounding air to be dragged around with it. Thus this concentric movement of air establishes in effect a weak vortex, see Fig. 14.36(a). If the rotating wheel is in contact with the ground it will roll forwards which makes windtunnel testing under these conditions difficult; this problem is overcome by using a supportive wheel and floor rig. The wheel is slightly submerged in a well opening equal to the tyre width and contact patch length for a normal loaded wheel and a steady flow of air is blown towards the frontal view of the wheel. With the wheel rig simulating a rotating wheel in contact with the ground, the wheel vortex air movement interacts and distorts the parallel main airstream.

Typical Suv Coefficient Drag
Fig. 14.33 (a and b) Effect of rear end tail extension on drag coefficient

The air flow pattern for an exposed wheel can be visualized and described in the following way. The air flow meeting the lower region of the wheel will be stagnant but the majority of the airstream will flow against the wheel rotation following the contour of the wheel until it reaches the top; it then separates from the vortex rim and continues to flow towards the rear but leaving underneath and in the wake of the wheel a series of turbulent vortices, see Fig. 14.36(b). The actual point of separation will creep forwards with increased rotational wheel speed. Air pressure distribution around the wheel will show a positive pressure build-up in the stagnant air flow front region of the wheel, but this changes rapidly to a high negative pressure where the main air flow breaks away from the wheel rim, see Fig. 14.36(c). It then declines to some extent beyond the highest point of the wheel, and then remains approximately constant around the rear wake region of the wheel. Under these described conditions, the exposed rotating wheel produces a resultant positive upward lift force which tends to reduce the adhesion between the tyre tread and ground.

Fig. 14.34(a and b) Effect of underbody roughness on drag coefficient

Reducing pressure

Air dam

(rear & partial sides)

Reducing pressure

Air dam

(front & partial sides)

'Upthrust 1 r positive I lift (+ve)

'Upthrust 1 r positive I lift (+ve)

Air dam

(rear & partial sides)

Reducing pressure

Wake negative pressure (-ve)

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Front end Semi high vented stagnant pressure

(a) Rear end underbody air dam

High speed low pressure

Air dam

(front & partial sides)

Reducing pressure

Wake negative pressure (-ve)

High speed low pressure

Low pressure

Rear end vented

High speed low pressure

Low pressure

Rear end vented

(b) Front end underbody air dam

(b) Front end underbody air dam

0 100 200 Dam height (mm)

Fig. 14.35(a-c) Effects of underbody front and rear end air dams relative to the lift and drag coefficient

Concentric streamlines

Weak vortex

Concentric streamlines

Weak vortex

Velocity gradient

(a) Wheel rotation in still air away from the ground

Separation vortices

Velocity gradient

(a) Wheel rotation in still air away from the ground

Separation vortices

Positive lift (+ve)

Point of separation

Airstream

Low pressure region

High pressure region

Positive lift (+ve)

Point of separation

Airstream

Low pressure region

High pressure region

Resultant upward

Pressure distribution

Resultant upward

Pressure distribution

lift (+ve) 1 Ù

T /

/

(b) Air flow pattern with wheel rolling on the ground

. Negative pressure (-ve)

Direction of motion

Positive pressure (+ve)

(c) Air pressure distribution with wheel rolling on the ground

Fig. 14.36 (a-c) Exposed wheel air flow pattern and pressure distribution

14.5.3 Partial enclosed wheel air flow pattern

(Figs 14.37(a and b) and 14.38(a-c)) The air flow passing beneath the front of the car initially moves faster than the main airstream, this therefore causes a reduction in the local air pressure. At the rear of the rotating wheel due to viscous drag air will be scooped into the upper space formed between the wheel tyre and the wheel mudguard arch (see Fig. 14.37(a and b)). The air entrapped in the wheel arch cavity circulates towards the upper front of the wheel due to a slight pressure build-up and is then expelled through the front end wheel to the mudguard gap which is at a lower pressure in both a downward and sideward direction. Decreasing the clearance between the underside and the ground and shielding more of the wheel with the mudguard tends to produce a loss of momentum to the air so that both

Fig. 14.37(a and b) Wheel arch air flow pattern aerodynamic lift CLW and drag CDW coefficients, and therefore forces, are considerably reduced Fig. 14.38(a-c).

14.5.4 Rear end spoiler (Fig. 14.39(a-c)) Generally when there is a gentle rear end body profile curvature change, it will be accompanied with a relatively fast but smooth streamline air flow over this region which does not separate from the upper surface. However, this results in lower local pressures which tend to exert a lift force (upward suction) at the rear end of the car. A lip, see Fig. 14.39(a), or small aerofoil spoiler, see Fig. 14.39(b), attached to the rear end of the car boot (trunk) interrupts the smooth streamline air flow thereby slowing down the air flow and correspondingly raising the upper surface local air pressure which effectively increase the downward force known as negative lift. A typical relationship between rear lift, front lift and drag coefficients relative to the spoiler lip height is shown in Fig. 14.39(c). The graph shows a general increase in negative lift (downward force) by increasing the spoiler lip height. However, this is at the expense of a slight rise in the front end lift coefficient, whereas the drag coefficient initially decreases and then marginally rises again with increased spoiler lip height. It should be appreciated that the break-up of the smooth streamline air flow and the increase in rear downward pressure should if possible be achieved without incurring too much, if any, increase in front end positive lift and aerodynamic drag.

14.5.5 Negative lift aerofoil wings

A negative lift wing is designed when attached to the rear end of the car to produce a downward thrust thereby enabling the traction generated by the rear driving wheels to be increased, or if a forward negative lift wing is fitted to improve the grip of both front steering wheels.

With the negative lift wing the aerofoil profile is tilted downward towards the front end with the negative and positive aerofoil section camber at the top and bottom respectively, see Fig. 14.40(a). The airstream therefore moving underneath the

(b) Front view

(b) Front view

0

Fig. 14.38 (a-c) Effect of underside ground clearance on both lift and drag coefficients

aerofoil wing has to move further and faster than the airstream flowing over the upper surface; the pressure produced below the aerofoil wing is therefore lower than above. Consequently there will be a resultant downthrust perpendicular to the cord of the aerofoil (see Fig. 14.40(b)) which can be resolved into both a vertical downforce (negative lift) and a horizontal drag force. Enlarging the tilt angle of the wing promotes more negative lift (downthrust) but this is at the expense of increasing the drag force opposing the forward movement of the wing, thus a compromise must always be made between improving the downward wheel grip and the extra drag force opposing the motion of the car. Racing cars have the aerofoil wing over the rear wheel axles or just in front or behind them, see Fig.

14.40(c). However, the drag force produces a clockwise tilt which tends to lift the front wheels of the ground, therefore the front aerofoil wings (see Fig. 14.40(c)) are sometimes attached low down and slightly ahead of the front wheels to counteract the front end lift tendency.

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