146 Afterbody drag

14.6.1 Squareback drag (Figs 14.41 and 14.42) Any car with a rear end (base) slope surface angle ranging from 90° to 50° is generally described as a squareback style (see Fig. 14.42). Between this angular surface inclination range for a squareback car there is very little change in the air flow pattern

High speed Reduced speed low pressure increase in pressure

High speed Reduced speed low pressure increase in pressure

0 20

80 100

40 60 Lip height (mm)

14.39(a-c) Effect of rear end spoiler on both lift and drag coefficients

0 20

High speed , low pressure

High speed , low pressure

o Turbulent

Reduced speed increase in pressure

Aerofoil spoiler o Turbulent

80 100

40 60 Lip height (mm)

14.39(a-c) Effect of rear end spoiler on both lift and drag coefficients

and therefore there is virtually no variation in the afterbody drag (see Fig. 14.41). With a parallel sided squareback rear end configuration, the whole rear surface area (base area) becomes an almost constant low negative pressure wake region. Tapering the rear quarter side and roof of the body and rounding the rear end tends to lower the base pressure. In addition to the base drag, the afterbody drag will also include the negative drag due to the surrounding inclined surfaces.

14.6.2 Fastback drag (Figs 14.41 and 14.43) When the rear slope angle is reduced to 25° or less the body profile style is known as a fastback, see Fig. 14.43. Within this much reduced rear end inclination the airstream flows over the roof and rear downward sloping surface, the airstream remain ing attached to the body from the rear of the roof to the rear vertical light-plate and at the same time the condition which helps to generate attached and trailing vortices with the large sloping rear end is no longer there. Consequently the only rearward suction comes from the vertical rear end projected base area wake, thus as the rear end inclined angle diminishes, the drag coefficient decreases, see Fig. 14.41. However, as the angle approaches zero there is a slight rise in the drag coefficient again as the rear body profile virtually reverts to a squareback style car.

14.6.3 Hatchback drag (Figs 14.41, 14.44 and 14.45)

Cars with a rear sloping surface angle ranging from 50° to 25° are normally referred to as hatchback

Slower airstream higher pressure

Direction of motion

Slower airstream higher pressure

Direction of motion

Airstream

Aerofoil

Faster airstream lower pressure

Airstream

Aerofoil

Faster airstream lower pressure

¡a) Air streamlining for an inclined negative lift aerofoil wing

Direction of air flow

Higher pressure

Higher pressure

Drag

Resultant reaction

Down-thrust (negative lift)

(b) Lift and drag components on an inclined negative lift wing

Down-thrust (negative lift)

Drag

Resultant reaction

(b) Lift and drag components on an inclined negative lift wing

Front negative lift wing

Rear negative lift wing

Negative lift

Front negative lift wing

Rear negative lift wing

Negative lift

Fig. 14.40 (a-c) Negative lift aerofoil wing considerations

Negative lift (down-

thrust) (c) Racing car incorporating negative lift wings

Fig. 14.40 (a-c) Negative lift aerofoil wing considerations style, see Fig. 14.44. Within this rear end inclination range air flows over the rear edge of the roof and commences to follow the contour of the rear inclined surface; however, due to the steepness of the slope the air flow breaks away from the surface. At the same time some of the air flows from the higher pressure underfloor region to the lower pressure roof and rear sloping surface, then moves slightly inboard and rearward along the upper downward sloping surface. The intensity and direction of this air movement along both sides of the rear upper body edging causes the air to spiral into a pair of trailing vortices which are then pushed downward by the downwash of the airstream flowing over the rear edge of the roof, see Fig. 14.45. Subsequently these vortices re-attach themselves on each side of the body, and due to the air's momentum these vortices extend and trail well beyond the rear of the car. Hence not only does the rear negative wake base area include the vertical area and part of the rearward projected slope area where the airstream separates from the body profile, but it also includes the trailing conical vortices which also apply a strong suction pull against the forward motion of the car. As can be seen in Fig. 14.41 there is a critical slope angle range (20 to 35°) in which the drag coefficient rises steeply and should be avoided.

0 30 60 90

Slope angle (deg)

Fig. 14.41 Effect of rear panel slope angle on the afterbody drag

Fig. 14.42 Squareback configuration
Fig. 14.43 Fastback configuration

Fig. 14.44

Hatchback configuration

Negative pressure

Hatchback configuration

Fig. 14.45 Hatchback transverse and trailing vortices

14.6.4 Notchback drag (Figs 14.46,14.47(a and b) and 14.48(a and b))

A notchback car style has a stepped rear end body profile in which the passenger compartment rear window is inclined downward to meet the horizontal rearward extending boot (trunk) lid (see Fig. 14.46). With this design, the air flows over the rear roof edge and follows the contour of the downward sloping rear screen for a short distance before separating from it; however, the downwash of the airstream causes it to re-attach itself to the body near the rear end extended boot lid. Thus the basewake area will virtually be the vertical rear boot and light panel; however, standing vortices will be generated on each side of the body just inboard on the top surface of the rear window screen and boot lid, and will then be projected in the form of trailing conical vortices well beyond the rear end of the boot, see Fig. 14.19(b). Vortices will also be created along transverse rear screen to boot lid junction and across the rear of the panel light.

Experiments have shown (see Fig. 14.47(a)) that the angle made between the horizontal and the inclined line touching both the rear edges of the roof and the boot is an important factor in determining the afterbody drag. Fig. 14.47(b) illustrates the effect of the roof to boot line inclination; when this angle is increased from the horizontal the drag coefficient commences to rise until reaching a peak at an inclination of roughly 25°, after which the drag coefficient begins to decrease. From this it can be seen that raising the boot height or extending the boot length decreases the effective inclination angle $e and therefore tends to reduce the drag coefficient. Conversely a very large effective inclination angle $e will also cause a reduction in the

Fig. 14.46 Notchback configuration

Various boot heights

10 20 30 40

Effective slope angle (Oe) deg

Fig. 14.47(a and b) Influence of the effective slope angle on the drag coefficient

Various boot heights

10 20 30 40

Effective slope angle (Oe) deg

Fig. 14.47(a and b) Influence of the effective slope angle on the drag coefficient drag coefficient but at the expense of reducing the volume capacity of the boot. The drag coefficient relative to the rear boot profile can be clearly illustrated in a slightly different way, see Fig. 14.48(a). Here windtunnel tests show how the drag coefficient can be varied by altering the rear end profile from a downward sloping boot to a horizontal boot and then to a squareback estate shape. It will be observed (see Fig 14.48(b)) that there is a critical increase in boot height in this case from 50 to 150 mm when the drag coefficient rapidly decreases from 0.42 to 0.37.

14.6.5 Cabriolet cars (Fig. 14.49) A cabriolet is a French noun and originally referred to a light two wheeled carriage drawn by one horse. Cabriolet these days describes a car with a folding roof such as a sports (two or four seater) or roadster (two seater) car. These cars may be driven with the folding roof enclosing the cockpit or with the soft roof lowered and the side screen windows up or down. Streamlining is such that the air flow follows closely to the contour of the nose and bonnet (hood), then moves up the windscreen before overshooting the screen's upper horizontal edge (see Fig. 14.49). If the rake angle of the windscreen is small (such as with a high mounted off road four wheel drive vehicle) the airstream will be deflected upward and rearward, but with a large rake angle windscreen the airstream will not rise much above the windscreen upper leading edge as the air flows over the open driver/passenger

0.42

0.40

50 100 150 200 250 Boot (trunk) height (h)mm

Effect of elevating the boot (trunk) height on the drag coefficient

compartment towards the rear of the car. A separation bubble forms between the airstream and the exposed and open seating compartment, the downstream air flow then re-attaches itself to the upper face of the boot (trunk). However, this bubble is unstable and tends to expand and burst in a cyclic fashion by the repetition of separation and re-attachment of the airstream on top of the boot (trunk), see Fig. 14.49. Thus the turbulent energy causes the bubble to expand and collapse and the fluctuating wake area (see Fig. 14.49), changing between h1 and h2 produces a relatively large drag resistance. With the side windscreens open air is drawn into the low pressure bubble region and in the process strong vortices are generated at the side entry to the seating compartment; this also therefore contributes to the car drag resistance. Typical drag coefficients for an open cabriolet car are given as follows: folding roof raised and side screens up CD 0.35, folding roof down and side screens up CD 0.38, and folding roof and side screens down CD 0.41. Reductions in the drag coefficient can be made by attaching a header rail deflector, streamlining the roll over bar and by neatly storing or covering the folding roof, the most effective device to reduce drag being the header rail deflector.

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