144 Car body drag reduction

14.4.1 Profile edge rounding or chamfering

There is a general tendency for aerodynamic lift and drag coefficients to decrease with increased edge radius or chamfer: experiments carried out showed for a particular car shape (see Fig. 14.27(a)) how the drag coefficient was reduced from 0.43 to 0.40 with an edge radius/chamfer increasing from zero to 40 mm (see Fig. 14.27(b)), and there was a slightly greater reduction with chamfering than rounding the edges; however, beyond 40 mm radius there was no further advantage in increasing the edge radius or chamfer.

14.4.2 Bonnet slope and windscreen rake (Fig. 14.28(a-c))

Increasing the bonnet (hood) slope angle a from zero to roughly 10° reduces the drag coefficient, but beyond 10° the drag reduction is insignificant, see Fig. 14.28(b). Likewise, increasing the rake angle y reduces the drag coefficient (see Fig. 14.28(c)) particularly when the rake angle becomes large;

Direction of air flow

Turbulent flow over upper surface

Vortices

Turbulent flow over upper surface

Vortices

Fig. 14.24(a and b) Air flow over a flat plate and aerofoil inclined at a small angle

(a) Inclined plate

Direction of Smooth flow over air flow upper surface

Up wash

Direction of Smooth flow over air flow upper surface

Up wash

Increased air speed lower pressure

Decreased air speed higher pressure

Down wash

(b) Inclined aerofoil

Increased air speed lower pressure

Decreased air speed higher pressure

Down wash

(b) Inclined aerofoil

Fig. 14.24(a and b) Air flow over a flat plate and aerofoil inclined at a small angle

Angle of attack (0) deg

Fig. 14.25 Lift and drag coefficient versus angle of inclination (attack)

Angle of attack (0) deg

Fig. 14.25 Lift and drag coefficient versus angle of inclination (attack)

however, very large rake angles may conflict with the body styling.

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