14110 Flow separation and reattachment

The stream of air flowing over a car's body tends to follow closely to the contour of the body unless there is a sudden change in shape, see Fig. 14.11(a). The front bonnet (hood) is usually slightly curved and slopes up towards the front windscreen, from here there is an upward windscreen tilt (rake), followed by a curved but horizontal roof; the rear windscreen then tilts downwards where it either merges with the boot (trunk) or continues to slope gently downwards until it reaches the rear end of the car.

The air velocity and pressure therefore reaches its highest and lowest values, respectively, at the top of the front windscreen; however, towards the rear of the roof and when the screen tilts downwards

(a) Notch front and rear windscreens

Attached flow

Attached flow

(b) Very streamlined shape

Fig. 14.11 (a and b) Flow separation and reattachment

(b) Very streamlined shape

Fig. 14.11 (a and b) Flow separation and reattachment there will be a reduction in air speed and a rise in pressure. If the rise in air pressure towards the rear of the car is very gradual then mixing of the air-stream with the turbulent boundary layers will be relatively steady so that the outer layers will be drawn along with the main airstream, see Fig. 14.11(b). Conversely if the downward slope of the rear screen/boot is considerable, see Fig. 14.11(a), the pressure rise will be large so that the mixing rate of mainstream air with the boundary layers cannot keep the inner layers moving, consequently the slowed down boundary layers thicken. Under these conditions the mainstream air flow breaks away from the contour surface of the body, this being known as flow separation. An example of flow separation followed by reattachment can be visualized with air flowing over the bonnet and front windscreen; if the rake angle between the bonnet and windscreen is large, the streamline flow will separate from the bonnet and then reattach itself near the top of the windscreen or front end of the roof, see Fig. 14.11(a). The space between the separation and reattachment will then be occupied by circulating air which is referred to as a separation bubble, and if this rotary motion is vigorous a transverse vortex will be established.

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