I recently bought a roof rack for my car and to my delight discovered it likes to whistle when my car approaches 100 kilometres per hour. But why does the whistling stop whenever I go under a bridge?
• To produce a noise, the roof rack needs to be struck at a suitable angle by a sufficiently strong, stable wind. The same effect comes into play when you blow across the mouth of a bottle. Taut strings also do interesting things in winds at various angles.
When your car travels at speed in an open space, a stable slipstream develops, which sustains your roof rack’s whistle. A bridge, especially one without much space beneath, disrupts this flow around your car and redirects the wind. If this changes the angle at which the slipstream meets the rack then the whistle stops, at least temporarily.
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To pin down this behaviour, you can easily experiment with roof racks of different sizes and shapes and drive your car in a range of environments. To fresh winds and bridges new.
Jon Richfield, Somerset West, South Africa