Windy problem is solved

December 10, 1999

The answer was blowing in the wind. Experts have for the first time calculated the mathematically correct way to fly a flag and fill a sail.

Alistair Fitt and PhD students Martin Pope and Tim Lattimer, of Southampton University's industrial applied mathematics group, have solved the equations of motion to describe how flags and sails will behave in variable winds.

The fluid dynamics research, which will be published early next year, has tackled a particularly tough area of applied mathematics, namely solving partial singular integro-differential equations.

The upshot is being able to predict the effect of a certain wind given a flag's weight and bending stiffness. The equations are more complicated for a sail but still give an indication as to whether it will fill properly, flap or even collapse.

"A sail is just a flag pinned at both ends and has only a certain number of positions it can take - something most sailors know from experience. The equations of motion can predict the stability," said Dr Fitt.

The mathematician believes the research could influence racing yacht sail design but is more confident that it will find a practical role in industry, helping predict the behaviour of artificial fibres blown by airstreams inside machines. This will allow engineers to design more effective and efficient spinning and drying plants.

Steve Farrar

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