Silencing wind turbines with textured blades

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Studies of the structure of owl wings has led to innovation on surface finishes for wind turbine blades so they produce less noise.

A newly-designed material, which mimics the wing structure of owls, could help make wind turbines, computer fans and other types of fan blades much quieter. Early wind tunnel tests of the coating have shown a substantial reduction in noise without any noticeable effect on aerodynamics.

Since wind turbines are heavily braked in order to minimise noise, the addition of this new surface would mean that they could be run at much higher speeds – producing more energy while making less noise. For an average-sized wind farm, this could mean several additional megawatts of electricity.

The surface has been developed by researchers at the University of Cambridge, in collaboration with researchers at three institutions in the USA. “Many owls – primarily large owls like barn owls or great grey owls – can hunt by stealth, swooping down and capturing their prey undetected,” said Professor Nigel Peake of Cambridge’s Department of Applied Mathematics and Theoretical Physics. “While we’ve known this for centuries, what hasn’t been known is how or why owls are able to fly in silence.”

Peake used high resolution microscopy to examine owl feathers in fine detail. It was observed that the flight feathers on an owl’s wing have a downy covering, which resembles a forest canopy when viewed from above. In addition to this fluffy canopy, owl wings also have a flexible comb of evenly-spaced bristles along their leading edge, and a porous and elastic fringe on the trailing edge.

“No other bird has this sort of intricate wing structure,” says Peake. “Much of the noise caused by a wing originates at the trailing edge where the air passing over the wing surface is turbulent. The structure of an owl’s wing serves to reduce noise by smoothing the passage of air as it passes over the wing – scattering the sound so their prey can’t hear them coming.”

In order to replicate the structure, the researchers looked to design a covering that would ‘scatter’ the sound generated by a turbine blade in the same way. Early experiments included covering a blade with material similar to that used for wedding veils, which despite its open structure, reduced the roughness of the underlying surface, lowering surface noise by as much as 30dB.

The researchers then developed a prototype material made of 3D-printed plastic and tested it on a full-sized segment of a wind turbine blade. In wind tunnel tests, the treatment reduced the noise generated by a wind turbine blade by 10dB, without any appreciable impact on aerodynamics.

While the coating still needs to be optimised, it could be used on a range of different types of wings and blades. The next step is to test the coating on a functioning wind turbine.

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