The team developed the material by carefully studying how owls hunt and fly in silence and used high-resolution microscopy to examine owl feathers in fine detail.
They 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.
Professor Nigel Peake of Cambridge’s Department of Applied Mathematics and Theoretical Physics, who led the research, said that the unique structure of the wing gives it some very interesting properties. “No other bird has this sort of intricate wing structure,” he said. “Much of the noise caused by a wing – whether it’s attached to a bird, a plane or a fan – 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.
While the ‘wedding veil’ worked remarkably well, it is not suitable to apply to a wind turbine or aeroplane. Using a similar design, 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.’
Early wind tunnel tests of the material have demonstrated that it could 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, and incorporating it onto an aeroplane would be far more complicated than a wind turbine, 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. According to the researchers, a significant reduction in the noise generated by a wind turbine could allow them to be spun faster without any additional noise, which for an average-sized wind farm, could mean several additional megawatts worth of electricity.