The wings of owls inspire windmill designers. Owls fly almost silently, while windmills make a lot of noise. By including characteristics of the wings, the whooshing sound of the blades is reduced.
One of the most common complaints about wind turbines is noise. The whizzing and swishing of the blades through the air can be maddening. It is a whooshing noise, reminiscent of the sound of airplanes and drones. To reduce this noise nuisance and to comply with the noise standards, designers are inspired by the wings of owls, which fly almost silently.
Researchers from Xi’an Jiaotong University in China have recently taken an extra step in this direction. They let in a publication in the scientific journal Physics of Fluids see that the whooshing sound can be further reduced by adopting even more features of the owl’s wing.
By far the largest part of a windmill’s noise is generated at the tip of the blades when they move through the air at high speed. The mechanical part of the wind turbine itself is not that loud. The sound you hear comes from vortices that are created in the air as they flow along the top and bottom of the blades.
When those air vortices reach the tip of the blade, they collide with each other, releasing energy in the form of sound. It is difficult to reduce this noise, due to the chaotic and uncontrollable nature of the air vortices. To find solutions for this, scientists like to look to nature, where millions of years of evolution have led to ingenious adaptations.
During a bird’s flight, the air rushes across the wings in a similar manner. With most birds this makes an audible sound, but not with owls that hunt at night. Their wings make no flapping sound, no rustling. This is partly because the trailing edges of their wings are rough and frayed. If you zoom in on it you will see a jagged structure. The serrations gradually merge the swirling air currents. As a result, the sound waves partially cancel each other, so that much less sound is audible.
Sound waves extinguish
To reduce the disturbing noise from windmills, researchers have been using the owl wing technique for a number of years now. They do this by placing a serrated edge on the back of the end of the blades. This serrated edge, just like the plumage of the owl, ensures that the air vortices are combined in such a way that the sound waves can cancel each other out. This ensures less noise pollution. But the cartels don’t always work well. How well they can extinguish the sound depends on the weather conditions and the sound frequency of the air vortices.
Researchers from Xi’an Jiaotong University zoomed in even further on owls’ wings to reduce noise pollution more effectively. They write that the serrations look more like an owl’s wing if you don’t make them tight triangular, but give them a bulge on one side.
Using theoretical calculations and computer models, the researchers show that such convex serrations can reduce the noise of rotating blades by a few decibels compared to the current serrations. In their publication, they emphasize that the results depend on the conditions in which blades rotate. Further research and tests in wind tunnels are thus needed to confirm these findings.
Cost picture
‘There are also other ways to further suppress the noise,’ says sound researcher Francesco Avallone of TU Delft, who was not involved in the study. For example, by placing a comb-like structure between the serrations, the airflow can be further controlled and the windmills are therefore quieter. ‘When developing such new techniques, you have to take into account that you run the risk that it is too expensive to produce, so that the industry is not interested in it.’
The owl-like serrated edges can therefore produce wonderful results in computer models, but in the end the reduction of noise pollution must still prove practical and affordable.