Scientists from Chiba University in Japan and Shanghai Jiao Tong University in China studied the serrations on the leading edges of owls’ wings, gaining new insight into how they work to make the bird’s flight silent, reports Wind Power Engineering & Development.
The scientists were curious about what made the owl such a silent hunter, and after some extensive studies, they published their conclusions in the journal, Bioinspiration and Biomimetics. The answer lies in the owl’s unique wing features, and this new knowledge could have important implications for the aircraft, drone and wind turbine industry.
If you have ever had the opportunity to see the nocturnal hunting bird in flight, you may have noticed how very quiet owls are. The owl is a stealth hunter, and despite its large size, it flies through the air on silent wings, looking for its nightly meal.
“Owls are known for silent flight, owing to their unique wing features, which are normally characterized by leading edge serrations, trailing edge fringes and velvet-like surfaces,” says lead author, Professor Hao Liu of Chiba University in Japan.
Finding the right bird to study
While all birds have wings, that doesn’t mean they are all quiet in flight. Some are downright noisy, such as pigeons. And even falcons can be a bit noisy. With pigeon, the problem is with their small wings and large body sizes. This means pigeons have to flap their wings a lot, and this causes turbulence which can be very noisy, reports New Atlas.
Falcons and other raptors may have larger wings to go along with their bigger bodies, but they still need to push a large amount of air to gain the speed needed in gaining height, and this turbulence is noisy, too.
The scientists looked at owls, known for their silent flight. And they asked themselves, what made this possible? They found that even with microphones focused on an owl in flight, almost no sound was recorded as they flap their wings. Part of the answer is that owl wings are designed to generate a lot of lift with very little flapping, but that’s just part of the answer.
“We wanted to understand how these features affect aerodynamic force production and noise reduction, and whether they could be applied elsewhere,” Professor Hao Liu said.
Finding the answer through advanced computer modeling
The scientists analyzed mathematical models of owls’ wings, both with a serrated leading edge and without using large eddy simulations used in computational fluid dynamics to simulate air flows and Particle-Image Velocimetry (PIV), an optical method for studying air flows.
What they found was interesting. The leading-edge serrations acted passively in controlling how air flows over an owl’s wings. According to the report, “Specifically, it controls the transition between how air flows in a streamlined fashion close to the wing, or laminar flow, and how this breaks up into turbulence on the upper wing surface.”
This means that the serrated-edge wings play a critical role in aerodynamic force and sound production. Another impressive feature is the angle of attack that owls can reach in flight. This is where the implications for use of this knowledge in designing rotors for airplane propellers, drones, and wind turbines come into play.
“These owl-inspired leading edge serrations, if applied to wind turbine blades, aircraft wings or drone rotors, could provide a useful biomimetic design for flow control and noise reduction. At a time when issues of noise are one of the main barriers to the building of wind turbines, for example, a method for reducing the noise they generate is most welcome,” Professor Liu says.
Professor Liu also noted that the short YouTube video in this article was chosen because it clearly shows the leading edge of the owl’s wings in action.
