Uncovering the secret to noiseless owl flight

If you’ve ever been startled by an owl which suddenly appears close to you as if out of nowhere, you’ll have discovered why these birds are renowned in the animal kingdom for their ability to fly noiselessly.

It’s clearly an asset having the ability to locate their prey using their exceptional hearing while remaining undetected.

Exactly how owls’ wings make no noise in flight remains a mystery. Engineers who uncover the secret could apply it to a host of transport developments and other engineering problems.

New research, published in the journal Bioinspiration & Biometrics, focuses on the shape of the edges of the birds’ wings.

Previous studies have found a link between noiseless flight and the presence of micro-fringes in owl wings. These are referred to as “trailing-edge” (TE) fringes. These appear to be the crucial factor in quiet owl flight.

“Despite many efforts by many researchers, exactly how owls achieve silent flight is still an open question,” says senior author Professor Hao Liu from the Graduate School of Engineering at Chiba University in Japan. “Understanding the precise role of TE fringes in their silent flight will enable us to apply them in developing practical low-noise fluid machinery.”

Liu and colleagues constructed two 3D models of a real owl wing. One had TE fringes, the other did not.

They then conducted fluid flow simulations at the speed of the gliding flight of approach of a real owl.

The simulations showed the TE fringes reduced noise levels of owl wings especially at high angles of attack. They also revealed wings with TE fringes had aerodynamic performance comparable to those without the fringes.

Owl flight wings feathers heat map aerodynamics diagram
The micro-fringes on owl wings effectively suppress the noise while maintaining the aerodynamic performance of the wings comparable to that of a wing without the fringes. Credit: Hao Liu / Chiba University.

Two mechanisms were identified by the team relating to how the TE fringes impact airflow.

First, the fringes reduce the fluctuations in airflow by breaking up the vortices that form at the trailing edge of the wing.

Second, they reduced the interactions between the flow at the feathers and wing tips. This suppresses the “shedding” of vortices at the wingtip. This shedding, sometimes referred to as “leakage”, is the effect you can sometimes see on aeroplanes where white trails appear at the tips of the wings.

Together, these mechanisms enhance the effects of TE fringes on owl wings, improving both their aerodynamics and noise reduction.

“Our findings demonstrate the effect of complex interactions between the TE fringes and the various wing features, highlighting the validity of using these fringes for reducing noise in practical applications such as drones, wind turbines, propellers and even flying cars,” Liu says.

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