Uncovering the secrets behind the silent flight of owls

Owls’ wings make no noise while flying, enabling them to accurately locate their prey using their exceptional hearing ability while remaining undetected. This unique ability depends on many factors and has long been a hot research subject.

While many studies have linked the trailing-edge (TE) micro-fringes in owl wings to their silent flight, the exact mechanism the fringes play in suppressing noise produced by wing flap-induced air movement has been unclear. Now, a team of researchers has uncovered the effects of these micro-fringes on the sound and aerodynamic performance of owl wings through computational fluid dynamic simulations. Their findings can inspire biomimetic designs for the development of low-noise fluid machinery.

While many studies have evaluated the fringes using flat plates and airfoils, their effects on the interactions of feathers and the different wing features in real owl wings remained unknown.

Dr. Hao Liu, a professor at the Graduate School of Engineering and leader of the Center for Aerial Intelligent Vehicles at Chiba University, Japan, with colleagues including Dr. Jaixin Rong from the Graduate School of Engineering and Dr. Yajun Jiang and Dr. Masashi Murakami from the Graduate School of Science, investigated how TE fringes influence the sound and aerodynamic performance of owl wings.

To understand how owl wings work, the team constructed two three-dimensional models of a real owl wing – one with and the other without TE fringes – with all its geometric characteristics. They used these models to conduct fluid flow simulations that combined the methods of large eddy simulations and the Ffowcs-Williams-Hawkings analogy. The simulations were conducted at the gliding flight speed of a real owl approaching.

Simulations revealed the TE fringes reduced the noise levels of owl wings, particularly at high angles of attack, and maintained aerodynamic performance comparable to owl wings without fringes.

The team identified two complementary mechanisms through which the TE fringes influence airflow. First, the fringes reduce the fluctuations in airflow by breaking up TE vortices. Second, they reduce flow interactions between feathers at the wingtips, suppressing the shedding of wingtip vortices. Synergistically, these mechanisms enhance the effects of TE fringes, improving aerodynamic force production and reducing noise.

Emphasizing the significance of these results, Prof. Liu says, “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.”

Chiba University