Owls produce negligible noise while flying. While many studies have linked the micro-fringes in owl wings to their silent flight, the precise mechanisms have been unclear. Now, a team of researchers has uncovered the consequences of those micro-fringes on the sound and aerodynamic performance of owl wings through computational fluid dynamic simulations. Their findings can encourage biomimetic designs for the event of low-noise fluid machinery.
Owls are fascinating creatures that may fly silently through a few of the quietest places. Their wings make no noise while flying, enabling them to accurately locate their prey using their exceptional hearing ability while remaining undetected. This unique ability will depend on many aspects and has long been a hot research subject.
Studies have found associations between the power to fly silently and the presence of micro-fringes in owl wings. These trailing-edge (TE) fringes play a vital role in suppressing the noise produced by wing flap-induced air movement.
Studying these fringes can result in the event of promising methods to scale back noise attributable to fluid machinery. While many studies have evaluated these fringes using flat plates and airfoils, their exact mechanisms and effects on the interactions of feathers and the several wing features in real owl wings remained unknown.
To unravel the secrets of silent owl wings, Professor Hao Liu along with his 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 at Chiba University in Japan, investigated how TE fringes influence each the sound and aerodynamic performance of owl wings.
When asked concerning the motivation behind their study, Prof. Liu says, “Despite many efforts by many researchers, exactly how owls achieve silent flight continues to be an open query. Understanding the precise role of TE fringes of their silent flight will enable us to use them in developing practical low-noise fluid machinery.” Their findings were published within the journal Bioinspiration & Biomimetics on November 17, 2023.
To know how owl wings work, the team constructed two three-dimensional models of an actual owl wing — one with and the opposite without TE fringes — with all its geometric characteristics. They used these models to conduct fluid flow simulations that combined the methods of enormous eddy simulations and the Ffowcs-Williams-Hawkings analogy. The simulations were conducted on the speed of the gliding flight of approach of an actual owl.
Simulations revealed that 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 the trailing edge vortices. Second, they reduce the flow interactions between feathers on the wingtips, thereby suppressing the shedding of wingtip vortices. Synergistically, these mechanisms enhance the consequences of TE fringes, improving each aerodynamic force production and noise reduction.
Emphasizing the importance of those results, Prof. Liu says, “Our findings reveal the effect of complex interactions between the TE fringes and the varied wing features, highlighting the validity of using these fringes for reducing noise in practical applications comparable to drones, wind turbines, propellers and even flying cars.”
Overall, this study deepens our understanding of the role of TE fringes within the silent flight of owls and may encourage biomimetic designs that could lead on to the event of low-noise fluid machinery.
