“The owl is almost completely silent in flight,” says Anupam Sharma, an Iowa State University assistant professor of aerospace engineering and Walter W. Wilson Faculty Fellow. Sharma started working in aeroacoustics – the noise associated with air flow – during graduate school and a previous position at General Electric. “Owls are not only silent in gliding flight, but also in flapping flight.”

Three components contribute to owls’ silent flight – the leading edge of the wing has small, fine, comb-like structures; the feathers at the wing trailing edge end in a pliable and porous fringe; and the flight feathers have a downy coat.

To learn how these features manipulate air flow, turbulence, and pressure to produce silent flight, Sharma is scanning owl wing specimens, creating digital models, and running multi-day simulations that use more than 16,000 processers provided by supercomputing facilities at Argonne National Laboratory in Lemont, Illinois.

3D-printed models of aircraft propeller blades with serrated leading edges inspired by owl wings.

Computer simulations show that owl-inspired airfoils substantially reduce the unsteady pressure on the back end of a blade surface. Researchers also found that the sound radiated by the owl-inspired design was reduced by up to 5dB across a wide frequency range without sacrificing aerodynamic performance.

The downy coat of owl wings inspired researchers at Virginia Tech to design model airfoils with a regular series of small, thin finlets and canopies near the blade trailing edge, running parallel to the airflow. Both research teams compared the performance of the owl-inspired airfoils to a standard, flat-surfaced airfoil. Tests of 3D-printed airfoils with a serrated leading edge inspired by owl wings showed the serrations substantially reduced airfoil noise. Experiments performed by Virginia Tech researchers agreed with the Iowa State simulations, showing the designs reduced noise and the finlet spacing on the airfoil is important.

Sharma says, “The results of this research could have an impact on the design of silent air vehicles with application in national defense, commerce, and transportation.”

Microscopic views of a barn owl wing show (a) leading edge comb, (b) downy coat on flight feathers, and (c) trailing edge fringe.

However, don’t expect airfoils on next-generation aircraft to resemble owl wings.

“Our approach is bio-inspired as opposed to bio-mimicry,” Sharma says. “Our designs won’t look like owl wings. We’re studying the physical mechanisms behind the owl’s silent flight. Then we’re taking simplified geometries inspired by the owl wings and applying those to aircraft wings, rotor blades of jet engines, and wind turbines.”

Iowa State University www.iastate.edu Virginia Tech https://vt.edu