The research of aeroacoustics specialists from Iowa State University could have a big impact on the design of wind turbines, aircrafts and other machines
In case you werenâ€™t aware, owls are incredibly silent flyers. Despite their size, they make surprisingly little noise when flying and this impressive characteristic of the nocturnal bird made Iowa State researchers want to take a closer look at its mechanics. As a result, assistant professor of aerospace engineering and Walter W. Wilson Faculty Fellow Anupam Sharma and his team developed innovatively designed 3D printing models of propeller blades, which could lead to quieter aircrafts, wind turbines etc. â€œThe owl is almost completely silent in flight,â€ says Sharma, who started working in aeroacoustics during graduate school and a previous position at General Electric. â€œOwls are not only silent in gliding flight, but also in flapping flight, which is amazingâ€, he adds.
The components that contribute to the owlsâ€™ silent flight are three: small, fine, comb-like structures on the leading edge of the wing; a pliable and porous fringe on feathers at the trailing edge of the wing; and a downy coat on the birdâ€™s flight feathers. In order to understand how these features manipulate air flow, turbulence, and pressure to produce silent flight, the researchers have 3D scanned various owl wing specimens, turning them into digital 3D models, and run multi-day simulations that use more than 16,000 processers provided by supercomputing facilities at Argonne National Laboratory in Lemont, Illinois. Using these simulations, they have made interesting discoveriesâ€”not just in terms of bird anatomy, but also regarding concepts that could be applied in real-world mechanical engineering, as owl-like designs can reduce sound by up to 5 dB across a wide frequency range without having an effect on aerodynamic performance.
At the beginning, it was difficult to replicate the soft and ultimately organic nature of wings and feathers for the metallic propeller of a turbine. However, the research team managed to do so, by 3D printing airfoils with a serrated leading edge, designed to resemble the fine comb-like structures on the owlâ€™s wings. The simulations showed that this way the noise was reduced significantly when compared to airfoils with a flat leading edge, while the unsteady pressure on the back end of the blade surface was also diminished. So even though there's a big material difference, copying the subtle form of the owl wing has produced a comparable effect. 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.
â€œThe results of this research could be applied to aircraft wings, rotor blades of jet engines, and wind turbines. They could also have an impact on the design of silent air vehicles with application in national defense, commerce, and transportation,â€ Sharma says. However, he clarifies that â€œour approach is bio-inspired as opposed to bio-mimicry and the designs wonâ€™t look like owl wings. Weâ€™re studying the physical mechanisms behind the owlâ€™s silent flight, taking simplified geometries afterwards, inspired by the owl wings.â€ For his research study, Sharma has been awarded with a five-year $500,000 CAREER grant by the National Science Foundation - the foundationâ€™s most prestigious award for early career faculty - and another $100,000 grant by the Iowa Space Grant Consortium.
Iowa State researchers, left to right, Bharat Agrawal, Andrew Bodling and Anupam Sharma. Photo by Christopher Gannon.
3-D printed models of aircraft propeller blades with serrated leading edges inspired by owl wings.
The owl hush kit â€“ unique feather adaptations that enable its silent flight.
Top: barn owl wing specimen. Bottom: Photographs through a microscope of (a) leading edge comb, (b) downy coat on flight feathers, and (c) trailing edge fringe.
Results of numerical analysis: (a) investigation of a blade geometry with serrations inspired by the leading edge comb of the owl,
(b) finlets inspired by the downy coat on flight feathers, (c) visualization of the radiated sound field for a model problem,
and (d & e) typical noise reduction observed in the simulations.