Engineering news
A team of US researchers have created a 3D printed wing attachment inspired by the downy canopy of owl feathers that can reduce wind turbine noise by 10dB, without impacting aerodynamics.
Many species of owl can hunt in effective silence by suppressing their noise at sound frequencies over the range that can be heard by humans (above 1.6kHz). The mechanical engineering team at Lehigh P.C. Rossing College of Engineering and Applied Science, in Bethlehem, Pennsylvania, have successfully mimicked the mechanisms behind the acoustics of owl flight in order to improve man-made aerodynamic design of wind turbines, aircraft, naval ships and vehicles.
The researchers from Lehigh, Virginia Tech, Florida Atlantic University and University of Cambridge, specifically looked at the velvety down that makes up the upper wing surface of many large owls. This is a unique physical attribute, even among birds, that contributes to owls' noiseless flight.
As seen under a microscope, the down consists of hairs that form a structure similar to that of a forest canopy. The hairs initially rise almost perpendicular to the feather surface but then bend over in the air flow direction to form a canopy with interlocking barbs at their tops to create ‘cross-fibres’.
The researchers tested mesh fabrics that mimic the effect of the canopy, such as wedding veil material placed over sandpaper to create "roughness", suspended in simulated air flows at the Virginia Tech wall-jet wind tunnel.
"The most effective of our designs mimics the downy fibres of an owl's wing, but with the cross-fibres removed," Justin W. Jaworski, assistant professor of mechanical engineering and mechanics at Lehigh, said. "The canopy of the owl wing surface pushes off the noisy flow. Our design mimics that but without the cross fibres, creating a unidirectional fence - essentially going one better than the owl."
Following their experiments, the team created a 3D printed, plastic attachment consisting of small "finlets" that can be attached to a wing. The researchers said that the finlet may be retrofitted to an existing wing design and used in conjunction with other noise-reduction strategies to achieve even greater noise suppression.
They have also investigated how mimicking the downy canopy of owl feathers can reduce roughness and trailing-edge noise. In particular, trailing-edge noise is prevalent in low-speed applications and sets their minimum noise level.
The ability to reduce wing noise has implications beyond wind turbines, and can be applied to other aerodynamic situations such as the noise created by air seeping through vehicle door and window spaces.
The findings will be published in two forthcoming papers – one called Bio-inspired trailing edge noise control in the American Institute of Aeronautics and Astronautics Journal and the other called Bio-inspired canopies for the reduction of roughness noise in the Journal of Sound and Vibration.