Natures wonders: silent owl flight (Introduction)

by David Turell , Saturday, December 03, 2016, 18:33 (2910 days ago) @ David Turell

Owls are birds of prey, but a good meaty meal depends on keeping those flapping wings as quiet as possible. The wings are large compared to body weight so they glide long distances, but they have to cut the air from time to time, and they have arranged to be quiet:

http://www.wsj.com/articles/the-secret-to-flying-as-silently-as-an-owl-1480631284

"One reason that large owls are such fearsome hunters is their ability to swoop down on prey in virtual silence. But how do they do it?

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"For one, they have a large wing surface area relative to their body weight, which allows for nearly effortless gliding. Previous research has shown that some special characteristics of owl feathers also play an important role. These traits include a comb-like fringe on the front of owls’ feathers, another fringe at their trailing edge and a downy layer of hairs that forms a sound-reducing canopy over the wings.

"In previous owl-inspired work, the team of scientists reported that blanketing rough surfaces with a synthetic version of this canopy cuts the noise from air flow. Now they have found that the feathers’ downy material appears to change the air flow over a wing in ways that reduce “trailing edge noise,” a significant source of sound whenever a blade or airfoil cuts through air. The phenomenon occurs when the back edge of an airfoil converts air turbulence into noise."

Comment: In evolution was this developed in one step, a saltation, or bit by bit? As usual I suspect saltation, as I don't imagine the owl's tried out different trailing edge alterations which requires figuring out how to develop the proteins to make the fine hairs and develop the mechanisms to produce the hairs, the usual problem of speciation. Note the biomimetic advances found by this study: making wind turbines quieter:

"Through trial and error, the scientists reproduced some of the noise-damping qualities of the owl’s downy layer by means of a plastic overlay made with a 3-D printer. They didn’t need to cover the entire wing surface to suppress trailing edge noise; it was enough to attach a strip of this material with a set of tiny fins extending past the back edge, where the problem occurs. “The most effective of our designs,” says Justin Jaworski, one of the scientists, “mimics the downy fibers of an owl’s wing but with the cross-fibers removed.”

"The scientists tried out their creation in a Kevlar wind-tunnel test designed to keep air in but let sound out. In a test using a fixed airfoil, the experimental configuration reduced noise by as much as 10 decibels. “That’s the difference between a normal conversation and running a vacuum cleaner,” says Dr. Jaworski.

"Why do tiny fins make such a big difference? William Devenport, another of the scientists, suggests that putting “finlets” on the trailing edge may break up the usual turbulence into smaller, quieter eddies.

"The wing addition added trivial weight, wasn’t hard to produce and had no material affect on the aerodynamics of the airfoil being tested. It just made things far quieter.

"Dr. Jaworski says that the technology might mitigate noise produced by turbine blades and other relatively slow-moving airfoils, since owllike canopies can be retrofitted onto existing blades as well as incorporated into new ones. Noise concerns, he notes, have held back the adoption of wind turbines as sources of sustainable energy."

Comment: Evolutionary mechanisms are smarter than we are, so we have to learn from nature to find what works. Not by chance.