Owls and Stealthy Wind Turbines

July 23, 2015

Bell, Book and Candle (1958, Richard Quine, Director) is one of my favorite classic films.[1] Among its cast of many stars is Kim Novak, who plays a modern witch assisted in her magic by her "familiar," a cat named, Pyewacket. The idea of a familiar is not recent. It goes back to Greek mythology, where the goddess, Athena, was often accompanied by a small owl of the species, Athene noctua (see figure).

Athena's owl, as pictured on a tetradrachm, a four drachma silver coin, ca. 480-420 BC. These coins were known colloquially as glaukes (γλαυκες, owls) (Photo by Marie-Lan Nguyen, via Wikimedia Commons.)

Since Athena was the goddess of wisdom, mathematics, arts, and crafts, the owl is even today associated with wisdom, as in the expression, "A wise old owl." The reason for the pairing of Athena and an owl is not known. What is known is the importance of birds in the culture of ancient Greece, as exemplified by the Aristophanes play, "The Birds (414 BC)."

Athena's owl is a member of the true owls, one of the two families of owl, the barn owl being the other. Barn owls are successful hunters of barnyard pests, such as rodents. It's strange to think that such a creature, ill-shaped for flight, could ever catch a rapid rodent. These large birds, however, make little noise while swooping down to capture their prey. An understanding of how they can fly in near silence may have application to reducing noise from wind turbines.[2-3]

A barn owl of the species, Tyto alba. These owls are also known as "church owls" and "screech owls." (Illustration by Thomas Bewick from the History of British Birds, 1847, via Wikimedia Commons.)

Wind power is a renewable energy technology, and that makes wind farms an attractive investment. As wind farms have proliferated, some side effects of wind power have been discovered. One of the most publicized is that of bird and bat strikes, but another vexing problem is wind power noise. The low frequency "thump-thump" sound of turbine blades propagates long distances, and this has reportedly caused sleep problems for nearby residents.[4]

In a paper presented at the American Institute of Aeronautics and Astronautics (AIAA) Aeroacoustics Conference (June 22-26, 2015, Dallas, Texas), scientists at the University of Cambridge, Virginia Tech, Lehigh University, and Florida Atlantic University, report how they used high resolution microscopy to observe the fine details in owl feathers.[3] By replicating this structure, they developed a prototype noise-reducing coating for wind turbine blades.[2]

Detail of a wind farm at Bønnerup Strand, Denmark. The blades of a wind turbine periodically disturb airflow, causing turbulence to create sound, which is an amplitude modulated pressure wave. (Photo by Dirk Goldhahn, via Wikimedia Commons.)

The research team found that an owl's wing feathers have a downy covering resembling a forest canopy. Along with this fluffy canopy, the wings have a flexible comb of evenly-spaced bristles along their leading edge. They also have an elastic and porous fringe on the trailing edge. Says Nigel Peake, a professor in the Cambridge Department of Applied Mathematics and Theoretical Physics.

"No other bird has this sort of intricate wing structure... Much of the noise caused by a wing - whether it's attached to a bird, a planes or a fan - originates at the trailing edge where the air passing over the wing surface is turbulent. The structure of an owl's wing serves to reduce noise by smoothing the passage of air as it passes over the wing - scattering the sound so their prey can't hear them coming."[2]

The research team endeavored to simulate this structure in a turbine blade coating. Coating blades with nothing more complicated than a mesh material similar to a wedding veil lowered the surface noise by as much as 30 decibel (dB).[2] They upgraded the design by 3D printing a pattern in plastic. They tested the pattern on a segment of a full-sized turbine blade in a wind tunnel and found a noise reduction of 10 dB without any great affect on aerodynamics.[2]

Microscope image of a flight surface feather of a Great Grey Owl. These feathers are unlike those of other birds. (Cambridge University photo by J. Jaworski.)

The next step, of course, would be fully-coated blades on a functioning wind turbine. While one object of the study is noise reduction at present rotation rates, the coatings could serve to allow faster rotation below established noise limits. The increased power from faster spin could amount to several megawatts for an average wind farm.[2] The 2014 estimated electrical demand in the United States was 1,415 billion kilowatt-hours.[5] This research was funded by the U.S. National Science Foundation and the U.S. Office of Naval Research.[2]

References:

1. Bell, Book and Candle (1958, Richard Quine, Director) and the Internet Movie Database.
2. Silent flights: How owls could help make wind turbines and planes quieter, University of Cambridge Press Release, June 22, 2015.
3. Web Site of the American Institute of Aeronautics and Astronautics (AIAA) Aeroacoustics Conference, Dallas, Texas, June 22-26, 2015.
4. Advantages and Challenges of Wind Energy, US Department of Energy, Office of Energy Efficiency & Renewable Energy Web Site.
5. Frequently Asked Questions - How is electricity used in U.S. homes, US Energy Administration Web Site.