I know almost nothing about fluid dynamics but my article about wingtip vortices piqued my interest in the subject.
Last weekend I learned about this amazing phenomenon, the von Kármán vortex street, animated above by Cesareo de La Rosa Siqueira.
Von Kármán vortex streets occur when a fluid flows past a stationary object and generates a long line of vortices that swirl in opposite directions. The phenomenon was named for Theodore von Kármán, the man who described it, and is probably called a street because it looks like one.
We usually don’t see von Kármán vortex streets in the air but it’s important that engineers plan for them. If a tall structure is uniformly straight or uniform structures are grouped too closely the vortices can make them fall down. That’s what happened in November 1965 when three of the eight Ferrybridge cooling towers collapsed in high winds. They could individually withstand the wind speed but not the vortex.
On a small scale, von Kármán vortex streets make wires sing in the wind. On a large scale they’re visible from outer space when clouds blow past a tall island.
Here’s a picture taken from the space shuttle that shows cloud cover blowing past Rishiri Island, Japan. When the wind encounters Mt. Rishiri the clouds form a von Kármán vortex street on the downwind side.
There are more than twenty islands that reliably generate von Kármán vortex streets. Click here to see more pictures from NASA.
(Vortex animation by Cesareo de La Rosa Siqueira via Wikimedia Commons. Space shuttle photo from NASA via Wikimedia Commons. Click on the images to see the originals)
Very cool,
The Tacoma Narrows Bridge collapsed the same year it was constructed and they believe possibly because of the resonance of the vortices you describe.
Very powerful they can be.
http://en.wikipedia.org/wiki/Tacoma_Narrows_Bridge
http://www.youtube.com/watch?v=j-zczJXSxnw
Gene
Gene, I’ve since learned that the Tacoma Narrows Bridge collapse was due “aerodynamic flutter” rather than a Karman vortex street. Since I’m not an engineer or physicist this technicality is lost on me.
Kate: You impress me more every day. You could have been a physicist. Vortex technology is utilized in mining engieering,especially dealing in ventilation issues.
I learned something new today….Thank You Kate!
Speaking of ‘Walking Down Vortex Street’……It has reminded me of the many forms that Starlings take when flocking at Sunset, looking for a roost.
The collective noun for this activity is a Mumuration of Starlings… By the very nature of this swirling, I have often thought that a Vortex of Starlings might be more descriptive.
The image below of Starlings over Rome, starts to illustrate this pattern…
Enjoy
Tom
http://www.tera-form.com/wp-content/uploads/2011/03/Starlings-Flock.jpg
Very cool, Tom. You might enjoy this video I posted in Feb 2011
http://www.birdsoutsidemywindow.org/2011/02/17/starlings-under-pressure/
I haven’t seen anything this awesome in the U.S. with starlings, though we have them.