Ever seen the
Tacoma Narrows bridge?
Probably the perfect situation for vortex shedding and resonance. If you notice the swing on the left at the beginning of the video, it moves at the same frequency, but in smaller motions. So the wind is probably blowing from the left to the right, so as a vortex is shed off the fist swing, it pushes that swing a tiny bit in the opposite direction, and that vortex then pushes the second swing (like wake turbulence flipping a small aircraft). So, as the first swing starts swinging and shedding vorticies, the second swing is doing the same thing, only it's also being affected by the vorticies from the first swing, which increases it's swinging amplitude even more. If you notice, at the beginning of the video, all three swings move in a relatively similar fashion, until the second swing really starts to go, and the flow around the third swing is disturbed so much that it isn't affected by the vortex shedding of the first two swings. The shedding frequency is the resonant frequency of the shedding structure, so the frequency at which the vorticies hit the second swing in addition to the vorticies being shed off the second swing is exactly the right freqency to make it resonate, which is why it swings back an forth, and once it gets going, the shedding off just the second swing is probably enough to keep it going.
That would be my guess.
Vortex SheddingOr, as one of my professors says, "It's probably just the magical pixies of digital image land".
Posted: Thu Oct 25, 2007 6:39 am
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