I'd never seen this before. Amazing.
[YOUTUBE]http://www.youtube.com/v/TMCf7SNUb-Q[/YOUTUBE]
For an explanation:
http://www.earthtrust.org/delrings.html
Quote:
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The silver rings, as it turns out, are "air-core vortex rings", and the helices are a similar phenomenon. Invisible, spinning vortices in the water are generated from the tip of a dolphin's dorsal fin when it is moving rapidly and turning. According to Hans: "Being unstable without a boundary nearby, the vortex line tends to form into a more stable form such as a helix. When the dolphins break the line, the ends are drawn together into closed rings. Owing to the Bernoulli effect, the higher velocity fluid around the core of the vortex is at a lower pressure than the fluid circulating farther away. Air is injected into the rings via bubbles released from the dolphin's blowhole." The energy of the water vortex is enough to keep the bubbles from rising for a reasonably long period--on the order of 10 seconds. There also seems to be a separate mechanism for producing small rings, which a dolphin can accomplish by a quick flip of its head.
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