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This (see http://www.youtube.com/watch?v=TMCf7SNUb-Q&feature=related) is so cool, I wanted to show it.

As a pro forma question for this forum, any idea what is going on with the breaking of larger, presumably vortex related bubble rings and reconnection making the smaller rings? The dolphins appear to be making smaller vortices by suddenly moving their snouts through a ring.

On a simpler level, why do the rings get larger? Any other good questions come to mind?

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I suggest you changed the title a little. As it stands it is way too confusing and I only clicked on this question by accident, thinking it was off-topic. Something like "Why do the bubble rings I saw in a video behave the way they do?" –  Marek Nov 17 '10 at 9:36
Thanks to Robert Cartaino for changing the title. –  j.c. Nov 19 '10 at 3:28
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1 Answer

up vote 6 down vote accepted

Heh, I usually show this movie during my presentations.
First of all, this has nothing to do with surface tension. Those are just vortex rings that are only visualised by dolphins by filling their cores with air (you can see this happening about 0:12).
At first the rings grow to optimize vortex core size, but the most increase is made by the fact that dolphins make water flow through the ring (you can see they kinda push the rings through the water). The speed of each point on a vortex ring in localised induction approximation is $\dot{\vec{s}}=\frac{\beta}{R}\hat{\mathbf{b}}$, where $\hat{\mathbf{b}}$ is a binormal vector of the ring, $\beta$ is constant and $R$ is the diameter of the ring; so when the dolphin pushes the ring its effective speed decreases and so its diameter must increase. Not counting friction, free ring holds its diameter.

The making of small rings is due to the fact that dolphins distort the ring making it reconnect with itself. Two rings are created in the process, small and big, yet the big degenerates in eddies created by the dolphin swimming past it to play with the smaller one.

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Very interesting. I suppose I should delete my "answer" then. By the way, where can one learn more about this stuff? Could you provide some reference? –  Marek Nov 17 '10 at 10:25
@Marek The bible is Vortex Dynamics by P. G. Saffman. In general this is widely used in superfluid turbulence (where the assumptions behind vortex line theory which are quite idealistic in classical fluid mechanics are almost satisfied); here The Book is Quanized Vortices in Helim II by R. J. Donnelly. "Vortex filament theory" is a good search keyword. –  mbq Nov 17 '10 at 11:58
thank you, I'll look into it. It's completely outside my area of research but I'd like to know at least something about the topic. –  Marek Nov 17 '10 at 14:27
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