# Why do those dolphin bubble rings I saw in a video behave the way they do?

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

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.