Fun with Vortex Rings

I decided to squeeze in a little bit about vortex rings into this morning’s lecture, partly because they illustrate the connections between fluid vorticity and magnetism, and partly because they’re fun…

Here’s an animation of a vortex ring showing how the fluid elements move around it (you might need to click on it to make it animate):

It’s quite easy to generate vortex rings in everyday situations, the simplest way being when a mass of fluid is impulsively pushed from an enclosed space through a narrow opening. In this case the poloidal flow is set in motion, at least in part, by interaction between the outer parts of the fluid mass and the edges of the opening. This results in fluid elements travelling in little circles, like those above, around a “core”; the direction of the vorticity is at right angles to these circles, i.e. in the toroidal direction. A vortex line can be formed from by joining together the vorticity vectors from each little circle to form a circle defining the core of the vortex ring. The behaviour of vortex lines in flows like this is entirely analogous to that of magnetic field lines. In this case, the vortex line follows the motion of the fluid, which is at right angles to it, so it propagates more-or-less without disruption. This is how most vortex ring toys work, such as shown in the two examples here; the second is far more dramatic!

The last video features some naturally-occurring vortex rings (as well as some  distinctly man-made examples). What I didn’t realise until I found this video last night is that whales and dolphins know how to make vortex rings too, only underwater. Why do they do this? Is there an evolutionary explanation? I doubt it! I think they’re just having fun.

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8 Responses to “Fun with Vortex Rings”

  1. Anton Garrett Says:

    Gandalf!

  2. telescoper Says:

    I was going to include that clip, but I suspect that some sort of special photographic effects may have been used in its creation.

  3. Nobody tell dolphins about how mushroom clouds are formed by incandescent vortex rings. A dolphin nuclear weapons program is the last thing anyone needs.

  4. Plants also produce vortex rings of a sort. Sphagnum moss produces vortex rings in the seeds when they are explosively expelled from the seed pod.

    • telescoper Says:

      I wonder whether such phenomena can be created by farting? Sounds like a good topic for an undergraduate project, I think….

  5. Bob Anderson (@doubledodge) Says:

    They were having a hilarious time on QI at the week-end, the show closed with the panel members slowly disappearing behind an increasingly thick pall of smoke. Very impressive.

    • telescoper Says:

      It’s a curious coincidence that this sort of thing was featured on QI this week. I hadn’t known until some students pointed it out during the lecture. I rarely watch TV, actually.

  6. […] The physics is quite simple, really, but the result is fascinating to watch. If you agree, you’ll also be interested in this old post of mine about vortex rings. […]

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