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A Mechanics of a ring vortex

  1. Sep 3, 2016 #1
    If you look at this YouTube video you will see an example of a ring vortex made by a dolphin (there are quite a few other examples). The problem is I can't understand why the ring doesn't immediately float to the surface. Only when it breaks up do the bubbles rise. The ring appears to counteract expected behaviour. Can anyone explain this?
    thanks
    Nick
     
  2. jcsd
  3. Sep 6, 2016 #2

    PeterO

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    If you stand 50m from the tee at a Golf Major, you will notice that the balls that have just been hit do not immediately fall to the ground - they actually "come down" perhaps 280 yards from the tee (230 yards from you).
    Why should these bubbles behave so differently to the balls.
    They do behave somewhat differently, in that they don't travel 200+ yards before heading for the surface.
     
  4. Sep 6, 2016 #3

    A.T.

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    Because it's a different situation. A ball in free fall accelerates at 1g downwards, regardless of it's horizontal movement. Those air pockets are not always accelerating upwards at the same rate.
     
    Last edited: Sep 6, 2016
  5. Sep 6, 2016 #4

    A.T.

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    The movement of the air pocket is dominated by the flow of the water, which is being modified by the dolphins. The inertia and angular momentum of the water are modulating the effect of the buoyant force.
     
  6. Sep 6, 2016 #5
    I also thought it may be dominated by a modified flow of water produced during the ring's creation but then if you look at this air vortex cannon video, about 1m 40 secs in, and look at the residual smoke there doesn't appear to be any modifying air flow associated with the smoke ring. I would suspect, in the case of the water scenario, there will be no modifying flow.
     
  7. Sep 6, 2016 #6

    A.T.

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    What do you mean by that? The vortex constitutes a circular flow.

    When an air bubble raises in still water, the water above the bubble flows below the bubble due to unbalanced gravity. But the water around that air ring is circulating fast, with pressure gradients different from still water.
     
  8. Sep 6, 2016 #7
    I thought that your comment, "The movement of the air pocket is dominated by the flow of the water, which is being modified by the dolphins." inferred that there was a movement of water (underwater current) along the direction of the forward motion of the ring which was somehow counteracting the natural tendency for the vortex "bubble" to rise.
     
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