On 09/08/10 17:36, RussellMc wrote:
> It is now obvious that this should have been in [TECH]
> Who would have though ? :-).
>
>   =20
>>> They aren't that hard to blow yourself
>>>       =20
>   =20
>> Yes; I've seen scuba divers blow air rings.  Usually straight upward,
>> and without any of the fine control that the dolphins show.
>>     =20
>   =20
>> A really interesting question is whether the dolphins got to observe a
>> human blowing air rings before they tried it themselves...
>>     =20
> The paper notes that rings form spontaneously from air bubble of more
> than a certain size.
> Ref as before, from
>
>         http://faculty.vetmed.ucdavis.edu/faculty/bjmccowan/Pubs/McCowane=
tal.JCP.2000.pdf
>
> Pressure inside the bubble equals the mean pressure outside it, and
> pressure below is higher than pressure above due to hydrostatic head.
> There is a net uplift in the middle of the bubble at the bottom which
> tends to flatten the bubble and when the surface tension is exceeded
> by the hydrostatic forces the middle "punches through and a torus
> forms. I suppose that the surface tension tends to rapidly pull the
> inner towards the outsides, extending the hole diameter. For rings
> blown at an angle forces will differ from this, but a central water
> jet or even the tendency to extend outwards from a high speed central
> air stream probably (and apparently) cause the middle to rupture. Once
> any sort of middle hole is formed surface tension forces will act to
> equalise forces throughout. The flow through the centre rolls the ring
> and starts the vortex.
>
> Their description comes from a Scientific American article which is
> barred to all those not paid up for access.
> Ref at end\
>   =20
I'd have to say growing up practically living in a diving pool that I'd=20
dispute this "any bubble" claim.
They may be right about the size but the circumstances of the formation=20
are critical, if there is any real turbulence then the bubble will just=20
shred into smaller fragments, or adopt a "jellyfish" shape.
It takes some practice to get right, if what they say is true then any=20
exhalation underwater would cause rings.

You need to get the air into the bubble and to cut it off cleanly (hence=20
the sound).

> Sounds unlikely?
> Sure - but here's a video thereof or an extremely good makeup.
> Note circle of bursting of bubbles in ring on water surface and
> initialsomewhat spiral shape - this may be due to formation motion or
> making an intended path for fish to be driven down.
>               http://www.youtube.com/watch?v=3DvJvfjiCTvq4
>
>   =20
Note they are blowing normal bubbles not rings. Different species do it=20
in different ways, some species do it themselves, they will swim in a=20
circle whilst blowing to make their own ball, then come up through it.=20
The video shows all the whales blowing a column in synchronisation to=20
form the ball.

--=20
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