Re: [Phys-L] half vortex ring

[John Denker <jsd@av8n.com>]

On 12/03/2014 05:35 AM, LaMontagne, Bob wrote:

> I found this interesting. 
>
> https://www.youtube.com/watch?v=pnbJEg9r1o8

Yes, interesting.

> The presenter gives more than a "Gee-Whiz" explanation.

However, not an entirely correct explanation.

1) Tell me again why a vortex ring stays together?  Really?

2) The words say "circular" but the image shows the fluid 
 flowing in a /helical/ pattern:
   https://www.youtube.com/watch?v=pnbJEg9r1o8#t=0m42s

3) It is not even a law of physics but rather a theorem of
 mathematics that a vortex line can /never/ end.  Therefore
 it is exceedingly misleading to suggest that there can be
 any such thing as "half" of a vortex ring.

 I would have emphasized that the vortex lines necessarily
 form a complete loop, of which half is underwater.

 Even then, it's so misleading that I would not show this
 video to a student who was not already familiar with
 regular old vortices in a homogeneous medium.