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This question is closely related to this one; The optics of vortex (in water): why there is a bright ring? In this case "physics girl" gives somehow an plausible explanation, and it's only debatable if this is the case or not.

But lets reverse the case by providing a laminar flow on the surface instead of draining the flow from the bottom. This is shown in this picture on the right compared to a "draining version" on left;

enter image description here

This reversed case must obviously influence to the optics and bending of the light at the surface. But this doesn't seem to have any influence to the optics of this ring;

enter image description here

In this case the flow is present all the time, and the "pit" sould be more or less constant. But the shade and the bright ring is there only when the rotation is aloud to be fully developed.

I made a video about this in Youtube. You see the shade and the ring best at 1:45 -> Other points are 4:00 and 1:10. A short version showing only these are here. Few fotos showing the surface;
enter image description here

The video where these pics comes is here; There is slow motion from 20 sec forward.

Please note that when the flow is laminar there is absolutely no air entrainment. This is a further sign from the solid nature of the fluid, and the surface-tension aspect of turbulence; this is related to this answer; Have we figured out how to analyze turbulent fluids?

Question; - Why here's still the shade and the bright ring? - Why isn't it there all the time?

I have tried to look the surface shape, and it's pretty near like this; the vortex only makes it "smooth"; there is no waves.

enter image description here

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  • $\begingroup$ I see no vortices in this (long) video; or tell at which second. I just see a ripple, and the caustic it generates at the bottom, that your drawing explain (but for the fact that the ray are not obliged to exactly converge in a same point :-) ). So please make the question more clear ! $\endgroup$ – Fabrice NEYRET Dec 9 '15 at 11:18
  • $\begingroup$ @FabriceNEYRET Thanks for interest. There is rotation which creates the shade (as in third foto above to be seen at 1:45-> youtu.be/_h41tBF9m0Q?t=124 the surface rotation and shade can be best seen in 1:10 -> youtu.be/_h41tBF9m0Q?t=70 on shade creation is again to be seen in 4:00 -> the blurring of the flow is caused by the wind $\endgroup$ – Jokela Dec 9 '15 at 11:33

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