I think I undersand how a vortex ring is formed when we use a long tube:like this

Vorticity is "created" by the velocity gradient caused by the viscosity and then, when the gaz goes out, the Kelvin theorem says that vorticity is conserved.

But most of the videos I see, use a box without a tube and yet, a vortex ring is still created (When I would have thought that the velocity would have "crash" into the wall...). Is it because there is a $U_y(x)$ velocity gradient (not the one I drew on the second picture) on the walls (where I added a question mark on the picture)?

enter image description here

  • $\begingroup$ I am wondering if the velocity between the two question marks is a constant. $\endgroup$ – Amey Joshi Sep 29 at 3:44

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