Ok, I've spent 2 hours now looking at vortices, and CFD vortex simulations, knotted vortecies, etc, and thinking about a nagging question I have with all of them. They all seem to create hollow toroidal shells.

Where is the optimal place to put particles such as smoke so that they occupy the center of the torus?

I can't seem to find any time reversed CFD simulations that track these particles back to where they started.