The hollow shells are just contours used for visualization, the vortex rings are not actually hollow (as in no fluid in the region). As for tracking particles

The hollow shells are just contours used for visualization, the vortex rings are not actually hollow (as in no fluid in the region). As for tracking particles near vortex rings, I thing you're looking for something like http://shaddenlab.berkeley.edu/uploads/shaddendabirimarsden06.pdf http://shaddenlab.berkeley.edu/uploads/shaddendabirimarsden0...