The 'Rankine vortex' is a simple model for a vortex in which the centre is in solid body rotation and the rest is a free, irrotational* vortex.
From the looks of their video, the bottle forms most of the solid body core. The velocity in the rest of the fluid goes as u~1/r and is pure tangential (streamlines are circles around the vortex centre).
This isn't anything special, it's just a pretty regular unforced vortex.
*irrotational here means that the vorticity (defined at a point) is zero, not that the system doesn't look spinny.
From the looks of their video, the bottle forms most of the solid body core. The velocity in the rest of the fluid goes as u~1/r and is pure tangential (streamlines are circles around the vortex centre).
This isn't anything special, it's just a pretty regular unforced vortex.
*irrotational here means that the vorticity (defined at a point) is zero, not that the system doesn't look spinny.