Archive for Rankine Vortex

Solitons in a Swimming Pool

Posted in The Universe and Stuff with tags , , , on December 1, 2014 by telescoper

Here’s a fascinating physics video for your amusement and edification:

I think the video explains what is going on pretty well, but if you want more information (including some connections with other bits of physics) you can look at this paper (whence I’ve nicked the following pictures and description).

Basically what is happening is that the motion of the plate through the water in the pool creates a connected pair of Rankine Vortices, rotating in opposite directions. The kinetic energy and the angular momentum the vortices quickly decay so the structure turns into a pair of dimpled singular surfaces called Falaco Solitons. Although unstable, these structures can survive for several minutes if the water in the pool is otherwise still:


Severing the thin, string-like, structure that connects the two dimples will make them vanish extremely rapidly.

The spooky-looking black discs you can see apparently floating on the surface are actually an optical artefact, formed on the floor of the pool by Snell refraction through the rotationally-induced dimpled surface:


The physics is quite simple, really, but the result is fascinating to watch. If you agree, you’ll also be interested in this old post of mine about vortex rings.