If you like Lord of the Rings as much as I do (let’s be real, you don’t), then you’re familiar with the film’s scene in which Bilbo and Gandalf take turns blowing smoke rings (I know, I know, Gandalf blows a ship, not a ring). Maybe you’ve even seen one of your old uncles partaking in this old-fashioned hobby.
I don’t know that I can think of any two things less alike than spry young dolphins and old hobbits, but they do actually have a very similar behavior: creating toroidal vortices. Say what? In layman’s terrestrial terms, you know them as smoke rings, but this phenomenon can occur in any fluid, even seawater.
The basics of this motion involve two structural components. First you have the ‘vortex.’ A vortex consists of a fluid moving around a central point, like you get in a tornado. Second, the specific shape this vortex is in is called a toroid – you may not have learned this one in your shapes class, but it’s really just a pretentious word for a doughnut. These rings form when a fluid undergoes a burst in speed relative to the fluid around it. At the edges of this burst, frictional forces slow the parcel down, but at different rates at different regions. Along the sides, viscous shearing causes the outside flow to fold back in on itself, into the faster moving flow on the inside. As the ring spins faster, the pressure drops enhancing the stability of the shape.
These flows would be invisible, but hobbits and dolphins have the propensity to show off. That’s why they create them using smoke, or in the dolphin’s case inserting a pocket of air into the water doughnut.
Dolphins, and many other cetaceans, are capable of creating these vortices by quickly blowing air through their “pipe” of a mouth. As far as scientists know, there’s not really a use for this ability other than play. They can move the rings, change their shape, and even pull smaller vortices from larger ones. Watch this adorable video of some dolphins playing with these vortices, and don’t feel bad that they know more about physics than you. (You may want to mute the video – the music is pretty terrible.)