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What is a vortex ring?
A vortex ring is the phenomenon where a quantity of fluid or gas in a toroid (donut) shape, travels through a medium of fluid or gas, while spinning like a thick circular bracelet that is being rolled off of a person's arm. (Except the spin is in the opposite direction as when a bracelet is rolled off in this way.)
How do vortex rings work?
Let's begin with the most well known example of a vortex ring, the smoke ring. A smoke ring starts off simply as a discrete puff of smoke that is pushed quickly and abruptly through a circular opening. When the puff of smoke hits the standing air in the room, the result is pretty much the same as if the puff of smoke is just a circular ball of smoke, standing still, and the air around it is the thing that is moving, rushing towards the ball of smoke like an oncoming wind. This "wind" hits the ball of smoke and begins pushing the outer edges back. The parts of the outer edge that are pushed back curl back into the ball, initiating the donut shape. Once the puff of smoke has instead become a donut, or toriod, shape, this shape is perpetuated in the following way. The oncoming "wind" is pushing on the outer and inner edges of the ring at the same speed, which tries to make the ring spin in both and inward and an outward direction at the same time. However there is more "outer edge" than there is "inner edge," so that's why the push of the "wind" causes the ring to continue spinning in the same direction, which is, with the outer edge travelling in the direction of the "wind."
A bubble ring works pretty much just the same way, only underwater. Bubble rings are actually made of two different rings, one inside the other, spinning in the same direction. The outer ring is made of water, and the inner one is made of air. (Think of a fat round bracelet that has been baked into the center of a bagel.) The air stays in the middle of a bubble ring because it is lighter than water, and the center of any vortex is a low-pressure area because of centrifugal force. You know the little miniature tornado you can make by stirring your glass of water quickly? Well, if you could put a top on your glass of water, and get the water to spin even faster, then you'd have a straight column of air in the middle of the glass, not a "V", or tornado, shape. A bubble ring is kind of like a long glass of water, spinning like this, that has been curved all the way around in a circle until it meets itself again.
Some bubble rings start out as just spinning air, and then the spin of the air gets the water near it to begin spinning also. The bubble rings that dolphins create by releasing air from their blow holes straight up towards the surface are examples of this. Other bubble rings start off as spinning water rings, and are not visible to us until some air is released near enough to the ring to get sucked into the middle. Dolphins have been seen to create rings like this by giving a certain kick with their tail fins, then swimming up to the invisible water-only vortex ring and releasing a short pulse of air that gets pulled into the ring. (Think how cool it would be to be able to "see" with echolocation, the way a dolphin can, and see a water-only vortex ring travelling through the water!)
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