A recent conversation with Jessica triggered thoughts about vortex rings.
These things turn up all over the place in nature and are fascinating to watch. Probably the most familiar is the smoke ring. It turns out that you can build a very inexpensive vortex ring geneator, but first it probably makes sense to talk about what they are.
A vortex ring is basically a doughnut shaped bit of fluid or gas that is travels through a medium of fluid or gas while spinning about its direction of travel. If you watch these closely. I don't condone smoking, but you have probably watched a smoker try this ...
The smoke ring starts off as a ball of smoke that is abrupted puffed an circular opening that the smoker forms with his lips. It is generally easier to think about what happens by considering the world viewed from the smoke ball's point of view. The smoke ball finds itself out of the smoker's mouth with a sudden breeze striking it. The breeze starts pushing the outer edges of the ball back and these edges curl into the ball transforming it into a doughnut shape (technically a toroid).
The amazing thing is that this shape is stable for quite a long time. Consider the world from the new smoke ring's point of view for awhile. The breeze in front of the ring pushes that the inner and outer edges of the ring at the same speed which tries to get the ring to spin in an inner and outer direction at the same time. The outer edge is longer than the inner edge so the force of the breeze causes the ring to spin with the outer edge travelling in the direction of the wind. Moving back to our point of view the outer edge of the ring travels backwards and the inner edge moves foward pulling the ring through the air.
If you look at the ring carefully you will notice that it is rotaing around the axis of travel ... if you point your left thumb in the direction of travel, the motion of the ring about the axis that your thumb points out is the direction that your fingers naturally curl. (physicists are cringing at the use of the left hand here:-)
So we can't have you or your friends dying of cancer, so here is a simple vortex generator. I used to make these as a teenager and I'm something of a maladroit, to anyone can make them.
1. cut the end from a soup can
2. cover one end with cardboard with a 1" round hole cut in the center
3. cover the other end with a tight membrane made from rubber. Cutting a rubber balloon in half and tightly stretching the membrane across the opening works well. You can use rubber bands to hold the rubber sheet in place.
4. that's it! strike the rubber sharply with a finger or hand and you will generate a vortex that can easily travel 10 or 15 feet. Someone else will feel it as a puff of air. You can put some smoke in the can with an incense stick held in the can for awhile. I used to make smoke as a kid with hcl and amonia, but that may be a bit on the dangerous side.
Details in the smoke ring (if you go that route) will show up well if you try this in a darkened room with a bright light aimed at the ring.
This sort of thing scales well. Steve writes that he has seen 55 gallon drum sized genertors that generated vortex rings strong enough to knock a hat off a kid 15 feet away. You strike a membrane on the back of the drum with a mallet.
Of course there are any number of fun things you can do with this theme. Controlling the impulse gives enormous control over the resulting ring. One can use a waveform generator and a speaker. You could even send a signal over the Internet and blow on someone.
Put a bit of perfume -- or skunk - extract in the generator and startle someone.
To appreciate these beasts mathematically all you need is experience with partial differential equaltions and a bit of physical intuition. Pick up a fluid mechanics book and get into this deeply. The beauty of nature is that it works at several levels of understanding.
Oh yeah -- you can do this in water with your mouth. Dolphins love to blow rings from their blowholes. I spent some time as a kid trying to do this without much luck, but have seen people do it beautifully.
9:53:45 AM 
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