Vortex rings --- from hookahs to powerful air cannons
Jearl Walker www.flyingcircusofphysics.com
September 2010 A smoke ring is blown with a strong puff from a mouth filled with smoke, as you can see in this video of a smoker with a hookah.
http://www.youtube.com/watch?v=q2ZjH3rEbRk
As the smoke and air leave through the rounded opening of the mouth, the flow near the lips is slowed by friction, and so the flow through the center of the opening tends to outrun it. This tendency causes the flow to curl outward around the lips, thus starting the vortex motion.
When blown correctly, the air swirls around as if it circles a solid circular ring --- moving away from the smoker on the inside of the ring and moving toward the smoker on the outside. Here is a cross section of the ring seen from the side.
The ring itself gradually moves away from the smoker, generally growing somewhat larger in diameter. The structure is called a ring vortex (or a vortex ring). The smoke merely acts as a tracer, making the air motion visible.
If a smoke ring approaches a wall, the friction on the airflow from the wall causes the ring to expand. The rate at which the air swirls decrease, somewhat like the rate at which an ice-skater spins on point decreases when arms are extended outward. The ring can also be controlled by moving a nearby hand toward it, thus pushing air against the ring. Here is a link that shows a young man forcing a ring to descend around the neck of a beer bottle.
http://www.youtube.com/watch?v=s6sTQwbJxOk&feature=related
Air Cannons
Larger vortex rings can be formed with an air cannon, which is a box with a circular opening at the front and a flexible covering (such as a plastic garbage bag) fitted across the otherwise open back. When the flexible covering is pulled to the rear and released, it pushes a stream of air through the circular opening. Just as with a blown smoke ring, the flow forms a ring vortex but without the benefit of a tracer. With an air cannon, you can startle someone across the room with a large ring vortex that approaches with no warning.
Here is link that shows a moderate size air cannon. In one part of the video, notice how a trailing but faster-moving vortex ring shoots through the leading vortex ring. In a laboratory situation, we would probably see that as the trailing vortex approaches the leading vortex, the trailing one shrinks and rotates faster while the leading one expands and rotates slower.
http://www.youtube.com/watch?v=GGW5aaW0uDA&feature=related “Big smoke gun”
Here is another large air cannon but without the smoke.
http://www.youtube.com/watch?v=am12NZwr3Fk&NR=1 “Candle cannon: behind the scenes”, head-on view of the vortex
Here is an air cannon that is large enough to require several people to generate the rings.
http://www.youtube.com/watch?v=7bvtvWxtI78&NR=1 “Giant vortex box: 20 meter test”
Powerful air cannon
Here is my favorite, one that resembles the big, bad wolf in the story of the three little pigs, a favorite in the English language. In the fable, the wolf blows down the straw house of the first pig and then the wood house of the second pig, but he cannot blow down the strong brick house of the third pig. However, in front of the air cannon powered by an explosion of acetylene and air, not even a brick wall stands a chance.
http://www.youtube.com/watch?v=hDx-mQqzYNw Vortex cannon” from “Bang! Goes the theory” television show
Explosions
Smoke rings can also be produced by large explosions, as in this video.
http://www.youtube.com/watch?v=tQWKWtL1aMM “Coffee can fireball (5 Charcoal Cremora)”
(Obviously, this is very dangerous, so leave this to the experts.) So much thermal energy is transferred to the air at the center of the explosion that the hot air quickly rises in a channel. However, because it encounters drag from the cooler air surrounding the channel, it tends to curl over like the smoke being blown by the hookah smoker. If everything goes right, the hot air forms a vortex ring, with the smoke from the explosion acting as a tracer.
More links:
ttp://www.youtube.com/watch?v=KpajwCoGOiY “Rauchringkanone” living room vortex cannon from a drum set
http://www.youtube.com/watch?v=lMLop6MIwUU&feature=related “Huge smoke ring generator”
http://www.youtube.com/watch?v=iYbTJOgxBHA&feature=related “Paper cup air cannon”, MacDonald’s cup
http://www.youtube.com/watch?v=1iUqq2mSsJw&feature=related backyard vortex cannon
http://www.youtube.com/watch?v=pQIywqFVZRA “Big smoke ring”, smoke ring from an explosion
References
Dots · through ··· indicate level of difficulty
Journal reference style: author, journal, volume, pages (date)
··· Maxworthy, T., “The structure and stability of vortex rings,” Journal of Fluid Mechanics, 51, part 1, 15-32 (1972)
··· Maxworth, T., “Some experimental studies of vortex rings,” Journal of Fluid Mechanics, 81, part 3, 465-495 (1977)
· Jenkins, D. C., “A Thanksgiving turkey shoot---physics style,” Physics Teacher, 26, 516-517 (November 1988)
· Bouffard, K., “The vortex cannon,” Physics Teacher, 38, No. 1, 18 (January 2000)
··· Niemi, A. J., “Exotic statistics of leapfrogging vortex rings,” Physics Review Letters, 94, article # 124502 (4 pages) (1 April 2005)
· Bouffard, K., “The vortex cannon,” Physics Teacher, 38, No. 1, 18 (January 2000)
x
Do you want the pub-trick stories here at the FCP site? Use the following links and then scroll down a page or search for "pub trick". Keep in mind my point --- anyone can do a pub trick but the real trick is to explain it without bluffing or just waving your hands in the air. Physics = the power to explain.
Chapter 1 archives
Water and the disappearing cigarette click this
Balancing a coin on a folded paper edge click this
Lifting a bottle with a thumb and one finger, click this
Hanging spoons from the nose, click this
Hanging bottle caps on your face, click this
Standing eggs on end click this
Removing a lighter from under a bottle click this
Removing a bill from between balanced bottles click this
Removing the cork from a wine bottle click this
Balancing a hammer and a lorry click this
Champagne cork as a morter round click this
Removing a coin from under a mug click this
Hanging a bottle on the wall click this
Matchstick rocket click this
Transferring a steel ball between beer mats click this
Tying a ring hitch click this
Chapter 2 archives
Reversing an egg in a tequila glass click this
Blowing out a candle click this
Escape from a cellophane pocket click this
Exchanging water and whiskey click this
1000 drops from an empty bottle click this
Yard of ale and beer boot click this
Tia Maria wormy action click this
Vortex in a bottle and the vortex beer bottle click this
Using glug-glug to clear beer foam click this
Inverted can of Red Bull click this
Chapter 3 archives
Moving match sticks on a glass rim
Chapter 4 archives
Slam-freezing a beer or soda
Chapter 5 archives
Rotating a matchstick balanced within a glass container
x
The other stories this month
Use the "News/Updates" button on the left side of the screen to see the other new stories for this month:
1. Pub trick --- making a coin rattle on the top of an open beer bottle
2. Turning rude and crude noises into music
3. Pattern formed in the shadow of a solid ball
4. Article of the month --- my article from May 1987. Ways in which things can disappear, like Chesire cat in Alice's Adventures in Wonderland.
x