Vortex Cannon

Activity/Demo overview: The vortex cannon can be used to fire smoke rings, knock objects over using airflow, or even shoot rings of fire. This can be used to demonstrate the principles of air pressure and air resistance. We have used this demo in the Wonders of Physics Annual Show, as well as for smaller groups like the PEOPLE program participants.

Activity/Demo Instructor Notes: 

Summary:

1. 1. The vortex cannon works by creating a slow-moving ring of air.
   2. Use all of the vortex cannon sizes.
   3. To make smoke rings, make sure the smoke machine is working properly.
   4. The larger cannons can be used to knock things down with the air flow.
   5. The trash can cannon can be used to do rings of fire with butane gas.
   6. This demo is great for audience interaction, so interact and have fun!

1. The vortex cannon is essentially a cylinder, typically with one end closed and one end open. In the closed end, a circular opening is cut, leaving about 10% of the closed face remaining as a border. The open end is sealed and covered with a taut, flexible material (diaphragm) like a drum. When you pull back and release or hit the diaphragm, air is pushed through the cylinder toward the circular opening. However, once it reaches the circular opening, the air scraping past the edge gets slowed down, while the air going through the middle moves faster. The slow air along the edge has a higher pressure than the fast air in the middle, causing the air to curl back on itself, and creates a ring/vortex of moving air that can trap smoke or other, heavier-than-air gasses in it and travel quite a substantial distance before dissipating.
2. The vortex cannon itself has a few different variations. They range in size from a small, red, plastic cup with a latex balloon stretched over the opening, to an oatmeal container, to a multi-gallon bucket, to a metal trash can with a garbage bag over the opening. They all work about equally well in terms of producing smoke rings, so it would be good to start with a smaller one and work your way up to the largest ones. For dramatic effect, you can keep the larger ones hidden and ask the audience whether they think bigger rings can be produced. A good joke about the red, plastic cup and the oatmeal container cannons is that you can make those cannons at home and the materials can be found at your local grocery store or frat house.
3. In order to produce the smoke rings, you need smoke from the smoke machine. Make sure there is enough fluid in the machine before running it and start the machine at least 5-10 minutes before it is needed. It needs time to warm up! However, don’t leave it running forever since it can overheat or periodically release smoke without user input. To fill a vortex cannon with smoke, simply hold the cannon’s opening directly over the smoke output. The small cannons really only need a half-second’s worth of smoke, while the larger ones need maybe a second or two.
4. The larger vortex cannons can also be used to knock things down. The air flowing through the center of the ring will be moving fast enough and with enough force to knock over a Styrofoam coffee cup, for example. You can have an audience member balance the cup on their head and then try to knock it off. You can be about 10 feet from them for this to work. If you have difficulty aiming, use smoke to produce rings that can help you track your aim. Just be sure to ask the audience member if they are okay getting smoke in their face!
5. If you are able to access the methane gas supply and the appropriate ring-shaped pipe, you can create rings of fire with the trash can vortex cannon. Steve Narf has butane, which is an invisible, but flammable, gas. Near the opening of the cannon, there are small holes in which someone can spray the butane. You will need someone to do this while you hold the cannon since it would be hard to reach yourself and the butane leaks out quickly. Using one can, the person assisting you should fill about 5 seconds worth of butane (maybe more if the can is running low) into the cannon. The ring pipe must be connected to the methane gas supply and lit with a match prior to filling the butane. It is recommended to have the ring oriented facing the audience so they can see the ring of fire straight on, as it will not last long. For safety, the person firing the cannon should stand back about a meter from the ring pipe, and nobody should be within a meter of the setup when the cannon is fired. When you do fire the cannon, do so directly level with the ring pipe such that the vortex/ring will line up with the lit flame. When the butane ignites, it will create a ring of fire that travels half a foot to a foot before dissipating. The butane might also “backfire” towards you, which is why keeping a safe distance is important. You can expect to only get a few rings of fire from one fill of butane.
6. The vortex cannon is a really fun demonstration because it elicits a great deal of audience interaction. As mentioned above, you can not only ask the audience for things they want to see (like bigger rings, or lighting the rings on fire), but also knock things off their heads. Even just firing the smoke rings close enough to be touched engages the audience, and lots of kids will reach out and try to disrupt the rings. Use this excitement to keep the energy going! I suggest only explaining how it works (see item 1 above for an explanation you could say) after you’ve done one or two of the cannons so that they are already engaged and ready to see/hear more. If you can, allow audience members to try the cannons themselves. You should operate the smoke machine, but people and kids especially love to shoot the smoke rings and even fire them at each other. As long as you keep an eye on things and don’t let any damage come to the cannons, this is totally okay. Just remember to keep a positive attitude and have fun with it. When you have fun, the audience will have fun too!

Sam Kramer, 2022-23 Wonders of Physics Outreach Fellow