VORTEX CANNONS

By Edwin Wise

Photograph by Michael Thad Carter

DOING DONUTS

A smoke ring, or vortex, is a beguiling thing to watch: a coherent, moving structure made out of thin air. Here are 3 cannons that can throw these “chunks of air” across a room.

I’d been aware of vortex cannons for quite some time, mostly as an interesting toy, and after I kept stumbling on the concept while touring the web, I decided I had to build my own. There’s something fascinating about taking a substance as ubiquitous and amorphous as air and transforming it into a coherent and persistent structure, almost like a crystal. I also hoped to find some use for my cannon, perhaps to propel scents or give the touch of a ghost in a haunted house.

An extreme example of the vortex’s “reach out and touch someone” power is the military vortex gun, which uses explosives to drive air rings that can knock people over, or at least generate enough sound pressure to render an area uninhabitable. And the Shockwave Cannon created by Survival Research Labs delivers an invisible boot to the head.

This project doesn’t go that far, but it will show you how to make 3 different cannons: a 5-minute version, a computer-controlled version driven by a subwoofer, and a powerful one based on my Boom Stick project (MAKE, Volume 13, page 114).

Set up: p.117 Make it: p.118 Use it: p.121

TOROID AFFAIRS

TOROID

TUB THUMPING CANNON

BARKING TUBE CANNON

BIG BAD BOOM CANNON

Illustation by Damien Scogin

➊ Puff of air travels forward

➋ Low pressure aft forms toroid vortex

➌ Trash-bag diaphragm makes puffs

➍ Subwoofer pumps series of vortices

➎ Audio gear controls vortex frequency

➏ Fog machine makes vortex visible

➐ PVC air cannon makes strong vortex

SET UP.

Photography by Edwin Wise

MATERIALS

TUB THUMPING CANNON

[A] 32-gallon plastic trash can with lid

[B] 45-gallon trash bag or tarp

[C] Ball, 1"–2" diameter

[D] Bungee cords (2), 1 small, 1 large

BARKING TUBE CANNON [MATERIALS NOT SHOWN]

½" plywood, 24"×24" sheet

¾" polystyrene insulating foam

Duct tape

Strong adhesive such as Liquid Nails or epoxy

12"-diameter cardboard tube, about 2’ long

12"-diameter subwoofer speaker

Audio wave signal source I used a laptop running Tone Generator from NCH Software (nch.com.au/tonegen).

Audio power amplifier

BIG BAD BOOM CANNON

This cannon is based on the Boom Stick project from MAKE, Volume 13. See makezine.com/15/vortex for the materials list and instructions.

TOOLS

[E] Jigsaw

[F] Party/Halloween fog machine

[G] Ruler

[H] Sharp knife

[I] Felt-tip marker

[J] 12" compass, or string, a washer, and a wood screw

MAKE IT.

BUILD YOUR AIR TOROID CANNONS

Time: Various Complexity: Easy

START ≫

1. TUB THUMPING VORTEX CANNON

This simple cannon, which you can make in 5 minutes, is a good way to get started.

1a. Cut a 6" hole in the bottom of the trash can. That’s it — you’re done! Now you can fill the can with fog from the party fogger and generate vortexes by thumping on the lid.

Photography by Edwin Wise

1b. The trash can’s lid will only take this abuse for so long before it cracks and falls apart. You can replace it with a snappy new plastic diaphragm. Start by gathering the center of a trash bag or tarp around a ball and using a small bungee cord to tie it off into a knob.

TIP: A light thump makes a slow-moving vortex, while a hard thump makes a fast-moving but less stable vortex.

1c. Duct-tape the diaphragm all around the top of the can with the knob in the center, leaving some slack. Stretch the large bungee cord across the handles. To fire, pull back and release the knob.

NOTE: You can experiment by cutting the hole larger. This makes larger vortexes that move more slowly.

2. BARKING TUBE VORTEX CANNON

This computer-controlled cannon was inspired by Bill Beaty’s Amateur Science website (amasci.com/amasci.html).

2a. Tape a 12" subwoofer into 1 end of a 12" cardboard tube.

2b. Wire the speaker to an amplifier, and wire the amp to your audio source. I used a laptop running Tone Generator, downloadable at nch.com.au/tonegen.

2c. Cut a plywood or foam ring for the exhaust end of the barrel, with an inside diameter between 4" and 6". Glue and tape it into place. This ring creates the vortex, and controls its size and velocity.

2d. Fill the barrel with fog and play sawtooth waves in the 1Hz to 10Hz range. Admire your vortices. Experiment with the frequency; when it gets too fast, the rings blur together and lose their structure.

3. BIG BAD BOOM VORTEX CANNON

My Boom Stick PVC air cannon, from MAKE, Volume 13, is great for generating air vortices. You just need to add a bit more plumbing and a large cardboard tube. See makezine.com/15/vortex for details.

USE IT.

FLYING FACTS FOR ROLLING DONUTS

EXPLORING THE VORTEX

With the computer-controlled Barking Tube, I wanted to experiment with different waveforms that would slowly pull the speaker cone back, and then fire it forward in a shaped pulse. I wrote a waveform generator in Java, but unfortunately found that the speaker wouldn’t follow the form. Instead, the cone moves in one “bounce” which pulls the smoke ring back into the cannon before it can detach and fly away. Either the audio synth in my computer or the amplifier itself filtered out most of the signal’s very-low-frequency components, distorting my waveform beyond any use. So I ended up using regular sawtooth waves, which work, but the only parameter to play with is the frequency. More work is needed.

The Big Bad Boom version will work outside, but the others work only in still air; a breeze will break up the vortex structure. In general, faster and smaller vortices dissipate sooner than slower and larger ones. In a controlled, still environment, a big, slow vortex can travel a long distance. In my not-so-controlled garage, I got Big Bad Boom vortices to travel 10’–20’.

The cannons are primarily meant for fun, and to illustrate the physics of the vortex. Any potential practical uses come from their ability to convey a blob of anything that’s about the same density as air through space, including things like bad smells, tear gas, or pepper spray. But with these, there is easily as much risk to the launcher as there is to the target.

Meanwhile, I am wondering whether it’s possible to launch a ring vertically, laden with a fine flammable powder such as powdered creamer, to form a flaming donut.

Happy vortexing!

TOP: Tub-thumping a fog vortex at the camera. BOTTOM: The Boom Stick air cannon connected to the vortex tube.

For videos of vortex cannons in action, along with more vortex resources, see makezine.com/15/vortex.