Experimenting with a vortex cannon

[Feathers]

Brainwave

You've all heard of the "airzookas" and the vortex-generators they make out of trash cans and some canvas, but that all bores me a bit...

I found this video awhile ago:



I was amazed and fascinated, and immediately found the full BBC "Bang Goes the Theory" episode.

It looked like some very sophisticated valves and equipment on the combustion chamber, and I began to doubt that the "clever, but not that clever" host built it himself.

Further digging revealed that the device in the video is actually a prototype "anti-hail shockwave cannon" built by Newton Systems International llc

Youtube is chalk-full of videos of these "shockwave cannons"



Here you can see an old commercial hail cannon firing a powerful, fast-moving vortex ring across a field, making a very distinct screaming/whistling noise.

The manufacturers assume ignorance on the part of their agricultural customers, and claim that the whistling/screaming is a "shockwave", which it clearly (check the slow-mo's on the first video) is not. Its a vortex ring moving at such speed, and with such a fast-moving internal flows that it makes noise.



All this, I found about 6 months before building my first piston-valve air cannon.



I am a believer.

This valve works wonders, and before I shot anything, I did some dry-shots with a 2, and 3 inch barrel.

Upon blasting it into the backyard at ~80 PSI with the 3" barrel, I heard that familiar screaming/whistling noise.

This indicated to me that the shape of the barrel, and the sharp, explosion-like release of air had aligned to form a fast-moving, audible vortex flying across my backyard.

I was thrilled, and immediately bought some 4" PVC and made a barrel which "stepped-up" from 2-3-4" over about 10" length, then a 2 foot 4" barrel.



You'll notice that the air tank is shorter, I built the barrel last week, and shortened the tank just yesterday for reasons I will discuss.

With this new barrel the cannon would, 2/3 times, produce the whistling vortex. However, when I tried to direct the vortex at something, it would veer off- course and take a different route across the backyard.

I also noticed that there is a "sweet spot" around 80 PSI. Much lower, and the vortex would not whistle, and travel slowly. Much higher than 80 and vortecies become more hit-and-miss, then at around 100 PSI I get nothing more than the normal "BANG".

Lesson learned, I decided to take a tip from the "big guns".

I looked at a picture of a commercial hail cannon, and scaled the barrel to a size which "seemed about right" given the size of my cannon. This turned out to be a 40" conical tapered barrel with a 7.5" major diameter, and a 1.8" minor diameter.

I proceeded to carve a big foam form out of pink insulation foam, cover it with clear packing tape, and then 3 coats of silicone-based mold release agent (it looked like a big pink parking cone)

I set a 2" thread-to-pipe PVC coupler over the top (to be fiberglassed into the Assembly), and fiberglassed the whole thing. Here is the result:



To get the form out of the finished part, I just screwed it onto the air-cannon, filled it up to ~20 PSI, and fired it out .

This new barrel worked great, and I was now creating powerful, audible, vortecies repeatedly, and with reasonable accuracy all the way up to ~90 PSI.

However, I'm still hitting some sort of barrier regarding it's performance.

I've done some testing (videos to come!) by filling the barrel with smoke from a fog machine, and filming the shot (focusing on the end of the barrel, and 4-5 feet past that) with my Go Pro HD Hero at 60 frames per second, and analyzing it frame by frame.

It looks to me that a contributing factor may be the increased volume of air leaving the barrel at ~100 PSI, which is actually interrupting the stable formation of the vortex.

Looking at the slow-mo's on the first video (BBC's "Bang Goes the Theory"), I see a vortex leave, and then a little bit of flame, but not much more exits the barrel. On my slow-motion analysis of the ~100 PSI shots however, I see the vortex leave, but in the midst of a giant, turbulent plume of air (visualized by the fog).

For this reason, I shortened the air tank, hoping to retain the "bang", or spike of pressure that the acetylene/oxygen hail cannons use to form a vortex, but reduce the amount of gas that folows.

Video of the more successful tests at 100 PSI with the shorter tank:

[Johansson]

Great work, I have been fashinated by vortex cannons for quite some time so thank you for posting this!

[Feathers]

SO.

Where I left off, I belive I was complaining about the shape of my cannon, the speed of the valve, and the size of the air tank.

And I've decided to upgrade to a 4" coaxial piston valve cannon design.

This will give a more uniform flow into the barrel from all directions, provide the smaller tank capacity, and the 1" increase in piston diameter will deliver 80% more force, hopefully opening the valve that much faster.

Here is the new piston assembley. You may notice I borrowed matti's idea, adding a conical diffuser to the barrel seal. This will hopefully help "untangle" airflow into the barrel, and subsequently the diffuser.



Here you can see the full assembley. I cut up a 4" male to 2" female coupling to use as a centering ring for the "barrel" tube. I also trimmed the edges of the vanes of the centering ring to airfoils, to help with smooth airflow. May be overkill, but it can't hurt.



This is a view down the barrel, at the diffuser on the end of the piston.



And the new aft closure. Two O-rings and eighteen 1/4"-20 stainless screws.



And finally, a finished cannon at the end of the night (2:00 AM)



Just have to work out a bumper for the piston, lube it up, and do a pressure test.

Hopefully a new video tomorow!

[turbochris]

I made an aluminum piston for my potato gun. It worked great. It blew the breach off from the valve hammering open so fast. Actually split a 4 inch PCV to pipe thread coupler. Thanks for posting this.

[Feathers]

So overall, the last co-axial air cannon design ended up performing no better than the original "T-valve" cannon design.

Reaching the "safe" operating limit of my PVC cannons at around 120 PSI, Air has been exhausted as a source of energy.

On to acetylene!!

Now I won't be using straight oxygen and acetylene, that would be too dangerous. The mix would detonate every shot, and I'd have broken cannons and broken me.

The commercial hail cannons don't use oxy-acetylene either. The two banks of bottles stored on site are acetylene, and high pressure air (3,000 PSI).

The high pressure air is used to help thoroughly mix acetylene in the large combustion chambers, as the delivery pressure of the acetylene is much lower (100-300 PSI depending on temperature) because it is stored as a dissolved gas in acetone, in a porous matrix inside the tank, rather than under pressure.

So I went ahead and built a short and squatty PVC combustion chamber, attempting to match up the scale of my finished rig to that of a commercial hail cannon as closely as possible.

The short combustion chamber, and central ignition are critical factors in avoiding detonation, where flame front velocity exceeds the local speed of sound, and the resulting spike in pressure does very bad things to the container.

Here it is.