SpudFiles

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:

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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"

[youtube][/youtube]

Here you can see an old commercial hail cannon firing a powerfull, 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 beliver.

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 alligned 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 proceded to carve a big foam form out of pink insulation foam, cover it with clear packing tape, and then 3 coats of silicone-based mould 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 assembley), 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 powerfull, 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 (focussing 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 interupting 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.

And that is where my adventure has lead to this point. Tomorow I will do testing with the smaller tank, and try to upload some videos!

Because personally, I myself were looking at this thread, and there were no videos, I'd get pretty bored pretty quick.

More to come!

What a refreshingly interesting first post, looking forward to see some videos

A similar previous project which was apparently not too successful: http://www.spudfiles.com/forums/fus-ro- ... 23754.html

Using a pneumatic, your best be is probably to use higher pressures and a very high flow fast valve (for which read "burst disk") in order to mimic the sort of flows being generated in combustion cannons.

I have tested the same kind of cannon, with fiberglass cone that was just about the same size as yours, I tried combustion, but that did not work and neither did pneumatic one.
Also pressure is more important here then chamber volume

There is lot of things that you need to know when making the cone.. the cone works like de laval nozzle, aka. convergent divergent nozzle.. meaning that you speed up the flow and reduce pressure. For the nozzle to work, you need to get the flow to mach 1 before the "barrel"... and I think your not going to get that cone to work with pneumatic cannon.

The cone length is important, it needs to be long enough to reduce the pressure to atmospheric.. If the cone is too short the flow is "under expanded" and for the vortex ring you need "fully expanded" flow.

If the cone is too short, you only get a loud bang expanding in all directions and that will not go very far..


* I have one vortex cannon, that weights 100kg and is over 3 meters long.. its a 5.5litres piston valved hybrid that has valve opening of 90mm. But im not getting good results yet, I need to get bigger cone for it.. and I made my cones for that hybrid one from 3mm steel plate

Thanks for the input!

Matti,

I am currently a student in the aerospace engineering program at the University of Kansas, so I actually know what your talking about!
I'm sure that supersonic flow is possible with >100 PSI, our supersonic wind tunnel at the university utilizes a vacuum resivoir (-30PSI) behind the test section, and can simulate ~mach 1.5.

The limiting factor as far as mach number at the breech is that pesky piston, which I need to get far out of there as fast as possible.

The piston travels about 2" at this point. Its all set up on a big threaded rod, so I can extend the travel a bit.

As well, I might try to shave some weight off of the piston, to get it moving faster.

Like I said, the cannon works beautifully up until ~90 PSI, but thats not good enough .

Also, and just as you said, I belive that my barrel is under-expanded at the higher pressures. Reviewing my 60 fps video frame by frame, its clear that the plume containing the vortex at 90-100 PSI expands to nearly twice the major diameter of the barrel as soon as it exits.

I see more pink foam and fiberglass in my future...

Thanks!

Well, good to hear that you understood what I was talking about, hope it helped !

About the valve.. if it has piston travel of 2" I dont think that making it any longer will help, that should have full flow easily with that travel. But you might want to think about making some sort of small cone to the piston head to help the turbulence at the valve outlet.. not sure how to describe what i mean.. oh.. look at that pic under my name "matti", It has a green piston with the design I mean.

Also minimizing the pilot volume of your pistonvalve might help little, try connecting the sprinkler valve directly to the end of the T-fitting.



Please let me know if you start getting some results

I see exactly what you mean, almost like those conical "aerospike" rocket nozzles that have cropped up in recent years.

That may be on the list, but for now with the smaller tank volume, its making great, accurate, and repeatable vortecies at 100-110 PSI.

Counting frames, and with a 65 foot shot to a tarp in my backyard, they appear to be traveling at ~60 MPH.

The only reason I'm not going any higher is because I'm worried about the structural integrity of my cannon! (mainly the shear stresses in the PVC surrounding the 10 bolts securing the aft closure.)

I will probably try an extended barrel to get the expansion right, if I decide to test higher Pressures.

Working on a video right now.

Thanks!

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

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Interesting stuff, do you think this design would work better with a smaller, high pressure burst disk gun? Or would the rupturing disk interfere with the vortex?

Also, wouldn't it be easier to cut your cone from some sheet metal?

Excellent work! You have inspired me to try once again, this time with a bigger cone.

Very nice, also took a look at the other projects on your channel, well done!

Very nice! It's great to see a new idea being properly explored on the site. I've experimented with vortex cannons before, but only the little ones made of a bucket and a membrane. Nothing that can do more than ruffle someone's hair.

About the supersonic flow... I'm definitely uneducated when it comes to fluid flow, but my understanding was that with a pneumatic cannon you can NOT exceed the speed of sound in the gas you are using. Perhaps this is an incorrect assumption however, as rocket engines and the like clearly have a supersonic exhaust.

Great work!

Insomniac wrote:my understanding was that with a pneumatic cannon you can NOT exceed the speed of sound in the gas you are using.

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You can go supersonic with compressed air alone...

So that's a PCP gun then? I see. Wikipedia still says that the limiting factor in a firearm is the speed of sound in the working gas.

I'm not doubting the video, just wondering why the limiting speed is above the SOS in the gas. No doubt there's some interesting physics going on.

Higher end PCPs with ultra efficient (Airforce Condor) or brutally overkill (Career) valves regularly exceed the speed of sound with compressed air alone.

Insomniac wrote:No doubt there's some interesting physics going on.

Something that has fascinated spudders since days of yore

The speed of a projectile is limited to the speed of sound in the propellant gas. In an air cannon, this would be at or around the speed of sound.

In a combustion gun this can be significantly higher than the speed of sound, depending on the temperature of the combusted gasses (the speed of sound in a gas depends mainly on temperature, very mildly on pressure, and the composition of the gasses), such as in a conventional firearm.

Thats why "light gas guns" use hydrogen as a propellant to simulate impacts at orbital velocities, as the speed of sound in hot, compressed hydrogen is so high.


The reasons for this, I'm not exactly sure of.

I do know that compressed air is used to operate supersonic wind tunnels. Just a little bit of google searching to confirm that.

Thats about it

I wish I knew more about this!