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Interesting expansion pattern

Post questions and info about pneumatic (compressed gas) powered cannons here. This includes discussion about valves, pipe types, compressors, alternate gas setups, and anything else relevant.
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Interesting expansion pattern

Unread postAuthor: saefroch » Sat Dec 04, 2010 8:06 pm

I recently filmed a test shot (no projectile, just a visual effect experiment) and captured a still from it that rather confuses me. Anyone care to explain? Based on the sheer coolness of the event, I'm going to see if I can replicate it.
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Last edited by saefroch on Sat Dec 04, 2010 10:47 pm, edited 1 time in total.
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Unread postAuthor: jr » Sat Dec 04, 2010 8:40 pm

could use a little more info....

what are you shooting?
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Unread postAuthor: saefroch » Sat Dec 04, 2010 8:47 pm

No projectile, but a barrel full of homemade airfloat charcoal.
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Unread postAuthor: Hotwired » Sat Dec 04, 2010 9:08 pm

So why does that surprise you?

It's just a pile of low mass particles, dust really, fire a pile of it out of a barrel, it hits still air and it drops dead in a sideways mushroom patten out of the barrel.

Unless you've bound the dust together the particles scatter and lose energy very rapidly.
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Unread postAuthor: Technician1002 » Sat Dec 04, 2010 9:44 pm

Did it eventually turn into a vortex ring?
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Unread postAuthor: saefroch » Sat Dec 04, 2010 10:47 pm

It's a pile of low-mass particles, yes. They hit air, yes. But I'm pretty sure they didn't drop dead in a sideways mushroom pattern. I think they formed a sphere, since the cloud that formed afterwards was a surprisingly regular.

No, sadly it didn't turn into a vortex ring. It continued to expand outwards and quickly became irregular.

The original post has been edited to show the sequence of 4 frames I find to be most significant, as the frames after just show the cloud expanding slowly.

I think it'd also be interesting for somebody to replicate this with either high-speed, a large barrel volume, or both. The amount of airfloat charcoal I can project is limited by my barrel volume, so for me it's minimal.
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Unread postAuthor: ilovefire » Sat Dec 04, 2010 11:57 pm

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Unread postAuthor: jackssmirkingrevenge » Sun Dec 05, 2010 12:29 am

this is from a howitzer:
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Unread postAuthor: saefroch » Sun Dec 05, 2010 12:34 pm

Indeed, those are the shapes I'd expect, rather like a sphere with a hole being punched in the middle. But a near-perfect sphere? That's what confuses me, especially because the cloud of particles was ahead of the expanding mass of air, so shouldn't it have an even more significant hole in it?
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Unread postAuthor: jackssmirkingrevenge » Sun Dec 05, 2010 3:20 pm

Image

[youtube]http://www.youtube.com/watch?v=LrWZ0DhXVHU&playnext=1&list=PLFBC89229BBAF5502&index=52[/youtube]

[youtube]http://www.youtube.com/watch?v=uf941T7pFTY&feature=related[/youtube]

Using 57mm deck guns as an example as they don't have a muzzle brake to interfere with the blast pattern, there's no evidence of a "hole" through the cloud.
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Unread postAuthor: saefroch » Sun Dec 05, 2010 3:32 pm

So the spherical cloud is the natural formation without a muzzle brake?
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Unread postAuthor: jackssmirkingrevenge » Sun Dec 05, 2010 3:35 pm

saefroch wrote:So the spherical cloud is the natural formation without a muzzle brake?


Pretty much, at least in all the stills I've seen:

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Unread postAuthor: saefroch » Sun Dec 05, 2010 3:57 pm

And in those stills, all the deformation is caused by the projectile's interaction with the cloud, I assume. I'll have to do another filming against something better so the muzzle's location in relationship to the cloud is apparent. I made about 2lbs of airfloat, so that should hold me for a while :P.
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Unread postAuthor: jackssmirkingrevenge » Sun Dec 05, 2010 4:19 pm

Out of curiousity I powdered some coffee and shot it out a 6mm pneumatic, looking at it on the high speed it produced a conical pattern, perhaps the particles are too dense. Oh, and now my room has a lovely aroma :D
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Unread postAuthor: saefroch » Sun Dec 05, 2010 4:24 pm

What did you use to grind it? Coffee also has very different properties when ground into a fine powder (espresso grind) than charcoal. I know, because I used my hand-powered burr grinder to produce the airfloat :D. For one, coffee generates a LOT of static potential when you grind it. So much so that it will actually increase in volume over the dry beans. Charcoal also increases in volume, but is not nearly as cohesive as a powder.

I think I might do that with some coffee, but maybe not as I might be shooting upwards of $10 out the barrel :?
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