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Chamber:Barrel and GGDT?

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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Unread postAuthor: Ragnarok » Sat Jul 25, 2009 4:16 pm

To be fair, no simulation should be taken as undeniably accurate.
I'll happily praise GGDT for proving itself at subsonic velocities (dependent on accurate user input, of course), but with the above caveat in mind, I don't get why people are so quick to take the results of GGDT as next to law.

I don't see anyone taking the results they get with a pinch of salt, which they should for any velocity range - but when they should definitely be doing when the results are Mach 1 or greater in the propellant gas, they seem even less likely to do so.
History (both that of spudgunning alone, and that of engineering in general) has shown it's no simple task to break the sound barrier.

The other thing that gets on my nerves a little is that people seem to think that GGDT is a substitute for genuine real world results.
As always guys: Simulation < Genuine results

Anyway, ranting over.
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Unread postAuthor: btrettel » Sat Jul 25, 2009 4:32 pm

Having looked into the problem in some detail, I'm fairly convinced that some sort of CFD simulation is necessary for any reasonably accurate simulation results at transonic speeds and faster. I'm not sure what approach GGDT or your Apocalypse modeler Rag takes, but this is what I plan for BAGS 2, at least in some extremely simplified manner, which still would be enormously complicated.
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All spud gun related projects are currently on hold.

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Unread postAuthor: Technician1002 » Sat Jul 25, 2009 4:45 pm

D_Hall wrote:
Technician1002 wrote:My current project is a 3 inch porting.. :D I do intend to waste a 2 inch barrel on it with a venturi entrance for an attempt at SOS. GGDT predicts well over SOS on a light projectile at 200 PSI.

Note that I'll be one of the first to admit that GGDT gets a bit iffy at supersonic velocities. I've done my best while trying to keep the sim generic and it's numbers are better than nothing but... Well, I wouldn't hang my hat on them.


Good point, but with GGDT predictions of well into supersonic, just crossing at supersonic is the reason to build and test. I know the theory on the wall of supersonic, but jet engines seem to be able to fly supersonic with most of the air going through the compressor at supersonic speeds. I hope to have a double verification chronograph in place for the testing. Both optical and wire break. I am building to find out.

Other places with supersonic air flow is high pressure flows from compressed air tanks with the valve knocked off and compressed air jet packs. This is not with light gas or elevated temperatures.

Air expanding in a burst creates a shock wave that travels at speed of sound. Air moving near the speed of sound then going through a venturi exceeds the speed of sound as it accelerates and drops pressure. This is what I am looking at in getting a golf ball supersonic maybe. I just gotta test. To test, I gotta build.

If my photography is any indication, I have gotten well over supersonic air blasts from the 2 inch cannon. At 1,000 frames a sec, the front can be seen at over Mach 1.5. The flow looks to be traveling at over 4.5 feet in 2 ms.
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Attachments
3 frames.jpg
3 frames of video 1 ms apart. Shot at 1,000 frames per second. How fast is the flow? The marked pieces of tape on the ruler are 1 foot apart.
3 frames.jpg (24.04 KiB) Viewed 260 times
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Unread postAuthor: jimmy101 » Sat Jul 25, 2009 4:59 pm

Ragnarok wrote:The other thing that gets on my nerves a little is that people seem to think that GGDT is a substitute for genuine real world results.
As always guys: Simulation < Genuine results

True enough. However, in some circumstances "simulation" gives better and/or more useful results than "genuine results".

Usually, in a simulation you can make small changes in the parameters and get answers much quicker than with "genuine results". To the extent that folks are too lazy to get "genuine results", "simulation" is the only answer.

An example would be the "mine sized chamber affect". Nobody, that I know of, has ever done a Latke type study on a variable chamber size on a fixed barrel combustion. The HGDT, and other, simulations are the only data we have.

In the real world, engineers generally use both simulation and "genuine results" to model complex systems. Both have their advantages and their disadvantages and both are useful. You get a better understanding of any system when you have both "genuine" and "simulation" results.

In the case of GGDT, I suspect GGDT is probably about as good as actual results for subsonic shots. When GGDT disagrees with actual results, I would first suspect that the parameters to GGDT were incorrect (valve flow and opening time, ammo friction and mass …) before suspecting a fundamental flaw in GGDT. Furthermore, when GGDT disagrees with actual results I would want to know if the difference is statistically significant. You really need to collect several replicates of the "genuine" data and do a statistical analysis.

A couple more big advantages of "simulation" versus "genuine" results; (1) Simulations don't have shot-to-shot variability. (2) Simulations give you some idea of why one set of parameters behaves differently than another. "Why" is often a more useful result than is an absolutely accurate prediction.
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Unread postAuthor: D_Hall » Sat Jul 25, 2009 5:34 pm

btrettel wrote:If won't break down if you do the differentiation properly with the chain rule. You still differentiate with respect to the radius.

Sure, but as you said yourself... You still differentiate with respect to the radius. In other words, you have to account for the fact that you made an algebraic substitution. Keeping it in the "radius world" makes for a much cleaner calculation that keeps the relationships more clear.
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Unread postAuthor: D_Hall » Sat Jul 25, 2009 5:38 pm

btrettel wrote:I'm fairly convinced that some sort of CFD simulation is necessary for any reasonably accurate simulation results at transonic speeds and faster. I'm not sure what approach GGDT or your Apocalypse modeler Rag takes, but this is what I plan for BAGS 2, at least in some extremely simplified manner, which still would be enormously complicated.

Agree.

And for what it's worth, GGDT uses simple compressible flow theory for a streamtube. Flow is assumed to be laminar and of a uniform velocity (ie, a streamtube). Nothing horribly exotic. I make no claims as to being a compressible flow guru in any way shape or form. I know the basics; nothing more.
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Unread postAuthor: Ragnarok » Sat Jul 25, 2009 6:03 pm

jimmy101 wrote:However, in some circumstances "simulation" gives better and/or more useful results than "genuine results".

A perfectly valid point. I like anyone use modellers. However, I remember what the DT in GGDT stands for: Design Tool.

If I'm designing a cannon, projectile, or whatever, I'll almost certainly hit a few simulators before I even think about picking up any tools.
However, when I've actually got the cannon/projectile/whatever, I don't use simulated results to determine its performance. I find real world results.

My issue is with people who say: "This is my cannon. GGDT says it shoots at 762 fps".
Sure, it's a ballpark figure, but should be treated as no closer. Without an accurate valve flow coefficient - which can only be determined experimentally - there's guestimation in the inputs, so the outputs are going to be flawed. Garbage In, Garbage Out - as they say.

That doesn't even count where people have rounded barrel IDs or guessed projectile weights. I use diameters and projectile weights with accuracies of 99% or more - and even then I only take the results as of being rough estimates.

~~~~~

And before anyone points out anything about the fact the LRC will be taking into account all kinds of minor things that will be insignificant next to input inaccuracy... it does that because I'm weird, not because it's actually a good idea.

To bring it back to Jimmy's argument for simulators, it's actually very interesting seeing how much each individual element affects the end results - "What happens if I double the projectile lift?", "How much does the Coriolis 'force' actually affect this?", "What if it were 10 degrees hotter that day?", "What if it's spinning off-vertical?", "What if it were rifled rather than drag stabilised?"

The really surprising thing is how much these things can change the results.
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Unread postAuthor: SpudJoe » Sat Jul 25, 2009 7:11 pm

So we went overkill then? Ugh...

We really just wanted to make a totally badass gun to break the speed of sound. We're planning on pumping that 3" chamber to 150psi or so. We were also going to use a ball valve (so simple!) but after reading around, it seems thats not the best idea. I think we're going to try burst disc instead (which also seems really easy and is apparently much better?). Correct me if I'm wrong, but isn't a burst disc method basically just placing burst disc material in between a male->female threaded coupling? The wiki is not very descriptive.

So maybe we'll go with a 20' barrel. I'm not sure where to pick up something that big, I might have to couple two 10' pieces (is that what most people do for extra long barrels?)

Thanks for the input!
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Unread postAuthor: spudtyrrant » Sat Jul 25, 2009 7:21 pm

uh maybe if you got a 50 foot barrel on there it might come close but a lot of factors come into making a cannon break the speed of sound and your cannon just doesn't have what it takes you are much more likely to do it with a high pressure lower volume gun with a very good valve(such as a burst disk like you described) if you wanted to even come close you would have to make you cannon unreasonably large to do so
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Unread postAuthor: Technician1002 » Sat Jul 25, 2009 7:50 pm

spudtyrrant wrote:uh maybe if you got a 50 foot barrel on there it might come close but a lot of factors come into making a cannon break the speed of sound and your cannon just doesn't have what it takes you are much more likely to do it with a high pressure lower volume gun with a very good valve(such as a burst disk like you described) if you wanted to even come close you would have to make you cannon unreasonably large to do so


Maybe not.. Model a 700 cubic inch tank in GGDT with a 3 inch burst disk, 2.5 inch barrel, 6 cubic inches of dead space for a venturi transition into the barrel and a light 2 oz tennis ball at 200 PSI at 15 feet. :D It's one of the planned experiments with the 3 inch I'm building. Golf balls and marshmallows are also on the list.
http://www.ferguson.com/index.shtml sells pipe in 20 foot lengths.

It's where I got my tennis ball barrel. Thinwall stuff. :D

My tanks are posted elsewhere. The little tank isn't all that big.
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Last edited by Technician1002 on Sat Jul 25, 2009 7:52 pm, edited 1 time in total.
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Unread postAuthor: SpudJoe » Sat Jul 25, 2009 7:52 pm

So was the guy at spudtech just BSing when he claimed he broke the sound barrier with this gun?

http://www.spudtech.com/images/products ... onfull.jpg

We're probably gonna go with 10-15' of 3" I guess. Hopefully we'll come close to the speed of sound anyway. I'm sure we'll all be happy no matter how it comes out, we'll know not to go so big in the future (and save some money in the proces!)

Thanks[/img]
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Unread postAuthor: D_Hall » Sat Jul 25, 2009 8:51 pm

SpudJoe wrote:So was the guy at spudtech just BSing when he claimed he broke the sound barrier with this gun?

In a word... Yes.

When pushed for his muzzle velocity, data, etc. what he finally would admit to was that the gun was REALLY REALLY LOUD. For some reason, he felt that REALLY REALLY LOUD was sufficient proof to claim supersonic.

:roll:

Was the gun supersonic? Possibly. But he had exactly zero data to back up his claims.
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