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Help with 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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Help with GGDT

Unread postAuthor: apocavorme » Sun Sep 18, 2011 4:19 pm

Hi, my first post, i read a lot here and tried to install ggdt but i got the following error:

anyone could help me? im using windows vista 64.
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Unread postAuthor: Gun Freak » Sun Sep 18, 2011 4:20 pm

Right click -> Run as Administrator
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Unread postAuthor: DYI » Sun Sep 18, 2011 4:26 pm

When I used Vista, running it as an Admin didn't solve the problem. In that case, you'll have to disable UAC altogether.
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Unread postAuthor: apocavorme » Sun Sep 18, 2011 6:35 pm

Thank you! i runned as admin and it worked!
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Unread postAuthor: apocavorme » Wed Sep 21, 2011 12:50 pm

hi again, i was using ggdt and i have a doubt... in a barrel sealing valve, why the pressure curve of the reservatory is diferent from the barrel pressure curve? The valve remains open all the time as shown on valve position curve...
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Unread postAuthor: jackssmirkingrevenge » Wed Sep 21, 2011 12:56 pm

The "reservatory" (I think you mean "chamber", did you translate "serbatoio" ;)) is losing pressure and the barrel is gaining pressure, but due to all the varialbles the rate of loss and gain will not be identical - the pressurised volume of the barrel expands as the projectile moves for example.
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Unread postAuthor: DYI » Wed Sep 21, 2011 1:11 pm

GGDT essentially models two different bodies of gas, the barrel and the chamber, with the valve parameters and pressure differential determining the rate of mass flow out of the barrel into the chamber. Its output is a numerical approximation of the solution of a small system of ODEs which describe this process. This turns out to be a useful model for low speed pneumatics (with appropriate fudge factors added in).

In reality, the gas has mass, and the pressure varies continuously between points inside the launcher at a given time - there is no true "barrel pressure", because at a given time, the pressure will be (usually) higher at the breech than it is at the projectile base.
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Unread postAuthor: apocavorme » Wed Sep 21, 2011 9:24 pm

jackssmirkingrevenge wrote:The "reservatory" (I think you mean "chamber", did you translate "serbatoio" ;)) is losing pressure and the barrel is gaining pressure, but due to all the varialbles the rate of loss and gain will not be identical - the pressurised volume of the barrel expands as the projectile moves for example.


...hehe serbatoio is italian, i didnt know... i was thinking in portuguese :lol:
i see the things are a little more complex than what i was imagining... i looked in a book but i only see explanation for uncompressible fluids... not the case...

DYI : is there a analitic way to solve the equations? i woud like to understand a little more... can you indicate me what to read?

Thank you all and sory for bothering...
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Unread postAuthor: DYI » Wed Sep 21, 2011 9:56 pm

There may be an analytical solution to the system used in GGDT, but it's not likely. I can definitively say that for the case of "proper" modeling of gas flow, as I described above, no analytical solution is known, even for the one dimensional case with many assumptions which simplify the system (this is a system of partial differential equations often called the Navier-Stokes equations).

If you're interested in learning more, John Anderson has two excellent texts you can look into - "Modern Compressible Flow", and "Computational Fluid Dynamics - The Basics with Applications". Unfortunately, internal ballistics is a terrible, terrible problem which the simplest CFD methods aren't readily applicable to. The methods which I've found to be most useful for the one dimensional case are pure Lagrangian approaches (one is referenced in btrettel's "Open Source Pneumatic Simulation" thread), and the ALE (Arbitrary Lagrangian Eulerian) method. I can post more on both tomorrow, if you like.
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Unread postAuthor: apocavorme » Wed Sep 21, 2011 10:44 pm

yes.. i study some eletromagnetic problems that may have some similarity with fluiddynamics... i use comsol to model problems but never looked to cfd mudules... im a beginer at fem and fdtd, not able to program hard yet, but if i understand the physics equations, could try it at comsol. i t would be good;
and thak you! hehe :lol:
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Unread postAuthor: apocavorme » Wed Sep 21, 2011 11:00 pm

i saw btrettel's thread, hi tech thing! hehe... and the mentioned paper; i ll start reading about... first i need to simulate the simplest case.. then i ll evolve... very good stuff.. any news about that code?
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Unread postAuthor: jackssmirkingrevenge » Wed Sep 21, 2011 11:29 pm

apocavorme wrote:...hehe serbatoio is italian, i didnt know... i was thinking in portuguese :lol:


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