Help with GGDT

[apocavorme]

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.

[Gun Freak]

Right click -> Run as Administrator

[DYI]

When I used Vista, running it as an Admin didn't solve the problem. In that case, you'll have to disable UAC altogether.

[apocavorme]

Thank you! i runned as admin and it worked!

[apocavorme]

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...

[jackssmirkingrevenge]

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.

[DYI]

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.

[apocavorme]

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
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...

[DYI]

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.

[apocavorme]

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

[apocavorme]

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?

[jackssmirkingrevenge]

...hehe serbatoio is italian, i didnt know... i was thinking in portuguese