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April 5, 2011, 11:20 
insulated container divided into equal volumes by a partition

#1 
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Alain
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Hi,
my goal is to model the following problem: A gas is confined to one side of a rigid, insulated container divided into equal volumes by a partition. The other side is initially evacuated. The partition is removed, and the gas expands to fill the entire container. Assuming ideal gas behavior, determine the final equilibrium pressure. I tried two ways to do this: first using two domains of fluid with an fluidfluid interface, then using cell expression language to do a global initialization for a single fluid domain. But my results until now have been unsuccessful. Anybody knows how to simulate this kind of problem? Last edited by Alain_aftch; April 10, 2011 at 23:32. 

April 5, 2011, 16:27 

#2 
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I guess the first question is why not just use the back of an envelope rather than a CFD solver! I assume you have your reasons?
Did you run this as steadystate? In steadystate mass is not going to get conversed during convergence, so I think you would need to run this as a transient case. Unfortunately that would be difficult to converge due to the high mach numbers in the evacuated space. Perhaps there's an expert parameter for the steady state case  anybody? 

April 5, 2011, 18:51 

#3 
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Glenn Horrocks
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There are no expert parameters which will help. Stumpy's comment is correct  sounds like a first year uni thermodynamics question which can be solved in a few minutes with the ideal gas laws.... So why bother with CFD?
It will be a tricky CFD simulation with all sorts of nasty initial transients. 

April 6, 2011, 08:44 

#4 
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Not sure if this would work, but if you set it up as a multiphase case with initial conditions set so the volume fraction of gas is 1, then you may be able to run steadystate. For steadystate multiphase cases in a closed system a global correction is applied to converse the mass of each phase.


April 10, 2011, 02:35 
simulate a burst disc

#5  
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Alain
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Quote:


April 10, 2011, 02:40 
something like that

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Alain
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April 10, 2011, 06:49 

#7 
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Glenn Horrocks
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What are you trying to get out of the analysis? Your first post only mentioned the final pressure, but now you are talking about a burst disc.


April 10, 2011, 15:51 

#8 
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Alain
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i'm trying to get the simulation of a pressure vessel failure and the release/dispersion of fluid into a low pressure chamber. Imagine that the fail is due to a burst disc. For this reason, I started this work with the first problem, which I know the theoretical answers.
Last edited by Alain_aftch; April 10, 2011 at 23:25. 

April 10, 2011, 22:21 

#9 
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Glenn Horrocks
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I assume you mean the final steady state mixed result for the "first problem".
So is your question how can you make the opening suddenly appear? 

April 10, 2011, 23:23 
yes

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Alain
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April 11, 2011, 06:28 

#11 
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Glenn Horrocks
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Lots of ways.
1) Define an initial condition with different conditions on each side. This is the easiest way. 2) Do a simulation to define the initial conditions where they are not connected (maybe a wall in the gap). Then use this as an initial condition on a simulation with the gap connected. 3) Put a source term, probably a momentum source, in the gap which initially stops flow (or at least greatly slows it down) and then dissappears. 4) Use a GGI and a sliding interface to slide it open. 5) Use moving mesh to open a connection up. I could probably think of more if I could be bothered. But option 1 is simplest so go with that if it is suitable. 

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