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-   -   boundaries problem? (http://www.cfd-online.com/Forums/cfx/98005-boundaries-problem.html)

pato March 1, 2012 05:36

boundaries problem?
 
Hi, iīm sorry to post a thread again but i donīt manage to solve the problem.
Iīm modelating a flow in porous media. The flow enters in horizontal direction, and as it goes through the channel, exits in vertical direction. In my first model the flow exit vertically trogh little holes in a perforated plate(air diffuser, and everything went allright. Now my task is to change this perforated plate for porous media. I Use exactly the same model, with same boundary conditions, but with porous media, i don get convergence, having floating point problem, overflow. Is it normal that with the same geom(ecept the porous layer instead of perforated layer), i have problems to achieve results??

Here my boundary conditions:

Steady state simulation
inlet: mass flow
opening: static pressure (with 0 Pa overflow problem)
Advection scheme : high resolution
Auto timescale
turbulence model:k-epsilon, (trying now with SST)
porous media: no permeability, no reisistance loss coefficient

In perforated plate i had no outlet, using the opening as an outlet. That should also work now, shouldnīt it?

How could i achieve a good convergence, or at least reliable results?

ghorrocks March 1, 2012 18:10

This is discussed in your other thread. Porous regions are handled totally differently to interface conditions and has its own issues with numerical stability. The numerical stability of porous regions has been improved in V14 so it sounds like a good time to upgrade.

pato March 1, 2012 18:41

Thank you for your answer. Nevertheless i have no acces to v14. Could anybody help me to treat this problem with v12??
any tips??should i use same boundary conditions as in normal fluid media??

ghorrocks March 2, 2012 07:52

If you do not need to use the porous approach then don't use it if it causes problems. Other than that, I can only recommend the normal things for when you have numerical instability - smaller time steps, better mesh quality, better initial guess, double precision etc etc.

evcelica March 2, 2012 22:46

I think setting the permeability to ZERO could be a problem!!!

You are trying to flow through an IMPERMEABLE membrane, that could probably give you some problems.
If you look at the momentum equation for a porous media, you are dividing by zero, which is usually a bad thing.....

ghorrocks March 3, 2012 06:14

Did he say zero permeability? I did not see that. Yes, that would definitely be a problem.

pato March 5, 2012 13:02

actually i didnīt set any value for permeability.
Is it the same as permeability 0?
Is it normal to have floating point problem then for very small permeabilities, like 1e-7mm2 or smaller?

evcelica March 5, 2012 18:07

My mistake,
I just saw no permeability and assumed you input a value of zero. If you did not insert a permeability then no, that is not the same as setting it to zero, that part of the equation is just omitted from the equations then.

I have noticed is very hard / impossible to converge when you have very small values of permeability since you start approaching the divide by zero problem.

The "CFX-Solver Theory Guide" will tell you how this is all calculated mathematically, and could perhaps shine some light onto what is causing the problem.


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