boundaries problem?

pato · March 1, 2012, 04:36

Hi, i´m sorry to post a thread again but i don´t manage to solve the problem.
I´m 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, shouldn´t it?

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

ghorrocks · March 1, 2012, 17:10

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, 17:41

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, 06:52

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, 21:46

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, 05:14

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

pato · March 5, 2012, 12:02

actually i didn´t 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, 17:07

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.

March 1, 2012, 04:36	boundaries problem?	#1
pato Member albert Join Date: Jan 2012 Posts: 31 Rep Power: 14	Hi, i´m sorry to post a thread again but i don´t manage to solve the problem. I´m 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, shouldn´t it? How could i achieve a good convergence, or at least reliable results?

Thread Tools	Search this Thread
Show Printable Version Email this Page	Search this Thread: Advanced Search
Display Modes
Linear Mode Switch to Hybrid Mode Switch to Threaded Mode

Similar Threads
Thread	Thread Starter	Forum	Replies	Last Post
[Commercial meshers] Handling cyclic BC from gambit to openfoam for a cascade airfoil problem - OF 1.6	maverick	OpenFOAM Meshing & Mesh Conversion	2	June 18, 2011 04:36
Velocity profiles problem behind the elbow (3D problem)	kabat73	FLUENT	8	May 9, 2010 04:26
periodic boundaries - flow through a net	PK	FLUENT	0	July 12, 2007 11:58
Adiabatic and Rotating wall (Convection problem)	ParodDav	CFX	5	April 29, 2007 19:13
Is this problem well posed?	Thomas P. Abraham	Main CFD Forum	5	September 8, 1999 14:52

March 1, 2012, 17:10		#2
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,696 Rep Power: 143	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.

March 1, 2012, 17:41		#3
pato Member albert Join Date: Jan 2012 Posts: 31 Rep Power: 14	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??

March 2, 2012, 06:52		#4
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,696 Rep Power: 143	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.

March 2, 2012, 21:46		#5
evcelica Senior Member Erik Join Date: Feb 2011 Location: Earth (Land portion) Posts: 1,167 Rep Power: 23	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.....

March 3, 2012, 05:14		#6
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,696 Rep Power: 143	Did he say zero permeability? I did not see that. Yes, that would definitely be a problem.

March 5, 2012, 12:02		#7
pato Member albert Join Date: Jan 2012 Posts: 31 Rep Power: 14	actually i didn´t 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?

March 5, 2012, 17:07		#8
evcelica Senior Member Erik Join Date: Feb 2011 Location: Earth (Land portion) Posts: 1,167 Rep Power: 23	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.