CFD Online Logo CFD Online URL
www.cfd-online.com
[Sponsors]
Home > Forums > OpenFOAM

unexpected flow speeds within porous zones

Register Blogs Members List Search Today's Posts Mark Forums Read

Reply
 
LinkBack Thread Tools Display Modes
Old   March 5, 2012, 05:56
Default unexpected flow speeds within porous zones
  #1
New Member
 
BT
Join Date: Jan 2011
Posts: 16
Rep Power: 5
MasterCooler is on a distinguished road
Dear openFoam-Community,

I am just analyzing flows through porosities and thereby I have found out that the flow speed within the porosity seems to have a strange and unexpected characteristic.

For example in the case attached (see picture, x direction flow speed in lila with values on the left vertical axes, pressure in black with values on the right vertical axes, picture of the porosity in x direction in the top part in order to show the cell resolution) the flow through the porosity is in x-direction only. Flows in y and z dirction are very close to 0. So I would expect that the flow through the porosity is at constant level and independly from the x coordinate.

But the result from plot over a line shows another behaviour whereby the flow speed degreases first and come only back to a stable behaviour (as expected) after 4 or 5 cells in x direction. Briefly before the flow leaves the porosity a similar beahviour can be observed. At the same time velocities in y and z direction are very close to 0 (not shown in the picture) what also seems to be a conflict in continuity from my point of view. On the other side pressure behaviour is close to the expectations.

At the beginning I assumed that that resolution of my porosity wasn't fine enough and only the first cell in x direction would be affect so to speak as a transition effect but now I have refined the mesh and can see that the effect doesn't depend from the mesh fineness too much.

Therefore I want to kindly ask the following questions:
1) How can the observed effect be explained?
2) What about law of continuity conservation, is it violated?
3) Can I trust in the flow speed values reached in the center of the porosity or which flow speed can be seen as representative?

Many thanks in advance for your helping!

Cheers
Ben Theobald
Attached Images
File Type: png flowWithinPorosity_800x600.png (67.2 KB, 79 views)
MasterCooler is offline   Reply With Quote

Old   March 5, 2012, 15:04
Default
  #2
Assistant Moderator
 
Bernhard Gschaider
Join Date: Mar 2009
Posts: 3,803
Rep Power: 38
gschaider will become famous soon enoughgschaider will become famous soon enough
Quote:
Originally Posted by MasterCooler View Post
Dear openFoam-Community,

I am just analyzing flows through porosities and thereby I have found out that the flow speed within the porosity seems to have a strange and unexpected characteristic.

For example in the case attached (see picture, x direction flow speed in lila with values on the left vertical axes, pressure in black with values on the right vertical axes, picture of the porosity in x direction in the top part in order to show the cell resolution) the flow through the porosity is in x-direction only. Flows in y and z dirction are very close to 0. So I would expect that the flow through the porosity is at constant level and independly from the x coordinate.

But the result from plot over a line shows another behaviour whereby the flow speed degreases first and come only back to a stable behaviour (as expected) after 4 or 5 cells in x direction. Briefly before the flow leaves the porosity a similar beahviour can be observed. At the same time velocities in y and z direction are very close to 0 (not shown in the picture) what also seems to be a conflict in continuity from my point of view. On the other side pressure behaviour is close to the expectations.

At the beginning I assumed that that resolution of my porosity wasn't fine enough and only the first cell in x direction would be affect so to speak as a transition effect but now I have refined the mesh and can see that the effect doesn't depend from the mesh fineness too much.

Therefore I want to kindly ask the following questions:
1) How can the observed effect be explained?
2) What about law of continuity conservation, is it violated?
3) Can I trust in the flow speed values reached in the center of the porosity or which flow speed can be seen as representative?

Many thanks in advance for your helping!

Cheers
Ben Theobald
Is it possible that what you're seeing is the same problem as this one I reported some time ago http://www.openfoam.org/mantisbt/view.php?id=134 ?

It seems that OpenCFD doesn't think that it is a problem (we can discuss about the fix that was provided, but the problem is there)
gschaider is offline   Reply With Quote

Old   March 6, 2012, 03:15
Default
  #3
New Member
 
BT
Join Date: Jan 2011
Posts: 16
Rep Power: 5
MasterCooler is on a distinguished road
Many thanks for the hint. I'll check if the described fix helps in OF2.1 as well and give a feedback.
MasterCooler is offline   Reply With Quote

Old   April 13, 2012, 07:29
Default
  #4
New Member
 
Join Date: Jan 2012
Posts: 7
Rep Power: 4
Kalas is on a distinguished road
Have you tested the proposed fix? I think I have encountered the same bug. I have a case which I tested in both OpenFOAM and Fluent and the velocity profile looks very different in the porous interfaces.
Kalas is offline   Reply With Quote

Old   April 16, 2012, 04:50
Default
  #5
New Member
 
BT
Join Date: Jan 2011
Posts: 16
Rep Power: 5
MasterCooler is on a distinguished road
Hey Kalas,

I am sorry but I haven't tested it so far. So it is still on my to do but not forgotten. In coming weeks I will have simulation projects again and then I will test the fix and report the result. If you should test it before it would be nice to read the result on your side.

Cheers
Ben
MasterCooler is offline   Reply With Quote

Old   April 20, 2012, 02:29
Default
  #6
New Member
 
Join Date: Jan 2012
Posts: 7
Rep Power: 4
Kalas is on a distinguished road
Hello again!

Now I have tested the proposed fix, but my results were the same as before. I am not sure I applied it correctly though. I am using rhoPorousMRFSimpleFoam and changed pEqn.H as suggested above, is that the correct way to implement the fix?

Cheers,
Klas
Kalas is offline   Reply With Quote

Old   April 20, 2012, 04:10
Default
  #7
Assistant Moderator
 
Bernhard Gschaider
Join Date: Mar 2009
Posts: 3,803
Rep Power: 38
gschaider will become famous soon enoughgschaider will become famous soon enough
Quote:
Originally Posted by Kalas View Post
Hello again!

Now I have tested the proposed fix, but my results were the same as before. I am not sure I applied it correctly though. I am using rhoPorousMRFSimpleFoam and changed pEqn.H as suggested above, is that the correct way to implement the fix?

Cheers,
Klas
- Apply patch
- Check that it really hit the files (strange things happen)
- recompile all the relevant libraries (or do ./Allwmake in $FOAM_SRC. That should recompile everything affected
- recompile the solver (not sure whether the relevant code parts are inlined or in the binary of the library)
gschaider is offline   Reply With Quote

Old   April 20, 2012, 05:08
Default
  #8
New Member
 
Join Date: Jan 2012
Posts: 7
Rep Power: 4
Kalas is on a distinguished road
Hmm ok,

I don't have access to the global installation and hoped it would suffice to make a new binary for the solver. But just to be clear what I did, I edited $FOAM_SOLVERS/compressible/rhoPorousFoam/rhoPorousMRFSimpleFoam/pEqn.H where I changed the line U -= trAU()*fvc::grad(p); to U = (fvc::reconstruct(phi))/rho;. Im pretty sure this made it into the file because i changed some output text which was changed in the solver output after I ran the recompiled solver from my $FOAM_USER_APPBIN. This solver works but gives exactly the same solution as the original..
Kalas is offline   Reply With Quote

Old   April 20, 2012, 07:02
Default
  #9
New Member
 
BT
Join Date: Jan 2011
Posts: 16
Rep Power: 5
MasterCooler is on a distinguished road
Hey Kalas,

besides, which version are you using? OF2.1?

Regards
Ben
MasterCooler is offline   Reply With Quote

Old   April 20, 2012, 08:33
Default
  #10
New Member
 
Join Date: Jan 2012
Posts: 7
Rep Power: 4
Kalas is on a distinguished road
Yep, I'm using 2.1.
Kalas is offline   Reply With Quote

Old   April 23, 2012, 19:22
Default
  #11
Assistant Moderator
 
Bernhard Gschaider
Join Date: Mar 2009
Posts: 3,803
Rep Power: 38
gschaider will become famous soon enoughgschaider will become famous soon enough
Quote:
Originally Posted by Kalas View Post
Hmm ok,

I don't have access to the global installation and hoped it would suffice to make a new binary for the solver. But just to be clear what I did, I edited $FOAM_SOLVERS/compressible/rhoPorousFoam/rhoPorousMRFSimpleFoam/pEqn.H where I changed the line U -= trAU()*fvc::grad(p); to U = (fvc::reconstruct(phi))/rho;. Im pretty sure this made it into the file because i changed some output text which was changed in the solver output after I ran the recompiled solver from my $FOAM_USER_APPBIN. This solver works but gives exactly the same solution as the original..
Forgot. The fix is only in the solver code. So the part about $FOAM_SRC was unnecessary

You should see some difference. Make sure with 'which rhoPorousFoam' that the new solver is actually used (or add some output to make sure)
gschaider is offline   Reply With Quote

Old   May 2, 2012, 07:48
Default
  #12
New Member
 
Join Date: Jan 2012
Posts: 7
Rep Power: 4
Kalas is on a distinguished road
Hi again!

I changed some output text so that I'm sure it is the changed solver being used. The solution is not exactly the same, but I still have oscillating velocities in the interface when entering the porous zone.
Kalas is offline   Reply With Quote

Reply

Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are On
Pingbacks are On
Refbacks are On


Similar Threads
Thread Thread Starter Forum Replies Last Post
Compressible flow through porous media? JoFFe CFX 2 November 1, 2010 05:50
flow through porous fibers ranjith Main CFD Forum 1 May 24, 2008 09:05
Porous medium flow shailesh OpenFOAM Running, Solving & CFD 8 September 14, 2007 05:18
porous media flow Siraj CD-adapco 4 February 24, 2006 22:24
air flow through porous pipe faiz rauf FLUENT 3 August 11, 2004 15:08


All times are GMT -4. The time now is 01:44.