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

GAMG hexa vs. tetrahedron meshes

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

Like Tree6Likes
  • 6 Post By MartinB

Reply
 
LinkBack Thread Tools Display Modes
Old   July 16, 2010, 12:14
Default GAMG hexa vs. tetrahedron meshes
  #1
Senior Member
 
Claus Meister
Join Date: Aug 2009
Location: Wiesbaden, Germany
Posts: 241
Rep Power: 8
idrama is on a distinguished road
Hello Foamers!

Has anybody experience to set up GAMG for hexa or tetraeder? On tet, Do I have to increase, for instance, smoother iteration or merge levels?

Cheers in advance
idrama is offline   Reply With Quote

Old   July 16, 2010, 13:23
Default
  #2
Senior Member
 
Martin
Join Date: Oct 2009
Location: Aachen, Germany
Posts: 252
Rep Power: 11
MartinB is on a distinguished road
Hi Claus,

in my opinion it doesn't matter for GAMG if you are using hexas or tetras. A good choice of settings for my projects is:
Code:
solvers
{
    U                                   // linear equation system solver for U
    {
        solver          smoothSolver;   // solver type
        smoother        GaussSeidel;    // smoother type
        tolerance       1e-06;          // solver finishes if either absolute
        relTol          0.01;           // tolerance is reached or the relative
                                        // tolerance here
        nSweeps         1;              // setting for smoothSolver
        maxIter         100;            // limitation of iterations number
    }
    pd                                  // linear equation system solver for pd
    {
        solver          GAMG;           // very efficient multigrid solver
        tolerance       1e-07;          // solver finishes if either absolute
        relTol          0.001;          // tolerance is reached or the relative
                                        // tolerance here
        minIter         3;              // a minimum number of iterations
        maxIter         100;            // limitation of iterions number
        smoother        DIC;            // setting for GAMG
        nPreSweeps      1;              // 1 for pd, set to 0 for all other!
        nPostSweeps     2;              // 2 is fine
        nFinestSweeps   2;              // 2 is fine
        scaleCorrection true;           // true is fine
        directSolveCoarsestLevel false; // false is fine
        cacheAgglomeration on;          // on is fine; set to off, if dynamic
                                        // mesh refinement is used!
        nCellsInCoarsestLevel 500;      // 500 is fine,
                                        // otherwise sqrt(number of cells)
        agglomerator    faceAreaPair;   // faceAreaPair is fine
        mergeLevels     1;              // 1 is fine
    }
}
Important to note:
- nPreSweeps must be 0 for other equation systems than pressure
- to use the minIter option you must patch GAMG sources in most OpenFOAM versions
- cacheAgglomeration must be switched off when using dynamic mesh refinement

Playing with the options don't give remarkable speedup for my problems.

Martin
MartinB is online now   Reply With Quote

Old   July 17, 2010, 05:19
Default
  #3
Senior Member
 
Claus Meister
Join Date: Aug 2009
Location: Wiesbaden, Germany
Posts: 241
Rep Power: 8
idrama is on a distinguished road
Thanks MartinB, the informations are valueable for me! However, there is one question concernins the option nCellsInCorseLeve: Why the sqrt of the cells? Have you an paper or something like that?

Cheers
idrama is offline   Reply With Quote

Old   July 17, 2010, 06:03
Default
  #4
Senior Member
 
Martin
Join Date: Oct 2009
Location: Aachen, Germany
Posts: 252
Rep Power: 11
MartinB is on a distinguished road
Hi Claus,
I found the hint for sqrt(#cells) in the forum some time ago and made a note in my fvSolution template without paying too much attention to it...

This might be an interesting thread (especially the links to the PDFs), if you want to use some variants of AMG:
New AMG Solvers in OF141dev SVN
However it could be some work to integrate these solvers and preconditioners in the newer OpenFOAM versions.

Martin
MartinB is online now   Reply With Quote

Old   July 19, 2010, 08:59
Default
  #5
Senior Member
 
Claus Meister
Join Date: Aug 2009
Location: Wiesbaden, Germany
Posts: 241
Rep Power: 8
idrama is on a distinguished road
Do you probaly mean: If I have n processors then I have to take (total no. cells)^(1/n)?
idrama is offline   Reply With Quote

Old   July 19, 2010, 09:30
Default
  #6
Senior Member
 
Martin
Join Date: Oct 2009
Location: Aachen, Germany
Posts: 252
Rep Power: 11
MartinB is on a distinguished road
I mean: sqrt(1000000) = 1000
But: I never tested it in more detail... my value of 500 was fine, changing it had no big influence on my computation speed.
You can test it out: start your simulation, change the value after a few iterations and check the new iteration times. Then make another change and check again...

Many other posts in the forum prefer values of 10 to 30 for nCellsInCoarsestLevel...
http://www.cfd-online.com/OpenFOAM_D...tml?1162930507

Martin
MartinB is online now   Reply With Quote

Old   September 1, 2013, 05:54
Default
  #7
Super Moderator
 
Bruno Santos
Join Date: Mar 2009
Location: Lisbon, Portugal
Posts: 8,258
Blog Entries: 34
Rep Power: 84
wyldckat is just really nicewyldckat is just really nicewyldckat is just really nicewyldckat is just really nice
FYI: For future reference, the link on the previous post is for the following thread in the new forum format: AMG versus ICCG
wyldckat 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
pisoFoam floating point error - GAMG sErik OpenFOAM Running, Solving & CFD 8 January 14, 2010 11:43
Hexa Block meshes in ANSYS Meshing? siw ANSYS Meshing & Geometry 3 July 31, 2009 10:40
MRFSimpleFoam amp cyclic patches david OpenFOAM Running, Solving & CFD 36 October 21, 2008 21:55
Differences between serial and parallel runs carsten OpenFOAM Bugs 11 September 12, 2008 11:16
Hexa meshes with ICEM CFD 5.1 Tim Guo CFX 8 June 29, 2005 09:27


All times are GMT -4. The time now is 03:10.