# best values for GAMG solver (nCellsInCoarsestLevel , nPreSweeps 0, nPostSweeps)

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 May 5, 2013, 13:52 best values for GAMG solver (nCellsInCoarsestLevel , nPreSweeps 0, nPostSweeps) #1 Senior Member   Join Date: Jun 2011 Posts: 151 Rep Power: 7 what is the best values for GAMG parameters specially for nCellsInCoarsestLevel , nPreSweeps 0, nPostSweeps is there a formula to calculate those based on the number of cells ? Regards

 May 9, 2013, 11:17 #2 Senior Member     Daniel P. Combest Join Date: Mar 2009 Location: St. Louis, USA Posts: 584 Rep Power: 20 Try this GAMG hexa vs. tetrahedron meshes There are several other threads, but the settings and concepts from this thread I use with my own simulations. songwukong likes this. __________________ Dan Find me on twitter @dancombest and LinkedIn

 May 10, 2013, 12:09 #3 Senior Member   Join Date: Jun 2011 Posts: 151 Rep Power: 7 Dear Daniel thanks so much for your answer. in most of OF tutorials the value of nCellsInCoarsestLevel is set to 10 (or 20) but in your link it is set to 500 (or sqrt(#cell)) do you know why OF used the 10 or 20 values ? Regards

 May 10, 2013, 12:23 #4 Senior Member     Daniel P. Combest Join Date: Mar 2009 Location: St. Louis, USA Posts: 584 Rep Power: 20 This is one of those situations where something has been tested and found to work for a particular situation. You can always try the sqrt(ncells), 10, 20, 50 or 100 and then see which one is the best for your case. I have read that 10 or 20 for larger cases is too low and can be inefficient. Hence why I stay a bit higher with sqrt(nCells). However, its a case by case change. __________________ Dan Find me on twitter @dancombest and LinkedIn

 May 11, 2013, 12:24 #5 Senior Member     Kyle Mooney Join Date: Jul 2009 Location: Amherst, MA USA - San Diego, CA USA Posts: 320 Rep Power: 10 I've seen similar changes in convergence speed by altering the nCellsInCoarsestLevel setting. In once case I had pressure converge twice as fast for a relatively large case @ 600k cells on about 50 processors. I don't have the solver settings on hand unfortunately. It is definitely worth investigating before starting big parametric or large parallel cases.

May 12, 2013, 01:09
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Quote:
 Originally Posted by chegdan This is one of those situations where something has been tested and found to work for a particular situation. You can always try the sqrt(ncells), 10, 20, 50 or 100 and then see which one is the best for your case. I have read that 10 or 20 for larger cases is too low and can be inefficient. Hence why I stay a bit higher with sqrt(nCells). However, its a case by case change.

Hi Daniel

by increasing the nCellsInCoarsestLevel value, the convergency increased but the high value of nCellsInCoarsestLevel may be increase the cpu time for solution
in other word by the high value of nCellsInCoarsestLevel solution converged, certainly
am I right ?

Regards

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