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

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Old   May 5, 2013, 13:52
Default best values for GAMG solver (nCellsInCoarsestLevel , nPreSweeps 0, nPostSweeps)
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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 ?

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Old   May 9, 2013, 11:17
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Try this

http://www.cfd-online.com/Forums/ope...on-meshes.html

There are several other threads, but the settings and concepts from this thread I use with my own simulations.
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Old   May 10, 2013, 12:09
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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 ?

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Old   May 10, 2013, 12:23
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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.
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Old   May 11, 2013, 12:24
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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.
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Old   May 12, 2013, 01:09
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Quote:
Originally Posted by chegdan View Post
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 ?

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Old   February 9, 2017, 07:42
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Quote:
Originally Posted by chegdan View Post
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.
Does anyone know where this recommendation (nCellsInCoarsestLevel = sqrt(ncells) comes from?

Best regards,

Kate
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Old   October 5, 2019, 09:46
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Remember that the OF tutorials are only demo tutorials. In openFoam there is no default options like in Fluent or any other commercial software; don't take the tutorial options as the one that work for sure!
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Old   October 7, 2019, 05:34
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In most of OF tutorials the value of nCellsInCoarsestLevel is set to 10 (or 20).
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