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-   -   Do residuals mean the same in PCG and AMG? (http://www.cfd-online.com/Forums/openfoam-programming-development/106381-do-residuals-mean-same-pcg-amg.html)

santiagomarquezd August 27, 2012 14:08

Do residuals mean the same in PCG and AMG?
 
Hi all, I'm doing a comparison between PCG and AMG. The problem is to solve with laplancianFoam and all Dirichlet BC's with both methods. I solved asking for the same absolute residuals of 1E-06 and then solved the problem using a direct solver in octave extracting the matrix and RHS with gdbOF.
The max absolute error is about 5E-5 with PCG and 2E-2 with AMG for a problem of 1000x1000 cells. This is the difference, same residuals in PCG and AMG lead to different quality solution. Next are the logs:

PCG:

diagonalPCG: Solving for T, Initial residual = 1, Final residual = 9.99042e-07, No Iterations 2231
ExecutionTime = 223.14 s ClockTime = 223 s

AMG:

GAMG: Solving for T, Initial residual = 1, Final residual = 5.69744e-07, No Iterations 12
ExecutionTime = 57.95 s ClockTime = 58 s

So that:

1. Is the definition of residuals different between PCG and AMG? or
2. I'm setting something bad in AMG and it is not converging at all

Any clues?

Thx in advance.

santiagomarquezd August 31, 2012 17:46

I found that you have to ask for lower residuals in AMG than PCG to obtain the similar max error in the solution.

Any clues?


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