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smoothSolver diverges - solution in using PBiCG solver?

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Old   December 15, 2009, 15:49
Default smoothSolver diverges - solution in using PBiCG solver?
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Dr. Alexander Vakhrushev
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Hi all!
I have 2 points to discuss:

1st one:

I wrote a heat transfer solver based on pimpleFoam, adding heat transfer equation.
I use high Courant numbers (up to 5) to speed up calculations. Flow simulation runs without problems (GAMG solver for pressure, smoothSolvers for U and turbulent parameters). By analogy I used smoothSolver also for TEqn in my solver:
Code:
    T
    {
        solver          smoothSolver;
        smoother      GaussSeidel;
        tolerance       1e-05;
        relTol          0;
    nSweeps        1;
    }
But it diverges just at the beggining of simulation with error in GaussSeidel smoother method.

Then I just switched TEqn solver to PBiCG with DILU preconditioner

Code:
    T
    {
        solver          PBiCG;
        preconditioner    DILU;
        tolerance       1e-05;
        relTol          0;
    }
and every thing runs Ok. I compared transient solution at different times with pisoFoam solution + heat transfer, and all fields correspond realy good to each other.
So what is the reason for such behavior?

2nd point:

Why am I using pimpleFoam? I solve more complicated task with solidification and turbulent flow. For some cases I search for steady state. One approache is to use steady state solvers, another - to apply transient or cvazi-transient solvers to reach steady state when solution will become stable.

I tried to use simpleFoam just to test such posibility for my task. It reached solution which transient solvers achieve and do not change later, but instanteniously solution deverged! Residuals got a big jump and solution crashed... Am wondering why it happend? Stable solution for that simmetric geometry which I used was assimetric, because there are 3 jets and in 2D they cannot be stable because two zones of low pressure do not have a connection. Is assymethry of solution the case for steady state solver to deverge?

So now I an forced to use PISO or PIMPLE technics to solve my problem... But it is time-consuming!

So, waiting for your opinion! Have a good day!
__________________
Best regards,

Dr. Alexander VAKHRUSHEV

Christian Doppler Laboratory for "Advanced Process Simulation of
Solidification and Melting"

Simulation and Modelling of Metallurgical Processes
Department of Metallurgy
University of Leoben

Franz-Josef-Str. 18
A - 8700 Leoben
Österreich / Austria
Tel.: +43 3842 - 402 - 3125
http://smmp.unileoben.ac.at
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Old   August 20, 2013, 16:22
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Jackie Chou
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Hello, Dr. Alexander VAKHRUSHEV,

I have met the same situation as you had. Have you got an idea about it?


Jackie
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Old   August 21, 2013, 11:17
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Dr. Alexander Vakhrushev
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Hi Jackie!

My post was from 2009, since that OpenFOAM improved a lot in numerics. So I do not observe that now in my simulations... Sometimes under-relaxation really helps to avoid simulation from explosion.

Cheers,
Alexander
__________________
Best regards,

Dr. Alexander VAKHRUSHEV

Christian Doppler Laboratory for "Advanced Process Simulation of
Solidification and Melting"

Simulation and Modelling of Metallurgical Processes
Department of Metallurgy
University of Leoben

Franz-Josef-Str. 18
A - 8700 Leoben
Österreich / Austria
Tel.: +43 3842 - 402 - 3125
http://smmp.unileoben.ac.at
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Old   September 11, 2013, 12:44
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Hi Dr.Alexander
Can you tell me what the difference is GAMG algorithms algorithms AMG?
THANKS
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