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December 17, 2007, 11:39 
Hello,
When running turbFoa

#1 
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nicolas
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Hello,
When running turbFoam, I get output from the DILUPBiCG solver: "DILUPBiCG: Solving for Ux, Initial residual = 1, Final residual = 7.53093e07, No Iterations 4". But when running interFoam, i dont get any output from the momentum equations; the code looks similar. What governs the type of output? Thanks Nico 

December 17, 2007, 12:15 
Hi Nico,
i suppose you are

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Christian Winkler
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Hi Nico,
i suppose you are running the dambreak testcase, where the momentum predictor in the fvSolution file is switched off. So only the pressure equation is solved. The correct velocity results from the pressurevelocity correction. You can change that in the PISO section: PISO { momentumPredictor yes; ... } regards Christian 

December 18, 2007, 08:50 
perfect, thank you very much

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nicolas
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perfect, thank you very much


September 2, 2009, 00:01 

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Santiago Marquez Damian
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Thanks to Caw for the answer, i was about to post the same question. I have some related questions:
1. What is the purpose of the momentum predictor? 2. Where I can find some theory about it? 3. With your clue we are able to see what are the residuals for the momentum equation, but, What parameters we have to change to set up the residuals limits in order to force the solver to go with the residuals below a given value (as is usual in commercial CFD codes)? Thanks in advance.
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Santiago MÁRQUEZ DAMIÁN, Ph.D. Research Scientist Research Center for Computational Methods (CIMEC)  CONICET/UNL Tel: 543424511594 Int. 7032 Colectora Ruta Nac. 168 / Paraje El Pozo (3000) Santa Fe  Argentina. http://www.cimec.org.ar 

September 2, 2009, 00:26 

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Sandy Lee
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Quote:


September 2, 2009, 21:48 

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Sandy Lee
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If the momentum predictor is switched off, the velocity fields will be gotten explicitly from the pressure equation, I guess, it is easier to implement parallel computation because only a pressure equation is solved. However, I think, if the momentum predictor is switched on, maybe we can get more efficiently during an unparellel simulation??


September 2, 2009, 23:16 

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Daniel WEI (老魏)
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It's all about SIMPLE/PISO algorithm.
Quote:
Quote:
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September 7, 2009, 12:31 

#8 
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Santiago Marquez Damian
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Thanks Daniel for your answers, it's a pleasure for me to interact with somebody in China (actually mi first time I guess). About answers 1 & 2 I'll read these thesis, I promise. And respect answer 3, you're right, I found the info in the pdf manuals. Regards.
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Santiago MÁRQUEZ DAMIÁN, Ph.D. Research Scientist Research Center for Computational Methods (CIMEC)  CONICET/UNL Tel: 543424511594 Int. 7032 Colectora Ruta Nac. 168 / Paraje El Pozo (3000) Santa Fe  Argentina. http://www.cimec.org.ar 

September 23, 2009, 09:01 

#9 
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Santiago Marquez Damian
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Daniel, I've been reading some thesis, specially the one from Jasak, it's awesone, none more clear about FVM and errors. Now I'm dealing with the interpretation of some lines of icoFoam solver, particularly with A and H operators. If we recall the annotated version from OpenFOAM Wiki:
Code:
//set up the linear algebra for the momentum equation. The flux // of U, phi, is treated explicity using the last known value of U. fvVectorMatrix UEqn ( fvm::ddt(U) + fvm::div(phi, U)  fvm::laplacian(nu, U) ); Code:
// solve using the last known value of p on the RHS. This gives us // a velocity field that is not divergence free, but approximately satisfies // momentum. See Eqn. 7.31 of Ferziger & Peric solve(UEqn == fvc::grad(p)); Code:
//  PISO loop take nCorr corrector steps for (int corr=0; corr<nCorr; corr++) { // from the last solution of velocity, extract the diag. term from the // matrix and store the reciprocal note that the matrix coefficients are // functions of U due to the nonlinearity of convection. volScalarField rUA = 1.0/UEqn.A(); Code:
// take a Jacobi pass and update U. See Hrv Jasak's thesis eqn. 3.137 and // Henrik Rusche's thesis, eqn. 2.43 UEqn.H is the righthand side of the // UEqn minus the product of (the offdiagonal terms and U). Note that // since the pressure gradient is not included in the UEqn. above, this // gives us U without the pressure gradient. Also note that UEqn.H() is // a function of U. U = rUA*UEqn.H(); Regards.
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Santiago MÁRQUEZ DAMIÁN, Ph.D. Research Scientist Research Center for Computational Methods (CIMEC)  CONICET/UNL Tel: 543424511594 Int. 7032 Colectora Ruta Nac. 168 / Paraje El Pozo (3000) Santa Fe  Argentina. http://www.cimec.org.ar 

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