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February 28, 2013, 12:02 
why PIMPLE doesn't converge at each time step but no diverging?

#1 
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Ehsan
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Location: Iran
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I have set Pimple nCorrectors to 10 but despite of this high value pimple doesn't converge at each time step.my problem is transient.
what does that message about not converging mean?then why diverging doesnt occurs immediately after not converging at a time step? Code:
PIMPLE: iteration 9 DILUPBiCG: Solving for Ux, Initial residual = 1.660614399e06, Final residual = 1.382107362e16, No Iterations 2 DILUPBiCG: Solving for Uy, Initial residual = 2.725991237e05, Final residual = 1.71994151e15, No Iterations 2 DILUPBiCG: Solving for h, Initial residual = 4.640570901e07, Final residual = 2.664924362e17, No Iterations 2 FDICPCG: Solving for p, Initial residual = 1.311912104e06, Final residual = 3.564637383e15, No Iterations 6 FDICPCG: Solving for p, Initial residual = 7.244953397e14, Final residual = 2.568884643e15, No Iterations 1 diagonal: Solving for rho, Initial residual = 0, Final residual = 0, No Iterations 0 time step continuity errors : sum local = 2.343009192e16, global = 1.367661713e17, cumulative = 2.07229617e08 rho max/min : 2.417134107 2.398184636 FDICPCG: Solving for p, Initial residual = 2.735098395e09, Final residual = 5.153284331e15, No Iterations 4 FDICPCG: Solving for p, Initial residual = 5.535348519e14, Final residual = 2.142343159e15, No Iterations 1 diagonal: Solving for rho, Initial residual = 0, Final residual = 0, No Iterations 0 time step continuity errors : sum local = 2.334873446e16, global = 1.47642379e17, cumulative = 2.072296172e08 rho max/min : 2.417134107 2.398184636 FDICPCG: Solving for p, Initial residual = 1.335350283e11, Final residual = 6.963764942e16, No Iterations 3 FDICPCG: Solving for p, Initial residual = 4.134830709e14, Final residual = 1.474027945e15, No Iterations 1 diagonal: Solving for rho, Initial residual = 0, Final residual = 0, No Iterations 0 time step continuity errors : sum local = 2.301559708e16, global = 1.40876864e17, cumulative = 2.072296173e08 rho max/min : 2.417134107 2.398184636 PIMPLE: iteration 10 DILUPBiCG: Solving for Ux, Initial residual = 1.82942849e06, Final residual = 5.541810383e17, No Iterations 2 DILUPBiCG: Solving for Uy, Initial residual = 2.959945528e05, Final residual = 2.277521743e15, No Iterations 2 DILUPBiCG: Solving for h, Initial residual = 6.035283727e07, Final residual = 9.136129994e17, No Iterations 2 FDICPCG: Solving for p, Initial residual = 1.595530764e06, Final residual = 3.504793444e15, No Iterations 6 FDICPCG: Solving for p, Initial residual = 7.14289137e14, Final residual = 2.634399827e15, No Iterations 1 diagonal: Solving for rho, Initial residual = 0, Final residual = 0, No Iterations 0 time step continuity errors : sum local = 2.339840533e16, global = 1.340257095e17, cumulative = 2.072296174e08 rho max/min : 2.417134107 2.398184636 FDICPCG: Solving for p, Initial residual = 2.868202145e09, Final residual = 5.345704264e15, No Iterations 4 FDICPCG: Solving for p, Initial residual = 5.420100184e14, Final residual = 1.936677383e15, No Iterations 1 diagonal: Solving for rho, Initial residual = 0, Final residual = 0, No Iterations 0 time step continuity errors : sum local = 2.324211337e16, global = 1.342398081e17, cumulative = 2.072296176e08 rho max/min : 2.417134107 2.398184636 FDICPCG: Solving for p, Initial residual = 1.966838831e11, Final residual = 6.116982397e16, No Iterations 3 FDICPCG: Solving for p, Initial residual = 4.085579847e14, Final residual = 1.43049529e15, No Iterations 1 diagonal: Solving for rho, Initial residual = 0, Final residual = 0, No Iterations 0 time step continuity errors : sum local = 2.304000431e16, global = 1.357813178e17, cumulative = 2.072296177e08 rho max/min : 2.417134107 2.398184636 PIMPLE: not converged within 10 iterations Code:
solvers { p { solver PCG; preconditioner FDIC; tolerance 1e14; relTol 0; nsweeps 1000; maxIter 25000; /*solver GAMG; smoother DICGaussSeidel; cacheAgglomeration true; nCellsInCoarsestLevel 200; nPreSweeps 0; nPostSweeps 2; nFinestSweeps 2; agglomerator faceAreaPair; mergeLevels 2; tolerance 1e012; relTol 0;*/ } pFinal { $p; relTol 0; } "rho.*" { $p; tolerance 1e12; relTol 0; } "(UehRkepsilonomega)" { solver PBiCG; preconditioner DILU; tolerance 1e12; relTol 0; maxIter 25000; } "(UhRkepsilonomega)Final" { $U; relTol 0; } } PIMPLE { momentumPredictor yes; nOuterCorrectors 10; nCorrectors 5; nNonOrthogonalCorrectors 1; rhoMin rhoMin [ 1 3 0 0 0 ] .6; rhoMax rhoMax [ 1 3 0 0 0 ] 8.5; //convergence 1e16; residualControl { "(pUkomega)" { relTol 0; tolerance 0.0000000001; } } } /*relaxationFactors { fields { p 0.9; } equations { U .99; k .99; omega .99; h .99; e .99; }*/ 

March 1, 2013, 16:49 

#2 
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Santiago Marquez Damian
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What value of residuals are expected for convergence?
Regards.
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March 2, 2013, 01:35 

#3 
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Ehsan
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what do you mean?should not all values converge?
The PIMPLE says that it doesn't converge as its shown but values are converged(final residuals are low) then what does that mean that PIMPLE doesn't converge? 

March 2, 2013, 08:17 

#4 
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Bruno Santos
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Greetings to all!
@Ehsan: I don't have a test case for confirming this myself, but I believe the issue is as follows:
Bruno
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March 2, 2013, 09:08 

#5 
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Ehsan
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thank you dear Bruno.
Please verify im correct or not:variables set in residualControl check for initial residuals that be lower than set value. But tolerances of variables that set in other dictionaries are related to final residuals.is it ok? Then 1)how can i be certain that solution at each time step is converged? 2)What is better for tolerances in separate dictionaries for p,U,k,omega,... 3)which value for tolerances in residualControl is appropriate for sureness of convergency at each time step? 4)whats the usage and beneficient of residualControl totally? 

March 2, 2013, 09:38 

#6 
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Bruno Santos
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Hi Ehsan,
Questions 1 to 3  read this excellent post on the topic of convergence: Convergence Question 4: AFAIK, it's an automated feature that hardcore CFD users rarely use . It will stop when the defined value is reached, but it cannot decide whether it's the right time to stop or not... Hardcore CFD users look at the residual plots and decide from there when it's converged or what needs adjusting. I think the decision process is described in the post above. Good luck! Best regards, Bruno
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I'll be at OFW11 in Portugal 

March 2, 2013, 10:43 

#7 
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Santiago Marquez Damian
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Hi, my question was about what value you set in the residualControl option.
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 

May 19, 2013, 14:16 

#8 
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Ehsan
Join Date: Oct 2012
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Hi dear Santiago
sorry for delay in reply. in fact i have seen your message now accidently. thanks.I never use residualControl anymore. I think its for steady_state problems rather than transients. is my conclusion true? thanks for your before reply again.
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