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June 14, 2021, 08:02 
Pressure won't converge using SimpleFoam

#1 
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Join Date: Sep 2020
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Hi All,
I'm running a case in simpleFoam and my pressure residuals won't converge. A similar case to this run a number of years ago had converged at this stage. I have checked the mesh, and there are no issues with skewness or nonOrthogonality so I know the error doesn't lie there. my fvScheme and fvSolution are as follows fvSolution solvers { p { solver GAMG; tolerance 1e6; relTol 0.1; smoother GaussSeidel; nPreSweeps 0; nPostSweeps 2; cacheAgglomeration on; agglomerator faceAreaPair; nCellsInCoarsestLevel 10; mergeLevels 1; } U { solver smoothSolver; smoother GaussSeidel; tolerance 1e6; relTol 0.1; nSweeps 1; } k { solver smoothSolver; smoother GaussSeidel; tolerance 1e6; relTol 0.1; nSweeps 1; } epsilon { solver smoothSolver; smoother GaussSeidel; tolerance 1e6; relTol 0.1; nSweeps 1; } } SIMPLE { nNonOrthogonalCorrectors 0; residualControl { p 1e3; U 1e4; "(kepsilon)" 1e4; } } relaxationFactors { fields { p 0.3; } equations { U 0.7; k 0.7; epsilon 0.7; } } cache { grad(U); } fvScheme ddtSchemes { default steadyState; } gradSchemes { default Gauss linear; } divSchemes { default none; div(phi,U) bounded Gauss upwind; div((nuEff*dev(T(grad(U))))) Gauss linear; div(phi,epsilon) bounded Gauss upwind; div(phi,k) bounded Gauss upwind; div((nuEff*dev2(T(grad(U))))) Gauss linear; } laplacianSchemes { default Gauss linear limited corrected 0.333; } interpolationSchemes { default linear; } snGradSchemes { default limited corrected 0.333; } fluxRequired { default no; p; } If anyone could advise, that would be greatly appreciated 

June 14, 2021, 08:05 

#2 
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Join Date: Sep 2020
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This is the most recent time step of the output
smoothSolver: Solving for Ux, Initial residual = 6.26867549999e05, Final residual = 4.83802208018e06, No Iterations 4 smoothSolver: Solving for Uy, Initial residual = 4.91362392681e05, Final residual = 3.40070029735e06, No Iterations 4 smoothSolver: Solving for Uz, Initial residual = 0.000164303461286, Final residual = 9.92105112991e06, No Iterations 4 GAMG: Solving for p, Initial residual = 0.00202976194843, Final residual = 0.000165595182502, No Iterations 2 time step continuity errors : sum local = 8.95914605453e09, global = 2.94382119613e10, cumulative = 2.68013954291e06 smoothSolver: Solving for epsilon, Initial residual = 0.000134760412047, Final residual = 7.63620479172e06, No Iterations 2 smoothSolver: Solving for k, Initial residual = 0.000571158570201, Final residual = 3.84860947038e05, No Iterations 4 ExecutionTime = 88904.63 s ClockTime = 90881 s 

June 15, 2021, 08:13 

#3 
Senior Member
Join Date: Apr 2020
Location: UK
Posts: 462
Rep Power: 11 
Some suggestions:
 Check your boundary conditions. Are they identical to the previous set up? Has OF changed in its treatment of those boundary types since you last ran the case?  do you get the same problem if you change the pressure solver from GAMG to PCG?  How good is your mesh quality? Maybe also look at the pressure field in paraview to see if there is a bump/discontinuity/irregularity in the pressure field  that might help you identify where the solution is stalling, and therefore help find the reason. Good luck & let us know when you find the solution! 

June 15, 2021, 08:31 

#4 
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Daniel
Join Date: Jan 2021
Posts: 39
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Hey Richard,
I think here lies the problem: relaxationFactors { fields { p 0.3; } equations { U 0.7; k 0.7; epsilon 0.7; } } At first, 0.3 in fields for p is low, try a higher one. Your residuals look good, so this hould help. Also put a factor in Equations for p in, like this: relaxationFactors { fields { p 0.7; } equations { U 0.7; k 0.7; p 0.7; epsilon 0.7; } } 

June 24, 2022, 09:28 

#5  
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Join Date: Dec 2019
Location: Cologne, Germany
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Quote:
2. it is the first time i hear that pEqn should be underrelaxed. where did you get that one? 

June 24, 2022, 12:17 

#6 
Senior Member

For relaxation, see e.g. https://en.wikipedia.org/wiki/SIMPLE_algorithm and references cited.


July 4, 2022, 06:17 

#7  
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Join Date: Dec 2019
Location: Cologne, Germany
Posts: 306
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Quote:
URF for pequation in OF will violate the mass conservation, so only the pField should be underrelaxed. p_field_URF + U_eqn_URF = 1, this is a good rule. @Richard97 stalling residuals are not telling if a solution is converging or not, i know that this behaviour is annoying. you should place samplePoints in your domain and track the iteration behaviour of those points. if they are not changing much, probably your problem is converged, considering your flow physics and numerics. 

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convergence, pressure, simple foam 
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