# OF8 - pimpleFoam: Increase in initial p residual at final iteration of each time step

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 February 17, 2024, 13:27 OF8 - pimpleFoam: Increase in initial p residual at final iteration of each time step #1 Member   sirLentschi Join Date: Nov 2010 Posts: 86 Rep Power: 15 Hello, I am currently running an LES WALE case in OF8 using pimpleFoam. An initial solution was adopted from a corresponding RANS simulation. I am encountering a significant increase in initial p residuals during the final iteration of each time step (from the beginning, up to the current time step = 407): Code: ```Courant Number mean: 0.008399714967 max: 0.7193162975 132 Time = 5e-06 133 134 PIMPLE: Iteration 1 135 smoothSolver: Solving for Ux, Initial residual = 3.798020874e-06, Final residual = 2.178881283e-09, No Iterations 136 smoothSolver: Solving for Uy, Initial residual = 1.23923313e-06, Final residual = 6.212075878e-10, No Iterations 137 smoothSolver: Solving for Uz, Initial residual = 3.326645723e-06, Final residual = 1.500835019e-09, No Iterations 138 GAMGPCG: Solving for p, Initial residual = 0.1039002852, Final residual = 7.321548563e-05, No Iterations 3 139 GAMGPCG: Solving for p, Initial residual = 0.007223483423, Final residual = 5.231179486e-06, No Iterations 4 140 GAMGPCG: Solving for p, Initial residual = 0.001099740598, Final residual = 4.479327689e-07, No Iterations 4 141 time step continuity errors : sum local = 8.324539494e-12, global = 2.845274255e-13, cumulative = 2.845274255e-13 142 GAMGPCG: Solving for p, Initial residual = 0.03220724529, Final residual = 2.08032012e-05, No Iterations 3 143 GAMGPCG: Solving for p, Initial residual = 0.002289461202, Final residual = 1.813159765e-06, No Iterations 5 144 GAMGPCG: Solving for p, Initial residual = 0.0003657092185, Final residual = 6.333790375e-07, No Iterations 3 145 time step continuity errors : sum local = 1.177320667e-11, global = -2.500815743e-13, cumulative = 3.444585116e-14 .... 182 PIMPLE: Iteration 5 183 smoothSolver: Solving for Ux, Initial residual = 3.571453512e-06, Final residual = 1.389921103e-09, No Iterations 184 smoothSolver: Solving for Uy, Initial residual = 5.873638525e-07, Final residual = 9.005931028e-09, No Iterations 185 smoothSolver: Solving for Uz, Initial residual = 1.298302045e-06, Final residual = 4.90217827e-10, No Iterations 186 GAMGPCG: Solving for p, Initial residual = 0.000810497471, Final residual = 3.461697627e-07, No Iterations 3 187 GAMGPCG: Solving for p, Initial residual = 4.458844283e-05, Final residual = 9.773047859e-08, No Iterations 3 188 GAMGPCG: Solving for p, Initial residual = 6.643134197e-06, Final residual = 7.799485967e-08, No Iterations 1 189 time step continuity errors : sum local = 1.893199055e-12, global = -2.463948232e-15, cumulative = -2.505635775e-1 190 GAMGPCG: Solving for p, Initial residual = 0.0002424780533, Final residual = 1.0771658e-07, No Iterations 3 191 GAMGPCG: Solving for p, Initial residual = 1.31896789e-05, Final residual = 5.020704946e-08, No Iterations 2 192 GAMGPCG: Solving for p, Initial residual = 1.971721138e-06, Final residual = 2.429364249e-08, No Iterations 1 193 time step continuity errors : sum local = 5.912169006e-13, global = -6.088530994e-15, cumulative = -3.114488874e-1 194 PIMPLE: Converged 195 Doing final iteration 196 PIMPLE: Iteration 6 197 smoothSolver: Solving for Ux, Initial residual = 1.315012105e-06, Final residual = 1.081018863e-09, No Iterations 198 smoothSolver: Solving for Uy, Initial residual = 2.160959541e-07, Final residual = 4.776109417e-09, No Iterations 199 smoothSolver: Solving for Uz, Initial residual = 4.772566881e-07, Final residual = 9.535555309e-09, No Iterations 200 GAMGPCG: Solving for p, Initial residual = 0.3212578807, Final residual = 0.000273160544, No Iterations 3 201 GAMGPCG: Solving for p, Initial residual = 0.02012123337, Final residual = 1.017517052e-05, No Iterations 4 202 GAMGPCG: Solving for p, Initial residual = 0.00291566844, Final residual = 1.634454767e-06, No Iterations 4 203 time step continuity errors : sum local = 3.007467842e-11, global = 1.636254419e-13, cumulative = 1.324805532e-13 204 GAMGPCG: Solving for p, Initial residual = 0.1235330451, Final residual = 0.0001013779859, No Iterations 3 205 GAMGPCG: Solving for p, Initial residual = 0.007228274664, Final residual = 3.421793683e-06, No Iterations 4 206 GAMGPCG: Solving for p, Initial residual = 0.001138691056, Final residual = 6.050579309e-07, No Iterations 4 207 time step continuity errors : sum local = 1.114932749e-11, global = 2.85878474e-14, cumulative = 1.610684005e-13 208 PIMPLE: Converged in 6 iterations 209 ExecutionTime = 220.96 s ClockTime = 221 s``` Excerpt of fvSolution: Code: ``` 20 p 21 { 22 solver PCG; 23 preconditioner 24 { 25 preconditioner GAMG; 26 tolerance 1e-7; 27 relTol 0; 28 nVcycles 2; 29 smoother GaussSeidel; 30 nPreSweeps 0; 31 nPostSweeps 2; 32 nFinestSweeps 2; 33 cacheAgglomeration on; 34 nCellsInCoarsestLevel 10; 35 agglomerator faceAreaPair; 36 mergeLevels 1; 37 } 38 tolerance 1e-7; 39 relTol 0.001; 40 minIter 1; 41 } 42 43 pFinal 44 { 45 \$p; 46 tolerance 1e-06; 47 relTol 0; 48 } 60 PIMPLE 61 { 62 nOuterCorrectors 50; 63 momentumPredictor true; 64 nCorrectors 2; 65 nNonOrthogonalCorrectors 2; 66 67 outerCorrectorResidualControl 68 { 69 "(U|(p).*)" 70 { 71 tolerance 1e-3; 72 relTol 0; 73 } 74 } 75 } 78 relaxationFactors 79 { 80 fields 81 { 82 p 0.7; 83 "(p)Final" 1; 84 } 85 equations 86 { 87 U 0.7; 88 omega 0.7; 89 k 0.7; 90 91 "(U|k|omega)Final" 1; 92 } 93 }``` Any hints? I am a bit at a loss. Many thanks! //Markus

 February 23, 2024, 14:07 #2 Member   sirLentschi Join Date: Nov 2010 Posts: 86 Rep Power: 15 No hints in this round?

 February 27, 2024, 05:46 #3 Senior Member   Join Date: Apr 2020 Location: UK Posts: 670 Rep Power: 14 This usually occurs because the intermediate PIMPLE iterations are run with underrelaxation, to help smooth the convergence, whilst the final PIMPLE is run without relaxation to enforce continuity. Looking at your fvSolution file - this is exactly what you have got. Post edit: see my post below - I meant "time accuracy" and not "continuity"; brain was on holiday! dlahaye likes this. Last edited by Tobermory; March 1, 2024 at 05:48.

March 1, 2024, 04:50
#4
Senior Member

Domenico Lahaye
Join Date: Dec 2013
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@tobermory
Quote:
 whilst the final PIMPLE is run without relaxation to enforce continuity.
Could you pls. elaborate on how you relate relaxation on p in the final PIMPLE with continuity. Do you encounter cases in which relaxation on p in the final PIMPLE is required to obtain convergence?

Many thanks. All the best.

 March 1, 2024, 05:47 #5 Senior Member   Join Date: Apr 2020 Location: UK Posts: 670 Rep Power: 14 Apologies Domenico - that was careless of me - relaxation of the p field has nothing to do with continuity (that's related to the p equation factor, which you'll note is always 1). What I meant to say was enforce "time accuracy" rather than "continuity". In other words, if you use a value of less than 1 for the p field relaxation factor, then you are artificially holding back the development of the pressure field. That may not be a problem, if you are searching for a steady state solution ... but will introduce some inaccuracy if you are trying for a true transient solution. I'll add a caution to my earlier post. dlahaye likes this.

 March 1, 2024, 14:28 #6 Member   sirLentschi Join Date: Nov 2010 Posts: 86 Rep Power: 15 Thank you very much for your clarification. I assumed that the relaxation has something to do with this issue but was not sure.