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January 22, 2018, 17:07 
LES, Courant Number, Crash, Sudden

#1 
Senior Member
Hasan K.J.
Join Date: Dec 2011
Location: Bristol, United Kingdom
Posts: 200
Rep Power: 15 
Hey All,
I have gone through other threads with simlar questions about crashing it hasnt helped me hence asking again here I have two LES simulation that are acting up with me, let me talk about one here, I am using pimpleFoam and I have set up time step such that to main a CFL value of 2 but suddenly it starts crashing I have a time step of 4e7 lower than which it will take way too long to complete the simulation The simulation runs without any issues for 0.1638s after which it crashes how do i solve the issue any idea I have used also tried using this runtime modifiable the simulation crashes much sooner with the time step going out of propotion. runTimeModifiable yes; maxDeltaT 1e7; maxCo 6;//0.9 adjustTimeStep yes; still the simulation crashes if I keep the CFL under 1 and the simulation will take forever to complete. I dont mind having a high CFL number like 810 but I dont want the simulations to crash what do i do about this ? check Mesh Results: Code:
Mesh stats points: 7229736 faces: 21118815 internal faces: 20597481 cells: 6945066 faces per cell: 6.00661 boundary patches: 6 point zones: 0 face zones: 1 cell zones: 1 Overall number of cells of each type: hexahedra: 6911856 prisms: 0 wedges: 0 pyramids: 0 tet wedges: 0 tetrahedra: 0 polyhedra: 33210 Breakdown of polyhedra by number of faces: faces number of cells 7 26865 8 2106 9 3186 11 1053 Checking topology... Boundary definition OK. Cell to face addressing OK. Point usage OK. Upper triangular ordering OK. Face vertices OK. Number of regions: 1 (OK). Checking patch topology for multiply connected surfaces... Patch Faces Points Surface topology CURV1 78240 79112 ok (nonclosed singly connected) INLET 5859 6104 ok (nonclosed singly connected) TOP 39690 41216 ok (nonclosed singly connected) OUTLET 5859 6104 ok (nonclosed singly connected) SYM 195843 197368 ok (nonclosed singly connected) SYM1 195843 197368 ok (nonclosed singly connected) Checking geometry... Overall domain bounding box (0.5 0.3 1.38675e17) (1.5 0.3 0.036) Mesh (nonempty, nonwedge) directions (1 1 1) Mesh (nonempty) directions (1 1 1) Boundary openness (4.44428e17 2.79828e17 1.34681e14) OK. Max cell openness = 2.21707e07 OK. Max aspect ratio = 231.527 OK. Minimum face area = 1.63904e10. Maximum face area = 4.73102e05. Face area magnitudes OK. Min volume = 7.23718e15. Max volume = 6.30802e08. Total volume = 0.0431456. Cell volumes OK. Mesh nonorthogonality Max: 66.8039 average: 9.43428 Nonorthogonality check OK. Face pyramids OK. Max skewness = 2.38317 OK. Coupled point location match (average 0) OK. Mesh OK. End Code:
Courant Number mean: 0.00476427 max: 2.39386 Time = 0.16831 smoothSolver: Solving for Ux, Initial residual = 6.17299e06, Final residual = 2.37401e10, No Iterations 1 smoothSolver: Solving for Uy, Initial residual = 1.66203e05, Final residual = 1.96127e10, No Iterations 1 smoothSolver: Solving for Uz, Initial residual = 0.000423954, Final residual = 7.96985e10, No Iterations 1 GAMGPCG: Solving for p, Initial residual = 0.49493, Final residual = 0.00339957, No Iterations 4 time step continuity errors : sum local = 2.14037e10, global = 1.12945e13, cumulative = 8.27299e10 GAMGPCG: Solving for p, Initial residual = 0.436823, Final residual = 8.13999e08, No Iterations 27 time step continuity errors : sum local = 5.1405e15, global = 2.77939e16, cumulative = 8.27299e10 bounding k, min: 1.70026 max: 143.203 average: 0.201061 ExecutionTime = 13111.3 s ClockTime = 13419 s Courant Number mean: 0.00476427 max: 2.40022 Time = 0.1683104 smoothSolver: Solving for Ux, Initial residual = 6.17539e06, Final residual = 2.2862e10, No Iterations 1 smoothSolver: Solving for Uy, Initial residual = 1.66265e05, Final residual = 1.97553e10, No Iterations 1 smoothSolver: Solving for Uz, Initial residual = 0.000424722, Final residual = 7.98927e10, No Iterations 1 GAMGPCG: Solving for p, Initial residual = 0.494715, Final residual = 0.00372712, No Iterations 4 time step continuity errors : sum local = 2.35334e10, global = 7.43158e13, cumulative = 8.26556e10 GAMGPCG: Solving for p, Initial residual = 0.437231, Final residual = 9.57638e08, No Iterations 34 time step continuity errors : sum local = 6.05988e15, global = 2.28986e16, cumulative = 8.26556e10 bounding k, min: 2.40413 max: 135.996 average: 0.201166 ExecutionTime = 13125.1 s ClockTime = 13433 s Courant Number mean: 0.00476427 max: 2.22604 Time = 0.1683108 smoothSolver: Solving for Ux, Initial residual = 6.16541e06, Final residual = 2.34176e10, No Iterations 1 smoothSolver: Solving for Uy, Initial residual = 1.66186e05, Final residual = 1.99606e10, No Iterations 1 smoothSolver: Solving for Uz, Initial residual = 0.000425189, Final residual = 8.04212e10, No Iterations 1 GAMGPCG: Solving for p, Initial residual = 0.494838, Final residual = 0.00405709, No Iterations 4 time step continuity errors : sum local = 2.5623e10, global = 5.33397e13, cumulative = 8.26023e10 GAMGPCG: Solving for p, Initial residual = 0.438004, Final residual = 9.12675e08, No Iterations 31 time step continuity errors : sum local = 5.76987e15, global = 1.11952e15, cumulative = 8.26024e10 bounding k, min: 1.69341 max: 146.946 average: 0.201293 ExecutionTime = 13138.4 s ClockTime = 13446 s Courant Number mean: 0.00476427 max: 2.31384 Time = 0.1683112 smoothSolver: Solving for Ux, Initial residual = 6.16279e06, Final residual = 2.38931e10, No Iterations 1 smoothSolver: Solving for Uy, Initial residual = 1.66167e05, Final residual = 1.95725e10, No Iterations 1 smoothSolver: Solving for Uz, Initial residual = 0.000425232, Final residual = 8.02747e10, No Iterations 1 GAMGPCG: Solving for p, Initial residual = 0.494657, Final residual = 0.00373473, No Iterations 4 time step continuity errors : sum local = 2.35479e10, global = 2.51002e14, cumulative = 8.26049e10 GAMGPCG: Solving for p, Initial residual = 0.437436, Final residual = 9.06846e08, No Iterations 26 time step continuity errors : sum local = 5.72547e15, global = 9.29442e17, cumulative = 8.26049e10 bounding k, min: 1.68987 max: 168.615 average: 0.20146 ExecutionTime = 13150.1 s ClockTime = 13458 s Courant Number mean: 0.00476427 max: 2.38364 Time = 0.1683116 smoothSolver: Solving for Ux, Initial residual = 6.16212e06, Final residual = 2.40648e10, No Iterations 1 smoothSolver: Solving for Uy, Initial residual = 1.6612e05, Final residual = 1.9752e10, No Iterations 1 smoothSolver: Solving for Uz, Initial residual = 0.00042509, Final residual = 8.20654e10, No Iterations 1 GAMGPCG: Solving for p, Initial residual = 0.493943, Final residual = 0.00313908, No Iterations 4 time step continuity errors : sum local = 1.97732e10, global = 8.09449e13, cumulative = 8.26858e10 GAMGPCG: Solving for p, Initial residual = 0.436808, Final residual = 9.37087e08, No Iterations 24 time step continuity errors : sum local = 5.91186e15, global = 3.492e16, cumulative = 8.26858e10 bounding k, min: 2.00964 max: 199.534 average: 0.201708 ExecutionTime = 13161.3 s ClockTime = 13469 s Courant Number mean: 0.00476427 max: 2.4211 Time = 0.168312 smoothSolver: Solving for Ux, Initial residual = 6.15957e06, Final residual = 2.41124e10, No Iterations 1 smoothSolver: Solving for Uy, Initial residual = 1.66092e05, Final residual = 2.04054e10, No Iterations 1 smoothSolver: Solving for Uz, Initial residual = 0.000424818, Final residual = 8.40124e10, No Iterations 1 GAMGPCG: Solving for p, Initial residual = 0.492384, Final residual = 0.00329825, No Iterations 4 time step continuity errors : sum local = 2.07622e10, global = 2.00169e13, cumulative = 8.27058e10 GAMGPCG: Solving for p, Initial residual = 0.436385, Final residual = 7.85718e08, No Iterations 29 time step continuity errors : sum local = 4.95646e15, global = 2.0603e16, cumulative = 8.27059e10 bounding k, min: 1.68348 max: 277.497 average: 0.202139 ExecutionTime = 13173.4 s ClockTime = 13482 s Courant Number mean: 0.00476427 max: 2.6113 Time = 0.1683124 smoothSolver: Solving for Ux, Initial residual = 6.15557e06, Final residual = 2.49922e10, No Iterations 1 smoothSolver: Solving for Uy, Initial residual = 1.66031e05, Final residual = 2.07838e10, No Iterations 1 smoothSolver: Solving for Uz, Initial residual = 0.000424472, Final residual = 8.41308e10, No Iterations 1 GAMGPCG: Solving for p, Initial residual = 0.492943, Final residual = 0.00398935, No Iterations 4 time step continuity errors : sum local = 2.51402e10, global = 1.85571e13, cumulative = 8.27244e10 GAMGPCG: Solving for p, Initial residual = 0.437938, Final residual = 8.82808e08, No Iterations 27 time step continuity errors : sum local = 5.56824e15, global = 2.41116e16, cumulative = 8.27244e10 bounding k, min: 1.6807 max: 452.143 average: 0.202836 ExecutionTime = 13185.5 s ClockTime = 13494 s Courant Number mean: 0.00476427 max: 2.90376 Time = 0.1683128 smoothSolver: Solving for Ux, Initial residual = 6.16156e06, Final residual = 2.51566e10, No Iterations 1 smoothSolver: Solving for Uy, Initial residual = 1.66091e05, Final residual = 2.05866e10, No Iterations 1 smoothSolver: Solving for Uz, Initial residual = 0.000424155, Final residual = 8.46042e10, No Iterations 1 GAMGPCG: Solving for p, Initial residual = 0.494805, Final residual = 0.00412264, No Iterations 4 time step continuity errors : sum local = 2.60347e10, global = 2.3208e13, cumulative = 8.27476e10 GAMGPCG: Solving for p, Initial residual = 0.439378, Final residual = 9.88441e08, No Iterations 26 time step continuity errors : sum local = 6.2528e15, global = 1.20021e16, cumulative = 8.27476e10 bounding k, min: 1.67949 max: 681.507 average: 0.203954 ExecutionTime = 13197.5 s ClockTime = 13506 s Courant Number mean: 0.00476427 max: 3.30989 Time = 0.1683132 smoothSolver: Solving for Ux, Initial residual = 6.17584e06, Final residual = 2.41709e10, No Iterations 1 smoothSolver: Solving for Uy, Initial residual = 1.66272e05, Final residual = 2.03654e10, No Iterations 1 smoothSolver: Solving for Uz, Initial residual = 0.000424275, Final residual = 8.59482e10, No Iterations 1 GAMGPCG: Solving for p, Initial residual = 0.498405, Final residual = 0.00364243, No Iterations 4 time step continuity errors : sum local = 2.31002e10, global = 9.33976e14, cumulative = 8.27382e10 GAMGPCG: Solving for p, Initial residual = 0.441554, Final residual = 8.01972e08, No Iterations 28 time step continuity errors : sum local = 5.09744e15, global = 4.27189e16, cumulative = 8.27382e10 bounding k, min: 1.67916 max: 998.119 average: 0.205817 ExecutionTime = 13210 s ClockTime = 13519 s Courant Number mean: 0.00476427 max: 3.86591 Time = 0.1683136 smoothSolver: Solving for Ux, Initial residual = 6.18222e06, Final residual = 2.35813e10, No Iterations 1 smoothSolver: Solving for Uy, Initial residual = 1.66425e05, Final residual = 2.1045e10, No Iterations 1 smoothSolver: Solving for Uz, Initial residual = 0.000424955, Final residual = 8.9451e10, No Iterations 1 GAMGPCG: Solving for p, Initial residual = 0.501903, Final residual = 0.00342728, No Iterations 4 time step continuity errors : sum local = 2.18593e10, global = 2.33473e13, cumulative = 8.27149e10 GAMGPCG: Solving for p, Initial residual = 0.444615, Final residual = 8.72048e08, No Iterations 27 time step continuity errors : sum local = 5.58744e15, global = 4.67401e16, cumulative = 8.27148e10 bounding k, min: 2.75056 max: 1607.42 average: 0.209344 ExecutionTime = 13221.8 s ClockTime = 13531 s Courant Number mean: 0.00476428 max: 4.63102 Time = 0.168314 smoothSolver: Solving for Ux, Initial residual = 6.17363e06, Final residual = 2.46097e10, No Iterations 1 smoothSolver: Solving for Uy, Initial residual = 1.66389e05, Final residual = 2.25103e10, No Iterations 1 smoothSolver: Solving for Uz, Initial residual = 0.000425524, Final residual = 9.87208e10, No Iterations 1 GAMGPCG: Solving for p, Initial residual = 0.504331, Final residual = 0.00363937, No Iterations 4 time step continuity errors : sum local = 2.33834e10, global = 7.90824e13, cumulative = 8.26357e10 GAMGPCG: Solving for p, Initial residual = 0.449269, Final residual = 9.41598e08, No Iterations 30 time step continuity errors : sum local = 6.07633e15, global = 1.16056e15, cumulative = 8.26356e10 bounding k, min: 3.24369 max: 3210.82 average: 0.216819 ExecutionTime = 13234.2 s ClockTime = 13544 s Courant Number mean: 0.00476428 max: 5.70862 Time = 0.1683144 smoothSolver: Solving for Ux, Initial residual = 6.16872e06, Final residual = 2.60786e10, No Iterations 1 smoothSolver: Solving for Uy, Initial residual = 1.66395e05, Final residual = 2.48458e10, No Iterations 1 smoothSolver: Solving for Uz, Initial residual = 0.000425701, Final residual = 1.22359e09, No Iterations 1 GAMGPCG: Solving for p, Initial residual = 0.508905, Final residual = 0.00352578, No Iterations 4 time step continuity errors : sum local = 2.28998e10, global = 7.03264e13, cumulative = 8.25653e10 GAMGPCG: Solving for p, Initial residual = 0.456481, Final residual = 8.32348e08, No Iterations 24 time step continuity errors : sum local = 5.43514e15, global = 8.80714e16, cumulative = 8.25652e10 bounding k, min: 5.62525 max: 7858.24 average: 0.233945 ExecutionTime = 13245.3 s ClockTime = 13555 s Courant Number mean: 0.00476428 max: 7.32406 Time = 0.1683148 smoothSolver: Solving for Ux, Initial residual = 6.17624e06, Final residual = 2.89695e10, No Iterations 1 smoothSolver: Solving for Uy, Initial residual = 1.66664e05, Final residual = 3.23421e10, No Iterations 1 smoothSolver: Solving for Uz, Initial residual = 0.000425773, Final residual = 1.89709e09, No Iterations 1 GAMGPCG: Solving for p, Initial residual = 0.51658, Final residual = 0.00304282, No Iterations 4 time step continuity errors : sum local = 2.00963e10, global = 1.67691e14, cumulative = 8.25669e10 GAMGPCG: Solving for p, Initial residual = 0.466839, Final residual = 7.83318e08, No Iterations 22 time step continuity errors : sum local = 5.21692e15, global = 2.25184e16, cumulative = 8.25669e10 bounding k, min: 15.909 max: 18005.5 average: 0.275304 ExecutionTime = 13256.4 s ClockTime = 13566 s Code:
ddtSchemes { default backward; } gradSchemes { default cellMDLimited Gauss linear 0.5; grad(p) cellMDLimited Gauss linear 0.5; grad(U) cellMDLimited Gauss linear 0.5; } divSchemes { default none; div(phi,U) Gauss linearUpwindV grad(U); div(phi,k) Gauss limitedLinear 1; div(phi,omega) Gauss limitedLinear 1; div(phi,B) Gauss limitedLinear 1; div(phi,nuTilda) Gauss limitedLinear 1; div(B) Gauss linear; div((nuEff*dev(T(grad(U))))) Gauss linear; } laplacianSchemes { default Gauss linear limited 1.0; } interpolationSchemes { default linear; interpolate(U) linear; } snGradSchemes { default limited 1.0; } fluxRequired { default no; p ; } Code:
solvers { p { solver PCG;//GAMG preconditioner { preconditioner GAMG; smoother GaussSeidel; nPreSweeps 0; nPostSweeps 2; nVcycles 2; cacheAgglomeration true; nCellsInCoarsestLevel 50; agglomerator faceAreaPair; mergeLevels 1; tolerance 1e07; relTol 0.01; } maxIter 100; tolerance 1e06; relTol 0.01;//0.05 } pFinal { solver PCG; preconditioner { preconditioner GAMG; smoother GaussSeidel; nPreSweeps 0; nPostSweeps 2; nVcycles 2; cacheAgglomeration true; nCellsInCoarsestLevel 50; agglomerator faceAreaPair; mergeLevels 1; tolerance 1e07; relTol 0.01; } maxIter 100; tolerance 1e07; relTol 0.0; } k { solver PBiCG; preconditioner DILU; tolerance 1e09; relTol 0.01; } kFinal { solver PBiCG; preconditioner DILU; tolerance 1e09; relTol 0; } omega { solver PBiCG; preconditioner DILU; tolerance 1e09; relTol 0.01; } omegaFinal { solver PBiCG; preconditioner DILU; tolerance 1e09; relTol 0; } U { solver smoothSolver; smoother DILUGaussSeidel; tolerance 1e07; relTol 0.01; } UFinal { solver smoothSolver; smoother DILUGaussSeidel; tolerance 1e07; relTol 0.0; } B { solver PBiCG; preconditioner DILU; tolerance 1e07; relTol 0; } nuTilda { solver PBiCG; preconditioner DILU; tolerance 1e07; relTol 0; } nuTildaFinal { solver PBiCG; preconditioner DILU; tolerance 1e07; relTol 0; } } PISO { nCorrectors 2; nNonOrthogonalCorrectors 0; } PIMPLE { nOuterCorrectors 1; nCorrectors 2; nNonOrthogonalCorrectors 0; pRefCell 0; pRefValue 0; } relaxationFactors { fields { "(p)" 0.3; } equations { "(U)" 0.7; "(UFinal)" 0.7; } } // ************************************************************************* // Kind Regards, Hasan K.J
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January 23, 2018, 00:09 

#2 
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Uwe Pilz
Join Date: Feb 2017
Location: Leipzig, Germany
Posts: 744
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> Courant Number mean: 0.00476427 max: 2.39386
That may be to high. I have LES simulations which require a max Co of 0.1 or even lower. Sometimes a larger maxCo can be used, but the simulation runs faster with a lower one (less correction steps).
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Uwe Pilz  Die der Hauptbewegung überlagerte Schwankungsbewegung ist in ihren Einzelheiten so hoffnungslos kompliziert, daß ihre theoretische Berechnung aussichtslos erscheint. (Hermann Schlichting, 1950) 

January 23, 2018, 02:54 

#3 
Senior Member
Join Date: Aug 2014
Location: Germany
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Your Courant Number is too high for LES. Even for implicit RANS something like CFL = 8 can be critical.
To be safe in LES you can use a DNS time step leading to CFL numbers <<1. I also see that your not converging in each time step. SO watch out for that too. 

October 31, 2019, 04:01 

#4  
Senior Member
Jianrui Zeng
Join Date: May 2018
Location: China
Posts: 157
Rep Power: 8 
Quote:
Besides, I set the residual criteria of p for 1e6 and u/h for 1e7. I am wondering whether it is enough for DNS or not. Thank you very much. 

November 21, 2019, 04:57 

#5 
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R Kyle
Join Date: Feb 2017
Location: Scotland
Posts: 11
Rep Power: 9 
Probably too late to respond, but might be useful for future...
To use PIMPLE correctly you need to set "nOuterCorrectors" to something like 50 or 100. That's the maximum number of steadystate loops it will do before moving on in time, but will move on in time if it converges before then. If you have that set to 1, you're operating in PISO mode and so yes, a Co of more than 1 lead to issues. Try setting nOuterCorrectors to 50, that might help. You'll then see many more iterations per time step, but as expected with PIMPLE (merged PISOSIMPLE). Excellent explanation and demo of PIMPLE by T. Holzmann, Mathematics, Numerics, Derivations and OpenFOAM, 2018.. Link here. 

November 22, 2019, 02:05 

#6 
Senior Member
Ruiyan Chen
Join Date: Jul 2016
Location: Hangzhou, China
Posts: 162
Rep Power: 10 
Does the "bounding k, xxx" message show up at the biginning of the simulation or from the time the simulation crashes? I doubt that it is there for the whole time, which suggests that something is wrong with your settings. Also, the residuals for pressure seem to not change at all, maybe the pressure boundary conditions need more caution?
I personlly find that setting maxCo is not as good as using a fixed time step (and carefully increase it if the courant number is below, say 1.0). For several cases I find that the max CFL number exceeds the maxCo I set so now I'm always using fixed time step for LES. By the way you obviously need more nOuterCorrectors. 

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courant number, crash, les, sudden 
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