# Improve simpleFoam convergence

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 July 19, 2010, 14:08 Improve simpleFoam convergence #1 Senior Member   Daniele Join Date: Feb 2010 Posts: 134 Rep Power: 8 Hi How can I improve simpleFoam convergenge? Now time simulation is 30min, can i improve it? Thanks

 July 20, 2010, 02:40 #2 Senior Member   matej forman Join Date: Mar 2009 Location: Brno, Czech Republic Posts: 104 Rep Power: 9 Sure you can. Have a faster computer. No, well, yes, you can make it run faster but you need to give us more information. Your question could be paraphrased as: "I have a car and it takes 30 minutes to get to work, could it be faster?" Post here fvSchemes, fvSolution, and screenshot of the residual plot, tell us what is the case (turbulent? what is Re?,backward facing step or swirler or flow around car, plane, building, channel flow), what are the BCs and if the variables (like velocity) are oscillating (e.g. post a plot of a velocity probe). matej

July 20, 2010, 05:25
#3
Senior Member

Daniele
Join Date: Feb 2010
Posts: 134
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you're right!! And I like your metaphor! It is a turbulent case (k-epilon model). It is similar to a backward facing step, i have a atmospheric bl with a obstacle on bottom.

top bottom outlet inlet
k symmetry wallFunction zerogradient fixed value
U symmetry 0 zerogradient log profile
eps symmetry wallfunction zerogradient imposed profile

Thanks
Attached Files
 Schemes.tar.gz (872 Bytes, 240 views) Res.pdf (7.3 KB, 331 views)

 July 20, 2010, 05:59 #4 Senior Member   Martin Join Date: Oct 2009 Location: Aachen, Germany Posts: 252 Rep Power: 12 Hi Daniele, I would reduce the relaxation factors to: Code: ```relaxationFactors { p 0.3; U 0.7; k 0.5; epsilon 0.5; R 0.5; nuTilda 0.5; }``` And I would change the solvers to: Code: ``` p { solver GAMG; tolerance 1e-7; relTol 0.001; minIter 5; maxIter 100; smoother GaussSeidel; // DIC; //DICGaussSeidel; //FDIC; nPreSweeps 1; nPostSweeps 3; nFinestSweeps 3; scaleCorrection true; directSolveCoarsest false; cacheAgglomeration on; nCellsInCoarsestLevel 50; // 500 agglomerator faceAreaPair; mergeLevels 1; // 3 } U { solver smoothSolver; smoother GaussSeidel; tolerance 1e-6; relTol 0.01; nSweeps 1; maxIter 100; };``` The solvers for k, epsilon, R and nuTilda the same as for U. Martin bennn likes this.

July 20, 2010, 06:44
#5
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Daniele
Join Date: Feb 2010
Posts: 134
Rep Power: 8
Thanks
The execution time is reduced from 2239 sec to 662, but after the obstacle the solution it isn't accurate

Daniele
Attached Files
 wss.pdf (4.6 KB, 318 views)

 July 20, 2010, 06:53 #6 Senior Member   Martin Join Date: Oct 2009 Location: Aachen, Germany Posts: 252 Rep Power: 12 Hi Daniele, your convergence criteria in fvSolution with 1e-3 is rather low... Restart your simulation from the latest timestep with convergence criteria of 1e-5. Martin

 July 20, 2010, 07:03 #7 Senior Member   Daniele Join Date: Feb 2010 Posts: 134 Rep Power: 8 Thanks I try it. Uhm In my project the obstacle on the bottom change its position, and then i calculate new solution at new position, this is do in a evolutive algorithm, so the iteration after the first must be very fast less than a minute. can i initialize the new mesh with the last time of previous simulation? Daniele

 July 20, 2010, 07:25 #8 Senior Member   Martin Join Date: Oct 2009 Location: Aachen, Germany Posts: 252 Rep Power: 12 I think, you can use the "mapFields" utility. With "mapFields -help" you can see the parameters necessary. In your "system" directory you need a file "mapFieldsDict": Code: ```/*--------------------------------*- C++ -*----------------------------------*\\ | ========= | | | \\ / F ield | OpenFOAM: The Open Source CFD Toolbox | | \\ / O peration | Version: 1.6 | | \\ / A nd | Web: www.OpenFOAM.org | | \\/ M anipulation | | \*---------------------------------------------------------------------------*/ FoamFile { version 2.0; format ascii; class dictionary; location "system"; object mapFieldsDict; } // * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * // // This dictionary is necessary to map fields from another simulation to this // one. The results from the source will be interpolated to the mesh in this // case directory. // Usually patchMap and cuttingPatches will be empty. // In the case that you want to use the output of one simulation as the // input for the next one, define patchMap as (inlet outlet). patchMap ( ); // usually empty cuttingPatches ( ); // * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //``` Navigate to your new (!) case directory. Use the mapFields command like this: mapFields /home/daniele/workinghere/firstRun -sourceTime 1234 Check the mapped results with paraFoam. Start the simulation in your new case directory. Martin

 July 20, 2010, 09:54 #9 Senior Member   Daniele Join Date: Feb 2010 Posts: 134 Rep Power: 8 Hi If I start new simulation with last time of the previous simulation time execution is 93s... I would about 30s 40s; if I use mapfield time execution is 400s. There is a thing that I dont understand: I start a simulation, then I copy last time directory in 0 directory, and restart the same case, and time execution is 30s, but if I start from solution because time is so high?

 July 20, 2010, 10:52 #10 Senior Member   Martin Join Date: Oct 2009 Location: Aachen, Germany Posts: 252 Rep Power: 12 It's not necessary to copy last time directory to "0" directory, you can edit the "system/controlDict" to start from your latest time directory: Code: ```startFrom startTime; // firstTime, startTime, latestTime startTime 0; // set > 0 to continue``` Replace "startFrom startTime;" with "startFrom latestTime;" and the simulation continues. Or replace "startTime 0;" with "startTime 1000;" to start from iteration 1000. You can use the mapField command with the parameter "-consistent" if your mesh is exactly the same. Otherwise if the meshes are different, the values are interpolated and the solver needs some time steps to adjust the solution again. May be I didn't understand your questions right... you don't speak german, do you? Martin

 July 20, 2010, 11:28 #11 Senior Member   Daniele Join Date: Feb 2010 Posts: 134 Rep Power: 8 I understand. I try it. Two mesh are very similar, top inlet and outlet are identical, the obstacle position on bottom si traslate of some cm. I did a simulation, and then I start a new simulation with the same mesh but startTime is latestTime of previous sim, and time execution is 40s, is normal? I don't change nothing, and i start from convergence solution, time execution should not be faster? p.s i don't speak german, and my english is very bad...you are very kind, and your advice are very useful.

 July 20, 2010, 12:23 #12 Senior Member   Martin Join Date: Oct 2009 Location: Aachen, Germany Posts: 252 Rep Power: 12 It's difficult to say if 40s is normal: usually one can observe that the time necessary for a single iteration step (i.e. solving for Ux, Uy, Uz, p ...) is higher in the beginning of the simulation (at least for a steady state simpleFoam simulation). With reaching convergence the time for each single iteration step drops significantly. Next to the time used for a single iteration step the number of iterations used to reach convergence is important. For my simulations here (simpleFoam without turbulence) I get convergence in a "fresh" simulation after 800 iterations. When using mapFields I can reduce the number of iterations to 400 iterations, although the first few single iteration steps do need the same execution time. Another important issue in your case is the question: when is your simulation converged good enough? I think a convergence tolerance of 1e-3 is not enough, but may be, 1e-4 is fine and 1e-5 is an overkill? You can save quite a lot of iterations as soon as you know which accuracy you really need... Some more ideas to speed up your simulations: Are you running your simulation in parallel mode? If not, and if you have a multicore or multiprocessor machine, use the parallelism! An important parameter which reduce the number of iterations is the relaxation factor: you can try to increase these factors! If you are familiar with OpenFOAM's c++ programming language you might try to use an dynamically increasing relaxation factor during runtime. There is at least one thread in this forum where this idea is discussed. If you can't speed up the simulations time directly you can try to use your hardware more efficiently: create a shell script or a python script that runs dozens of simulations over night. I use this technique quite often... automatic mesh creation with blockMesh is not hard, and with the "sample" utility you can do automatic postprocessing, too. Hope these ideas are useful Martin

 July 20, 2010, 14:06 #13 Senior Member   Martin Join Date: Oct 2009 Location: Aachen, Germany Posts: 252 Rep Power: 12 Here are some more ideas: I suppose you are using hexahedra elements. If not, try switching to hexahedra. Reduce the mesh size, i.e. the number of elements. Run your simulation until convergence. Then use a finer mesh and map the previous results... Optimize your mesh so it really fits the needs of your case: a fine mesh in the important areas (behind the obstacle for example), a coarser mesh elsewhere. With the refineMesh utility you can refine the whole mesh immediately. With refineWallLayer you can split the cells next to a patch. If you have hexahedra and you want to refine locally only, I can upload an utility... Although this is usually done to avoid divergence in complex cases: run simpleFoam without turbulence first, at least for let's say 200 iterations. Then switch on turbulence and continue the simulation. For my previous suggestion of using mapFields you can try this: just use U and p for the mapping and leave the turbulence fields. Of course you can combine some of these strategies, too... Martin

 July 21, 2010, 02:50 #14 Senior Member   matej forman Join Date: Mar 2009 Location: Brno, Czech Republic Posts: 104 Rep Power: 9 Daniele, martin gave you extensive advice to which there's not much to add. Just a few bits: a) the precision of the result - using upwind scheme for convection terms will gain you speed, but not precision. The good news is, you may change the scheme on the fly using some utility or script. Other issue with precision might be the use of k-eps model and consequently wall functions in OpenFOAM. b) I understand that you have solved solution, then you move the obstacle, map thesolution and run. well your solution is converged, but the flowfield is different as the obstacle is moved so the solution will take a little longer at the beginning. Alternativelly to Martin's suggestion with switching off the turbulence, I would make it bit easier with changing the viscosity which means changing the turbulence level. You are after two rabbits here. The precision one is going one direction while the speedy one is running slightly other direction. It's hard to get both at the same time. good luck matej

 July 21, 2010, 03:00 #15 Senior Member   Daniele Join Date: Feb 2010 Posts: 134 Rep Power: 8 Dear Martin Thanks for everything. You were very patient and kind to me. I try different and valuable advice you gave me. Daniele

 July 22, 2010, 07:08 #16 Senior Member   Daniele Join Date: Feb 2010 Posts: 134 Rep Power: 8 Hi I refine my mesh only where is necessary and i reduce my domain size. I calculated a first solution with a convergence tolerance of 1e-4, and solution is very good. I try to increase tolerance (1e-3,1e-2) for the following simulation that I inizialized with mapFields, and the time simulation is very good 20s, and 10s, and the solution quality is good. Is it wrong calculate the new solution from a solution more precise,but raising tolerance? Daniele

 July 22, 2010, 07:24 #17 Senior Member   Martin Join Date: Oct 2009 Location: Aachen, Germany Posts: 252 Rep Power: 12 Hi Daniele, it is not wrong, but you should be aware of the results quality! Try this: write your results every 50th iteration. You can do this with the following entry in "system/controlDict": writeInterval 50; Then check the results in paraview, i.e. have a careful look at how much the results change from let's say timestep 200 to timestep 250, or timestep 200 to 800... it is useful to reduce the number of colors to 10 (in Color Scale Editor, enable "Discrete Colors" with Solution of 10). You can see the amount of change in the fields... then decide for your particular problem, if it is ok or not. I am doing flow channel shape optimization here, and I can say quite early if the new geometry is better or worse than the latter one. But when reaching the optimum, I must decrease the convergence tolerances more and more. Martin Last edited by MartinB; July 22, 2010 at 08:36. Reason: tolerances must be decreased to get better results, not increased, of course...

 July 22, 2010, 07:28 #18 Senior Member   matej forman Join Date: Mar 2009 Location: Brno, Czech Republic Posts: 104 Rep Power: 9 It is completely wrong. You've chosen the wrong rabbit to chase. The tolerance we are talking about is the tolerance with which the solver solves the linearized set of N-S equations. When you release the tolerance, the solver will be faster, but the price.... You can play with a tolerance but when you want reliable solution you should lower it back towards the end of the solution of each of your case. good luck matej

 July 22, 2010, 07:38 #19 Senior Member   Daniele Join Date: Feb 2010 Posts: 134 Rep Power: 8 Dear Matey But if i inizialized my simulation with a field very precise, I must not use necessarily a very low tolerance or not?

July 22, 2010, 08:02
#20
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Daniele
Join Date: Feb 2010
Posts: 134
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Post my solution. The source for mapfields utility is the same for all. I change tolerance convergence.
Attached Files
 conf.pdf (7.1 KB, 271 views)

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