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simpleFoam tutorial PitzDaily using Reynolds stress tensor (LRR RASModel) 

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March 2, 2022, 04:36 

#21 
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Carlos Rubio Abujas
Join Date: Jan 2018
Location: Spain
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Hi lpz456,
Sadly, I've never succeded in making this tutorial to work. Even trying relaxation factor as low as 0.05 during 20k iteration the solution converges and maintain this "transient" behaviour. I found curious that the turbulent fields (R, epsilon, nut) seem to be converged, but velocity ad pressure keeps oscilating. I've tried it on v7 and v2012 in case is some implementation error, but no luck. I guess there is something on the setting I'm mising, but I cannot figure out what. 

March 2, 2022, 12:03 

#22 
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Carlos Rubio Abujas
Join Date: Jan 2018
Location: Spain
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After some time I tried again to run this case, with some minor advances:
1. I figured out that R relaxationFactor sould be in "equations" instead of "fields" As far as I know, the differente is on fields the new value is applied directly to the new field, where on the case of "equations" it is used to relax the solving matrix. 2. I've observed that sometimes the tolerance of the R fields drops abruptly to 1e10 or so and is kept stuck due to solver settings. To avoid this I set a minimum of 1 iteration on each field on the solvers. I algo tried to tight the relative and absolute tolerance of the solvers. 3. I've check the results of the kEpsilon case against some literature found on internet. I've found a comparative on the results for LRR, kEpsilon and kOmega. Here the link. Sadly, I did not find enough information in this file to replicate the results on LRR. When comparing the results, the kEpsilon case is quite similar to the results presented here. The results on velocity and nut are pretty similar among the three turbulence models. As on the current approach we are preinitiallizing the fields with the results of kEpsilon I expected minor changes on these fields. However what I've seen is a drastic variation on all the profiles, which makes me doubt about the usefullness of the preinitialization. Following this line of reasoning, as I consider that mi initial guess is pretty close to the final results, I imposed a strong relaxation factor on U and p, hoping that the turbulence fields can update fast enought to keep them close to the initial value. However I had no success. I share the current state of mi fvSolution file in case it can be helpfull for anyone trying to replicate this tutorial. system/fvSolution Code:
/** C++ **\ =========  \\ / F ield  OpenFOAM: The Open Source CFD Toolbox \\ / O peration  Website: https://openfoam.org \\ / A nd  Version: 7 \\/ M anipulation  \**/ FoamFile { version 2; format ascii; class dictionary; location "system"; object fvSolution; } solvers { p { solver GAMG; tolerance 1e08; relTol 0.01; smoother GaussSeidel; } "(Ukepsilonomegafv2)" { solver smoothSolver; smoother symGaussSeidel; tolerance 1e08; relTol 0.001; minIter 1; } R { solver GAMG; smoother GaussSeidel; tolerante 1e08; relTol 0.001; minIter 1; } } SIMPLE { nNonOrthogonalCorrectors 0; consistent yes; } relaxationFactors { fields { p 0.01; } equations { U 0.05; R 0.01; epsilon 0.05; } } // ************************************************************************* // 

March 6, 2022, 20:15 

#23  
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Quote:
Recently, I also noticed the link you provided. In fact, there is information worthy of attention in the link. The openfoam version in the link is 1.7.1. By comparing with openfoamv2106, you will find that the LRR model has changed significantly. There is an item called wall reflection in the new version. I tried to close the wall reflection term in the turbulenceproperties file and use the uniform initial field of R. The convergence has been significantly improved! Although there are still some unsteady properties. Therefore, at present, I locate the convergence problem in the LRR model itself (especially the wall reflection term). I intend to study the equation itself first, and then solve the convergence problem. Good luck, too! 

March 6, 2022, 21:14 

#24 
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Unfortunately, after consulting fluent's theory manual, I found that the wall reflection term is switched on by default. In the case I studied, the simulation by fluent has good convergence, but by openfoam always diverges.


August 4, 2023, 14:29 

#25  
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Lennin Amorim
Join Date: Oct 2022
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Quote:
I've had abysmal convergence problems with the running first the kEpsilon tutorial first and using the R field for the last iteration. I managed to get much better results runing the simpleFoam postProcess func R latestTime command on a clean kepsilon case, and then changing the turbulence model. https://www.cfd.at/sites/default/fil...ExampleSix.pdf 

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