# How to get steady result in pitzDaily using LES model

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 October 15, 2018, 05:19 How to get steady result in pitzDaily using LES model #1 New Member   Yuan Jiahui Join Date: Oct 2018 Posts: 9 Rep Power: 6 Hi Foamers, I am trying to use simpleFoam to solve pitzDaily with LES model and compare the results between LES model and RAS model (k-epsilon model). And I change part codes in tutorials(incompressibel/pisoFoam/les/pitzDaily), but the caculation is not convergent no matter how long the running time I set. Here is my changes: 1.fvsolution solvers { p { solver GAMG; tolerance 1e-06; relTol 0.1; smoother GaussSeidel; } pFinal { \$p; smoother DICGaussSeidel; tolerance 1e-06; relTol 0; } "(U|k|B|nuTilda|s)" { solver smoothSolver; smoother GaussSeidel; tolerance 1e-05; relTol 0; } } SIMPLE { nNonOrthogonalCorrectors 0; consistent yes; residualControl { p 1e-2; "(pFinal|U|B|nuTilda|s)" 1e-3; } } relaxationFactors { equations { U 0.9; // 0.9 is more stable but 0.95 more convergent ".*" 0.9; // 0.9 is more stable but 0.95 more convergent } } 2.conrtolDict application simpleFoam; startFrom startTime; startTime 0; stopAt endTime; endTime 2000; deltaT 1; writeControl timeStep; writeInterval 100; purgeWrite 0; writeFormat ascii; writePrecision 6; writeCompression off; timeFormat general; timePrecision 6; runTimeModifiable true; Please tell why I can not get steady result in LES model Best regards, yuanjiahui

 October 16, 2018, 05:01 #2 Senior Member   Robert Join Date: May 2015 Location: Bremen, GER Posts: 292 Rep Power: 10 Do your calculations with a transient solver like pisoFoam or pimpleFoam. Then compare similar time steps. yuanjiahui and Mahmoud Abbaszadeh like this. __________________ If you liked my answer to your question, please consider leaving a "Like" in return

 October 16, 2018, 07:05 #3 Senior Member   Join Date: Mar 2014 Posts: 108 Rep Power: 11 You have to use a transient solver for LES but for comparison with RAS results, you can add "fieldAverage" function within controlDict for getting the mean values of U and p. yuanjiahui likes this.

 October 16, 2018, 07:48 #4 New Member   Yuan Jiahui Join Date: Oct 2018 Posts: 9 Rep Power: 6 Hi mzzmrt.Can I get a steady result with a transient solver like pisoFoam in LES model?

October 16, 2018, 07:57
#5
New Member

Yuan Jiahui
Join Date: Oct 2018
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Quote:
 Originally Posted by RobertHB Do your calculations with a transient solver like pisoFoam or pimpleFoam. Then compare similar time steps.
Hi RobertHB. I intended to caculate with simpleFoam, but then I found that LES model does not work with simpleFoam. So can I get steady result with pisoFoam in LES model and compare it with RANS model?

 October 16, 2018, 08:36 #6 Senior Member   Robert Join Date: May 2015 Location: Bremen, GER Posts: 292 Rep Power: 10 In short, no. pisoFoam calculates a transient, time dependant solution. And you use LES to resolve the transient flow without averaging like RAS does. If you want a steady, time independant, solution use simpleFoam and a RAS model. If you want to compare LES and RAS, you can do so by comparing the steady RAS and the transient RAS and LES results. yuanjiahui likes this. __________________ If you liked my answer to your question, please consider leaving a "Like" in return

October 16, 2018, 08:57
#7
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Yuan Jiahui
Join Date: Oct 2018
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Quote:
 Originally Posted by RobertHB In short, no. pisoFoam calculates a transient, time dependant solution. And you use LES to resolve the transient flow without averaging like RAS does. If you want a steady, time independant, solution use simpleFoam and a RAS model. If you want to compare LES and RAS, you can do so by comparing the steady RAS and the transient RAS and LES results.
I sorry to bother you again. But I do not understand your last words "you can do so by comparing the steady RAS and the transient RAS and LES results". The problem for me is that how to caculate a case like pitzDaily with both LES and RANS and compare the final results of them.

regards

October 16, 2018, 09:20
#8
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Robert
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Quote:
 Originally Posted by yuanjiahui The problem for me is that how to caculate a case like pitzDaily with both LES and RANS and compare the final results of them.
Do two simulations, one with pisoFoam RAS and a second one with pisoFoam LES.
Conduct both for the same runtime.
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 October 16, 2018, 13:02 #9 Senior Member   Join Date: Mar 2014 Posts: 108 Rep Power: 11 Add this to your controlDict, solve LES with pisoFoam and check the results in paraview: functions { fieldAverage1 { type fieldAverage; functionObjectLibs ("libfieldFunctionObjects.so"); enabled true; outputControl outputTime; fields ( U { mean on; prime2Mean off; base time; } p { mean on; prime2Mean off; base time; } ); } } RobertHB and yuanjiahui like this.

October 16, 2018, 20:37
#10
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Yuan Jiahui
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Quote:
 Originally Posted by RobertHB Do two simulations, one with pisoFoam RAS and a second one with pisoFoam LES. Conduct both for the same runtime.
No, one is with simpleFoam RAS and the second one is with pisoFoam LES and there is measurement data in steady state need to be comparaed.

October 17, 2018, 03:06
#11
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Robert
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Quote:
 Originally Posted by yuanjiahui No, one is with simpleFoam RAS and the second one is with pisoFoam LES and there is measurement data in steady state need to be comparaed.
Compare whatever you want, but notice that LES is a transient turbulence simulation, not steady. So either compare different timesteps to your steady solution or use a long runtime and the average approach suggested by mzzmrt. You can also use the "Temporal Statistics" filter of paraview to get an average over the total runtime.
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