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How to get steady result in pitzDaily using LES model

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Old   October 15, 2018, 06:19
Default How to get steady result in pitzDaily using LES model
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Yuan Jiahui
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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
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Old   October 16, 2018, 06:01
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Do your calculations with a transient solver like pisoFoam or pimpleFoam. Then compare similar time steps.
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Old   October 16, 2018, 08:05
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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.
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Old   October 16, 2018, 08:48
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Hi mzzmrt.Can I get a steady result with a transient solver like pisoFoam in LES model?
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Old   October 16, 2018, 08:57
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Quote:
Originally Posted by RobertHB View Post
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?
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Old   October 16, 2018, 09:36
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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.
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Old   October 16, 2018, 09:57
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Quote:
Originally Posted by RobertHB View Post
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
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Old   October 16, 2018, 10:20
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Quote:
Originally Posted by yuanjiahui View Post
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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Old   October 16, 2018, 14:02
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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;
}
);
}
}
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Old   October 16, 2018, 21:37
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Quote:
Originally Posted by RobertHB View Post
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.
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Old   October 17, 2018, 04:06
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Quote:
Originally Posted by yuanjiahui View Post
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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