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Old   June 5, 2012, 13:27
Default Ansys/Fluent and OpenFoam comparison, LES, XiFoam, Smagorinsky
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achinta
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Hi All,
I would to compare results of Fluent and OpenFoam for Cold simulation of EV burner. In Fluent, the simulation was done using Smagorinsky-Lilly model. I would like to compare it with Smagorinsky model of OpenFoam. I have boundary conditions given for Fluent and I need to convert it to OpenFOAM input. Could someone give me hints about conversion. I have the follwing parameters for ANSYS and I need equivalent parameters for OpenFOAM (if it exists):
1)critical rate of strain
2)progress variable (I think its regress variable in OF---b=1-c)
3)mean mixture fraction
4)Mean fraction variance
5)Constants of Smagorinsky model-- Cs, Cfvar,PDF Schmidt number
6)Turbulent length scale constant
7) Turbulent flame speed constant
8)Stretch Factor coefficient
9)Turbulent Schmidt number

Btw,
1) Do I have to change /constant/LESProperties file for a given problem?
2) Do I have to change anything except nu in constant/transportProperties file?

P.S: My next step would be to compare combustion simulation of ANSYS and XiFoam. I might need some help there too

Kind regards,
Achinta

Last edited by achinta; June 5, 2012 at 13:32. Reason: improvement
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Old   June 7, 2012, 11:15
Default may be able to help
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Hey Achinta,

Actually I am new here and also to the field of OpenFOAM, Combustion and LES, which is the reason why I am writing this.

I need some problems, just to practice and I think you can provide me with one here.

If your geometry and the FLUENT case/data files are not confidential, I would request you to send it over to me at

turbulentreactingflows@gmail.com

First we should go only after LES (cold flow) and then Combustion with LES using OpenFOAM. I can't promise anything at the moment but will try to help as long as time permits.

Lets get started,
cheers
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Old   June 7, 2012, 12:25
Default again
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Hi again Achinta,

I feel that the tone of my previous post was not quite pleasant. I am really sorry for that.

Basically the point is that if you can send me your ANSYS FLUENT case (for BC's) files then I may be able to help you with OpenFOAM (both LES alone and LES with Combustion).

I hope it will benefit both of us

cheers
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Old   June 8, 2012, 12:08
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Hi there,
Sorry for late reply. I was bit busy. I am working for a company and the mesh file is confidential. Sorry, i cannot send you those files. I am new to LES too and i dont have any extra tutorials. please send me some if you have ..
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Old   June 8, 2012, 12:28
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Hello everyone,
I am simulating cold combustion . it diverged. Please tell me if i made any mistake.


Kind regards,
Achinta

Last edited by achinta; June 23, 2012 at 15:21.
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Old   June 11, 2012, 13:48
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Hello everyone,
I am able to set boundary conditions and run the code. But it diverges after 15 time steps. I had used the following schemes:

-----
ddtSchemes
{
default backward;
}

gradSchemes
{
default Gauss linear;
grad(p) Gauss linear;
grad(U) Gauss linear;
}

divSchemes
{
default none;
div(phi,U) Gauss Linear;
div(phi,k) 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 none;
laplacian(nuEff,U) Gauss linear corrected;
laplacian((1|A(U)),p) Gauss linear corrected;
laplacian(DkEff,k) Gauss linear corrected;
laplacian(DBEff,B) Gauss linear corrected;
laplacian(DnuTildaEff,nuTilda) Gauss linear corrected;
}

interpolationSchemes
{
default linear;
interpolate(U) linear;
}

snGradSchemes
{
default corrected;
}

fluxRequired
{
default no;
p ;
}

-----
here is fvSolution file:
------
p
{
solver PCG;
preconditioner DIC;
tolerance 1e-06;
relTol 0.1;
}

pFinal
{
solver PCG;
preconditioner DIC;
tolerance 1e-06;
relTol 0;
}

U
{
solver PBiCG;
preconditioner DILU;
tolerance 1e-05;
relTol 0;
}

k
{
solver PBiCG;
preconditioner DILU;
tolerance 1e-05;
relTol 0;
}

B
{
solver PBiCG;
preconditioner DILU;
tolerance 1e-05;
relTol 0;
}

nuTilda
{
solver PBiCG;
preconditioner DILU;
tolerance 1e-05;
relTol 0;
}
}

PISO
{
nCorrectors 2;
nNonOrthogonalCorrectors 0;
}
-------------


With the above settings and time step of 1micro second, the solution diverged in 15 time steps.

1) I refered some threads which suggested to use 'filteredLinear' for convective schemes. So I changed "div(phi,U) Gauss filteredLinear;" (Should I change other divergence schemes too?)

2) Some threads suggested GAMG for p and pFinal. I made some changes in tolerance limits too. here is the file:

--------
p
{
solver GAMG;
tolerance 1e-6;
relTol 0.1;

smoother GaussSeidel;
nPreSweeps 0;
nPostSweeps 2;
nFinestSweeps 2;

cacheAgglomeration true;

nCellsInCoarsestLevel 0;
agglomerator faceAreaPair;
mergeLevels 1;
};

pFinal
{
solver GAMG;
tolerance 1e-7;
relTol 0;

smoother GaussSeidel;
nPreSweeps 0;
nPostSweeps 2;
nFinestSweeps 2;

cacheAgglomeration true;

nCellsInCoarsestLevel 0;
agglomerator faceAreaPair;
mergeLevels 1;
};

U
{
solver PBiCG;
preconditioner DILU;
tolerance 1e-06;
relTol 0;
}

k
{
solver PBiCG;
preconditioner DILU;
tolerance 1e-06;
relTol 0;
}

B
{
solver PBiCG;
preconditioner DILU;
tolerance 1e-06;
relTol 0;
}

nuTilda
{
solver PBiCG;
preconditioner DILU;
tolerance 1e-06;
relTol 0;
}
}

PISO
{
nCorrectors 3;
nNonOrthogonalCorrectors 1;
}


-----

I increased time step to 5 microseconds since there are many corrector loops. But the problem still exists and the solution is diverging.

I kindly request someone with LES experience to help me out. What could be the reason? Is it because of Smagorinsky model? Should I use OneEqEddy LES model?

Or is it because of Boundary/ initial conditions?

Should i use a smaller time step? The mesh is really fine (6 million cells) and uses hexahedral and tetrahedral cells.

I am using pisoFoam (transient calculations. So its bit tricky). Should i use pimple instead of piso?

Regards,
Achinta

Last edited by achinta; June 11, 2012 at 16:39. Reason: improvement
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