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Old   December 16, 2010, 11:21
Default Need some details in SimpleFOAM
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Vishal Jambhekar
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Hi All,

I have set up problem to simulate flow over a city using exp prof at inlet. Unfortunately I dnot know the Char. Length for Epsilon calculation. However I have calculated it using values of using ratio of (nut/nu).

but in tranport properties i see following :

nu0, nuInf m and n . I dont understand which value means what.....???

And as I run the simulation I cant even even notice the solver solving for nuTIlda. can someone tell me does the solver solves for nuTilda in SimpleFoam...
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Old   December 19, 2010, 08:39
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David Boger
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It sounds to me like the coefficients you are referring to belong to one of the non-Newtonian viscosity models. They should be listed within a dictionary with a name such as CrossPowerLaw, in which case the corresponding source code shows how the parameters are used:
Code:
Foam::viscosityModels::CrossPowerLaw::calcNu() const
{
    return (nu0_ - nuInf_)/(scalar(1) + pow(m_*strainRate(), n_)) + nuInf_;
}
But whether you are even using these or not depends on which transport model you have selected in transportProperties; e.g. if you are using
Code:
transportModel  Newtonian;
then the coefficients you are asking about are irrelevant.

As far as the solution of nuTilda, the only models I know of that solve for nuTilda are the RAS and LES versions of the Spalart-Allmaras turbulence model, so the code will only solve for that variable if you have selected Spalart-Allmaras as the turbulence model. For example, in your RASProperties file, you might specify
Code:
RASModel            SpalartAllmaras;
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Old   December 23, 2010, 04:48
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Vishal Jambhekar
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Thanks Bogar,

But I am using "Newtonian" Fluid and "Standard k-epsilon model". but In that case i dont know how to calculate epsilon as i dont know the char. length for the domain.

I was trying to calculate it using formula specified at following link and give it as inlet condition.(List of values). There they use "mut" for calculation of "epsilon".


[IMG]file:///tmp/moz-screenshot-2.png[/IMG]http://www.cfd-online.com/Wiki/Standard_k-epsilon_model


However I am trying to validate results for Fluent against OF, where we have following condotions for fluent.

Terbulent Intensity : 3
Standard k-epsilon model.


Note : I am giving list of values for U, K and Epsilon at inlet
I guess terbulent intensity here belongs to the ration of mut/mu. which should be same for incompressible fluid. How can I represtne the same inlet conditions in OF...???

following is the error i am getting :

[1] #0 Foam::error:rintStack(Foam::Ostream&) in "/home/vj/OpenFOAM/OpenFOAM-1.7.1/lib/linux64GccDPOpt/libOpenFOAM.so"
[1] #1 Foam::sigFpe::sigFpeHandler(int) in "/home/vj/OpenFOAM/OpenFOAM-1.7.1/lib/linux64GccDPOpt/libOpenFOAM.so"
[1] #2 __restore_rt at sigaction.c:0
[1] #3 Foam::PBiCG::solve(Foam::Field<double>&, Foam::Field<double> const&, unsigned char) const in "/home/vj/OpenFOAM/OpenFOAM-1.7.1/lib/linux64GccDPOpt/libOpenFOAM.so"
[1] #4 Foam::GAMGSolver::solveCoarsestLevel(Foam::Field<d ouble>&, Foam::Field<double> const&) const in "/home/vj/OpenFOAM/OpenFOAM-1.7.1/lib/linux64GccDPOpt/libOpenFOAM.so"
[1] #5 Foam::GAMGSolver::Vcycle(Foam::PtrList<Foam::lduMa trix::smoother> const&, Foam::Field<double>&, Foam::Field<double> const&, Foam::Field<double>&, Foam::Field<double>&, Foam::Field<double>&, Foam::PtrList<Foam::Field<double> >&, Foam::PtrList<Foam::Field<double> >&, unsigned char) const in "/home/vj/OpenFOAM/OpenFOAM-1.7.1/lib/linux64GccDPOpt/libOpenFOAM.so"
[1] #6 Foam::GAMGSolver::solve(Foam::Field<double>&, Foam::Field<double> const&, unsigned char) const in "/home/vj/OpenFOAM/OpenFOAM-1.7.1/lib/linux64GccDPOpt/libOpenFOAM.so"
[1] #7 Foam::fvMatrix<double>::solve(Foam::dictionary const&) in "/home/vj/OpenFOAM/OpenFOAM-1.7.1/lib/linux64GccDPOpt/libfiniteVolume.so"
[1] #8 Foam::lduMatrix::solverPerformance Foam::solve<double>(Foam::tmp<Foam::fvMatrix<doubl e> > const&) in "/home/vj/OpenFOAM/OpenFOAM-1.7.1/lib/linux64GccDPOpt/libincompressibleRASModels.so"
[1] #9 Foam::incompressible::RASModels::kEpsilon::correct () in "/home/vj/OpenFOAM/OpenFOAM-1.7.1/lib/linux64GccDPOpt/libincompressibleRASModels.so"
[1] #10 main in "/home/vj/OpenFOAM/OpenFOAM-1.7.1/applications/bin/linux64GccDPOpt/simpleFoam"
[1] #11 __libc_start_main in "/lib64/libc.so.6"
[1] #12 Foam::regIOobject::writeObject(Foam::IOstream::str eamFormat, Foam::IOstream::versionNumber, Foam::IOstream::compressionType) const in "/home/vj/OpenFOAM/OpenFOAM-1.7.1/applications/bin/linux64GccDPOpt/simpleFoam"
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Old   December 23, 2010, 10:11
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David Boger
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There are other threads on the forum that already discuss aspects of your question. See this one for example, which also discusses a way in OpenFOAM to specify the turbulence intensity as a boundary condition.

I don't think there's any way to get around the fact that you'll need to "know" or make some assumptions about the turbulence length scale in your domain. Presumably, Fluent is making some assumption about it if you're not specifying it some way; you'll have to check the Fluent documentation to find out.

As far as the definition of turbulence intensity, it is not mut/mu but rather can be related to something proportional to the square root of k. You should be able to easily find the definition on-line.
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Old   March 10, 2011, 04:15
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Vishal Jambhekar
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Hi Boger,

Thanks a lot, can you please look into this. I have complete details for my case here

Help with k epsilon values of turbulence
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