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-   -   What is the c3 coefficient in k-epsilon model? (http://www.cfd-online.com/Forums/openfoam/65144-what-c3-coefficient-k-epsilon-model.html)

Marcin June 5, 2009 08:24

What is the c3 coefficient in k-epsilon model?
 
Hi everyone!

I've tried to find some informations about the coefficients in the k-epsilon turbulence model and i found the c3, a coefficient which I can't find enywhere else. Does anybody know know what is it? And how big should be the value of it? I think the default value is -0.33. Why negative?

Thank's for any help.

Best regards,

Marcin

sega June 7, 2009 16:58

As "mentioned" in the code the default value for c3 is -0.33 and is corresponding to compressible flow only.
It is included in the dissipation equation for epsilon.

Marcin June 9, 2009 02:12

Hallo Sebastian,

thank you for your reply. I have also found these informations in the code. The problem is, I don't uznderstand them. I have comared some equations implemented in OpenFOAM with other, commercial codes (for example CFX) and most of them are simmilar or the same exept for the dissipation equation. I have also compared this equation with some literature and I haven't found anywhere else the C3 coefficient exept for some papers, where the C3 was 0.85. I have also searched for -0.33 value but without success. I would be pleased for some help.

Best regards,

Marcin

sega June 9, 2009 05:59

You habe obviously done far more research on this topic than me.
I'm quite new to turbulence and have only dealt with incompressible fluids so far.
Tough enough without compressible fluids ;)

Marcin June 10, 2009 02:11

Hallo Sebastian,

thak you anyway for your help. All my simulations are compressible and turbulent. The problem is, i have done some simulations of a stator blade of a compressor and comared the results of OpenFOAM with CFX. All values like velocities, pressure, mach number, kinetic energy and temperature of both simulations are quite similar with small or very small differences. The only problematic value is the dissipation. The difference between these codes is quite big with a value between 1e4 and 1e6 in depand of the position. I have no idea what could be the cause. When i change the C3 coefficient from -0.33 to 0.85 it doesn't seem to make a difference in the simulation, but I would anyway like to know, what it is.

Regards,

Marcin

waynezw0618 June 11, 2009 01:48

Hi Marcin

How do you give the inlet boundary value of k and epsilon for you blade?

i am doing the simulation with water pump.it is easy to give a value for SST ,but for k-epsilon,i always get divgence. i tried two ways

1 calculating it from the according the Programme guide P84

2 post the CFX result to get the k and epsilon value

but both leaded to divgence within 100 steps

how can I do

wayne
Quote:

Originally Posted by Marcin (Post 218745)
Hallo Sebastian,

thak you anyway for your help. All my simulations are compressible and turbulent. The problem is, i have done some simulations of a stator blade of a compressor and comared the results of OpenFOAM with CFX. All values like velocities, pressure, mach number, kinetic energy and temperature of both simulations are quite similar with small or very small differences. The only problematic value is the dissipation. The difference between these codes is quite big with a value between 1e4 and 1e6 in depand of the position. I have no idea what could be the cause. When i change the C3 coefficient from -0.33 to 0.85 it doesn't seem to make a difference in the simulation, but I would anyway like to know, what it is.

Regards,

Marcin


Marcin June 15, 2009 02:31

Dear Wayne,

I had also problems with the inlet boudary conditions for k and epsilon. I have also tried to set up predetermined values, but it doesn't seem to work in both codes. The one way you can loose it is:

the boudary condition for k:

type turbulentIntensityKineticEnergyInlet;
intensity 0.05;
value $internalField;

and for epsilon:

type turbulentMixingLengthDissipationRateInlet;
mixingLength 0.005;
value $internalField;

This is also the default adjustment in CFX in inlet. The problem is, that I get with these settings different results for k and epsilon in OpenFOAM and CFX and I don't know where these diffrences come from.
I would be pleased, if you could inform me about your results and how you solve your problem.

Regards, Marcin

rob3rt June 16, 2010 08:39

Hi Marcin,

I have a problem with kEpsilonCoeffs. I randomly selected some numbers (i.e. 0.09, 0.14, -0.33, etc.....) and it generates this error:

Selecting incompressible transport model Newtonian
Selecting RAS turbulence model kEpsilon


--> FOAM FATAL ERROR:
Attempt to return primitive entry ITstream : ::kEpsilonCoeffs, line 19, IOstream: Version 2.0, format ASCII, line 0, OPENED, GOOD
primitiveEntry 'kEpsilonCoeffs' comprises
on line 19 the doubleScalar -0.33
as a sub-dictionary

From function const dictionary& primitiveEntry::dict()
in file db/dictionary/primitiveEntry/primitiveEntry.C at line 107.

FOAM aborting

#0 Foam::error::printStack(Foam::Ostream&) in "/home/roberto/OpenFOAM/OpenFOAM-1.6.x/lib/linux64GccDPOpt/libOpenFOAM.so"
#1 Foam::error::abort() in "/home/roberto/OpenFOAM/OpenFOAM-1.6.x/lib/linux64GccDPOpt/libOpenFOAM.so"
#2 Foam::primitiveEntry::dict() in "/home/roberto/OpenFOAM/OpenFOAM-1.6.x/lib/linux64GccDPOpt/libOpenFOAM.so"
#3 Foam::dictionary::subDict(Foam::word const&) in "/home/roberto/OpenFOAM/OpenFOAM-1.6.x/lib/linux64GccDPOpt/libOpenFOAM.so"
#4 Foam::boussinesq::RASModel::RASModel(Foam::word const&, Foam::GeometricField<Foam::Vector<double>, Foam::fvPatchField, Foam::volMesh> const&, Foam::GeometricField<double, Foam::fvsPatchField, Foam::surfaceMesh> const&, Foam::GeometricField<double, Foam::fvPatchField, Foam::volMesh> const&, Foam::transportModel&) in "/home/roberto/OpenFOAM/OpenFOAM-1.6.x/lib/linux64GccDPOpt/libboussinesqRASModels.so"
#5 Foam::boussinesq::RASModels::kEpsilon::kEpsilon(Fo am::GeometricField<Foam::Vector<double>, Foam::fvPatchField, Foam::volMesh> const&, Foam::GeometricField<double, Foam::fvsPatchField, Foam::surfaceMesh> const&, Foam::GeometricField<double, Foam::fvPatchField, Foam::volMesh> const&, Foam::transportModel&) in "/home/roberto/OpenFOAM/OpenFOAM-1.6.x/lib/linux64GccDPOpt/libboussinesqRASModels.so"
#6 Foam::boussinesq::RASModel::adddictionaryConstruct orToTable<Foam::boussinesq::RASModels::kEpsilon>:: New(Foam::GeometricField<Foam::Vector<double>, Foam::fvPatchField, Foam::volMesh> const&, Foam::GeometricField<double, Foam::fvsPatchField, Foam::surfaceMesh> const&, Foam::GeometricField<double, Foam::fvPatchField, Foam::volMesh> const&, Foam::transportModel&) in "/home/roberto/OpenFOAM/OpenFOAM-1.6.x/lib/linux64GccDPOpt/libboussinesqRASModels.so"
#7 Foam::boussinesq::RASModel::New(Foam::GeometricFie ld<Foam::Vector<double>, Foam::fvPatchField, Foam::volMesh> const&, Foam::GeometricField<double, Foam::fvsPatchField, Foam::surfaceMesh> const&, Foam::GeometricField<double, Foam::fvPatchField, Foam::volMesh> const&, Foam::transportModel&) in "/home/roberto/OpenFOAM/OpenFOAM-1.6.x/lib/linux64GccDPOpt/libboussinesqRASModels.so"
#8
in "/home/roberto/OpenFOAM/OpenFOAM-1.6.x/applications/bin/linux64GccDPOpt/my_porousSimpleFoam8"
#9 __libc_start_main in "/lib/libc.so.6"
#10
in "/home/roberto/OpenFOAM/OpenFOAM-1.6.x/applications/bin/linux64GccDPOpt/my_porousSimpleFoam8"
Aborted

Do you have any suggestions??

Thanks very much in advance.


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