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November 27, 2007, 09:19 
I'm working on an airfoil prof

#1 
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Emanuele
Join Date: Mar 2009
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I'm working on an airfoil profile NACA0012. I wrote my mesh in Gambit and then i imported it in OpenFOAM. I'm going to solver it with simpleFOAM (incompressible and steady flow, using kepsilon model) but the solution isn't accurate i think. At the angle of attack = 15° not stall and the total force on the profile is very low (i've calculated them using ssimpleFoam found in the board). I'd like to know:
1) what are the correct boundary condition on k and epsilon (in what way i give the correct k and epsilon fixed values on inlet patch and in internal field?) 2) do i have to use another model of turbulence? Thanks in advantage Emanuele 

November 27, 2007, 11:15 
I think, you must use freestre

#2 
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Matvej Kraposhin
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Location: Moscow, Russian Federation
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I think, you must use freestream BC for infinity field, standard wall BC for walls on profile and inlet for inlet patch.
inlet: U  fixedValue, p  zeroGradient, k  fixedValue, epsilon  fixedValue infinity field  U  freestream, p  freestreamPressure, k and epsilon  zeroGradient. profile walls  U  fixedValue (0 0 0), p  zeroGradient, k and epsilon  zeroGradient 

November 27, 2007, 11:48 
what are the fixed values of k

#3 
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Emanuele
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what are the fixed values of k and epsilon to setting? is there a mode to calculate them?


November 27, 2007, 21:44 
what kind of model of turbulen

#4 
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Emanuele
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what kind of model of turbulence is better??
and for the better what are the boundary condition on the k epsilon or another variable? what is the better schemes to use? 

November 28, 2007, 03:44 
For k/epsilon, I normally spec

#5 
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Mark Olesen
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For k/epsilon, I normally specify turb. intensity and the mixing length. I don't say that you have good values for these, but I find them a bit easier to visualize than the raw tke and dissipation.
The boundary conditions for 10% turbulence and 5mm length scale would look something like this: // k dimensions [ 0 2 2 0 0 0 0 ]; internalField uniform 1; ... inlet { type turbulentIntensityKineticEnergyInlet; intensity 0.1; value 1; } // epsilon dimensions [ 0 2 3 0 0 0 0 ]; internalField uniform 200; ... inlet { type turbulentMixingLengthDissipationRateInlet; mixingLength 0.005; value uniform 200; } NB: The stability of your calculation can be greatly affected by the initialization of epsilon. 

December 2, 2007, 10:31 
the value on inlet patch of k

#6 
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Emanuele
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the value on inlet patch of k and epsilon affect very greatly the solution...also using komega model you can choose the initial values very well....in what way can i fix this initial values (of k epsilon and omega) to obtain the correct solutions??


December 3, 2007, 05:52 
Hello,
what does
"valu

#7 
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Markus Rehm
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Location: Erlangen (Germany)
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Hello,
what does "value 1;" in k and "internalField uniform 200;" in epsilon stand for? It is the same as internalField, why do we need it in the BCdescription? Regards Markus 

May 19, 2010, 04:58 

#8 
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Hi at all,
i'd like to continue this thread because I have a lot of trouble with bounding k/epsilon although there are so many threads concerning this topic. so far I know that the initialization of k and epsilon is very important to achieve a stable calculation. but actually I don't really know what went wrong. I would like to simulate a flow throw a piston valve using the simpleFoam solver with the RNG k_epsilon model. Here are my settings. Maybe you can take a look what should be changed to get a stable calculation: p Code:
boundaryField { inlet { type zeroGradient; } outlet { type fixedValue; value uniform 476.2; } Code:
boundaryField { inlet { type fixedValue; value uniform (0 0 1); } outlet { type zeroGradient; } Code:
internalField uniform 0.0003; boundaryField { inlet { type fixedValue; value uniform 0.0003; } outlet { type zeroGradient; } Code:
internalField uniform 1.35e4; boundaryField { inlet { type fixedValue; value uniform 1.35e4; } outlet { type zeroGradient; } Code:
internalField uniform 0.1; boundaryField { inlet { type freestream; freestreamValue uniform 0.1; value uniform 0.1; } outlet { type freestream; freestreamValue uniform 0.1; value uniform 0.1; Code:
application simpleFoam; startFrom startTime; startTime 0; stopAt endTime; endTime 7.2e2; deltaT 3e6; writeControl timeStep; writeInterval 1000; purgeWrite 0; writeFormat ascii; writePrecision 6; writeCompression uncompressed; timeFormat general; timePrecision 6; graphFormat raw; runTimeModifiable yes; Code:
ddtSchemes { default steadyState; } gradSchemes { default Gauss linear; grad(p) Gauss linear; grad(U) Gauss linear; } divSchemes { default none; div(phi,U) Gauss upwind; div(phi,k) Gauss upwind; div(phi,epsilon) Gauss upwind; div(phi,R) Gauss upwind; div(R) Gauss linear; div((nuEff*dev(grad(U).T()))) Gauss linear; } laplacianSchemes { default none; laplacian(nuEff,U) Gauss linear corrected; laplacian((1A(U)),p) Gauss linear corrected; laplacian(DkEff,k) Gauss linear corrected; laplacian(DepsilonEff,epsilon) Gauss linear corrected; laplacian(DREff,R) Gauss linear corrected; } interpolationSchemes { default linear; interpolate(U) linear; } snGradSchemes { default corrected; } fluxRequired { default no; p; } Code:
solvers { p PCG { preconditioner DIC; tolerance 1e06; relTol 0.01; }; U PBiCG { preconditioner DILU; tolerance 1e05; relTol 0.1; }; k PBiCG { preconditioner DILU; tolerance 1e05; relTol 0.1; }; epsilon PBiCG { preconditioner DILU; tolerance 1e05; relTol 0.1; }; R PBiCG { preconditioner DILU; tolerance 1e05; relTol 0.1; }; } SIMPLE { nNonOrthogonalCorrectors 1; } relaxationFactors { p 0.3; U 0.7; k 0.05; epsilon 0.05; R 0.7; } Regards! 

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