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 nuovodna November 27, 2007 09:19

I'm working on an airfoil prof

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?

Emanuele

 mkraposhin November 27, 2007 11:15

I think, you must use freestre

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

 nuovodna November 27, 2007 11:48

what are the fixed values of k

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

 nuovodna November 27, 2007 21:44

what kind of model of turbulen

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?

 olesen November 28, 2007 03:44

For k/epsilon, I normally spec

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.

 nuovodna December 2, 2007 10:31

the value on inlet patch of k

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??

 markusrehm December 3, 2007 05:52

Hello, what does "valu

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 BC-description?

Regards Markus

 bephi May 19, 2010 04:58

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;     }```
U
Code:

```boundaryField {     inlet     {         type            fixedValue;         value          uniform (0 0 -1);     }     outlet     {         type            zeroGradient;     }```
k
Code:

```internalField  uniform 0.0003; boundaryField {     inlet     {         type            fixedValue;         value          uniform 0.0003;     }     outlet     {         type            zeroGradient;     }```
epsilon
Code:

```internalField  uniform 1.35e-4; boundaryField {     inlet     {         type            fixedValue;         value          uniform 1.35e-4;     }     outlet     {         type            zeroGradient;     }```
nut
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;```
controlDict
Code:

```application            simpleFoam; startFrom            startTime; startTime            0; stopAt                endTime; endTime                7.2e-2; deltaT                3e-6; writeControl            timeStep; writeInterval            1000; purgeWrite            0; writeFormat            ascii; writePrecision            6; writeCompression        uncompressed; timeFormat            general; timePrecision            6; graphFormat            raw; runTimeModifiable        yes;```
fvSchemes
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((1|A(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; }```
fvSolution
Code:

```solvers {     p PCG     {         preconditioner  DIC;         tolerance        1e-06;         relTol          0.01;     };     U PBiCG     {         preconditioner  DILU;         tolerance        1e-05;         relTol          0.1;     };     k PBiCG     {         preconditioner  DILU;         tolerance        1e-05;         relTol          0.1;     };     epsilon PBiCG     {         preconditioner  DILU;         tolerance        1e-05;         relTol          0.1;     };     R PBiCG     {         preconditioner  DILU;         tolerance        1e-05;         relTol          0.1;     };     } SIMPLE {     nNonOrthogonalCorrectors 1; } relaxationFactors {     p              0.3;     U              0.7;     k              0.05;     epsilon        0.05;     R              0.7; }```
If there is something else to change or improve in my settings please let me know! I'll be very thankful!

Regards!

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