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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?

Thanks in advantage

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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