CFD Online Discussion Forums - NACA0012 with simpleFOAM

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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

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

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?

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?

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.

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??

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

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;

    }

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!