[Arzed23]

Hello all,

I've been trying to validate my numerical scheme for my research project using the "Aerodynamic Characteristics of Seven Symmetrical Airfoil Sections Through 180-Degree Angle of Attack for Use in Aerodynamic Analysis of Vertical Axis Wind Turbines" paper by Sheldahl and Klimas: http://www.osti.gov/scitech/servlets/purl/6548367/

However, I can only get my lift coefficient angle of attack curves to line up for so much of the curves as shown in the curves here:

My research project follows on from Gawad's work (http://www.tsest.org/index.php/TCMS/article/view/158), who managed to achieve the validation I'm trying to sort out.

Is there something inherently wrong with the data, or is there something wrong with my simulation parameters? Below are my case files, sorry if this post has become quite long-winded, this is my first post!

Thanks guys!

/*--------------------------------*- C++ -*----------------------------------*\
| =========                 |                                                 |
| \\      /  F ield         | OpenFOAM: The Open Source CFD Toolbox           |
|  \\    /   O peration     | Version:  3.0.1                                 |
|   \\  /    A nd           | Web:      www.OpenFOAM.org                      |
|    \\/     M anipulation  |                                                 |
\*---------------------------------------------------------------------------*/
FoamFile
{
    version     2.0;
    format      ascii;
    class       volScalarField;
    location    "0";
    object      epsilon;
}
// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //

dimensions      [0 2 -3 0 0 0 0];

// epsilon = 4.5E-5 U^3/c = 4.5E-5 25^3/1 = 0.703125 

//****** Re = 700,000

// PREV CALCULATIONS
// L is the characteristic length (as per Reynolds number calculations, i.e. airfoil chord length in this case)
// From calculations in k case file, k = 0.603552
// epsilon = 0.164k^1.5 / 0.07L = 0.164*(0.603552^1.5)/(0.07*1) = 1.098544

internalField   uniform 1.0655;

boundaryField
{
    inlet
    {
        type            fixedValue;
        value 		uniform 1.0655;
    }

    outlet
    {
        type            zeroGradient;
    }

    top
    {        
	type            fixedValue;
        value           uniform 1.0655;
    }

    bottom
    {        
	type            fixedValue; 
        value           uniform 1.0655;
    }

    final_validation
    {
        type            epsilonWallFunction;
        value           uniform 1.0655;
    }

    #include "include/frontBackPatches"
}
// ************************************************************************* //

/*--------------------------------*- C++ -*----------------------------------*\
| =========                 |                                                 |
| \\      /  F ield         | OpenFOAM: The Open Source CFD Toolbox           |
|  \\    /   O peration     | Version:  3.0.1                                 |
|   \\  /    A nd           | Web:      www.OpenFOAM.org                      |
|    \\/     M anipulation  |                                                 |
\*---------------------------------------------------------------------------*/
FoamFile
{
    version     2.0;
    format      ascii;
    class       volScalarField;
    object      k;
}
// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //

// k = 10^-4 U^2 = 10^-4 25^2 = 0.0625

// *** Re = 700,000

// PREVIOUS CALCS
// Re = U_ref*D/v = 25*1/(1 E-05) = 2,500,000
// turbulence intensity, I = 0.16* Re^(-1/8) = 0.16*(2,500,000)^(-1/8) = 0.025373 (6 dp)
// k = 3/2*(U_ref*I)^2 = 3/2*(100*0.028453)^2 = 0.603552 (6 dp)

dimensions      [0 2 -2 0 0 0 0];

internalField   uniform 6.8426;


boundaryField
{
    inlet
    {
        type            fixedValue;
        value 		uniform 6.8426;
    }

    outlet
    {
        type            zeroGradient;
    }

    top
    {
        type            fixedValue; 
        value           uniform 6.8426;
    }
    bottom
    {
        type            fixedValue; 
        value           uniform 6.8426;
    }
    final_validation
    {
        type            kqRWallFunction;
        value           uniform 6.8426;
    }

    #include "include/frontBackPatches"
}
// ************************************************************************* //

/*--------------------------------*- C++ -*----------------------------------*\
| =========                 |                                                 |
| \\      /  F ield         | OpenFOAM: The Open Source CFD Toolbox           |
|  \\    /   O peration     | Version:  3.0.1                                 |
|   \\  /    A nd           | Web:      www.OpenFOAM.org                      |
|    \\/     M anipulation  |                                                 |
\*---------------------------------------------------------------------------*/
FoamFile
{
    version     2.0;
    format      ascii;
    class       volScalarField;
    location    "0";
    object      nut;
}
// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //

dimensions      [0 2 -1 0 0 0 0];

internalField   uniform 0; // copied from motorbike case (uses S-A, not k-e, but originally 0.14)

boundaryField
{
    inlet
    {
        type            calculated;
        value		uniform 0;
    }
    outlet
    {
        type            calculated;
        value		uniform 0;
    }
    top
    {
        type            calculated; 
        value           uniform 0;
    }
    bottom
    {
        type            calculated; 
        value           uniform 0;
    }
    final_validation
    {
        type            nutkWallFunction;
        value           uniform 0;
    }

    #include "include/frontBackPatches"
}

// ************************************************************************* //

/*--------------------------------*- C++ -*----------------------------------*\
| =========                 |                                                 |
| \\      /  F ield         | OpenFOAM: The Open Source CFD Toolbox           |
|  \\    /   O peration     | Version:  3.0.1                                 |
|   \\  /    A nd           | Web:      www.OpenFOAM.org                      |
|    \\/     M anipulation  |                                                 |
\*---------------------------------------------------------------------------*/
FoamFile
{
    version     2.0;
    format      ascii;
    class       volScalarField;
    object      p;
}
// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //

dimensions      [0 2 -2 0 0 0 0];

internalField   uniform 0;

boundaryField
{
    inlet
    {
        type            zeroGradient;
    }

    outlet
    {
        type            fixedValue;
	value		uniform 0;
    }

    top
    {
        type            zeroGradient;
    }
    bottom
    {
        type            zeroGradient;
    }
 
    final_validation
    {
        type            zeroGradient;
    }

    #include "include/frontBackPatches"
}
// ************************************************************************* //

/*--------------------------------*- C++ -*----------------------------------*\
| =========                 |                                                 |
| \\      /  F ield         | OpenFOAM: The Open Source CFD Toolbox           |
|  \\    /   O peration     | Version:  3.0.1                                 |
|   \\  /    A nd           | Web:      www.OpenFOAM.org                      |
|    \\/     M anipulation  |                                                 |
\*---------------------------------------------------------------------------*/
FoamFile
{
    version     2.0;
    format      ascii;
    class       volVectorField;
    object      U;
}
// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //

dimensions      [0 1 -1 0 0 0 0];

internalField   uniform (0 0 0); //initial internal velocity

// wing travelling at 71.1943 m/s in the x direction at Re = 700,000

boundaryField
{
    inlet
    {
        type            fixedValue;
        value uniform (71.1943 0 0);
    }

    outlet
    {
        type            zeroGradient;
    }

    top
    {
        type            fixedValue; 
        value           uniform (0 0 0);
    }
    bottom
    {
        type            fixedValue;
        value           uniform (0 0 0);
    }
    final_validation
    {
	type		fixedValue;
	value 		uniform (0 0 0);
    }

    #include "include/frontBackPatches"
}

// ************************************************************************* //

[arjun]

can't help you much but make sure you first get the results with same version. Things can sometimes change drastically between versions.

So to start i would just make sure that i get the same results on both versions, that is the one he used and current one.

[Arzed23]

Not sure why the images didn't come up but here are my lift coefficient angle of attack curves, and Gawad's for comparison:

Gawad (kEpsilon)
http://imgur.com/FFHPhaM

kOmegaSST:
http://imgur.com/cEmZOBe

kEpsilon:
http://imgur.com/dyUVS0E

Forgot to attach a picture of the mesh so attached is one below. For reference, the chord length, C is 152.4 mm (6 inch) with the a span length of 4C. The box dimensions are 8C long, 4C high, 4C wide. The image below is a slice as the rectangular box nature of the mesh makes it harder to visualise:

overall domain
http://imgur.com/DjQ8jFa

close up of airfoil
http://imgur.com/KFKk0eK

snappHexMexDict

/*--------------------------------*- C++ -*----------------------------------*\
| =========                 |                                                 |
| \\      /  F ield         | OpenFOAM: The Open Source CFD Toolbox           |
|  \\    /   O peration     | Version:  3.0.1                                 |
|   \\  /    A nd           | Web:      www.OpenFOAM.org                      |
|    \\/     M anipulation  |                                                 |
\*---------------------------------------------------------------------------*/
FoamFile
{
    version     2.0;
    format      ascii;
    class       dictionary;
    object      snappyHexMeshDict;
}
// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //

// Which of the steps to run
castellatedMesh true;
snap            true;
addLayers       false;


// Geometry. Definition of all surfaces. All surfaces are of class
// searchableSurface.
// Surfaces are used
// - to specify refinement for any mesh cell intersecting it
// - to specify refinement for any mesh cell inside/outside/near
// - to 'snap' the mesh boundary to the surface
geometry
{
    final_validation.stl
    {
        type triSurfaceMesh;
        name final_validation;
    }

    /*refinementBox_1
    {
        type searchableBox;
        min (-0.09 -0.15 0.0); //1 cell to the left
        max (0.7 0.12 0.9144); //twice the chord length to the right
    }

    refinementBox_2
    {
        type searchableBox;
        min (-0.06 -0.12 0.0); //1 cell to the left
        max ( 0.65 0.09 0.9144); //twice the chord length to the right
    }

    refinementBox_3
    {
        type searchableBox;
        min (-0.03 -0.09 0.0); //1 cell to the left
        max ( 0.6 0.06 0.9144); //twice the chord length to the right
    }*/

    refinementBox_1
    {
        type searchableBox;
        min (-0.4572 -0.3048 0.0); //1 cell to the left
        max (0.762 0.3048 0.6096); //twice the chord length to the right
    }

    refinementBox_2
    {
        type searchableBox;
        min (-0.36 -0.24 0.0); //1 cell to the left
        max ( 0.74 0.24 0.6096); //twice the chord length to the right
    }

    refinementBox_3
    {
        type searchableBox;
        min (-0.24 -0.18 0.0); //1 cell to the left
        max ( 0.7 0.18 0.6096); //twice the chord length to the right
    }
};



// Settings for the castellatedMesph generation.
castellatedMeshControls
{

    // Refinement parameters
    // ~~~~~~~~~~~~~~~~~~~~~

    // If local number of cells is >= maxLocalCells on any processor
    // switches from from refinement followed by balancing
    // (current method) to (weighted) balancing before refinement.
    maxLocalCells 10000000;

    // Overall cell limit (approximately). Refinement will stop immediately
    // upon reaching this number so a refinement level might not complete.
    // Note that this is the number of cells before removing the part which
    // is not 'visible' from the keepPoint. The final number of cells might
    // actually be a lot less.
    maxGlobalCells 200000000;

    // The surface refinement loop might spend lots of iterations refining just a
    // few cells. This setting will cause refinement to stop if <= minimumRefine
    // are selected for refinement. Note: it will at least do one iteration
    // (unless the number of cells to refine is 0)
    minRefinementCells 10;

    // Allow a certain level of imbalance during refining
    // (since balancing is quite expensive)
    // Expressed as fraction of perfect balance (= overall number of cells /
    // nProcs). 0=balance always.
    maxLoadUnbalance 0.10;


    // Number of buffer layers between different levels.
    // 1 means normal 2:1 refinement restriction, larger means slower
    // refinement.
    nCellsBetweenLevels 3;



    // Explicit feature edge refinement
    // ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

    // Specifies a level for any cell intersected by its edges.
    // This is a featureEdgeMesh, read from constant/triSurface for now.
    features
    (
	{file "final_validation.eMesh"; level 5;}
    );



    // Surface based refinement
    // ~~~~~~~~~~~~~~~~~~~~~~~~

    // Specifies two levels for every surface. The first is the minimum level,
    // every cell intersecting a surface gets refined up to the minimum level.
    // The second level is the maximum level. Cells that 'see' multiple
    // intersections where the intersections make an
    // angle > resolveFeatureAngle get refined up to the maximum level.

    refinementSurfaces
    {
        final_validation
        {
            // Surface-wise min and max refinement level
            level (5 5);

            // Optional specification of patch type (default is wall). No
            // constraint types (cyclic, symmetry) etc. are allowed.
        }
    }

    // Resolve sharp angles
    resolveFeatureAngle 80; //was 30 by default


    // Region-wise refinement
    // ~~~~~~~~~~~~~~~~~~~~~~

    // Specifies refinement level for cells in relation to a surface. One of
    // three modes
    // - distance. 'levels' specifies per distance to the surface the
    //   wanted refinement level. The distances need to be specified in
    //   descending order.
    // - inside. 'levels' is only one entry and only the level is used. All
    //   cells inside the surface get refined up to the level. The surface
    //   needs to be closed for this to be possible.
    // - outside. Same but cells outside.

    refinementRegions
    {
        refinementBox_1
        {
            mode inside;
            levels ((1.0 1));
        }

        refinementBox_2
        {
            mode inside;
            levels ((1.0 2));
        }
	
        refinementBox_3
        {
            mode inside;
            levels ((1.0 3));
        }
    }


    // Mesh selection
    // ~~~~~~~~~~~~~~

    // After refinement patches get added for all refinementSurfaces and
    // all cells intersecting the surfaces get put into these patches. The
    // section reachable from the locationInMesh is kept.
    // NOTE: This point should never be on a face, always inside a cell, even
    // after refinement.
    locationInMesh (-0.001 -0.001 0.001);


    // Whether any faceZones (as specified in the refinementSurfaces)
    // are only on the boundary of corresponding cellZones or also allow
    // free-standing zone faces. Not used if there are no faceZones.
    allowFreeStandingZoneFaces true;
}

// Settings for the snapping.
snapControls
{
    //- Number of patch smoothing iterations before finding correspondence
    //  to surface
    nSmoothPatch 3;

    //- Relative distance for points to be attracted by surface feature point
    //  or edge. True distance is this factor times local
    //  maximum edge length.
    tolerance 2.0;

    //- Number of mesh displacement relaxation iterations.
    nSolveIter 30;

    //- Maximum number of snapping relaxation iterations. Should stop
    //  before upon reaching a correct mesh.
    nRelaxIter 5;

    // Feature snapping

        //- Number of feature edge snapping iterations.
        //  Leave out altogether to disable.
        nFeatureSnapIter 10;

        //- Detect (geometric only) features by sampling the surface
        //  (default=false).
        implicitFeatureSnap false;

        //- Use castellatedMeshControls::features (default = true)
        explicitFeatureSnap true;

        //- Detect points on multiple surfaces (only for explicitFeatureSnap)
        multiRegionFeatureSnap false;
}



// Settings for the layer addition.
addLayersControls
{
    // Are the thickness parameters below relative to the undistorted
    // size of the refined cell outside layer (true) or absolute sizes (false).
    relativeSizes false;

    // Per final patch (so not geometry!) the layer information
    layers
    {
	    final_validation.stl
	    {
		nSurfaceLayers 10;
	    }
    }

    // Expansion factor for layer mesh
    expansionRatio 1.3;

    // Wanted thickness of final added cell layer. If multiple layers
    // is the thickness of the layer furthest away from the wall.
    // Relative to undistorted size of cell outside layer.
    // See relativeSizes parameter.
    finalLayerThickness 0.01524;

    // Minimum thickness of cell layer. If for any reason layer
    // cannot be above minThickness do not add layer.
    // Relative to undistorted size of cell outside layer.
    minThickness 0.00015;

    // If points get not extruded do nGrow layers of connected faces that are
    // also not grown. This helps convergence of the layer addition process
    // close to features.
    // Note: changed(corrected) w.r.t 17x! (didn't do anything in 17x)
    nGrow 0;

    // Advanced settings

    // When not to extrude surface. 0 is flat surface, 90 is when two faces
    // are perpendicular
    featureAngle 60;

    // At non-patched sides allow mesh to slip if extrusion direction makes
    // angle larger than slipFeatureAngle.
    slipFeatureAngle 30;

    // Maximum number of snapping relaxation iterations. Should stop
    // before upon reaching a correct mesh.
    nRelaxIter 3;

    // Number of smoothing iterations of surface normals
    nSmoothSurfaceNormals 1;

    // Number of smoothing iterations of interior mesh movement direction
    nSmoothNormals 3;

    // Smooth layer thickness over surface patches
    nSmoothThickness 10;

    // Stop layer growth on highly warped cells
    maxFaceThicknessRatio 0.5;

    // Reduce layer growth where ratio thickness to medial
    // distance is large
    maxThicknessToMedialRatio 0.3;

    // Angle used to pick up medial axis points
    // Note: changed(corrected) w.r.t 17x! 90 degrees corresponds to 130 in 17x.
    minMedianAxisAngle 90;


    // Create buffer region for new layer terminations
    nBufferCellsNoExtrude 0;


    // Overall max number of layer addition iterations. The mesher will exit
    // if it reaches this number of iterations; possibly with an illegal
    // mesh.
    nLayerIter 50;
}



// Generic mesh quality settings. At any undoable phase these determine
// where to undo.
meshQualityControls
{
    #include "meshQualityDict"


    // Advanced

    //- Number of error distribution iterations
    nSmoothScale 4;
    //- Amount to scale back displacement at error points
    errorReduction 0.75;
}


// Advanced

// Write flags
writeFlags
(
    scalarLevels
    layerSets
    layerFields     // write volScalarField for layer coverage
);


// Merge tolerance. Is fraction of overall bounding box of initial mesh.
// Note: the write tolerance needs to be higher than this.
mergeTolerance 1e-6;


// ************************************************************************* //

While I'm at it, here are my fvSchemes, fvSolution, controlDict files:

fvSchemes

/*--------------------------------*- C++ -*----------------------------------*\
| =========                 |                                                 |
| \\      /  F ield         | OpenFOAM: The Open Source CFD Toolbox           |
|  \\    /   O peration     | Version:  4.1                                   |
|   \\  /    A nd           | Web:      www.OpenFOAM.org                      |
|    \\/     M anipulation  |                                                 |
\*---------------------------------------------------------------------------*/
FoamFile
{
    version     2.0;
    format      ascii;
    class       dictionary;
    object      fvSchemes;
}
// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //

ddtSchemes
{
    default         steadyState;
}

gradSchemes
{
    default         Gauss linear;
    grad(U)         cellLimited Gauss linear 1;
}

divSchemes
{
    default         none;
    div(phi,U)      bounded Gauss linearUpwindV grad(U);
    div(phi,k)      bounded Gauss upwind;
    div(phi,epsilon)  bounded Gauss upwind;
    div((nuEff*dev2(T(grad(U))))) Gauss linear;
}

laplacianSchemes
{
    default         Gauss linear corrected;
}

interpolationSchemes
{
    default         linear;
}

snGradSchemes
{
    default         corrected;
}

wallDist
{
    method meshWave;
}


// ************************************************************************* //

fvSolution

/*--------------------------------*- C++ -*----------------------------------*\
| =========                 |                                                 |
| \\      /  F ield         | OpenFOAM: The Open Source CFD Toolbox           |
|  \\    /   O peration     | Version:  4.1                                   |
|   \\  /    A nd           | Web:      www.OpenFOAM.org                      |
|    \\/     M anipulation  |                                                 |
\*---------------------------------------------------------------------------*/
FoamFile
{
    version     2.0;
    format      ascii;
    class       dictionary;
    object      fvSolution;
}
// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //

solvers
{
    p
    {
        solver           GAMG;
        tolerance        1e-7;
        relTol           0.01;
        smoother         GaussSeidel;
    }

    U
    {
        solver           smoothSolver;
        smoother         GaussSeidel;
        tolerance        1e-8;
        relTol           0.1;
        nSweeps          1;
    }

    k
    {
        solver           smoothSolver;
        smoother         GaussSeidel;
        tolerance        1e-8;
        relTol           0.1;
        nSweeps          1;
    }

    epsilon
    {
        solver           smoothSolver;
        smoother         GaussSeidel;
        tolerance        1e-8;
        relTol           0.1;
        nSweeps          1;
    }
}

SIMPLE
{
    nNonOrthogonalCorrectors 3;
    consistent yes;
}

relaxationFactors
{
    equations
    {
        U               0.9;
        k               0.9;
        epsilon           0.9;
    }
}

cache
{
    grad(U);
}

// ************************************************************************* //

controlDict

/*--------------------------------*- C++ -*----------------------------------*\
| =========                 |                                                 |
| \\      /  F ield         | OpenFOAM: The Open Source CFD Toolbox           |
|  \\    /   O peration     | Version:  3.0.1                                 |
|   \\  /    A nd           | Web:      www.OpenFOAM.org                      |
|    \\/     M anipulation  |                                                 |
\*---------------------------------------------------------------------------*/
FoamFile
{
    version     2.0;
    format      ascii;
    class       dictionary;
    location    "system";
    object      controlDict;
}
// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //

libs
(
    "libOpenFOAM.so"
    "libincompressibleTurbulenceModel.so"
    "libincompressibleRASModels.so"
);

application     simpleFoam;

startFrom       latestTime;

startTime       0;

stopAt          endTime;

endTime         10000;

deltaT          1;

writeControl    timeStep;

writeInterval   1000;

purgeWrite      0;

writeFormat     ascii;

writePrecision  6;

writeCompression off;

timeFormat      general;

timePrecision   6;

runTimeModifiable true;

functions
{
    #include "forceCoeffs"
}
// ************************************************************************* //

[Arzed23]

Thank you very much for the response Arjun, I'm sorry but I don't think I quite understand what you mean by versions, do you mean comparing his simulation to mine?

Cheers,

Ben

[Arzed23]

After A LOT of mesh mainpulation, I've managed to get a decent validation going on with the k Omega SST model: http://imgur.com/MWvnH4Z

An example of how the residuals look like is as follows: http://imgur.com/O9nCHP1

The residuals seem okay asides from the pressure one which in spite of going down, heavily fluctuates; this I'm unsure how to remedy.

Pressure residual aside, could there be anything else causing the model to over-predict the lift coefficients past the actual stall point? Thanks!