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-   -   [snappyHexMesh] non uniform mesh near the stl object (https://www.cfd-online.com/Forums/openfoam-meshing/233526-non-uniform-mesh-near-stl-object.html)

vava10 January 31, 2021 15:41

non uniform mesh near the stl object
 
2 Attachment(s)
Hey,

I wanted to create a uniform mesh around the kayak. Since there is an air-water interface in my case I used topoSet and refineMesh (twice) at the interface to get a gradual change to a finer mesh from the first course mesh generated using blockMesh. Then I use snappyHexMesh to generate a mesh around the stl object. But I am unable to get a uniform mesh around the object. I will attach the codes which I used for the meshing I am using OpenFOAM for the first time and I wanted to create a uniform mesh around the kayak. Since there is an air-water interface in my case I used topoSet and refineMesh (twice) at the interface to get a gradual change to a finer mesh from the first course mesh generated using blockMesh. Then I use snappyHexMesh to generate a mesh around the stl object. But I am unable to get a uniform mesh around the object. I will attach the codes which I used for the meshing along with the resultant mesh I got. I am not that experienced in the meshing. Can anyone suggest me how to get a uniform mesh and why I am not getting a uniform mesh around the object

blockMesh

Code:


/*--------------------------------*- C++ -*----------------------------------*\
| =========                |                                                |
| \\      /  F ield        | OpenFOAM: The Open Source CFD Toolbox          |
|  \\    /  O peration    | Version:  v2012                                |
|  \\  /    A nd          | Website:  www.openfoam.com                      |
|    \\/    M anipulation  |                                                |
\*---------------------------------------------------------------------------*/
FoamFile
{
    version    2.0;
    format      ascii;
    class      dictionary;
    object      blockMeshDict;
}
// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //

scale  1;

vertices
(
    (-10.4 0 -7.8)//0
    (10.4 0 -7.8)//1
    (-10.4 3.4 -7.8)//2
    (10.4 3.4 -7.8)//3
       
    (-10.4 0 0)//4
    (10.4 0 0)//5
    (-10.4 3.4 0)//6
    (10.4 3.4 0)//7
       
    (-10.4 0 5.2)//8
    (10.4 0 5.2)//9
    (-10.4 3.4 5.2)//10
    (10.4 3.4 5.2)//11
 
);

blocks
(
    hex (0 1 3 2 4 5 7 6)(70 11 26) simpleGrading (1 1 1)
    hex (4 5 7 6 8 9 11 10)(70 11 17) simpleGrading (1 1 1)
);

edges
(
);

boundary
(
    atmosphere
    {
        type patch;
        faces
        (
            (8 9 11 10)
        );
    }
    outlet
    {
        type patch;
        faces
        (
            (0 4 6 2)
            (4 8 10 6)
           
        );
    }
    inlet
    {
        type patch;
        faces
        (
            (1 3 7 5)
            (5 7 11 9)
           
        );
    }
    /*bottom
    {
        type patch;
        faces
        (
            (0 2 3 1)
        );
    }*/
    side
    {
        type symmetry;
        faces
        (
            (0 2 3 1)
            (0 1 5 4)
            (4 5 9 8)
            (2 6 7 3)
            (6 10 11 7)
        );
    }
   
);

mergePatchPairs
(
);

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


toposet.1

Code:


/*--------------------------------*- C++ -*----------------------------------*\
| =========                |                                                |
| \\      /  F ield        | OpenFOAM: The Open Source CFD Toolbox          |
|  \\    /  O peration    | Version:  v2012                                |
|  \\  /    A nd          | Website:  www.openfoam.com                      |
|    \\/    M anipulation  |                                                |
\*---------------------------------------------------------------------------*/
FoamFile
{
    version    2.0;
    format      ascii;
    class      dictionary;
    location    "system";
    object      topoSetDict;
}
// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //

actions
(
    {
        name    c0;
        type    cellSet;
        action  new;
        source  boxToCell;
        box    (-10.4 0 -2.0) (10.4 3.4 2.0);
    }
);

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

toposet.2

Code:

/*--------------------------------*- C++ -*----------------------------------*\
| =========                |                                                |
| \\      /  F ield        | OpenFOAM: The Open Source CFD Toolbox          |
|  \\    /  O peration    | Version:  v2012                                |
|  \\  /    A nd          | Website:  www.openfoam.com                      |
|    \\/    M anipulation  |                                                |
\*---------------------------------------------------------------------------*/
FoamFile
{
    version    2.0;
    format      ascii;
    class      dictionary;
    location    "system";
    object      topoSetDict;
}
// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //

actions
(
    {
        name    c0;
        type    cellSet;
        action  new;
        source  boxToCell;
        box    (-10.4 0 -1.0) (10.4 3.4 1.0);
    }
);

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

refineMEsh

Code:

/*--------------------------------*- C++ -*----------------------------------*\
| =========                |                                                |
| \\      /  F ield        | OpenFOAM: The Open Source CFD Toolbox          |
|  \\    /  O peration    | Version:  v2012                                |
|  \\  /    A nd          | Website:  www.openfoam.com                      |
|    \\/    M anipulation  |                                                |
\*---------------------------------------------------------------------------*/
FoamFile
{
    version    2.0;
    format      ascii;
    class      dictionary;
    location    "system";
    object      refineMeshDict;
}
// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //

set            c0;

coordinateSystem global;

globalCoeffs
{
    tan1            (1 0 0);
    tan2            (0 1 0);
}

patchLocalCoeffs
{
    patch          outside;
    tan1            (1 0 0);
}

directions      ( tan1 tan2 normal );

useHexTopology  yes;

geometricCut    no;

writeMesh      no;


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


snappyHexMesh
Code:

/*--------------------------------*- C++ -*----------------------------------*\
| =========                |                                                |
| \\      /  F ield        | OpenFOAM: The Open Source CFD Toolbox          |
|  \\    /  O peration    | Version:  v2012                                |
|  \\  /    A nd          | Website:  www.openfoam.com                      |
|    \\/    M anipulation  |                                                |
\*---------------------------------------------------------------------------*/
FoamFile
{
    version    2.0;
    format      ascii;
    class      dictionary;
    object      snappyHexMeshDict;
}
// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //

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


// 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
{
    kayak.stl
    {
        type triSurfaceMesh;
        name hull;

        patchInfo
        {
            type wall;
        }
    }
    refinementBox
        {
                type searchableBox;
                min (-0.80403 0 -0.581307);
                max (5.76206 0.409332 0.612993);
               
        }
}


// Settings for the castellatedMesh 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 100000;

    // 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 8000000;

    // 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 0;

    // 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 "kayak.eMesh";
            level 2;
        }
    );



    // 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
    {
        hull
        {
            // Surface-wise min and max refinement level
            level (5 5);
        }
    }

    resolveFeatureAngle 30;


    // 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
        {
            mode inside;
            levels ((1e15 2));
        }
    }


    // 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 (-0.7 0 0);


    // 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 4.0;
    tolerance 1.0;

    //- Number of mesh displacement relaxation iterations.
    nSolveIter 100;

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

    nFeatureSnapIter 10;
}



// 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 true;

    // Per final patch (so not geometry!) the layer information
    layers
    {
        hull
        {
            nSurfaceLayers 6;

        }
    }

    // Expansion factor for layer mesh
    expansionRatio 1.2;


    // 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 0.7;

    // Minimum thickness of cell layer. If for any reason layer
    // cannot be above minThickness do not add layer.
    // See relativeSizes parameter.
    minThickness 0.25;

    // 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 1.7.x! (didn't do anything in 1.7.x)
    nGrow 0;


    // Advanced settings

    // When not to extrude surface. 0 is flat surface, 90 is when two faces
    // are perpendicular
    featureAngle 180;

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

    // 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 1.7.x! 90 degrees corresponds to 130
    // in 1.7.x.
    minMedialAxisAngle 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;

    // Max number of iterations after which relaxed meshQuality controls
    // get used. Up to nRelaxIter it uses the settings in meshQualityControls,
    // after nRelaxIter it uses the values in meshQualityControls::relaxed.
    nRelaxedIter 20;
}



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


// Advanced

// Flags for optional output
// 0 : only write final meshes
// 1 : write intermediate meshes
// 2 : write volScalarField with cellLevel for postprocessing
// 4 : write current intersections as .obj files
debug 0;


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


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

Thank you in advance


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