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-   -   SnappyHexMesh refine but does not snap (https://www.cfd-online.com/Forums/openfoam-meshing-snappyhexmesh/110581-snappyhexmesh-refine-but-does-not-snap.html)

malaboss December 14, 2012 12:37

SnappyHexMesh refine but does not snap
 
1 Attachment(s)
Hi FOAMers,

I am trying tu study an air foil case in 2D.
The geometry is defined by an stl file.
To design the mesh I decided to use snappyHexMesh.

However when i run it, i get a refined geometry but not snapped.
I already saw few threads about it but no solution given what right for my case.

I use Open Foam 2.1.1
My background mesh size is closed to 1x1x1
I defined a refinement box around the air foil
I defined well the location in mesh

And i just don't know what i am doing wrong.

You can find enclosed my blockMeshDict and my snappyHexMeshDict, and a picture of the situation.

blockMeshDict
Code:

/*---------------------------------------------------------------------------*\
| =========                |                                                |
| \\      /  F ield        | OpenFOAM: The Open Source CFD Toolbox          |
|  \\    /  O peration    | Version:  1.0                                  |
|  \\  /    A nd          | Web:      http://www.openfoam.org              |
|    \\/    M anipulation  |                                                |
\*---------------------------------------------------------------------------*/

FoamFile
{
    version        2.0;
    format          ascii;

    root            "";
    case            "";
    instance        "";
    local          "";

    class          dictionary;
    object          blockMeshDict;
}

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


convertToMeters 1;

vertices       
(
    (-17000.0 3500  -5750.0)
    ( 8000.0 3500  -5750.0)
    ( 8000.0  6000  -5750.0)
    (-17000.0  6000  -5750.0)
    (-17000.0 3500  6750.0)
    ( 8000.0 3500  6750.0)
    ( 8000.0  6000  6750.0)
    (-17000.0  6000  6750.0)
);

blocks         
(
    hex (0 1 2 3 4 5 6 7) (10 1 5) simpleGrading (1 1 1)
);

edges         
(
);

patches       
(
    patch maxY
    (
        (3 7 6 2)
    )
    patch minX
    (
        (0 4 7 3)
    )
    patch maxX
    (
        (2 6 5 1)
    )
    patch minY
    (
        (1 5 4 0)
    )
    patch minZ
    (
        (0 3 2 1)
    )
    patch maxZ
    (
        (4 5 6 7)
    )
);

mergePatchPairs
(
);


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

snappyHexMeshDict
Code:

/*---------------------------------------------------------------------------*\
| =========                |                                                |
| \\      /  F ield        | OpenFOAM: The Open Source CFD Toolbox          |
|  \\    /  O peration    | Version:  1.0                                  |
|  \\  /    A nd          | Web:      http://www.openfoam.org              |
|    \\/    M anipulation  |                                                |
\*---------------------------------------------------------------------------*/

FoamFile
{
    version        2.0;
    format          ascii;

    root            "/home/penfold/mattijs/foam/mattijs2.1/run/icoFoam";
    case            "cavity";
    instance        "system";
    local          "";

    class          dictionary;
    object          autoHexMeshDict;
}

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

// Which of the steps to run
castellatedMesh true;  // true;
snap            true;  // true;
addLayers      true;  // 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
{
    surfacemesh.stlb
    {
        type triSurfaceMesh;
        name stlSurface;
    }

    refinementBox
    {
        type searchableBox;
        min (0 3425 0);
        max (2418.4 16500.4 2171.11);
    }
};



// Settings for the castellatedMesh generation.
castellatedMeshControls
{
        allowFreeStandingZoneFaces true;
    // Refinement parameters
    // ~~~~~~~~~~~~~~~~~~~~~

    // While refining maximum number of cells per processor. This is basically
    // the number of cells that fit on a processor. If you choose this too small
    // it will do just more refinement iterations to obtain a similar mesh.
    maxLocalCells 1000000;

    // 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 2000000;

    // 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;

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



    // 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 "someLine.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
    {
        stlSurface
        {
            // Surface-wise min and max refinement level
            level (2 4);  // 5 6
        }
    }

    // Resolve sharp angles
    resolveFeatureAngle 15;  // 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;  // inside;
            levels ((1.0e+15 4));  //1E15 4
        }
    }


    // 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 (-3023.55 4000 -103.606);
}



// Settings for the snapping.
snapControls
{
    //- Number of patch smoothing iterations before finding correspondence
    //  to surface
    nSmoothPatch 10;

    //- Relative distance for points to be attracted by surface feature point
    //  or edge. True distance is this factor times local
    //  maximum edge length.
    tolerance 1.0;

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

    //- Maximum number of snapping relaxation iterations. Should stop
    //  before upon reaching a correct mesh.
    nRelaxIter 10;
}



// Settings for the layer addition.
addLayersControls
{
    relativeSizes true;  //true;  // v 1.6
    // Per final patch (so not geometry!) the layer information
    layers
    {
        //minZ
        //{
        //    nSurfaceLayers 3;
        //}
        stlSurface_patch0  // a_surface_name
        {
            nSurfaceLayers 5;
        }
    }

    // Expansion factor for layer mesh
    expansionRatio 1.4;  // 1.0;

    //- 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.
    //finalLayerRatio FINALLAYERRATIO;  // 0.3;
    finalLayerThickness 0.6;  // 0.3;    // v 1.6

    //- 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 0.001;  // 0.1;

    //- 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.
    nGrow 1;  // 1;


    // Advanced settings

    //- When not to extrude surface. 0 is flat surface, 90 is when two faces
    //  make straight angle.
    featureAngle 179;  // 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
    minMedianAxisAngle 130;

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

    // Overall max number of layer addition iterations  // v 1.6
    nLayerIter 50;  // v 1.6
}



// Generic mesh quality settings. At any undoable phase these determine
// where to undo.
meshQualityControls
{
    //- Maximum non-orthogonality allowed. Set to 180 to disable.
    maxNonOrtho 65;

    //- Max skewness allowed. Set to <0 to disable.
    maxBoundarySkewness 20;
    maxInternalSkewness 4;

    //- Max concaveness allowed. Is angle (in degrees) below which concavity
    //  is allowed. 0 is straight face, <0 would be convex face.
    //  Set to 180 to disable.
    maxConcave 80;

    //- Minimum projected area v.s. actual area. Set to -1 to disable.
    minFlatness 0.5;

    //- Minimum pyramid volume. Is absolute volume of cell pyramid.
    //  Set to very negative number (e.g. -1E30) to disable.
    minVol 1e-13;

    //- Minimum face area. Set to <0 to disable.
    minArea -1;

    //- Minimum face twist. Set to <-1 to disable. dot product of face normal
    //- and face centre triangles normal
    minTwist 0.02;

    //- minimum normalised cell determinant
    //- 1 = hex, <= 0 = folded or flattened illegal cell
    minDeterminant 0.001;

    //- minFaceWeight (0 -> 0.5)
    minFaceWeight 0.02;

    //- minVolRatio (0 -> 1)
    minVolRatio 0.01;

    //must be >0 for Fluent compatibility
    minTriangleTwist -1;

        //- Minimum quality of the tet formed by the face-centre
    //  and variable base point minimum decomposition triangles and
    //  the cell centre. This has to be a positive number for tracking
    //  to work. Set to very negative number (e.g. -1E30) to
    //  disable.
    //    <0 = inside out tet,
    //      0 = flat tet
    //      1 = regular tet
    minTetQuality 1e-30;

    // Advanced

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


// 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;


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


5tern November 12, 2014 10:08

Same here
 
Hey,

did you find a solution then?
I have the same problem for my 2D-geometry.

Similar cases only give solutions for 3D (featured edge).

Thanks in advance

Tobi November 25, 2014 18:04

Hi Stern,
As you See from the date, it's a long time ago. On my homepage you find 2d mesh generation tutorials ...

Good luck

5tern December 9, 2014 04:23

Found Problem
 
Hello Tobias,

thank you for your reply. I found my .stl-file to be the problem, for I assumed to get the 3D-effect simply by extruding after snapping. But snappy already requires a volume mesh.

Tobi December 10, 2014 02:44

Yes sHM require a 3d mesh.
Where is the problem?

5tern December 10, 2014 06:27

Hello Tobi,

you're right and there was no problem except me forgetting to extrude the 2D-mesh BEFORE snapping :-) I just wrote it in case someone experiences the same "problem".

Tobi December 10, 2014 06:31

Quote:

Originally Posted by 5tern (Post 523263)
Hello Tobi,

you're right and there was no problem except me forgetting to extrude the 2D-mesh BEFORE snapping :-) I just wrote it in case someone experiences the same "problem".

Well you also can make the following steps:
  • castellatedMesh
  • snap the mesh
  • (flattenMesh [optional])
  • extrudeMesh by using front or back patch


That is my standard procedure if I want to generate normal 2d or rotational 2d meshes.
Never did extruding before snapping.


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