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-   -   [snappyHexMesh] Aiming for a coarser mesh (https://www.cfd-online.com/Forums/openfoam-meshing/219509-aiming-coarser-mesh.html)

Sorabh July 30, 2019 02:12

Aiming for a coarser mesh
 
Hello,
I am aiming at a coarser mesh than what I have right now, in order to use the mppicDyMFoam which demands that the mesh size be larger than the particle size.

While running the snappyHexMesh, I get the following information:
Code:

Checking faces in error :
    non-orthogonality > 65  degrees                        : 0
    faces with face pyramid volume < 1e-09                : 634018
    faces with face-decomposition tet quality < -1        : 0
    faces with concavity > 80  degrees                    : 0
    faces with skewness > 4  (internal) or 20  (boundary) : 0
    faces with interpolation weights (0..1)  < 0.05        : 0
    faces with volume ratio of neighbour cells < 0.01      : 0
    faces with face twist < 0.01                          : 0
    faces on cells with determinant < 0.001                : 0

This line is what concerns me:
Code:

faces with face pyramid volume < 1e-09                : 634018
How do I reduce this many invalid faces to zero? And what is the method to make the mesh coarser?

This is my snappyHexMeshDict:

Code:

/*--------------------------------*- C++ -*----------------------------------*\
| =========                |                                                |
| \\      /  F ield        | OpenFOAM: The Open Source CFD Toolbox          |
|  \\    /  O peration    | Version:  3.0.x                                |
|  \\  /    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
{
    innerCylinder.stl
    {
        type        triSurfaceMesh;
        name        innerCylinder;
        regions
        {
            ascii
            {
                name      innerCylinder;
            }
        }
    }
    innerCylinderSmall.stl
    {
        type        triSurfaceMesh;
        name        innerCylinderSmall;
        regions
        {
            ascii
            {
                name      innerCylinderSmall;
            }
        }
    }
    outerCylinder.stl
    {
        type        triSurfaceMesh;
        name        outerCylinder;
        regions
        {
            ascii
            {
                name      outerCylinder;
            }
        }
    }
    propellerTip.stl
    {
        type        triSurfaceMesh;
        name        propellerTip;
        regions
        {
            ascii
            {
                name      propellerTip;
            }
        }
    }
    propellerStem1.stl
    {
        type        triSurfaceMesh;
        name        propellerStem1;
        regions
        {
            ascii
            {
                name      propellerStem1;
            }
        }
    }
    propellerStem2.stl
    {
        type        triSurfaceMesh;
        name        propellerStem2;
        regions
        {
            ascii
            {
                name      propellerStem2;
            }
        }
    }
    propellerStem3.stl
    {
        type        triSurfaceMesh;
        name        propellerStem3;
        regions
        {
            ascii
            {
                name      propellerStem3;
            }
        }
    }
};



// 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 1000000;

    // 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;

    // 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 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        "innerCylinderSmall.eMesh";
            level      4;
        }
        {
            file        "outerCylinder.eMesh";
            level      0;
        }
        {
            file        "propellerTip.eMesh";
            level      4;
        }
//        {
//            file        "propellerStem1.eMesh";
//            level      4;
//        }
        {
            file        "propellerStem2.eMesh";
            level      4;
        }
        {
            file        "propellerStem3.eMesh";
            level      4;
        }
    );



    // 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
    {
        innerCylinderSmall
        {
            level      (4 4);

            faceType    boundary;
            cellZone    innerCylinderSmall;
            faceZone    innerCylinderSmall;
            cellZoneInside  inside;
        }
        outerCylinder
        {
            level      (0 0);
        }
        propellerTip
        {
            level      (4 5);
        }
        propellerStem1
        {
            level      (4 4);
        }
        propellerStem2
        {
            level      (4 4);
        }
        propellerStem3
        {
            level      (4 4);
        }
    }

    // Resolve sharp angles
    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
    {
        innerCylinder
        {
            mode        inside;
            levels      ((1E15 3));
        }
        innerCylinderSmall
        {
            mode        inside;
            levels      ((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 (0.05 0.05 0.03);


    // 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 false;
}



// 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; // 1.0;

    //- Number of mesh displacement relaxation iterations.
    nSolveIter 300;

    //- 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 true;

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

        // Detect features between multiple surfaces
        // (only for explicitFeatureSnap, default = false)
        multiRegionFeatureSnap true;
}



// 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
    {
    }

    // Expansion factor for layer mesh
    expansionRatio 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.
    // See relativeSizes parameter.
    finalLayerThickness 0.3;

    // 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 0.1;

    // 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.
    // 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 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
{
    //- 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 pyramid volume. Is absolute volume of cell pyramid.
    //  Set to a sensible fraction of the smallest cell volume expected.
    //  Set to very negative number (e.g. -1E30) to disable.
    minVol 1e-9;//1e-13;

    //- 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 -1; // 1e-30;

    //- 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 0.01;

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

    //- minFaceWeight (0 -> 0.5)
    minFaceWeight 0.05;

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

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


    // Advanced

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

    // Optional : some meshing phases allow usage of relaxed rules.
    // See e.g. addLayersControls::nRelaxedIter.
    relaxed
    {
        //- Maximum non-orthogonality allowed. Set to 180 to disable.
        maxNonOrtho 75;
    }
}



// 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.

Carlo_P July 31, 2019 09:27

Hey Das,
Coarser Mesh:


the faster way to create a coarse mesh is to change you blockMeshDict.


In blockMeshDict, you have to find
blocks(hex (0 1 2 3 4 5 6 7) ($xcells $ycells $zcells) simpleGrading (n1 n2 n3));


where n1 n2 n3 are some number that indicate the division of the block.
If you reduce them, you will have a coarser mesh.




Reduce face with low volume:
This part is very tricky. Normally, it depends a lot on the quality of the .stl.
You can try to export in different quality, maybe also in .obj if you can.
Another way, but it doens't improve the quality of mesh, is to change the value in the meshQualityControls: instead of 1e-9, you can use e-15 or very negative one to disable the control.


It is not the best way! This doesn't change the quality of the mesh.


Cheers,
Carlo


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