[pconen]

Dear Foamers,


i'm face a Problem using sHM.
My mesh is for a mixing process with very large but also many small cells. I recognized that there are Cells that are really strange. They seem to be really flat or nearly double lined as you can see in the picture. I would like to know why these cells are generated and how I can improve this.



Here you can see my snappyHexMeshDict:

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
{
    Mischstrecke.stl
    {
        type triSurfaceMesh;
    name Mischstrecke;

        regions
        {
            inlet_exhgas    { name inlet_exhgas; }
            inlet_air        { name inlet_air; }
            inlet_adblue    { name inlet_adblue; }
            outlet        { name outlet; } 
            wall        { name wall; }
            injector        { name injector; }
        }
    }
/*
    refinementBox //USER DEFINED REGION NAME
    {
    type searchableBox;            //REGION DEFINED BY BOUNDING BOX
    min (-0.16  0.16 -1.00);         //parameters
    max ( 0.16 -0.16 -1.30);
    }

*/
    //- Refine a bit extra around the small centre hole
    box
    {
    type searchableCylinder;
    point1 (0 0.012 -1.105);
    point2 (0 0.310 -1.105);
    radius 0.022;
/*
        type searchableBox;         //Sphere
    min (-0.160 -0.100 -1.150);    //centre (0 0 -1.136);
    max ( 0.160  0.310 -1.080);    //radius 0.300;
*/
    }

    cyla            //DynFaecher
    {
    type searchableCylinder;
    point1 (0 0 -1.150);
    point2 (0 0 -1.00);
    radius 0.165;
    }

    cylb            //StatFaecher; links
    {
    type searchableCylinder;
    point1 (0 0 -2.300);
    point2 (0 0 -2.000);
    radius 0.165;
    }

    cylc            //StatFaecher; rechts
    {
    type searchableCylinder;
    point1 (0 0 -2.750);
    point2 (0 0 -2.450);
    radius 0.165;
    }

    cyld            //Injektorkopf
    {
    type searchableCylinder;
    point1 (0 0 -1.140);
    point2 (0 0 -1.090);
    radius 0.012;
    }
};



// 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 2000000;

    // 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 3000000;

    // 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 25;

    // 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 "Mischstrecke.eMesh";
            level 3;
        }

    );



    // 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
    {
        Mischstrecke
        {
            // Surface-wise min and max refinement level
            level (0 0);
            regions 
            {
                inlet_exhgas           { level (0 0); patchInfo { type patch; }}
                inlet_air              { level (1 1); patchInfo { type patch; }}
                inlet_adblue         { level (2 2); patchInfo { type patch; }}
                outlet             { level (0 0); patchInfo { type patch; }}
                wall                { level (1 1); patchInfo { type wall; }}
        injector        { level (4 4); gapLevelIncrement 3; patchInfo { type wall; }}
        //- Optional increment (on top of max level) in small gaps
        
            }        
        }
    }

    // Resolve sharp angles
    resolveFeatureAngle 45;


    // 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
    {
         box
        {
            mode inside;
            levels ((1 1));    //levels ((1E15 6));
        }

         cyla
        {
            mode inside;
            levels ((2 2));    //levels ((1E15 6));
        }

         cylb
        {
            mode inside;
            levels ((2 2));    //levels ((1E15 6));
        }

         cylc
        {
            mode inside;
            levels ((2 2));    //levels ((1E15 6));
        }

         cyld
        {
            mode inside;
            levels ((4 4));    //levels ((1E15 6));
        }
    }


    // 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 0 -1.3);


    // 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 15;

    //- Relative distance for points to be attracted by surface feature point
    //  or edge. True distance is this factor times local
    //  maximum edge length.
    tolerance 1.2;

    //- Number of mesh displacement relaxation iterations.
    nSolveIter 100;        //30-300 (higher better, increasing time)

    //- Maximum number of snapping relaxation iterations. Should stop
    //  before upon reaching a correct mesh.
    nRelaxIter 10;        //5 or higher

    // Feature snapping

        //- Number of feature edge snapping iterations.
        //  Leave out altogether to disable.
        nFeatureSnapIter 20;    //10 or higher

        //- 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
    {
        inlet_exhgas
        {
            nSurfaceLayers 0;
        }
        inlet_air
        {
            nSurfaceLayers 0;
        }
        inlet_adblue
        {
            nSurfaceLayers 0;
        }
        outlet
        {
            nSurfaceLayers 0;
        }
        wall
        {
            nSurfaceLayers 0;
        }
        injector
        {
            nSurfaceLayers 0;
        }
    }

    // Expansion factor for layer mesh
    expansionRatio 1;//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 0.0005;

    // 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 0.00005;

    // 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 0;

    // Advanced settings

    // When not to extrude surface. 0 is flat surface, 90 is when two faces
    // are perpendicular
    featureAngle 180;//75

    // 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 8;// 3;

    // Number of smoothing iterations of surface normals
    nSmoothSurfaceNormals 6;// 1;

    // Number of smoothing iterations of interior mesh movement direction
    nSmoothNormals 8;// 3;

    // Smooth layer thickness over surface patches
    nSmoothThickness 10;

    // Stop layer growth on highly warped cells
    maxFaceThicknessRatio 0.8;// 0.5;

    // Reduce layer growth where ratio thickness to medial
    // distance is large
    maxThicknessToMedialRatio 0.6;// 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;
    nMedialAxisIter    4;


    // 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;

    nRelaxedIter 20;
}



// 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;
    //
    maxNonOrtho 75; //90;
    minVol 1e-13;
    minTetQuality 1e-15;
    maxConcave 80;
    minArea -1;
    maxInternalSkewness 4;    //4
    maxBoundarySkewness 20;    //20
    minFaceWeight 0.02;
    minVolRatio 0.01;
    minTwist 0.02;
    minTriangleTwist -1;
    minDeterminant 0.001;
    relaxed {}
}


// Advanced

// Write flags
writeFlags
(
//    scalarLevels    // write volScalarField with cellLevel for postprocessing    
//    layerSets       // write cellSets, faceSets of faces in layer
//    layerFields     // write volScalarField for layer coverage
);

// 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-7;

I hope I used tis forum in a correct way. If you need more information to help me, please let me know.

[prota]

Hello!
I faced with the same problem. It's just a paraview visualization issue. Try to check and uncheck 'decompose polyhedra' and 'crinkle slice' options and see not projection but actual cells.



Here I have default options: https://imgur.com/fq1cOS2
And here I enabled 'crinkle slice': https://imgur.com/quXszn3

I hope it's still relevant answer.