[Starcatcher]

Hi folks,

i tried to use snappyHexMesh. Look how my boundary layer looks like. Lok at the corners. how can i repair this. what setting could be responsible for this?

Thank you


temp.jpg

[Vinzmann]

Hello starcatcher,

could you please post your sHMDict?

Best regards,
Vinz

[Starcatcher]

yes, sorry:

FoamFile
{
    version 2.0;
    format ascii;
    class dictionary;
    location system;
    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
{
    inlet.stl
    {
        type    triSurfaceMesh;
        name    inlet;
    }

    outlet1.stl
    {
        type    triSurfaceMesh;
        name    outlet1;
    }

    wand.stl 
    {
        type    triSurfaceMesh;
        name    wand;
    }
    

      
    

}

// 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 500000000;

    // 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 0;

    // 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 "inlet.eMesh";
            level 3;
        }
        {
            file "outlet1.eMesh";
            level 2;
        }

    {
        file "wand.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
    {
        inlet
        {
            level (2 2);
            patchInfo { type patch; }
        }

        outlet1
        {
            level (2 2);
            patchInfo { type patch; }
        }

        wand
        {
            level (2 2);
            patchInfo { type wall; }
        }

    }

    // Resolve sharp angles
    resolveFeatureAngle 60;


    // 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
    {
      refineBox1.stl{ mode inside; levels ((3 3)) ;}
      refineBox2.stl{ mode inside; levels ((3 3)) ;}
      refineBox3.stl{ mode inside; levels ((3 3)) ;}
      refineBox_out1.stl{ mode inside; levels ((3 3)) ;}
      
    //    refinement_begasung
    //    {
    //        mode inside;
    //        levels ((2.0 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 (0.5 0.5 0.5);


    // 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 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
    {
      wand{nSurfaceLayers 5;}      
    }

    // 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 0.6;

    // 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 0.01;

    // 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 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 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;


    nRelaxedIter 20;
    writeVTK false;
    noErrors false;
    layerRecovery 1;
    growZoneLayers false;
    projectGrownUp 0.0;
}



// 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-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 -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 0.05;

    // 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;

    //- If >0 : preserve single cells with all points on the surface if the
    //  resulting volume after snapping (by approximation) is larger than
    //  minVolCollapseRatio times old volume (i.e. not collapsed to flat cell).
    //  If <0 : delete always.
    // minVolCollapseRatio 0.5;

    // 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;
    }
}


// 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;

[Starcatcher]

this problem does not appear when i choose only one face. In this case i chose all the four asimuthal faces

[Starcatcher]

Here, i have a more simple geometry of a block. as you can see the lower and left wall are assigned with a layer. this works ok on the outer parts of the walls but look terrible, where these two walls meet.
Untitled.jpg

[AtoHM]

First thing to try: look into the parameter 'featureAngle'. For perpendicular faces with merged layers, it should be > 90 deg.


Layer generation is sometimes very tricky with snappyHexMesh. There are numerous threads here on the forum where you can find recommendations on resolving issues. There is no a single set of parameters that works well for all geometries.