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[snappyHexMesh] Layer coverage at 98% with collapses at corners

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Old   August 28, 2021, 06:48
Default Layer coverage at 98% with collapses at corners
  #1
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Hello,

I am trying to understand different entries in snappyHexMeshDict "addLayersControls" section.

So, I created a geometry with sharp corners (90 degree), and to increase the boundary layer coverage I did the following:
  1. changed featureAngle to be larger than 90
  2. reduced minThickness to be a very small value
  3. edited meshQualityDict's minVol to be 1e-30
  4. increased some iterations entries such as nRelaxIter, nLayerIter (honestly, not knowing what they do exactly).

I've managed to get a 98.1% coverage, due to some collapses in corners. while the layers were added in some corners successfully, other corners are collapsed completely, as shown in the attachments.

So why did snappyHexMesh failed to add layers in some corners when succeeded in adding layers for corners with same angle?

And one last question, when I try to increase the layers number nSurfaceLayers, the coverage percentage is reduced drastically (~ 70%), so my question is for OpenFOAM professionals who work with cases with y+ ~ 1. Is mesh preparation for such small y+ values possible using snappyHexMesh, and how to achieve it?




Code:
/*--------------------------------*- C++ -*----------------------------------*\
| =========                 |                                                 |
| \\      /  F ield         | OpenFOAM: The Open Source CFD Toolbox           |
|  \\    /   O peration     | Version:  4.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 false;
snap            false;
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
{
    regionsSTL.stl
    {
        type triSurfaceMesh;
        regions
        {

            inlet               { name inlet; }
            outlet              { name outlet; }
            walls               { name walls; }
        }
    }
};



// 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 200000000;

    // 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 100000000;

    // 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)

    // if you have less than 'minRefinementCells' cells to refine, stop refinement
    minRefinementCells 5;

    // 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 "regionsSTL.eMesh";
            level 1;
        }
    );



    // 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
    {
        regionsSTL.stl 
        {
            // Surface-wise min and max refinement level
            level (0 0);
            regions
            {
                inlet   { level (0 0); patchInfo { type patch; } }
                outlet   { level (0 0); patchInfo { type patch; } }

                walls { level (3 3); patchInfo { type walls;  } }
            }
        }
    }

    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
    {
    }


    // 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.
    // This is an outside point locationInMesh (-0.033 -0.033 0.0033);
    locationInMesh (0.291667 0.291667 0.291667); // Inside point

    // 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 5;

    //- Relative distance for points to be attracted by surface feature point
    //  or edge. True distance is this factor times local
    //  maximum edge length.
    tolerance 1.1;

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

    //- 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 8;

        //- Detect (geometric) features by sampling the surface
        implicitFeatureSnap false;

        //- Use castellatedMeshControls::features
        explicitFeatureSnap true;

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

    // Per final patch (so not geometry!) the layer information
    layers
    {
        walls
        {
            nSurfaceLayers 3;  
        }
    }

    // Expansion factor for layer mesh
    expansionRatio 1.4;


    // Wanted thickness of final added cell layer. If multiple layers
    // is the thickness of the layer furthest away from the walls.
    // See relativeSizes parameter.
    finalLayerThickness 0.3;

    // Minimum thickness of cell layer. If for any reason layer
    // cannot be above minThickness do not add layer.
    // See relativeSizes parameter.
    minThickness 0.00000001;

    // 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;


    // Advanced settings

    // When not to extrude surface. 0 is flat surface, 90 is when two faces
    // are perpendicular
    featureAngle 95;

    // Maximum number of snapping relaxation iterations. Should stop
    // before upon reaching a correct mesh.
    nRelaxIter 50;

    // Number of smoothing iterations of surface normals
    nSmoothSurfaceNormals 5;

    // Number of smoothing iterations of interior mesh movement direction
    nSmoothNormals 10;

    // Smooth layer thickness over surface patches
    nSmoothThickness 5;

    // 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 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 200;

    // 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 60;
}



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

    // 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 65;
    }

    // Advanced

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


// 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
);


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


// ************************************************************************* //
Attached Images
File Type: png 1.png (93.9 KB, 138 views)
File Type: png 2.png (109.1 KB, 122 views)
File Type: png 3.png (48.1 KB, 97 views)
File Type: png 4.png (51.6 KB, 86 views)
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Old   December 4, 2023, 20:04
Default
  #2
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John Doe
Join Date: Jun 2013
Posts: 4
Rep Power: 12
apek_great is on a distinguished road
Hi, I have the same problem. I am using snappyhexmesh with OF dev. I want to add layers (5 layers) around a cube that is mounted on a floor. However, the boundary layer has collapsed near the bottom edge of the cube attached on the floor (refer to the figures attached).



Anyone here have suggestions for improvement?


castellatedMesh true;
snap true;
addLayers true;

geometry
{
building
{
type triSurfaceMesh;
file "model.stl";
}
};

castellatedMeshControls
{
// Refinement parameters
// ~~~~~~~~~~~~~~~~~~~~~
maxLocalCells 100000;
maxGlobalCells 2000000;
minRefinementCells 10;
maxLoadUnbalance 0.10;
nCellsBetweenLevels 3;

// Explicit feature edge refinement
// ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
features
(
{
file "model.eMesh";
level 0;
}
);

// Surface based refinement
// ~~~~~~~~~~~~~~~~~~~~~~~~
refinementSurfaces
{
building
{
level (3 3);
}
}

resolveFeatureAngle 30;

// Region-wise refinement
// ~~~~~~~~~~~~~~~~~~~~~~
refinementRegions
{

}

// Mesh selection
// ~~~~~~~~~~~~~~
locationInMesh (0 1.2 0);
allowFreeStandingZoneFaces true;
}

snapControls
{
nSmoothPatch 3;
tolerance 2.0;
nSolveIter 30;
nRelaxIter 5;

// Feature snapping
nFeatureSnapIter 10;
implicitFeatureSnap false;
explicitFeatureSnap true;
multiRegionFeatureSnap false;
}

addLayersControls
{
relativeSizes false;

layers
{
"(building).*"
{
nSurfaceLayers 5;
}
"(floor).*"
{
nSurfaceLayers 5;
firstLayerThickness 0.005;
}
}
expansionRatio 1.1;
firstLayerThickness 0.0025;
minThickness 1e-16;
nGrow 0;

// Advanced settings
featureAngle 300;
slipFeatureAngle 30;
nRelaxIter 3;
nSmoothSurfaceNormals 1;
nSmoothNormals 3;
nSmoothThickness 10;
maxFaceThicknessRatio 0.5;
maxThicknessToMedialRatio 0.3;
minMedianAxisAngle 90;
nBufferCellsNoExtrude 0;
nLayerIter 50;
}

meshQualityControls
{
#include "meshQualityDict"
}

writeFlags
(
scalarLevels
layerSets
layerFields
);

mergeTolerance 1e-6;
Attached Images
File Type: jpg no of surface layer 2.jpg (96.3 KB, 14 views)
File Type: jpg layer collapse 2.jpg (163.3 KB, 14 views)
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