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[snappyHexMesh] SHM Bad quality snapping/terracing

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Old   April 20, 2022, 20:20
Default SHM Bad quality snapping/terracing
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Hello all, I am relatively new to OpenFoam, so I do apologise if this is an issue with an obvious solution.

I am currently trying to model a domain for flow, consisting of a top flow region, a bottom flow region and a cooling hole connecting the two, as shown in the screenshot (domain.png). Each region is composed of 6 surfaces, these being the sides, inlet and outlet, with either top or bottom depending on the region, and a "wall" surface connected to the hole surface.

So far, when I've tried to use snappyHexMesh to create a mesh for it, the castellation step seems fine with no errors, but after snapping I end up with a poor quality mesh where the hole surface appears to be composed of "terraces" with the smooth curve being split into cells on multiple levels (terracing.png). The mesh has many illegal faces and fails mesh checks, and during the meshing process the error message "Did no successfully snap mesh. Continuing to snap to resolve easy surfaces but the resulting mesh will not satisfy your quality constraints" is repeatedly displayed.

This issue seems to persist regardless of surface refinement level for the hole surface. Curiously, when I tried meshing the bottom region + hole surface the snapping seemed to take place successfully, and same with the top region + hole surface and the hole on its own, which made me think that it may have been a cell count limit issue, which I attempted to mitigate by massively increasing the parameters maxLocalCells and maxGlobalCells to no avail.

Following various suggestions from forum posts, I have also tried:
- decreasing the Snap controls tolerance
- Nsolveiter up to 300
- Using surfaceFeaturesDict
- resolveFeatureAngle down to 10
unfortunately to no effect.

I also attach my blockMeshDict and snappyHexMeshDict below

Code:
/*--------------------------------*- C++ -*----------------------------------*\
  =========                 |
  \\      /  F ield         | OpenFOAM: The Open Source CFD Toolbox
   \\    /   O peration     | Website:  https://openfoam.org
    \\  /    A nd           | Version:  9
     \\/     M anipulation  |
\*---------------------------------------------------------------------------*/
FoamFile
{
    format      ascii;
    class       dictionary;
    object      blockMeshDict;
}
// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //

scale 1;

vertices
(
    (0 0 0)
    (635 0 0)
    (635 199.435 0)
    (0 199.435 0)
    (0 0 200)
    (635 0 200)
    (635 199.435 200)
    (0 199.435 200)
);

blocks
(
    hex (0 1 2 3 4 5 6 7) (60 20 20) simpleGrading (1 1 1)
);

edges
(
);

boundary
(
);

// ************************************************************************* //
Code:
/*--------------------------------*- C++ -*----------------------------------*\
  =========                 |
  \\      /  F ield         | OpenFOAM: The Open Source CFD Toolbox
   \\    /   O peration     | Website:  https://openfoam.org
    \\  /    A nd           | Version:  9
     \\/     M anipulation  |
\*---------------------------------------------------------------------------*/
FoamFile
{
    format      ascii;
    class       dictionary;
    object      snappyHexMeshDict;
}
// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //

// Which of the steps to run
castellatedMesh false;
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
{
    bottom_bottom
    {
        type triSurfaceMesh;
        file "bottom_bottom.stl";
    }
    bottom_inlet
    {
        type triSurfaceMesh;
        file "bottom_inlet.stl";
    }
    bottom_outlet
    {
        type triSurfaceMesh;
        file "bottom_outlet.stl";
    }
    bottom_sides
    {
        type triSurfaceMesh;
        file "bottom_sides.stl";
    }
    bottom_wall
    {
        type triSurfaceMesh;
        file "bottom_wall.stl";
    }
    /*holetop
    {
        type triSurfaceMesh;
        file "hole_top.stl";
    }
    holebottom
    {
        type triSurfaceMesh;
        file "hole_bottom.stl";
    }*/
    holesurf
    {
        type triSurfaceMesh;
        file "newholesurf_notobo.stl";
    }
    top_inlet
    {
        type triSurfaceMesh;
        file "top_inlet.stl";
    }
    top_outlet
    {
        type triSurfaceMesh;
        file "top_outlet.stl";
    }
    top_top
    {
        type triSurfaceMesh;
        file "top_top.stl";
    }
    top_sides
    {
        type triSurfaceMesh;
        file "top_sides.stl";
    }
    top_wall
    {
        type triSurfaceMesh;
        file "top_wall.stl";
    }
};



// 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 100000000;

    // 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 2000000000;

    // 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;

    // 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 3;



    // Explicit feature edge refinement
    // ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

    // Specifies a level for any cell intersected by its edges.
    // This is a featureEdgeMesh, read from constant/geometry for now.
    features
    (
        {
            file "newholesurf_notobo.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
    {
            bottom_bottom
	    {
	    	level (1 1);
	    }
	    bottom_inlet
	    {
	    	level (1 1);
	    }
	    bottom_outlet
	    {
	    	level (1 1);
	    }
	    bottom_sides
	    {
	    	level (1 1);
	    }
	    bottom_wall
	    {
	    	level (2 2);
	    }
	    /*holetop
	    {
	    	level (2 2);
	    }
	    holebottom
	    {
	    	level (2 2);
	    }*/
	    holesurf
	    {
	    	level (3 3);
	    }
	    top_inlet
	    {
	    	level (1 1);
	    }
	    top_outlet
	    {
	    	level (1 1);
	    }
	    top_top
	    {
	    	level (1 1);
	    }
	    top_sides
	    {
	    	level (1 1);
	    }
	    top_wall
	    {
	    	level (2 2);
	    }
    }

    // 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
    {
        /*refinementBox
        {
            mode    inside;
            level   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 (300 69 100);


    // 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 15; //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; //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 20;

        //- Detect (geometric only) features by sampling the surface
        //  (default=false).
        implicitFeatureSnap true;

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

        //- 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 true;

    // Per final patch (so not geometry!) the layer information
    layers
    {
        "(lowerWall|motorBike).*"
        {
            nSurfaceLayers 1;
        }
    }

    // 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 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 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
{
    #include "meshQualityDict"
}


// 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;


// ************************************************************************* //
I can also upload the .stl files I've used in a followup post, but has anyone else had a similar issue or perhaps a solution? Thank you for your patience and kind help!
Attached Images
File Type: jpg domain.jpg (23.0 KB, 15 views)
File Type: jpg terracing.jpg (123.4 KB, 23 views)
File Type: jpg castellated.jpg (137.6 KB, 28 views)
File Type: jpg bottom+hole.jpg (146.2 KB, 32 views)
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