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[snappyHexMesh] Edge Refinement

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Old   May 5, 2017, 17:12
Default Edge Refinement
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Franco
Join Date: Sep 2015
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Hi foamers,

I'm having hard times with SHM and extrudeMess, since I'm trying to create a 2D mesh from SHM with extrudeMesh. The mesh is fine (I don't get any error with checkMesh), however, the refinement seems not to work on the edges. As you can see from the images, there are some very sharp edges at the four corner of the mash. I improved the feature level, but I can't get rid of the problem. I'm quite sure that there is a problem with the introduction of the viscous layers (when the layers are not added, I don't get this error), however, I can't understand which parameter I should change in addLayersControls (I tried with featureAngle, without success). Could you please help me?

Thanks a lot!!

Franco

Code:
/*--------------------------------*- C++ -*----------------------------------*\
| =========                 |                                                 |
| \\      /  F ield         | OpenFOAM: The Open Source CFD Toolbox           |
|  \\    /   O peration     | Version:  4.1                                   |
|   \\  /    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       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
{
    Air.stl
    {
        type    triSurfaceMesh;
        name    Air;
    }
    Base1.stl
    {
        type    triSurfaceMesh;
        name    Base1;
    }
    Base2.stl
    {
        type    triSurfaceMesh;
        name    Base2;
    }
    Base3.stl
    {
        type    triSurfaceMesh;
        name    Base3;
    }
    Base4.stl
    {
        type    triSurfaceMesh;
        name    Base4;
    }
    Base5.stl
    {
        type    triSurfaceMesh;
        name    Base5;
    }
    Bottom.stl
    {
        type    triSurfaceMesh;
        name    Bottom;
    }
    Left.stl
    {
        type    triSurfaceMesh;
        name    Left;
    }
    Outlet.stl
    {
        type    triSurfaceMesh;
        name    Outlet;
    }
    RCF1.stl
    {
        type    triSurfaceMesh;
        name    RCF1;
    }
    RCF2.stl
    {
        type    triSurfaceMesh;
        name    RCF2;
    }
    RCF3.stl
    {
        type    triSurfaceMesh;
        name    RCF3;
    }
    RCF4.stl
    {
        type    triSurfaceMesh;
        name    RCF4;
    }
    Right.stl
    {
        type    triSurfaceMesh;
        name    Right;
    }
    Top.stl
    {
        type    triSurfaceMesh;
        name    Top;
    }

    refinementBox
    {
        type    searchableBox;
        min     (0.05 0    0);
        max     (0.64 0.15 0.01);
    }
};



// 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 2000000;

    // 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;

    // Number of buffer layers between different levels.
    // 1 means normal 2:1 refinement restriction, larger means slower
    // refinement.
    nCellsBetweenLevels 1;
    //maxLoadUnbalance 0.10;



    // 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 "Base1.eMesh"; level 5;} 
        {file "Base2.eMesh"; level 5;} 
        {file "Base3.eMesh"; level 5;} 
        {file "Base4.eMesh"; level 5;} 
        {file "Base5.eMesh"; level 5;} 
        {file "Bottom.eMesh"; level 5;} 
        {file "Left.eMesh"; level 5;} 
        {file "Outlet.eMesh"; level 5;} 
        {file "RCF1.eMesh"; level 5;} 
        {file "RCF2.eMesh"; level 5;} 
        {file "RCF3.eMesh"; level 5;} 
        {file "RCF4.eMesh"; level 5;} 
        {file "Right.eMesh"; level 5;} 
        {file "Top.eMesh"; level 5;} 
);



    // 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
    {
        Base1
        {
            // Surface-wise min and max refinement level
            level (4 5);
        }
        Base2
        {
            // Surface-wise min and max refinement level
            level (4 5);
        }
        Base3
        {
            // Surface-wise min and max refinement level
            level (4 5);
        }
        Base4
        {
            // Surface-wise min and max refinement level
            level (4 5);
        }
        Base5
        {
            // Surface-wise min and max refinement level
            level (4 5);
        }

        Bottom
        {
            // Surface-wise min and max refinement level
            level (4 5);
        }
        Outlet
        {
            // Surface-wise min and max refinement level
            level (4 5);
        }
        Top
        {
            // Surface-wise min and max refinement level
            level (4 5);
        }

        Left
        {
            // Surface-wise min and max refinement level
            level (5 5);
        }
        Right
        {
            // Surface-wise min and max refinement level
            level (5 5);
        }
        RCF1
        {
            // Surface-wise min and max refinement level
            level (5 6);
        }
        RCF2
        {
            // Surface-wise min and max refinement level
            level (5 6);
        }
        RCF3
        {
            // Surface-wise min and max refinement level
            level (5 6);
        }
        RCF4
        {
            // Surface-wise min and max refinement level
            level (5 6);
        }

    }

    // Resolve sharp angles on fridges
    resolveFeatureAngle 30; //was 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
    {
        
        Air
        {
            mode inside;
            levels ((1e15 5));
        }

        /*refinementBox
        //Air
        {
            mode inside;
            levels ((1e15 5));
        }*/
    }


    // 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 (0.1 0.1 0.005);


    // 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 300;

    //- Maximum number of snapping relaxation iterations. Should stop
    //  before upon reaching a correct mesh.
    nRelaxIter 5;
    nFeatureSnapIter 15; // default is 10
}



// 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
    {
        Left
        {
            nSurfaceLayers 3;
        }
        Right
        {
            nSurfaceLayers 3;
        }
        Base1
        {
            nSurfaceLayers 3;
        }
        Base2
        {
            nSurfaceLayers 3;
        }
        Base3
        {
            nSurfaceLayers 3;
        }
        Base4
        {
            nSurfaceLayers 3;
        }
        Base5
        {
            nSurfaceLayers 3;
        }

    }

    // Expansion factor for layer mesh
    expansionRatio 1.15;

    // 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 0.4;

    // 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.
    // See relativeSizes parameter.
    minThickness 0.2;

    // 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; // was 60

    // 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 16x! 90 degrees corresponds to 130 in 16x.
    minMedianAxisAngle 80; //was 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-13;

    //- Minimum quality of the tet formed by the face-centre
    //  and variable base point minimum decomposition triangles and
    //  the cell centre.  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;


    // Advanced

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


// Advanced

// 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 (12.8 KB, 65 views)
File Type: png 2.png (12.3 KB, 65 views)
File Type: png 3.png (18.3 KB, 54 views)
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Old   May 6, 2017, 17:18
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Franco
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The problemhas been quickly solved by changing the factors in the snapcontroldict.

BTW, the introduction of the viscous layers does not affect the sharp edges shown in the figures (when addlayers is false, the sharp edges do not go way).
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Old   December 2, 2017, 07:07
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Deep
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This looks interesting. I am facing the exact same problem with SHM. Could you discuss more about what factors you changed? Perhaps, share your new shm dict ?
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Old   December 2, 2017, 13:30
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Franco
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Dear Deep,

Unfortunately I'm not working anymore on this case and I do not remember which of the four factors I changed: I think it was tolerance or nSolveIter, but its a guess.

If that does not work, I would consider changing the size of the blocks in the blockMeshDict file (I would decrease their size and defining a low level of refinement for the surfaces, edges...).

Hope It helps,

Franco
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extrude mesh, extrude2dmesh 2d mesh, snapphhexmesh

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