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SnappyHexMesh... Domain boundaries are uneven

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Old   January 20, 2016, 11:43
Default SnappyHexMesh... Domain boundaries are uneven
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Hello,

I'm working with a fairly simple geometry (kind of cubic). Since I only have square angles in my domain, I first thought I could avoid the snapping phase of the SnappyHexMesh utility. (Snapping takes a while and introduces all sorts of deformations, and I end up with a large amount of polyhedras).

The problem occurs when a boundary is refined to different levels over its surface.
Here's a screenshot attached. Look at the top surface, there's a discontinuity.

First question: is there a way to avoid that discontinuity without snapping ? Why can't SHM just conserve the part of domain that intersects the STL ?

Second question: Does that discontinuity in the mesh have a large impact on the solution (ex: strong local residuals) ?

Just tell me if you need more files/info. I am not familiar with posting.

Thank you
Fred




Code:
/*--------------------------------*- C++ -*----------------------------------*\
| =========                 |                                                 |
| \\      /  F ield         | OpenFOAM: The Open Source CFD Toolbox           |
|  \\    /   O peration     | Version:  2.4.0                                 |
|   \\  /    A nd           | Web:      www.OpenFOAM.org                      |
|    \\/     M anipulation  |                                                 |
\*---------------------------------------------------------------------------*/
FoamFile
{
    version     2.0;
    format      ascii;
    class       dictionary;
    object      snappyHexMeshDict;
}

// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //

geometrie_etudiee "PoulaillerVOL.stl";
geometrie_nom "PoulaillerVOL";
geometrie_extract "PoulaillerVOL.extendedFeatureEdgeMesh";
surface_raffinee "PoulaillerVOL";
surface1     "HX_ESA_1_IN";
surface1stl  "HX_ESA_1_IN.stl";
surface2     "HX_ESA_1_OUT";
surface2stl  "HX_ESA_1_OUT.stl";
surface3     "HX_ESA_2_IN";
surface3stl  "HX_ESA_2_IN.stl";
surface4     "HX_ESA_2_OUT";
surface4stl  "HX_ESA_2_OUT.stl";
surface5     "HX_ESA_1b";
surface5stl  "HX_ESA_1b.stl";
surface6     "HX_ESA_2b";
surface6stl  "HX_ESA_2b.stl";

//Comment concatonner en c++ ? surface1 = ''HX_ESA1_IN''&''HX_ESA2_IN''  ?

// Which of the steps to run
castellatedMesh true;
snap            false;
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
{
    $geometrie_etudiee
    {
        type triSurfaceMesh;
        name $geometrie_nom;
    }
    $surface1stl
    {
        type triSurfaceMesh;
        name $surface1;
    }
    $surface2stl
    {
        type triSurfaceMesh;
        name $surface2;
    }
    $surface3stl
    {
        type triSurfaceMesh;
        name $surface3;
    }
    $surface4stl
    {
        type triSurfaceMesh;
        name $surface4;
    }
    $surface5stl
    {
        type triSurfaceMesh;
        name $surface5;
    }
    $surface6stl
    {
        type triSurfaceMesh;
        name $surface6;
    }
    refinementBox1 //Bande pleine hauteur HX1 niveau 3)
    {
        type searchableBox;
        min (22.4 0 0);
        max (28.4 11 3);
    }
    refinementBox2 //Bande pleine hauteur HX2 niveau 3)
    {
        type searchableBox;
        min (48 0 0);
        max (54 11 3);
    }
    refinementBox3 //Bande top-tier HX1 niveau 3
    {
        type searchableBox;
        min (20 0 0);
        max (30 11 3);
    }
    refinementBox4 //Bande top-tier HX2 niveau 3
    {
        type searchableBox;
        min (46 0 0);
        max (56 11 3);
    }
    refinementBox5 //Zone entree/sortie niveau 5
    {
        type searchableBox;
        min (24.7 6.75 1.3);
        max (26 8 3);
    }
    refinementBox6 //Zone entree/sortie niveau 5
    {
        type searchableBox;
        min (50 6.75 1.3);
        max (51.5 8 3);
    }
    refinementBox7 //contour HX1 niveau 5 (17 janvier)
    {
        type searchableBox;
        min (24.5 7 1.5);
        max (26.2 11 2);
    }
    refinementBox8 //contour HX2 niveau 5 (17 janvier)
    {
        type searchableBox;
        min (50 7 1.5);
        max (51.6 11 2);
    }

//refinementSphere2
//    {
//        type searchableSphere;
//       centre (50.8 7.47 1.825);
//       radius 1;
//    }

};

//----------------------------------------------------------------------------------------//
// CASTELLING------------------------------------------------------------------------------//
//----------------------------------------------------------------------------------------//

// 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 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 1; //original = 1

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


    // 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 $geometrie_extract;
            level 0; //original = 0
        }
    );

    // 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
    {
        $geometrie_nom
        {
            // Surface-wise min and max refinement level
            level (1 1); //was (2 2)
        }
        $surface1
        {
            // Surface-wise min and max refinement level
            level (5 5); //was (2 2)
        }
        $surface2
        {
            // Surface-wise min and max refinement level
            level (5 5); //was (2 2)
        }
        $surface3
        {
            // Surface-wise min and max refinement level
            level (5 5); //was (2 2)
        }
        $surface4
        {
            // Surface-wise min and max refinement level
            level (5 5); //was (2 2)
        }
    }

    resolveFeatureAngle 95;   // Les arêtes ayant un angle supérieur seront raffiné au level max.


    // Region-wise refinement (OVERRIDES SURFACE-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
    {
/*        refinementBox3
        {
            mode inside;
            levels ((1E15 3));
        }
        refinementBox4
        {
            mode inside;
            levels ((1E15 3));
        }
      refinementBox5
        {
            mode inside;
            levels ((1E15 3));
        }
        refinementBox6
        {
            mode inside;
            levels ((1E15 3));
        }
*/
        refinementBox7
        {
            mode inside;
            levels ((1E15 3));
        }
        refinementBox8
        {
            mode inside;
            levels ((1E15 3));
        }

//box 3 et 4 : niveau 4
//box 5 et 6 : niveau 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.
    // This is an outside point locationInMesh (-0.033 -0.033 0.0033);
    locationInMesh (1 1 1); // 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 true;
}

//----------------------------------------------------------------------------------------//
// SNAPPING-------------------------------------------------------------------------------//
//----------------------------------------------------------------------------------------//

// Settings for the snapping.
snapControls
{
    //- Number of patch smoothing iterations before finding correspondence
    //  to surface
//snappywiki: This option changes how many times the exterior (the boundary wall) should be iterated. The more iterations, the smoother the mesh will become
    nSmoothPatch 3;  //original 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 1.0; //original 1

    //- Number of mesh displacement relaxation iterations.
//snappyWiki: Cette option change combien de fois le snapping des points aura lieu:
//- plus le nombre est élevé, meilleure sera la qualité du mesh
//- plus ce nombre est élevé, plus le mesh sera équidistant par rapport à la frontière
//- plus le nombre est élevé, plus l'étape du snapping sera longue
    nSolveIter 100;  //original 300

    //- Maximum number of snapping relaxation iterations. Should stop
    //  before upon reaching a correct mesh.
//SnappyWiki: This option changes how many times the mesh will run a relaxing script that removes some bad mesh point
    nRelaxIter 5;

    // Feature snapping

        //- Number of feature edge snapping iterations. "Morph iterations"
        //  Leave out altogether to disable. 
        nFeatureSnapIter 10;

        //- 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 true;
}

//----------------------------------------------------------------------------------------//
// ADD LAYER------------------------------------------------------------------------------//
//----------------------------------------------------------------------------------------//

// 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
    {
        "sphere.*"  //Ecrire ici le nom de la patch finale (ex:   "sphere.*"    )
        {
            nSurfaceLayers 5;  //Nombre de couches à ajouter pour la patch mentionnée
        }
    }

    // Expansion factor for layer mesh
    expansionRatio 1.1;


    // 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.
    // See relativeSizes parameter.
    minThickness 0.05;  //original 0.25

    // 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 (for adding layers)

    // When not to extrude surface. 0 is flat surface, 90 is when two faces
    // are perpendicular
    featureAngle 30;

    // Maximum number of snapping relaxation iterations. Should stop
    // before upon reaching a correct mesh. (Le maximum d'itérations 
    // n'est pas atteint si un mesh correcte est trouvé)
    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
    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;

    // 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 20;
}

//----------------------------------------------------------------------------------------//
// QUALITY CONTROLS-----------------------------------------------------------------------//
//----------------------------------------------------------------------------------------//

// Generic mesh quality settings. At any undoable phase these determine
// where to undo.
meshQualityControls
{
    #include "meshQualityDict"  //La plupart des parametres sont dans ce dictionnaire

    // 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 0;  //original 75
    }

    // Advanced

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

//----------------------------------------------------------------------------------------//
// AUTRES PARAMÈTRES----------------------------------------------------------------------//
//----------------------------------------------------------------------------------------//

// 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 question forum snapping.png (4.6 KB, 43 views)
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