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gaps for close surfaces- meshing with sHM

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Old   November 28, 2016, 03:10
Default gaps for close surfaces- meshing with sHM
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Jan Goebel
Join Date: Mar 2009
Location: Mannheim, Baden Wuerttemberg, Germany
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Dear community,

maybe someone of you had the same problem and can help me out. Copying the snappyHexmeshDicht from the sample cases and giving the first try on meshing some more complicated geometry, I'm facing problems with near surfaces producing small gaps in the mesh. Closing the gaps with
Code:
 gapLevel (2 0 8); gapMode mixed;
in refinementRegions closes the gaps, but results in faces with pyramid volumes and won' t fullfill the mesh quality criteria.
The same hapens, when adding
Code:
 gapLevelIncrement ...
to refinement surfaces. I even added a thin walled cone to this region in order to fill it with elelemts, but without success.

What am I doing wrong ? Is there a quick workaround for getting a good mesh ?

attached you can find some screenshots and here's my sHMdict- file:

Code:
/*--------------------------------*- C++ -*----------------------------------*\
| =========                 |                                                 |
| \\      /  F ield         | OpenFOAM: The Open Source CFD Toolbox           |
|  \\    /   O peration     | Version:  v1606+                                |
|   \\  /    A nd           | Web:      www.OpenFOAM.com                      |
|    \\/     M anipulation  |                                                 |
\*---------------------------------------------------------------------------*/
FoamFile
{
    version     2.0;
    format      ascii;
    class       dictionary;
    object      snappyHexMeshDict;
}
// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //

// Which of the steps to run
castellatedMesh true;
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
{
	inlet.stl {type triSurfaceMesh; name inlet;}
	outlet.stl {type triSurfaceMesh; name outlet;}
	symmetry.stl {type triSurfaceMesh; name symmetry;}
	visc_pipe_in.stl {type triSurfaceMesh; name visc_pipe_in;}
	visc_in_lower.stl {type triSurfaceMesh; name visc_in_lower;}
	visc_in_top.stl {type triSurfaceMesh; name visc_in_top;}
	visc_out_lower.stl {type triSurfaceMesh; name visc_out_lower;}
	visc_out_top.stl {type triSurfaceMesh; name visc_out_top;}
	volume.stl {type triSurfaceMesh; name volume;}
//	refinementBox_in_top {type searchableBox; min (-0.013159 0.0641 -0.01325); max (0.013159 0.0771497 0);}
//	refinementBox_in_lower {type searchableBox; min (-0.00755 0.0483 -0.00755); max (0.00755 0.066503 0);}
//	refinementBox_volume {type searchableBox; min (-0.01365 0.005 -0.01365); max (0.06 0.09025 4.369e-12);}
	cone {type searchableCone; point1 (0 0.0689 0); radius1 0.01365; innerRadius1 0.013344; point2 (0 0.0714 0); radius2 0.01365; innerRadius2 0.013344;}
}


// 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 5000000;

    // 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 300000000;

    // 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 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 "inlet.eMesh"; level 2;}
        {file "outlet.eMesh"; level 2;}
        {file "symmetry.eMesh"; level 2;}
        {file "visc_pipe_in.eMesh"; level 2;}
        {file "visc_in_lower.eMesh"; level 2;}
        {file "visc_out_lower.eMesh"; level 2;}
        {file "visc_in_top.eMesh"; level 2;}
        {file "visc_out_top.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 (0 0);}
	outlet {level (0 0);}
	symmetry {level (0 0);}
       	visc_pipe_in {level (0 2);}
       	visc_in_lower {level (3 3);}
       	visc_out_lower {level (1 2);}
       	visc_in_top {level (1 2);}
       	visc_out_top {level (1 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_in_top {mode inside; levels ((10000 0)); gapLevel (2 0 8); gapMode mixed;}
//		refinementBox_in_lower {mode outside; levels ((10000 0)); gapLevel (5 0 10); gapMode mixed;}
//   		cone {mode inside; levels ((0.0003 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 (0.025 0.07945 -0.0001); //inner coarse poor
    locationInMesh (0 0.0667 -1e-6);





    // 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 10; //3

    // Number of smoothing of internal points on refinement interfaces 
    nSmoothInternal     $nSmoothPatch; 

    //- Relative distance for points to be attracted by surface feature point
    //  or edge. True distance is this factor times local
    //  maximum edge length.
    tolerance 5.0; //4.0

    //- Number of mesh displacement relaxation iterations.
    nSolveIter 300; //30

    //- Maximum number of snapping relaxation iterations. Should stop
    //  before upon reaching a correct mesh.
    nRelaxIter 8;

    // 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 true;

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

        //- Detect points on multiple surfaces (only for explicitFeatureSnap)
        multiRegionFeatureSnap false; //false

	// When to run face splitting (never at first iteration, always 
	// at last iteration). Is interval. Default -1 (disabled) 
        // Recommendation: set to half the number of feature snap iterations 
//	nFaceSplitInterval  5; 	
}



// 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 false;

    // Per final patch (so not geometry!) the layer information
    layers
    {
	visc_in_lower
		{nSurfaceLayers 3;}
	visc_in_top
		{nSurfaceLayers 3;}
	visc_out_lower
		{nSurfaceLayers 3;}
	visc_out_top
		{nSurfaceLayers 3;}
	visc_pipe_in
		{nSurfaceLayers 3;}
    }

    // Expansion factor for layer mesh
    expansionRatio 1.3;

    // 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 0.000514098;
//    finalLayerThickness 0.00016;

    // 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 0.00018;
//    minThickness 0.00008;

    // 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 190;

    // At non-patched sides allow mesh to slip if extrusion direction makes
    // angle larger than slipFeatureAngle.
    slipFeatureAngle 75;

    // 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 130;


    // 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

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


// 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;


// ************************************************************************* //
Thank you in advance,

jango
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