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Snapping not enough (even with surfaceFeatureExtract)

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Old   November 7, 2013, 18:59
Default Snapping not enough (even with surfaceFeatureExtract)
  #1
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Aron Lentsch
Join Date: Nov 2013
Posts: 1
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ironclad is on a distinguished road
Hello!

I working on my first sHM case, a 30 wedge of a Laval nozzle and I am stuck since days with getting the mesh properly snapped all the way to the edges. Even after disabling the mesh quality controls and changing the snap controls one-by-one, I fail to line up the mesh fully with the edges. I created an edge mesh with surfaceFeatureExtract and use the eMesh file.

As you can see on the attached images, the edges of the mesh remain wavy instead of being straight as those from the STL file. This waviness stems from the castellated mesh. But why is the mesh not fully snapped to the edges? I tried increasing nSolveIter up to 300 without any noticeable change.

I would be most grateful for any educated hint how I could progress my case.

Cheers!

snappyHexMeshDict:
Code:
/*--------------------------------*- C++ -*----------------------------------*\
| =========                 |                                                 |
| \\      /  F ield         | OpenFOAM: The Open Source CFD Toolbox           |
|  \\    /   O peration     | Version:  2.1.1                                 |
|   \\  /    A nd           | Web:      www.OpenFOAM.org                      |
|    \\/     M anipulation  |                                                 |
\*---------------------------------------------------------------------------*/
FoamFile
{
    version     2.0;
    format      ascii;
    class       dictionary;
    object      autoHexMeshDict;
}

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

// 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
{
    fluid.stl
    {
        type triSurfaceMesh;
        name fluid;
    }
    wall.stl
    {
        type triSurfaceMesh;
        name wall;
    }
};



// 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 0;
    minRefinementCells 10;

    // Number of buffer layers between different levels.
    // 1 means normal 2:1 refinement restriction, larger means slower
    // refinement.
    //nCellsBetweenLevels 1;
    nCellsBetweenLevels 3;

    // 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;



    // 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 "fluid.eMesh";
            level 0;
//            level 1;
//            level 5;  ... level defined the refinement of the mesh on edges
        }
    );



    // 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
    {
        fluid
        {
            // Surface-wise min and max refinement level
//            level (2 2);
//            level (1 1);  // version 1.3
//            level (4 4);  // ... the more often the refinement the better the snapping
//            level (0 1);
            level (1 1);
        }
    }

    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
    {
//        refineHole
//        {
//            mode inside;
//            levels ((1E15 3));
//        }
        wall
        {
            mode distance;
            levels ((0.002 2));
        }

    }


    // 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 (0.0 0.0 0.01); // 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;
}



// Settings for the snapping.
snapControls
{
    //- Number of patch smoothing iterations before finding correspondence
    //  to surface
    nSmoothPatch 3;
    //nSmoothPatch 10; ... no difference

    //- Relative distance for points to be attracted by surface feature point
    //  or edge. True distance is this factor times local
    //  maximum edge length.
    tolerance 4.0;
    //tolerance 4.0; // ... no change -> see bad_snapping.png
    //tolerance 0.1;  //... much worst !!!

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

    //- Maximum number of snapping relaxation iterations. Should stop
    //  before upon reaching a correct mesh.
    //nRelaxIter 16;
    //nRelaxIter 5;
    nRelaxIter 50;

    //- Highly experimental and wip: number of feature edge snapping
    //  iterations. Leave out altogether to disable.
    nFeatureSnapIter 10; // if not defined result is clearly worst
}



// 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
    {
        "30M_3Dwedge01"
        {
            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.
    //  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 0.001;

    //- 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
    //  make straight angle.
    featureAngle 30;

    //- 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
    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;
}



// 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;
    maxNonOrtho 180;

    //- Max skewness allowed. Set to <0 to disable.
    //maxBoundarySkewness 20;
    //maxInternalSkewness 4;
    maxBoundarySkewness -1;
    maxInternalSkewness -1;

    //- 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;
    maxConcave 180;

    //- 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;
    minVol 1e-30;

    //- 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;
    minTwist -2;

    //- 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;

    //- if >0 : preserve single cells with all points on the surface if the
    //  resulting volume after snapping (by approximation) is larger than
    //  minVolCollapseRatio times old volume (i.e. not collapsed to flat cell).
    //  If <0 : delete always.
    //minVolCollapseRatio 0.5;


    // Advanced

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



    // 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 75;
        maxNonOrtho 180;
    }
}


// Advanced

// Flags for optional output
// 0 : only write final meshes
// 1 : write intermediate meshes
// 2 : write volScalarField with cellLevel for postprocessing
// 4 : write current intersections as .obj files
debug 0;


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


// ************************************************************************* //
blockMeshDict:
Code:
/*--------------------------------*- C++ -*----------------------------------*\
| =========                 |                                                 |
| \\      /  F ield         | OpenFOAM: The Open Source CFD Toolbox           |
|  \\    /   O peration     | Version:  2.1.1                                 |
|   \\  /    A nd           | Web:      www.OpenFOAM.org                      |
|    \\/     M anipulation  |                                                 |
\*---------------------------------------------------------------------------*/
FoamFile
{
    version     2.0;
    format      ascii;
    class       dictionary;
    object      blockMeshDict;
}

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

convertToMeters 0.001;

// Max radius of wedge
//r   30;
r   31;

// Total length
// (throat is at coordinate 0/0/0)
lsub   118;
lsup   22;

// Wedge angle
adeg  31;
//adeg  35;

/* - - -  CALCULATIONS  - - - - - - - - - - - - - - - - - - - - - */

arad  #calc "degToRad($adeg/2.0)";

//s     #calc "$r*tan($arad)";
yp    #calc "$r*sin($arad)";
yn    #calc "-1.0*$yp";
//zr    #calc "$r*cos($arad)";
//z     #calc "$zr+$s*sin($arad)";
ln    #calc "-1.0*$lsub";

vertices
(
    ($ln    $yn   0)
    ($lsup  $yn   0)
    ($lsup  $yp   0)
    ($ln    $yp   0)
    ($ln    $yn   $r)
    ($lsup  $yn   $r)
    ($lsup  $yp   $r)
    ($ln    $yp   $r)
);

blocks
(
    hex (0 1 2 3 4 5 6 7) (40 12 24) simpleGrading (0.4 1 1)
);

edges
(
);

boundary
(
    frontAndBack
    {
        type patch;
        faces
        (
            (3 7 6 2)
            (1 5 4 0)
        );
    }
    inlet
    {
        type patch;
        faces
        (
            (0 4 7 3)
        );
    }
    outlet
    {
        type patch;
        faces
        (
            (2 6 5 1)
        );
    }
    lowerWall
    {
        type wall;
        faces
        (
            (0 3 2 1)
        );
    }
    upperWall
    {
        type patch;
        faces
        (
            (4 5 6 7)
        );
    }
);

// ************************************************************************* //
Attached Images
File Type: png no_good_snapping_01.png (10.0 KB, 139 views)
File Type: png no_good_snapping_02.png (3.7 KB, 134 views)
File Type: png no_good_snapping_03.png (5.1 KB, 143 views)
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Old   November 8, 2013, 03:58
Default
  #2
Member
 
phil
Join Date: Oct 2013
Posts: 36
Rep Power: 5
gelbebanane is on a distinguished road
Hey,
I had the same problem as u or lets say my problem slightly got better by adding following options in my sHMDict. You can find these options also referred in the original sHM Source Code Folder (in my case: /sw/OpenFOAM/OpenFOAM-2.2.x/applications/utilities/mesh/generation/snappyHexMesh/snappyHexMeshDict)

Code:
    // 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;
Try both, the explizit and implizit snapping feature. In my case i think the implizit is a tic better. Another way, but i do not know if it will work is, when you edit your blockMeshDict from your hexagonal setup to an cylindric geometry.

Hope this can help you

Greetings
sajjadli likes this.

Last edited by gelbebanane; November 8, 2013 at 05:24. Reason: blockMesh idea
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