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[snappyHexMesh] How to use sHM and extrudeMesh for two-dimensional meshing

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Old   February 28, 2021, 16:46
Default How to use sHM and extrudeMesh for two-dimensional meshing
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Jared Bland
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Hello,


I'm trying to get a basic mesh working using snappyHexMesh by extending the pitzDaily tutorial into my own geometry, a simplified version of the car in the snappyHexMesh documentation. When I run sHM, the resulting mesh is two layers thick, and I'm not sure how to use sHM to generate only one cell thick meshes. When I attempted to use extrudeMesh, the resulting mesh is also two layers thick, and so I'm not entirely sure what it's doing. I've attempted it both with and without overwrite options. Additionally, I've attempted to play with the various levels, but either achieved the same thing, or a grossly distorted version of the geometry.



Here are some photos of the mesh. snappyHexMesh seems to perform the castellation fine as well as the snapping.






I've generated the background blockMesh, then the surfaceFeatureExtract, followed by snappyHexMesh and finally extrudeMesh.

Here is my code:



blockMeshDict

Code:
/*--------------------------------*- C++ -*----------------------------------*\
| =========                 |                                                 |
| \\      /  F ield         | OpenFOAM: The Open Source CFD Toolbox           |
|  \\    /   O peration     | Version:  v2006                                 |
|   \\  /    A nd           | Website:  www.openfoam.com                      |
|    \\/     M anipulation  |                                                 |
\*---------------------------------------------------------------------------*/
FoamFile
{
    version     2.0;
    format      ascii;
    class       dictionary;
    object      blockMeshDict;
}
// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //

// scale   0.001;

vertices
(    
    (0   0  0) // 0 = (xmin ymin zmin)
    (60  0  0) // 1 = (xmax ymin zmin)
    (60  1  0) // 2 = (xmax ymax zmin)
    (0   1  0) // 3 = (xmin ymax zmin)
    (0   0 40) // 4 = (xmin ymin zmax)
    (60  0 40) // 5 = (xmax ymin zmax)
    (60  1 40) // 6 = (xmax ymax zmax) 
    (0   1 40) // 7 = (xmin ymax zmax)
);



blocks
(
    hex (0 1 2 3 4 5 6 7) (60 1 40) simpleGrading (1 1 1)
);
// define the shape as one large cube with vertices 0 through 7
// Grading chooses number of slices in x, y, z, repsectively 
//// Here chosen s.t. it's evenly sliced into 1mm cubes
// simpleGrading controls the cell expansion reatios for each direction in the block
//// Expansion ratin enables the meh to be graded or refined in specified directs 

edges
(
);

boundary
(
    walls
    {
        type patch;
        faces
        (
            (0 4 7 3)
            (1 2 6 5)
            (0 1 5 4)
            (0 3 2 1)
            (3 7 6 2)
            (4 5 6 7)
        );
    }
);

// ************************************************************************* //
surfaceFeatureExtractDict

Code:
/*--------------------------------*- C++ -*----------------------------------*\
| =========                 |                                                 |
| \\      /  F ield         | OpenFOAM: The Open Source CFD Toolbox           |
|  \\    /   O peration     | Version:  v2006                                 |
|   \\  /    A nd           | Website:  www.openfoam.com                      |
|    \\/     M anipulation  |                                                 |
\*---------------------------------------------------------------------------*/
FoamFile
{
    version     2.0;
    format      ascii;
    class       dictionary;
    object      surfaceFeatureExtractDict;
}
// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //

// scale   0.001; 


volumeAscii.stl
{
  extractionMethod extractFromSurface;
  extractFromSurfaceCoeffs
  {
    includedAngle 180; 
  }
  subsetFeatures
  {
    nonManifoldEdges yes;
    openEdges        yes;
  }
  writeObj           yes;
}

inletAscii.stl
{
  extractionMethod extractFromSurface;
  extractFromSurfaceCoeffs
  {
    includedAngle 180; 
  }
  subsetFeatures
  {
    nonManifoldEdges yes;
    openEdges        yes;
  }
  writeObj           yes;
}

outletAscii.stl
{
  extractionMethod extractFromSurface;
  extractFromSurfaceCoeffs
  {
    includedAngle 180; 
  }
  subsetFeatures
  {
    nonManifoldEdges yes;
    openEdges        yes;
  }
  writeObj           yes;
}

topAndBottomAscii.stl
{
  extractionMethod extractFromSurface;
  extractFromSurfaceCoeffs
  {
    includedAngle 180; 
  }
  subsetFeatures
  {
    nonManifoldEdges yes;
    openEdges        yes;
  }
  writeObj           yes;
}

frontAscii.stl
{
  extractionMethod extractFromSurface;
  extractFromSurfaceCoeffs
  {
    includedAngle 180; 
  }
  subsetFeatures
  {
    nonManifoldEdges yes;
    openEdges        yes;
  }
  writeObj           yes;
}

backAscii.stl
{
  extractionMethod extractFromSurface;
  extractFromSurfaceCoeffs
  {
    includedAngle 180; 
  }
  subsetFeatures
  {
    nonManifoldEdges yes;
    openEdges        yes;
  }
  writeObj           yes;
}


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

snappyHexMeshDict
Code:
/*--------------------------------*- C++ -*----------------------------------*\
| =========                 |                                                 |
| \\      /  F ield         | OpenFOAM: The Open Source CFD Toolbox           |
|  \\    /   O peration     | Version:  v2006                                 |
|   \\  /    A nd           | Website:  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       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
{
    wing_5degrees.obj
    {
        type    triSurfaceMesh;
        name    wing;
    }

    refinementBox
    {
        type    box;
        min     (-1 -1 -1);
        max     ( 5  1  1);
    }
}


// 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 100;

    // Number of buffer layers between different levels.
    // 1 means normal 2:1 refinement restriction, larger means slower
    // refinement.
    nCellsBetweenLevels 6;



    // 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 ();



    // 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
    {
        wing
        {
            // Surface-wise min and max refinement level
            level (5 5);
        }
    }

    // Resolve sharp angles on fridges
    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;
            levels ((1e15 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.
    locationInMesh (-0.5 0 0);


    // 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 4.0;

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

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

    // Per final patch (so not geometry!) the layer information
    layers
    {
        wing
        {
            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 0.7;

    // 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 0.25;

    // 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.
    // Note: changed(corrected) w.r.t 1.7.x! (didn't do anything in 1.7.x)
    nGrow 0;

    // Advanced settings

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


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

extrudeMeshDict
Code:
/*--------------------------------*- C++ -*----------------------------------*\
| =========                 |                                                 |
| \\      /  F ield         | OpenFOAM: The Open Source CFD Toolbox           |
|  \\    /   O peration     | Version:  v2006                                 |
|   \\  /    A nd           | Website:  www.openfoam.com                      |
|    \\/     M anipulation  |                                                 |
\*---------------------------------------------------------------------------*/
FoamFile
{
    version     2.0;
    format      ascii;
    class       dictionary;
    object      extrudeMeshDict;
}
// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //

constructFrom    patch;
sourceCase     "/home/OpenFOAM/user-v2006/run/pitzDaily";
sourcePatches  (front);

// If construct from patch: patch to use for back (can be the same as sourcePatch)
exposedPatchName  back;

// Flip surface normals before usage.
// Valid only for extrude from surface or patch
flipNormals      false;

// Linear extrusion in point-normal direction
extrudeModel linearDirection;

 patchType     empty; 

nLayers             1;

expansionRatio    1.0;

linearDirectionCoeffs
{
  direction (0 1 0);
  thickness     1;
}

// Do front and back need to be merged?
// Usually only makes sense for 360 degree wedges
mergeFaces      false;

// Merge small edges. Fraction of the bounding box.
mergeTol            0;


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


Any advice would be greatly appreciated.
Attached Images
File Type: png extrudeMeshSide.png (38.1 KB, 70 views)
File Type: png extrudeMeshTop.png (6.4 KB, 70 views)
File Type: png extrudeMeshCloseup.png (68.8 KB, 80 views)
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