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[snappyHexMesh] cracks running through mesh

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Old   July 31, 2020, 10:40
Default cracks running through mesh
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Georg Berghammer
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Hello,

I am new to OpenFoam and I am tiring to Perform an integration of an .stl geometry (created via FreeCAD) and a refinement box into my blockMesh via snapyHexMesh.

When do this I get strange cracks running through my mesh (See attached Image).

I guess it is a problem in my snappyHexMeshDict

I have tried to skip surfaceFeatureExtract and .emesh part.
- tried to use “admesh -fdv file.stl” on my geometry.
- tried using a super simple geometry.


Does someone know where my mistake could be?


snappyHexMeshDict
Code:
/*--------------------------------*- C++ -*----------------------------------*\

| =========                 |                                                 |

| \\      /  F ield         | OpenFOAM: The Open Source CFD Toolbox           |

|  \\    /   O peration     | Version:  4.x                                   |

|   \\  /    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       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

{

    Rohr.stl

    {

        type triSurfaceMesh;

        name Rohr;

    }

    ground.stl

    {

        type triSurfaceMesh;

        name ground;

    }

    boxRefinement1  // User defined region name 

    { 

        type   searchableBox;       // region defined by bounding box 

        min    (-0.2 -1.0 10.0); 

        max    (2.2 1.0 14.0); 

    } 

    boxRefinement2  // User defined region name 

    { 

        type   searchableBox;       // region defined by bounding box 

        min    (-0.3 -2.0 10); 

        max    (18.0 2.0 17.0); 

    } 

    boxRefinement3  // User defined region name 

    { 

        type   searchableBox;       // region defined by bounding box 

        min    (-1.0 -6.0 10.0); 

        max    (43.0 6.0 22.0); 

    } 

    

};







// 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 1000000000;



    // 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 2000000000;



    // 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 5;







    // 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 "Rohr.eMesh";

        level 7;

    }

    {

        file "ground.eMesh";

        level 1;

    }

    );







    // 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

    {

        Rohr

        {

            // Surface-wise min and max refinement level

            level (7 7);

        }

        ground

        {

            // Surface-wise min and max refinement level

            level (1 1);

        }            

    }



    // 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

    {

    boxRefinement1  // User defined region name 

    { 

     mode inside;

     levels ((1.0 6)); 

    } 

    boxRefinement2  // User defined region name 

    { 

     mode inside;

     levels ((1.0 4)); 

    } 

    boxRefinement3  // User defined region name 

    { 

     mode inside;

     levels ((1.0 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 (5.0001 3.0001 31.123);





    // 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 30;



    //- Maximum number of snapping relaxation iterations. Should stop

    //  before upon reaching a correct mesh.

    nRelaxIter 5;



    // 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 false;



        //- Use castellatedMeshControls::features (default = true)

        explicitFeatureSnap true;



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

        multiRegionFeatureSnap true;

}







// 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

    {

    }



    // Expansion factor for layer mesh

    expansionRatio 1.2;



    // 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 ;



    // 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 ;

    firstLayerThickness ;



    // 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 180;



    // At non-patched sides allow mesh to slip if extrusion direction makes

    // angle larger than slipFeatureAngle.

    slipFeatureAngle 30;



    // 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 90;





    // Create buffer region for new layer terminations

    nBufferCellsNoExtrude 1;





    // 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;



    //- 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;

}





// 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;





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

surfaceFeatureExtractDict

Code:
/*--------------------------------*- C++ -*----------------------------------*\

| =========                 |                                                 |

| \\      /  F ield         | OpenFOAM: The Open Source CFD Toolbox           |

|  \\    /   O peration     | Version:  4.x                                   |

|   \\  /    A nd           | Web:      www.OpenFOAM.org                      |

|    \\/     M anipulation  |                                                 |

\*---------------------------------------------------------------------------*/

FoamFile

{

    version     2.0;

    format      ascii;

    class       dictionary;

    object      surfaceFeatureExtractDict;

}

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

ground.stl

{

    // How to obtain raw features (extractFromFile || extractFromSurface)

    extractionMethod    extractFromSurface;



    extractFromSurfaceCoeffs

    {

        // Mark edges whose adjacent surface normals are at an angle less

        // than includedAngle as features

        // - 0  : selects no edges

        // - 180: selects all edges

        includedAngle   150;

    }



    subsetFeatures

    {

        // Keep nonManifold edges (edges with >2 connected faces)

        nonManifoldEdges       no;



        // Keep open edges (edges with 1 connected face)

        openEdges       yes;

    }





    // Write options



        // Write features to obj format for postprocessing

        writeObj                yes;

}



Rohr.stl

{

    // How to obtain raw features (extractFromFile || extractFromSurface)

    extractionMethod    extractFromSurface;



    extractFromSurfaceCoeffs

    {

        // Mark edges whose adjacent surface normals are at an angle less

        // than includedAngle as features

        // - 0  : selects no edges

        // - 180: selects all edges

        includedAngle   150;

    }



    subsetFeatures

    {

        // Keep nonManifold edges (edges with >2 connected faces)

        nonManifoldEdges       no;



        // Keep open edges (edges with 1 connected face)

        openEdges       yes;

    }





    // Write options



        // Write features to obj format for postprocessing

        writeObj                yes;

}







// ************************************************************************* //
Attached Images
File Type: png DN80_Vertikal_10m_100ms_geschlossen_Gitter..-run.png (139.8 KB, 31 views)
DeGeorg is offline   Reply With Quote

Old   July 31, 2020, 17:08
Default
  #2
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Bloerb will become famous soon enough
Are you refering to those lines running through your mesh? These are due to the fact that the mesh is not perfectly orthogonal with your slice. Shift the slice in z direction slightly and they should move. You can also check the crinkle clip option of the slice and you'll see that this is just paraview playing tricks on you.



Basically you are slightly cutting into the cells in front of this row of cells.
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