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[snappyHexMesh] Collapsing Layers

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Old   July 10, 2015, 12:17
Default Collapsing Layers
  #1
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Tim
Join Date: Mar 2015
Posts: 7
Rep Power: 9
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Hey everyone!

I'm currently working on a simulation of the wind flow over a suburban area. The simulation domain consists of a bigger lake with some islands in it. Of course I want to add layer to the ground surface. But as you can see in the attached images, layers are collapsing.

Now I already tried to modify the featureAngle setting but without success. I also tried the solution given in this thread. But the changeDirectory method hasn't worked as well.

I attached a second image showing the surface mesh of the ground surface and now I wonder if those cells are the reason for the layers to collapse. I am not that experienced with snappy so I would be more than happy for an advise or two

Quick second question: I have several .stl files for the forests in the computed area. I am going to write those areas to cellZones. So that I can later define them as a porous medium. Now if I mesh them relatively coarse in the refinement process, will the layer generation again look for those forest-defining stl's and refine the inside or not? Or are the layers even overriding the cellZones?

Thanks in advance,
Tim


Code:
 /*--------------------------------*- C++ -*----------------------------------*\
| =========                 |                                                 |
| \\      /  F ield         | OpenFOAM: The Open Source CFD Toolbox           |
|  \\    /   O peration     | Version:  2.2.2                                 |
|   \\  /    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 false;
snap            false;
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
{
    
    
    domain.stl
    {
        type triSurfaceMesh;
        name domain;
    }
    
    
    
        sueden  
    {
        type searchablePlane;
        planeType pointAndNormal;
        pointAndNormalDict    
            {
                basePoint (372424 5808836 0);    
                normalVector (0 -1 0);
            } 
    }
    
    osten  
    {
        type searchablePlane;
        planeType pointAndNormal;
        pointAndNormalDict    
            {
                basePoint (377998 5808836 0);    
                normalVector (1 0 0);
            } 
    }
    
    norden  
    {
        type searchablePlane;
        planeType pointAndNormal;
        pointAndNormalDict    
            {
                basePoint (372424 5813998 0);    
                normalVector (0 1 0);
            } 
    }
    
    westen  
    {
        type searchablePlane;
        planeType pointAndNormal;
        pointAndNormalDict    
            {
                basePoint (372424 5808836 0);    
                normalVector (-1 0 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 20000000;

    // 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 20000000;

    // 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 5;

    // 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 3;



    // 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
    {
          
    
    
    
    domain
    {
        level (2 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
    {
        
    
    
    

   }


    // 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 (374001.1 5812001.1 149.9);


    // 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 5;

    //- 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 40;

    //- Maximum number of snapping relaxation iterations. Should stop
    //  before upon reaching a correct mesh.
    nRelaxIter 10;

    // Feature snapping

        //- Number of feature edge snapping iterations.
        //  Leave out altogether to disable.
        nFeatureSnapIter 30;

        //- Detect (geometric only) features by sampling the surface
        //  (default=false).
        implicitFeatureSnap true;

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

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



// 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
    {
        domain
        {
            nSurfaceLayers 5;
        }
    
    
    // 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 (374001.1 5810001.1 299.9);


    // 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 5;

    //- 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 40;

    //- Maximum number of snapping relaxation iterations. Should stop
    //  before upon reaching a correct mesh.
    nRelaxIter 10;

    // Feature snapping

        //- Number of feature edge snapping iterations.
        //  Leave out altogether to disable.
        nFeatureSnapIter 30;

        //- Detect (geometric only) features by sampling the surface
        //  (default=false).
    }

    // 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.
    // is the thickness of the layer furthest away from the wall.
    // See relativeSizes parameter.
    finalLayerThickness 0.8;

    // 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 0.001;

    // 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.
    // Note: changed(corrected) w.r.t 17x! (didn't do anything in 17x)
    nGrow 1;

    // 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 1.0;

    // Reduce layer growth where ratio thickness to medial
    // distance is large
    maxThicknessToMedialRatio 1.0;

    // 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
{
    //- Maximum non-orthogonality allowed. Set to 180 to disable.
    maxNonOrtho 65;

    //- Max skewness allowed. Set to <0 to disable.
    maxBoundarySkewness 10;
    maxInternalSkewness 2;

    //- 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. This has to be a positive number for tracking
    //  to work. 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 0.02;

    //- minimum normalised cell determinant
    //- 1 = hex, <= 0 = folded or flattened illegal cell
    minDeterminant 0.001;

    //- minFaceWeight (0 -> 0.5)
    minFaceWeight 0.02;

    //- 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

// 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;


// ************************************************************************* //
Attached Images
File Type: jpg layer_1.jpg (46.8 KB, 561 views)
File Type: jpg layer_2.jpg (98.3 KB, 451 views)
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Old   July 13, 2015, 09:01
Default
  #2
Member
 
DanielP
Join Date: Jan 2015
Posts: 33
Rep Power: 9
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Hello Tim,

I notice that certains parameters that should be under the AddLayerControls dictionary are defined outside the dictionary. See below the parameters that should be moved. I have modified the sub-dictionary for AddLayersControls.

__________________________________________________ ___

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
{
domain
{
nSurfaceLayers 5;
}

// 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 (374001.1 5810001.1 299.9);


// 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;


// 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.
// is the thickness of the layer furthest away from the wall.
// See relativeSizes parameter.
finalLayerThickness 0.8;

// 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 0.001;

// 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.
// Note: changed(corrected) w.r.t 17x! (didn't do anything in 17x)
nGrow 1;

// 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 1.0;

// Reduce layer growth where ratio thickness to medial
// distance is large
maxThicknessToMedialRatio 1.0;

// 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;
}
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Old   July 13, 2015, 14:15
Default
  #3
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Tim
Join Date: Mar 2015
Posts: 7
Rep Power: 9
Tim_Simon is on a distinguished road
Hello Daniel,

thank you for your reply. I fixed the snappyHexMeshDict. Don't know how this happenend. The layer generation does work better now but unfortunatley layers are still collapsing over the domain. I have attached several images showing some of the problematic areas.

I'm trying my best to find parameters that suit my surface but to this point without success. It seems that the slightest change of topography (heights or depths) is causing massive trouble.

Kind regards,
Tim


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;
}
// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //

// 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
{
    domain.stl
    {
        type triSurfaceMesh;
        name domain;
    }

};



// 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 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 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
    (
        /*{
            
        }*/
    );



    // 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
    {
        domain
        {
            // Surface-wise min and max refinement level
            level (2 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
    {

    }


    // 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 (374001.1 5812001.1 199.1);


    // 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 false;
}



// 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
    {
        domain
        {
            nSurfaceLayers 12;
        }
    }

    // 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 0.5;

    // 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 0.001;

    // 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.
    // 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 30;

    // Number of smoothing iterations of surface normals
    nSmoothSurfaceNormals 10;

    // Number of smoothing iterations of interior mesh movement direction
    nSmoothNormals 30;

    // Smooth layer thickness over surface patches
    nSmoothThickness 50;

    // Stop layer growth on highly warped cells
    maxFaceThicknessRatio 1;

    // Reduce layer growth where ratio thickness to medial
    // distance is large
    maxThicknessToMedialRatio 1;

    // 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 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;


// ************************************************************************* //
Attached Images
File Type: jpg layer_3.jpg (42.6 KB, 205 views)
File Type: jpg layer_4.jpg (51.0 KB, 189 views)
File Type: jpg layer_5.jpg (58.6 KB, 201 views)
File Type: jpg layer_6.jpg (48.1 KB, 214 views)
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Old   July 13, 2015, 21:40
Default
  #4
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DanielP
Join Date: Jan 2015
Posts: 33
Rep Power: 9
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Hello Tim,

Regarding the boundary layers parameters.Note the following :

-relativeSizes: This parameter will control the finalLayerThickness parameter. By setting it to true, the finalLayerThickness is divided by the length of the patch, in other words the last boundary layer will be 0.5/patch lenght and not 0.5.

-To calculate a reasonable finalLayerThickness, take the length of one cubic element from your blockMesh mesh and divide by 2 power (refinement) or
length/2^2. This will give you an estimation of the length of the Snap mesh individual element. Take a percentage of this length (10-15%).

-Try lowering your expansionRatio to 1.00-1.08.

-The minMedianAxisAngle since very high. Try a lower angle, see 5 degree.
This will control the extrusion of the boundary layer when the Snap mesh is translated towards the center.

Also, try to generate 1 perfect boundary layer.

This link might be also useful:

http://www.openfoamwiki.net/index.php/SnappyHexMesh

Thanks,

Daniel
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Old   July 29, 2015, 10:16
Default
  #5
New Member
 
Tim
Join Date: Mar 2015
Posts: 7
Rep Power: 9
Tim_Simon is on a distinguished road
Hey Daniel, thanks again for your help.

As I understand the finalLayerThickness refers to the already refined cell size on that specific surface and not the initial cell size of blockMesh?

I have looked up the snappy tutorials you linked for me. I am currently using the '3d meshing of a helix with layer generation' as a template. The results I am getting are way better than before but not as good as they need to be.

I managed to generate that one perfect layer but with increasing the number of layers problems begin to show up again. I am using an expansionRatio of 1.0. I have attached images of the same location for 5, 10 and 15 layers. I need layers up to about 40-50 meters of my domain and at the current state my layer thickness is at 1m. Having an expansionRatio anything more than 1 seems to dramatically worsen the results.

I'm still trying to experiment with different settings in the Dict but it seems that I am not doing it right :/ Mesh generation from the attached pictures is made with the following settings:

Dict:

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;
}
// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //

// Which of the steps to run
castellatedMesh false;
snap            false;
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
{
    domainWoEascii.stl
    {
        type triSurfaceMesh;
        name domainWoE;
    }
    
    waldE.stl
    {
        type triSurfaceMesh;
        name waldE;
    }
    
    E_unten.stl
    {
        type triSurfaceMesh;
        name E_unten;
    }
    
    E_oben.stl
    {
        type triSurfaceMesh;
        name E_oben;
    }

};



// 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 10000000;

    // 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 200000000;

    // 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 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
    (
        /*{
            
        }*/
    );



    // 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
    {
        domainWoE
        {
            // Surface-wise min and max refinement level
            level (3 3);

        }
    
    waldE
        {
            // Surface-wise min and max refinement level
            level (3 3);

        }
    
    E_unten
        {
            // Surface-wise min and max refinement level
            level (3 3);
        faceZone E_unten;
        cellZone E_unten;
        zoneInside true;

        }
    
    E_oben
        {
            // Surface-wise min and max refinement level
            level (3 3);
        faceZone E_oben;
        cellZone E_oben;
        zoneInside true;

        }
    }

    // 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
    {
    
    E_unten
        {
        mode inside;
        levels ((3 3));
    }
    
    E_oben
        {
        mode inside;
        levels ((3 3));
    }

    }


    // 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 (374001.1 5812001.1 199.1);


    // 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 false;
}



// 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
    {

    waldE
        {
            nSurfaceLayers 10;
        finalLayerThickness 0.2;
        expansionRatio 1;
        }
    
    domainWoE
        {
            nSurfaceLayers 10;
        finalLayerThickness 0.2;
        expansionRatio 1;
        }
    }

    // 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 0.6;

    // 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 0.001;

    // 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.
    // 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 60;

    // Maximum number of snapping relaxation iterations. Should stop
    // before upon reaching a correct mesh.
    nRelaxIter 4;

    // Number of smoothing iterations of surface normals
    nSmoothSurfaceNormals 1;

    // Number of smoothing iterations of interior mesh movement direction
    nSmoothNormals 1;

    // Smooth layer thickness over surface patches
    nSmoothThickness 1;

    // Stop layer growth on highly warped cells
    maxFaceThicknessRatio 0.5;

    // Reduce layer growth where ratio thickness to medial
    // distance is large
    maxThicknessToMedialRatio 1;

    // Angle used to pick up medial axis points
    // Note: changed(corrected) w.r.t 17x! 90 degrees corresponds to 130 in 17x.
    minMedianAxisAngle 20;


    // 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 50;
}



// Generic mesh quality settings. At any undoable phase these determine
// where to undo.
meshQualityControls
{
    #include "meshQualityDict.layer"
    
    // 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;


// ************************************************************************* //
Quality controls for steps 1 and 2:

Code:
/*--------------------------------*- C++ -*----------------------------------*\
| =========                 |                                                 |
| \\      /  F ield         | OpenFOAM: The Open Source CFD Toolbox           |
|  \\    /   O peration     | Version:  2.3.1                                 |
|   \\  /    A nd           | Web:      www.OpenFOAM.org                      |
|    \\/     M anipulation  |                                                 |
\*---------------------------------------------------------------------------*/
FoamFile
{
    version     2.0;
    format      ascii;
    class       dictionary;
    object      meshQualityDict;
}
// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //

//- 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. This has to be a positive number for tracking
//  to work. Set to very negative number (e.g. -1E30) to
//  disable.
//     <0 = inside out tet,
//      0 = flat tet
//      1 = regular tet
minTetQuality 1e-15;

//- 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 0.02;

//- Minimum normalised cell determinant. This is the determinant of all
//  the areas of internal faces. It is a measure of how much of the
//  outside area of the cell is to other cells. The idea is that if all
//  outside faces of the cell are 'floating' (zeroGradient) the
//  'fixedness' of the cell is determined by the area of the internal faces.
//  1 = hex, <= 0 = folded or flattened illegal cell
minDeterminant 0.001;

//- Relative position of face in relation to cell centres (0.5 for orthogonal
//  mesh) (0 -> 0.5)
minFaceWeight 0.05;

//- Volume ratio of neighbouring cells (0 -> 1)
minVolRatio 0.01;

//- Per triangle normal compared to average normal. Like face twist
//  but now per (face-centre decomposition) triangle. Must be >0 for Fluent
//  compatibility
minTriangleTwist -1;


//- if >0 : preserve 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 0.1;

relaxed
(
    
);

// ************************************************************************* //
Quality controls for layer addition:

Code:
/*--------------------------------*- C++ -*----------------------------------*\
| =========                 |                                                 |
| \\      /  F ield         | OpenFOAM: The Open Source CFD Toolbox           |
|  \\    /   O peration     | Version:  2.3.1                                 |
|   \\  /    A nd           | Web:      www.OpenFOAM.org                      |
|    \\/     M anipulation  |                                                 |
\*---------------------------------------------------------------------------*/
FoamFile
{
    version     2.0;
    format      ascii;
    class       dictionary;
    object      meshQualityDict;
}
// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //

//- Maximum non-orthogonality allowed. Set to 180 to disable.
maxNonOrtho 180;//65;

//- Max skewness allowed. Set to <0 to disable.
maxBoundarySkewness -1;// 20;
maxInternalSkewness -1; //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 180;//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 -1e30; //1e-13;

//- Minimum quality of the tet formed by the face-centre
//  and variable base point minimum decomposition triangles and
//  the cell centre. This has to be a positive number for tracking
//  to work. Set to very negative number (e.g. -1E30) to
//  disable.
//     <0 = inside out tet,
//      0 = flat tet
//      1 = regular tet
minTetQuality -1e30; //1e-15;

//- 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 0.02;

//- Minimum normalised cell determinant. This is the determinant of all
//  the areas of internal faces. It is a measure of how much of the
//  outside area of the cell is to other cells. The idea is that if all
//  outside faces of the cell are 'floating' (zeroGradient) the
//  'fixedness' of the cell is determined by the area of the internal faces.
//  1 = hex, <= 0 = folded or flattened illegal cell
minDeterminant 0.001;

//- Relative position of face in relation to cell centres (0.5 for orthogonal
//  mesh) (0 -> 0.5)
minFaceWeight 0.05;

//- Volume ratio of neighbouring cells (0 -> 1)
minVolRatio 0.01;

//- Per triangle normal compared to average normal. Like face twist
//  but now per (face-centre decomposition) triangle. Must be >0 for Fluent
//  compatibility
minTriangleTwist -1;


//- if >0 : preserve 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 0.1;

relaxed
(
    
);

// ************************************************************************* //
Best,
Tim
Attached Images
File Type: jpg 5layer_2.jpg (49.6 KB, 256 views)
File Type: jpg 10layer_2.jpg (61.3 KB, 251 views)
File Type: jpg 15layer_2.jpg (50.3 KB, 263 views)
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Old   August 2, 2015, 22:51
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DanielP
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danielpiaget is on a distinguished road
Hello Tim,

Since you need a very thick boundary layers relative to your domain, I suggest that you changed the following points:

-Change nCellsBetweenLevels to 1
-reduce the refinement to level to 2 for all the surfaces
-Set the tolerance to 1
- I have notice that you using the explicitFeatureSnap without the features
being defined in the features section of the castellatedMeshControls.Please see
this thread:
http://www.cfd-online.com/Forums/ope...-new-post.html

-You can try and set the relativeSize to false. This way all you declared values for the boundary layers are not relative but true values.

-Lower the slipFeatureAngle to say 10. This will control of the points are extruded between patches.

-Lower the minMedianAxisAngle to 5.

The values of the finalLayerThickness and expansionRatio are use in calculating the finalThickness of the total number of specified boundary layers. This specified value is use when snappyHexMesh tries to generated the boundary layers.

Note also that the Snap mesh must be displace in order to make room for
the Addlayer mesh. The internal displacement of the points are limited by the
mesh quality parameters that control the quality of the elements.

Thanks,

Daniel
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