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[snappyHexMesh] sHM too many cells

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Old   July 5, 2017, 07:32
Default sHM too many cells
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Thomas
Join Date: May 2017
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Knapsack is on a distinguished road
Hi guys,

that's my first entry here!

To my problem. In my SnappyHexMeshdict I've defined an amount of 15mio cells, but it seems to produce way more than that. CheckMesh tells me it's about 52~mio cells. Am I misunderstanding the definition of maxGlobalCells or what am I doing wrong here?

snappyHexMeshDict:
Code:
/*--------------------------------*- C++ -*----------------------------------*\
| =========                 |                                                 |
| \\      /  F ield         | OpenFOAM: The Open Source CFD Toolbox           |
|  \\    /   O peration     | Version:  4.1                                   |
|   \\  /    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
{
    SHM_1.stl
    {
    type triSurfaceMesh;

    regions
    {
        tyre { name tyre; }
    }    
    }
    SHM_2.stl
    {
    type triSurfaceMesh;

    regions
    {
        nozzle_body { name nozzle_body; }
        nozzle_inlet { name nozzle_inlet; }    
    }
    }
    ground_nozzle_tyre
    {
        type searchableBox;
        min ( -1.8 -0.9 0 );
        max ( 3.2 0.9 0.01 );
    }
    ground_xmin
    {
        type searchableBox;
        min ( -6.4 -0.9 0 );
        max ( -1.8 0.9 0.01 );
    }
    ground_xmax
    {
        type searchableBox;
        min ( 3.2 -0.9 0 );
        max ( 12.8 0.9 0.01 );
    }
    ground_ymin
    {
        type searchableBox;
        min ( -6.4 -6.4 0 );
        max ( 12.8 -0.9 0.01 );
    }
    ground_ymax
    {
        type searchableBox;
        min ( -6.4 0.9 0 );
        max ( 12.8 6.4 0.05 );
    }
};



// 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 1000000;

    // 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 15000000;

    // 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
    (
    {
        file "SHM_1.eMesh";
        levels ((0.02 4));
    }

    {
        file "SHM_2.eMesh";
        levels ((0.02 4));
    }
    );



    // 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
    {
    SHM_1.stl
    {
        level (3 4);

        regions
        {
        tyre { level (3 4); patchInfo { type wall;}}
        }
    }
    
    SHM_2.stl
    {
        level (3 4);

        regions
        {
        nozzle_body { level (3 4); patchInfo { type wall;}}
        nozzle_inlet { level (3 4); patchInfo { type patch;}}
        }
    }
    }

    // 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
    {
        ground_nozzle_tyre { mode inside; levels (( 1e15 5 ) ); }
    ground_ymin { mode inside; levels (( 1e15 5 ) ); }    
    ground_ymax { mode inside; levels (( 1e15 5 ) ); }  
    ground_xmin { mode inside; levels (( 1e15 5 ) ); }    
    ground_xmax { mode inside; levels (( 1e15 5 ) ); }
    }


    // 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 ( -2.55453 1.215 1.041564);


    // 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 false;
}



// 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 1.2;

    //- 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
    {
        SHM_1.stl_tyre  { nSurfaceLayers 2; }
    SHM_2.stl_nozzle_body { nSurfaceLayers 2; }
    SHM_2.stl_nozzle_inlet { nSurfaceLayers 2; }
    ground_nozzle_tyre { nSurfaceLayers 1; }
    ground_xmin { nSurfaceLayers 1; }   
        ground_xmax { nSurfaceLayers 1; } 
        ground_ymin { nSurfaceLayers 1; } 
        ground_ymax { nSurfaceLayers 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 0.3;

    // 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 0.1;

    // 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 60;

    // 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 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
{
        relaxed
        {
        }

        maxNonOrtho 65;
        maxBoundarySkewness 20;
        maxInternalSkewness 4;
        maxConcave 80;
        minFlatness 0.5;
        minVol 1e-13;
        minArea -1;
        minTwist 0.05;
        minDeterminant 0.001;
        minFaceWeight 0.05;
        minVolRatio 0.01;
        minTriangleTwist -1;
        minTetQuality -1;
        nSmoothScale 4;
        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;


// ************************************************************************* //
checkMesh:
Code:
/*---------------------------------------------------------------------------*\
| =========                 |                                                 |
| \\      /  F ield         | OpenFOAM: The Open Source CFD Toolbox           |
|  \\    /   O peration     | Version:  4.x                                   |
|   \\  /    A nd           | Web:      www.OpenFOAM.org                      |
|    \\/     M anipulation  |                                                 |
\*---------------------------------------------------------------------------*/
Build  : 4.x-422f1770e08a
Exec   : checkMesh
Date   : Jul 04 2017
Time   : 20:52:07
Host   : "zeus133"
PID    : 25384
Case   : /home/kilotait/OF_Projects/02_try
nProcs : 1
sigFpe : Enabling floating point exception trapping (FOAM_SIGFPE).
fileModificationChecking : Monitoring run-time modified files using timeStampMaster
allowSystemOperations : Allowing user-supplied system call operations

// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //
Create time

Create polyMesh for time = 0

Time = 0

Mesh stats
    points:           57407690
    faces:            165491129
    internal faces:   162103124
    cells:            54045732
    faces per cell:   6.06143
    boundary patches: 9
    point zones:      0
    face zones:       0
    cell zones:       0

Overall number of cells of each type:
    hexahedra:     52908466
    prisms:        17711
    wedges:        0
    pyramids:      0
    tet wedges:    19
    tetrahedra:    0
    polyhedra:     1119536
    Breakdown of polyhedra by number of faces:
        faces   number of cells
            4   931
            5   1119
            6   16622
            7   56
            8   224
            9   1089412
           10   4
           12   9634
           15   1459
           18   75

Checking topology...
    Boundary definition OK.
    Cell to face addressing OK.
    Point usage OK.
    Upper triangular ordering OK.
    Face vertices OK.
    Number of regions: 1 (OK).

Checking patch topology for multiply connected surfaces...
                   Patch    Faces   Points                  Surface topology
              wall-y-max    87075    91464  ok (non-closed singly connected)
              wall-x-min    17634    18505  ok (non-closed singly connected)
              wall-x-max    17631    18501  ok (non-closed singly connected)
              wall-y-min     6075     6256  ok (non-closed singly connected)
      wall-z-min-nearCar  3199872  3206681  ok (non-closed singly connected)
              wall-z-max    12150    12376  ok (non-closed singly connected)
                    tyre    19341    20784  ok (non-closed singly connected)
             nozzle_body    27933    28395  ok (non-closed singly connected)
            nozzle_inlet      294      556  ok (non-closed singly connected)

Checking geometry...
    Overall domain bounding box (-3.2 -1.8 -0.001) (6.4 1.8 3.199)
    Mesh has 3 geometric (non-empty/wedge) directions (1 1 1)
    Mesh has 3 solution (non-empty) directions (1 1 1)
    Boundary openness (5.87436e-15 -1.90617e-13 7.39134e-12) OK.
    Max cell openness = 3.43864e-16 OK.
    Max aspect ratio = 5.20353 OK.
    Minimum face area = 5.94441e-07. Maximum face area = 0.00511448.  Face area magnitudes OK.
    Min volume = 7.46551e-10. Max volume = 0.00020505.  Total volume = 110.53.  Cell volumes OK.
    Mesh non-orthogonality Max: 64.3153 average: 3.59958
    Non-orthogonality check OK.
    Face pyramids OK.
    Max skewness = 2.50984 OK.
    Coupled point location match (average 0) OK.

Mesh OK.

End
I hope you can help me out with this or explain my misunderstanding.

Thanks in advance guys!
Thomas

Last edited by Knapsack; July 5, 2017 at 08:53.
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Old   July 6, 2017, 09:06
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Hi,
I might be wrong, but I think that the maxGlobalCells parameter only applies to the first step of snappy (castellatedMesh).
When you add the boundary layers, more cells are added, and that could be the cause of your high cells count.
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Old   July 8, 2017, 08:41
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Thomas
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It really differs in the outcome. If i set the maxGlobalCells to 15-20 mio cells, the outcome is nearly the same size.
If it set it to 30 mio cells it nearly "explodes" to ~60 mio cells in the end.
But i will give it a try! Thanks

Last edited by Knapsack; July 10, 2017 at 07:57.
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