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[snappyHexMesh] SnappyHexMesh no layers and no decent mesh for complex geometry

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Old   February 24, 2015, 17:33
Default SnappyHexMesh no layers and no decent mesh for complex geometry
  #1
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Hi FOAMers,

I try to mesh a complex delta wing with snappy. However, the layers it produces around it look horrible. So I decided to go a step back and just try to proper mesh a straight 3d wing. But again I cannot get layers around it and I just dont understand why anymore.

Code:
/*--------------------------------*- C++ -*----------------------------------*\
| =========                 |                                                 |
| \\      /  F ield         | OpenFOAM: The Open Source CFD Toolbox           |
|  \\    /   O peration     | Version:  2.1.0                                 |
|   \\  /    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;  //create mesh
snap            true;    //cuts mesh at surface interface
addLayers       true;    //creates layers on surface


// 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
{
    UCAV.stl        //must be stored in triSurfaceMesh
    {
        type triSurfaceMesh;
               
    name UCAV;

    }


//scales and cleans stl file (run before importing)
// surfaceConvert constant/triSurface/X48b.stl constant/triSurface/X48big.stl -clean -scale 100


    

    refinementBox    //some volume not defined as fine region..yet.
    {
        type searchableBox;
    min (-0.2 -0.05 -0.7); 
    max ( 0.7 0.05 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 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 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 3; // The minimum amount of cells required between refinement region the more you have the more space needed to reach refinement max to refiement min



    // Explicit feature edge refinement
    // ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~


//If you have special features (sharp angles, trailing edges...) 
//You can isoilate these features with 

///surfaceFeatureExtract -writeObj -includedAngle 150  constant/triSurface/X48B.stl X

// (any 2 line of cells that make an angle above 150 will be included in this //extractedfeatureset) (outside angle between cells) or feature that have less then 10 elements

    // Specifies a level for any cell intersected by its edges.
    // This is a featureEdgeMesh, read from constant/triSurface for now.
    features
   (
       {
            //name.obj

           file "UCAV.eMesh";
           level 3;  //refinement for all extracted surfaces in name.obj files and name.emesh file
       }
    );



    // 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
    {
        UCAV //patch you want to refine
        {
//        mode distance;
// volume {
//        mode distance; 
//        level 
//        (0.0006 4) (0.002 3) (0.01 2);
//        ((0.0006 9) (0.002 8) (0.01 7));        
            // Surface-wise min and max refinement level
        //;level(min ref level, max ref level)
        //which is which is based on the the resolve feature angle below
            level (3 3);
        }
    }

    // Resolve sharp angles
    resolveFeatureAngle 180; //cells above angle use the max ref level, cells below use min//was 105


    // 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
        {
//    volume
//        {
//            mode distance;
//            levels ((0.0006 9) (0.002 8) (0.01 7));
//        }
//          
            mode inside;
            levels ((1E15 3));
        }
    UCAV
    {
            mode distance;
            levels ((0.0006 4) (0.0018 3));
    }

    }
        //say 3 levels away from a surface
        //distance (1e-04 1e-03, 1e-02,1e-01)
        //distance away from surface corresponding to a refinement level (must be in order so OF knows which refinement level) the number of distance dictates how many levels of refinement at a given distance

//levels (max min)  aka anything inside the refinebox (in this case) must have level 4 refiment



    // 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 (-0.2 0.0 -0.5); //importnant only when you have multiple domains
    // point states what is the inner region of the mesh so just make sure it's within the domain and not on or in a surface


    // 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; //??????? someone find out
}

//--------------------------------------------------------------------------------------------------

// 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 10;     //tolerance*cellsize on surface = distance to look for closest cell             distance relative to local cell size to look for adjacent cells

    //- Number of mesh displacement relaxation iterations.
    nSolveIter 300;    //deforms entire mesh to make good quality iterations

    //- Maximum number of snapping relaxation iterations. Should stop
    //  before upon reaching a correct mesh.
    nRelaxIter 5;    //relaxing sharp intersection makes smooth

    //- Highly experimental and wip: number of feature edge snapping
    //  iterations. Leave out altogether to disable.
    //  Do not use here since mesh resolution too low and baffles present
    nFeatureSnapIter 10;

    //this is iterations to snap solely on the extracted special features
        

}



// Settings for the layer addition.
addLayersControls
{

    //adds boundary layers to a given surface


    // Are the thickness parameters below relative to the undistorted
    // size of the refined cell outside layer (true) or absolute sizes (false).
    relativeSizes true;        //relative to local cell size at surface
    //if this if false you would have to specify in meters the finallayerthickness, with it on we just use a factor of the local cell size
    // Per final patch (so not geometry!) the layer information
    layers
    {

    UCAV
        {
            nSurfaceLayers 10;
        }
    }

    // Expansion factor for layer mesh
    expansionRatio 1.15; //cells are growing in height by a factor of 1.2 away from the surface

    //- 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 0.2;    //0.3*(local cell size) is the final layer thickness

    //- 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 0.01;        //not enforced but seeks to meet this

    //- 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; //sometimes cells don't extrude away from surface due to cells meeting at a sharp angle this forces the cells to grow a layer away from the surface


    // Advanced settings

    //- When not to extrude surface. 0 is flat surface, 90 is when two faces
    //  make straight angle.
    featureAngle 180;  //this is how OF k  nows whether to not extrude a layer (nGrow forces this layer to extrude and disregards feature angle

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

    // Number of smoothing iterations of surface normals
    nSmoothSurfaceNormals 50; //was 1

    // Number of smoothing iterations of interior mesh movement direction
    nSmoothNormals 30; //was 3

    // Smooth layer thickness over surface patches
    nSmoothThickness 50; //was 10

    // Stop layer growth on highly warped cells
    maxFaceThicknessRatio 0.3;

    // Reduce layer growth where ratio thickness to medial
    // distance is large
    maxThicknessToMedialRatio 0.5;

    // Angle used to pick up medial axis points
    // Note: changed(corrected) w.r.t 17x! 90 degrees corresponds to 130 in 17x.
    minMedianAxisAngle 80;


    // 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 10; //was 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 90; // was 65

    //- Max skewness allowed. Set to <0 to disable.
    maxBoundarySkewness 90; //was 20
    maxInternalSkewness 90; // was 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 90; //was 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 0.05;

    //- 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 0.9; //was 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; //as 0 it will always show mesh before snapping, after snapping, and after 


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


// ************************************************************************* //
I also get all the time the following warning when using snappy and I just cannot get rid of it:
Code:
--> FOAM Warning : Displacement (-6.68027e-06 -2.56825e-07 -1.16867e-05) at mesh point 326884 coord (0.000462189 -6.66163e-06 -0.49958) points through the surrounding patch faces
Is there anyone who can give me some hints on how to fix this issue?

I have read through quite a lot of threads here and try to apply them to my case without success.

Also what is the recommended value for maxLocalCells and maxGlobalCells when the dimensions of my blockMesh are 4.5m, 4m, 1m as the computer I use seems to take forever when running it for maxLocalCells 3000000 and maxGlobalCells 6000000. Are there any recommendation for the highest refinement level I have to use at different stages in order to avoid that the meshing process kills itself because it runs out of memory?

Thank you very much in advance for your help!!!!!!

Smoothing displacement ...
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Old   February 25, 2015, 08:05
Default Finding Problems
  #2
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Bernhard Linseisen
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Hi there!

Although at some points I have similar problems, I blame mainly my lack of experience with the tool for it. So please do not take everything I write to be absolute and sole truth but rather as guessing with some background.

1. For cell faces pointing in strange directions, you might want to have a look at http://www.sourceflux.de/blog/tag/snappyhexmesh/
One of the blog-entries there also describes what to do in order to avoid wrong-directed faces. Maybe that is of help.

2. Concerning numbers of cells: As far as I experienced, something like half a million cells per GB of RAM seems to be a rough measure, as the meshing process sometimes produces more cells than what you have specified. At least I tried to use maxGlobalCells 8000000 on a 16GB-machine (parallel use of snappy on 4 processors) and it was killed by a signal 9 mpirun error (too high resource demand). The last cell-count provided by the log is slightly above 11 million cells.

3. As you are trying to get a grasp of some basic steps as well: You can also do different steps separately. If you have a seemingly good mesh after castellation or snapping and you just want to change the layering, it is good to avoid these two steps for saving time.
Just run these two steps once for the good mesh and then switch the entries at the beginning of your snappyHexMeshDict to "false". This way the last mesh is reused for layering and you don't have to redo castellation and snapping. Of course it helps to do these steps without the "-overwrite" option as long as you have not yet reached a good mesh...

Let us know if you find out what causes the layering failure, please!

Cheers,
Bernhard
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