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-   -   Collapsing layers (http://www.cfd-online.com/Forums/openfoam-meshing-snappyhexmesh/111997-collapsing-layers.html)

michi.ka January 19, 2013 11:04

Collapsing layers
 
1 Attachment(s)
Hello,

I created a 2D mesh using snappyHexMesh.
My problem is that the added layers collapse at the end of the layer surface, see attached picture in the lower right corner.

Is there any possiblity to avoid collapsing layers?

Here's my snappyHexMeshDict:

Code:

/*--------------------------------*- C++ -*----------------------------------*\
| =========                |                                                |
| \\      /  F ield        | OpenFOAM: The Open Source CFD Toolbox          |
|  \\    /  O peration    | Version:  2.1.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      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
{
    flowRegion.stl
     
    {
        type triSurfaceMesh;
    regions       
    {
    BLADE1 {name BLADE1;}


   
        BLADE2 {name BLADE2;}
        }
    }
    blade1.stl
    {
    type triSurfaceMesh;
    regions {}
    }   
   
    blade2.stl
    {
    type triSurfaceMesh;
    regions {}
    }   
       
 
};



// 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 100000000;

    // 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 120000000;

    // The surface refinement loop might spend lots of iterations refining just
    // a few cells. This setting will cause refinement to stop if <=
    // minimumRefine are selected for refinement. Note: it will at least do one
    // iteration (unless the number of cells to refine is 0)
    minRefinementCells 100;

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



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

    // Specifies a level for any cell intersected by its edges.
    // This is a featureEdgeMesh, read from constant/triSurface for now.
    features (
        {
        file "flowRegion.eMesh";
        level 0;
        }


    );



    // 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
    {
        flowRegion.stl
        {
            // Surface-wise min and max refinement level
            level (0 0);
        regions
        {
        BLADE1 {level (0 0);}
           
                BLADE2 {level (0 0);}

            }

        }
   
    }

    // Resolve sharp angles on fridges
    resolveFeatureAngle 30;


    // Region-wise refinement
    // ~~~~~~~~~~~~~~~~~~~~~~

    // Specifies refinement level for cells in relation to a surface. One of
    // three modes
    // - distance. 'levels' specifies per distance to the surface the
    //  wanted refinement level. The distances need to be specified in
    //  descending order.
    // - inside. 'levels' is only one entry and only the level is used. All
    //  cells inside the surface get refined up to the level. The surface
    //  needs to be closed for this to be possible.
    // - outside. Same but cells outside.

    refinementRegions
    {
     
    blade1.stl
    {
    mode distance;
    levels ((10 2) (20 1));
    }

    blade2.stl
    {
    mode distance;
    levels ((10 2) (20 1));
    }
    }


    // 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 10 10);    //Netzpunkt im Teil des Netztes welches Řbrigbleibt, darf nicht auf einem face liegen


    // 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 5; //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 1.0; //1.0 4.0

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

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

    nFeatureSnapIter 10;
}



// 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
    {
      BLADE1

        {
            nSurfaceLayers 2;
        }
   
      BLADE2

        {
            nSurfaceLayers 2;
      }

   
    }

    // Expansion factor for layer mesh
    expansionRatio 1.1;

    //- 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  0.37;

    //- Minimum thickness of cell layer. If for any reason layer
    //  cannot be above minThickness do not add layer.
    //  Relative to undistorted size of cell outside layer.
    //  See relativeSizes parameter.
    minThickness 0.01;

    //- 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
    //  make straight angle.
    featureAngle 180;

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

    // Number of smoothing iterations of surface normals
    nSmoothSurfaceNormals 10; //1

    // Number of smoothing iterations of interior mesh movement direction
    nSmoothNormals 15; //3

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

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

    // Reduce layer growth where ratio thickness to medial
    // distance is large
    maxThicknessToMedialRatio 0.9;  //0.3

    // Angle used to pick up medial axis points
    // Note: changed(corrected) w.r.t 16x! 90 degrees corresponds to 130 in 16x.
    minMedianAxisAngle 90; //50

    // 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; //65

    //- Max skewness allowed. Set to <0 to disable.
    maxBoundarySkewness 20;//20
    maxInternalSkewness 4; //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; //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; //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; //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;  //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; //-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;


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


Thanks,
Michael

[IMG]file:///home/itsnas/haemmerl/OpenFOAM/haemmerl-2.1.x/run/testcase/AGTURBO/WM_LES/layers.png[/IMG][IMG]file:///home/itsnas/haemmerl/OpenFOAM/haemmerl-2.1.x/run/testcase/AGTURBO/WM_LES/layers.png[/IMG]

Eloise January 21, 2013 09:44

Hi Michael,

Here is my suggestion:
1) Run snappyHexMesh steps castellation and snap only
Code:

castellatedMesh true; snap true; addLayers false;

2) with the changeDictionary utility, change your lower patch from "patch" or "wall" to "empty"
3)
Run snappyHexMesh step layers only
Code:

castellatedMesh false; snap false; addLayers true;

4)
with the changeDictionary utility, change your lower patch from "empty" to "patch" or "wall"

This worked well for me as long as I ran in serial, I got the idea from this thread

If you get a way to have it to work in parallel meshing, I'd be interested :)
Good luck!
Elo´se

michi.ka January 22, 2013 06:55

2 Attachment(s)
Thanks for your reply!

The layers look great now!
But the boundary that I've changed to empty now is not plane anymore but deformed.
The first image shows the deformed surface, image 2 shows how it was before.

Any ideas how to fix this?

Eloise January 22, 2013 07:08

Hi Michael,

I also observed some deformations with this process and, just as you, I'm not fully satisfied with the resulting mesh. But I haven't made further progress on that case yet.
If someone has a solution for it, I'd be glad to read about it!

Regards,
Elo´se

Chia February 3, 2013 18:26

Hi,
to avoid deformation of the mesh first you can try to run the castellated, snap and addLayers all separately. In snappyHexMesh there is also the entry nSmoothPatch in snapControls, which is the number of pre-smoothing iteration, you can try to set it equal to zero if you have sharp edges and you want to maintain it.

Regarding the collapsing of layers, the entry featureAngle has a key role, if you set it 180░ you should obtain no layers collapsed, but I have seen that you have already used this value.

I hope these advice help you.

Chiara

Chia February 3, 2013 18:33

You can also take a look to point 2 in this page:
http://openfoamwiki.net/index.php/SnappyHexMesh
--> A Comprehensive Tour of snappyHexMesh - 7th OpenFOAM Workshop (25 June 2012).
it helps me a lot to understand how sHM works.

Regards,
Chiara


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