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-   -   Snappy creates strange cells far away from boundary (http://www.cfd-online.com/Forums/openfoam-meshing-snappyhexmesh/113175-snappy-creates-strange-cells-far-away-boundary.html)

vainilreb February 14, 2013 05:26

Snappy creates strange cells far away from boundary
 
Hey everyone,

I'm working on a tunnel with three cars in it. I've created the outer geometry with blockMesh and now I'm trying to adapt the mesh to the cars' surfaces with snappyHexMesh.

I kept my car models as edgy as possible and tried to avoid any curvatures, so snappy only has to fit the intersected cells to the surface.
This works fine, but in some distance from the cars, it creates strange thin cells with extremely high aspect ratios.

In this distance I have a really nice orthogonal mesh and there should be no reason to split any cells there.

I've attached my snappyHexMeshDict. I hope you can help me.

Best regards,

Robert

Code:

/*--------------------------------*- C++ -*----------------------------------*\
| =========                |                                                |
| \\      /  F ield        | OpenFOAM: The Open Source CFD Toolbox          |
|  \\    /  O peration    | Version:  2.1.1                                |
|  \\  /    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 false;
snap            true;
addLayers      false;
 
 
geometry
{
 vehicles.stl
 {
      type triSurfaceMesh;
  name vehicles;
 }
 inlet.stl
 {
      type triSurfaceMesh;
  name inlet;
 }
 
};
 
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 50000000;
 
    // 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 5000000;
 
    // 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 0;
 
    // 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 1;
 
 
    // 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 "vehicles.eMesh";
          level 2;
 }
 {
          file "inlet.eMesh";
          level 2;
 }
    );
 
 
    // 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
    {
        inlet
        {
            // Surface-wise min and max refinement level
            level (2 2);
 }
        vehicles
        {
            // 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 (15 15 15);
 

    // 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 10;
 
    //- Relative distance for points to be attracted by surface feature point
    //  or edge. True distance is this factor times local
    //  maximum edge length.

    tolerance 3;
 
    //- Number of mesh displacement relaxation iterations.

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

    nRelaxIter 10;
 
    //- Number of feature snapping iterations to perform.
   
    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 true;
 
    // Per final patch (so not geometry!) the layer information

    layers
    {
 vehicles
 {
  nSurfaceLayers 3;
 }
    }
 
    // Expansion factor for layer mesh

    expansionRatio 1.5;
 
    //- Wanted thickness of final added cell layer. If multiple layers
    //  is the thickness of the layer furthest away from the wall.
    //  See relativeSizes parameter.

    finalLayerThickness 0.66;
 
    //- 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 0.2;
 
    //- 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.

    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 10;
 
    // Number of smoothing iterations of surface normals

    nSmoothSurfaceNormals 10;
 
    // Number of smoothing iterations of interior mesh movement direction

    nSmoothNormals 10;
 
    // 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

    minMedianAxisAngle 90;
 
    // Create buffer region for new layer terminations

    nBufferCellsNoExtrude 1;
 
    // Overall max number of layer addition iterations

    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 40;

   
    //- Max skewness allowed. Set to <0 to disable.

    maxBoundarySkewness 15;

    maxInternalSkewness 3;
 
    //- 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;
 
    //- Minimum projected area v.s. actual area. Set to -1 to disable.

    minFlatness -1;
 
    //- 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-30;

    minTetQuality 1e-6;
 
    //- 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 -2;
 
    //- minimum normalised cell determinant
    //- 1 = hex, <= 0 = folded or flattened illegal cell

    minDeterminant 0.1;
 
    //- minFaceWeight (0 -> 0.5)

    minFaceWeight 0.1;
 
    //- minVolRatio (0 -> 1)

    minVolRatio 0.01;
 
    //must be >0 for Fluent compatibility

    minTriangleTwist -1;
 

    // Advanced
 
    //- Number of error distribution iterations

    nSmoothScale 2000;

    //- amount to scale back displacement at error points

    errorReduction 0.1;
}
 

// 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-9;
 
 
// ************************************************************************* //


vainilreb February 14, 2013 05:44

1 Attachment(s)
A screenshot of the problem:


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