[Trinxo123]

Hey,

I'm trying to mesh a ventilation system using SnappyHexMesh. The problem comes with the impeller. This is just a twisted sheet of metal, so both the leading and trailing edges are completely sharp. The problem is at the edges, where the snapping is not great, even though I'm using surfaceExtractFeatures to get the edges and a pretty fine level for the impeller edges (10). Please find attached pictures and the snappyHexMeshDict, I have tested multiple settings as implicit feature snapping/ explicit, increasing the number of iterations for snapping and reducing the allowed skewness in the quality part, nothing is really improving the snapping on the impeller edges. The more problematic points are the outer vertex at the tip (see picture).

Till now, I'm not considering increasing the level of refinement of the edge as it is already very fine and that would imply (maybe) an excessive number of cells.
Thanks everyone for helping.

snappy Dict:

/*--------------------------------*- C++ -*----------------------------------*\
| =========                 |                                                 |
| \\      /  F ield         | OpenFOAM: The Open Source CFD Toolbox           |
|  \\    /   O peration     | Version:  v2406                                 |
|   \\  /    A nd           | Website:  www.openfoam.com                      |
|    \\/     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
{
    impellerTotal.stl
    {
        type        triSurfaceMesh;
        name        impeller;
    }
    engine_bench.stl
    {
        type        triSurfaceMesh;
        name        engine;
    }
    jetfanDuct.stl
    {
        type        triSurfaceMesh;
        name        externalDuct;
    }
    cylinder_support.stl
    {
        type        triSurfaceMesh;
        name        mRFzone;
    }

    entry_support.stl
    {
        type        triSurfaceMesh;
        name        inletF;
    }

    exit_support.stl
    {
        type        triSurfaceMesh;
        name        outletF;
    }
    refinementBox
    {
        type        searchableCylinder;
        point1      (1.5 0 0);
        point2      (-3.5 0 0);
        radius      1.2;
    }
    innerRefinement
    {
        type        searchableCylinder;
        point1      (1.4 0 0);
        point2      (-1.6 0 0);
        radius      0.7;
    }
}


// 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 800000;

    // 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 50000000;

    // 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 2; 


    // 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        "impellerTotal.eMesh";
           level       10;
       }

       {
           file        "engine_bench.eMesh";
           level       8;
       }

       {
           file        "jetfanDuct.eMesh";
           level       6;
       }
       
       {
           file        "exit_support.eMesh";
           level       6;
       }

       {
           file        "entry_support.eMesh";
           level       6;
       }

       {
           file        "cylinder_support.eMesh";
           level       6;
       }
    );



    // 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
    {
        impeller
        {
            level       (6 8);

            patchInfo
            {
              type wall;
            }
        }
        engine
        {
            level       (6 7);

            patchInfo
            {
              type wall;
            }
        }
        externalDuct
        {
            level       (5 5);

            patchInfo
            {
              type wall;
            }
        }

        mRFzone
        {
            level          (6 6);
            faceType       internal;
            cellZone       rotatingZone;
            faceZone       rotatingFzone;
            cellZoneInside inside;
        }

        inletF
        {
            level       (5 5);
            faceType    internal;
            faceZone    inletDuct;
        }

        outletF
        {
            level       (5 5);
            faceType    internal;
            faceZone    outletDuct;
        }
    }

    // Resolve sharp angles
    resolveFeatureAngle 15;


    // 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
        {
            mode        inside;
            levels      ((1E15 3));
        }

        innerRefinement
        {
            mode        inside;
            levels      ((5 5));
        }

        mRFzone
        {
            mode        inside;
            levels      ((6 6));
        }

    }


    // 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 (1 0 0);


    // 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 3.0; // 1.0;

    //- Number of mesh displacement relaxation iterations.
    nSolveIter 300;

    //- Maximum number of snapping relaxation iterations. Should stop
    //  before upon reaching a correct mesh.
    nRelaxIter 6;

    // 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 features between multiple surfaces
        // (only for explicitFeatureSnap, default = false)
        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
    {
      impeller
      {
         nSurfaceLayers 5;
      }
      engine
      {
         nSurfaceLayers 6;
      }
      externalDuct
      {
        nSurfaceLayers 5;
      }
    }

    // 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 0.1;

    // 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;

    // 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 1.7.x! (didn't do anything in 1.7.x)
    nGrow 0;

    // Advanced settings

    // When not to extrude surface. 0 is flat surface, 90 is when two faces
    // are perpendicular
    featureAngle 350;

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

    // 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 1.7.x! 90 degrees corresponds to 130
    // in 1.7.x.
    minMedialAxisAngle 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
{
    //- Maximum non-orthogonality allowed. Set to 180 to disable.
    maxNonOrtho 65;

    //- Max skewness allowed. Set to <0 to disable.
    maxBoundarySkewness 10;
    maxInternalSkewness 5;

    //- 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 -1; // 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 0.01;

    //- 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 0.75;

    // Optional : some meshing phases allow usage of relaxed rules.
    // See e.g. addLayersControls::nRelaxedIter.
    relaxed
    {
        //- Maximum non-orthogonality allowed. Set to 180 to disable.
        maxNonOrtho 75;
    }
}



// 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:

Time = 2

Mesh stats
    points:           2130516
    faces:            5141188
    internal faces:   4806028
    cells:            1551077
    faces per cell:   6.4131026
    boundary patches: 4
    point zones:      1
    face zones:       3
    cell zones:       1

Overall number of cells of each type:
    hexahedra:     1097718
    prisms:        50980
    wedges:        0
    pyramids:      0
    tet wedges:    1787
    tetrahedra:    0
    polyhedra:     400592
    Breakdown of polyhedra by number of faces:
        faces   number of cells
            4   56228
            5   46885
            6   120971
            7   3159
            8   1854
            9   89841
           10   331
           11   1650
           12   51072
           13   22
           14   542
           15   26195
           16   6
           17   90
           18   1744
           21   2

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
            defaultFaces     1000     1002      ok (closed singly connected)
                impeller   269977   356132  ok (non-closed singly connected)
                  engine    26440    30469  ok (non-closed singly connected)
            externalDuct    37743    41039  ok (non-closed singly connected)
                    ".*"   335160   428168      ok (closed singly connected)


Checking faceZone topology for multiply connected surfaces...
                FaceZone    Faces   Points                  Surface topology
           rotatingFzone    10520    10765  ok (non-closed singly connected)
               inletDuct     1764     1901  ok (non-closed singly connected)
              outletDuct     2184     2321  ok (non-closed singly connected)

Checking basic cellZone addressing...
                CellZone        Cells       Points       VolumeBoundingBox
           rotatingZone      1250619      1756451   0.42326486 (-0.1239693 -0.61256844 -0.61244237) (0.25 0.61256849 0.61250001)

Checking basic pointZone addressing...
               PointZone  PointsBoundingBox
            frozenPoints       0(1e+150 1e+150 1e+150) (-1e+150 -1e+150 -1e+150)

Checking geometry...
    Overall domain bounding box (-15.000286 -4.999914 -4.999885) (5.0000088 4.9999108 4.9999398)
    Mesh has 3 geometric (non-empty/wedge) directions (1 1 1)
    Mesh has 3 solution (non-empty) directions (1 1 1)
    Boundary openness (-3.5531312e-17 -3.2523627e-17 1.9716543e-17) OK.
    Max cell openness = 4.4048629e-16 OK.
    Max aspect ratio = 7.0443819 OK.
    Minimum face area = 9.8053389e-10. Maximum face area = 1.0009453.  Face area magnitudes OK.
    Min volume = 7.7218743e-11. Max volume = 1.0003527.  Total volume = 1998.6224.  Cell volumes OK.
    Mesh non-orthogonality Max: 64.990768 average: 14.02511
    Non-orthogonality check OK.
    Face pyramids OK.
 ***Max skewness = 9.4475423, 110 highly skew faces detected which may impair the quality of the results
  <<Writing 110 skew faces to set skewFaces
    Coupled point location match (average 0) OK.

Failed 1 mesh checks.