[EVBUCF]

Hello all,

I am using SnappyHexMesh for the first time since the previous cases I have ran seem to give an inaccurate boundary layer and I was hoping to fix this with SHM. I am importing a .stl file which was exported from a SolidWorks CAD model. The problem I seem to be having is that the layers which I have defined do not fully surround the surface. I have attached pictures to show what I mean.

Another problem is that there are small dimples that form on the surface which are undesirable. How can I smooth these bumps out?

Lastly, it seems like SHM likes to refine the mesh a lot on the symmetry line of the aircraft which I am meshing. The image attached should show you what I mean. Is this ok? How do I correct this?

Here is my SHMDict:

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
{
    FullX48b.stl        //must be stored in triSurfaceMesh
    {
        type triSurfaceMesh;
            //NEW NAME FOR PATCH    
    name FullX48b;

    }


//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 (-1.0 -0.7 0.0);
        max ( 8.0  0.7 2.5);
    } */
};



// 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 100000;

    // 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 6; // 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 -including angle 150 -minElement 10  constant/triSuraface/name.obj
// (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 "name.eMesh";
        //    level 2;  //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
    {
        //X48b //patch you want to refine
FullX48b
        {
            // 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 (6 8);
        }
    }

    // Resolve sharp angles
    resolveFeatureAngle 30; //cells above angle use the max ref level, cells below use min


    // 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 4));
        //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 0 0); //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 5;     //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 0;    //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
    {

    FullX48b_FullX48B
        //X48b
        {
            nSurfaceLayers 7;
        }
    }

    // Expansion factor for layer mesh
    expansionRatio 1.2; //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 0.3;    //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 0.1;        //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 179;  //this is how OF knows 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 8;

    // 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 5;

    // 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 30;
}



// 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 20;
    maxInternalSkewness 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;

    //- 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 0.02;

    //- minimum normalised cell determinant
    //- 1 = hex, <= 0 = folded or flattened illegal cell
    minDeterminant 0.001;

    //- minFaceWeight (0 -> 0.5)
    minFaceWeight 0.02;

    //- 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;
}


// 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;

bumps.jpg

surface.jpg

LE.jpg

TE.jpg