[New-to-CFD]

I have a handful of STL I'm trying to work into my mesh using snappyHexMesh.

Basically I'm using them to mesh to the surface of the STLs so the domain is cut out around these objects.

Everything is working fine but when the objects are too close together, the surfaces get merged together. See below image.


Screenshot from 2020-05-01 14-15-14.png

Is there a setting in the snappyHexMeshDict I can vary to prevent this? Or would meshing one after another prevent this?

I've ran surfaceFeatureExtract and am using the eMesh files for features snapping, and the stls as refinmentSurfaces. My dict is below:

/*--------------------------------*- C++ -*----------------------------------*\
| =========                 |                                                 |
| \\      /  F ield         | OpenFOAM: The Open Source CFD Toolbox           |
|  \\    /   O peration     | Version:  2.3.1                                 |
|   \\  /    A nd           | Web:      www.OpenFOAM.org                      |
|    \\/     M anipulation  |                                                 |
\*---------------------------------------------------------------------------*/
FoamFile
{
    version     2.0;
    format      ascii;
    class       dictionary;
    object      snappyHexMeshDict;
}
// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //

//https://cfd.direct/openfoam/user-guide/v7-snappyHexMesh/

castellatedMesh true;
snap            true;
addLayers       true;

geometry
{
    unit.stl //file
    {
        type triSurfaceMesh;
        name unit; //user defined name
    }
    
        unit1.stl //file
    {
        type triSurfaceMesh;
        name unit1; //user defined name
    }

/*    sphere1*/
/*    {*/
/*        type    searchableSphere;*/
/*	    centre  (5.0 -0.49 0.3825);*/
/*        radius  0.15; */
/*    }*/
/*    */
/*    sphere2*/
/*    {*/
/*        type    searchableSphere;*/
/*	    centre  (5.0 -0.49 0.3825);*/
/*        radius  0.10; */
/*    }*/
};

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 500000;

    // 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;

    // 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 "unit.eMesh";
            level 5; //0 forces to snap to edges
        }
        {
            file "unit1.eMesh";
            level 5; //0 forces to snap to edges
        }
    );



    // 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
    {
        unit
        {
            // Surface-wise min and max refinement level
            level (5 5);
        }
        unit1
        {
            // Surface-wise min and max refinement level
            level (5 5);
        }
    }

    // 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
    {
/*        sphere1*/
/*        {*/
/*            mode inside; //distance*/
/*            levels ((2 2)); //((x y) (x y)) level X within Y m*/
/*        }*/

/*        sphere2*/
/*        {*/
/*            mode inside; //distance*/
/*            levels ((2 2)); //((x y) (x y)) level X within Y m*/
/*        }*/
    }


    // 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 -0.5 0.5); //inside = internal mesh, outside = external


    // 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 2;

    //- Number of mesh displacement relaxation iterations.
    nSolveIter 10;

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

    //- Highly experimental and wip: number of feature edge snapping
    //  iterations. Leave out altogether to disable.
    //  Of limited use in this case since faceZone faces not handled.
    nFeatureSnapIter 10;
    
    //- Detect (geometric only) features by sampling the surface
    //  (default=false).
    implicitFeatureSnap false;

    //- Use castellatedMeshControls::features (default = true)
    explicitFeatureSnap true;

    //- Detect points on multiple surfaces (only for explicitFeatureSnap)
    multiRegionFeatureSnap true;
}



// Settings for the layer addition.
addLayersControls
{
    relativeSizes true;

    // Per final patch (so not geometry!) the layer information
    layers
    {
    unit
	    {
	     nSurfaceLayers 2;
	    }
    }

    // Expansion factor for layer mesh
    expansionRatio 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.
    finalLayerThickness 0.5;

    // 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 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
    // are perpendicular
    featureAngle 30;

    // 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 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 50;
}



// Generic mesh quality settings. At any undoable phase these determine
// where to undo.
meshQualityControls
{
    #include "meshQualityDict"
    
    //- Number of error distribution iterations
    nSmoothScale 5;
    //- amount to scale back displacement at error points
    errorReduction 0.75;
}


// Advanced

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

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

Any suggestions appreciated.


UPDATE - I've tried playing with the new SHM settings shown here:

https://www.openfoam.com/releases/op...processing.php

but don't seem to do what I want..fiddling around....

[rideste]

Hi, i have same problem.
Did you solve the problem?