[Friendly]

Hi everyone,


I am new to SnappyHexMesh and try to generate a mesh. It works quite well, but I want to add a refinement layer to the boundary.


I have already refined the surface with a level of 3 and nCellsBetweenLevel 7. But I want to add a new layer to the boundary with nCellBetweenLevel 3 on top. Is this somehow possible?


Thanks in advance!

[Swift]

Hi,

I think you can use the following to achieve what you want:

    // 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
    //   increasing 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
    {
        //box1x1x1
        //{
        //    mode inside;
        //    levels ((1.0 4));
        //}
        //sphere.stl
        //{
        //    mode distance;
        //    levels ((1.0 5) (2.0 3));
        //}
    }

See here for more details.

Thomas

[Friendly]

Hi and thanks for your reply.


This would be a refinement in a box or a sphere. But I want to refine my boundary layer also for other geometries who can be much more complicated.

[Swift]

Hi,

Have you tried it?

The example showed

sphere.stl

Presumably it would work for any geometry so long as you have that geometry as an stl file. This would allow you to handle your complicated geometry.

The import part of the example I showed is:

        
{
         mode distance;
        levels ((1.0 5) (2.0 
}

Let me know how it goes.

Thomas

[Friendly]

I tried it but nothing at all hapoened? Here is my snappyHexMeshDict:

/*--------------------------------*- C++ -*----------------------------------*\
| =========                 |                                                 |
| \\      /  F ield         | OpenFOAM: The Open Source CFD Toolbox           |
|  \\    /   O peration     | Version:  4.1                                   |
|   \\  /    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 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
{
    body.stl
    {
        type    triSurfaceMesh;
        name    body_local;
    }

    box
    {
    type searchableBox;
    min (0.3 -0.1 -0.1);
    max (0.7 0.3 0.1);

    }
    
    box2
    {
    type searchableBox;
    min (0.385 -0.1 -0.1);
    max (0.75 0.3 0.1);

    }
                  

    
};



// 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 0;

    // Number of buffer layers between different levels.
    // 1 means normal 2:1 refinement restriction, larger means slower
    // refinement.
    nCellsBetweenLevels 7;
    



    // 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 "body.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
    {
            body_local
        {
            // Surface-wise min and max refinement level
            level (2 2);

        }
    }

    // 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
    {
        box
        {
            mode inside;
            levels ((1 1));
        }
        box2
        {
            mode inside;
            levels ((2 2));
        }
        
        body.stl
        {
            mode distance;
            levels ((1 5) (2 7));
        }
    }


    // 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 (0.41 0.051 0.005);


    // 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 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;

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

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

    nFeatureSnapIter 100;
    
    //Detect (geometric only) features by
    //sampling the surface (default=false).

    implicitFeatureSnap false;

    // Use castellatedMeshControls::features
    // (default = true)

    explicitFeatureSnap true;
    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
    {
        body
        {
            nSurfaceLayers 3;
        }
    }

    // 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.
    // See relativeSizes parameter.
    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.
    // See relativeSizes parameter.
    minThickness 0.1;

    // 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 20;

    // 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
    // Note: changed(corrected) w.r.t 16x! 90 degrees corresponds to 130 in 16x.
    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
{
    //- 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 70;

    //- 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.  Set to very negative number (e.g. -1E30) to
    //  disable.
    //     <0 = inside out tet,
    //      0 = flat tet
    //      1 = regular tet
    minTetQuality 1;

    //- 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;

    //- 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 0.5;

    //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

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


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

[Yann]

Hello Friendly,

I'm not exactly sure to understand what you want to to in the first post.

Nevertheless, about your last post, it doesn't work because you renamed your body.stl to body_local. Hence you have to use that name in your refinementRegions section :

        body_local
        {
            mode distance;
            levels ((1 5) (2 7));
        }

[Friendly]

Yeah you are right, I already fixed it.



Its just not clear for me, what is the difference between the refinement on a surface and the refinement of the region with the distance mode.


Is there any difference?

[Yann]

Yes there is a difference:

• surfaceRefinement controls the refinement of the cells intersecting the surface described in your STL file. It means these cells will be refined to the level range you have set in the surfaceRefinement section, and the transition from these levels of refinement to the level 0 is controlled by the nCellsBetweenLevel parameter, which is a global parameter for the whole mesh. It means you can't control locally the transition between the refined surface and the surrounding mesh.
• refinementRegion in distance mode allows to have a finer control of the level of refinement of the cells surrounding a specific surface according to prescribed distance and level of refinement. This is useful to create refined volumes of cells around an object or in specific regions, like refining the volume in the wake of an object, independently of the nCellsBetweenLevels transition mechanism.
• Keep in mind that snappyHexMesh always keep the most restrictive parameter: if a zone is refined due to the nCellsBetweenLevels parameter, you can't use refinementRegion to "unrefine" it.

It depends a lot on your geometry and what you try to achieve.

It might not make a big difference on a simple geometry, but it makes a big one on complex geometries involving a big range of refinement levels.

Imagine a complex geometry such as a car: you might need to refine some surfaces up to...lets say level 10, to properly represent small details on the geometry. If you use a big value for nCellsBetweenLevels to have smooth transitions, it means this parameter will be applied to every level transition from 10 to 0 and these small refined details will have a huge impact on the size of your mesh. In this case, you should better use a smaller value for nCellsBetweenLevels and use refinementRegions to specify your own distance and level of refinement around your geometry. (And possibly use different parameters with different STL files to have a finer control on the zones you want to refine or not)


Yann

[Friendly]

Thank you very much for the detailed explanation!


Its helped a beginner like me a lot!