[belier1988]

Hi Foamers,

I'm interested in creating a mesh with snappyHexMesh around a square cylinder. I plan to run some steady simulations later using simpleFoam with wall models.

In attachment is a snapshot of my mesh using the ExtractCellsByRegion filter in ParaView.
I want to generate boundary layer meshes near the surface.
What seems to be problematic to me is that the boundary layer cells near the corner seem be too stretched and they deform outer cells relatively far away from the square. The checkMesh result seems not too bad though I think.

Here is my snappyHexMeshDict file

/*--------------------------------*- C++ -*----------------------------------*| =========                 |                                                 |
| \      /  F ield         | OpenFOAM: The Open Source CFD Toolbox           |
|  \    /   O peration     | Version:  2.2.0                                 |
|   \  /    A nd           | Web:      www.OpenFOAM.org                      |
|    \/     M anipulation  |                                                 |
\*---------------------------------------------------------------------------*/
FoamFile
{
    version     2.2;
    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
{
    square3.stl
    {
        type triSurfaceMesh;
        name poteaux;

        patchInfo
        {
            type wall;
        }
    }

    refinementBox0
    {
        type searchableBox;
        min (-1.0 -1.0 0.0);
        max ( 1.0  1.0 1.0);
    }

    refinementBox1
    {
        type searchableBox;
        min (-0.5 -0.5 0.0);
        max ( 0.5  0.5 1.0);
    }
};

// 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 1000000;

    // 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 10000000;

    // 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 10;



    // 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 "square3.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
    {
        poteaux
        {
            // Surface-wise min and max refinement level
            level (2 2);
        }
    }

    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
    { 
//  refinementBox0
//  {
//      mode inside;
//      levels ((1E15 1));
//  }
//  refinementBox1
//  {
//      mode inside;
//      levels ((1E15 2));
//  }
    }


    // 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 (0.4875 0.4875 0.0125);


    // 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 1;

    //- Relative distance for points to be attracted by surface feature point
    //  or edge. True distance is this factor times local
    //  maximum edge length.
    //    tolerance 4.0;
    tolerance 1.0;

    //- Number of mesh displacement relaxation iterations.
    nSolveIter 1;

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

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



// 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
    {
        poteaux
        {
            nSurfaceLayers 5;
        }
    }

    // Expansion factor for layer mesh
    expansionRatio 1.05;

    //- 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 1.0;

    //- Minimum thickness of cell layer. If for any reason layer
    //  cannot be above minThickness do not add layer.
    //  See relativeSizes parameter.
    minThickness 1e-12; // 1e-12; do nothing
    
    //- 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
    //  make straight angle.
    //- Increase to avoid boundary layer collapsing close to the corners.
    featureAngle 180; //30 is too low. above 180 does nothing.

    //- 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 0.2;

    // 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;

    // Max number of iterations after which relaxed meshQuality controls
    // get used. Up to nRelaxIter it uses the settings in meshQualityControls,
    // after nRelaxIter it uses the values in meshQualityControls::relaxed.
    nRelaxedIter 20;
}



// 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;	// NB: mettere valore del checkMesh post castellated e separare 3 fasi

    //- Max skewness allowed. Set to <0 to disable.
    maxBoundarySkewness 4;
    maxInternalSkewness 2;

    //- 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.  Set to very negative number (e.g. -1E30) to
    //  disable.
    //     <0 = inside out tet,
    //      0 = flat tet
    //      1 = regular tet
    minTetQuality 1e-30; // ( prima era 1e-30: abilitato );

    //- 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;	// 0.05;	// -1; disattivo

    //- 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;

    //- if >0 : preserve single cells with all points on the surface if the
    //  resulting volume after snapping (by approximation) is larger than
    //  minVolCollapseRatio times old volume (i.e. not collapsed to flat cell).
    //  If <0 : delete always.
    //minVolCollapseRatio 0.5;


    // 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 180;
    }
}


// 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;

writeFlags
(
    scalarLevels    // write volScalarField with cellLevel for postprocessing
    layerSets       // write cellSets, faceSets of faces in layer
    layerFields     // write volScalarField for layer coverage
);

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


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

and the checkMesh output

/*---------------------------------------------------------------------------*\
| =========                 |                                                 |
| \\      /  F ield         | OpenFOAM: The Open Source CFD Toolbox           |
|  \\    /   O peration     | Version:  4.1                                   |
|   \\  /    A nd           | Web:      www.OpenFOAM.org                      |
|    \\/     M anipulation  |                                                 |
\*---------------------------------------------------------------------------*/
Build  : 4.1
Exec   : checkMesh
Date   : Sep 01 2020
Time   : 16:11:27
Host   : "cal"
PID    : 16449
Case   : snappy
nProcs : 1
sigFpe : Enabling floating point exception trapping (FOAM_SIGFPE).
fileModificationChecking : Monitoring run-time modified files using timeStampMaster
allowSystemOperations : Allowing user-supplied system call operations

// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //
Create time

Create polyMesh for time = 0

Time = 0

Mesh stats
    points:           562739
    faces:            1624881
    internal faces:   1590633
    cells:            531195
    faces per cell:   6.05336
    boundary patches: 7
    point zones:      0
    face zones:       0
    cell zones:       0

Overall number of cells of each type:
    hexahedra:     521364
    prisms:        0
    wedges:        0
    pyramids:      0
    tet wedges:    0
    tetrahedra:    0
    polyhedra:     9831
    Breakdown of polyhedra by number of faces:
        faces   number of cells
            6   2376
            9   5655
           12   1607
           15   193

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
                  outlet     4096     4225  ok (non-closed singly connected)
                   inlet     4096     4225  ok (non-closed singly connected)
                  bottom     4096     4225  ok (non-closed singly connected)
                     top     4096     4225  ok (non-closed singly connected)
                   front     4096     4225  ok (non-closed singly connected)
                    back     4096     4225  ok (non-closed singly connected)
                 poteaux     9672     9674      ok (closed singly connected)

Checking geometry...
    Overall domain bounding box (-1.6 -1.6 -1.1) (1.6 1.6 2.1)
    Mesh has 3 geometric (non-empty/wedge) directions (1 1 1)
    Mesh has 3 solution (non-empty) directions (1 1 1)
    Boundary openness (-6.65212e-17 7.18065e-18 -9.29091e-18) OK.
    Max cell openness = 3.55874e-16 OK.
    Max aspect ratio = 2.54668 OK.
    Minimum face area = 0.00010526. Maximum face area = 0.004037.  Face area magnitudes OK.
    Min volume = 1.44408e-06. Max volume = 0.000218858.  Total volume = 32.6656.  Cell volumes OK.
    Mesh non-orthogonality Max: 59.9574 average: 5.69625
    Non-orthogonality check OK.
    Face pyramids OK.
    Max skewness = 1.254 OK.
    Coupled point location match (average 0) OK.

Mesh OK.

End

So, about the mesh quality, do you see any way to improve it? Any remarks, suggestions are welcome, I'm all ears.
Have a nice day.

[simrego]

Hi!


When you are adding boundary layers, the goal is mostly to have the proper y+ value near the wall. In your case it seems like the inserted cells are nearly the same size as the original cell size near the wall thus with your boundary layers you only align the cells with the flow but since it is a square in a cartesian mesh, it is already done after the snapping.

Try to increase expansionRatio (1.2 or maybe 1.3). Also you can try to decrease the finalLayerThickness (0.8 - 0.9, or maybe 0.5). Play a bit with these values and your mesh will be better. But don't worry if your cells will collapse. In snappyHexMesh the proper boundary layer insertion is not the most trivial and easiest thing but with a simple geometry like this it should be fine.


If it helped, feel free to share your resulted mesh

[Propanotriol]

I suppose that the flow goes normal to the squared cylinder's longitudinal axis. In this case, the corners are not at all OK.

Moreover, you need to take into account several aspects when meshing. The aspect ratio but also the size ratio between adjacent cells. A good strategy is to have a well refined mesh where the gradients are larger and also over the wake of the squared cylinder.

You may find this link interesting

https://www.semanticscholar.org/pape...6bb2843cd0b3f9

https://escholarship.org/content/qt1...d2b8000f17.pdf