[Counterdoc]

Hi,


I tried to mesh a car with snappyHexMesh and at the first look it came out quite well. But as I checked some parts of it I found that they are super coarse and I don't know why. Additionally I found that the bottom of the tyres are not closed? I put them exactly on z 0.0.

What could cause this? Would addLayers solve it?

/*--------------------------------*- C++ -*----------------------------------*\
| =========                 |                                                 |
| \\      /  F ield         | OpenFOAM: The Open Source CFD Toolbox           |
|  \\    /   O peration     | Version:  v2206                                 |
|   \\  /    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       false;


// 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
{
    clio3rs.stl
    {
        type triSurfaceMesh;
        name clio3rs;
    }

    refinementBox
    {
        type box;
        min  (-0.8027 -0.8881 0.0);
        max  (3.1784 0.8883 1.465);
    }
}


// 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 10000000;

    // 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 3;



    // 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 "clio3rs.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
    {
        clio3rs
        {
            // Surface-wise min and max refinement level
            level (4 6);
        }
    }

    // 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
    {
        //refinementBox
        //{
        //    mode inside;
        //    levels ((1E15 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 (-1.95 1.45 0.01);


    // 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 8;

    //- 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 80;

    //- Maximum number of snapping relaxation iterations. Should stop
    //  before upon reaching a correct mesh.
    nRelaxIter 8;

    // Feature snapping

    //- Number of feature edge snapping iterations.
    //  Leave out altogether to disable.
    nFeatureSnapIter 20;

    //- 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 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
    {
        clio3rs
        {
            nSurfaceLayers 3;
        }
    }

    // Expansion factor for layer mesh
    expansionRatio 1.0;

    // 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 0.3;

    // 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;

    // 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 130;

    // At non-patched sides allow mesh to slip if extrusion direction makes
    // angle larger than slipFeatureAngle.
    slipFeatureAngle 30;

    // 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 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 60;
}



// Generic mesh quality settings. At any undoable phase these determine
// where to undo.
meshQualityControls
{
    #include "meshQualityDict"


    // Advanced

    //- Number of error distribution iterations
    nSmoothScale 4;
    //- Amount to scale back displacement at error points
    errorReduction 0.75;
}


// Advanced

// Write flags
writeFlags
(
    scalarLevels
    layerSets
    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;


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

Thank you!


Best,
Marius

[AtoHM]

I see nothing unusual, so just some thoughts:
1) try minRefinementCells > 0 (1?)
2) try surfaceRefinement with min and max level 5 6 or 4 5 and not two levels apart
3) do you reach 10M cells in the process? Then increase maxGlobalCells.



Regarding the wheel: Does the blockMesh really cover the geometry completely?

[Counterdoc]

Thanks for the help.


I just increased minRefinementCells to 10 and also adjusted surfaceRefinement to 5 6.


Regarding 3)
I reach 10M cells, yes. But as soon as I increase the value for maxGlobalCells snappyHexMesh just crashes and closes the terminal window. I have no idea why. I started with 8 cores in parallel meshing and reduced it to 4, but no luck.


Setup is:
Ryzen 7 3800X - 8 physical cores at 4.7 GHz
32 GB DDR4 3200 RAM


I can see that the RAM hits the 32 GB while meshing and uses the swap partition.


Oh and regarding the blockMesh. The car is at Z 0.0 and the blockMesh also ends at Z 0.0. I just set Z to -1.0E-4

[linnemann]

You could scale the mesh/geometry to mm so that snappy has larger numbers to work with.


Precission is better when 1 is 1.0 and not 0.001


It does not always help, but I've found it genereally performs a bit better.


Sometimes I wonder if Snappy were to be faster if everything internally was integers. just scale the mesh geometry so 1mm is 1000 then precission could still be ok when converting to m again.

[AtoHM]

Ok, so the cell cap is most likely the reason you have these big cells, once snappy hits the cap, it just stops refinement.
Regarding blockMesh: indeed, I always provide a slightly bigger blockMesh than the bounding box of the stl to avoid such effects. So, this should be fixed now as well?
But it appears to be a memory issue now and I don't know how to resolve that other than getting more resources.
Interesting remarks from linnemann, will there be an influence on the memory there?

[Counterdoc]

Currently snappyHexMesh is running with 40M cells and it still reaches that limit. But I guess that is normal for a whole car that should have a fine mesh. I have seen a bachelor thesis from an Audi student with a 70M cells mesh.



Regarding memory I think I will upgrade the RAM to 64 GB. For more I would have to build a new PC or switch the whole calculations to a server with big resources.

[linnemann]

Rule of thumb, 1GB of memory per 1mil cells.

[Counterdoc]

Originally Posted by linnemann

Rule of thumb, 1GB of memory per 1mil cells.

Good tip, thank you! I will upgrade the RAM :-)