[Irish09]

Hello Foamers!

After working through this meshing process on the internal flow of a diesel injector I felt inclined to share my results as internal flow applications of SHM seem to inspire many questions on the forums.

!!Disclaimer, I am no SHM expert, so if anyone wants to elaborate on any of these issues please do!!

To begin with 2D slice of the 3D injector file I began with:
Injector_2D_Slice.jpg

Due to the sharp angles that are present in the geometry as well as circular shapes I needed to use surfaceFeatureExtract twice:

surfaceFeatureExtract -includedAngle 180 -writeObj  constant/triSurface/SprayA210675doublesmoothascii.stl  outputfile

surfaceFeatureExtract -includedAngle 150  -writeObj constant/triSurface/SprayA210675doublesmoothascii2.stl  outputfile2

(Note SprayA210675doublesmoothascii.stl and SprayA210675doublesmoothascii2.stl are actually the same stl file, but if I didn't do it this way then the two surface extractions overwrote each other)

You can then check to see what surfaces were included in the output files you created. Looking at the file "filename_edgeMesh.obj" and taking a slice of the resulting image:

surfaceedges1.jpg

You can see that the horizontal lines were were not captured by the surface edge program. Looking at the second _edgeMesh.obj file:

surfaceedges2.jpg

The horizontal edges have now been captured. Note a few extra things have been captured that overlap with the first surface extraction, but that is fine.

Next make sure that your blockmeshdict creates a mesh of sufficient density, ie make sure that the interior of the geometry you want to capture has nodes in it.

There are a few things to make sure you edit before running a case like this. First make sure you load your geometry:

geometry
{
    SprayA210675doublesmoothascii.stl
    {
        type triSurfaceMesh;
        name nozzle;
    }
};

Then specify which edges are being refined, ie the .eMesh files you create (which should be located in the /constant/triSurface/ directory):

    // 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 "SprayA210675doublesmoothascii.eMesh";
            level 0;
        }
        {
            file "SprayA210675doublesmoothascii2.eMesh";
            level 0;
        }
    );

Note that the level for each of the .eMesh files is set to 0, which is counterintuitive based on the refinementSurfaces section. However, if a value >0 is chosen then the edges will not get snapped to correctly in my experience thus far.

Specify how much refinement you want to perform (Note SHM takes the name of the stl you provide, in this case nozzle, and adds an underscore. Since I want to refine all surfaces I can use the wildcard * to make sure all surfaces are refined):

    refinementSurfaces
    {
      "nozzle_*"
      {
          // Surface-wise min and max refinement level
          level (8 9);
      }

    }

Make sure the location in the mesh is defined to be within the geometry which you input:

    locationInMesh (0.07 0.0 1.3);

Enable the edge snapping feature:

    //- Highly experimental and wip: number of feature edge snapping
    //  iterations. Leave out altogether to disable.
    nFeatureSnapIter 10;

Specify how many layers, if any, you want to create near the edges:

    layers
    {
        "nozzle_*"
        {
           nSurfaceLayers 4;
        }
    }

For completeness, the entire snappyHexMeshdict is incldued here:

/*--------------------------------*- C++ -*----------------------------------*\
| =========                 |                                                 |
| \\      /  F ield         | OpenFOAM: The Open Source CFD Toolbox           |
|  \\    /   O peration     | Version:  2.0.0                                 |
|   \\  /    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
{
    SprayA210675doublesmoothascii.stl
    {
        type triSurfaceMesh;
        name nozzle;
    }
};

// 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 20;

    // 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 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 "SprayA210675doublesmoothascii.eMesh";
            level 0;
        }
        {
            file "SprayA210675doublesmoothascii2.eMesh";
            level 0;
        }
    );



    // 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
    {
      "nozzle_*"
      {
          // Surface-wise min and max refinement level
          level (8 9);
      }

    }

    // Resolve sharp angles
    resolveFeatureAngle 180;


    // 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
    {

    }


    // 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 (0.07 0.0 1.3);


    // 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 4.0;

    //- 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.
    //  Do not use here since mesh resolution too low and baffles present
    nFeatureSnapIter 10;
}



// 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
    {
        "nozzle_*"
        {
           nSurfaceLayers 4;
        }
    }

    // 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.
    //  is the thickness of the layer furthest away from the wall.
    //  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.
    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
    //  make straight angle.
    featureAngle 90;

    //- 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
{
    //- Maximum non-orthogonality allowed. Set to 180 to disable.
    maxNonOrtho 65;

    //- Max skewness allowed. Set to <0 to disable.
    maxBoundarySkewness 3.5;
    maxInternalSkewness 2.5;

    //- 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-30;

    //- 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;


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


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

The resulting mesh:
mesh1.pngmesh2.jpg

It's not totally complete as I need to reduce the number of cells (currently 2.25 million), but this shows the overall process and what things to consider.

Some thoughts to consider if you may still be having problems:

1) Look at the bounding box you are using with your desired geometry. Could you make it smaller in order to reduce the initial mesh size?

2) Are you going to exceed the maxGlobalCells value? SHM will not abort if you exceed this value, but rather just stop creating new cells. By increasing the initial mesh density defined in the blockmeshdict you increase the chances of this value being exceeded.

3) If you increased the initial mesh density, how many refinement levels are you performing? By increasing the initial density of the mesh, the refinement levels do not need to be as large. Refinement levels dictate how many times each cell is split into smaller cells. So if you increase the density of the initial mesh and keep the refinement level at say 10 or 11, then you may be increasing the resulting mesh calculation much more than you may think.

4) All of these things could contribute to SHM eating up all of your memory and aborting the operation. If you are getting the memory error then try reducing the number of refinement levels, the initial mesh density, and even possibly the number of edge layers being created. Get it to a point where it will successfully run and then if necessary increase these values to get the desired mesh resolution.

5) If you are unsure if your memory is being completely utilized watch your SHM operation using the 'top' command. In my experience SHM aborted right when my memory maxed out.

[romant]

Hej,

thanks for sharing this. I like the idea of the double feature edge extraction and your thoughts on the the snapping edge feature.

I have one more question, which of the .obj files do you usually inspect in order to see if you featureSurface extraction was good or not?

[Irish09]

Originally Posted by romant

Hej,

thanks for sharing this. I like the idea of the double feature edge extraction and your thoughts on the the snapping edge feature.

I have one more question, which of the .obj files do you usually inspect in order to see if you featureSurface extraction was good or not?

Roman,

I'm glad you find it useful, I had a hard time finding internal flows to base my mesh on so figured I would help provide one.

The .obj file from surfacefeatureextract that I have been using is the "filename_edgeMesh.obj", which paraview can open directly. Though I'm sure there are other options available, like just looking at the .eMesh file. I believe I have seen some discussion in the forums about trying to open an .eMesh in paraview, but I didn't investigate it much.

[wyldckat]

Greetings to all!

FYI: you can convert "eMesh" files to "obj" and vice-versa:

surfaceFeatureConvert relative/path/file.eMesh file.obj

Best regards,
Bruno

[Turbulence]

HI Everyone

I want to improve the cell density in certain regions of my mesh(created using SHM).
It would be great if anyone could help me understand how refinement levels work.

Thank You
Turbulence.

[wyldckat]

Hi Turbulence,

You should find answers here: http://openfoamwiki.net/index.php/Sn...als_and_Guides - read the tutorial "A Comprehensive Tour of snappyHexMesh".

Best regards,
Bruno