[JulytoNovember]

I'm new to snappyhexmesh, I have 5 unconnected regions in 5 files. 2 rotors, 2 AMI, 1 outer cylinder. I used the propeller snappyhexmeshdict to try to mesh my stl files. Do I need to create all the meshes separately? Or can I mesh the 5 region as 1 system, even though they are not connected?

[kwardle]

It is not exactly clear what you are looking for--more info please. Pictures may be helpful. When you say 'unconnected region' what exactly do you mean? If you are saying that you have each surface or patch defined in its own file as a separate body then you may be able to just concatenate the files into one stl. That said, snappy requires a closed stl so you need to be careful that the original stl is good quality.
Again, more info gets you better help. Less info, and only few people are willing to take a wild stab in the dark...

[JulytoNovember]

Thank you for your reply. By unconnected I mean the patches share no faces. When I run my snappyhexmesh, the only part that is shown is the lower AMI. The blockMesh background doesn't change at all. Here is my snappyhexmeshdict and a pic of the lower AMI. My stl files are closed and I'm not getting any error messages from my snappyhexmesh log file. Any ideas of where I'm going wrong?

/*--------------------------------*- C++ -*----------------------------------*\
| =========                 |                                                 |
| \\      /  F ield         | OpenFOAM: The Open Source CFD Toolbox           |
|  \\    /   O peration     | Version:  2.1.1                                 |
|   \\  /    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       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
{
    Cylinder.stl
    {
        type        triSurfaceMesh;
        name        Cylinder;
    }
    rotor1.stl
    {
        type        triSurfaceMesh;
        name        rotor1;
    }
    rotor2.stl
    {
        type        triSurfaceMesh;
        name        rotor2;
    }
    AMI1.stl
    {
        type        triSurfaceMesh;
        name        AMI1;
    }
    AMI3.stl
    {
        type        triSurfaceMesh;
        name        AMI3;
    }
};

// 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 20000000;

    // 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 10;

    // 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        "Cylinder.eMesh";
            level       2;
        }
        {
            file        "rotor1.eMesh";
            level       4;
        }
        {
            file        "rotor2.eMesh";
            level       4;
        }
        {
            file        "AMI1.eMesh";
            level       4;
        }
        {
            file        "AMI3.eMesh";
            level       4;
        }
    );



    // 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
    {
        Cylinder
        {
            level       (0 0);
        }
        rotor1
        {
            level       (5 5);
        }
        rotor2
        {
            level       (5 5);
        }
    AMI1
        {
            level       (4 4);
            cellZone    AMI1;
            faceZone    AMI1;
            cellZoneInside  inside;
        }
        AMI3
        {
            level       (4 4);
            cellZone    AMI3;
            faceZone    AMI3;
            cellZoneInside  inside;
        }
    }

    // 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
    {
        AMI1
        {
            mode        inside;
            levels      ((1E15 4));
        }
        AMI3
        {
            mode        inside;
            levels      ((1E15 4));
        }
        Cylinder
        {
            mode        inside;
            levels      ((1E15 4));
        }
    }


    // 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 -0.75 0);


    // 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 4.0; // 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 5;

    //- 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 20;
}



// 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
    {
    }

    // 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 0.3;

    //- 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 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 30;

    //- Maximum number of snapping relaxation iterations. Should stop
    //  before upon reaching a correct mesh.
    nRelaxIter 3;

    // 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 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 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. 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 -1; // 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 0.01;

    //- 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;


    // 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 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 .stl 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;


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

[kwardle]

Originally Posted by JulytoNovember

By unconnected I mean the patches share no faces.

But presumably they share edges? That is what I mean, the edges and edge nodes need to match up perfectly for snappy to catch the internal volume. From what you have described I think you have included each of your patches as a separate stl file. Probably what has happened is that your locationInMesh is inside AMI1 so that is the only one being kept, but it is not a closed volume so the castellated mesh does not get cut right.

What you want to do is combine all of these into a single closed stl. You can just concatenate them and give each "solid" the name of the patch in the stl file for each solid part. As in:

...
solid rotor1
<facet stuff>
endsolid rotor1
solid rotor2
<facet stuff>
endsolid rotor2
...

Perhaps you have already named these in each individual stl? Just take a look at the first line--it will read "solid <name>".

After you have a combined stl file--let's call it "full.stl"--then you read in just that one stl file into snappy as but then have separate 'region'
full.stl
{
type triSurfaceMesh;
name full;

regions
{
cylinder
{
name cylinder;
}
rotor1
{
name rotor1;
}
...[repeat for all patches]...
}
}

The reason you have to add the region defs and names for each patch is because as a default it will name each as full_rotor1, full_rotor2, etc. You could just change this in you polyMesh/boundary file after it is created, but this saves a step later on. As an alternative you could keep the full.stl unamed and then make your patches later on by running surfaceToPatch using your individual stl files.

Based on the still somewhat sketchy information you have given about what your geometry actually looks like this is my best guess. Hope this makes sense and is helpful.

[JulytoNovember]

Thank you for your help. You answered my question, I didn't mean to be so vague. I just commented out all but one surface, then added 1 new surface when the previous surface was visually correct.

[halvo912]

Hey all,


Lurker and new to posting. I had a question about this case and was wondering if anyone may be able to help.



It was advised to merge all stl files into one and then name the separate regions in the snappyhexmeshdict, but how does the dict know which solids you're referring to in the stl file? Say for example, I have 4 different solids combined in one stl file. How does the dict know which of the 4 I'm referring to when I create the definitions? Are there names for the solids attached somewhere in the stl file?


Thank you

[AtoHM]

Given you have an ASCII STL, try opening it up with a text editor
If its binary, you can try to open it in Paraview and save as ASCII, I am not sure if it carries over the names correctly, though.