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Old   November 19, 2015, 11:40
Default snappyHexMesh - backward facing step
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Hi all,

I want to mesh a backward facing step-STL geometry using snappyHexMesh.
How can I prevent snappyHex mesh from rounding off the edges like it does right now?
(I just want 90 Edges)
(see picture)

(I am aware that there are easier ways to mesh such a rectangular geometry but I would like to try snappyHexMesh here)

What is the meaning of the following warning?

--> FOAM Warning : 
    From function autoSnapDriver::calcNearestFace(..)
    in file autoHexMesh/autoHexMeshDriver/autoSnapDriverFeature.C at line 404
    Did not find surface near face centre (1.49788 0.02185 0.00625)


/*--------------------------------*- C++ -*----------------------------------*\
| =========                 |                                                 |
| \\      /  F ield         | OpenFOAM: The Open Source CFD Toolbox           |
|  \\    /   O peration     | Version:  2.4.0                                 |
|   \\  /    A nd           | Web:                      |
|    \\/     M anipulation  |                                                 |
    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
        type triSurfaceMesh;
        name KANAL;
        type triSurfaceMesh;
        name INFLOW;
        type triSurfaceMesh;
        name INFLOW;
        type triSurfaceMesh;
        name WALLINLETLOWER;
        type triSurfaceMesh;
        name STEPWALL;
        type triSurfaceMesh;
        type triSurfaceMesh;
        name UPERWALLL;
        type triSurfaceMesh;
        name SYMMETRY1;
        type triSurfaceMesh;
        name SYMMETRY2;

    //- Refine a bit extra around the small centre hole
//     refineHole
//     {
// //         type searchableSphere;
// //         centre (0 0 -0.012);
// //         radius 0.003;
//     }

// Settings for the castellatedMesh generation.

    // 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 1;

    // Explicit feature edge refinement
    // ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

    // Specifies a level for any cell intersected by its edges.
    // This is a featureEdgeMesh, read from constant/triSurface for now.
            file "KANAL.extendedFeatureEdgeMesh";
            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.

            // Surface-wise min and max refinement level
            level (0 0);

    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.

//         refineHole
//         {
//             mode inside;
//             levels ((1E15 3));
//         }

    // 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.
    // This is an outside point locationInMesh (-0.033 -0.033 0.0033);
    locationInMesh (0.5 0.1 0.0005); // Inside point

    // 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.
    //- 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 5;

    // Feature snapping

        //- Number of feature edge snapping iterations.
        //  Leave out altogether to disable.
        nFeatureSnapIter 10;

        //- Detect (geometric) features by sampling the surface
        implicitFeatureSnap false;

        //- Use castellatedMeshControls::features
        explicitFeatureSnap true;

        //- Detect features between multiple surfaces
        //  (only for explicitFeatureSnap, default = false)
        multiRegionFeatureSnap true;

// Settings for the layer addition.
    // 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
//         "flange_.*"
//         {
//             nSurfaceLayers 1;
//         }

    // 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.
    // See relativeSizes parameter.
    minThickness 0.25;

    // 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.
    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 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
    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.
    #include "meshQualityDict"

    // Optional : some meshing phases allow usage of relaxed rules.
    // See e.g. addLayersControls::nRelaxedIter.
        //- Maximum non-orthogonality allowed. Set to 180 to disable.
        maxNonOrtho 75;

    // Advanced

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

// Advanced

// Write flags
    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;

// ************************************************************************* //
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Old   November 20, 2015, 05:24
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Niels Nielsen
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Well as you stated you are really using a hammer where you should use a screwdriver.

My best advice would be to use another tool for such geometry and use SHM where that excels.

SHM is not the best at capturing sharp edges and requires a lot of tweaking to be perfect.

You could also try cfMesh as that gives better results with sharp edges IMO.

PS. I do not do personal support, so please post in the forums.
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Old   November 20, 2015, 09:20
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You are absolutely right I am trying to do this in SALOME now. Do you think that's an appropriate choice?
The problem which broad me here is that I want to create a mesh with high wall resolution while keeping a low aspect ratio (I read that it has to be below 10).
Is that correct? Do I have to keep such a low AR or is there any guideline available?
In the pitzyDaily-Tut they achieve a max AR of ~7.
Also is there a possibility to show the AR in paraFoam or select based on AR even if it doesn't exceed checkMesh-maximum?

Thanks for your advice!
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Old   November 20, 2015, 09:29
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Salome would be a very good choice.

Regarding the other questions.

You can have very high AR as long as you are sure the flow is only in one direction. If the flow have a slight angle compared the the mesh direction and you have high AR then you will get numerical errors/instability.

There is a whole discussion about high AR when dealing with eg. wing profiles.
As you can imagine it can be hard to estimate separation point on the profile with a very high AR as the flow has a tendency to follow the mesh shape.

When you do checkMesh and you have high AR you can have that write out a cellSet/faceSet and you can do foamToVTK -cellSet <nameOfCellSet>
The name will be output from checkMesh.

Then you can load the VTK file in paraview and also the complete mesh, color the high AR with a different color and zoom into the area of interest.

If there is only one cell it can be hard to spot so turn off the big mesh and do a "Reset" of the camera. That will zoom into what mesh is visible.
Attached Images
File Type: png 2015-11-20 14_26_19-ParaView 4.3.1 64-bit.png (2.2 KB, 13 views)
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PS. I do not do personal support, so please post in the forums.
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Old   November 20, 2015, 10:57
Default underdeterminedCells
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That makes sense, so I should pay attention to AR especially in regions where eddys occur.

There is one thing which bothers me about checkMesh.
It gives a ***-warning "underdeterminedCells"?
What is the meaning of that?

I found this post
What is an under-determined cell?

When you have a look at the source code of the check mesh utility, you will see that there are two kinds of underdetermined cells.

The first kind of cells do have two or less free internal faces that do not belong to boundary patches. This can lead to numerical errors, for example when a tet element has 3 boundary faces.

About the second kind of cells I cannot tell you much. I still don't know what's going on there. But it has something to do with the ration of one face area and the complete element area.
So I tried spliting those cells and they are no longer underdetermined but I don't know exactly why that is?
Do you know something about it?
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