[crizpi21]

Hi,

I am trying to simualte the flow around a cube with rounded edges using OpenFOAM and I need to model the boundary layer on the surface of the cube.

I am using snappyHexMesh to generate the mesh and have sucessfully defined a region with a finer mesh around the cube (first 2 steps: castellating and snapping), see attached file mesh_cube.jpg. However, the last step (addying layers) is not working as I want. My objective is to have 20 layers around the surface of the cube with a very small thickness to accurately model the boundary layer and fulfill the y+< 1 requirement for the first layer. So far, no layers have been created. Find a link to my case with all the files here: https://drive.google.com/drive/folde...qV?usp=sharing

I also copy here my snappyHexMesh file:

/*--------------------------------*- C++ -*----------------------------------*\
| =========                 |                                                 |
| \\      /  F ield         | OpenFOAM: The Open Source CFD Toolbox           |
|  \\    /   O peration     | Version:  2.2.1                                 |
|   \\  /    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       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
{
    cube.stl
    {
        type triSurfaceMesh;
        name cube;
    }

    refinementBox1
    {
        type searchableBox;
        min (18.5 15.25 5.25);
        max (29 19.75 9.75);
      }
};



// PHASE 1. CASTELLATEDMESH settings
castellatedMeshControls
{

    // Refinement parameters

    maxLocalCells 10000000;
    maxGlobalCells 20000000;
    minRefinementCells 0; //antes tenía 10, del tío random de internet
    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.
     features
    (
        {
            file "cube.eMesh";//file "cube.extendedFeatureEdgeMesh"
            level 3; //antes tenía 0, ERRROOOOOOR
        }
    );


    // 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
    {
        cube
        {
            // Surface-wise min and max refinement level
            level (3 3); //antes tenía 7 7

            // Optional specification of patch type (default is wall). No
            // constraint types (cyclic, symmetry) etc. are allowed.
            patchInfo
            {
                type wall;
                inGroups (cubeGroup);
            }
        }
    }

    // Resolve sharp angles
    resolveFeatureAngle 10; //con 10 OK, edges whose adjacent surface normals are at angle > value are resolved


    // 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
    {
        //cube
        //{
         //mode distance;
         //levels ((0.01 3)); //antes 0.0006 6
        //}
        refinementBox1
        {
            mode inside;
            levels ((1E15 3)); //antes 5, 4 me genera ~20 celdas en square, OK
        }

    }


    // 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 (9.23149e-05 0.0025 0.0025); //antes 0 0 1


    // 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;
}



// Phase 2: SNAPPING Settings.
snapControls
{
    //- Number of patch smoothing iterations before finding correspondence to surface
    nSmoothPatch 5; //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 2.0;//1.0

    //- Number of mesh displacement relaxation iterations.
    nSolveIter 30;//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 15; //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 false; //true
}



// PHASE 3: LAYERING Settings
addLayersControls
{
    // Are the thickness parameters below relative to the undistorted
    // size of the refined cell outside layer (true) or absolute sizes (false).
    relativeSizes false;

    // Per final patch (so not geometry!) the layer information
    layers
    {
        cube

      {
        nSurfaceLayers 20;
      }
    }

    // Expansion factor for layer mesh
    expansionRatio 1.1;

    // 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.
    firstLayerThickness 1.573E-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 2.9E-4;

    // 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 1;

    // Advanced settings

    // When not to extrude surface. 0 is flat surface, 90 is when two faces
    // are perpendicular
    featureAngle 180; //antes a 30

    // 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 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 180;


    // 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 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;


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

Can anyone help me with this?

Thanks!

Cristina

[sk11]

Which version of OpenFoam you are using?
Did you check the timesteps created with SHM? or did you use -overwrite flag?

I tested with your case with some minor modification in surfaceFeatureExtractDict to cope up with version and I found shm creating layers.
Regards,
sarath

[crizpi21]

Originally Posted by sk11

Which version of OpenFoam you are using?
Did you check the timesteps created with SHM? or did you use -overwrite flag?

Hi Sarath, I am using OpenFOAM v1712 and I run SHM without overwrite, to look specifically at the last time step (addying layers). I kept on trying and, after some modifications, I am now able to visualise them in Paraview.

However, my problem now is that layers are collapsing. My objective is generate a create between 10 and 20 layers and the first layer around the obstacle should have a thickness around 1E-5 m (to fulfill y+<1 criteria).

So far I have tried with 10 layers and first layer thicknesses between 2E-4 m to 8E-4 m (which correspond to relative values of 0.01 to 0.04, respectively). In all cases my cells start to collapse as shown in the picture and I get less than 10 layers (around 6, increasing a bit as I increase first layer thickness). slice1.1_thickness004.PNG

I have also tried with different number of layers but I had similar results. Do you have any suggestions on how to solve this? I really need a very fine first layer (although I am still far) and I don't understand why this is behaving like this or which parameteres to modify.

Find a link to my modified case: https://drive.google.com/drive/folde...w__7ukbSA2ZvqV

Any advice would be helpful,
Thanks in advance

Cristina

[sk11]

You can specify layer thickness by these following ways :

- expansionRatio and finalLayerThickness (cell nearest internal mesh)
- expansionRatio and firstLayerThickness (cell on surface)
- overall thickness and firstLayerThickness
- overall thickness and finalLayerThickness
- overall thickness and expansionRatio

Try changing your configuration according to desired thickness.
regards,
Sarath

[crizpi21]

Originally Posted by sk11

You can specify layer thickness by these following ways :

- expansionRatio and finalLayerThickness (cell nearest internal mesh)
- expansionRatio and firstLayerThickness (cell on surface)
- overall thickness and firstLayerThickness
- overall thickness and finalLayerThickness
- overall thickness and expansionRatio

Try changing your configuration according to desired thickness.
regards,
Sarath

Hi, as I mentioned, I have tried several cases changing expansionRatio and firstLayerThickness according to the thickness I need (E-5). However, with a tichkness of E-4 and expansion ratio between 1.1 and 1.2, the layers collapse and when I go to lower thicknesses, they are not even created. It looks like there may be a limit in the values you can put. Do you have any clue?

Thanks,
Cristina

[vmanikanth]

Hello, Did you solved your problem. If yes please put your solution here. I am facing similar issue