CFD Online Logo CFD Online URL
www.cfd-online.com
[Sponsors]
Home > Forums > Software User Forums > OpenFOAM > OpenFOAM Meshing & Mesh Conversion

[snappyHexMesh] Error defining boundary layer around cube snappyHexMesh

Register Blogs Members List Search Today's Posts Mark Forums Read

Reply
 
LinkBack Thread Tools Search this Thread Display Modes
Old   July 7, 2018, 17:05
Default Error defining boundary layer around cube snappyHexMesh
  #1
Member
 
Cristina Hernandez
Join Date: May 2018
Posts: 35
Rep Power: 3
crizpi21 is on a distinguished road
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:

Code:
/*--------------------------------*- 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
crizpi21 is offline   Reply With Quote

Old   July 9, 2018, 08:14
Default
  #2
New Member
 
Sarath
Join Date: Mar 2017
Location: Spain
Posts: 18
Rep Power: 4
sk11 is on a distinguished road
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
sk11 is offline   Reply With Quote

Old   July 9, 2018, 14:38
Default
  #3
Member
 
Cristina Hernandez
Join Date: May 2018
Posts: 35
Rep Power: 3
crizpi21 is on a distinguished road
Quote:
Originally Posted by sk11 View Post
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
crizpi21 is offline   Reply With Quote

Old   July 10, 2018, 05:15
Default
  #4
New Member
 
Sarath
Join Date: Mar 2017
Location: Spain
Posts: 18
Rep Power: 4
sk11 is on a distinguished road
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
sk11 is offline   Reply With Quote

Old   July 10, 2018, 07:08
Default
  #5
Member
 
Cristina Hernandez
Join Date: May 2018
Posts: 35
Rep Power: 3
crizpi21 is on a distinguished road
Quote:
Originally Posted by sk11 View Post
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
crizpi21 is offline   Reply With Quote

Reply

Thread Tools Search this Thread
Search this Thread:

Advanced Search
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are On
Refbacks are On


Similar Threads
Thread Thread Starter Forum Replies Last Post
Error - Solar absorber - Solar Thermal Radiation MichaelK CFX 12 September 1, 2016 06:15
Wrong flow in ratating domain problem Sanyo CFX 17 August 15, 2015 07:20
Radiation interface hinca CFX 15 January 26, 2014 18:11
[snappyHexMesh] Boundary layer grid for Wigley hull by snappyHexMesh novak1203 OpenFOAM Meshing & Mesh Conversion 8 June 27, 2013 13:03
[snappyHexMesh] Boundary layer in a pipe Clementhuon OpenFOAM Meshing & Mesh Conversion 6 March 12, 2012 13:41


All times are GMT -4. The time now is 16:16.