[saeed jamshidi]

Hello,
I'm simulating flow over cylinder in order to learn openfoam. However, would you please let me know how i could alter snappyhexMesh parameters in order to nail to my desired mesh (specially close to the cylinder wall).
Mind you, I have attached my mesh and desired mesh:).Thank you for your time.

/*--------------------------------*- C++ -*----------------------------------*\
| =========                 |                                                 |
| \\      /  F ield         | OpenFOAM: The Open Source CFD Toolbox           |
|  \\    /   O peration     | Version:  v2112                                 |
|   \\  /    A nd           | Website:  www.openfoam.com                      |
|    \\/     M anipulation  |                                                 |
\*---------------------------------------------------------------------------*/
FoamFile
{
    version     2.0;
    format      ascii;
    class       dictionary;
    object      snappyHexMeshDict;
}
// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //

castellatedMesh true;
snap            true;
addLayers       true;

geometry
{
    cylinder
    {
        type    searchableCylinder;
        point1  (0 0 -0.01);
        point2  (0 0 0.01);
        radius  0.02;
    }
    refinementBox 
    {
        type searchableBox;
        min (-0.2 -0.2 -0.01);
        max (0.5 0.2 0.01);
    }
};


castellatedMeshControls
{
    maxLocalCells       100000;
    maxGlobalCells      200000000;
    minRefinementCells  10;
    maxLoadUnbalance    0.10;
    nCellsBetweenLevels 3;      // important
    resolveFeatureAngle 30;
    allowFreeStandingZoneFaces true;
    features
    ();
    refinementSurfaces
    {
        cylinder
        {
            
			level (4 4);
            patchInfo
            {
                type     wall;
                inGroups (cylinderGroup);
            }
        }
    };
	
    refinementRegions
	{
	    refinementBox
	    {
	  
	       mode inside;
		   levels ((1 1));
		
	    }
	  
	}
	
    locationInMesh (0.1 0.1 0);
}


snapControls
{
    nSmoothPatch    3;
    tolerance       2.0;
    nSolveIter      30; //
    nRelaxIter      5;   //
	nFeatureSnapIter 10; //
	implicitFeatureSnap false;
	explicitFeatureSnap true;
    multiRegionFeatureSnap false;
	
}


// Controls for boundary layer addition
addLayersControls
{
    relativeSizes true;
    layers
    {
        "cylinder"
        {
            nSurfaceLayers 7;
        }
        
    }
    expansionRatio 1.2;
    finalLayerThickness 0.3;
    minThickness 0.1;
    nGrow 0;
    featureAngle 30;
    slipFeatureAngle 80;
    nRelaxIter 3;
    nSmoothSurfaceNormals 1;
    nSmoothNormals 3;
    nSmoothThickness 10;
    maxFaceThicknessRatio 0.5;
    maxThicknessToMedialRatio 0.3;
    minMedianAxisAngle 90;
    nBufferCellsNoExtrude 0;
    nLayerIter 50;
}



// Controls for mesh quality
meshQualityControls
{
    maxNonOrtho 65;
    maxBoundarySkewness 20;
    maxInternalSkewness 4;
    maxConcave 80;
    minVol 1e-18;
    minTetQuality 1e-9;
    minArea -1;
    minTwist 0.05;
    minDeterminant 0.001;
    minFaceWeight 0.05;
    minVolRatio 0.01;
    minTriangleTwist -1;

    // Advanced
    nSmoothScale 4;
    errorReduction 0.75;

    relaxed
    {
        maxNonOrtho 75;
    }
}

// Advanced
debug 0;
mergeTolerance 1e-6;

[AtoHM]

Thats as good as a snappyHexMesh-mesh gets. Do you have any specific part you want another way? To me, it looks fine.

[saeed jamshidi]

Dear AtoHM, thank you for your response. My first goal is to understand snappyHexMesh nature deep as much as possible.
As you can see in zoomed area in the second mesh, except for boundary layers there are three step in meshing.
The point is this jump from smaller grids to larger grids is smooth and each cell just were splited to four cells (not two cells). In contrast, in my mesh there are cells that were splited to two cells.
I just want to know how I could do that. I don't know whether I could convey my mean or not?!
Thanks

[AtoHM]

Ah, I get it.
Do not worry, what you see is just a visual thing coming from a Paraview setting. You should deactivate the 'decompose polyhedra' option to get a better representation, as shown here: How to visualize polyhedron mesh in paraview


Just to clarify, snappy does neither split in half nor into 4, it's always 1 to 8 because we are in 3d. This is called an octree and shown e.g. here: https://www.cradle-cfd.com/technolog...etail0004.html

[saeed jamshidi]

Dear AtoHM, thank you again. It was really helpful.
I have attached my result; Maybe someone would see it!

[AtoHM]

Glad to help, now, go and play with

nCellsBetweenLevels 3;

and see what happens