[Dreoasteh]

Hi everyone,

I've been trying to mesh a nozzle for an LES case. The coarsest mesh is 1.5M cells (first image below - its a slice of the nozzle contraction and exit). Then I refined the blockMesh so as to produce 3M and 6M cell meshes (second and third images).

As you can see the outline becomes increasingly crooked in the contraction and sharp edge of the exit. It is as if the snapping processes did not work correctly for the finer meshes and it no longer follow the STLs. This can be seen clearly in the fourth image, which is a detail view of the nozzle exit for the 6M mesh. The snapping is distorted and is much worse than in the 1.5M mesh in the fifth image.

In case there is an error in the snappyHexMeshDict, I have included it below:

/*--------------------------------*- C++ -*----------------------------------*\
| =========                 |                                                 |
| \\      /  F ield         | OpenFOAM: The Open Source CFD Toolbox           |
|  \\    /   O peration     | Version:  2.4                                   |
|   \\  /    A nd           | Web:      www.OpenFOAM.org                      |
|    \\/     M anipulation  |                                                 |
\*---------------------------------------------------------------------------*/
FoamFile
{
    version     2.0;
    format      ascii;
    class       dictionary;
    object      snappyHexMeshDict;
}

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

castellatedMesh true;
snap            true;
addLayers       true;

geometry
{
    inj.stl 
    {
        type triSurfaceMesh;
        name inj;
    }
    nozExt.stl
    {
        type triSurfaceMesh;
        name nozExt;
    }
    nozInt.stl
    {
        type triSurfaceMesh;
        name nozInt;
    }
    nozCon.stl
    {
        type triSurfaceMesh;
        name nozCon;
    }
    refinement1
    {
        type searchableCylinder;
        point1 (0.0 0.0 0.0);
        point2 (0.142711875 0.0 0.0);
        radius 0.035;
    }
    refinement2
    {
        type searchableCylinder;
        point1 (0.02 0.0 0.0);
        point2 (0.11 0.0 0.0);
        radius 0.025;
    }
    auxHollowCylinder.stl
    {
        type triSurfaceMesh;
        name auxHollowCylinder;
    } 
    refinement5
    {
        type searchableCylinder;
        point1 (0.0  0.0 0.0);
        point2 (0.065 0.0 0.0);
        radius 0.006;
    }
    refinement6
    {
        type searchableCylinder;
        point1 (0.025 0.0 0.0);
        point2 (0.030 0.0 0.0);
        radius 0.0025;
    }
};

castellatedMeshControls
{
    maxLocalCells 10000000;
    maxGlobalCells 20000000;
    minRefinementCells 0;
    nCellsBetweenLevels 2;

    features
    (
        
        {file "nozExt.eMesh";level 3;}
        {file "nozInt.eMesh";level 4;}
        {file "nozCon.eMesh";level 5;}
        {file "inj.eMesh";level 3;}
        
    );

    refinementSurfaces
    {
        nozExt
        {
            level (3 3);
            patchInfo
            {
                type    wall;
            }
        }
        nozInt
        {
            level (4 4);
            patchInfo
            {
                type    wall;
            }
        }
        nozCon
        {
            level (5 5);
            patchInfo
            {
                type    wall;
            }
        }        
        inj
        {
            level (3 3);
            patchInfo
            {
                type    patch;
            }
        }
    }

    resolveFeatureAngle 30;

    refinementRegions
    {

        refinement1
        {
            mode inside;
            levels ((1E15 2));
        }
        refinement2
        {
            mode inside;
            levels ((1E15 3));
        }
        auxHollowCylinder
        {
            mode inside;
            levels ((1E15 5));
        }        
        refinement5
        {
            mode inside;
            levels ((1E15 4));
        }
        refinement6
        {
            mode inside;
            levels ((1E15 4));
        }
        nozCon
        {
            mode distance;
            levels ((0.0007 5));
        }        
    }

    locationInMesh (0.05 -0.0000 -0.0000);
    allowFreeStandingZoneFaces true;
}

snapControls
{
    nSmoothPatch 3;
    tolerance 1;
    nSolveIter 300;
    nRelaxIter 10;
    nFeatureSnapIter 15;
    implicitFeatureSnap false;
    explicitFeatureSnap true;
    multiRegionFeatureSnap false;

}

addLayersControls
{
    relativeSizes true;

    layers
    {
        nozInt
        {
            nSurfaceLayers 5;
            finalLayerThickness 0.3;
            minThickness 0.05;
        }
        walls
        {
            nSurfaceLayers 5;
            finalLayerThickness 0.3;
            minThickness 0.05;
        }        
        nozCon
        {
            nSurfaceLayers 5;
            finalLayerThickness 0.3;
            minThickness 0.05;
        }    
    }
    expansionRatio 1.2;
    finalLayerThickness 0.3;
    minThickness 0.05;
    nGrow 0;
    featureAngle 180;
    slipFeatureAngle 30;
    nRelaxIter 10;
    nSmoothSurfaceNormals 1;
    nSmoothNormals 3;
    nSmoothThickness 10;
    maxFaceThicknessRatio 0.5;
    maxThicknessToMedialRatio 0.3;
    minMedianAxisAngle 90;
    nBufferCellsNoExtrude 0;
    nLayerIter 150;
}

meshQualityControls
{
    maxNonOrtho 60;
    maxBoundarySkewness 20;
    maxInternalSkewness 4;
    maxConcave 80;
    minVol 1e-13;
    minTetQuality 1e-15;
    minArea -1;
    minTwist 0.02;
    minDeterminant 0.001;
    minFaceWeight 0.02;
    minVolRatio 0.01;
    minTriangleTwist -1;
    nSmoothScale 4;
    errorReduction 0.75;
}

writeFlags
(
);

mergeTolerance 1E-6;
// ************************************************************************* //

Any suggestions towards solving this issue are greatly appreciated

[linnemann]

Hi

If I were you I would try with cfmesh instead.
It's faster, better captures features and are better at layer coverage.

https://www.openfoam.com/releases/op...processing.php

https://cfmesh.com/meshing-software/

I'm in no way affiliated with them, but from my findings it makes a good mesh.

Automatic meshing strategies

[Dreoasteh]

Hi Niels,

Thanks for you advice. Indeed cfmesh produced much better quality in an incredibly smaller amount of time (1 min vs 1 hour!)

I would recommend anyone stuck with snappy to try it out