CFD Online Discussion Forums - [snappyHexMesh] Mesh does not follow feature edges

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Mesh does not follow feature edges

Hi all,

I'm relatively new to OpenFOAM, so unfortunately I feel completely lost with the following problem and have no idea which knobs to turn:

The case:

windAroundBuildings tutorial.
I only use a different OBJ file and adjust the dimensions of

Code:

backgroundMesh

and

Code:

refinementBox

in the

Quote:

blockMeshDict

and

Code:

snappyHexMeshDict

, respectively.

What you see in the screenshot:

The grey solid volume is from the OBJ and represents the building.
The white lines on the building are the feature edges as extracted by

Code:

surfaceFeatureExtract

.
The pink lines come from rendering the VTM file as "Feature Edges" in ParaView.

The problem is that the pink feature edges do not follow the actual features of the building nicely. I suspect that

Code:

snappyHexMesh

needs better refinement, but I don't know which parameter to adjust or whether this is even the correct assessment of the problem.

It does seem that some more refinement could help. You have the following options:
- make a finer background mesh by changing the number of cells in blockMeshDict
- volume refinement (the 'box' you're talking about)
- Refine cells on buildings' surfaces
- Refine cells on edges.

Those options are sorted by number of additional cells they will generate.
I'd recommend the 'surfaces' option, just add refinementSurfaces to castellatedMeshControls, then increase level step by step.

https://www.openfoam.comu/documentat...nementSurfaces

If you share your snappy dict, I can show you more precisely. :)

Quote:

Originally Posted by kandelabr (Post 833782)

If you share your snappy dict, I can show you more precisely. :)

Thanks a lot for the help! :) I inserted the snappy dict below. It's directly taken from the

Code:

windAroundBuildings

example.

Code:

/*--------------------------------*- 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;

}

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



#includeEtc "caseDicts/mesh/generation/snappyHexMeshDict.cfg"



castellatedMesh on;

snap            on;

addLayers       off;



geometry

{

    buildings.obj

    {

        type triSurfaceMesh;

        name buildings;

    }



    refinementBox

    {

        type box;

        min  (-97.1975 -110.911 0.0);

        max  (137.1975 150.911 78.0);

    }

}



castellatedMeshControls

{

    features

    (

      { file  "buildings.eMesh"; level 1; }

    );



    refinementSurfaces

    {

        buildings

        {

            level (3 3);

            patchInfo { type wall; }

        }

    }



    refinementRegions

    {

        refinementBox

        {

            mode inside;

            levels ((1E15 2));

        }

    }



    locationInMesh (1 1 1);

}



snapControls

{

    explicitFeatureSnap    true;

    implicitFeatureSnap    false;

}



addLayersControls

{

    layers

    {

        "CAD.*"

        {

            nSurfaceLayers 2;

        }

    }



    relativeSizes       true;

    expansionRatio      1.2;

    finalLayerThickness 0.5;

    minThickness        1e-3;

}



meshQualityControls

{}



writeFlags

(

    noRefinement

    // scalarLevels

    // layerSets

    // layerFields

);



mergeTolerance 1e-6;





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

Code:

    features

    (

      { file  "buildings.eMesh"; level 1; }

    );



    refinementSurfaces

    {

        buildings

        {

            level (3 3);

            patchInfo { type wall; }

        }

    }

Increase the bold numbers and see what happens. First try to set level 4; in features, then also level (4 4); also in surfaces.

Thanks, that helped and was also a good starting point to learn more about SnappyHexMesh. :)