[arezayan87]

Hi

I've stuck in a problem with [snappyHexMesh]. I need too mesh a big complex city, but when meshing process is finished the result is terrible!
the box-shaped buildings get deformed (see the picture attached )

I increased the level values in castellatedMeshControls block, in addition the value of the resolvedFeatureAngle is set 175. but no considerable changes occurred!!

is there any way to get ride of this tough situation?

[Alczem]

Hey,


If you can, share your snappyHexMeshDict, it will help people understand.


The resolveFeatureAngle works the other way I believe! Meaning that to detect more edges, you have to reduce this value (set it to 30 to resolve the edges of faces making an angle sharper than 30° for instance). By setting it to 175, you are only detecting angles sharper than 175°, which is... very sharp! Try reducing it.


Are you using implicit or explicit feature detection? What are your input geometry files (stl, obj...)?

[Yann]

This bit might help to understand the resolveFeatureAngle parameter: https://doc.openfoam.com/2306/tools/...veFeatureAngle

[AtoHM]

Looks suspiciously like a highly flat background mesh. Snappy is very bad at handling flat blockMesh cells as input. Try cubic background cells.

[arezayan87]

Originally Posted by Alczem

Hey,


If you can, share your snappyHexMeshDict, it will help people understand.


The resolveFeatureAngle works the other way I believe! Meaning that to detect more edges, you have to reduce this value (set it to 30 to resolve the edges of faces making an angle sharper than 30° for instance). By setting it to 175, you are only detecting angles sharper than 175°, which is... very sharp! Try reducing it.


Are you using implicit or explicit feature detection? What are your input geometry files (stl, obj...)?

about resolveFeatureAngle you're right, I set includedAngle 170.

I've attached my SHMDict here.

[arezayan87]

Originally Posted by AtoHM

Looks suspiciously like a highly flat background mesh. Snappy is very bad at handling flat blockMesh cells as input. Try cubic background cells.

Dear AtoHM

I don't understand what "flat blockMesh" is, I used blockMesh as backGround mesh. would you explain more about cubic background cell?

[AtoHM]

In the 2nd image looking at the defect regions it appears your background mesh cells are flat; meaning not as high (z) as they are wide (x and y extent). Snappy does work best with a uniform background mesh (x, y and z lengths identical == cubic cells).

[arezayan87]

Originally Posted by AtoHM

In the 2nd image looking at the defect regions it appears your background mesh cells are flat; meaning not as high (z) as they are wide (x and y extent). Snappy does work best with a uniform background mesh (x, y and z lengths identical == cubic cells).

Thanks for clear explanation about flat and cubic cells,
After trying with a cubic background cells snapControls does not work properly.
I mean the edges are coarse even in high level values. I guess snapping the edges is somehow disabled!!

this is my SHMDict:

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

castellatedMesh on;
snap            on;
addLayers       on;

geometry
{
    buildings.stl
    {
        type triSurfaceMesh;
        name buildings;
    }
/*
    ground.stl
    {
        type triSurfaceMesh;
        name buildings;
    }
*/
    refinementBox
    {
        type box;
        min  (-256   -272.5   0);
        max  (233.239 277.011  65);
    }

}

castellatedMeshControls
{
  nCellsBetweenLevels 15;
  resolveFeatureAngle 15;
    features
    (
      { file  "buildings.eMesh"; level 6; }
    );

    refinementSurfaces
    {
        buildings
        {
            level (4 4);
            patchInfo { type wall; }
        }

        ground
        {
            level (1 2);
            patchInfo { type wall; }
        }
    }

    refinementRegions
    {
        refinementBox
        {
            mode inside;
            levels ((1E15 2));
        }
    }

    locationInMesh (2 0 0.2);
}

snapControls
{
    nSolveIter 100;
    //- Maximum number of snapping relaxation
    //iterations. Should stop before upon
    //reaching a correct mesh.
    nRelaxIter 15;
    // Feature snapping
    //Number of feature edge snapping iterations.
    nFeatureSnapIter 10;
    explicitFeatureSnap    true;
    implicitFeatureSnap    false;
}

addLayersControls
{
    layers
    {
        "CAD.*"
        {
            nSurfaceLayers 3;
        }
    }

    relativeSizes       false;
    expansionRatio      1.2;
    featureAngle        160;
    finalLayerThickness 0.5;
    minThickness        1.8e-3;
//    Surface based refinement level (2 4);
}

meshQualityControls
{}

writeFlags
(
    noRefinement
    // scalarLevels
    // layerSets
    // layerFields
);

mergeTolerance 1e-6;

[Yann]

This looks like castellatedMesh.

If you ran snappy without the -overwrite option, make sure to move the to latest time step in ParaView in order to see your final mesh.

[arezayan87]

Originally Posted by Yann

This looks like castellatedMesh.

If you ran snappy without the -overwrite option, make sure to move the to latest time step in ParaView in order to see your final mesh.

Thanks dear Yann
That works
your solution helped me to tackle the problem

[arezayan87]

Hi after a long time

I have a problem with using snappyHexMesh for this case(p1.png). when I increase the level in castellatedMeshControls block (more than 5) snappy crashes and stop running. I don't understand the outputs because it doesn't say what the problem is explicitly!!


best wishes
Amirreza

[Yann]

Hello,

It would help to have the full snappy log file. (It is often useful to know what was going on prior to the crash)

Yann