[Hr_kules]

Hey,

i an older post i found a quote stating that it is possible to create a mesh in gmsh and import it to further create the boundary layer with snappy hex mesh.


Is this actually possible? If so, has anybody done that before and can tell how to do that? Or just share some experiences?

[Hr_kules]

I have just tried doing that with a simple pipe and it actually works.

[CaptCrash]

Would you be able to describe the process you used?

[Hr_kules]

Hey, yes of course!

I have created the mesh in GMSH with a script alone. The resulting mesh is located in the case in the constant folder -> constant/mesh.msh
I import the mesh with gmshToFoam constant/mesh.msh
Prvious command creates the polymesh folder with faces, cellzones and so on..
In the snappyHexMeshDict i start with following settings:

/*--------------------------------*- C++ -*----------------------------------*\
  =========                 |
  \\      /  F ield         | OpenFOAM: The Open Source CFD Toolbox
   \\    /   O peration     | Website:  https://openfoam.org
    \\  /    A nd           | Version:  9
     \\/     M anipulation  |
\*---------------------------------------------------------------------------*/
FoamFile
{
    format      ascii;
    class       dictionary;
    object      snappyHexMeshDict;
}
// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //

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

castellatedMesh off;
snap            off;
addLayers       on;

Note that the castellated mesh and snap procedures are turned off. However all the settings must still exist in the dict, although they are not needed:

geometry
{
    dummy
    {
        type            triSurfaceMesh;
        file            "dummy.stl";
        regions
        {
            dummy1.stl        {name dummy1;}
            dummy2.stl        {name dummy2;}
            dummy3.stl        {name dummy3;}
            dummy4.stl    {name dummy4;}            
        }
    }
};

castellatedMeshControls
{
    features
    (
    );

    refinementSurfaces
    {
        dummy
        {
            level          (0 0);
            regions
            {
                dummy1            { level  (1 1); patchInfo { type wall;  } } 
                dummy2            { level  (1 1); patchInfo { type wall;  } } 
                dummy3            { level  (1 1); patchInfo { type wall;  } } 
                dummy4            { level  (1 1); patchInfo { type wall;  } } 

            }
         }



    }

    locationInMesh (0.0 0.0 0.005);

    nCellsBetweenLevels 2;

    resolveFeatureAngle 15;
}

snapControls
{
    implicitFeatureSnap    true;
}

What you actually write doesn't matter expect that a dummy stl with whatever names you apply needs to exist, however nothing is done. Just make sure the settings and a consistent named stl exist.
In the last portion of the script:

addLayersControls
{
    layers
    {
        top
        {
            nSurfaceLayers  5;
            mergeFaces      true;
        }
        bottom
        {
            nSurfaceLayers  5;
            mergeFaces      true;
        }
        left
        {
            nSurfaceLayers  5;
            mergeFaces      true;
        }
        right
        {
            nSurfaceLayers  5;
            mergeFaces      true;
        }                        
    }
    relativeSizes   true;
    expansionRatio  1.2;
    finalLayerThickness 0.4;
    minThickness    0.001;
}

You can make references to the imported patches, given they were named in GMSH, if not, you should name them in GMSH before you import the mesh to OpenFoam.

For simple geometries you can simply run:

gmshToFoam constant/mesh.msh
createPatch -overwrite
snappyHexMesh -overwrite

Side note: Since i want some of the patches to be walls i use the createPatch dict to change the boundary type to wall and use the createFromPatch option of the createPatch dict.

The resulting mesh in my case was mesh with various elements mixed, the checkMesh command doesn't complain about the mesh and test runs were all quite successfull, so i hope this helps you! If you have any further questions i will try ma best to answer them!

[CaptCrash]

Thanks! That is very useful.