[TheMadHungarian]

Hi all, a longtime "lurker" here. I have learnt a lot from this community but there is one case now I need some special hints.


I'm using blockMeshDict to create a spherical mesh with a spherical void in the middle (a spherical shell in essence). The meshing goes great but when checking the inside of the mesh with Paraview and "Slice" or "Clip", I see several ill-formed cells. How do I fix those? I figured the mesher would be smart enough to create a bunch of slightly spherical "bricks" some some cells are wedge-shaped...I guess. I'm still getting good results from sonicFoam but these artifacts bother me.


The simulation I'm running is a transonic blast expansion, FYI. We're trying to determine the shock wave travel characteristics.


blockMesh runs without errors, checkMesh also "OK" everything, and I also ran renumberMesh...all good. Still getting those weird artifacts. What's the deal?????



So, I need inputs, comments, hints, etc.


Thanks!!!!!


Pictures attached are of the actual mesh in question.

FoamFile
{
    version     2.0;
    format      ascii;
    class       dictionary;
    object      blockMeshDict;
}
// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //

verbose yes;

geometry {
    sphere1 {
        type searchableSphere;
        centre (0 0 0);
        radius 3.5;
    }
    sphere2 {
        type searchableSphere;
        centre (0 0 0);
        radius 50;
    }
}

scale 1;

n1 20;

v    28.8675;//0.5773502;
mv  -28.8675; //-0.5773502;
vh   2.0207; //0.2886751;
mvh -2.0207; //-0.2886751;

a    35.3553; //0.7071067;
ma  -35.3553; //-0.7071067;
ah   2.47487; //0.3464101;
mah -2.47487; //-0.3464101;

vertices (
    ($mvh $mvh $mvh)
    ( $vh $mvh $mvh)
    ( $vh  $vh $mvh)
    ($mvh  $vh $mvh)
    ($mvh $mvh  $vh)
    ( $vh $mvh  $vh)
    ( $vh  $vh  $vh)
    ($mvh  $vh  $vh) //7

    ($mv $mv $mv) //8
    ( $v $mv $mv)
    ( $v  $v $mv)
    ($mv  $v $mv)
    ($mv $mv  $v)
    ( $v $mv  $v)
    ( $v  $v  $v)
    ($mv  $v  $v) //15
);

blocks (
    hex ( 9  8 12 13  1  0  4  5) ($n1 $n1 $n1) simpleGrading (1 1 1)
    hex (10  9 13 14  2  1  5  6) ($n1 $n1 $n1) simpleGrading (1 1 1)
    hex (11 10 14 15  3  2  6  7) ($n1 $n1 $n1) simpleGrading (1 1 1)
    hex ( 8 11 15 12  0  3  7  4) ($n1 $n1 $n1) simpleGrading (1 1 1)
    hex ( 8  9 10 11  0  1  2  3) ($n1 $n1 $n1) simpleGrading (1 1 1)
    hex (13 12 15 14  5  4  7  6) ($n1 $n1 $n1) simpleGrading (1 1 1)
);

edges (
    arc  8  9 (0 $ma $ma)
    arc 10 11 (0 $a $ma)
    arc 14 15 (0 $a $a)
    arc 12 13 (0 $ma $a)

    arc  8 11 ($ma 0 $ma)
    arc  9 10 ($a 0 $ma)
    arc 13 14 ($a 0 $a)
    arc 12 15 ($ma 0 $a)

    arc  8 12 ($ma $ma 0)
    arc  9 13 ($a $ma 0)
    arc 10 14 ($a $a 0)
    arc 11 15 ($ma $a 0)


    arc  0  1 (0 $mah $mah)
    arc  2  3 (0 $ah $mah)
    arc  6  7 (0 $ah $ah)
    arc  4  5 (0 $mah $ah)

    arc  0  3 ($mah 0 $mah)
    arc  1  2 ($ah 0 $mah)
    arc  5  6 ($ah 0 $ah)
    arc  4  7 ($mah 0 $ah)

    arc  0  4 ($mah $mah 0)
    arc  1  5 ($ah $mah 0)
    arc  2  6 ($ah $ah 0)
    arc  3  7 ($mah $ah 0)
);

faces (
    project ( 0 4 7 3) sphere1
    project (2 6 5 1) sphere1
    project ( 1 5 4  0) sphere1
    project (3 7 6 2) sphere1
    project ( 0 3 2 1) sphere1
    project (4 5 6 7) sphere1

    project (8 12 15 11) sphere2
    project (10 14 13 9) sphere2
    project (9 13 12 8) sphere2
    project (11 15 14 10) sphere2
    project (8 11 10 9) sphere2
    project (12 13 14 15) sphere2
);

boundary (
    atmosphere {
        type patch;
        faces
        (
            ( 8 12 15 11)
            (10 14 13  9)
            ( 9 13 12  8)
            (11 15 14 10)
            ( 8 11 10  9)
            (12 13 14 15)
        );
    }
    blast {
        type patch;
        faces
        (
            ( 0 4 7 3)
            (2 6 5 1)
            (1 5 4 0)
            (3 7 6 2)
            (0 3 2 1)
            (4 5 6 7)
        );
    }
);

checkMesh results show no issues:

Mesh stats
    points:           393682
    faces:            1161600
    internal faces:   1142400
    cells:            384000
    faces per cell:   6
    boundary patches: 2
    point zones:      0
    face zones:       0
    cell zones:       0

Overall number of cells of each type:
    hexahedra:     384000
    prisms:        0
    wedges:        0
    pyramids:      0
    tet wedges:    0
    tetrahedra:    0
    polyhedra:     0

Checking topology...
    Boundary definition OK.
    Cell to face addressing OK.
    Point usage OK.
    Upper triangular ordering OK.
    Face vertices OK.
    Number of regions: 1 (OK).

Checking patch topology for multiply connected surfaces...
    Patch               Faces    Points   Surface topology
    atmosphere          9600     9602     ok (closed singly connected)
    blast               9600     9602     ok (closed singly connected)

Checking geometry...
    Overall domain bounding box (-50 -50 -50) (50 50 50)
    Mesh has 3 geometric (non-empty/wedge) directions (1 1 1)
    Mesh has 3 solution (non-empty) directions (1 1 1)
    Boundary openness (2.65525e-016 2.44712e-016 2.10464e-016) OK.
    Max cell openness = 3.83177e-016 OK.
    Max aspect ratio = 5.79932 OK.
    Minimum face area = 0.0104915. Maximum face area = 4.08288.  Face area magnitudes OK.
    Min volume = 0.018498. Max volume = 4.59341.  Total volume = 522007.  Cell volumes OK.
    Mesh non-orthogonality Max: 50.2352 average: 8.22207
    Non-orthogonality check OK.
    Face pyramids OK.
    Max skewness = 0.470245 OK.
    Coupled point location match (average 0) OK.

Mesh OK.

End

[yambanshee]

This is likely nothing more than an artifact of how paraview cuts the mesh when you section it, and is not an accurate representation of what your mesh actually looks like. All my meshes look like this when viewing a slice. I just think that paraview doesn't do a good job of accurately showing the mesh when sliced

[guin]

Your mesh is fine. In order to see the mesh elements correctly do not forget to enable "Crinkle clip" (bottom left-hand side in attached snapshot)

[TheMadHungarian]

OK, thanks for the input...I checked the number of cells, it checks out.
I have the crinkle-cut vs. non-crinkle cuts attached....still does not look right, it must be a Paraview issue.


The odd thing is, when I slice or clip the mesh, I clip it right in the middle - right between the cells - so there should be no weird artifacts.


Andy

[guin]

It's a numerical issue form ParaView. The clip sets by default its origin at (0,0,0). Just shift it slightly -e.g. to (1e-6, 1e-6, 1e-6) - in order to help ParaView at identifying which cells are at which side of the clip.

[TheMadHungarian]

That reminds me: is there any utilities which convert OpenFOAM mesh to some other format, say Gmsh?
I want to visually check the mesh using anotjer format...

[TheMadHungarian]

Originally Posted by guin

It's a numerical issue form ParaView. The clip sets by default its origin at (0,0,0). Just shift it slightly -e.g. to (1e-6, 1e-6, 1e-6) - in order to help ParaView at identifying which cells are at which side of the clip.

Thank you guin! it worked !!!!!


Although it is weird that when Paraview "slices" a cell it shows a completely different view that one would expect when slicing a "brick" - it would still be a brick but smaller.
Oh well. Lesson learnt.


Thanks again.

[guin]

what about https://cpp.openfoam.org/dev/foamToStarMesh_8C.html ?

there are a couple of "foamTo<whatever>" utilities, just search for them in the online documentation.

[guin]

... among them also https://cpp.openfoam.org/dev/foamMeshToFluent_8C.html

[mAlletto]

I've slightly updated the above blockMesh in order that someone can specify freely the inner and the outer radius. Maybe it is useful for somebody

FoamFile
{
    version     2.0;
    format      ascii;
    class       dictionary;
    object      blockMeshDict;
}
// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //

verbose yes;

innerRadius 35;
outerRadius 50;

geometry {
    sphere1 {
        type searchableSphere;
        centre (0 0 0);
        radius $innerRadius;
    }
    sphere2 {
        type searchableSphere;
        centre (0 0 0);
        radius $outerRadius;
    }
}

scale 1;

n1 20;

v #eval{ $outerRadius*0.5773502 };
mv #eval{ -$outerRadius*0.5773502 };
vh #eval{ $innerRadius*0.5773502 };
mvh #eval{ -$innerRadius*0.5773502 };
a #eval{ $outerRadius*0.7071067 };
ma #eval{ -$outerRadius*0.7071067 };
ah #eval{ $innerRadius*0.7071067 };
mah #eval{ -$innerRadius*0.7071067 };

/*
v    28.8675;//0.5773502;
mv  -28.8675; //-0.5773502;
vh   2.0207; //0.2886751;
mvh -2.0207; //-0.2886751;

a    35.3553; //0.7071067;
ma  -35.3553; //-0.7071067;
ah   2.47487; //0.3464101;
mah -2.47487; //-0.3464101;
*/

vertices (
    ($mvh $mvh $mvh)
    ( $vh $mvh $mvh)
    ( $vh  $vh $mvh)
    ($mvh  $vh $mvh)
    ($mvh $mvh  $vh)
    ( $vh $mvh  $vh)
    ( $vh  $vh  $vh)
    ($mvh  $vh  $vh) //7

    ($mv $mv $mv) //8
    ( $v $mv $mv)
    ( $v  $v $mv)
    ($mv  $v $mv)
    ($mv $mv  $v)
    ( $v $mv  $v)
    ( $v  $v  $v)
    ($mv  $v  $v) //15
);

blocks (
    hex ( 9  8 12 13  1  0  4  5) ($n1 $n1 $n1) simpleGrading (1 1 1)
    hex (10  9 13 14  2  1  5  6) ($n1 $n1 $n1) simpleGrading (1 1 1)
    hex (11 10 14 15  3  2  6  7) ($n1 $n1 $n1) simpleGrading (1 1 1)
    hex ( 8 11 15 12  0  3  7  4) ($n1 $n1 $n1) simpleGrading (1 1 1)
    hex ( 8  9 10 11  0  1  2  3) ($n1 $n1 $n1) simpleGrading (1 1 1)
    hex (13 12 15 14  5  4  7  6) ($n1 $n1 $n1) simpleGrading (1 1 1)
);

edges (
    arc  8  9 (0 $ma $ma)
    arc 10 11 (0 $a $ma)
    arc 14 15 (0 $a $a)
    arc 12 13 (0 $ma $a)

    arc  8 11 ($ma 0 $ma)
    arc  9 10 ($a 0 $ma)
    arc 13 14 ($a 0 $a)
    arc 12 15 ($ma 0 $a)

    arc  8 12 ($ma $ma 0)
    arc  9 13 ($a $ma 0)
    arc 10 14 ($a $a 0)
    arc 11 15 ($ma $a 0)


    arc  0  1 (0 $mah $mah)
    arc  2  3 (0 $ah $mah)
    arc  6  7 (0 $ah $ah)
    arc  4  5 (0 $mah $ah)

    arc  0  3 ($mah 0 $mah)
    arc  1  2 ($ah 0 $mah)
    arc  5  6 ($ah 0 $ah)
    arc  4  7 ($mah 0 $ah)

    arc  0  4 ($mah $mah 0)
    arc  1  5 ($ah $mah 0)
    arc  2  6 ($ah $ah 0)
    arc  3  7 ($mah $ah 0)
);

faces (
    project ( 0 4 7 3) sphere1
    project (2 6 5 1) sphere1
    project ( 1 5 4  0) sphere1
    project (3 7 6 2) sphere1
    project ( 0 3 2 1) sphere1
    project (4 5 6 7) sphere1

    project (8 12 15 11) sphere2
    project (10 14 13 9) sphere2
    project (9 13 12 8) sphere2
    project (11 15 14 10) sphere2
    project (8 11 10 9) sphere2
    project (12 13 14 15) sphere2
);

boundary (
    atmosphere {
        type patch;
        faces
        (
            ( 8 12 15 11)
            (10 14 13  9)
            ( 9 13 12  8)
            (11 15 14 10)
            ( 8 11 10  9)
            (12 13 14 15)
        );
    }
    blast {
        type patch;
        faces
        (
            ( 0 4 7 3)
            (2 6 5 1)
            (1 5 4 0)
            (3 7 6 2)
            (0 3 2 1)
            (4 5 6 7)
        );
    }
);

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