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-   -   "hex" blocks that give prism and tet wedge cells (http://www.cfd-online.com/Forums/openfoam-meshing-blockmesh/106424-hex-blocks-give-prism-tet-wedge-cells.html)

MaryBau August 28, 2012 15:07

"hex" blocks that give prism and tet wedge cells
 
Hi;

I am doing a simple mesh using blockMesh. I define only two "hex" blocks, but I end up with cells of the type prism and tet wedge. Any idea what could be the reason for this?

Further notes:
-If I fix all the expansion ratios to 1, the mesh works fine and I do not have any errors in checkMesh -allGeometry -allTopology, so I do not think the definition of the blocks is wrong.
-The height of the first cell has to be really small (z+(1)~0.3) because I am using kwSST model with rugosity. So, I have to refine one of the blocks in the z direction. In that case, I get prism and tet wedges cells.

Here is my blockMeshDict:
Quote:

.....
convertToMeters 1;


// ******Coordinates of blocks
x0 0;
x1 3000;

y0 -250; //Spanwise
y1 250;

z0 0; //Height
z1 100; // Added another block to have only refinement in the bottom part to reach z+=0.3 without having really distorted cells at the top of the domain
z2 500;

//*****Number of cells*****

Nx 250; // Number of x-cells
Ny 80; // Number of y-cells
Nz1 110; // Number of z-cells
Nz2 35;

//*****Grading values******
Gx 1.0; //x-grading
Gy 1.0; //y-grading
Gz1 1100920; //z-grading Equivalent to cell-to-cell expansion k : 1.13613
Gz2 1; //z-grading

vertices
(
($x0 $y0 $z0) //0
($x1 $y0 $z0) //1
($x1 $y1 $z0) //2
($x0 $y1 $z0) //3
($x0 $y0 $z1) //4
($x1 $y0 $z1) //5
($x1 $y1 $z1) //6
($x0 $y1 $z1) //7
($x0 $y0 $z2) //8
($x1 $y0 $z2) //9
($x1 $y1 $z2) //10
($x0 $y1 $z2) //11
);

blocks
(
hex (0 1 2 3 4 5 6 7) ($Nx $Ny $Nz1) simpleGrading ($Gx $Gy $Gz1)
hex (4 5 6 7 8 9 10 11) ($Nx $Ny $Nz2) simpleGrading ($Gx $Gy $Gz2)
);

edges
(
);

boundary
(
inlet
{
type mappedPatch;
offset (3000 0 0 );
sampleMode nearestCell;
sampleRegion region0;
samplePatch none;

faces
(
(0 4 7 3)
(4 8 11 7)
);
}
outlet
{
type patch;
faces
(
(1 2 6 5)
(5 6 10 9)
);
}
bottom
{
type wall;
faces
(
(0 3 2 1)
);
}
top
{
type symmetryPlane;
faces
(
(8 9 10 11)
);
}
front
{
type cyclic;
neighbourPatch back;
faces
(
(0 1 5 4)
(4 5 9 8)
);
}
back
{
type cyclic;
neighbourPatch front;
faces
(
(3 7 6 2)
(7 11 10 6)
);
}
);

mergePatchPairs
(
);

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


and this is the output of checkMesh -allGeometry -allTopology:

Quote:

Mesh stats
points: 2956446
faces: 8755970
internal faces: 8632150
cells: 2900000
boundary patches: 6
point zones: 0
face zones: 0
cell zones: 0

Overall number of cells of each type:
hexahedra: 2888192
prisms: 11736
wedges: 0
pyramids: 0
tet wedges: 72
tetrahedra: 0
polyhedra: 0

Checking topology...
Boundary definition OK.
Cell to face addressing OK.
Point usage OK.
Upper triangular ordering OK.
Face vertices OK.
Topological cell zip-up check OK.
Face-face connectivity OK.
Number of regions: 1 (OK).

Checking patch topology for multiply connected surfaces ...
Patch Faces Points Surface topology Bounding box
inlet 10160 10368 ok (non-closed singly connected) (0 -250 0.000715361) (0 250 500)
outlet 10160 10368 ok (non-closed singly connected) (3000 -250 0.000715361) (3000 250 500)
bottom 20000 20331 ok (non-closed singly connected) (0 -250 0) (3000 250 0.000715361)
top 20000 20331 ok (non-closed singly connected) (0 -250 500) (3000 250 500)
front 31750 32128 ok (non-closed singly connected) (0 -250 0.000715361) (3000 -250 500)
back 31750 32128 ok (non-closed singly connected) (0 250 0.000715361) (3000 250 500)

Checking geometry...
Overall domain bounding box (0 -250 0) (3000 250 500)
Mesh (non-empty, non-wedge) directions (1 1 1)
Mesh (non-empty) directions (1 1 1)
Boundary openness (-1.01591e-15 -3.39516e-14 0) OK.
***High aspect ratio cells found, Max aspect ratio: 2.20513e+06, number of cells 1100000
<<Writing 1100000 cells with high aspect ratio to set highAspectRatioCells
Minumum face area = 3.40116e-05. Maximum face area = 143.785. Face area magnitudes OK.
Min volume = 0.000408139. Max volume = 898.658. Total volume = 7.5e+08. Cell volumes OK.
Mesh non-orthogonality Max: 89.9979 average: 0.736219
*Number of severely non-orthogonal faces: 712.
Non-orthogonality check OK.
<<Writing 712 non-orthogonal faces to set nonOrthoFaces
***Error in face pyramids: 52 faces are incorrectly oriented.
<<Writing 52 faces with incorrect orientation to set wrongOrientedFaces
Max skewness = 0.333333 OK.
Coupled point location match (average 3.55271e-18) OK.
***Error in face tets: 192 faces with low quality or negative volume decomposition tets.
<<Writing 72 faces with low quality or negative volume decomposition tets to set lowQualityTetFaces
*Edges too small, min/max edge length = 1.08837e-05 12, number too small: 923346
<<Writing 923346 points on short edges to set shortEdges
All angles in faces OK.
Face flatness (1 = flat, 0 = butterfly) : average = 1 min = 1
All face flatness OK.
Cell determinant (wellposedness) : minimum: 6.64998e-23 average: 1.93353
***Cells with small determinant found, number of cells: 1542144
<<Writing 1542144 under-determined cells to set underdeterminedCells
***Concave cells (using face planes) found, number of cells: 22044
<<Writing 22044 concave cells to set concaveCells

Failed 5 mesh checks.

End


Thanks in advance!!

MaryBau January 30, 2013 14:46

I was using OFv2.1.0 when I was getting the error explained in my previous post.

Now, I used the same blockMeshDict in OFv2.1.1 and OFv1.7.1 and I did not have that problem!!!


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