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 xcells
Ny 80; // Number of ycells
Nz1 110; // Number of zcells
Nz2 35;
//*****Grading values******
Gx 1.0; //xgrading
Gy 1.0; //ygrading
Gz1 1100920; //zgrading Equivalent to celltocell expansion k : 1.13613
Gz2 1; //zgrading
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 zipup check OK.
Faceface 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 (nonclosed singly connected) (0 250 0.000715361) (0 250 500)
outlet 10160 10368 ok (nonclosed singly connected) (3000 250 0.000715361) (3000 250 500)
bottom 20000 20331 ok (nonclosed singly connected) (0 250 0) (3000 250 0.000715361)
top 20000 20331 ok (nonclosed singly connected) (0 250 500) (3000 250 500)
front 31750 32128 ok (nonclosed singly connected) (0 250 0.000715361) (3000 250 500)
back 31750 32128 ok (nonclosed singly connected) (0 250 0.000715361) (3000 250 500)
Checking geometry...
Overall domain bounding box (0 250 0) (3000 250 500)
Mesh (nonempty, nonwedge) directions (1 1 1)
Mesh (nonempty) directions (1 1 1)
Boundary openness (1.01591e15 3.39516e14 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 = 3.40116e05. 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 nonorthogonality Max: 89.9979 average: 0.736219
*Number of severely nonorthogonal faces: 712.
Nonorthogonality check OK.
<<Writing 712 nonorthogonal 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 3.55271e18) 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 = 1.08837e05 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: 6.64998e23 average: 1.93353
***Cells with small determinant found, number of cells: 1542144
<<Writing 1542144 underdetermined 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!!
