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Old   June 20, 2009, 08:44
Default blockMesh: block with 6 vertexes
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Daniele
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Dear all,
I tried to create a 2D mesh with the geometry of figure "scheme.jpg" (flow around a thin airfoil).
scheme.jpg
In particular it has two blocks with only 6 vertexes (blocks 1 and 5). I defined them in according to U-136 (user guide). I checked many times the blockMeshDict and it seems to be correct (both blocks and patches).
Nevertheless when I run blockMesh I obtain the following:

// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //
Create time


Reading block mesh description dictionary

Creating block mesh

Creating blockCorners

Creating curved edges

Creating blocks

Creating patches

Creating block mesh topology
--> FOAM Warning :
From function cellModel::mag(const labelList&, const pointField&)
in file meshes/meshShapes/cellModel/cellModel.C at line 134
zero or negative pyramid volume: -4.6643e-08 for face 0
--> FOAM Warning :
From function cellModel::mag(const labelList&, const pointField&)
in file meshes/meshShapes/cellModel/cellModel.C at line 134
zero or negative pyramid volume: -4.6401e-08 for face 1
--> FOAM Warning :
From function cellModel::mag(const labelList&, const pointField&)
in file meshes/meshShapes/cellModel/cellModel.C at line 134
zero or negative pyramid volume: -5.43305e-09 for face 2
--> FOAM Warning :
From function cellModel::mag(const labelList&, const pointField&)
in file meshes/meshShapes/cellModel/cellModel.C at line 134
zero or negative pyramid volume: -8.7611e-08 for face 3
--> FOAM Warning :
From function cellModel::mag(const labelList&, const pointField&)
in file meshes/meshShapes/cellModel/cellModel.C at line 134
zero or negative pyramid volume: -4.6522e-08 for face 4
--> FOAM Warning :
From function cellModel::mag(const labelList&, const pointField&)
in file meshes/meshShapes/cellModel/cellModel.C at line 134
zero or negative pyramid volume: -4.6522e-08 for face 5
--> FOAM Warning :
From function blockMesh::createTopology(IOdictionary&)
in file createTopology.C at line 412
negative volume block : 0, probably defined inside-out
--> FOAM Warning :
From function cellModel::mag(const labelList&, const pointField&)
in file meshes/meshShapes/cellModel/cellModel.C at line 134
zero or negative pyramid volume: -0.000493484 for face 0
--> FOAM Warning :
From function cellModel::mag(const labelList&, const pointField&)
in file meshes/meshShapes/cellModel/cellModel.C at line 134
zero or negative pyramid volume: -0.000526919 for face 1
--> FOAM Warning :
From function cellModel::mag(const labelList&, const pointField&)
in file meshes/meshShapes/cellModel/cellModel.C at line 134
zero or negative pyramid volume: -3.57818e-06 for face 2
--> FOAM Warning :
From function cellModel::mag(const labelList&, const pointField&)
in file meshes/meshShapes/cellModel/cellModel.C at line 134
zero or negative pyramid volume: -0.00101682 for face 3
--> FOAM Warning :
From function cellModel::mag(const labelList&, const pointField&)
in file meshes/meshShapes/cellModel/cellModel.C at line 134
zero or negative pyramid volume: -0.000510201 for face 4
--> FOAM Warning :
From function cellModel::mag(const labelList&, const pointField&)
in file meshes/meshShapes/cellModel/cellModel.C at line 134
zero or negative pyramid volume: -0.000510201 for face 5
--> FOAM Warning :
From function blockMesh::createTopology(IOdictionary&)
in file createTopology.C at line 412
negative volume block : 9, probably defined inside-out

Default patch type set to empty
--> FOAM Warning :
From function polyMesh:olyMesh(... construct from shapes...)
in file meshes/polyMesh/polyMeshFromShapeMesh.C at line 576
Found 22 undefined faces in mesh; adding to default patch.

Check block mesh topology

Basic statistics
Number of internal faces : 14
Number of boundary faces : 32
Number of defined boundary faces : 32
Number of undefined boundary faces : 0

Checking patch -> block consistency

Creating block offsets

Creating merge list .

Creating points

Creating cells

Creating patches

Creating mesh from block mesh

Default patch type set to empty

Writing polyMesh

end
----------------------------------------------------
---------------------------------------------------

There are 22 undefined faces; this is OK, because:
20 faces build the patches perpendicular to the 3rd dimension,
2 zero area faces belong to blocks with 6 vertexes.

A detail of the region with feces from 0 to 5 (nose of the airfoil) is in the file tec.jpg.
tec.jpg

Again when I run checkMesh I obtain:

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

Create time

Create polyMesh for time = constant

Time = constant

Mesh stats
points: 6500
faces: 12730
internal faces: 6212
cells: 3160
boundary patches: 3
point zones: 0
face zones: 0
cell zones: 0

Number of cells of each type:
hexahedra: 3142
prisms: 18
wedges: 0
pyramids: 0
tet wedges: 0
tetrahedra: 0
polyhedra: 0

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

Checking patch topology for multiply connected surfaces ...
Patch Faces Points Surface
profilo 116 236 ok (not multiply connected)
esterno 80 160 ok (not multiply connected)
defaultFaces 6322 6500 ok (not multiply connected)

Checking geometry...
Domain bounding box: (-1.54976 -1.54906 0) (1.54999 1.55 0.01)
Boundary openness (1.21489e-19 4.14491e-19 1.148e-16) OK.
***High aspect ratio cells found, Max aspect ratio: 1146.73, number of cells 3
<<Writing 3 cells with high aspect ratio to set highAspectRatioCells
***Zero or negative face area detected. Minimum area: 0
<<Writing 2 zero area faces to set zeroAreaFaces
Min volume = 1.9843e-11. Max volume = 0.000234574. Total volume = 0.0753615. Cell volumes OK.
Mesh non-orthogonality Max: 75.5066 average: 12.3191
*Number of severely non-orthogonal faces: 2.
Non-orthogonality check OK.
<<Writing 2 non-orthogonal faces to set nonOrthoFaces
Face pyramids OK.
Max skewness = 2.85261 OK.
All angles in faces OK.
Face flatness (1 = flat, 0 = butterfly) : average = 1 min = 1
All face flatness OK.

Failed 2 mesh checks.

End
-------------------------------------------------------------
-------------------------------------------------------------

Does someone know the reason of this problem?

Many thanks,

Daniele
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Old   June 20, 2009, 15:05
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Hi dani,

I'm new using openFOAM but I think those warnings are from blocks 0 and 9, the compiler says that in warning 7 and in the last one.

From function blockMesh::createTopology(IOdictionary&)
in file createTopology.C at line 412
negative volume block : 0, probably defined inside-out

I got the same problem meshing my airfoil and it was solved implementing the right hand coordinate system for each block it's in U-130 page, although I got another problem the mesh doesn't respect the airfoil boundary (the external mesh should not pass the airfoil profile) I have started another forum about that yesterday.

Good luck with your mesh,
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Old   June 21, 2009, 03:52
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Daniele
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Hi erodv,
thanks for the reply.

Yes, the problem is on block 0 and block 9, but the 'strange' thing is that all my block definitions use the right hand coordinate system (according to U-130). Furthermore the patches are correctly oriented.

So I have a doubt: can I use a block with 6 vertexes in such a problem?
In other words: can I use a block with 6 vertexes only in axi-symmetric cases (and so not in my 2D problem)?

Regards,

Daniele

(P.S: I'm going to answer to your thread!)
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Old   June 25, 2009, 13:13
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Johan Stander
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Dear Daniele,

Your vertex ordering in the hex() command may be wrong. In OF we use a righthand orientation system. This means you should specify the vertexes in hex() in an anti-clockwise direction, first the front patch vertex set than the rear patch vertex set. Hope it helps ;-)
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