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November 2, 2007, 09:16 
To All,
I'm modeling extern

#1 
Member
Doug Baldwin
Join Date: Mar 2009
Posts: 53
Rep Power: 8 
To All,
I'm modeling external aerodynamic flow about a body. I understand that for accurate drag assessment the mesh should extend from the body by 20 times the reference length, y+ should be less than 5, and the cell height should grow by a factor of about 1.15. My question is that the above constraints inevitably lead to smallEdges and highAspectRatioCells, yes? And is this ok? For instance, for a given geometry of air I can generate a mesh with no errors by increasing y+ and increasing the cell height growth factor. But, for a more refined mesh with the above constraints, checkMesh issues warnings and errors. checkMesh outputs are... For very coarse mesh: Mesh stats points: 58116 edges: 159043 faces: 143936 internal faces: 114112 cells: 43008 boundary patches: 1 point zones: 0 face zones: 0 cell zones: 0 Number of cells of each type: hexahedra: 43008 prisms: 0 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 zipup check OK. Face vertices OK. Faceface connectivity OK. Number of regions: 1 (OK). Checking patch topology for multiply connected surfaces ... Patch Faces Points Surface all 29824 29826 ok (closed singly connected surface) Checking geometry... Domain bounding box: (150.156 0.743357 174.114) (149.844 150 125.886) Boundary openness (1.62991e16 3.78198e16 2.14744e16) OK. Max cell openness = 3.20853e15 OK. Max aspect ratio = 129.482 OK. Minumum face area = 1.91668e05. Maximum face area = 1135.09. Face area magnitudes OK. Min volume = 3.67946e07. Max volume = 3217.21. Total volume = 6.45353e+06. Cell volumes OK. Mesh nonorthogonality Max: 80.4968 average: 27.0736 *Number of severely nonorthogonal faces: 599. Nonorthogonality check OK. <<Writing 599 nonorthogonal faces to set nonOrthoFaces Face pyramids OK. Max skewness = 3.82417 OK. Min/max edge length = 0.00129032 172.098 OK. All angles in faces OK. Face flatness (1 = flat, 0 = butterfly) : average = 0.999757 min = 0.943597 All face flatness OK. Mesh OK. End But, for the more refined mesh... Mesh stats points: 1452900 edges: 4324771 faces: 4291136 internal faces: 4224448 cells: 1419264 boundary patches: 1 point zones: 0 face zones: 0 cell zones: 0 Number of cells of each type: hexahedra: 1419264 prisms: 0 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 zipup check OK. Face vertices OK. Faceface connectivity OK. Number of regions: 1 (OK). Checking patch topology for multiply connected surfaces ... Patch Faces Points Surface all 66688 66690 ok (closed singly connected surface) Checking geometry... Domain bounding box: (150.156 0.74336 174.114) (149.844 150 125.886) Boundary openness (5.17672e17 7.27314e16 1.80348e16) OK. ***High aspect ratio cells found, Max aspect ratio: 3723.68, number of cells 519 <<Writing 519 cells with high aspect ratio to set highAspectRatioCells Minumum face area = 5.83592e09. Maximum face area = 275.784. Face area magnitudes OK. Min volume = 6.86967e11. Max volume = 1096.84. Total volume = 6.45353e+06. Cell volumes OK. Mesh nonorthogonality Max: 89.4941 average: 26.6997 *Number of severely nonorthogonal faces: 27612. Nonorthogonality check OK. <<Writing 27612 nonorthogonal faces to set nonOrthoFaces Face pyramids OK. Max skewness = 2.99001 OK. *Edges too small, min/max edge length = 4e06 22.4771, number too small: 307632 <<Writing 322167 points on short edges to set shortEdges All angles in faces OK. Face flatness (1 = flat, 0 = butterfly) : average = 0.999975 min = 0.98297 All face flatness OK. Failed 1 mesh checks. End Any and all advice welcomed. Thank you, Doug 

November 6, 2007, 22:22 
Doug,
Typically, in automot

#2 
New Member
Rajneesh
Join Date: Mar 2009
Posts: 28
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Doug,
Typically, in automotive industry people use prism cells on the body and then Hex in the rest of flow domain to avoid quality issues. For my external aero simulations, with Tgrid/Fluent surface mesh is pure trias followed by several layers of prism layers and then tetra or hex in rest of the flow domain. Quality is pretty much independent of the mesh size. Surface mesh quality is of bigger concern. what kind ( low/high speed, streamlined/bluff body) simulation are you doing?  Rajneesh 

November 7, 2007, 18:03 
Rajneesh,
Thank you for off

#3 
Member
Doug Baldwin
Join Date: Mar 2009
Posts: 53
Rep Power: 8 
Rajneesh,
Thank you for offering your experience. I'm simulating Re=1e7 streamlined flow. My mesh is pure quad on the surface wall, with a uniform column of hexs from each surface quad all the way out to the boundary patch. The boundary patch has exactly the same number of quads as the surface wall. checkMesh is perfect if I go out to say 3x reference length. But at up to 20x reference length the ratio between short edges and long edges produces short edge errors, and aspect ratio errors are also generated. The potentialFoam solver works anyway, and the results appear correct. I'm interested in knowing if I have uniformly short edges near the body and uniformly long edges far away from the body will this degrade the quality of the results. My basic question is if I know the mesh quality is perfect in checkMesh for a mesh of say 3x reference length, and I simply add every larger cells beyond 3x which eventually causes short edge and aspect ratio errors in checkMesh, should I be concerned? I could probably chase the cause of error and eventually have a perfect checkMesh out to 20x, but I'd like to first know if this is worth the time and effort. Doug 

November 7, 2007, 20:17 
Doug,
I've run simpleFoam o

#4 
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Patrick Bourdin
Join Date: Mar 2009
Posts: 40
Rep Power: 8 
Doug,
I've run simpleFoam on some wing cases, using the SA model with a y+ of about 1 at the walladjacent cells and a stretching factor of 1.1 (hex in the boundary layer, tet in the far field). Checkmesh did not complain about too high aspect ratio though. here's the output of checkmesh for one of my case: /**\  =========    \ / F ield  OpenFOAM: The Open Source CFD Toolbox   \ / O peration  Version: 1.4.1   \ / A nd  Web: http://www.openfoam.org   \/ M anipulation   \**/ Exec : checkMesh . stfwSym_RF00_RA00 Date : Oct 16 2007 Time : 18:03:58 Host : kittyhawk PID : 27593 Root : /users/aexpb/OpenFOAM1.4.1/aexpb1.4.1/run/simpleFoam Case : stfwSym_RF00_RA00 Nprocs : 1 Create time Create polyMesh for time = constant Time = constant Mesh stats points: 1114235 edges: 4231194 faces: 5316839 internal faces: 5268373 cells: 2199879 boundary patches: 7 point zones: 0 face zones: 0 cell zones: 0 Number of cells of each type: hexahedra: 853440 prisms: 30440 wedges: 0 pyramids: 48376 tet wedges: 0 tetrahedra: 1267623 polyhedra: 0 Checking topology... Boundary definition OK. Point usage OK. Upper triangular ordering OK. Topological cell zipup check OK. Face vertices OK. Faceface connectivity OK. Number of regions: 1 (OK). Checking patch topology for multiply connected surfaces ... Patch Faces Points Surface symmetry4 15074 11062 ok (not multiply connected) outflow5 1108 602 ok (not multiply connected) velocityinlet6 1126 611 ok (not multiply connected) slipwall7 1238 658 ok (not multiply connected) aftWinglet8 6330 5616 ok (not multiply connected) frontWinglet9 6318 5610 ok (not multiply connected) wing10 17272 17496 ok (not multiply connected) Checking geometry... Domain bounding box: (20 5.721684313e13 20) (20 20 20) Boundary openness (3.50775689274e17 1.03061502414e15 4.01765248617e17) OK. Max cell openness = 1.99760162922e14 OK. Max aspect ratio = 454.261326444 OK. Minumum face area = 2.01963520532e10. Maximum face area = 3.65068621543. Face area magnitudes OK. Min volume = 1.85425452264e15. Max volume = 2.39605975495. Total volume = 31999.9963006. Cell volumes OK. Mesh nonorthogonality Max: 89.2519784873 average: 27.8654976223 *Number of severely nonorthogonal faces: 100029. Nonorthogonality check OK. <<Writing 100029 nonorthogonal faces to set nonOrthoFaces Face pyramids OK. Max skewness = 2.91618646831 OK. *Edges too small, min/max edge length = 1.09452909612e05 3.42547059472, number too small: 270345 <<Writing 285910 points on short edges to set shortEdges All angles in faces OK. Face flatness (1 = flat, 0 = butterfly) : average = 0.999985939551 min = 0.977107638673 All face flatness OK. Mesh OK. End Despite the warning about the small edges, the run went fine. Patrick 

November 7, 2007, 21:08 
Patrick,
Great! I now know

#5 
Member
Doug Baldwin
Join Date: Mar 2009
Posts: 53
Rep Power: 8 
Patrick,
Great! I now know to focus only on the Aspect Ratio, which I can readily correct. Question, is there a benefit to you using tet in the far field rather than hex? For my meshing method, pure hex is easier. I could use tet in the far field but that would require two meshes then meshMerge and meshStitch. Thanks for sharing your experience. Doug 

November 7, 2007, 21:22 
Doug,
My using tet in the f

#6 
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Patrick Bourdin
Join Date: Mar 2009
Posts: 40
Rep Power: 8 
Doug,
My using tet in the far field was more a matter of convenience (my geometry was quite complex and didn't allow me to extrude the structured surface grid up to the farfield boundary of the computational domain). If you can stick to a pure hex grid, then stick to it (hex are better than tet as far as accuracy is concerned). Patrick 

March 16, 2011, 06:29 

#7 
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Vic
Join Date: Feb 2011
Posts: 5
Rep Power: 6 
Hi Patrick,
I am not an expert but I just want to post a question: the max Aspect ratio of 454 is OK? I was told that the maximum Aspect ratio should be kept within 150, although I totally know not why this number is like this. Should you give me any suggestions? Thanks! Vic 

January 20, 2012, 14:21 

#8 
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Saurabh
Join Date: Jan 2012
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Mine is even worse! CheckMesh gives:
***High aspect ratio cells found, Max aspect ratio: 8.41538e+193, number of cells 8460 <<Writing 8460 cells with high aspect ratio to set highAspectRatioCells Minumum face area = 5.24051e07. Maximum face area = 1.11681e06. Face area magnitudes OK. Min volume = 2e300. Max volume = 2e300. Total volume = 1.692e296. Cell volumes OK. Mesh nonorthogonality Max: 180 average: 169.488 ***Number of nonorthogonality errors: 22956. <<Writing 22956 nonorthogonal faces to set nonOrthoFaces ***Error in face pyramids: 50760 faces are incorrectly oriented. <<Writing 27804 faces with incorrect orientation to set wrongOrientedFaces Max skewness = 0.704293 OK. Dont know why Im getting this. I have very benign mesh with all cells near to 1 cubic mm. Can it be because some of my edges in blockMesh are arc. In fact many of them.
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February 15, 2012, 04:45 

#9 
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It may caused by the too slim cells, check you mesh and modify the node distribution at the problematic regions. Besides, edges are following arcs as you said, orientation problem is very prone to be there.


February 15, 2012, 12:43 

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Saurabh
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Thanks Rikio I will try it
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