[ICEM] Prism on complex geometry

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November 4, 2010, 04:43
Prism on complex geometry
#1
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Hi all,

i am struggling at the moment in generating a good mesh for a very complex shaped natural fracture out of CT data.

It is possible to generate a pure tetra mesh using the robust(octree) mesher.
the delauney mesher fails, that is, it creates some really strange (huge) elements?!

The problem is now: i can only handle something around 14Mio elements with fluent on my machine. It would be very nice to have 3 or 4 boundary layers on the walls, but the prism mesher always fails. Since my geometry is really complex, i want to know if you think it is possible at all to generate it? If not, a hexa-mesh won´t be also not possible??

shohin
Attached Images
 core08_overview.jpg (86.6 KB, 185 views) core8_mesh_close.jpg (83.9 KB, 136 views)

 November 6, 2010, 16:36 Outside the box. #2 Retired from CFD Online     Simon Pereira Join Date: Mar 2009 Location: Ann Arbor, MI Posts: 2,660 Blog Entries: 1 Rep Power: 38 Hmm, very thin and you want several layers thick to better capture flow gradients between the walls? Just thinking outside of the box here, but have you considered taking the surface mesh from one side and extruding it thru the thickness? You can extrude with an equation to simulate a double sided growth ratio. If it is not a uniform thickness, but it is close, you could project the new mesh to the wall... Delaunay starts from a surface mesh, so it wouldn't generate huge elements. Those would be created by a mis use of its first step to create the surface mesh. Octree can work directly from faceted geometry to create volume mesh and surface mesh. You can also use curvature and proximity size functions to make sure your mesh is fine enough... (Under Global Mesh Params) Beyond that, I would need to get my hands on the model to diagnose the prism failure problem.

 November 6, 2010, 19:18 #3 New Member   Join Date: Apr 2010 Posts: 17 Rep Power: 8 hi, thanks for your answer! well, i can generate now both with delauney and octree pure tetra meshes with a desired number of elements, thats not the problem. unfortunately my hardware restricts using 40Mio or so.... but i can not really generate enough elements in very thin areas. maybe i should play around with the proximity and curvature feature a little more. beyond, i manage to create prism meshs, both from one layer and splitting and from lets say 3 layers initially. i can also load it without error in fluent, but it is not converging. i assume the quality of the mesh is to bad, i have some tetra elements with a quality of 1e-5!!! your idea with the surface mesh sounds interesting! could you please give me some hints how to manage this. i will study the manual about extruding... the problem will be, i think, that the thickness is not at all uniform!!

 November 7, 2010, 20:06 oops, on second thought... #5 Retired from CFD Online     Simon Pereira Join Date: Mar 2009 Location: Ann Arbor, MI Posts: 2,660 Blog Entries: 1 Rep Power: 38 Um, never mind. I typed the last bit based on memory and without looking at your model recently. I just went back and looked at your model again... That would take a long time to interactive hexa block. I would probably stick with tetra prism unless you have a bunch of time (a day or two and a lot of hexa splits.) or don't mind ignoring some details. Hexa is patch independent and doesn't have to capture everything... I had originally suggested the extrude because you have the surface mesh already. How important is it to capture the thickness variation? An extrude would be pretty painless once you deleted the side you were not interested in.

 November 8, 2010, 09:52 #6 New Member   Join Date: Apr 2010 Posts: 17 Rep Power: 8 Hey, first, thanks for your thorough answer the hexa method really sounds interesting, though very time consuming, furthermore i am absolutely new to hexa meshing etc....so thats why i would prefer the tetras. As i said, i can generate prism layers (2 or 3 on both sides, the remaining interior filled with tetras) but i still get some errors while prism meshing, so not everything seems to work perfectly. Nevertheless, the mesh is imported into fluent w/o errors, but i do not get convergence (just a test)...so i assume the mesh is anything but perfect at the moment. it seems to be that i have some very bad tetras (bad quality: 1e-5), maybe that is the problem right now. The extruding idea seems also interesting...i thought about extruding via element normals on both sides....but the splitting of the big surface (= wall; bottom= inlet, top = outlet) into to 2 surfaces is not so easy. i am also not so sure if this will be the best approach....but thats only a thought. unfortunately the thickness is one of the major features i want to capture, thats why i am so much concerned about enough elements to examine flow and solute transport in the interior (maybe i should stick for the moment with a pure tetra mesh which gives me some results, but for sure not the best....)

 November 8, 2010, 13:50 Prism settings... #7 Retired from CFD Online     Simon Pereira Join Date: Mar 2009 Location: Ann Arbor, MI Posts: 2,660 Blog Entries: 1 Rep Power: 38 It could be that different prism settings are what you need...

 November 8, 2010, 14:48 #8 New Member   Join Date: Apr 2010 Posts: 17 Rep Power: 8 yes, that could be true...but which settings!?!?!? i already played around with them and it is not really becoming better.....

 November 9, 2010, 13:04 Settings... #9 Retired from CFD Online     Simon Pereira Join Date: Mar 2009 Location: Ann Arbor, MI Posts: 2,660 Blog Entries: 1 Rep Power: 38 Lets focus the questions and the info you provide on the prism errors... Some photos of cut planes zoomed in so I can see the problem areas would help me diagnose and suggest alternate settings. It would also help to know what settings you did use...

 November 10, 2010, 12:54 #10 New Member   Join Date: Apr 2010 Posts: 17 Rep Power: 8 ...ok, i will provide pics and details tomorrow....looking forward to your suggestions! thanks a lot for your help

November 16, 2010, 05:45
#11
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sorry for the delay...i had some severe problems in the lab with this borecore...it is leaking at the moment due to high fluid pressure
attached find some first pics about the settings...maybe this gives you a first impression about the settings...i will provide some cut planes later on...

in this example i only generated one layer of prisms and splitt them later...but it makes no difference if i generate 3 layers initially....i think the biggest problem are some very very bad tetrahedra which i cannot remove/remesh etc...
Attached Images
 icem_shot1.jpg (89.2 KB, 82 views)

 November 16, 2010, 15:35 Solutions need problems... #12 Retired from CFD Online     Simon Pereira Join Date: Mar 2009 Location: Ann Arbor, MI Posts: 2,660 Blog Entries: 1 Rep Power: 38 Yea, I would need to look at the cross section... These settings would be fine for some models. When I see the problems I can suggest the solutions...

November 24, 2010, 13:12
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hey,
so sorry for the delay again!

i can say that i "accidently" got a mesh with 4 prisms on each wall, which has a reasonable mesh quality (mean ~ 0.70). i still have some bad tetras, a few pyramids, and some quads with a quality of around 0.01.
the quality was more or less a results of extensive smoothing (~ 1000s) of all the elements together up to a quality of 0.2.
the final mesh has ~10 Mio elements which is more or less the max i can reasonably handle with my hardware (also considering the calculation times in fluent).
i want to add, that i had to use the octree volume mesh, because when using delauney i always got some problems in the fluent mesh check (degenerated contact points)....

nonetheless, fluent reads in without problems and converges fine!
so, i will live with that for the moment, until i have more hardware resources available which offers me more flexibility for the mesh size/type.

nethertheless, i attached a pic of an arbitrary cutplane.....

thanks again, PSYMN, for your great support!!!!

shohin
Attached Images
 mesh.jpg (83.1 KB, 87 views)

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