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gnuboard May 16, 2010 20:10

Creation of hexa dominant mesh and prism layer
Hi everyody,

TEST GEOMETRY: a sphere in a rectangular box

WANTED MESH DESCRIPTION: I want to create an hexa-dominant mesh with ICEM-cfd, with an prism layer on the sphere.

COMPUTER SYSTEM: windows7 32 bits, 4 gig of RAM (3.5gig really)

ANSYS ICEM-CFD V12 is used

THE PROBLEM: 1. if i start the meshing by create the hexa core in the box, and after, to do the prism layer on the sphere, the prism creation fail.

To partly solve my problem:
1.I create an octree tetra mesh all over the box
2.I create the prism layer
3.I transform the tetra in hexa (12 tetra for 1 hexa)

With this solution, the problem is the memory fail. The first mesh is tetra, so i need to create a lot more tetra to finally got only some hexa. If i could create the hexa first, i could get more refinement.

Do you have an other idea to create this kind of mesh?



PSYMN May 17, 2010 09:37

The ICEM CFD Prism algorithm can not move hexa's out of the way as it inflates the prisms. Therefore, you need to make sure that you don't convert tetra to hexa before prism inflation.

With ICEM CFD, there are always other ways... ;)

1) You could try a Hexa (blocking tab) mesh. This would be really easy if you really wanted to do a sphere in a box... What is your real application? Aircraft shapes, wings, etc. are also pretty easy to do with Hexa blocking. It gives a pure hexa mesh very quickly with the best boundary layers possible and it has very low memory requirements (the lowest of any meshing tool I know).

2) You could try hexa core... Personally, I prefer the transitions in the 12 to 1 conversion, but Hexa Core will use less memory (cartesian algorithm). To use it, start with a Tetra prism mesh, but with a large max size in the volume (to reduce the amount of mesh generated in the volume). Then generate prisms... Then go back into Params by parts and set a max size and turn on hexcore for the volume parts you want to have hexa core in... (other hexa core settings are under global params => volume params => Cartesian => Hexa Core.) This will dump your octree tetras (but keep the surface mesh and prisms). It then uses a cartesian algorithm to generate the hexa core of the right size in the volume, which it then steps back a few layers from the pre existing mesh. Then it uses a delaunay algorithm to fill in the gap between the Hexas and Prisms with Tetras...

3) The ICEM CFD Hexa core isn't as good as the TGrid Hexa Core. TGrid is a bit of a memory pig, but if it is all the same to you, you should try it out. TGrid Hexa core is ideal for Fluent in that it supports hanging nodes, even with adjacent tetras (ICEM Hexa Core only supports haning nodes within the Cartesian region, but fails if they are at the surface adjacent to tetras). The TGrid Hexa Core is also able to go to the walls if they are flat.

4) You could try subdividing your geometry into smaller chunks that your memory capacity could handle. Again, the practicality of this would be dependent on the geometry...

gnuboard May 17, 2010 11:27


The sphere is a test model, my real application is automotive application.

I'm trying to replicate the same type of mesh that i saw in this article (but my model is very very simpler) or search "Automotive CFD" in google, this is the first link.

Tonight, i'll try your first tips.

Could you explain more your tips # 4, it seems very interesting.

Thanks for your clear answer.

PSYMN May 17, 2010 12:36

4) Subdividing the geometry works best if you have natural locations for the subdivision... A vehicle subdivision is somewhat less natural, but you might find that you could mesh the majority of the far field with Hexa and then mesh the complex near car region with Tetra, add prisms and then do the tet to hex... This would be at a reduced memory level from your current plan. Pictures of this done with a race car can be found in my ICEMCFD Tips and Tricks 2008 pdf...

I will find the link later and post it, or you can look thru my older posts since I have posted it many times before.

As for the article, it compared ICEM CFD/Tgrid with Harpoon. You will need to be more specific about what part of that you wanted to reproduce.

gnuboard May 17, 2010 15:23

I found your tips and tricks in the thread "[ICEM] Hybrid mesh". I'll look at this tonight.

Scuse me, my last last question on the article was very fuzzy. To answering you, i want to reproduce the mesh as described in the paragraph 2.4 (maybe not the same size) and have an prism layer on the outer surface. I'll only study the outer body.


PSYMN May 18, 2010 10:27

Side link...
While working on Joel's model (for this thread... )

I did tet to hex conversion around a prism layer and am linking it here in case it helps.

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