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-   -   Tetra-Prism-Meshing-Extreme! (http://www.cfd-online.com/Forums/cfx/22364-tetra-prism-meshing-extreme.html)

Quest March 29, 2006 05:31

Tetra-Prism-Meshing-Extreme!
 
Hello, I want to model a simple flow in a cylindrical pipe. The problem is the length is about 20 times longer than the diameter. The boundary layer is very thin (Re > 2E6). But the mesh elements are getting too many as I make a fine resolution over the boundary layer. I have not been able to make good mesh yet. I think the height of the prisms must raise very quickly but I don't know exactly how to do this. Can somebody give me some help.

Chebeba March 29, 2006 05:51

Re: Tetra-Prism-Meshing-Extreme!
 
Assuming you are using ICEM: Prism height growth is controlled by Mesh/Global Mesh Parameters/Prism Settings/Height Ratio.

Default is 1.2, set it to 1.5 for VERY quick height growth.

/C

James Date March 29, 2006 07:10

Re: Tetra-Prism-Meshing-Extreme!
 
Use ICEM-Hexa with an O-grid, This will keep the mesh size down and give you the best control in the boundary layer.

Quest March 29, 2006 09:06

Re: Tetra-Prism-Meshing-Extreme!
 
Unfortunately in my special case O-Grid doesn't work well.

@Chebeba Yes I am using ICEM 10.0. But what do I have to do next? The manual doesn't help me much. Some tips would be nice.

Chebeba March 29, 2006 09:50

Re: Tetra-Prism-Meshing-Extreme!
 
? eh what do you mean "do next"? You mean you haven't even gotten as far as running the prism mesher or what?

Rough steps (for an unstruct mesh):

- Create a surface grid on the pipe with your desired resolution. Use the Surface Mesh tools or create a blocking and convert the Pre-Mesh to a surface mesh.

- Set prism parameters in global mesh settings.

- Run prism mesher.

- Fill the rest of the domain with tetras using the tetra volume mesh from surface mesh.

In case don't like the Tutorial Manual, check out the videos at http://www-harwell.ansys.com/demoroom/demos.php?id=39 there might be something for you there.

Quest March 29, 2006 10:24

Re: Tetra-Prism-Meshing-Extreme!
 
I let the prism mesher run but the results were not good enough. So I changed some parameters and now the prism mesher doesn't work. It breaks up without an error message without any prism element. I don't know what's wrong.

Chebeba March 30, 2006 01:14

Re: Tetra-Prism-Meshing-Extreme!
 
The mesher is not robust. You will have to provide it with a geometry, a surface mesh and a set of parameters that allows it to do what you say it should do without running into strange situations.

Look at your surface mesh and your parameters, and ask yourself:

Can all surface cells be extruded to the full height I need without degenerating? (The global prism mesh settings panel will calculate total height for you if you leave that field empty and press calculate).

Is the angle between any surface mesh face allways less than 270 degrees (i e no convex corners sharper than 90 degrees)?

For concave corners, is the size of the first row of shells next to the corner curve big enough to accomodate the full height of the prisms growing from the opposite surface?

If the answer to any of these questions is No, your chances of having the mesher run successfully are slim.

In other words, don't expect miracles from the mesher. It just extrudes columns from the surface, it's not intelligent about what to do if there is no obvious topological way to do this.

And agreed, the error messages are completely non-informative. They remind me of my first Mac, whitch used to say "An Error of type -1 has occured." whenever it gave up...

/C


Quest March 30, 2006 03:05

Re: Tetra-Prism-Meshing-Extreme!
 
Thanks a lot to you.

I've installed SP1 and SP2 and it is working much better now. So I will continue trying.

Also very funny are the corrected bugs from the release version(especially 1,5,9):

1.Prism from surface mesh does not work in the release version

2.Tetra cannot represent some geometry because of the default ?Fix Non-manifold? option

3.?Remesh Center? is disabled for Hex Dominant Mesher when running from geometry

4.Min Prism Quality does not work correctly in the release version

5."Tetra + Prism layers" does not work in the release version

6.Can't extrude prism layers on standalone surface mesh

7.Hexa-core mesher does not work from the mesh with prism elements

8.Parallel Tetra crash under IBM-AIX

9.Unable to read prism parameters from old prism_params files


KBanks April 3, 2006 12:48

Re: Tetra-Prism-Meshing-Extreme!
 
Hi, I'm intrigued as to why an o-grid in Hexa would not work for your purpose. If you're meshing a pipe - then an o-grid is almost certainly the best way.

leon April 13, 2006 09:53

Re: Tetra-Prism-Meshing-Extreme!
 
?? Which version of ICEM are you using? I'm using the version 5.0, but I could not find the option as you presented!!


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