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-   -   [ANSYS Meshing] Tetrahedral or Hexahedral (http://www.cfd-online.com/Forums/ansys-meshing/95525-tetrahedral-hexahedral.html)

Mina_Shahi December 21, 2011 10:02

Tetrahedral or Hexahedral
 
1 Attachment(s)
Dear Ansys users

I am going to mesh a 3D geometry, this geometry is rather simple one, and consisting 2 rectangulars part, 31 tubes with 1mm diameter are coneected to them acting as inlet boundaries, (see the attached file)

Attachment 10513

Now i am wondering that with these small tube which mesh method is better Tetrahedral or Hexahedral ? i am going to mesh it in Workbench and then do CFD analysis,

Far December 21, 2011 15:22

mix of both

Mina_Shahi December 22, 2011 06:52

Quote:

Originally Posted by Far (Post 336648)
mix of both

Thank you for the answer, so Hex DOMINANT should be Ok in ansys workbecnh, true??

PSYMN December 22, 2011 12:34

I agree with "Far", but don't recommend the Hexadominant method for CFD.

The Hexadominant method starts by paving quads at the walls and then marches inward with isotropic hexas which can crash somewhat badly in the middle. This is good for FEA structural analysis where most of the interesting stuff happens near the surface and uniform elements are sufficient to capture it all, but it is not good for CFD.

However, if you tried Multizone (or sweep) with an inflation layer, you could get a nice combination of swept hexas that would produce a good mesh for CFD.

Far December 22, 2011 12:42

What I wanted to say is "Always prefer the hexa mesh but when the quality cannot be maintained e.g. aspect ratio, skewness then it is better to switch to unstructured mesh".
Also in unimportant areas such as far-field use the unstructured mesh while using the structured mesh in boundary layer region, thereby inner mesh does not propagate in outer region.

Mina_Shahi December 22, 2011 17:36

Quote:

Originally Posted by PSYMN (Post 336762)
I agree with "Far", but don't recommend the Hexadominant method for CFD.

The Hexadominant method starts by paving quads at the walls and then marches inward with isotropic hexas which can crash somewhat badly in the middle. This is good for FEA structural analysis where most of the interesting stuff happens near the surface and uniform elements are sufficient to capture it all, but it is not good for CFD.

However, if you tried Multizone (or sweep) with an inflation layer, you could get a nice combination of swept hexas that would produce a good mesh for CFD.

Thanks for your suggestion, i tried Multizone or sweep but it gives errors even when i changed the grid size ... also Hexadominant is not good it gives a warrning. a warning message states that a low percentage of hex elements or poorly shaped hex elements may result. the only method which worked was Tetrahedrons. what do you think?

PSYMN December 22, 2011 17:51

Tetra is, by far, the easiest way to go and often a good way to start.

Your model looked simple enough to me that I thought MultiZone would have done nicely. Hexa is slightly better for quality and speed of convergence, but if the model is giving you trouble, go for Tetra/Prism...

One other thing you could try is "CutCel" hexa. It is as automatic as Tetra, but gives majority Hexa. It does use hanging nodes though so it really only works well with Fluent. Get 14.0 though, since it was improved a lot over 13.0 both in speed and robustness, especially when used with Prism.

Best regards,

Simon

Mina_Shahi December 22, 2011 17:58

Quote:

Originally Posted by PSYMN (Post 336808)
Tetra is, by far, the easiest way to go and often a good way to start.

Your model looked simple enough to me that I thought MultiZone would have done nicely. Hexa is slightly better for quality and speed of convergence, but if the model is giving you trouble, go for Tetra/Prism...

One other thing you could try is "CutCel" hexa. It is as automatic as Tetra, but gives majority Hexa. It does use hanging nodes though so it really only works well with Fluent. Get 14.0 though, since it was improved a lot over 13.0 both in speed and robustness, especially when used with Prism.

Best regards,

Simon

Did you use ANSYS workbench for meshing? there are 6 methods to make a mesh, CFX mesh, MultiZone, Tetra, Hexa dominant, ...

There is no ""CutCel" hexa" on that? By the way how Tetra works from this point :numerical error or dissipation error ?

Thank you again
Regards

PSYMN December 23, 2011 16:45

Cutcel is not quite like all the others. It is a global method, so you don't set a method on a part or body, instead just left click on "mesh" in the model tree and then look down in the details panel. You will find a pull down that lets you turn on Cutcel.

I don't know what you are asking here
Quote:

By the way how Tetra works from this point :numerical error or dissipation error ?

toda December 20, 2012 04:52

Warning in the Ansys Meshing
 
Hello everybody,

I`m new in ANSYS, when I mesh some geometry in the Ansys Meshing, there is a warning: "Inflation created stairstep mesh at the some locations". What the meaning of this warning? If I use this mesh on Fluent simulation, is there any effect about this warning?

Thank you very much for the reply.

PSYMN December 20, 2012 13:48

Quote:

Originally Posted by toda (Post 398382)
Hello everybody,

I`m new in ANSYS, when I mesh some geometry in the Ansys Meshing, there is a warning: "Inflation created stairstep mesh at the some locations". What the meaning of this warning? If I use this mesh on Fluent simulation, is there any effect about this warning?

Thank you very much for the reply.

It just means that your prism layers are not all fully developed. So if you wanted 15 layers everywhere, but there wasn't room for 15 layers in some places, it will "step down" to 14 or less. Some solvers don't mind stair stepping at all (CFX), Fluent prefers full layers, but it really depends on where the stair stepping is.

toda January 15, 2013 00:55

VOF models on venturi scrubber simulation
 
5 Attachment(s)
Dear all.


My simulation is about venturi scrubber. A venturi scrubber is designed to effectively use the energy from the inlet gas stream to atomize the liquid being used to scrub the gas stream, so the gas stream will more clean. The end of the inlet water pipe there is some small nozzles. This venturi have 2 inlet and 2 outlet. Two inlet are inlet water and inlet gas. Because flow in venturi scrubber is turbulent, I make inflation in region near the venturi scrubber wall. The boundary conditions are inlet water=velocity inlet, inlet gas=velocity inlet, outlet1 and outlet2 = pressure outlet. In Fluent, I use models: Multiphase VOF, Energy Equation, and Viscous is k-epsilon. Multiphase have 2 eulerian phase, that is air and water liquid. The solution method is PISO. The absolute criteria of convergence is 1e-4 except energy (energy is 1e-6). After 7903 iteration, the simulation still not convergen. The simulation also have reversed flow in outlet1 and outlet2. Anyone can help me to solve this problem?? Is the multiphase VOF true for this problem?? Any help is highly appreciated. Thanks.

I`m sorry if the discussion is out of ansys meshing topic.

Best Regards
Toda

toda January 15, 2013 00:59

VOF models on venturi scrubber simulation
 
2 Attachment(s)
The more attachments about my simulation.

Best Regards
Toda


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