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[ICEM] Meshing from Overcomplex CAD

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Old   January 3, 2014, 17:01
Question Meshing from Overcomplex CAD
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Daniel Wilde
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The vast majority of my experience in CFD uses the ANSYS suite of software for meshing and CFD solution within the power generation industry (primarily ICEM/Fluent). I have recently become involved in automotive work which uses a powerflow like approach for CFD and the methodology for generating mesh is...foreign.

My question is, does anyone have recommendations for handling overly complex CAD within the ANSYS suite (particularly ICEM)?

I know ICEM has octree and there is also the wrapper available, but when I try to even load the CAD into ICEM (on a respectable workstation) I crash or have to wait an hour until the geometry has been converted to .tin

ANSA, on the other hand, can bring in the geometry almost painlessly. I would much rather mesh in ICEM, but If I can't load the geometry then I have no choice.

I know a lot of you are thinking that the CAD should be simplified within CAD software, but for reasons that are beyond my control, there is no access to true CAD software.
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Old   January 5, 2014, 10:37
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Vangelis Skaperdas
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Dear QCFD,

Indeed ANSA is very powerful in reading really complex CAD data
and also perform clean up and de-featuring.
As you describe it is not always possible, or efficient, to perform
geometry simplification in CAD. The tasks of the CAD designer
and the CFD engineer are distinct and different,

And if you want to know something else, it is very easy also to generate
the mesh in ANSA too.

I would suggest that you seek for help in the following
PDF documents that you can directly access from ANSA in
Help>ANSA documentation index

1) ANSA for CFD Quick Start Guide
2) Tutorial Basic under TOPO MESH
3) Tutorial meshing for external aero under CFD

Give it and go and you will find that you will create hi quality
surface meshes at a fraction of the time

Best regards

Vangelis
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Old   January 5, 2014, 12:36
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you can use design modeller or space claim before importing mesh too icem cfd...
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Old   January 5, 2014, 12:38
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Vangelis,

Thank you for the guidance with using ANSA. I'm the short time I've been exposed to the software, I've seen it create clean mesh on complex geometry without much struggle.

I will definitely look into the ANSA resources you have indicated.

What I am really trying to figure out is if somewhere in thier suite of software, ANSYS has similar capabilities. ANSYS is a huge name in simulation and I am surprised by the apparent gap in capability compared to ANSA in handling complex geometry. (Automotive components with absolutely no defeaturing I'm the CAD)

Thank you,
QCFD
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Old   January 5, 2014, 12:48
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Far,

Yes my typical process has been to use dedicated software to make simplifications to the geometry before starting to work in ICEM or whatever mesher I am going to use. I am working on getting licensing for CAD software so I can revisit this approach and compare it to bringing the nonsimplified geometry straight into ANSA.

I am just surprised that ICEM can't even load the unsimplified geometry while ANSA can do it so easily. There must be different internal representation of geometry within the software?

Thank you,
QCFD
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Old   January 5, 2014, 12:51
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which format you are using? What message you are getting while loading the model?
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Old   January 5, 2014, 13:24
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The CAD is provided in either .JT or step format. ANSA has a "translator" that runs in a seperate GUI and translates the JT files into the .ansa format which can be opened quickly.

With ICEM I have to open the .stp directly. I know this is not a fair comparison since the translation is inside the GUI and working from .stp (heavy) instead of JT (light). However, The software will lock up for several hours working to translate .stp into .tin.

Again, when I regain access to CAD software, I will try converting to parasolid (.x_t) which I typically use, and see if ICEM performs better.

Perhaps the issue is largely file format, but once the geometry is loaded as a .tin, it still slows ICEM to a crawl to render while ANSA can still function.

I know there is the "triangulation tolerance" setting in ICEM which determines how "faceted" the geometry is represented and could speed up performance. Perhaps ANSA facets the geometry heavily to improve performance?
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Old   January 5, 2014, 13:38
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In fact, default tolerance is 0.001 and it should not be the issue. I dont know why icem is not loading geometry may you can try 0.1 tolerance before importing file !!! or may be ANSA is better there ...
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Old   January 6, 2014, 08:27
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Vangelis Skaperdas
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Hi QCFD,

Just for reference, JT files usually contain the model
in two formats, tesselated triangles (like STL mesh)
and NURBS geometry.
In ANSA translator there is the option which one (or both)
to tranlate.
In ANSA the tesselated data appear as FE-mod mesh (surface
mesh without associated geometry) and the NURBS as
geometry, so called Faces.

Have in mind that ANSA displays the model with a default resolution
length of 20 units (lets say millimetres)
We should not confuse this length with the tolerance length with
which topology is applied, which is something like 0.05

Hope this helps

cheers

Vangelis
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Old   January 6, 2014, 10:35
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Daniel Wilde
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Thank you Vangelis,

That is very helpful to know. I think a large part of my issue is that I'm so accustomed to the way ICEM handles geometry that I don't really understand what is going on behind the scenes ANSA and have made some wrong assumptions. That information answers some questions that I haven't even asked yet.
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Old   January 7, 2014, 03:51
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Vangelis Skaperdas
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You are welcome QCFD
Don't hesitate to ask more

Best regards

Vangelis
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