[giangcoikx]

Dear everyone,

Maybe I ask an old question but I would like to hear your points as an overview.

Currently I am doing on a small turbine research, what I am doing first is to create the turbine geometry from a bunch of coordinates on CATIA. Then I mesh it by Pointwise and eventually solve it by CFX.

My question is about the first step. It seems easy and convenient to draw on CATIA, it gives me a good surface quality. However, some experts said that the purpose of CATIA and other CAD softwares is for manufacturing, for this reason, its best tolerance is about 0.001 inches or 0.0254 mm, meanwhile for some turbulence model (which requires y+ around 1 or 0.001 mm), much lower than manufacturing standard. And due to this, there might appears some small wiggles and gaps on intersecting surfaces due to incompatibility that I don't see on CATIA, but might appears when I import it on Pointwise.

(I have experienced it once when I import igs file created in Solidworks. We divided a surface into 2 parts for meshing convenience purpose (quilts). But when we opened it in Pointwise, it appeared lamina boundaries on the cutting line although it had better be a lines only. And then if we deleted these lines, it appeared a gap.)

When I open the 747 tutorial igs file in Pointwise and in CATIA, it appeared that CATIA shows much more sub-surfaces on a big surface. Is this a potential source of incompatibility? If not, how should I understand?

The experts suggested me to draw a geometry directly on the meshing software. However, as I understand, is it difficult on Pointwise when the object has complex geometry like a engine turbine? If not, on the Axial turbine tutorial, which software did Pointwise draw it?

My private question, for for personal knowledge, do you think ANSYS ICEM has convenient tools for geometry creating? Please do not bias your company product.

In total, which CAD tools do you often use? How much did you have to adjust geometry again on meshing software, make it mesh-able?

P/s: I know a meshing object must be water-tight. Does it mean it must form a single model only? I mean for one body line a turbine, an airplane wing body.

Thank you very much for your effort to help me.
I am a newbie so I am confused about the working principles.: D

[dgarlisch]

You have asked the US$1,000,000 question!

Pointwise is constantly dealing with CAD tolerance issues all the time.

Pointwise customers are constantly faced with the task of creating a mesh on less than desirable (sloppy, low tolerance) CAD models. As you have noted, this can lead to some interesting challenges when your mesh spacing is much smaller than the CAD tolerance.

DISCLAIMER:
I am not a Catia or CAD app expert. However, I do have some light experience using various CAD systems.

Just a few weeks ago I was investigating this very topic. Based on my google search results, I am under the impression that some CAD systems do allow you to control the tolerance used in your CAD model. I do not know if Catia is one of them*.

Since you have the advantage of building your own CAD model, and you know the cell spacings you need for your grid, you can set the CAD tolerance to a tighter value. I would definitely suggest you do that! The main drawback with using a tighter tolerance is that CAD entities may use more RAM and operations like surface-surface intersections will take longer to compute.

I hope others who see this post will be able to offer specific information about setting the CAD tolerance in Catia or other apps.


I cannot suggest a CAD app. I do not know enough to give an informed answer.


Once you have constructed your CAD model with an appropriate, tighter tolerance, you will most likely want to take advantage of Solid Model meshing in Pointwise. There are several video tutorials that explain Solid Modelling better than I could.

Good luck!

* I found this Catia link that may help.