meshing problem with different mesh size

[dzheng]

Hi,

Recently I imported a complicated 3D model from other 3D-design software with extension .SLDASM (SolidWorks Assembly File) into GID, and tried to make its surface conformal and mesh it (I only need surface mesh). I think I already made it conformal for that when I check View -> Higher entities -> lines all lines are interior lines. The problem is with one component surface of this model, so I extract this component surface and save it in a single Gid file. I can mesh it successfully with setting mesh size 0.4 or larger than this value, but errors will be reported if I try to mesh it with setting mesh size smaller than 0.3. The error is : Couldn't mesh the surface. I don't know why this happens, I upload the file in .rar format, please check it and help to fix it.

Thank you very much!
Awaiting for your reply.

[escolano]

The causes of problems meshing CAD geometries could be near infinite.

Some of the particularities of your surface are:
1- It is a trimmed NURBS surface (untrimmed parametric surfaces are easier to be meshed, its parameter space domain is the whole square 1x1)
2- It has a short curve of length 0.03 units compared with the lenght of 12 units
3- It is a closed NURBS surface, closed in both parametric directions u and v (topologically it is like a sphere), then the inverse of the parametrization is not one-to-one (two points on the space parameter maps to the same 3D point), and it is difficult then to be meshed in the space parameter.
4- curves that are trimming the surface are not very accuratelly on the surface (e.g. some curve at distance 0.04 units of the surface)

Then depending on assigned mesh sizes, and all meshing parameters the process could be successful or not.

For example, if do you collapse the short line (1). e.g. collapsing with tolerance=0.04
Utilities->Preferences... Import and export-Import tolerance=User define, size=0.04
Geometry->Edit->Collapse->Model
then I am able to generate surface meshes with sizes 0.3, or 0.1 or others
but in the mesh I could visually detect the problem (4): the triangles of the surface change suddently its normal to be on the curve.

curve on surface gap

change_of_normals_because_curve-surface_gap.png (46.99 KiB) Viewed 8144 times

In particular, the trimming lines in this case are curves parallel to curves u=constant and v=constant, then it is possible to build a untrimmed surface, dividing the total closed surface (blue dark+cyan+pink surfaces) into three surfaces.
The pink part is similar to the original surface, but in this case is untrimmed and not closed, then is much less problematic to be meshed.

untrimmed_surface.png (50.98 KiB) Viewed 8144 times

I attach a GiD model with these untrimmed surfaces and the original trimmed one.

Assembly2_v32_support_v6_rebuild_parts.zip

(124.42 KiB) Downloaded 314 times

[dzheng]

Thank you very much for your fast response, your method is very helpful, I have tried as you said to build a untrimmed surface and divide the untrimmed surface into 3 parts, then I deleted the blue dark surface keeping cyan and pink parts which are interested in my case, the meshing now for these two adjoint surfaces can be set to very small size (i tried 0.01) without reporting any error, I think it should be fine to mesh the whole assembly surface. I have bothered by this problem for a lone time, thank you for rescue me.

best regards!

[dzheng]

I just encountered another problem: the intersection between the pink surface and the blue tube fails, the result is that, after intersection the pink part disappears and cant be meshed. My goal is to mount the blue tube onto the pink surface to make them a continuous surface (so that the surface is conformal). My method is to intersect the blue tube with the pink surface, and then delete two redundant surfaces, one is a small pink circle inside the blue tube and the other one is a segment of blue tube in the inner side of the pink surface. I failed because the pink surface required to be meshed disappeared . I also upload the .gid file.

Thank you for your attention.

Regards

[dzheng]

[escolano]

The attached model (Assembly2_v32_support_v6_untrimmed_devied_v1.gid) doesn't include the blue tube, it only has the pink untrimmed surface!!

If your tube is a 'closed single cylinder' you can try dividing the cylinder in two half-cylinders by the 0.5 parameter value, then maybe the intersection could be done.

[dzheng]

I am sorry to upload the wrong .gid file, too many modified gid files in the folder. I upload the right one.

I tried to divide the tube with parameter 0.3, 0.5 and 0.7, still the pink surface disappears after intersection, it does not help.

I made an surfaces intersection between the blue tube and an planar surface copied form the top surface of the part (it is a pink circle surface in the gid file), it has no problem. So I guess there are still problems with that pink curve surface that are interested.

thank you

regards

[escolano]

The difficulties to generate a calculation mesh (and a render mesh to visualize in smooth/flat render mode) of the surface are related with the 'too short line' number 1145 (near the maximum x).

If you really don't require this detail a solution could de collapse its points to delete it.

After intersect the surfaces, the pink surface must change from untrimmed to trimmed, because is holed by the tube (and the trimmed case is more difficult that the untrimmed one for several algorithms)

Another way to avoid the problem is divide the pink surface to isolate the problematic (short line) in a untrimmed surface, and the part to be intersected in another surface that will become trimmed, but at least doesn't has the 'short line' extra difficulty.

division_and_intersection.png (67.93 KiB) Viewed 8123 times

I attach the GiD model modified with this solution:

Assembly2_v32_support_v6_untrimmed_devied_antenna_v1.rar

(455.35 KiB) Downloaded 319 times

[dzheng]

Sorry for not reply fast due to another work on which I need to pay more attention.

Yes, it works with dividing the surface in v-sense, and after mounting the surface onto the model I can generate mesh by any size.

Thank you very much for helping.

Wish you everything goes well.

zheng