Personal tools
You are here: Home Forum Use Too small mesh elements

Too small mesh elements

Up to Use

Too small mesh elements

Posted by Annette Brask at November 28. 2019

Hi, 

I am working on generating a mesh for a quite complex geometry and some of the elements of the mesh end up too small for my purposes. The geometry is imported as a STEP file from Inventor. The algorithm I am using is NETGEN 1D-2D (with quadrilateral elements). I construct a hypothesis with minimum size 0.0125 like shown in the attached picture, but still there are some elements with an area of size 0.6*10^-7. Does anyone have an idea why these small elements may occur? Is there a method of preventing or removing elements under a certain size? 

 

Unfortunately, I can not include the CAD file due to confidentiality, but let me know if you need more information. 

 

Thank you in advance! 

 

Best regards, 

 

Annette

Attachments

Re: Too small mesh elements

Posted by Saint Michael at November 28. 2019

Hi Annette

Can you make a picture of too small elements with some surrounding mesh of good size?

St.Michael

Re: Too small mesh elements

Posted by Christophe Bourcier at November 29. 2019

Hi,

You can call merge nodes with 1e-5, 1e-4 or 1e-3 to remove the small elements.

Christophe

Re: Too small mesh elements

Posted by Annette Brask at December 02. 2019

Thank you for your replies St. Michael and Christophe!

 

St. Michael, I am not sure exactly where the elements are. I think they are too small to see. The problem occurs when I am using NEMOH and I get an error message that some of the elements are too small. Also, when I use the Controls--> Face controls --> Area function, there seems to be some elements with small areas compared to the largest ones (please see the attached picture). 

Christophe, how do you do this? Is there an automatic way of finding all elements that are smaller than a certain size or do I have to merge each one manually. It may be tricky for me to do them manually because the elements are too small to see. 

 

Best regards,

Annette

Attachments

Re: Too small mesh elements

Posted by Christophe Bourcier at December 02. 2019
To see the small elements, you can create a group of faces with a filter area < 1e-4 (Right click> Create group, on Face, on Filter, set Filter, Add, Area less than 1e-4).

If you can't see them, you can display their numbers by right-click Numbering> Display elements #.

From the menu Modification> Transformation> Merge nodes, 2nd radio button (Manual), then click Detect to see if some nodes are going to be merged. If there is none, you can increase the tolerance to 1e-4 or 1e-3 and click Detect again.

Re: Too small mesh elements

Posted by Annette Brask at Wednesday 15:03

Hi Christophe, 

 

Again, thank you for your quick reply! I have done what you instructed and these are useful applications to be aware of. 

The group I made consists of meshes that have an area smaller than 1e-5. The two upper pictures in the attached file show examples of the small elements as a part of the surrounding mesh and the lower picture shows a part of the group that was created. When I used the node merging application some elements were detected and removed. However, quite a few of the small elements were not detected when I used the node merging application, and click detect. I tried with tolerance 1e-6, 1e-5 and 1e-4. Are there some cases where it does not find them or may I be doing something wrong? 

 Link to picture: https://marinepowersystems-my.sharepoint.com/:i:/g/personal/annette_marinepowersystems_co_uk/EZJuSjxYGk1Dut61NtV8pGgBU_Tf-q6m9_lnVjcPHjUUlw?e=f3hT7J 

 

It may be easier to remake the geometry in Salome instead of importing it from Inventor. I will try that as well and see if I have any luck. 

Best regards, 

 

Annette

 

Attachments

Re: Too small mesh elements

Posted by Christophe Bourcier at Wednesday 19:37

These elements are not that small.

They can come from small edges on the shape (in GEOM, you can display vertices by right-click Display mode> Show vertices).

You can try different values of parameters for Netgen hypothesis to see if it helps.

Or instead of netgen, select gmsh with these options to get quads:

Re: Too small mesh elements

Posted by Annette Brask at Monday 16:01

Hi Christophe, 

 

I tried building the geometry in Salome instead, and now the meshing is working fine. There may have been something odd with the imported geometry. 

Thank you for all your help and suggestions! I definitely learned some new useful features. 

 

Best regards, 

 

 

Annette

Powered by Ploneboard
Document Actions