Difference between meshing in Mechanical and PyMAPDL

[bedermaus]

So the question is why when im meshing same geometry in Mechanical and PyMAPDL, getting not same results?
For meshing in PyMAPDL i used mapdl.vsweep('ALL'). How to produce same meshing as in Mechanical?

[mikerife]

Hi @bedermaus
Please see the following Help entry on a description of Mechanical vs MAPDL meshing:
https://ansyshelp.ansys.com/account/secured?returnurl=/Views/Secured/corp/v221/en/wb_msh/ds_meshing_white_paper.html

Could you please many more details on the issue. What result are you comparing? How are you post-processing in PyMAPDL? Is the result mesh size/shape/etc independent?

With respect to producing the same mesh: this will depend on the details of the mesh tools used. Some tools in Mechanical just don't exist in MAPDL. Some of the meshing options in Mechanical can be recreated in MAPDL if we attack it from another angle.

Mike

[bedermaus]

hi @mikerife
To produce a problem just import any .iges, .prt, etc. geometry to mechanical and to mapdl, and mesh it with default settings, the results will be not same at all, also solution. Is it possible to set same options for mesh like in mechanical and how? When i generate input .dat file from mechanical, tthere is no mesh options, mechanical just build nodes and elements. Of course if i will input this .dat file to mapdl, i will get same results, but im not looking to generate .dat file everytime when i switсh geometry. About pre-processing in mapdl, im using all same model properties as in .dat file from mechanical

[mikerife]

I finished writing this and just realized that I may have made a giant, bad assumption. I took "results" to mean FEA results like displacements, strains, etc. Now I get the feeling that "results" really meant only the actual mesh.

Hi @bedermaus Are you an Ansys customer? If so I suggest opening and Service Request and have an live discussion with technical support. This is not really a PyMAPDL issue. And really there are so many meshing options/features/functions/methods/etc that answering this type of general question via forum is near impossible.

If you can narrow this question down then we may be able to have a productive forum discussion. I don't think that anyone wants to sit down and write 6 pages on all everything there is to know about say shell meshing in Mechanical vs MAPDL only to find out that you only work with solids.

That being said...I don't think it's necessary to generate the same mesh. For any geometry there should be many meshes possible that capture the response and derived result fields accurately enough to meet your needs. So let me ask you - what is your needed level of accuracy? And how different are the results? If the results differ between the Mechanical and MAPDL meshes by say < 1% then I have to think "what's the issue?". But if the results are way different, for the sake of argument let's say they differ by 25%, then I would ask whether either mesh is accurate enough on its own to meet your needs. And please forgive this (it really sounds snarky when I say this to myself) but comparing inaccurate results is (probably) a waste of time. Especially on highly nonlinear and/or ill conditioned models.

And lastly if you are just sweep meshing solids then try:

1 In Mechanical turn off Defeaturing
2 In Mechanical change the Mesh Quality -> Error Limits to Aggressive Mechanical
3 Use edge size controls to constrain the mesh pattern
4 In MAPDL use size controls too

And 5 - give us all more to work with. Show some images of the meshes and where they are different.

Mike

[bedermaus]

Hi @mikerife
Im working not on public project, so unfortunately, i can't send any images. Also i don't have my personal Ansys "account".
I realized what happens, and its not about meshing... The problem that when i import geometry from CAD to Mechanical, it takes Length Unit from the CAD, like if i built model in 'mm'. in Mechanical it will be in 'mm'. And as i understand in MAPDL it imports in meters, and i cant control it with mapdl.units() command. So as i undestand i should control it by myself and just convert all the units by myself.
Fixed everything with converting meters to mm
mapdl.run('VLSCALE,ALL,,,.001,.001,.001,,0,1').
Thanks a lot for everything.

[mikerife]

Hi @bedermaus Ah ok, that makes sense. MAPDL is a unitless program. It expects the input to be in a consistent set of units but does no checking. That /UNITS command is a "book keeping" command. It is used to add units to the MAPDL database as a record of the users intent. But it does not change any input to meet the unit system given by the command.

Some of the CAD engines use a specific unit system internally. From what I know that is always metric (meters). So even if using some other units the CAD file output will be in meters, and when imported to MAPDL it will be in meters.

WB Mechanical is altogether different in its Unit setting actually scales all the inputs to match.

Mike