[Gmsh] About *Surface sets in Gmsh - Abaqus

[Christophe Geuzaine]

> On 6 Nov 2019, at 10:26, MA Ariza-Gracia <miguel.ariza at artorg.unibe.ch> wrote:
> 
> Dear all,
> 
> apologies, I just realized I crossed the subjects of two different (related questions). Following up using the previous geometry, though.
> 
> The problem is that when you create a volume mesh from some STL surfaces and export it to an Abaqus INP, the structure of the INP file is as follows:
> 
> **************************************************************************************************
> 
> *Heading
> 
> *NODE
> 
> <bunch of coordinates and labels>
> 
> *ELEMENT, type=CPS3, ELSET=Surface1  -> This is bad (in my application, of course)
> 
> <bunch of labels and connectivity>
> 
> *ELEMENT, type=C3D4, ELSET=Volume1
> 
> <bunch of labels and connectivity>
> 
> **************************************************************************************************
> 
> The main problem here is that a 2D triangular mesh is generated on top of the 3D solid mesh which is not the goal. The goal is, for example, to have a *SURFACE definition in which to apply a pressure boundary condition / contact definition WITHOUT adding an additional 2D mesh on top. This *SURFACE definition should be done with the facets of the elements that belong to the volume but are on the surface (hopefully, it was clear).
> 

I don't know the Abaqus format well enough to understand this...

Two choices : 

1) either patch our INP exporter (other people on the list might know better and could help). You could open an issue on our gitlab, as for developer access and create a branch with your proposed changes

2) you can also directly use the API to access all the mesh info, and then write the INP file yourself directly.


> 
> 
> In Abaqus, the surface definition can be Element-based or Node-based. For the Element-based, the *SURFACE is defined as a group of element sets and the facet that should be used for defining the surface. For example,
> 
> 
> **************************************************************************************************
> 
> *ELSET, ELSET=test-surface_S6, generate  -> This would generate an element set of 16 elements whose facets are in the boundary of interest
>   1,  61,   4
> *SURFACE, type=ELEMENT, name=test-surface -> This would generate the surface using the previous element 6 and facet S6 of the hexahedra (there is a convention in Abaqus for this)
> test-surface_S6, S6
> 
> **************************************************************************************************
> 
> Yesterday, I delved a bit in the source code and I think there is not such an option (only NSETS and ELSETS) but, if we would be able to retrieve (somehow) those elements with facets on the surface of interest, we could actually build the surface with a bit of post-processing (tedious but doable).
> 
> 
> 
> So, finally, my question: is there any way of retrieving elements (*ELSETS) and nodes (*NSETS) of ONLY a volume mesh in Gmsh? Take as an example the enclosed INP files (geometries in line with my previous email):
> 
> 1) thin-lens-duplicate-geo.inp: this file was exported after meshing the volume and without a physical group -> "Wrong" file definition
> 
> 2) thin-lens-volume-geo.inp: this file was exported after meshing WITH a physical group for the volume. Here, there is no wrong additional 2D elements, but we do not have any information about elements or nodes in a target surface. -> Can we obtain this information somehow in Gmsh?
> 
> Note: the *.geo files and the *.STL correspond to the ones used for generating the INP files.
> 
> 
> 
> This topic also raised an additional question for me:
> 
> 1) Can we have internal boundaries if there is, for example, an inclusion inside a lens? E.g., bubble of air. This would help defining internal contacts or pressures or the like.
> 

Yes, sure - just define this either as CAD entities, or as discrete entities like the 2 exterior surfaces, and build the appropriate volume(s).

Christophe



> 
> 
> Best regards and apologies for the crossed subject once again,
> 
> MA
> 
> <surface-sets-inquiry.zip>

— 
Prof. Christophe Geuzaine
University of Liege, Electrical Engineering and Computer Science 
http://www.montefiore.ulg.ac.be/~geuzaine