[vtk-developers] how to handle FEM data in VTK?

[Pierre JUILLARD]

Dear M. Gebbert,

I thank you very much for all these details.
I tried to organize all the information you gave me in the enclosed ppt
(also, I printed it in the enclosed pdf for an easier reading).

Could you please confirm me that this is "more or less" a good
representation of your own VTK structure? It raised some uncertainties from
my side, and if it doesn't bother you, I would like to ask further questions
on the following sujects.

=============================================
POINT DATA at integration point for shell and volume elements
=============================================

How should the difference concerning integration points between shell and
volume elements be dealt with?

In your e-mail, I don't see any distinction between shell and volume
elements: you seem to deal with them equally using the same data
structure.However, I made a difference in the pdf because some of the shell
integration point (the in-plane integration points) have at least one more
point data than the others (over-thickness integration points): the
thickness.
I guess there thus should be a distinction, shouldn't it?

As for volume elements, all integration points are considered equally.

=============================================
LOCAL COORDINATE SYSTEM for tensor results
=============================================

I really am a VTK beginner as I think you have guessed.
The thing is I don't know in which coordinate system are expressed vectors
and tensors, and if this coordinate system may be modified.
Is it indeed possible to use local coordinate system?
Several local coordinate definitions (attached to a single finite element)
are possible, based on the element connectivity.
The following one is most notably widely spread in explicit simulation for
quadrangles:

Considering a quadrangle made up with the nodes 1, 2, 3 and 4.
calling A the middle of segment [1,4]
calling B the middle of segment [1,2]
calling C the middle of segment [2,3]
calling D the middle of segment [3,4]
=> the X local axis may be given by: [A,C]
=> the Z local axis may be given by: AC x BD (vector product)
=> and finally the Y local axis may be given by: Z x X (vector product)

It is interesting as it allows to move a mesh (translation / rotation)
without having to update the vector and tensor data attached to the elements
(cells).

So, if the definition of the local coordinate system (in which are expressed
vector and tensor result for one cell) can be modified, my question is:
How should this definition be stored? How to make it dependent from the
element connectivity? Woudl you have some advices?

=============================================
CELL and POINT DATA in the first ugrid
=============================================

You were indicating me that you store some cell and point data in the first
ugrid.I think it is intersting to store nodal thickness (optionnal), element
ID, and material ID at this level. Had you other data in mind?

=============================================
correspondences between ugrids
=============================================

You were pointing me to use integration method IDs to have a link between
the 2nd and 3rd ugrid.
I also added the element IDs to have a link between the 1st and 2nd ugrid:
does this seem relevant to you?

=============================================
related works
=============================================

Finally, do you know if the LSDynaReader class produces the same kind of VTK
structure when reading LSDyna results? I tried to read the description page
of the vtkLSDynaReader class, and I am not really sure to understand what is
really done...

=============================================

Thank you again for all the advices you already gave me.

I know that you are only describing your method, and that several others
could most probably be found to handle FEM data in VTK. So You may be
surprised that I have so many questions.
The thing is, it really appears to me as a complex subject, and I would like
to have a very good understanding of the already "validated" methods such as
yours.

I thank you again for your help.
Best regards,

Pierre

PS: I opened the file you sent in your previous e-mail in ParaView: if I'm
correct, there is no mesh, but only integration points? Am I right?
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