Handling second order element

[Clemoreau]

Hello all,

I was wondering if meshio could handle second order elements (especially with .fem format). Because I read a mesh containing second order elements and the result is not what I expected : something seems not be working in the node's connectivity.

I don't know if it's a bug or if it's something which was not foreseen in the development. I could eventually contribute to the repository if necessary.

Thank you

[nschloe]

Should work, it's a bug if it doesn't. It looks like something with the node ordering is off...

[Clemoreau]

Ok, it's a bug then, do you want me to continue the discussion here or to open an issue ?

When I import one CHEXA element (second order -> 20 nodes), I obtain that :

I think you're right, the problem is in the node ordering

[nschloe]

The best thing would bit a PR straight away. 😎

[Clemoreau]

Ahah yes I guess so, I'll look at it,

Can you confirm that I'm looking at the right place in _nastran.py / function read_buffer() ? (it is for .fem file)

[nschloe]

meshio uses VTK ordering for everything, see here:

[Clemoreau]

Thank you for the reference ! The problem was effectively in the node ordering for hexahedron20 and wedge15. Now it's fixed.

I would like to add a parameter 'is_quadratic' to the write/read function to write/read a linear mesh from a quadratic mesh if False. Do you agree if I add that to my PR ? (default value would be True). Do you agree ?
The matter is that I often have to deal with quadratic mesh while I don't need such a level of detail and it's easier to work with linear mesh. It's a kind of mesh simplification.

Thank you !

[nschloe]

I would like to add a parameter 'is_quadratic'

That'd just complicate things. What we could do instead is add a command-line too, something like

meshio force-order 2 infile outfile

which forces all cells in infile to have a certain order. This could be used to reduce all cells to order 1, or augment all cells.

[Clemoreau]

Ok, I think that it's a bit too complicated for me, but I will consider it if I realise that it is useful in my work. It was just an idea but I'm not sure of how doing it.

Still, I corrected the ordering of the nodes for hexahedron20 and wedge15 on my fork and I would like to add these modifications to the main branch of meshio if you agree.

I create a PR for this purpose.

Thank you for your kind help :)

[nschloe]

Ok, I think that it's a bit too complicated for me

Shouldn't be too hard I think. Anyway, I appreciate the effort!