Adapt demo for a tube: boundary conditions


I’m relatively new to fenics and I have some questions. I want to simulate the eigenmodes of a tube (i.e. like this).

I thought that I can adapt the demo file “”. My tube mesh is not a shell mesh, but a structural mesh, and I think the equations should therefore be the same (is this right?). Now I am searching for the right boundary conditions.
I want to look at a free vibration, but when I set the b.c. to “None”, I get really strange torsional modes (look at the picture below).

So all in all the differences between my program and the demp program are the bc = None statement and the different mesh.

I am a bit confused, because I don’t get modes like modes 5 and 6 in the link above. It looks like I have to clamp the tube on the sides, but then I wouldn’t get bending modes where the ends are also deflected :thinking:

Maybe someone could give me a hint or tell me if I have some mistake in the thinking process. That would be nice :slight_smile:

greetings :wave: