Possibility of periodic boundary condition for cylindrical polar coordinates

[tbrandvik]

I am trying to implement a periodic boundary condition for the 3D elastic equation. The only difference compared to the existing example in the demos is that the boundaries are periodic in cylindrical polar coordinates (e.g. the slave nodes are at the same coordinates as the master nodes, but rotated through an angle). This leads to the following kind of relationship between the displacements of the periodic nodes, assuming the second node is the first node rotated about the x-axis:

$$ \begin{align} dy_2 &= a\cdot dy_1 + b\cdot dz_1 \ dz_2 &= c\cdot dy_1 + d\cdot dz_1 \end{align} $$

The coefficients (a, b, c, d) would be fixed and set by the rotation matrix.

I was wondering if anyone has any ideas on how to implement this with dolfinx_mpc? Any help would be greatly appreciated!