Tangent of line-elements with quadratic shape functions

[Ludwig H]

Hello,

I am currently working on a problem describing surface elasticity in two dimensions. Thus, dim=2 and spacedim=3. The boundary of the triangulation, where surface elasticity is defined, is described in dim=1 in spacedim=3.

I extract the boundary triangulation with "extract_boundary_mesh".

To describe the surface-elasticity, I need Vectors at the quadrature-points of the boundary, that are tangential to the boundary. In total, due to quadratic shape functions, I have 3 quadrature points on each line-element.

Is here any function like fe_values.get_tangential_vectors()?

Alternatively, vectors normal to the boundary would also be helpful. However, when the boundary is dim=1  a unique normal vector is not defined!

Does anyone have an idea how I could resolve the issue?

Thanks a lot,

Ludwig