[PL/BC] Reimplement Robin-type BC

The former implementation did not produce expected results for 2D and 3D case. Rewriting and implementing the Jacobian for the Newton scheme. Keeping K matrix for the Picard scheme as applying the BC to the r.h.s. only significantly increases number of non-linear iterations because the problem becomes non-linear.

A method of manufactured solutions is used to verify implementation of the Robin-type boundary condition. Grid convergence was assessed and the observed order of convergence is 1.99, which, as expected, is close enough to 2.

In the same setup a Neumann and Dirichlet-type boundary conditions can (and were) tested with same convergence order as the Robin-type BC.

The test is testing both non-linear solver schemes, Newton and Picard. With the new implementation of the Robin-type BC the convergence rates of the non-linear solver are satisfying, usually 2 iterations per time step for both schemes.

Four tests testing the axisymmetry were updated, three of them verified against a given manufactured solution, and the TES one is relying on the numerical results, i.e. the 3D solution was mapped to 2D axisymmetric case and is used as reference.

  1. Feature description was added to the changelog
  2. Tests covering your feature were added?

Review commit-wise. Many details are provided in the commit messages.

Edited by Dmitry Yu. Naumov

Merge request reports