The auxiliary system's purpose is to allow explicit calculations using nonlinear variables.
These values can be used by kernels, BCs and material properties.
Just couple to them as if they were a nonlinear variable.
They will also come out in the output file... useful for viewing things you don't solve for (e.g. velocity).
Auxiliary variables currently come in two flavors:
Element (constant or higher order monomials)
Nodal (linear Lagrange)
When using element auxiliary variables:
You are computing average values per element (constant) or Cholesky solve.
You can couple to nonlinear variables and both element and nodal auxiliary variables.
When using nodal auxiliary variables:
You are computing values at nodes.
You can only couple to nonlinear variables and other nodal auxiliary variables.
Auxiliary variables have "old" states just like nonlinear variables.
Further AuxVariable documentation
- Phase Field App