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Description

An Outflow boundary condition is a Dirichlet boundary condition that only allows a net flux in direction of the boundary normal vector, i.e., to the outside. This is particularly handy for simulation evaporative potentials. Depending on the actual potential at the boundary, setting a Dirichlet condition can lead to a flux into or out of the domain. An outflow boundary condition would prevent the inward flux and simply apply a (Neumann) no-flow condition in this case.

This boundary condition has already been implemented in !134 and has proven well in previous simulations.

Proposal

Add boundary condition type Outflow to Richards model. The boundary condition stores a matric head value which is only applied if the normal flux on the boundary is positive (i.e., outwards).

• Add OutflowBoundaryCondition to the permitted BC types of the Richards model.
• Check for Outflow boundary condition in Richards local operators. If the normal flux is negative, prohibit contribution to the local residual.
• Update documentation.

How to test the implementation?

Add a system test where an Outflow BC is applied and a comparable Dirichlet BC would lead to a flux. Then compare the result with the result of an ODE solver which applies the no-flow BC.

Related issues

See #