## Outflow conditions

When setting outflow conditions in the freeflow model, for example for the energy balance equation, this may result in an unexpected solution behavior. The problem is the fact that the outflow conditions only properly work if `\mathbf{v}\cdot \mathbf{n} \geq 0`

, because then the upwind quantity is calculated using the cell values. However, if `\mathbf{v}\cdot \mathbf{n} < 0`

, then the upwind quantity is calculated using boundary values. Currently, for outflow conditions these are equal to the cell values, which means that the upwinding is not done correctly. This could result in oscillations or (as in my test cases) to a very poor Newton convergence.

One solution would be to use Dirichlet values for the case that `\mathbf{v}\cdot \mathbf{n} < 0`

. However, requiring Dirichlet values at outflow conditions might be confusing....