Inconsistent normal directions in boundary conditions
Both the ExtrapolationOutflow and the FreeSlip boundary conditions require a normal direction.
Currently, the normal direction in the outflow condition has to point from the fluid to the wall (in the direction of flow) while the normal direction for the free slip condition has to point from the wall to the fluid.
It would be nice if the normals would have a consistent direction. Alternatively, the direction (wall to fluid / fluid to wall) could be specified explicitly together with the direction vector.