A correction to my submitted issue: I wrote that the `value` in the `write` funtion is needed "for restart and visualisation in paraview". In fact, restart will probably work correctly without the `value` written out because `evaluate()` is called by the `dict`-reading constructor and, indeed, this reconstructs the face values on the patch correctly using the `fixedValue` and slip condition. The point about paraview not showing the correct velocity on a patch when using the BC still holds. I have confirmed that, and that the addition of the `value` in `write` solves this (I used paraview, not paraFoam). So one could argue that this is a paraview OpenFOAM reader problem, rather than a problem with the BC implementation itself.