Boundary Patch
 

I am currently modelling a flow channel, where water is circulating. That channel is open at the top. Since the formation of waves etc. is not of interest in this case (mainly because of realatively low speed), a VOF solver is not necessary in my opinion and e.g. IcoFoam will do. So I thought of a patch as a border between water and air. That patch should be noGradient and fixedValue=zero for normal directions, but should make no limitation for tangential directions (since it is no wall, so the water should not be slowed by friction on the top). I was thinking about using the slip boundary patch but I am not sure if that is the right choice. Any suggestions?