how can i define it
 

a face not wall,fluide is water,this face is open to air,how can i define it ?

Re: how can i define it
 

You can definate it as a free surface using the "Symmetry" buondary consition. This will set all derivatives gradients equal equal zero normal to the boundary.

Of coure this does not allow for evaportation, surface deforemation, etc

-Dan