Hydrodynamic simulations of sloshing phenomena often involve the application
of slip boundary condition at the wetted surfaces. If these surfaces are
curved, the ambiguous nature of the normal vector in the discretized problem
can interfere with the application of such boundary condition. Even the use of
consistent normal vectors, preferred from the point of view of conservation, 
does not assure good approximation of the continuum slip condition in the
discrete problem, and non-physical recirculating flow fields may be observed.
As a remedy, we consider the Navier slip condition, and more successfully,
the so-called BC-free boundary condition.