[er_ijaz]

I remember reading some notes, zerogradient boundary condition should not be applied at the outlet for pressure.
Outflow condition refers to zerogradient for all properties except pressure.
I'm analysing flow through a pipe in which half of the pipe is water while upper half of the pipe is air.
At the outlet of the pipe I'm concerned about fixing the pressure as the hydrostatic pressure of the water varies with height and since the density changes with two phase, I'm worried of fixing the pressure at the outlet.
Also in openfoam we need to fix p-rgh value and interfoam computes the pressure based on the height.
I noticed in two tutorials of openfoam v4.0 , p_rgh is set as zerogradient in the outlet. I'm also convenient to use zerogradient at the outlet, but my concern is the 'rule' pressure should not be 'zerogradient' at the outlet.