outlet boundary condition of numerical wave tank
 

hello,every CFDer! I'm working at simulating a numerical wave tank with the commercial solver STAR-CCM+. There are somethings puzzled me, I hope you can give me some suggestions. My question is :
How should I define the outlet pressure condition of numerical wave tank?
I have tried two different BCs at the outlet.First, I prescribed the pressure at the outlet with hydrostatic pressure of undisturbed water surface(flat wave in STAR-CCM+), but I find the water level at the outlet will decrease gradually. Then, I tried another boundary condition, and I set zero gradient of pressure at the outlet with STAR-CCM+'s user defined field function (scalar->pressure->ignore boundary values) , but I find the water level at the outlet will increase gradually. Now I'm very puzzled. I hope somebody can help me to deal with this matter.

you may need run a longer time to reach the equilibrium state. If the outlet has higher hydraulic pressure as in you first case, the inflow is more than the outflow, the water surface rises such that inflow decreases and outflow increases. At one point, they reach equilibrium. In the other case, when you set zero gradient, the outflow is more than the inflow. Thus the surface decreasing so that inflow increases and the outflow decreases.