outlet 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.