zero pressure BC under tidal flow
 

Dear cfd users,

I have another problem with figuring out validation of simulation results. My situation is that flow comes in from the left side to the right side and after four seconds, fluid flows in reverse direction. I set up with inlet uniform velocity in time as an inlet BC, and with outlet zero pressure as an outlet BC. Zero pressure condtion remains for four seconds, but after the time, zero condition doesn't. My thought is that simulated solutions look fine and convered. The only thing I don't understand is about the outlet boudnary condition. Please let me know your precious advice if you experienced this before.

Best regards, Jeongho