[Tobi]

Dear Wang,

don't double post threads (see my PM). Take time to ask your question. If you don't take your time, no one else will do it. The pressure BC makes no sense - millions of possible pressures possible. However, you statement: dt = 6e-5 to satisfy Co<1. How do you think that is true? If you have any obstacle, the flow will accelerate somewhere etc (Bernoulli)... and then your Co criterion is not satisfied anymore. Error accumulation can be an outcome.

Good luck.