yes I've added report of mass flow rate to monitor the result. I'm still trying to find the optimal settings that are much stable and robust; however, the recommened values for factors seem not necessarily lead to good result. As the pic shows, residuals surged too quickly, even though my time step size was quite small. I used PISO scheme with PRESTO!, and under-relaxation factors of 0.5 for both pressure and momentum. The user guide says,
"When you use PISO neighbor correction, under-relaxation factors of 1.0 or near 1.0 are recommended for all equations."
hmm but I think 0.5 is aggressive enough. What should I do to overcome this... hope you can give any suggestion, thank you so much.