fluid two flow into a cylinder filled with fluid one how to set the B.C. of the fluid two


I am trying to use openLB to simulate a case like this: fluid two flow into a cylinder which is filled with fluid one.What I know is the outlet pressure, so I decide to set both of fluid one and two’s BC are pressure B.C… I meet a problem. I have no idea about how to set the pressure B.C. of the fluid two.

Based on some messages in the forum, I learn that if I add the pressure B.C. in the outlet, I need to define the Rho in the outlet. But, in the beginning fluid one does not reach the outlet. how can I define the outlet rho of fluid two? Could you please give me some advice and hint?

Thank you very much!