2d planar simulation setting
 

Hi, I am doing the VOF simulation in a T-shape pipe as shown in the pic. Now, I'd like to simplify the simulation as a 2d planar model along the pipe center line. But I am confused how to set the refernce value as the diameter for each pipe is different. Besides, how should I set the boundary condition for this 2d planar simulation? I used the velocity inlet and pressure outlet for 3d simulation at the pipe ends. Should I use the velocity inlet bc with the same value for 2d simulation or replace velocity inlet with massflow inlet? Thanks in advance.