Boundary condition and 2d simulation for venturi
 

Hi all,
I have a question about the boundary conditions used for venturi tubes with an injection inlet in the neck. In fact there is a main inlet where fluid 1 (say water) flows with a known velocity. After passing the converging section, there is an opening in the throat that can entrain fluid 2 (say air). Then two fluids mix and exit through divergent section.
I have tried pressure inlet=0 for the second inlet, though there is some continuity problems in the solution when impending a steady state convergence.
I also appreciate it if you could tell me whether this problem should be modelled in 2d or 3d. I believe it can be modelled in 2d but since the axisymetric is not clear I doubted a bit.
I have attached a schematic of the design

Thanks in advance