Self priming venturi scrubber
 

Hello everyone!
My problem is related to three phase flow in which air and dust are entering from the inlet of venturi while water has to enter in the domain through orifices around the throat section through due to difference between inside throat (negative pressure) and outside throat (in tank, hydrostatic pressure).
I am facing a difficulty in applying boundary condition for that....any suggestions?

That depends entirely what is controlling the flow. If it is a specified flow rate then use a flow rate boundary. If it is a pressure then use a pressure boundary. If you don't know what it is then you can't impose a boundary condition there and you need to move the boundary to somewhere you do know the conditions.