boundary conditions

OB

I have a solid zone defined as x min=0mm, x max =100mm, y min= -20mm, y max =0, And another fluid zone (air) on the solid zone (on top) defined as x min=0mm, x max =100mm, y min=0mm, y max =2.5mm, In the y=2.5mm .I want to put a fluid as boundary conditions (mass flow for example) this jet flow was displaced in the x direction (velocity v= 4mm/s), in this case we use an UDF, the radius of this jet is for example r=3mm .This fluid injected was put on the solid zone after the passage in the fluid zone (air). It is possible to do this approximation, and which model can we use (VOF….) Thank you very much.