fluid-structure interaction and fluid-fluid interaction

Dear all,

I am trying to solve a problem and I would be very grateful if you would help me. I am very new in comsol, and I also don't have a background in the matter of fluidics.
My structure could be described as something similar to a micro-funnel.
Let's say I have vertical tube where I introduce water at the top. Water falls because of gravity and at certain height of the tube there exist an obstacle with a very small aperture in the middle through which water can flow. You can find the geometry in the attachment.
I want to simulate the interaction of water on the surface of the obstacle, and calculate how fast the water flows through the hole.
I have used fluid-structure interaction physics, but what about initial boundary conditions? should I define an interaction between water and air to calculate how the flows moves?
Could someone help me please? I have just follow the example in the gallery of fluid-structure interaction, but I think I should make some changes to it, and I am not sure how.
Thanks a lot in advance