Interfaces? Help needed Urgently!
 

I basically already posted this question, but no one wants to reply to it.

Lets say i have a wheel. The wheel rotates inside a wheel housing which envelopes the top 2/3rds of the wheel, leaving the bottom 1/3rd exposed to the airflow.

The wheel is rotating, the housing is not, but both are moving through the air at x m/s.

Now i want to model the effects of a rotating wheel on the airflow BUT NOT the flow inside the wheel housing, so only the bottom 1/3rd.

I dont really understand the concept of interfaces, frankly it seems there are too many types and the whole system is massively overcomplicated.

How do i go about creating an interface between the bottom opening of the wheel housing which separates the fluid region from a region containing no fluid, but at the same time allows the wheel to rotate inside the housing?

why would you want to work with interfaces or even the area inside where there is no fluid domain.
Just use 1/3 of the wheel + a box of air around it and apply a custom function to this 1/3 wheel surface.