Reply To: Water is stuck in my water wheel’s blades

[Rob]

Have a look at the various Intro lectures for Fluent on Learning. An interface (what you have) has a wall associated with it for when the two sides don't line up, and otherwise works as an interior/internal boundary (cells on both sides & flow goes through it). 

Cell size needs to be roughly similar on both sides to help the solver maths. And with VOF aim for an aspect ratio as near 1.0 as you can get. You also need a fairly high mesh resolution to capture the free surface. 

Open channel is a VOF boundary option and is designed for channels with a preset height of water: they're covered in the User's and Theory Guides in Fluent's Help system.