Partitioning of 3D objects

I’m pretty new to using netgen and Python in general and so it wasn’t long, until I encountered my first problem.

I’m trying to simulate the flow of a fluid through a gap that gets narrower. I want to adapt the boundary conditions on the bottem, so that in one area the velocity of the fluid is 0 and in another area has some different value than 0, a slack so to speak.

Solving this would be fairly easy in 2D because the itutorial on this is very good, nevertheless partitioning the fluid in 3D into two parts isn’t. Maybe I’m overlooking something but I am trying for a few days now and I can’t seem to find a solution.

I hope someone can help me.
Thank you very much for your time.