For any rotating machinery model in COMSOL Multiphysics, you should finalize the geometry using Form Assembly and create identity pairs. To reduce the amount of identity pairs, both the stationary and rotating parts of the model are recommended to be consolidated into single domains. To enable continuous fluid flow across touching boundaries when defining the physics, you need to define a Flow Continuity boundary condition. For the rotating machinery example, we show how these settings are used to enable movement and flow within a micromixer.
For an in-depth look at the differences between Form Union and Form Assembly, please see the Form Union and Form Assembly Geometry Finalization Methods video.