As a thought experiment, I was thinking about how to actuate an passive impeller inside a tube from outside, contactless. Imagine a tube equipped with an impeller inside it. The impeller is mounted in a passive way such that it can freeley rotate around the tube's axis. That setup can be used to generate air or water flow for example but the impeller has to be actuated in order to do so.
What I was thinking about is to mount permanent magnets at the impeller's tips and surround the tube outside with a stator containing many electromagnets.
The interesting question here is how does such a setup behave knowing that additionally to an air gap (in the case we want air flow generation), we have now an additional layer of material separating the rotor and the stator (the tube's material).
Does the tube's material play a role? If yes, what are the properties of that material that have an effect (magnetic permeability? magnetic capacitance?, thickness?), I can imagine that it would behave differently when using PVC, Stainless Steel, Iron, Copper.
How does the performance compare to a normal brushless motor? Which affects the system's performance more: the increased distance between stator and rotor, or the additional material layer between them?
Do such actuation systems already exist? If yes how are they called?