The example geometry shows an axial mixer, which is supposed to have an external wall fixed and the internal shaft rotating.
We need to create a rotating zone as a separate CAD element. There are 2 ways of doing that.
a) you create it "internally", such that it covers only the inner part. That might be difficult if the clearance between the in-out walls is small. See Mesh M1 and simulation S1 M1
b) you create a cylinder that "cuts" the whole rotating element. See geometry G3, mesh M3 G3 and simulations S2, S3 and S4
b.1) We assign the rotation to the whole rotation zone. We want the external wall to be stagnant
b.2) The first idea to get a stagnant wall is to impose a counter-rotation, See simulation S2
b.3) Second, we assign a rotating wall velocity BC, but we fix rotation to zero. See simulation S3
b.4) We can also assign same rotation to the external wall. See simulation S4
c) Since the rotating zone covers just the "middle" of the domain, there need to be a non-rotating part of the mesh.
To fulfill the "fixed outer wall", the case with zero rotation (S3) is the correct one.