Yes, I was running into that message " A multi-region mesh was assigned - this analysis-type requires a single-region mesh". After trawling other posts last night for an answer, I came back to ask now and you’ve already provided one answer. Thanks @Retsam.
After adding a Cell Zone to my “Compressible 2 - two parts” sim, I have a new error message “One or more cell zones are adjacent to a flow region cavity. Cell zones faces must be inside the flow region.”
I read here that " The rotating zone volume should be 100% solid and should intersect with the flow volume (think of the rotating zone volume as a plain, 100% solid part)."
but perhaps my “Rotating Volume” is a surface not a solid?
Was it misleading of me to name it a Rotating “Volume” ? (I’m not familiar with naming conventions).
I’ve found " B) CONVERT THE SURFACE INTO A SOLID" in the knowledge base article “How to Create a Rotating Zone”, so I’ll try that next.