There is a lot to admire in this paper and I congratulate the authors in using experiment and CFD to contribute to the understanding of a complicated phenomenon. Once a flow becomes nonuniform, as it does for the flow described here, there a many more degrees of freedom and the flow can establish itself in many different ways. In consequence, flow that is stalled (in the sense that this takes for rotating stall in compressors) can exhibit many different arrangements. I would like to suggest that the rotating disturbance found here is a particular type of rotating stall corresponding to the special circumstances of an isolated rotor with large tip clearance.

The problem that I have with this paper and with others that have come from Germany over the last few years is the term “rotating instability.” One of the useful conclusions from the present paper, referred to in the...