The authors thank Prof. Cumpsty for his critical comments. After the discussion in Munich (ASME Turbo Expo 2000), it is disappointing that a major part of this comment still focuses on the chosen term “rotating instability” (RI). It is understood that a common term for a certain effect is needed in order to ensure efficient communication. This is the reason the authors used the questioned term, as it was already in use (“coined”) by several investigators to name the observed characteristic flow unsteadiness.
The authors agree that there might be better names for this effect. They do not agree that the term “rotating instability” is totally wrong. They are still convinced that the physics of the observed flow unsteadiness is different from the typical part-span rotating stall caused by overloaded blades.
Figure 1 shows the classification of different flow effects that can be observed close to the stability limit of...
