## 1 Introduction  
The operating range of an axial compressor is strictly limited by instabilities such as rotating stall. A huge effort has been made to gain detailed information about the flow topology of rotating stall cells, the stall inception process and a wide range of prestall phenomena representing various routes to stalled flows. The most common stall inception pattern, the spike stall inception, is linked to short length-scale disturbances of large amplitudes. Many proposed mechanisms of spike formation are associated with blockage growth, critical incidence, forward spillage and radial vorticity. Common to all investigated mechanisms for stall onset is the extremely transient character of the inception process. Small disturbances pass over into fully developed stall cells within a few rotor revolutions. This property has been a great challenge for all researchers who concentrate on stall warning. Regarding this topic, useful prestall activity was mainly taken into account when it could be identified at least hundreds of revolutions before stall. One of the earliest studies of stall warning signatures was made by [PERSON] et al. (1990) [1] by identifying prestall modal disturbances. Different correlations and stall warning devices emerged during the following 20 years [2; 3; 4]. In this context, [PERSON] et al. (2011) [5] paid attention to discrete disturbances which appear well before the onset of stall. The footprint of \"discrete prestall propagating disturbances\" was identified by areas of low pressure on the casing wall. Also referred to as \"blue holes\", the low pressure spots have been ascribed to one of the fundamental reasons for the rise in blade passing irregularity. In addition, [PERSON] et al. (2011) [5] focused on the level of irregularity being affected by the tip clearance height and eccentricity. The results demonstrate the future relevance of this particular type of prestall pattern, especially in the light of smaller engine cores and larger relative tip gaps [6]. Moreover, the identified prestall disturbances were linked to a broad peak in frequency spectra. This characteristic hump arising at approximately 40% of the blade passing frequency is an identification feature in one of the most controversially discussed areas of compressor research known as \"Rotating Instability\" (RI). Even the term \"Rotating Instability\" which was first introduced by [PERSON] and [PERSON] in 1997 [7] was considered inappropriate by parts of the community. However, [PERSON] and [PERSON] (1985) [8] observed \"the simultaneous existence of disturbances of different wavelength\" 12 years earlier. [PERSON] et al. (1991) [2] noted \"propagating coherent disturbances\" which neither fit to the modal nor the spike stall model and which appear in a broad operating range before the compressor drops into stall. All findings suggest that the knowledge about this special type of prestall phenomenon is older than the term \"Rotating Instability\". Over the years, many features of RI have transpired. The general findings in this subject can be summarised as follows:  
1. Rotating Instability occurs when the compressor is operating at off-design conditions. If the mass flow is reduced at a constant rotational speed, the nominally uniform still unstalled flow becomes unstable and transitions into a \"prestall condition\" which is significantly non-uniform. Meanwhile, the compressor is nominally unstalled. 2. The RI phenomenon can be located in the vicinity of the rotor tips, where the occurrence of RI is more likely with larger tip gaps [9]. 3. The inception of RI can be identified by a broad chain of peaks in the frequency spectrum taken by time-resolved pressure measurements near the rotor blade tips. All statements are indisputable in the class where RI is discussed. Early explanations on related physics focused on the unsteadiness due to the tip clearance vortex breakdown [10; 11; 12]. Later, it was found that these models do not provide a sufficient explanation on the formation mechanism of RI because [PERSON] et al.