## 6 Prestall Disturbances
### Prestall Propagating Disturbances; Numerical Results
Considering the dynamic process of the mode number, it is obvious that there is a link between the vortices and the spectral signature in the frequency domain. Grey-scaled static pressures on all surfaces reveal that each vortex filament induces a low pressure spot on the casing wall. This finding is in good agreement with the experimental results presented in Figure 4. The pressure signal also contained spikes of low static pressure. A detailed picture of the vortex topology is shown in Figure 10. The streakline pattern on the casing contains several features which can be ascribed to the subject of st  
Figure 9: Mode analysis and visualisation of flow disturbances (CFD). Also known as \"forward spilling\", the clearance flow is being redirected and is spilling around the leading edge of the neighbouring blade. This particular flow pattern arises when the clearance vortex trajectory is aligned in parallel with the rotor leading edge plane. As each passage is affected by spilling forward, the result is a continuous separation line upstream of the rotor. Furthermore, the discrete vortex filament spans between the casing and the suction side of the rotor blade, thus shielding the clearance flow from the main flow. Rotation of the vortex tube is indicated by white and black arrows. In general, the coherent flow topology strongly resembles a classical spike formation known from spike stall inception mechanisms [20]. A similar flow topology was also addressed by [PERSON] et al. (2001) [21] who described the occurrence of short length-scale stall cells when the compressor is operating in a mild stall condition. Intense areas of low pressure on the casing gave rise to the assumption that a \"separation vortex\" with one end attached to the casing and the other attached to the suction surface appears as a \"mini-cell\". However, in contrast to the findings of [PERSON] et al (2015) [20], the localised disturbance does not evolve into a stall cell after a few rotor revolutions. In the presented case, discrete vortex structures remain unchanged when the through-flow is constant--they represent indeed a kind of flow non-uniformity but they are not responsible for flow separation or part span stall. The vortex dynamics is depicted in Figure 11. Again, the vortices are visualised by the \(\lambda_{2}\)-isosurfaces. The motion of the vortex tubes is characterised as follows. While the end of the vortex tube at the casing moves circumferentially, the end on the suction-surface convects downstream. During this process, each vortex trajectory is periodically divided by the leading edges (vortex V3; circled in red). The grey-scaled static pressures show that the aerodynamic loading increases at the blade tip when the casing end of the vortex has just passed the leading edge. This finding confirms the potential impact of RI on aeroelasticity. Additionally, vortices are affected by a decay mechanism or they pass into one another. The vortex merging is exemplarily  
Figure 10: Flow mechanism of Rotating Instability (CFD). demonstrated by the vortices V1 and V2. The most important finding is that shedding of vorticity being known from spike formation in its embryonic stage is _not_ involved in the general flow topology of prestall vortices. The authors believe that this illustration allows the formation and the interaction mechanism of prestall disturbances to be shown for the first time.