Rotating instability (RI) as a prestall wave is closely related to tip clearance noise, blade vibration and rotating stall/surge of axial compressors, so lots of researchers have focused on it. In this paper, the experimental and numerical investigations for a subsonic compressor are carried out to explore the flow mechanism leading to the step change in aerodynamic modes of rotating instabilities at different rotor speeds. The measurement results show that RIs only appear at operating points in a narrow mass flow range near the stability limit of the test rotor both at a medium and at a high rotor speeds. The dominant mode order frequency of RI decreases from increased rotor speed. The simulations show that circumferential traveling waves also appear near the stability limit of the test rotor at different rotor speeds. The dominant frequency/mode of the circumferential traveling waves decrease with the raise of rotor speed. Details of the simulated flow fields reveal that the periodical behavior of TLF will form the flow unsteadiness in one passage. There is a time shift of pressure oscillations in between rotor passages. RI originates when the flow unsteadiness propagates circumferentially no matter at MRS or HRS. In the rotating frame, the time shift of pressure oscillations reduces and the period of the static pressure oscillation increases from the increased rotor speed. The variation of the time shift of pressure oscillations and the period of the static pressure oscillation lead to the step change in the dominant frequency/mode of RI.

It is well known that tip flow unsteadiness has profound effects on both performance and stability of axial compressors. A number of numerical simulations have been performed in transonic compressors to uncover the nature of tip flow unsteadiness. From this research, tip flow unsteadiness can be attributed to many factors, such as the movement of the primary and secondary leakage flow, the interaction between shock and vortex, and the tip leakage vortex breakdown. However, no final conclusion has yet been reached on this matter. The current investigation is carried out to explore the origin of tip flow unsteadiness from the perspective of the evolution and development of tip leakage vortex breakdown. In this paper, unsteady RANS simulations have been performed to investigate the fluid dynamic processes in a tip-critical transonic compressor, NASA Rotor 35. A vortex core visualization method based on an eigenvector method is introduced as an important tool to identify the vortex arising from tip leakage flow. As the flow rate varies, three critical operating points with distinctive features of flow unsteadiness are observed. At the first critical operating point, bubble-type breakdown occurs, and gives rise to a weak unsteadiness with high frequency in the rotor passage due to the oscillation of the recirculation region induced by the tip leakage vortex breakdown. At the second critical operating point, the vortex breakdown has transformed from bubble-type to spiral-type, which leads to the frequency of the pressure oscillation reduced almost by half and the amplitude increased significantly. At the third critical operating point, a new vortex that is perpendicular to the pressure surface comes into being in the tip region, which leads to a prominent pressure oscillation of the tip flow and another jump in amplitude. As a result, the evolution and development of tip leakage vortex breakdown are closely related to the tip flow unsteadiness of the investigated rotor.

Rotating instability (RI) is an obvious unsteady flow phenomenon occurring in the tip region of compressors, which is potentially linked to tip clearance flow noise, blade vibration and rotating stall/surge. The existing investigations for RI indicate the origins of RI are closely related to unsteady flow behaviors in given blade passages in the rotating reference frame, which depend on the design specifics of axial machines. However, no efforts are made to set up a quantitative link between the time scale of unsteady behavior in a given passage and the characteristic parameters of RI, let alone to define the fluid dynamic processes/events which are causally linked with the RI inception. This is the motivation for the current investigations. For the case shown in Part I, a quantitative link between the time scale of tip flow unsteadiness and the characteristic parameters of RI has been set up. The consistency between experimental and numerical results also demonstrates that multi-passage computations with the circumferential extent larger than the length scale of RI could shed light on the flow mechanisms relevant to the emergence of RI. In this part, systematic multi-passage simulations were thus laid out for a tip-critical transonic rotor, NASA Rotor 35, to identify the fluid dynamic processes for the inception of RI in presence of shock wave. It is found that the flow field becomes unsteady when the mass flow rate is below a critical value close to the stability limit. The flow unsteadiness in specific passages originates from the rotor tip region, but it is not synchronous with the occurrence of the circumferential travelling wave similar to RI as is the case in Part I. The origin of flow unsteadiness attributes to the periodic oscillation of a new vortex structure is termed as tip secondary vortex (TSV). The TSV is essentially a vortex segment arising from the spiral-type breakdown of TLV. The underlying flow mechanism for the inception of a rotating wave similar to RI is that the periodic oscillation of TSV is capable of inducing the flow blockage transfer against the rotor rotation direction. In conjunction with the investigation results reported in Part 1, a local inception criterion for RI is thus identified, which could be depicted as the initiation of the flow blockage transfer induced by the unsteady flow behavior in blade passages near tip.

Rotating instability (RI) is an obvious unsteady flow phenomenon occurring in the tip region of compressors, which is potentially linked to tip clearance flow noise, blade vibration and rotating stall/surge. The existing investigations for RI indicate the origins of RI are closely related to unsteady flow behaviors in given blade passages in the rotating reference frame, which depend on the design specifics of axial machines. However, no efforts are made to set up a quantitative link between the time scale of unsteady behavior in a given passage and the characteristic parameters of RI, let alone to define the fluid dynamic processes/events which are causally linked with the RI inception. This is the motivation for the current investigations. In Part I, the experimental and numerical investigations are carried out to investigate tip flow unsteadiness in a subsonic axial compressor rotor. The measurement results show RI appears at operating points near the stability limit of the test rotor. It becomes more pronounced with mass flow rate decreased. The corresponding computational experiments show that flow unsteadiness in given passages also appears close to the stability limit with its initial origination confined to the tip region. The appearance of tip flow unsteadiness is accompanied by a phase lag pattern in different passages across the circumference similar to the detection of stall flutter. The well-developed Fourier-decomposed method is thus used to evaluate the mode characteristics of circumferential traveling waves. It turns out the circumferential traveling wave rotating against the rotor rotation direction with the mode order of 4 is prominent in the flow field with its frequency in the absolute frame equivalent to the mean frequency value of RI detected in measurements. The further analyses of the simulated flow fields indicate that tip flow unsteadiness in a given passage attributes to the periodic oscillation of “secondary clearance flow”, which induces a blockage tranfer across the passage. The mode order and propagation speed of RI depend on the blockage transfer induced by the periodic oscillation of “secondary clearance flow” between two neighbouring passages along the whole circumference. The investigation results presented in the paper implies that one of early ideas to interpret the origin of RI might be altered to such an extent that it contains any unsteady behavior associated with tip leakage flow, rather than limited to “periodical oscillation of tip leakage vortex”.