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An experimental study is described to investigate the negative effects of the tip clearance gap on the aerodynamic and acoustic performance of axial turbomachines. In addition to the increased broadband levels reported in the literature when the tip clearance is enlarged, significant level increases were observed within narrow frequency bands below the blade passing frequency. Measurements of the pressure and velocity fluctuations in the vicinity of the blade tips reveal that the tip clearance noise is associated with a rotating flow instability at the blade tip, which in turn is only present under reversed flow conditions in the tip clearance gap. A turbulence generator inserted into the tip clearance gap is found to be effective in eliminating the tip clearance noise and in improving the aerodynamic performance.

This paper reports on an experimental and numerical investigation aimed at understanding the mechanisms of rotating instabilities in a low speed axial flow compressor. The phenomena of rotating instabilities in the current compressor were first identified with an experimental study. Then, an unsteady numerical method was applied to confirm the phenomena and to interrogate the physical mechanisms behind them. The experimental study was conducted with high-resolution pressure measurements at different clearances, employing a double phase-averaging technique. The numerical investigation was performed with an unsteady 3-D Navier-Stokes method that solves for the entire blade row. The current study reveals that a vortex structure forms near the leading edge plane. This vortex is the result of interactions among the classical tip-clearance flow, axially reversed endwall flow, and the incoming flow. The vortex travels from the suction side to the pressure side of the passage at roughly half of the rotor speed. The formation and movement of this vortex seem to be the main causes of unsteadiness when rotating instability develops. Due to the nature of this vortex, the classical tip clearance flow does not spill over into the following blade passage. This behavior of the tip clearance flow is why the compressor operates in a stable mode even with the rotating instability, unlike traditional rotating stall phenomena.

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An experimental investigation is described to study the azimuthal structure of the rotating blade flow instability which was recently observed in axial turbomachines. An azimuthal mode analysis is applied to the unsteady pressures at the casing wall immediately upstream of the inlet plane of the rotor. The rotating instability is characterized as an unstable source with a narrowband frequency and unsteady rotational speed. It is shown that there is only one dominant mode at each peak frequency of the rotating instability components, and subsequent modes are continuously numbered. The peaks due to the rotating instability modes are not tonal in nature but spread over a narrow band. The frequency variation observed can be explained by either a variation of the source frequency, a variation of the angular velocity of the modes, or a combination of both. Similar to rotating stall, which is a frozen flow pattern, the rotating instability source rotates in the same direction as the rotor at about half the shaft speed. The rotating instability components comprise azimuthal modes of much higher order than rotating stall. At highly throttled operation, both flow phenomena are observed at the same time. The mode orders of rotating instability and rotating stall are not harmonically related. In agreement with earlier studies, the tip clearance noise of axial flow machines is found to be generated by the interaction between the rotating instability pattern and the rotor blades. (Author)

This paper reports on an experimental and numerical investigation aimed at understanding the mechanisms of rotating instabilities in a low speed axial flow compressor. The phenomena of rotating instabilities in the current compressor were first identified with an experimental study. Then, an unsteady numerical method was applied to confirm the phenomena and to interrogate the physical mechanisms behind them. The experimental study was conducted with high-resolution pressure measurements at different clearances, employing a double phase-averaging technique. The numerical investigation was performed with an unsteady 3-D Navier-Stokes method that solves for the entire blade row. The current study reveals that a vortex structure forms near the leading edge plane. This vortex is the result of interactions among the classical tip-clearance flow, axially reversed endwall flow, and the incoming flow. The vortex travels from the suction side to the pressure side of the passage at roughly half of the rotor speed. The formation and movement of this vortex seem to be the main causes of unsteadiness when rotating instability develops. Due to the nature of this vortex, the classical tip-clearance flow does not spill over into the following blade passage. This behavior of the tip-clearance flow is why the compressor operates in a stable mode even with the rotating instability, unlike traditional rotating stall phenomena.