An investigation of the behavior of a transonic compressor rotor when operating close to stall is presented. The specific area of interest is the behavior and location of low-frequency instabilities when operating close to stall. When running close to stall compressors begin to exhibit non-periodic flow between the blade passages even when appearing to be operating in a stable steady-state condition. These frequencies are not geometrically fixed to the rotor and typically appear at 0.3–0.8 of the rotor speed. The presence of these low-frequency instabilities are known and detection are reasonably commonplace, however attempts to quantify the location and strength of these instabilities as stall is approached have proved difficult. In the test rotor probes were positioned in the case-wall upstream, downstream and over the rotor blade tips. Simultaneous data from all the probes was taken at successive steady-state settings each operating closer to stall. The simultaneous data is presented to show the development and distribution of the instabilities over the rotor as stall was approached. Initially the instabilities appeared within the rotor row and extended downstream. At operation closer to stall they protruded upstream and downstream. The greatest amplitude of the instabilities was within the blade row in the complex flow region that contained the tip-vortex interacting with the shock and the shock impinging on the blade suction surface. In the current rotor the data shows that the instabilities were present during steady-state operation when stall was approached even when stall was not imminent. In addition they do not behave in a linear manner as stall was approached.

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