This paper experimentally investigated the evolution of the tip clearance flow of a CRAC (Contra-Rotating Axial Compressor) test rig by means of high-response dynamic pressure measurements. The unsteady pressure field along both chordwise and circumferential directions in the tip clearance is recorded. The tip clearance vortex trajectory is captured using RMS (Root-Mean Square) method. Pressure spectrum analysis indicates that the unsteadiness of tip clearance vortex occurred when the flow coefficient approaches low enough even in the stable operating point. The unsteadiness of tip clearance vortex gets stronger as the flow coefficient drops until rotating stall occurs. According to this feature, the auto-correlation analysis and the cross-correlation analysis combined probability statistics method are used to work as pre-stall warning methods. In addition to, rotating instability which is caused by disturbances propagating along circumferential direction occurred at some flow condition.

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