Experiments measuring circumferential mode characteristics of compressors were made for the high demand for the number of sensors

and the aliasing of circumferential wave number when insufficient sensors were provided. A calibration and prediction of higher circumferential mode numbers was proposed using two sets of measuring points

unsteady pressure fluctuations near the tip region in an axial compressor were studied experimentally using this method and the upper limit of identifiable mode numbers was five times of the original method. Circumferential mode characteristics were captured both on blade passing frequency (BPF) and rotational instability frequency (RIF) under several flow conditions. The characteristic spectrum of rotating instability with broadband hump existed within a large range of flow conditions. Both frequency range and dominant circumferential wave number decreased with the flow rate

while at the same time circumferential angular velocity of rotating instability increased; strength of the pressure fluctuations varied significantly in time domain.