The unsteady flow phenomenon of a transonic axial compressor NASA Rotor 36 was investigated. Based on the unsteady simulation results

proper orthogonal decomposition (POD) modes and time coefficients were obtained with POD method

and the flow characteristics of the tip clearance flow field at near stall conditions were further analyzed. Results showed that: at near stall condition

the dominant frequency of tip clearance flow field was the tip leakage vortex frequency

about 0.6 times blade passing frequency; the POD mode with higher energy determined the fluctuation frequency and amplitude of tip leakage vortex

and the POD mode with lower energy determined the fine structure of the flow field; the first 5 POD modes reconstructed the flow field very well

providing a theoretical guidance for the establishment of the low-order model.