Investigation of unsteady characteristics in a counter-rotating compressor based on unsteady numerical simulation and mode decomposition methods

This study investigates the unsteady characteristics and its flow mechanisms in a counter-rotating axial flow compressor (CRAC) based on unsteady simulation, combining the proper orthogonal decomposition (POD) and dynamic mode decomposition (DMD). The change patterns of rotor-rotor interaction and tip leakage flow (TLF) as well as their unsteady characteristics are revealed. The dominant flow structures and their spatial-temporal features in two rotors are obtained based on the POD and DMD methods. The results show that the unsteady fluctuation of TLF in the front rotor (R1) and its own unsteadiness are very weak, and the unsteady characteristics in the R1 are mainly caused by rotor-rotor interaction. The unsteadiness in the rear rotor (R2) is mainly due to the effect of TLF and rotor-rotor interaction, besides, the unsteady fluctuation of TLF is progressively stronger than that of rotor-rotor interaction. The dominant frequency identified by both POD and DMD in the R1 is 1 times blade passage frequency (BPF) resulting from rotor-rotor interaction, which does not change with operating conditions. However, the dominant frequency in the R2 varies from 1BPF to 0.8BPF. The 0.8BPF corresponds to the unstable dominant flow structure induced by the spillage at the blade leading edge, which has a decisive effect on the compressor stability. The rotor-rotor interaction with strong unsteady fluctuation is dominant to the unsteady flow in the CRAC, and the TLF further aggravates local unsteadiness. In addition, more modes are needed to reconstruct the flow field in the R2. The maximum reconstruction error of POD modes is lower than that of the DMD when the mode number is equal. The reconstruction error of POD is higher for small-scale flow structures, while the opposite happens for the DMD.