Analysis of Dynamic Characteristics in the Tip Near-Stall Flow Field of a Transonic Axial Compressor Based on POD and DMD

Under the pre-stall condition of transonic axial compressors, intense unsteady flow within the tip clearance region acts as the primary mechanism initiating rotating stall and constraining the stable operating range of gas turbine engines. Conventional proper orthogonal decomposition (POD) alone only captures spatially coherent structures with optimal flow-field energy distribution, yet fails to directly characterize modal dynamic evolution and thus cannot fully uncover the inherent onset mechanism of compressor instability. Taking the NASA Rotor 67 transonic compressor rotor as the research object, this work obtains time-resolved unsteady flow-field data in the tip region near stall via high-fidelity numerical simulation. A combined POD and dynamic mode decomposition (DMD) framework is employed to synchronously extract spatial structural features and dynamic evolutionary laws of the flow field for instability mechanism analysis. Results demonstrate that the dominant POD mode corresponds to the tip secondary vortex induced by breakdown of tip leakage vortex, with a dominant frequency of 3495 Hz and its harmonics. DMD identifies a weakly growing dominant mode associated with the tip secondary vortex and a low-frequency mode linked to oscillations of the leakage vortex core; the mutual coupling constitutes the core trigger for flow-field instability.