Unsteady Numerical Investigation of Stall Onset and Propagation in a High-Speed Counter-Rotating Axial Compressor

Stall inception in high-speed counter-rotating axial compressors remains insufficiently understood because of the complex unsteady interactions between adjacent rotating blade rows. In this study, full-annulus unsteady Reynolds-averaged Navier–Stokes simulations combined with dense pressure monitoring, time-frequency analysis, and dynamic mode decomposition are used to investigate the evolution of a low-frequency stall precursor. A dominant disturbance at approximately 700 Hz in the stationary frame, corresponding to approximately 370 Hz in the rear-rotor-relative frame, is first detected in the rear-rotor region and subsequently appears at upstream and downstream monitoring stations. Its amplitude increases continuously over several rotor revolutions as the compressor approaches stall inception. Circumferential pressure signals exhibit an ordered phase progression around the annulus, indicating an annulus-scale, circumferentially organized disturbance rather than isolated local fluctuations. Dynamic mode decomposition associates the 370 Hz mode with periodic displacement and deformation of the rear-rotor shock system. Shock-position histories further reveal increasing oscillation amplitudes and circumferential phase shifts as stall is approached. Larger shock excursions are accompanied by stronger near-tip streamline deflection and precede substantial passage-blockage growth. These results identify the rear rotor as the earliest detected and primary amplification region of the precursor and support an interpretation involving progressively amplified shock-related unsteadiness and subsequent deterioration of passage flow capacity.