Flow instabilities such as rotating stall and surge limit the performance and stability of compressors. Although dynamic mode decomposition (DMD) has been widely applied to analyze unsteady compressor flows, most existing studies are restricted to single rotor configurations or single rotational speed and primarily focus on stall or mild surge regimes. Multi-stage coupling effects, rotational-speed-dependent instability mechanisms, and deep surge conditions remain insufficiently explored. In this study, full-annulus unsteady Reynolds-averaged Navier–Stokes simulations of a two-stage transonic fan are performed under multiple rotational speeds, and DMD is applied to extract the dominant modes during the deep surge process. A comparison between DMD and fast Fourier transform shows that the maximum relative frequency error reaches 11.7% at a low-frequency mode (0.015 blade passing frequency), while the error for other modes remains within 7%, confirming the reliability of the DMD analysis. The growth rate analysis indicates that at the design speed (1.0n), the second-stage rotor becomes unstable first, whereas at 0.8n, the first-stage rotor initiates instability. The dominant modes reveal three primary types of flow disturbances, including axial disturbances, circumferential disturbances, and self-excited tip leakage flow oscillations, which together form a self-sustained feedback loop driving the instability. The flow evolution further demonstrates that rotating stall reduces flow capacity, leading to blockage, and ultimately triggering severe reverse flow associated with deep surge. This study extends previous DMD-based investigations by revealing the instability onset locations in a two-stage fan and by capturing the full process from near-stall to deep surge.
Wang et al. (Mon,) studied this question.