Based on the reduced-order variational mode decomposition to represent the quasi-periodic components of the ship airwake

The ship airwake is a typical complex unsteady separated flow, in which quasi-periodic components induced by the periodic shedding and reorganization of vortical structures are embedded within the macroscopically stochastic fluctuations. Owing to the spectral aliasing effect arising from multiscale flow structures under a broadband turbulent background, the extraction of individual dynamical behaviors from such complex flow fields remains highly challenging. To address this issue, the Reduced-order Variational Mode Decomposition (RVMD) method is introduced in the present study to perform modal analysis of the unsteady spatiotemporal evolution of the SFS2 ship airwake. Compared with conventional decomposition methods, RVMD adaptively extracts locally narrowband spatiotemporal modes from non-stationary flow fields and directly links spatial structures to their temporal dynamics. Consequently, the dominant hidden dynamical behaviors within the airwake can be identified more effectively. The results demonstrate that, although the ship airwake exhibits globally stochastic turbulent characteristics, distinct periodic dynamical behaviors persist within the flow and manifest as organized coherent structures. Based on the intermodal correlation, frequency distribution, and spatial structural characteristics, the dominant dynamical behaviors are classified into three representative dynamical modes: the slowly varying mode, the vortex sliding mode, and the vortex shedding mode. The slowly varying mode characterizes relatively stable low-frequency quasi-steady recirculation features, the vortex-sliding mode reflects the downstream transport of shed vortices along the flight deck surface, and the vortex-shedding mode corresponds to the periodic shedding of the shear layer near the deck edge. Furthermore, comparative analyses under multiple inflow conditions (wind-over-deck angles of 0°, 15°, and 30°) preliminarily verify the structural robustness of these three dynamical modes under varying inflow conditions. This finding indicates that these modes are not incidental phenomena associated with a specific operating condition, but rather intrinsic dynamical attributes of the ship airwake.