Analysis of transient hydrodynamic modal characteristics of tidal current turbines under surge motion

Tidal current turbines operating on floating platforms are subject to surge motion, which can strongly influence wake evolution and turbine performance. To address this, this paper investigates the transient wake characteristics of tidal current turbines under surge motion by implementing the actuator line model within large eddy simulation. Dynamic mode decomposition (DMD) is applied to extract coherent flow structures and dominant dynamic modes under both fixed and surge conditions. The results indicate that the surge motion increases turbulence intensity, leading to a greater energy loss and faster wake recovery. The DMD method effectively identifies the primary dynamic modes in the wake. Under fixed conditions, modal energy is highly concentrated at the rotational frequency, blade passage frequency, and their harmonics, with the 1-2fn modes linked to support-structure vortex shedding. In contrast, surge motion excites additional modes at the first and second multiples of the surge frequency, as well as coupled modes with blade rotation, amplifying unsteady flow interactions. These findings highlight the critical role of surge-induced dynamics in wake evolution and provide quantitative insights for improving tidal turbine performance and array optimization.