Abstract
Surge and heave represent the primary motion components affecting floating tidal stream turbines in operational environments, yet their combined effects on performance and wake dynamics remain poorly understood. This study numerically investigates a horizontal-axis tidal stream turbine subjected to prescribed isolated surge, isolated heave, and coupled surge-heave motions. To this end, a dynamic actuator line model is developed within the OpenFOAM-v2406 framework and validated against published measurements of turbine performance and wake velocity. Results indicate that increasing motion amplitude or frequency induces modest reductions (typically within ∼10%) in mean power and thrust coefficients, whereas surge induces pronounced phase-locked fluctuations that can exceed 50% of the mean level. Heave alone produces comparatively weak performance fluctuations. Under coupled motions, the turbine performance remains largely surge-dominated, with limited sensitivity to amplitude ratios and phase differences between the surge and heave motions. In contrast, wake structures respond dynamically to both surge and heave components. Blade tip and root vortices persist to approximately one turbine diameter downstream. Beyond this, surge promotes toroidal vortex rings, while heave generates strip-like vortices. Their interaction yields a composite wake with enhanced asymmetry, which is quantified using vortex-core distributions and a wake-deflection coefficient. The results provide a phase-resolved characterization of how coupled motion kinematics govern performance unsteadiness and wake re-organization, offering guidance for performance and wake dynamics assessment of floating tidal turbines.