Abstract
In practical operation, the floating platform supporting vertical-axis tidal turbines exhibits multi-degree-of-freedom motions under combined wave and current loads. To investigate these effects, a two-dimensional lattice Boltzmann and large eddy simulation model was developed, coupling turbine rotation with three-degree-of-freedom oscillations (yaw, surge, and sway). This numerical method, integrated with a central composite design, systematically analyzes the synergistic effects of coupled motion frequencies on power and load characteristics. The coupled numerical results show good agreement with experimental data, validating the accuracy of the proposed approach for the hydrodynamic evaluation of vertical-axis tidal turbines. The three-degree-of-freedom motion frequencies are found to affect power characteristics in distinct ways. The time-averaged power coefficient is dominated by sway, exhibiting significant nonlinear and interaction effects, whereas the coefficient of variation of the power coefficient is dominated by surge, with a weaker but statistically significant contribution from sway. In terms of load responses, surge frequency dominates fluctuations in the rotational moment coefficient, while fluctuations in the yaw moment are influenced by a more balanced contribution of motion frequencies in all directions. Thrust and lateral force fluctuations are governed by multi-frequency coupling effects. The results of this research provide relevant data for the hydrodynamic analysis of vertical-axis tidal turbines under multi-degree-of-freedom platform motions and for related structural design considerations. They contribute to understanding energy capture performance and load behavior in realistic operating conditions.