Investigation of the floating IEA wind 15-MW RWT using vortex methods Part II: Wake impact on downstream turbines under turbulent inflow

The manuscript presents a novel numerical investigation on the impact of the wake of a floating IEA wind 15-MW reference wind turbine (RWT) on downstream machines using a state-of-the-art vortex solver coupled to a multi-body code. The coupling enables to account for the flexibility of the different turbine components as well as to include the effect of the controller and the dynamics of the floating support structure. First, the turbine is mounted on the WindCrete spar-buoy platform, and the wake impact on a second turbine positioned at different downstream positions is investigated and compared with the impact of the wake generated by a bottom-fixed machine. It is found that the faster breakdown of the vortex structures triggered by the motion of the floater in the upstream turbine increases the power production of a downstream machine, as well as its mean thrust level relative to normal operation downstream a bottom-fixed machine. It is demonstrated that this effect is drastically reduced with the increase of the turbulence intensity (TI). Further, simulations with a prescribed harmonic motion of the upstream turbine in surge and pitch under different turbulence levels are presented. It is found that the motion of the floater has a strong impact in the generated wake and consequently in the operation of downstream machines. In particular, downstream turbines experienced considerably higher blade loading that led to an increase of the aerodynamic power. Finally, aero-hydro-servo-elastic simulations of five turbines in a row have shown that the interaction between multiple floating machines is more dynamic than between bottom-fixed turbines. This has been mainly observed at high wind speeds, where the pitch and surge motions of the floater in turbines located deep inside the farm can be resonantly excited by the interaction with the wind farm flow. In practice, this means that the power and thrust variations increase with the turbine location depth inside the farm. Overall, the study highlights the importance of an accurate flow and wake modeling for the prediction of turbine-to-turbine interaction in a floating context.