The wake of a laboratory-scale floating offshore wind turbine model is investigated under prescribed sinusoidal surge, sway, roll, pitch, and yaw motions using large-eddy simulations coupled to an actuator-line model. The study aims to assess how the wake of a moving turbine evolves in a high-blockage-ratio wind tunnel and to compare the results with the literature on full-scale models and experiments. The present work covers sinusoidal motions in five of the six floating offshore wind turbine degrees of freedom in a single study and uses radial probes that sample circular two-dimensional cross-sections of the wake at several downstream positions instead of the commonly used linear probes. Two cases per degree of freedom are considered, corresponding to two distinct wake regimes: one with a low frequency and high amplitude and one with a high frequency and low amplitude. The low-frequency/high-amplitude cases exhibit wake behavior close to the fixed-bottom case, as the prescribed frequency falls outside the high-energy spectral range naturally developed by the fixed-bottom wake. Conversely, the high-frequency/low-amplitude cases, whose prescribed frequency is within this high-energy range, produce strongly amplified perturbations, more irregular wake boundaries, earlier tip and root vortex trail expansion and merger, sharper turbulence intensity peaks, and faster wake recovery. The amplification is concentrated at the tip and root vortex trails, where the shear flow instability is strongest. An exception is the high-frequency surge case, which hampers wake recovery at the simulated frequency. Despite the high blockage ratio and wake confinement, all phenomena identified are consistent with the literature, confirming that the fundamental floating-wind-turbine wake dynamics are captured in this setup. ... Read more

Two setups are used to investigate differences between modeling a wind turbine nacelle by means of an actuator-line model (ALM) and a wall-model (WM) using large-eddy simulations. One advantage of the ALM is that it requires a lower mesh refinement, making it less computationally costly. In the first setup, the nacelle is in standalone configuration and the ALM results show a much lower turbulence intensity and a significantly slower wake recovery when compared to the WM cases. In the second setup, the nacelle is in a rotor-nacelle assembly configuration and many variations of the ALM are tested in order to match the results from the experiment addressed in the OC6 task phase III. Contrary to previous findings that the nacelle might affect the turbine loads, this study shows that the improved match with the experiment stems from the increased mesh refinement in the nacelle region rather than the actual presence of the nacelle. Nevertheless, the wake profiles in the near-wake show a very good agreement between the ALM and WM, regardless of the refinement in the nacelle region. These cases also show a higher wake deficit than not using any nacelle at all. ... Read more

This study reports the results of the second round of analyses of the Offshore Code Comparison, Collaboration, Continued, with Correlation and unCertainty (OC6) project Phase III. While the first round investigated rotor aerodynamic loading, here, focus is given to the wake behavior of a floating wind turbine under large motion. Wind tunnel experimental data from the UNsteady Aerodynamics for FLOating Wind (UNAFLOW) project are compared with the results of simulations provided by participants with methods and codes of different levels of fidelity. The effect of platform motion on both the near and the far wake is investigated. More specifically, the behavior of tip vortices in the near wake is evaluated through multiple metrics, such as streamwise position, core radius, convection velocity, and circulation. Additionally, the onset of velocity oscillations in the far wake is analyzed because this can have a negative effect on stability and loading of downstream rotors. Results in the near wake for unsteady cases confirm that simulations and experiments tend to diverge from the expected linearized quasi-steady behavior when the rotor reduced frequency increases over 0.5. Additionally, differences across the simulations become significant, suggesting that further efforts are required to tune the currently available methodologies in order to correctly evaluate the aerodynamic response of a floating wind turbine in unsteady conditions. Regarding the far wake, it is seen that, in some conditions, numerical methods overpredict the impact of platform motion on the velocity fluctuations. Moreover, results suggest that the effect of platform motion on the far wake, differently from original expectations about a faster wake recovery in a floating wind turbine, seems to be limited or even oriented to the generation of a wake less prone to dissipation. ... Read more