Dynamics of floating-wind-turbine wakes in a wind tunnel setup

Abstract

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.