Wind tunnel experiments were performed, where the development of the wake of a model wind turbine was measured using stereo Particle Image Velocimetry to observe the influence of platform pitch motion. The wakes of a classical bottom fixed turbine and a streamwise oscillating turbine are compared. Results indicate that platform pitch creates an upward shift in all components of the flow and their fluctuations. The vertical flow created by the pitch motion as well as the reduced entrainment of kinetic energy from undisturbed flows above the turbine result in potentially higher loads and less available kinetic energy for a downwind turbine. Experimental results are compared with four wake models. The wake models employed are consistent with experimental results in describing the shapes and magnitudes of the streamwise velocity component of the wake for a fixed turbine. Inconsistencies between the model predictions and experimental results arise in the floating case particularly regarding the vertical displacement of the velocity components of the flow. Furthermore, it is found that the additional degrees of freedom of a floating wind turbine add to the complexity of the wake aerodynamics and improved wake models are needed, considering vertical flows and displacements due to pitch motion.

a b s t r a c tExperiments were performed using two model wind turbines operated in tandem with a bottom-fixed configuration and a floating configuration with both turbines allowed to freely oscillate in the streamwise direction. Wakes of both turbines were measured using stereoscopic Particle Image Velocimetry. Turbulent characteristics of the far wake of the first turbine acting as the inflow for the downwind turbine were characterized calculating the eddy viscosity and mixing length profiles from the obtained data. The influence of the far wake on the statistical properties of the near wake of the second turbine are compared between the fixed and oscillating configurations. The incoming mixing length clearly influences the Reynolds stresses and turbulence production of the near wake in the shear layer. Below, the connection between incoming mixing length and the near wake is less evident, due to the impact of the nacelle and rotation of the rotor. For the oscillating turbine, the Reynolds stresses and turbulence production in the near wake of the downwind turbine are damped. Vertical fluctuations were found to decrease though an increase in the mean vertical component. New challenges arise in the design of a floating offshore wind farms, in terms of farm layout and load estimations.

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Turbulence intermittency characteristics of the flow behind pitching and fixed wind turbines are assessed via hot-wire anemometry in a wind tunnel experiment. The pitching wind turbine model is free to oscillate in the streamwise direction to simulate pitch motion. Two inflow conditions are considered: 15% and 1.8% turbulent intensities. Empirical mode decomposition and Hilbert Huang transform are employed and validated by comparing the Hilbert energy spectrum with the Fourier energy spectrum. The extended self-similarity model indicates that pitching effects are more pronounced at locations where the flow is less turbulent due to its effect of being overshadowed by intermittency caused by tip vortex shedding. This agrees with arbitrary order Hilbert spectrum analysis (HSA) results. HSA is proven to be more accurate for scaling exponent estimation than structure functions as the latter results are significantly affected by the energetic scales. Premultiplied energy spectra show that pitch motion affects preferably large scales 0.1D−0.5D and the same amount of energy is contained on smaller scales compared to the fixed turbine, suggesting potential of higher power production. This work considers offshore wind turbine wakes by examining the pitch motion effects on the flow. Hence, results have direct implications on power production and quantification of fatigue loads due to pitch cyclic motion.

Turbulence intermittency in the wake behind a single floating wind turbine as well as merging wakes due to a pair of floating turbines is investigated using magnitude cumulant analysis and non-analytical cumulant analysis. This low-order statistical approach is used to compute the intermittency for its impact on fatigue loading and power output signals. In the near wake, a 60% increase in the intermittency coefficient compared to the inflow is found. Pitch motion causes a 17% increase in intermittency compared to fixed turbines. The pitch-induced intermittency depletes in the far-wake, and hence, investigating whether a pitch-induced intermittency of one turbine affects a successive one in a wind array setting is recommended. Non-local scale interactions near rotor tips are observed as undulations in the cumulant profiles, referred to as tip-effect fluctuations. The impact of turbulence intensity on intermittency is also examined, and a positive correlation between the two is found in the near-wake. In the far-wake, however, it is found to speed up the pitch-induced intermittency depletion. The wake merging region between two neighboring turbines experiences lower intermittency and damps tip-effect fluctuations. This work provides more reliable intermittency estimation by utilizing lower moment statistics. The findings aid description, turbulent loading quantification, and stochastic modeling for floating wind farm wakes as well as fixed ones for both single and merging wakes.