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Vortex-induced vibration (VIV) of wind turbine towers during installation is an aero-structural problem of significant practical relevance. Vibrations may happen in the tower structure, especially when the rotor-nacelle assembly is not yet attached to the tower or if the rotor blades are not yet connected to the tower-nacelle assembly. The complexity of aeroelastic phenomena involved in VIV makes modelling and analysis challenging. Therefore, the aim of the current research is to investigate the fundamental mechanisms causing the onset and sustenance of vortex-induced vibrations. To gain more understanding of the nature of vibrations, a methodology is established that distinguishes between different components of the forces at play. This approach allows for identifying how various force components impact the oscillation of a rigid body. The method is executed using the OpenFOAM open-source software. Numerical simulations are conducted on a two-dimensional smooth cylinder at both subcritical and supercritical Reynolds numbers to establish a correlation between wind turbine tower vibrations and the force mechanism. The analysis involves performing unsteady Reynolds-averaged Navier–Stokes (URANS) simulations using the modified pimpleFoam solver with the k–ω shear stress transport (SST) turbulence model. Both fixed and free-vibrating cases are studied for smooth cylinders. For the high-Reynolds-number cases, a setup matching the tower top segment of the IEA 15 MW reference wind turbine was chosen. Studying the flow around a cylinder at a subcritical Reynolds number reveals that the primary force involved is the vortex-induced force. The combined force due to viscosity, added mass, and vorticity contributes most to the overall force. For a freely vibrating cylinder with a single degree of freedom in the crossflow direction, the analysis indicates that the force component associated with the cylinder's motion is crucial and significantly affects the total force. Moreover, analysing the energy transfer between the fluid and the structure, a positive energy contribution by the vortex-induced force is observed on or before the dominant Strouhal velocity. This confirms observations at low Reynolds numbers in the literature that the vortex shedding predominantly contributes to the initiation of oscillations during VIV. The kinematic force contributes to the energy transfer of the system, but the mean energy transfer per cycle is negligible.

Waves have the potential to increase the power output of a floating wind turbine by forcing its rotor to move against the wind. Starting from this observation, we use four multi-physics models of increasing complexity to investigate the role of waves and platform movements in the energy conversion process of four floating wind turbines of 5–15 MW in the Mediterranean Sea. Progressively adding realism to our simulations, we show that large along-wind rotor movements are needed to increase the power output of a floating wind turbine; however, these are prevented by the current technology of spar and semi-submersible platforms. Wind turbulence is the main cause of power fluctuations for the four floating wind turbines we examined and is preponderant over the effect of platform motions due to waves. In a realistic met-ocean environment, the power curve of the floating wind turbines we studied is lower than that obtained with a fixed foundation, with reductions in the annual energy production of 1.5 %–2.5 %. The lower energy production is mainly ascribed to the platform mean tilt, which reduces the rotor's effective area.

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.

Aerodynamic-load calculation in aero-hydro-servo-elastic modeling tools has been recently validated against experiments for low-frequency platform motions but without considering the capability of active wind turbine controls. This work presents a control design framework that allows for including industry-standard wind turbine control functionalities in a model-scale rotor and its application to a 1:100 scaled version of the International Energy Agency (IEA) 15 MW turbine. Wind tunnel tests with a fixed foundation and steady wind show the scaled turbine reproduces the steady-state rotor speed–blade pitch–thrust–torque characteristics of the IEA 15 MW turbine, confirming the controller design method. Tests with a prescribed platform pitch motion are carried out to assess the turbine response and controller modeling in conditions representative of the normal operation of floating wind turbines. The blade element momentum model of OpenFAST is verified against the experiment, showing aerodynamic thrust and torque are predicted with higher accuracy in the below-rated than the above-rated region: in our simulation, the decrease in thrust oscillation amplitude due to blade pitch actuation is underpredicted. This, combined with uncertainty in modeling the blade pitch actuators, complicates the numerical–experimental simulation of the turbine aerodynamic response in above-rated operation.

This paper provides a summary of the work done within Phase III of the Offshore Code Comparison Collaboration, Continued, with Correlation and unCertainty (OC6) project, under the International Energy Agency Wind Technology Collaboration Programme Task 30. This phase focused on validating the aerodynamic loading on a wind turbine rotor undergoing large motion caused by a floating support structure. Numerical models of the Technical University of Denmark 10 MW reference wind turbine were validated using measurement data from a 1:75 scale test performed during the UNsteady Aerodynamics for FLOating Wind (UNAFLOW) project and a follow-on experimental campaign, both performed at the Politecnico di Milano wind tunnel. Validation of the models was performed by comparing the loads for steady (fixed platform) and unsteady (harmonic motion of the platform) wind conditions. For the unsteady wind conditions, the platform was forced to oscillate in the surge and pitch directions under several frequencies and amplitudes. These oscillations result in a wind variation that impacts the rotor loads (e.g., thrust and torque). For the conditions studied in these tests, the system aerodynamic response was almost steady. Only a small hysteresis in airfoil performance undergoing angle of attack variations in attached flow was observed. During the experiments, the rotor speed and blade pitch angle were held constant. However, in real wind turbine operating conditions, the surge and pitch variations would result in rotor speed variations and/or blade pitch actuations, depending on the wind turbine controller region that the system is operating. Additional simulations with these control parameters were conducted to verify the fidelity of different models. Participant results showed, in general, a good agreement with the experimental measurements and the need to account for dynamic inflow when there are changes in the flow conditions due to the rotor speed variations or blade pitch actuations in response to surge and pitch motion. Numerical models not accounting for dynamic inflow effects predicted rotor loads that were 9 % lower in amplitude during rotor speed variations and 18 % higher in amplitude during blade pitch actuations.

In the field of floating wind energy, large-scale wind turbine models deployed in natural environments represent a key link between small-scale laboratory tests and full-scale prototypes. While implying smaller cost, design and installation effort than a full-scale prototype, large-scale models are technologically very similar to prototypes, can be tested in natural sea and wind conditions, and reduce by a consistent amount the dimensional scaling issues arising in small-scale experiments. In this framework the presented work report the aerodynamic and control system assessment of a 1:15 model of the DTU 10 MW wind turbine installed on a multipurpose-platform model for fish farming and energy production. The model has operated for 6 months in a natural laboratory and has been exposed to fully natural and uncontrolled environmental conditions. Assessment is performed in terms of rotor thrust force and power controller parameters such as rotor speed, blade pitch and rotor power as a function of incoming wind speed.

The aerodynamics of floating turbines is complicated by large motions which are permitted by the floating foundation, and the interaction between turbine, wind, and wake is not yet fully understood. The object of this paper is a wind tunnel campaign finalized at characterizing the aerodynamic response of a 1:100 scale model of the IEA 15 MW subjected to imposed platform motion. The turbine aerodynamic response is studied focusing on thrust force, torque, and wake at 2.3D downwind the rotor. Harmonic motion is imposed in the surge, sway, roll, pitch, and yaw directions with several frequencies and amplitudes, which are selected to be representative of the two 15 MW floating turbines developed within the COREWIND project. Thrust and torque show large-amplitude oscillations with surge and pitch motion, the main effect of which is an apparent wind speed; oscillations in thrust and torque are negligible with the other motions, the main effect of which is to alter the wind direction. The thrust and torque response measured in the experiment is compared with predictions of a quasi-steady model, often used for control-related tasks. The agreement is good in the case of low-frequency surge motion, but some differences are seen in the pitch case. The quasi-steady model is not predictive for the response to wave-frequency motion, where blade unsteadiness may take place. Wake was measured imposing motion in five directions with frequency equal to the wave frequency. The axial speed is slightly lower with motion compared to the fixed case. The turbulence kinetic energy is slightly lower too. Wave-frequency motion seems to produce a more stable and lower flow mixing.

Floating offshore wind turbines are subjected to large motions due to the additional degrees of freedom of the floating foundation. The turbine rotor often operates in highly dynamic inflow conditions, and this has a significant effect on the overall aerodynamic response and turbine wake. Experiments are needed to get a deeper understanding of unsteady aerodynamics and hence leverage this knowledge to develop better models and to produce data for the validation and calibration of existing numerical tools. In this context, this paper presents a wind tunnel experiment about the unsteady aerodynamics of a floating turbine subjected to surge motion. The experiment results cover blade forces, rotor-integral forces, and wake. The 2D sectional model tests were carried out to characterize the aerodynamic coefficients of a low-Reynolds-number airfoil with harmonic variation in the angle of attack. The lift coefficient shows a hysteresis cycle close to stall, which grows in strength and extends in the linear region for motion frequencies higher than those typical of surge motion. Knowledge about the airfoil aerodynamic response was utilized to define the wind and surge motion conditions of the full-turbine experiment. The global aerodynamic turbine response is evaluated from rotor-thrust force measurements, because thrust influences the along-wind response of the floating turbine. It is found that experimental data follow predictions of quasi-steady theory for reduced frequency up to 0.5 reasonably well. For higher surge motion frequencies, unsteady effects may be present. The turbine near wake was investigated by means of hot-wire measurements. The wake energy is increased at the surge frequency, and the increment is proportional to the maximum surge velocity. A spatial analysis shows the wake energy increment corresponds with the blade tip. Particle image velocimetry (PIV) was utilized to visualize the blade-tip vortex, and it is observed that the vortex travel speed is modified in the presence of surge motion.

Floating wind turbines rely on feedback-only control strategies to mitigate the negative effects of wave excitation. Improved power generation and lower fatigue loads can be achieved by including information about incoming waves in the turbine controller. In this paper, a wave-feedforward control strategy is developed and implemented in a 10 MW floating wind turbine. A linear model of the floating wind turbine is established and utilized to understand how wave excitation affects rotor speed and so power, as well as to show that collective pitch is suitable for reducing the effects of wave excitation. A feedforward controller is designed based on the inversion of the linear model, and a gain-scheduling algorithm is proposed to adapt the feedforward action as wind speed changes. The performance of the novel wave-feedforward controller is examined first by means of linear analysis and then with non-linear time-domain simulations in FAST. This paper proves that including some information about incoming waves in the turbine controller can play a crucial role in improving power quality and the turbine fatigue life. In particular, the proposed wave-feedforward control strategy achieves this goal complementing the industry-standard feedback pitch controller. Together with the wave-feedforward control strategy, this paper provides some insights about the response of floating wind turbines to collective-pitch control and waves, which could be useful in future control-design studies.