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Design and verification of control strategies for floating wind turbines often makes use of aero-hydro-servo-elastic modeling tools. Aerodynamic loads calculation in these tools has been recently validated against experiments not including active wind turbine control. This work investigates the aerodynamic response of a floating wind turbine scale model with active control and platform pitch motion. This is done in wind tunnel testing and with modeling of the scaled system in the offshore tool OpenFAST. A control design framework is developed to include the reference wind turbine controller ROSCO in the wind tunnel experiment. With platform pitch motion, the turbine aerodynamic response is predicted by the numerical model with different accuracy depending on the turbine control regime. Below rated wind, oscillations of aerodynamic torque in simulations are of lower amplitude than in the experiment, also when dynamic inflow is considered in the aerodynamic model. Above rated wind, where the turbine is controlled with collective blade pitch actuation, the response is not quasi-steady, and differences between the experiment and simulation are larger than in below-rated wind, in particular for phase with respect to motion.

This study reports the results of the second round of analyses of the 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 both on 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 over-predict the impact of platform motion on the velocity fluctuations. Moreover, results suggest that, different from original expectations about a faster wake recovery in a floating wind turbine, the effect of platform motion on the far wake seems to be limited or even oriented to the generation of a wake less prone to dissipation.

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.

In the field of floating wind energy large-scale wind turbine models deployed in natural environment 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 is reporting 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 six 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.

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.

Experimental tests performed in a wind tunnel or in a natural laboratory represent a fundamental research tool to develop floating wind technologies. In order to obtain reliable results, the wind turbine scale model rotor must be designed so to obtain a fluid-structure interaction comparable to the one experienced by a real machine. This implies an aerodynamic design of the 3D blade geometry but, also, a structural project to match the main aeroelastic issues. For natural laboratory models, due to not controlled test conditions, the wind turbine rotor model must be checked also for extreme winds. The present paper will focus on all the strategies adopted to scale a wind turbine blade presenting two studied cases: the first is a 1:75 scale model for wind tunnel applications and the second a 1:15 model for natural laboratory tests.

Floating offshore wind turbines are subjected to large motions because of the additional degrees of freedom offered by the floating foundation. The rotor operates in highly dynamic inflow conditions and this is deemed to have a significant effect on the aerodynamic loads, as well as on the wind turbine wake. Floating wind turbines and floating farms are designed by means of numerical tools, that have to model these unsteady aerodynamic phenomena to be predictive of reality. Experiments are needed to get a deeper understanding of the unsteady aerodynamics, and hence leverage this knowledge to develop better models, as well as to produce data for the validation and calibration of the existing tools. This paper presents a wind-tunnel scale-model experiment about the unsteady aerodynamics of floating wind turbines that followed a radically different approach than the other existing experiments. The experiment covered any aspect of the problem in a coherent and structured manner, that allowed to produce a low-uncertainty data for the validation of numerical model. The data covers the unsteady aerodynamics of the floating wind turbine in terms of blade forces, rotor forces and wake. 2D sectional model tests were carried to study the aerodynamics of a low-Reynolds blade profile subjected to a harmonic variation of the angle of attack. The lift coefficient shows an hysteresis cycle that extends in the linear region and grows in strength for higher motion frequencies. The knowledge gained in 2D sectional model tests was exploited to design the rotor of a 1/75 scale model of the DTU 10MW that was used to perform imposed surge motion tests in a wind tunnel. The tower-top forces were measured for several combinations of mean wind speed, surge amplitude and frequency to assess the effect of unsteady aerodynamics on the response of the system. The thrust force, that plays a crucial role in the along-wind dynamics of a floating wind turbine mostly follows the quasi-steady theory. The near-wake of the wind turbine was studied by means of hot-wire measurements, and PIV was utilized to visualize the tip vortex. It is seen that the wake energy is increased in correspondence of the motion frequency and this is likely to be associated with the blade-tip vortex, which travel speed is modified in presence of surge motion.

Multi-purpose floating platforms are emerging as a promising concept in ocean engineering applications, thanks to their capability of ensuring system integration, cost reduction and modularization. However, their increasing complexity requires the development of numerical tools, which need to be validated experimentally through adequate physical models. New challenges hence arise, since the subsystems integrated in the structure generally follow different scaling laws and may need relatively large physical models to achieve a reliable similitude between the full-scale structure and its physical model counterpart. The latter issue can be critical, because indoor tests in wave tanks and basins constrain the scale factors to the size of the available facilities. Open-sea experiments, albeit challenging because of the uncontrolled environmental conditions, could be a valid complement to the traditional indoor tests. This article proposes a review of the multi-physics scaling strategies for the subsystems usually embedded in multi-purpose floating platforms, i.e. floating support, mooring system, wind turbine, wave energy converter and aquaculture facilities, by providing a critical analysis on the relevance of the scaling factor and of the scaling strategy. The paper may also serve as a guide for practical applications involving one or several of the considered subsystems.

The scope of this work is to investigate if and how it is possible to estimate the incident wave elevation on a floating wind turbine, with the purpose of improved control strategies. A Kalman based algorithm is proposed, which receives as input the rigid motions of the floater and estimates the wave elevation hitting the floating platform. The structure of the observer is described and the estimator is tested numerically on the OC3-Hywind platform coupled with the 5-MW reference wind turbine from NREL. Limitations to the estimation procedure are discussed. Finally the algorithm is tested on experimental data coming from a wave basin experimental campaign on a floating wind turbine model. The algorithm still needs improvements, but results are encouraging in the development of this technology.

This paper develops a novel feedforward control strategy for reducing structural loads caused by waves in floating offshore wind turbines. The proposed control strategy is based on the inversion of a linear model of the floating wind turbine, and a real-time forecast of the wave obtained from an upstream measurement is utilized to compute a collective pitch control action. Two feedforward controllers are considered: one is designed to cancel the rotor speed oscillations and one to lower the towertop fore-aft shear force. The feedforward control strategies are implemented in a 10MW floating wind turbine, complementing the standard feedback controller for generator speed regulation. Numerical simulations are carried out in FAST, in four operating conditions with realistic wind and waves, proving the proposed feedforward controller effectively mitigates the structural loads caused by waves. In detail, the feedforward action reduces the loads spectra in the frequency range where linear wave is active. The best performance is realized higher winds (the FA force is reduced up to 25% in 22 m/s wind), where the wave excitation is the strongest.

Optimising the O&M activities related to the proprietary infrastructure assets is crucial for a company successful management. Nowadays this topic is gaining even major hype due to the accessibility of new technologies such as big data. Such reasons push the industrial market towards a pursue of effectiveness and efficiency of maintenance strategies. This work inserts itself in this context: born from a collaboration with a company involved in the sector of telecommunications, it aims at the determination of an heuristic model able to estimate, as far as their towers are concerned, the probability of a structural failure. The initial phase is about the analysis of the company’s database to identify the variables to be considered in the study. Then the focus is shifted on the research of a synthetic index that could summarise the contribution of the aforementioned parameters. Subsequently, a sensitivity analysis aimed at testing the robustness of such index is carried out, followed by a subdivision of the entirety of the assets into homogeneous clusters in terms of loads and structural properties. The clusterization is intended to feed the “risk” axis of a risk-impact matrix to be used to tailor the O&M best practices to apply to the fleet of communication towers. The predictive capability of such representation will be assessed performing physical checks on a number of towers sampled from the matrix.

The paper presents the results from the multidisciplinary investigations performed on Baghetto Bridge. The structure, built in 1966, crosses the Adda river between the Municipalities of Castello dell'Acqua and Chiuro. It consists of three steel–concrete composite girders of different lengths supported by stone-masonry piers. The research program involved several investigations: (i) documentary research, (ii) ambient vibration testing, (iii) hydraulic study, and (iv) FE modelling and updating. After a concise review of the bridge history and a brief description of the structure, the hydraulic study based on existing topographic surveys is presented. This latter analyses allowed the estimation of hydraulic actions as well as the evaluation of possible hydro-geomorphologic scenarios. Subsequently, the complete results of the experimental tests are described and discussed. The modal parameters identified from the dynamic survey allowed validation of the numerical model, together with a more in-depth interpretation of the actual behaviour of the bridge. Moreover, the overall investigation allows to sound the presented multidisciplinary approach for the structural assessment of existing bridges.

Experimental activities on wind turbine models allow to speed up their design as well as the development of robust and effective control algorithms. Intending to have large information on load distribution on the blades, this work presents an experimental setup of a model wind turbine blade instrumented with two different types of optical fiber sensors (FOS). The first one is a traditional chain of Fiber Bragg Grating sensors, able to measure a large number of strain values; the second is based on Optical Backscatter Reflectometry (OBR), able to provide a continuous strain measurement. The experimental setup is calibrated to estimate load distribution along the blade and the two technologies are compared. The results of the experimental campaign suggest that optical fiber sensors could represent an interesting solution for real-time continuous load monitoring purposes.

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.

Two vertical-axis wind turbines (VAWTs) benefit from a power increase when placed side by side in close proximity. To study the potential of paired VAWTs for integration in wind farms, wind tunnel wake measurements of lift-driven VAWTs are compared for isolated and three counter-rotating configurations. Because the wake of an isolated VAWT is deflected, the direction of rotation significantly influences the wake of paired VAWTs. The wake of counter-rotating VAWTs where the adjacent blades move downwind, exhibits a similar length, width and replenishment as the wake of an isolated VAWT. The wake of counter-rotating VAWTs with adjacent upwind moving blades, however, significantly differs from an isolated VAWT wake. While its wake length is similar to an isolated VAWT wake, its width and replenishment are not. Because of appealing wake characteristics, paired VAWTs exhibit unique advantages for wind farm applications, and especially for offshore floating wind farms.

A key approach to large renewable power management is based on implementing storage technologies, including batteries, power-to-gas, and compressed air energy storage (CAES). This work presents the preliminary design and performance assessment of an innovative type of CAES, based on underwater compressed air energy storage (UW-CAES) volumes and intended for installation in the proximity of deep-water seas or lakes. The UW-CAES works with constant hydrostatic pressure storage and variable volumes. The proposed system is adiabatic, not using any fuel to increase the air temperature before expansion; a sufficient turbine inlet temperature (TIT) is instead obtained through a thermal energy storage (TES) system which recovers the compression heat. The system includes (i) a set of turbomachines (modular multistage compressor, with partial intercooling; expansion turbine); (ii) a TES system with different temperature levels designed to recover a large fraction of the compression heat, allowing the subsequent heating of air prior to the expansion phase; (iii) an underwater modular compressed air storage, conceived as a network of rigid but open tanks lying on the seabed and allowing a variable-volume and constant pressure operation. The compressor operates at variable loads, following an oscillating renewable power input, according to strategies oriented to improve the overall system dispatchability; the expander can be designed to work either at full load, thanks to the stability of the air flowrate and of the TIT guaranteed by the thermal storage, or at variable load. This paper first discusses in detail the sizing and off-design characterization of the overall system; then it simulates a case study where the UW-CAES is coupled to a wind farm for peak shaving and dispatchability enhancement, evaluating the impact of a realistic power input on performances and plant flexibility. Although the assessment shall be considered preliminary, it is shown that round-trip efficiency (RTE) in the range of 75–80% can be obtained depending on the compressor section configuration, making the UW-CAES a promising technology compared to electrochemical and pumped-hydrostorage systems. The technology is also applied to perform peak-shaving of the electricity production from an off-shore wind farm; annual simulations, based on realistic wind data and considering part-load operation, result in global RTE around 75% with a 10–15% reduction in the average unplanned energy injection in the electric grid. The investigated case study provides an example of the potential of this system in providing power output peak shaving when coupled with an intermittent and nonpredictable energy source.

This paper presents the methodological and technical aspects of the implementation of a hardware-in-the-loop experimental setup, suitable to simulate the dynamics of a floating system in wind tunnel experiments. The presented work is based on the specific application of a floating offshore wind turbine scale model, even if the approach holds also for floating systems in general, being subjected to the simultaneous action of wind and waves. The setup is based on a two degrees-of-freedom system, providing the motion of the floating system along surge and pitch. With reference to the implementation of the HIL test methodology the following topics are thoroughly analyzed: scaling issues, numerical issues, mechanical system characterization, force corrections, measurement, and signal processing. For each of these topics the methodological approach is presented, and the technical issues are described. A subset of results is then reported to show the effectiveness of such experimental apparatus, for different sea states: still water, regular, and irregular waves.

This paper presents the main results of experimental hybrid/HIL wind tunnel tests of the National Renewable Energy Laboratory (NREL) 5 MW wind turbine coupled with the DeepCwind semi-submersible floating platform. Experiments in still water and in irregular waves without wind are compared to corresponding FAST simulations in order to assess the effectiveness of the proposed HIL methodology and to highlight the main sources of uncertainty. Tests are repeated under three wind conditions, corresponding to different wind turbine operating points, to evaluate the effect of the aerodynamic force field on the overall system response. Experimental results are compared to the output of FAST simulations to investigate its prediction capability with respect to the influence of unsteady aerodynamic loads on the FOWT dynamics.

This paper is intended to present the authors’ know-how about hybrid/hardware-in-the-loop (HIL) wind tunnel testing of floating offshore wind turbines (FOWTs), that was acquired in the last decade through a very intense activity. Numerical codes commonly used within the scientific community to study FOWTs show significant uncertainties with respect to the prediction of some phenomena peculiar of these systems, like the rotor-wind dynamic interaction or the coupling between the machine controller and the low-frequency rigid-body motion modes of the platform. A hybrid/HIL methodology was developed by the authors to exploit the unique characteristics of the Politecnico di Milano wind tunnel to collect data useful to improve the existing knowledge about the above mentioned scientific questions.

The hybrid testing method developed by CENER for floating wind turbine scaled tests combining wind and waves (SIL) has been upgraded in order to introduce not only the wind turbine rotor thrust, but also the out-of-plane rotor moments (aerodynamic and gyroscopic). The former ducted-fan has been substituted by a multi-propellers actuator system. The new system has been completely developed, calibrated and used on a test campaign carried out at MARIN’s Concept Basin. It was installed on a 1/50 scaled model of the DeepCwind 5MW semisubmersible turbine built by MARIN within the EU MARINET2/Call No.3 under ACTFLOW project framework. The control strategy of the floating turbine was developed by POLIMI and TUDELFT and integrated into the SIL numerical model. The experiment has proved a good behaviour of the enhanced SiL method. It has revealed that the relative importance of gyroscopic moments is low in comparison with the aerodynamic rotor moments in the considered cases. The results also show how rotor moments are particularly important in the floating turbine dynamics in cases with large rotor load imbalances such as situations where one blade fails to pitch.

The design of control strategies for floating offshore wind turbines (FOWTs) is even more difficult than for onshore and bottom-fixed offshore ones and a recognized control strategy for FOWTs is currently lacking. In order to design effective control strategies, the additional dynamics of these systems should be taken into account in the models used to solve this task. This paper presents the analytical derivation of a novel model conceived for control design purposes. In detail, the model is based on a linear description of the highly non-linear phenomena that are relevant for an FOWT. The quasi-steady assumption is used to give a description of the aerodynamic loads and how these are influenced by the main control inputs. Hydrodynamic radiation and diffraction forces are introduced by means of linear-time-invariant parametric models. Simulation results shows that the proposed linear model is able to predict the structural response of the turbine system and the floating platform effectively in the case of control inputs, wind and wave disturbances. Compared to the nonlinear high-fidelity model, the proposed model shows similar results, however, without much complexity, which is promising in the desing of FOWT control strategies.

This work deals with an analytical linear coupled model describing the integrated aero-hydrodynamics of floating offshore wind turbines. Three degrees of freedom (platform surge, platform pitch and rotor azimuth) were considered with the goal of building a reduced-order model suitable for being integrated in control design algorithms as well as to be used for a straightforward evaluation and comprehension of the global system dynamics.

This paper presents the features and capabilities of a vertical axis wind turbine (VAWT) designed and built by the authors, which is meant to be the reference machine for open data code validation purposes under the project VODCA, VAWT Open Data for Code Assessment. The machine, whose design details are provided openly and summarized in the present document, has Individual Pitch Control (IPC) capability and it will be used for wind tunnel tests. Related results will be shared with participants who want to validate their own code, in terms of prediction of the aerodynamics or assessment of IPC strategies. The article reports a comprehensive overview of the state of the art of laboratory tests for VAWTs as well as the motivation of VODCA project, further to the need of code validation against experimental tests in controlled environment. The various phases of the project are also presented. Furthermore, the design of the carbon fiber blades, the structural components, as well as the mechatronics of the machine are summarized in this paper. Moreover, the experimental characterization of the machine's effective capabilities and properties, carried out after the completion of the building process, is reported and then the testing possibilities are defined and discussed.

Multi-purpose offshore structures are very complex systems to be designed as many different requirements have to be taken into account simultaneously. Experimental tests on scaled models can be very useful to verify structural behavior and to validate numerical models. The definition of the scale can play a key role in performing tests as high scales, closer to full-scale, are generally related to more reliable results. The present paper concerns the design of a wind turbine model for a large-scale model of a Multi-purpose offshore Platform developed within the EU project H2020 Blue Growth Farm. This project aims at developing an offshore farm, based on a modular floating structure that integrates wave energy converters and a wind turbine with aquaculture. The scale model of the complete structure will be deployed at the Natural Ocean Engineering Laboratory (NOEL). The paper will focus on the strategies adopted to scale 1:15 the DTU 10 MW wind turbine in order to represent both the main aeroelastic features and all the functionalities of a real machine. The complete design of the machine and its control will be also provided.

This study illustrates the aerodynamic and mechatronic design of a 1/75 scaled model of the DTU 10 MW wind turbine to perform wind tunnel tests in floating offshore configuration. Due to the strong discrepancy of the Reynolds number between full and model scale (up to 150), a dedicated low-Reynolds airfoil (SD7032) was chosen for the aerodynamic design of the blades, and the final shape was defined based on a dedicated optimisation algorithm which had as target the matching of the scaled thrust force and the first flap-wise bending frequency, as it is thoroughly explained in the study. Furthermore, the mechatronic design is reported in terms of the design choices adopted to get the best target-oriented functionalities to the model (i.e. individual pitch control, bandwidth) and to reduce as much as possible the weights, greatly affecting the aero-elastic scaling. The results gathered during experimental campaigns at Politecnico di Milano wind tunnel, are reported confirming the validity of the design and manufacturing choices.

Model testing of offshore structures has been standard practice over the years and is often recommended in guidelines and required in certification rules. The standard objectives for model testing are final concept verification, where it is recommended to model the system as closely as possible, and numerical code calibration. Model testing of floating offshore wind turbines is complex due to the response depending on the aero-hydro-servo-elastic system, but also due to difficulties to perform model tests in a hydrodynamic facility with correctly scaled hydrodynamic, aerodynamic and inertial loads. The main limitations are due to the Froude-Reynolds scaling incompatibility, and the wind generation. An approach to solve these issues is by use of hybrid testing where the system is divided in a numerical and a physical substructure, interacting in real-time with each other. Depending on the objectives of the model tests, parts of a physical model of a FOWT can then be placed in a wind tunnel or an ocean basin, where the rest of the system is simulated. In the EU H2020 LIFES50+ project, hybrid model tests were performed in the wind tunnel at Politecnico di Milano, as well as in the ocean basin at SINTEF Ocean. The model tests in the wind tunnel were performed with a physical wind turbine positioned on top of a 6DOF position-controlled actuator, while the hydrodynamic loads and the motions of the support structure were simulated in real-time. For the tests in the ocean basin, a physical floater with tower subject to waves and current was used, while the simulated rotor loads were applied on the model by use of a force actuation system. The tests in both facilities are compared and recommendations on how to combine testing methodologies in an optimal way are discussed.

A key approach to large renewable power management is based on implementing storage technologies, including batteries, power-to-gas and compressed air energy storage (CAES). This work presents the preliminary design and performance assessment of an innovative type of CAES, based on underwater storage volumes (UW-CAES) and intended for installation in the proximity of deep water seas or lakes. The UW-CAES works with constant hydrostatic pressure storage and variable volumes. The proposed system is adiabatic, not using any fuel to increase the air temperature before expansion; a sufficient TIT is instead obtained through a thermal energy storage system which recovers the compression heat. The system includes (i) a set of turbomachines (modular multi-stage compressor, with partial intercooling; expansion turbine); (ii) a thermal energy storage (TES) system with different temperature levels designed to recover a large fraction of the compression heat, allowing the subsequent heating of air prior to the expansion phase; (iii) an underwater modular compressed air storage, conceived as a network of rigid but open tanks lying on the seabed and allowing a variable-volume and constant pressure operation. The compressor operates at variable loads, following an oscillating renewable power input, according to strategies oriented to improve the overall system dispatchability; the expander can be designed to work either at full load, thanks to the stability of the air flow rate and of the TIT guaranteed by the thermal storage, or at variable load. The paper first discusses in detail the sizing and off-design characterization of the overall system; it is then simulated a case study where the UW-CAES is coupled to a wind farm for peak shaving and dispatchability enhancement, evaluating the impact of a realistic power input on performances and plant flexibility. Although the assessment shall be considered preliminary, it is shown that round trip efficiency in the range of 75%–80% can be obtained depending on the compressor section configuration; making the UW-CAES a promising technology compared to electrochemical and pumped-hydro storage systems. The technology is also applied to perform peak-shaving of the electricity production from a wind park; annual simulations considering part load operation result in global round trip efficiency around 75% with a 10 to 15% reduction in the average unplanned energy injection in the electric grid. The investigated case study provides an example of the potential of this system in providing power output peak shaving when coupled with an intermittent and non-predictable energy source.

During the last decades, a particular attention has been paid to the aerodynamic optimization of professional bike riders and their equipment, with a large use of experimental investigation as well as numerical simulations. Extensive studies have been carried out in wind tunnels all over the world by professional cycling teams to study the best set-up for time trial competitions. The standard approach is to compare different parts of the equipment or different positions and find out the optimized configuration by a trial and error procedure. This paper presents an attempt of helmet design based on CFD calculations and experimental validation of numerical results with a new experimental set-up able to measure the surface pressure on a real helmet.

The paper presents the complete activity carried out on two light and slender arched structures in twin arrangement: the two arches built for Expo 2015 in Milan. In particular, this study represents a rare occasion of comprehensive measurements of different types and scales on the same structure: this highlights the relevance of a structured and comprehensive approach, from wind tunnel to full-scale testing, as a valuable support to design, construction and maintenance of wind-sensitive constructions. These two structures were considered potentially critical, from an aerodynamic point of view, due to their rectangular cross-wind section, that can be prone to instability and VIV problems, and to their low frequencies and structural damping that can be easily excited by the wind. Wind tunnel tests performed at Politecnico di Milano wind tunnel, on aeroelastic models of the two arches confirmed the occurrence of galloping dynamic instability at wind speeds lower than the design value and gave indication concerning the required structural damping to control the phenomenon. This information was used in the design process of the TMD (Tuned Mass Damping) system used on the structures to suppress the vibrations. The behaviour of the real structures was then experimentally verified in order to check their structural parameters (frequencies, damping and modal shapes) with respect to the expected ones calculated using a FEM model, and to verify the proper behaviour of the designed TMD system. The data coming from the modal identification of the structures permitted also a final tuning of the TMDs. The two arches were then monitored for several months to observe their dynamic behaviour under different wind conditions. A wide database was gathered, collecting the strongest and most frequent environmental occurrences in the field. The accelerations registered during the considered period did not reach dangerous levels for the structures, in agreement with the expected results obtained from wind tunnel tests.

This paper describes the innovative experimental approach, introduced by the authors in the framework of the EU H2020 project LIFES50+, to perform scale model tests on floating offshore wind turbines (FOWTs). A 6-DOFs hardware-in-the-loop (HIL) system was designed and realized to reproduce the global FOWT response to wind and waves in the atmospheric boundary layer (ABL) of the Politecnico di Milano (PoliMi) wind tunnel. A 2-DOFs (surge and pitch) HIL system was used to perform preliminary tests, assessing the capabilities of the hybrid experimental methodology, and to gather data for the finalization of the 6-DOFs setup. Results from the first experimental campaign are discussed, showing the effect of aerodynamic loads on the coupled FOWT response.

The Flettner rotor is attracting increasing attention as a viable technology for wind-assisted ship propulsion. Nonetheless, the influence of the Reynolds number on the aerodynamic performance of rotating cylinders is still unclear and under debate. The present study deals with a series of wind-tunnel experiments on a large-scale Flettner rotor in which the forces and pressures acting on the cylinder were measured for Reynolds numbers as large as R e = 1 . 0 ⋅ 10 6 . The rotating cylinder used in the experimental campaign had a diameter of 1.0 m and span of 3.73 m. The results indicate that the lift coefficient is only affected by the Reynolds number in the critical flow region and below velocity ratio k = 2 . 5 . Conversely, in the velocity ratio range 1 k ≤ 2 . 5 , the drag coefficient is markedly influenced by the Reynolds number over the entire range of flow conditions analyzed. The power coefficient scales with the cube of the tangential velocity and it appears to be insensitive to the Reynolds number or whether the cylinder is spun in an air stream or in still air.

This article presents a hardware-in-the-loop (HIL) methodology developed at Politecnico di Milano (PoliMi) to perform wind tunnel tests on floating offshore wind turbines (FOWTs). The 6-DOFs HIL setup is presented, focusing on the main differences with respect to a previous 2-DOFs system. Aerodynamic, rotor and control related loads, physically reproduced by the wind turbine scale model, must be measured in real-time and integrated with the platform numerical model. These forces contribute to couple wind turbine and floating platform dynamics and their correct reproduction is of fundamental importance for the correct simulation of the floating system behavior. The procedure developed to extract rotor loads from the available measurements is presented, discussing its limitations and the possible uncertainties introduced in the results. Results from verification tests in no-wind conditions are presented and analyzed to identify the main uncertainty sources and quantify their effect on the reproduction of the floating wind turbine response to combined wind and waves.

Experiments on a large-scale Flettner rotor were carried out in the boundary-layer test section of Politecnico di Milano wind tunnel. The rotating cylinder used in the experimental campaign (referred to as Delft Rotor) had a diameter of 1.0 m and span of 3.73 m. The Delft Rotor was equipped with two purpose-built force balances and two different systems to measure the pressure on the rotor’s outer skin. The goal of the experiments was to study the influence of different Reynolds numbers on the aerodynamic forces generated by the spinning cylinder. The highest Reynolds number achieved during the experiments was \( {\text{Re}} = 1.0 \cdot 10^{6} \).

In this paper, a rare example of comparison between sectional and full-aeroelastic model tests is presented. Interestingly, the experiments were conducted in two very different wind tunnel facilities by different research teams. The study concerns two long-span steel arch structures recently built in Milan, Italy, for Expo 2015 World Fair. The structures have only aesthetic purposes and are therefore very flexible and light, which makes them sensitive to wind-induced excitation and prone to aeroelastic instabilities. In particular, in smooth flow an interesting phenomenon of interference between vortex-induced vibration and galloping was observed up to high values of the Scruton number. This aeroelastic instability is very dangerous as large-amplitude vibrations can occur in wind speed ranges where they are not expected, at least for what classical theories for vortex-induced vibration and quasi-steady galloping are concerned. Moreover, the provisions of Eurocode 1 resulted clearly unsuitable and non-conservative to address such a phenomenon. Despite the differences in the facilities and in the models, a good agreement was found between the results obtained in the two laboratories. The major discrepancies were observed in the transitional behavior for intermediate values of the Scruton number, the sectional model showing a more unstable behavior. The tests on the full-aeroelastic model also allowed considering the effect of the angle of wind exposure of the structures, both the in-plane and the out-of-plane vibrations of the arches and the dynamic response to turbulent wind. In particular, a set of tests in smooth flow was performed accounting for the presence of the other arch and of the surrounding buildings. A particular dynamic excitation of the in-plane flexural modes of the structures was observed in well defined ranges of flow speeds when one arch is in the wake of the other. Finally, both experimental campaigns highlighted the need for the installation of tuned mass dampers on the real structures to guarantee their safety. The effectiveness of these devices against the observed galloping-type instability was also verified through wind tunnel tests on the full-aeroelastic model.

This paper presents a new hardware-in-the-loop methodology for wave-basin scale-model experiments about floating offshore wind turbines and its application as a tool for the validation of control strategies. In the hardware-in-the-loop experiments, the physical Froude-scaled wind turbine model used in conventional scale-model tests is replaced by a numerical model, measurements and a multi-fan actuator. As usual, properly-scaled waves are generated in the wave basin and the floating platform is simulated by means of a scale-model. The hardware-in-the-loop methodology was used to recreate the interaction between the collective pitch controller and the platform pitch mode that, often observed in numerical studies. In addition, the blade-root load measurement available in the numerical model of the rotor was used to implement an individual pitch control strategy. Different from in conventional experiments, the hardware-in-the-loop methodology allows to recreate a realistic three-dimensional wind field that was used to demonstrate the effectiveness of the individual pitch control. The improved emulation of the rotor loads and wind field make the hardware-in-the-loop experimental methodology an effective tool for the development and validation of control strategies for floating offshore wind turbines.

Floating offshore wind turbines allow wind energy to be harvested in deep waters. However, additional dynamics and structural loads may result when the floating platform is being excited by wind and waves. In this work, the conventional wind turbine controller is complemented with a novel linear feedforward controller based on wave measurements. The objective of the feedforward controller is to attenuate rotor speed variations caused by wave forcing. To design this controller, a linear model is developed that describes the system response to incident waves. The performance of the feedback-feedforward controller is assessed by a high-fidelity numerical tool using the DTU 10MW turbine and the INNWIND.EU TripleSpar platform as references. Simulations in the presence of irregular waves and turbulent wind show that the feedforward controller effectively compensates the wave-induced rotor oscillations. The novel controller is able to reduce the rotor speed variance by 26%. As a result, the remaining rotor speed variance is only 4% higher compared to operation in still water., Comment: Initial submission to the Science of Making Torque from Wind (TORQUE) 2020 conference, 10 pages, 7 figures

Closely spaced counter-rotating vertical-axis wind turbines (VAWTs) exhibit a considerable power improvement compared to the same turbines in isolation. It thus makes sense to study their potential for wind farm production optimisation. With this objective in mind, the wake of an isolated VAWT is compared experimentally to the wake of counter-rotating VAWTs. Because of the unsteady aerodynamics, the wake of an isolated VAWT deflects towards the region behind its upwind moving blade. The direction of rotation thus directly influences the deflection of the wake. For paired configurations, it is possible to use this wake deflection as an advantage. A pair of counter-rotating VAWTs where the upwind moving blades are at the centre of the pair, exhibits a particularly narrow wake. With the benefit of a power improvement and a narrow wake, closely spaced counter-rotating VAWTs exhibit promising characteristics to improve the power density in a wind farm.