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Volterra series models are considered an attractive approach for modelling nonlinear aerodynamic forces for bridge decks since they extend the convolution integral to higher dimensions. Optimal identification of nonlinear systems is a challenging task since there are typically many unknown variables that need to be determined, and it is vital to avoid overfitting. Several methods exist for identifying Volterra kernels from experimental data, but a large class of them put restrictions on the system inputs, making them infeasible for section model tests of bridge decks. A least-squares identification method does not restrict the inputs, but the identified model often struggles with noisy (non-smooth) kernels, which is deemed to be unphysical and a sign of overfitting. In this work, regularised least-squares identification is introduced to improve the performance of model identification using least-squares. Standard Tikhonov regularisation and other penalty techniques that impose decaying kernels are also explored. The performance of the methodology is studied using experimental data from wind tunnel tests of a twin deck section. The regularised Volterra models show equal or better results in terms of modelling the self-excited forces, and the regularisation makes the models less prone to overfitting.

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. This research has been supported by the office of Energy Efficiency and Renewable Energy (grant no. DE-AC36-08GO28308). Peer Reviewed Roger Bergua1, Amy Robertson1, Jason Jonkman1, Emmanuel Branlard1, Alessandro Fontanella2, Marco Belloli2, Paolo Schito2, Alberto Zasso2, Giacomo Persico3, Andrea Sanvito3, Ervin Amet4, Cédric Brun5, Guillén Campaña-Alonso6, Raquel Martín-San-Román6, Ruolin Cai7, Jifeng Cai7, Quan Qian8, Wen Maoshi8, Alec Beardsell9, Georg Pirrung10, Néstor Ramos-García10, Wei Shi11, Jie Fu11, Rémi Corniglion12, Anaïs Lovera12, Josean Galván13, Tor Anders Nygaard14, Carlos Renan dos Santos14, Philippe Gilbert15, Pierre-Antoine Joulin15, Frédéric Blondel15, Eelco Frickel16, Peng Chen17, Zhiqiang Hu17, Ronan Boisard18, Kutay Yilmazlar19, Alessandro Croce19, Violette Harnois20, Lijun Zhang21, Ye Li21, Ander Aristondo22, Iñigo Mendikoa Alonso22, Simone Mancini23, Koen Boorsma23, Feike Savenije23, David Marten24, Rodrigo Soto-Valle24, Christian W. Schulz25, Stefan Netzband25, Alessandro Bianchini26, Francesco Papi26, Stefano Cioni26, Pau Trubat27, Daniel Alarcon27, Climent Molins27, Marion Cormier28, Konstantin Brüker28, Thorsten Lutz28, Qing Xiao29, Zhongsheng Deng29, Florence Haudin30, and Akhilesh Goveas31 1National Wind Technology Center, National Renewable Energy Laboratory, Golden, CO 80401, USA 2Department of Mechanical Engineering, Politecnico di Milano, Milan 20156, Italy 3Laboratory of Fluid-Machines, Dipartimento di Energia, Politecnico di Milano, Milan 20156, Italy 4Wind Department, Bureau Veritas, Paris 92937, France 5Marine Division, Research Department, Bureau Veritas, Saint-Herblain 44818, France 6Wind Turbine Technologies, Centro Nacional de Energías Renovables, Sarriguren 31621, Spain 7Integrated Simulation Department, China General Certification Center, Beijing 100013, China 8Research Institute, China State Shipbuilding Corporation, Chongqing 401122, China 9Offshore Technology Department, DNV, Bristol BS2 0PS, UK 10Department of Wind Energy, Technical University of Denmark, Lyngby 2800, Denmark 11State Key Laboratory of Coastal and Offshore Engineering, Dalian University of Technology, Dalian 116024, China 12Département Electrotechnique et Mécanique des Structures, Électricité de France, Paris 91120, France 13Wind Energy Department, eureka!, Errigoiti 48309, Spain 14Department of Wind Energy, Institute for Energy Technology, Kjeller 2027, Norway 15Département Mécanique des Fluides, IFP Energies nouvelles, Rueil-Malmaison 92852, France 16Research and Development, Maritime Research Institute Netherlands, Wageningen 6708, the Netherlands 17Marine, Offshore and Subsea Technology, Newcastle University, Newcastle NE1 7RU, UK 18Aerodynamic Department, Office National d’Etudes et de Recherches Aérospatiales, Paris 92190, France 19Department of Aerospace Science and Technology, Politecnico di Milano, Milan 20156, Italy 20Floating Offshore Group, PRINCIPIA, La Ciotat 13600, France 21Wind Energy Group, Shanghai Jiao Tong University, Shanghai 200240, China 22Department of Offshore Renewable Energy, Tecnalia Research & Innovation, Donostia-San Sebastián 20009, Spain 23Wind Energy Department, Netherlands Organisation for Applied Scientific Research, Petten 1755, the Netherlands 24Wind Energy Department, Technische Universität Berlin, 10623 Berlin, Germany 25Institute for Fluid Dynamics and Ship Theory, Hamburg University of Technology, 21073 Hamburg, Germany 26Department of Industrial Engineering, University of Florence, Florence 50139, Italy

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.

Currently, the energy and environmental efficiency of buildings has led to the development of cladding systems that may help to reduce the structure’s energy demand, using techniques such as the Permeable Double Skin Façade (PDSF). Given complex aerodynamic interactions, the presence of an external porous screen in addition to an inner skin may play a crucial role in the fluid-dynamic characterization of such buildings, making the definition of wind effects very complex. A new methodology for the quantitative assessment of the impact of wind-loading conditions on this particular type of cladding is presented. It is based on a combined experimental–numerical approach, essentially based on wind-tunnel tests on a rigid scale model and computational fluid dynamic simulations. A case study is proposed as an application of this methodology. Results include the design pressure values for the inner glazed façade and the permeable facade. An estimation of the flow rate across the porous skin is quantified using the numerical model.

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 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.

This paper introduces a new methodology to assess the influence of a windscreen on the crosswind performance of trains running on a bridge. Considering the difficulties encountered in both carrying out wind tunnel tests that consider the vehicle speed or complete CFD analyses, a simplified CFD approach is here discussed. Instead of simulating simultaneously the windscreen plus the moving train, the numerical problem is split into two parts: firstly, a simulation of the windshield alone is used to extract the perturbed velocity profile at the railway location; secondly, this profile used as an inlet condition for the wind velocity acting on an isolated train. The method is validated against a complete train plus windshield simulation in terms of pressure distribution and aerodynamic force coefficients on the train, and flow streamlines. This approach opens to the possibility of evaluating the aerodynamic performance of a vehicle on bridges considering bridge and vehicle as separated. Wind velocity profiles measured on the bridge during a wind tunnel campaign could be used as the initial condition for numerical simulations on vehicles.

The 15 Early Stage Researchers (ESRs) in the LIKE project investigate topics in which wind lidar play a significant role. This report provides the ESRs an introductory reading and gives a short introduction into the basic principles, as well as an overview on the practical application of lidar wind measurement technology for a wide range of research fields, including a corresponding literature review. Wherever possible, it will also give the ESRs recommendations on the use of lidars and related best practices and provide corresponding state-of-the-art documents in the attachment.

The disruptive potential of floating wind turbines has attracted the interest of both the industry and the scientific community. Lacking a rigid foundation, such machines are subject to large displacements whose impact on aerodynamic performance is not yet fully explored. In this work, the unsteady aerodynamic response to harmonic-surge motion of a scaled version of the DTU 10 MW turbine is investigated in detail. The imposed displacements have been chosen representative of typical platform motion. The results of different numerical models are validated against high-fidelity wind tunnel tests specifically focused on the aerodynamics. Also, a linear analytical model relying on the quasi-steady assumption is presented as a theoretical reference. The unsteady responses are shown to be dominated by the first surge harmonic, and a frequency domain characterization, mostly focused on the thrust oscillation, is conducted involving aerodynamic damping and mass parameters. A very good agreement among the codes, the experiments, and the quasi-steady theory has been found, clarifying some literature doubts. A convenient way to describe the unsteady results in a non-dimensional form is proposed, hopefully serving as a reference for future works.

The disruptive potential of floating wind turbines has attracted the interest of both industry and scientific community. Lacking a rigid foundation, such machines are subject to large displacements whose impact on the aerodynamic performance is not yet fully acknowledged. In this work, the unsteady aerodynamic response to an harmonic surge motion of a scaled version of the DTU10MW turbine is investigated in detail. The imposed displacements have been chosen representative of typical platform motions. The results of different numerical models are validated against high fidelity wind tunnel tests specifically focused on the aerodynamics. Also a linear analytical model, relying on the quasi-steady assumption, is presented as a theoretical reference. The unsteady responses are shown to be dominated by the first surge harmonic and a frequency domain characterization, mostly focused on the thrust oscillation, is conducted involving aerodynamic damping and mass parameters. A very good agreement among codes, experiments and quasi-steady theory has been found clarifying some literature doubts. A convenient way to describe the unsteady results in non-dimensional form is proposed, hopefully serving as reference for future work.

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 high-efficiency facades, such as porous double skins, have become increasingly popular due to the recent technological progress in architecture. The so-called porous double skin facade (DSF) systems, which are constituted by a permeable layer over a closed inner facade, are often adopted to reduce the system energy demand. However, as expected, the porous skin alters the wind-induced pressures acting on the inner facade. Therefore, the cladding loads for such a facade system has to be accurately estimated performing wind tunnel tests. Using the low-rise buildings of the New Bocconi Campus as a case study, we present the experimental wind tunnel methodologies utilized to assess the wind-induced peak pressures acting on the inner glazed skin of the porous double skin facade system designed for the case at hands. In particular, the reduction of both the positive and negative peak pressures estimated for the inner facade is addressed when comparing the standard facade to the porous DSF case. In addition, the valuable data set of the pressure signals acquired for the porous DSF system studied, allows one to investigate the dependence of the computed peak pressures on the averaging time utilized for the extreme value estimates.

Wind tunnel testing represents an established technique for the assessment of wind-induced pressure on cladding systems. Nonetheless, some physical events, such as the pressure peaks that occur on a building’s lateral facades, are not fully understood. To enable detailed analysis of the nature of these pressure peaks, we performed high-resolution pressure measurements on a high-rise building in two different wind tunnels: the closed-circuit wind tunnel of Politecnico di Milano, and the open-circuit Wall of Wind facility at Florida International University. The objective of the paper is to present the experimental set-up and the high-resolution pressure data, and to investigate the characteristics of the extreme suction events at individual pressure taps and their relevance for cladding design. We first compare the two atmospheric boundary layers, and subsequently present the pressure coefficients statistics. Then, we present probability density functions of the local and area-averaged pressure coefficients and visualize the space-time characteristics of two peak events to investigate their relevance for cladding design. The experiments provide consistent results and exhibit two types of suction events: one is characterized by an extremely short duration and spatial extension, while the other impacts a larger portion of the facade.

In the last decade, the bridge aerodynamics community has pivoted towards the investigation and prediction of nonlinear aerodynamic phenomena. Several nonlinear models have been suggested, but the research community has yet to conclude on the best types of models for the wide range of nonlinearity observed in the research. Multiple authors have indicated that Volterra models show promise in modeling nonlinear bridge aerodynamics. However, efficient data-driven identification of the Volterra model is challenging. A Laguerrian expansion basis is introduced in this work to alleviate the identification issue. The proposed method improves the identification of the Volterra models, leading to a reduction in computational effort and simplification of the least-squares problem by parameterizing the kernels. The method is also more robust to noise and small perturbations in the experimental data, leading to smooth, decaying kernels that are interpretable in a physical context. A relevant theoretical example is given to evaluate the applicability of the technique in bridge aerodynamics. Furthermore, the method is used to identify 1st− to 4th-order Volterra models on the experimental data of one degree of freedom and two degrees of freedom self-excited forces. The study shows that the technique can identify Volterra models with high fidelity when one degree of freedom and most two degrees of freedom motion data are considered. The model struggles slightly when considering the noisiest and most nonlinear two degrees of freedom scenarios for self-excited drag force.

This paper illustrates the methodology and the experimental verification of the design of a 1/75 aero-elastic scaled rotor of the DTU 10 MW reference wind turbine for wind tunnel tests. The aerodynamic design was focused on the minimization of the difference, in terms of thrust coefficient, with respect to the full scale reference. From the Selig low-Reynolds airfoils database, the SD7032 one was chosen for this purpose and a corresponding constant section wing was tested at DTU red wind tunnel, providing force and distributed pressure coefficients for the design, in the Reynolds range 30 − 250 × 10 3 and for different angles of attack. The aero-elastic design algorithm was set to define the optimal spanwise thickness over chord ratio (t/c), the chord length and the twist, in order to match at least the first flapwise scaled natural frequency. An aluminium mould for the carbon fibre autoclave process was CNC manufactured based on B-Splines CAD definition of the external geometry given as an output of the design procedure. Wind tunnel tests at were carried out Politecnico di Milano on the whole 1/75 wind turbine scale model, confirming the successful aerodynamic design and manufacturing approaches. The experimental modal analysis carried out to verify the structural consistency of the scaled blade is also reported.

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.

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.

Wind tunnel testing represents an established technique for the assessment of wind-induced pressure on cladding systems. Nonetheless, some physical events, such as the strong negative peaks of pressure that occur on a building’s lateral facades, are not fully understood. These events can be catastrophic for cladding systems, which motivates their further investigation. The objective of the present work is to study the nature of extreme suction events, by comparing high-resolution pressure tap measurements on a high-rise building collected at two different facilities: the closed-circuit wind tunnel of Politecnico di Milano, and the open-circuit Wall of Wind facility at Florida International University. We first compare the statistics of the two atmospheric boundary layers, and subsequently present results for the mean and root mean square pressure distributions and negative peak pressure events. The experiments provide consistent results, and they both exhibit two types of negative peak pressure events: one is characterized by an extremely short duration and spatial extension, while the other impacts a larger portion of the facade.

Permeable double skin facades (permeable DSF) are part of modern architectural concepts. An accurate assessment of the wind loads and the porosity effects on this kind of facades is crucial for a correct design and performance evaluation. Measuring and predicting DSF airflow is not a straightforward task due to the interaction between the wind turbulence and the outer porous skin. Moreover, Eurocode and many other National Codes do not supply any prescription about such issue. In this paper the comprehensive experimental study of wind loads acting on the porous double skin facade of the New Bocconi Campus, currently under construction in Milan, is reported. Cladding loads assessment has been carried out performing wind tunnel tests on properly scaled rigid models able to reproduce the aerodynamic behaviour of the porous medium. Peak pressures for the inner facade have been estimated and compared to the ones expected forcing a standard glazed facade.

Nine research teams organized a round-robin measurement campaign of the wake of two porous discs in a homogeneous and “low-turbulent’ flow. Mean streamwise velocity and turbulence intensity profiles at four diameters downstream of the discs were measured and compared through such metrics as the maximum of velocity deficit, the maximum of turbulence intensity, the wake width and the thrust coefficient. The dependence of these metrics onthe inflow conditions (freestream turbulence intensity and Reynolds number based on the disc diameter) is discussed.

Bridges crossing steep valleys in mountainous regions may be subjected to large angles of attack of the incoming wind flow, because of the complex orography of the region where the structure is located. Therefore, the aerodynamic analysis of bridges in this kind of regions should account for this peculiarity. In this paper, the aerodynamic characterization of a bluff deck section of a railway bridge, with angles of attack ranging from −6 to +6° is discussed. It is shown that standard analyses that consider only angles of attack near 0° only are not sufficient to ensure optimal aerodynamic performances. Indeed, with such large angles of attack the flow field near the deck may change consistently, also leading to aeroelastic instabilities or strong vortex-induced vibrations. To mitigate the aeroelastic issues, without an abrupt change of the deck section, several aerodynamic appendices were tested, and a tailored section with acceptable behavior was selected.

Design of large modern structures can be strongly affected by wind actions, in terms of dynamic loading as well as peak facade cladding load. The wind forcing can be described by means of CFD analysis as well by wind tunnel tests using a scaled model instrumented with pressure taps. The density and the distribution of pressure taps on the building surface is critical, as well as the averaging of the surface pressure time history for the definition of the wind actions. Dynamic actions are calculated using a modal approach, while pressure excitation is computed according to time equivalent area averaging. The definition of the time equivalent area averaging is carefully analysed, in order to provide pressure values consistent with the forcing area.

This paper presents the results of sectional wind tunnel studies of the Ewijk bridge, a 270 m span cable-stayed bridge crossing over the river Waal in Netherlands, which during its refurbishment was doubled with a close parallel bridge. The aerodynamics of parallel bridges may be complex and unpredictable, therefore specific wind tunnel tests were conducted. The tests were performed with sectional models in a 1:50 scale with the aim of identifying the aerodynamic interference effects between the two decks, in terms of aeroelastic stability, vortex-induced vibrations, and stationary aerodynamic loads. Several configurations were tested, since during various stages of refurbishment, the profile, the eigen-frequencies, the mass, and the relative position of the deck change. Results show that for the considered case, the interference effect is characterized by a significant change in the steady aerodynamic coefficients, vortex-induced vibrations of the upstream box-girder and forcing of the downstream bridge section, and no stability problems.

Wake models play a fundamental role in finding optimized solutions in wind farm control. In fact, they allow assessing how wakes develop and interact with each other with the agility required for real-time applications. In this paper, a Gaussian Wake Model (GWM) is implemented in the OpenFAST framework in a way such that its fidelity is increased with respect to previously implemented models, while enhancing its compatibility with control purposes. The OpenFAST tool is coupled with Floris, NREL’s software based on the GWM, in order to simulate the wake effect on downstream machines (in the case where the downstream rotor is fully covered by the wake, only partially covered by the wake, of the wake is generated by the interaction of more than one turbine), while the rotor aerodynamics is calculated using the BEMT on the actual rotor flow field. We intend this work as a starting point for developing and testing open/closed-loop control logics that will work in real wind farms. To show the suitability of the implementation, the entire model is then compared to Floris.

This work presents a selected set of results from an extensive experimental campaign aimed at characterizing the wake aerodynamics of a floating offshore wind turbines (FOWT) in dynamic conditions. The campaign was carried out in the Politecnico di Milano 14m x 4m atmospheric boundary layer test section, under UNAFLOW project (UNsteady Aerodynamics of FLoating Wind turbines). Wake measurements were carried out either through hot-wire anemometers with a traversing system as well as through Particle Image Velocimetry (PIV) technique. A 1/75 scale model of the DTU 10 MW reference wind turbine was adopted, which was mounted on a hydraulically actuated mechanism imposing sinusoidal surge motion at different frequencies and amplitudes, and for different tip speed ratios λ. Selected results are reported with the aim of giving a first interpretation of the novel set of measurements, also completing previous measurements conducted by the authors, to put the basis of a more comprehensive understanding of unsteady aerodynamics of FOWT for control and wind farm purposes.

The evaluation of the performance of a Darrieus Vertical Axis Wind Turbine is performed using an actuator line code. This allows to investigate the aerodynamics of the turbine and the interaction with the boundary conditions. Blade Pitch control, tip-effects modelling and blockage effects in an open-jet wind tunnel test section are studied and investigated. Pitch control allows to enhance the power extraction, tip effects allow to generate a more realistic model, while the modelling of the open-jet test section allows to validate the numerical approach on experimental data intrinsically characterized by not negligible blockage effects.

In the last ten years, four measurement campaigns were performed at Politecnico di Milano on two Darrieus Vertical Axis Wind Turbines (VAWT) for micro-generation of different architecture (H-shaped vs troposkien), but sharing the blade number (3), the blade profile (NACA 0021), and the swept area (1.5 m2). The experiments, carried out in the large-scale wind tunnel of Politecnico di Milano, included detailed wake measurements. This paper presents a review of the research activities related to velocity and turbulence measurements in the wake, proposing an analysis of both the technical aspects and the scientific outcomes of the investigation. In particular, the wakes of these turbines were measured on several surfaces downstream of the rotors for different tip speed ratios and different Reynolds numbers, searching for corresponding conditions between the two rotors. The paper first presents the technical issues involved in measuring the flow velocity in the wake of VAWT rotors with intrusive techniques such as hot wire anemometers and pressure probes. The second part of the paper proposes a comprehensive analysis of the wakes shed by the tested models. The wakes appear asymmetric and roughly follow the shape of the rotor, their width and velocity deficit being strongly dependent on the tip speed ratio. Flow angle measurements show the onset of large-scale tip vortices, for both the H-shape and the troposkien rotors, even though resulting from different aerodynamic mechanisms in the two architectures. A discussion on the impact of the wake features on the implementation of VAWTs in the urban environment concludes the paper.

UNAFLOW (UNsteady Aerodynamics for Floating Wind) is a joint EU-IRPWIND founded experiment on wind turbine rotor unsteady aerodynamics. It brings together four different academic contributors: Energy research Centre of the Netherlands (ECN), DTU Wind Energy, University of Stuttgart (USTUTT) and Politecnico di Milano (PoliMi) sharing knowledge both in numerical modelling and in experimental tests design, allowing direct numerical and experimental comparison. The experimental tests carried out for UNAFLOW are of the same type of the ones carried out during the ongoing EU H2020 project LIFES50+ [1], regarding both the unsteady behaviour of the 2d blade section and the entire turbine rotor, although with improved setup and wider test matrix. The project partners are already currently jointly collaborating in the AVATAR project [2], developing and validating numerical models of different accuracy level. The numerical models used in the UNALFOW project range from engineering tool (eg. BEM) to high fidelity CFD methods. Numerical simulations are used both in the design of experiment phase and in the results analysis allowing for an in depth understanding of the experimental findings through advanced modelling approach. The UNAFLOW project, together with a new understanding of the unsteady behaviour of the turbine rotor aerodynamics, will provide also an open database to be shared among the scientific community for future analysis and new models validation.

Summary This paper examines the aerodynamic properties of a rectangular tall building with sharp edges. The building has a section aspect ratio B/D = 2.6 and a height aspect ratio H/D = 9, giving in a quite slender prismatic configuration. The study was performed on sectional and three-dimensional models of the building. The sectional model was tested in a smooth flow condition, and the aerodynamic coefficients of the section were determined. Forces and pressure distributions were measured on the three-dimensional model to compare with the aerodynamic behavior of the sectional model. Both a rigid (non-moving) and a moving model of the building were tested to analyze possible aeroelastic effects. Different levels of incoming wind turbulence were tested, since this is a key parameter in the aerodynamic behavior of the rectangular prism. Copyright © 2015 John Wiley & Sons, Ltd.

This paper proposes a formulation for the assessment of the unsteady aerodynamics of floating offshore wind turbines, based on wind tunnel experiments through surge and pitch imposed motions on the 1/75 DTU 10 MW scale model. Rotor thrust and torque were analysed out of a set of different combinations of amplitudes and frequencies of the imposed mono-harmonic motion, following the idea of splitting these forces into steady and unsteady contributions, respectively through steady and unsteady aerodynamic coefficients. The latter were analysed, for different tip-speed ratios, both experimentally and numerically, with respect to a newly introduced parameter, the “wake reduced velocity”, which turned out to be effective in the description of the unsteady regime. Experimental results have shown good consistency of the formulation and put the basis for further studies on this topic, for the comprehension of this phenomenon and for the development of reduced-order models for control purposes, with the focus of the global system dynamics.

The structure studied in this paper is a 85 meters high tapered obelisk, made of reinforced concrete for the first 9 meters and of steel welded plates for the upper 76 meters. Its bluff cross section consists of two closely spaced triangular sections that are scaled linearly with the height from ground. Having a complex bluff section and being a light structure, aerodynamic instabilities and vortex induced vibrations were investigated using a mixed experimental-numerical approach, in order to define wind loads for the structural design, and possible interventions aimed at mitigating the dynamic effects due to wind action. Exploiting the regular tapering of the obelisk section, a rigid sectional model with constant section was initially tested in wind tunnel: static aerodynamic coefficients and vortex-induced response as a function of the Scruton number were measured for several angles of attack. The experimental sectional results were successively used to assess numerically the response of the structure considering tapering, the gradient of wind speed, the modal shapes, and the correlation of wind actions. Even though the numerical analysis showed that the tapering drastically reduces aerodynamic issues, the achieved results underlined the pressing need to increase the damping of the structure in order to avoid galloping and vortex induced vibrations for wind speeds within the design range; to this aim tuned mass dampers were designed.

This paper reports the results of a wake measurement campaign carried out at Politecnico di Milano wind tunnel to support current studies of the authors about unsteady aerodynamics of floating offshore wind turbines (FOWT) as well as evaluating experimental evidence for wind farm applications of FOWT. The wind turbine scale model adopted for these tests is the 1/75 DTU 10 MW wind turbine developed for the LIFES50+ project. In this study, imposed surge motions, at different frequencies and amplitudes, were provided at the base of the turbine’s tower, to investigate the influence of motion on the wake. Results reported show good agreement with the state-of-the-art of wind turbines wake aerodynamics in steady conditions; concerning the unsteady conditions (imposed motion) interesting data are reported and collected with respect to a ”wake reduced frequency” parameter which is, according to the authors, a straightforward value to estimate the ”level” of unsteadiness connected to a specific dynamic condition due to floating motion. Interesting and consistent phenomena are observed and commented, although opening to the need of further experimental and numerical investigations.

Monte-Carlo simulations are a well-established methodology to numerically assess the effect of variations of both mechanical and aerodynamic parameters on the aeroelastic stability of a suspended bridge. In this research paper we use this statistical approach to present a design methodology for the assessment of long span aeroelastic behavior that considers not only the flutter velocity as a unique safety index, as it is common practice, but also the trend of the total damping as a function of the wind velocity. Specifically, several significant in-service conditions are considered. Indeed, wind velocities at in-service conditions are events with a probability much higher than flutter condition, and in these cases a reliability analysis represents an added value in the design process. Therefore the statistical analysis of total damping at in-service conditions is suggested as a useful tool to judge the performance of a structure especially for those structures where the wind–structure interaction represents the capital aspect of the overall design as it happens for the very long span bridges. Given the probability distribution of the input parameters, the Monte Carlo simulations allows one to statistically investigate the aeroelastic stability of the structure, in terms of range of variation of the total damping and flutter velocity, through an eigenvalue analysis. It is also possible to state the probability of not matching the design technical specification and the probability of being in a certain prescribed variation band. The mathematical and statistical background of the design methodology are presented and a case of study is analyzed with reference to the Messina Straits Bridge. The test case was chosen to stress the methodology with a challenging design, where extreme structural and aerodynamic solutions are implemented to reach a main span length of 3300 m and where the performances of the structure are deeply bounded to the reliability of the aeroelastic design.

This paper presents a detailed experimental study finalized to evaluate the effect of the porosity on the dynamic behavior of a civil structure. The structure considered is an original architectonic complement erected on top of a new tall building. Its cladding, made of perforated steel plates, forms an intricate three-dimensional spiral characterized by five different diameters which decrease along the height of the structure. The main objective of the tests is to evaluate the effect of the porosity of the external perforated panels, comparing the dynamic response obtained from the in-service configuration (permeable panels) and the results obtained with non-porous panels. This comparison showed that the porosity of the panels is of great importance and can completely change the dynamic behavior of the whole spire. This study has a realistic relevance as the structure could be subjected to atmospheric icing which could block up the panels and change their porosity. Wind tunnel tests were performed on 1:50 scaled rigid and aeroelastic models in smooth and turbulent flow conditions. The effect of the porosity was estimated comparing wind actions in terms of global wind loads at the base of the spire, local wind load on a panel and the structure׳s proneness to vortex induced vibrations. The analysis carried out highlights on the great influence of porosity in the dynamic response of the structure, mainly in relation to vortex shedding induced vibrations, which were almost absent with porous panels, but very strong in the case of non-porous panels.

This paper presents the results of a multifaceted investigation into the vortex-induced vibrations of the stay cables of a roundabout flyover. Large cable vibrations were spotted during moderate wind speed conditions immediately after the completion of the structure, and after three weeks, because of concerns about the structural integrity of the cable system, temporary restrainers were applied. An investigation was conducted to assess the potential fatigue damage that occurred during that time, combining experimental measurements and a finite-element model of the cable system. As a result, the cable sockets were replaced, causing delays in the opening of the flyover and additional costs. Wind-induced vibration occurred as a result of the very low damping of the cables. Therefore, after the repair, to suppress the phenomenon two differently tuned mass dampers of the Stockbridge type were installed on each cable. The article describes the results of the investigation and presents the methodology us...

This paper illustrates the mechatronic design of the wind tunnel scale model of the DTU 10MW reference wind turbine, for the LIFES50+ H2020 European project. This model was designed with the final goal of controlling the angle of attack of each blade by means of miniaturized servomotors, for implementing advanced individual pitch control (IPC) laws on a Floating Offshore Wind Turbine (FOWT) 1/75 scale model. Many design constraints were to be respected: among others, the rotor-nacelle overall mass due to aero-elastic scaling, the limited space of the nacelle, where to put three miniaturized servomotors and the main shaft one, with their own inverters/controllers, the slip rings for electrical rotary contacts, the highest stiffness as possible for the nacelle support and the blade-rotor connections, for ensuring the proper kinematic constraint, considering the first flapwise blade natural frequency, the performance of the servomotors to guarantee the wide frequency band due to frequency scale factors, etc. The design and technical solutions are herein presented and discussed, along with an overview of the building and verification process. Also a discussion about the goals achieved and constraints respected for the rigid wind turbine scale model (LIFES50+ deliverable D.3.1) and the further possible improvements for the IPC-aero-elastic scale model, which is being finalized at the time of this paper.

This paper presents the first set of results of the steady and unsteady wind tunnel tests, performed at Politecnico di Milano wind tunnel, on a 1/75 rigid scale model of the DTU 10 MW wind turbine, within the LIFES50+ project. The aim of these tests is the validation of the open source code AeroDyn developed at NREL. Numerical and experimental steady results are compared in terms of thrust and torque coefficients, showing good agreement, as well as for unsteady measurements gathered with a 2 degree-of-freedom test rig, capable of imposing the displacements at the base of the model, and providing the surge and pitch motion of the floating offshore wind turbine (FOWT) scale model. The measurements of the unsteady test configuration are compared with AeroDyn/Dynin module results, implementing the generalized dynamic wake (GDW) model. Numerical and experimental comparison showed similar behaviours in terms of non linear hysteresis, however some discrepancies are herein reported and need further data analysis and interpretations about the aerodynamic integral quantities, with a special attention to the physics of the unsteady phenomenon.

This paper illustrates the aero-elastic optimal design, the realization and the verification of the wind tunnel scale model blades for the DTU 10 MW wind turbine model, within LIFES50+ project. The aerodynamic design was focused on the minimization of the difference, in terms of thrust coefficient, with respect to the full scale reference. From the Selig low Reynolds database airfoils, the SD7032 was chosen for this purpose and a proper constant section wing was tested at DTU red wind tunnel, providing force and distributed pressure coefficients for the design, in the Reynolds range 30-250 E3 and for different angles of attack. The aero-elastic design algorithm was set to define the optimal spanwise thickness over chord ratio (t/c), the chord length and the twist to match the first flapwise scaled natural frequency. An aluminium mould for the carbon fibre was CNC manufactured based on B-Splines CAD definition of the external geometry. Then the wind tunnel tests at Politecnico di Milano confirmed successful design and manufacturing approaches.

Global coefficients for unsteady aerodynamic forces are a powerful tool for the assessment of the aeroelastic response of bridges in turbulent winds. However, global coefficients do not provide any information about the phenomenology of the fluid–structure interaction. This paper introduces a distributed representation of global coefficients that, exploiting the knowledge of the unsteady pressure field along the contour of the deck, allows to understand more in detail the aerodynamics of the phenomenon. We use the distributed representation to analyze the unsteady forces acting on a simple closed-box single girder deck, and we focus the study on two aspects. First, we analyze the unsteady aerodynamic forces near the stall of the moment coefficient, highlighting that a specific area of the deck governs the phenomenon; then we focus the attention on the comparison between the phenomenology of self-excited and wind induced unsteady forces. Results show the effectiveness of the distributed representation in highlighting similarities and discrepancies between different wind–bridge interactions. The distributed representation results to be an interesting tool to perform the optimization of the aeroelastic performances of decks and to improve nonlinear models for the dynamic response of bridges in turbulent wind.

The present paper shows an experimental study about vortex induced vibrations on a rigid suspended cylinder characterized by high value of mass ratio (m⁎) and tested at high Reynolds number condition. Most of the available literature data have been obtained on high mass ratio models at quite low Reynolds number while the combination of high mass ratio and high Reynolds number it is not well documented. The data collected during the campaign performed at Politecnico di Milano wind tunnel highlight the different states experienced by the cylinder in the lock-in region showing unexpected high values of oscillation amplitude. The steady state response has been investigated both in terms of displacement and in terms of aerodynamic forces. The wake has been analyzed through anemometers placed downstream the cylinder: the harmonic content of the wake has been related to different fluid dynamic states observed in the lock-in.

This paper presents a procedure for the response prediction and reduction in high-rise buildings under multidirectional wind loads. The procedure is applied to a very slender tall building that is instructive. The structure is exposed to both cross-wind and along-wind loads obtained from pressure measurements on a rigid model (scaled 1 : 100) that was tested in a wind tunnel with two different configurations of the surroundings. In the theoretical formulation, dynamic equations of the structure are introduced by finite element and 3D lumped mass modeling. The lateral responses of the building in the two directions are controlled at the same time using tuned mass dampers (TMDs) and active tuned mass dampers (ATMDs) commanded by LQR and fuzzy logic controllers, while the effects of the uncontrolled torsional response of the structure are simultaneously considered. Besides their simplicity, fuzzy logic controllers showed similar trend as LQR controllers under multidirectional wind loads. Nevertheless, the procedure presented in this study can help decision makers, involved in the design process, to choose among innovative solutions like structural control, different damping techniques, modifying geometry, or even changing materials.