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Wind energy in the atmospheric boundary layer serves as the primary source for energy absorption and structural load on wind turbines. However, the impact of turbulent coherent structures on the aerodynamic performance and wake characteristics of wind turbines has not been comprehensively evaluated. In this study, the proper orthogonal decomposition (POD) method is employed to assess the influence of turbulent coherent structures of varying scales on the aerodynamic performance and wake characteristics of wind turbines in the neutral atmospheric boundary layer. The results show that turbulent coherent structures are the main factor that determines the wind velocity fluctuation, aerodynamic performance and wake characteristics of wind turbine in the atmospheric boundary layer. When considering the 13th or lower order POD mode, the wind velocity fluctuation increases with the increase of energy content (more POD modes) of the turbulent coherent structures. When considering the first 19 POD modes, the dynamic loads and power of wind turbine fluctuate with high frequencies, the thrust fluctuates in an amplitude range between 2.4 % and 13.9 % around the mean value, and the power fluctuates from 4.5 % to 28.6 % of the mean value. When considering the first 40 POD modes, the average power generation of the wind turbine increases by 26 % compared to the case with no turbulent structures considered. The study of turbine wake shows that turbulent coherent structures can expand the wind turbine wake approximately to a width of 2.5 D and a height of 3 D (D is the diameter of the wind turbine), offset the wake approximately to 2 D , and move forward the position of the wake vortex beginning to dissipation approximately to 7 D behind the wind turbine. In addition, turbulent coherent structures can accelerate the wake velocity recovery by increasing the momentum exchange between the atmospheric boundary layer and wind turbine wake. • A POD-based inflow generation method is established for wind turbine simulation. • Study into the coherent structures with various energy contributions on wind turbine performance. • Obtained the most influence energy contribution of coherent structures in the ABL. • Unveil the influence of coherent structures on the expansion and deflection of wind turbine wakes. [ABSTRACT FROM AUTHOR]

The use of wind energy, the area of wind farms, and the length of wind turbine blades continuously increase with the increase in energy demand. This study aims to simulate large wind farms in the atmospheric boundary layer with different double-rotor (DR) wind turbine arrangements using large eddy simulation. The effect of four DR wind turbine arrangements on the efficiency of a large wind farm was investigated in this study. All simulations were performed using the OpenFOAM solver. The performance of different turbine arrangements was investigated based on the mean velocity, the extent of the wake region, mean kinetic energy, and turbine power. Results showed that the arrangement of DR turbines with transverse spacing produces higher power than other layouts. Moreover, DR layouts produce the same power as single-rotor layouts with a short blade length. This scenario is important from an economic point of view. [ABSTRACT FROM AUTHOR]

Static axial induction control and tilt control are two strategies that have the potential to increase power production in wind farms, mitigating wake effects and increasing the available power for downstream turbines. In this study, wind tunnel experiments are performed to evaluate the efficiency of these two techniques. First, the axial induction of upstream turbines in wind farms comprising two, three, and five turbines is modified through the tip-speed ratio. This strategy is found to be ineffective in increasing power extraction. Next, the power extraction and flow through a two-turbine wind farm are evaluated, considering different tilt angles for the upstream turbine, under two levels of incoming flow turbulence intensities and turbine spacing distances. It is shown that forward tilting increases the overall power extraction by deflecting the wake downwards and promoting the entrainment of high-speed fluid in the upper shear layer, regardless of the turbine spacing distance and turbulence intensity level. Also, the wake is seen to recover faster due to the increased shear between the wake and the outer flow. Tilting a turbine backward deflects the wake upwards and pulls low-speed flow from under the turbine into the wake space, increasing the available power for downstream turbines, but it is not enough to increase global power extraction. Moreover, since the wake deflection under backward tilting is not limited by ground blockage, it leads to larger secondary steering compared with forward tilting. Finally, it is demonstrated that the secondary steering of the downstream turbine's wake influences the flow encountered by a turbine positioned farther downstream. [ABSTRACT FROM AUTHOR]

Understanding the unsteady characteristics of the mid-to-far wake and comprehension of the aerodynamic performance of floating offshore wind turbines is essential for the further development of offshore wind farms. In this perspective, a developed Actuator line model is utilized to analyze an offshore turbine on four different platforms. The model reliability was examined through three sets of validations involving turbine output in fixed and floating conditions and wake expansion in terms of size and rate. The affected relative velocity by the platform motion contributes to the wake deformation and temporal effects on induced velocity. Angular platform motions produce a non-axisymmetric helical wake that raises the chance of meandering wake patterns. It was founded although platform movement generally can boost the recovery of mean velocity value, it may amplify the amplitude of velocity deficit fluctuation at further downstream by encouraging interactions and merging vortex rings. Consequently, the wake propagates into a form of stronger circles whose period, strength, and center are functions of turbine movement and operation conditions. By providing a computationally efficient tool, the findings emphasize the importance of wake propagation in designing and assessing farm layouts that operate in the presence of significant multi-motions of floating offshore wind turbines. [ABSTRACT FROM AUTHOR]

The unsteady aerodynamic characteristics and interference effects of floating offshore wind turbine (FOWT) are mainly affected by the pitch motion of the ocean platform. Based on computational fluid dynamics (CFD) and advanced overset grid technology, the aerodynamic performance and wake characteristics of a fully configured wind turbine with rotating blades, nacelle , and tower are studied in this paper. The effects of the amplitude and frequency of pitch motion on the wind turbine aerodynamic loads and flow field are investigated herein in detail. The power and thrust between numerical simulation and experiment are compared. The results show that the grid and simulation parameters used in this study can accurately capture the aerodynamic characteristics and the flow field around wind turbines. The influence of the pitch amplitude and frequency on the performance of wind turbines is discussed. The complex flow interaction between the tip vortex, tower shedding vortex, and the turbulent wake was observed. The present results indicate that the pitch motion amplitude and frequency have a great influence on the power, thrust, and wake characteristics. [ABSTRACT FROM AUTHOR]

Recently, wind turbine research has switched focus to vertical axis wind turbines due to the extensive research that has been performed on horizontal axis wind turbines and the potential of vertical axis wind turbines in built-up areas. This study aims to analyse the performance of a small-scale hybrid vertical axis wind turbine that can switch from functioning as a Darrieus (lift) turbine to a Savonius (drag) turbine by rotating the blades. The turbine was analysed using 3D computational fluid dynamics (CFD) simulations in ANSYS Fluent as the primary method, and the findings were verified using wind tunnel experiments. During the analysis, design parameters such as the blade length, diameter, and number of blades were varied to determine if the design had room for improvement. It was found that the current design of the turbine has an optimal efficiency of 12.5% in the Darrieus configuration, which was found to increase when the diameter or blade length was increased. The Savonius configuration was found to be more efficient at low tip-speed ratios (<0.14), and its efficiency could be increased by adding more blades. The experiments found similar trends to the simulations; however, the efficiencies obtained were on average a tenfold increase from the simulation. Implementing the changes that increased efficiency leads to an increased wake recovery distance, making it less suitable for use in a wind farm. [ABSTRACT FROM AUTHOR]

Savonius vertical axis wind turbines have simple structures, can self-start in environments with low wind speed and strong turbulence intensity, and can be installed at low costs. Therefore, installation is possible in urban centers with low wind speeds, which may contribute to the construction of a decentralized power system. Savonius wind turbines are operated by drag force, with the blades moving in the same direction as the flow current providing the thrust force and those moving in the opposite direction of the wind being rotated by the drag force. In this study, the Savonius wind turbine design was examined to develop a stable wind turbine for use in urban centers at low wind speeds. The Savonius rotor design variables (aspect and overlap ratios) and blade forms (semi-circular, Bach, and elliptical type) were examined using computational fluid dynamics analysis. Moreover, a rotor capable of providing the target output was designed and maximum rotor efficiency of 18% was realized. Further, changes to the flow corresponding with various turbine layouts were analyzed to determine the arrangement that would maximize turbine performance. The results showed that the maximum efficiency of the turbines was in the 17–19% range and without significant variation. [ABSTRACT FROM AUTHOR]

Renewable energy has received a lot of attention. In recent years, offshore wind power has received particular attention among renewable energies. Fixed-type offshore wind turbines are now the most popular. However, because of the deep seas surrounding Japan, floating types are more preferable. The floating system is one of the factors that raises the cost of floating wind turbines. Vertical axis wind turbines (VAWT) have a low center of gravity and can tilt their rotors. As a result, a smaller floating body and a lower cost are expected. A mechanism called a floating axis wind turbine (FAWT) is expected to further reduce the cost. FAWT actively employs the features of VAWT in order to specialize itself in the area of offshore floating-type wind turbines. The lifting line theory simulation was used in this study to discuss the performance of the FAWT under the tilted conditions and its wake field. The results show that a tilted VAWT recovers faster than an upright VAWT. This suggests that FAWTs can be deployed in high density and efficiently generate energy as an offshore wind farm using VAWTs. [ABSTRACT FROM AUTHOR]

The wake of a wind turbine has an important influence on the output power of wind farms. Staggered height layout is an emerging method for the layout optimization of wind farms. In order to study the effect of a staggered height layout on the overall power output of wind farms in depth, we established a combination of two large wind turbines and three small wind turbines arranged laterally between the two large wind turbines, and set four working conditions with different distances between the small wind turbines and the downstream large wind turbines as the research objects. The wind turbine array is analyzed by numerical simulation The layouts add three small wind turbines between the two large wind turbines, and each row of small wind turbines has a different distance from the downstream large wind turbines. The results show that as the distance from the upstream large wind turbine increases, the power of the three small wind turbines on the downstream wind turbine tends to be positive. The numerical simulation suggests that under the inflow wind speed, the closer to the downstream large wind turbine, the higher the wind speed is at the hub height. [ABSTRACT FROM AUTHOR]

This paper presents a new method for approximately modeling 2‐D ideal steady fluid flows with finite vorticity induced by actuator curves of arbitrary shape. An actuator curve is an infinitesimally thin region containing a body force density acting on the flow, while the rest of the flow is force‐free. The approach can be used for any flow satisfying this description, but, like other actuator methods, it is naturally suited for modeling turbines or propellers. We derive a weak formulation of the governing equations in the Lagrange streamfunction ψ and solve it using finite elements. Compared to related methods such as the actuator cylinder (AC) approach, our formulation is uniquely suited for computations involving wake–wake or wake–turbine interactions. We validate the method by computing the flow through a single Darrieus vertical‐axis wind turbine (VAWT) and comparing with previous work. To demonstrate the ability to simulate interacting actuators of arbitrary configuration, we simulate a three‐VAWT array. The turbines are modeled in a freestream, and the loading is chosen to represent ideal airfoils. The standard VAWT results are consistent with previous work, validating the method. The three‐VAWT array demonstrates a higher efficiency than the single VAWT (0.56 vs. 0.52), with differing optmal tip speed ratios for the upwind and downwind turbines (upwind: 3.9, downwind: 3.1. The optimum for a single turbine is 3.6). The flow field of the three‐VAWT array shows expected features such as an acceleration of flow between the two counter‐rotating upwind turbines. [ABSTRACT FROM AUTHOR]

This study examines the wake of a porous disk that generates a velocity deficit equivalent to that of a wind turbine. Three‐dimensional unsteady numerical simulations based on the finite volume method are performed. The URANS‐SST (k–ω) model is applied for the turbulence closure. Two investigations are carried out in this study: (i) the influence of the disk porosity on the wake, for porosities values (p) ranging from 0 to 0.55 in the case of a perpendicular flow; and (ii) the influence of the yaw angle on the wake deviation, for yaw angles ranging from 5° to 30°. Good agreements with the available experimental data are obtained for the mean x‐velocity component. The results confirm that wake length increases as porosity decreases. For nonporous disks, most part of the fluid is deflected toward the mast and above the disk. The y‐velocity contours highlight two contra‐rotating vortices in the vicinity of the disk. In both cases (nonporous and porous disks), a high turbulent kinetic energy is obtained near the disk area, with a higher maximum value for the nonporous disk. [ABSTRACT FROM AUTHOR]

Wake generated by wind turbine can greatly influence the performance of downstream turbine. To better understand the wake self-similarity characteristics of vertical axis wind turbine (VAWT), the shear stress transport (SST) turbulence model with the addition of the γ-Reθ transition model is performed to model a twoblade VAWT at different operating conditions. The simulated blade surface pressure and torque are compared with existing experimental results for validation. Results show that, the simulated results after considering the transition model are more consistent with the experimental results. Analysis of the flow field shows that the average streamwise velocity of the wake in the horizontal plane under different tip speed ratios is asymmetry, but symmetric in the vertical plane. Further analysis indicates that, at different downstream positions, the nondimensional streamwise velocity deficit in the vertical plane remains self-similarity and basically coincides with the Gaussian distribution curve exclude the wake edges. In addition, the larger the tip speed ratio, the easier streamwise velocity deficit reach self-similarity state at downstream of the VAWT. The results of this study will be helpful to establish the wake model of the VAWT. [ABSTRACT FROM AUTHOR]

Data driven approaches are utilized for optimal sensor placement as well as for velocity prediction of wind turbine wakes. In this work, several methods are investigated for suitability in the clustering analysis and for predicting the time history of the flow field. The studies start by applying a proper orthogonal decomposition (POD) technique to extract the dynamics of the flow. This is followed by evaluations of different hyperparameters of the clustering and machine learning algorithms as well as their impacts on the prediction accuracy. Two test cases are considered: (1) the wake of a cylinder and (2) the wake of a rotating wind turbine rotor exposed to complex flow conditions. The training and test data for both cases are obtained from high fidelity CFD approaches. The studies reveal that the combination of a classification-based machine learning algorithm for optimal sensor placement and Bi-LSTM is sufficient for predicting periodic signals, but a more advanced technique is required for the highly complex data of the turbine near wake. This is done by exploiting the dynamics of the wake from the set of POD modes for flow field reconstruction. A satisfactory accuracy is achieved for an appropriately chosen prediction horizon of the Bi-LSTM networks. The obtained results show that data-driven approaches for wind turbine wake prediction can offer an alternative to conventional prediction approaches. [ABSTRACT FROM AUTHOR]

In order to predict the wake velocity of floating offshore wind turbines quickly and accurately, a three-dimensional analytical wake model (3DM_FP) is proposed in this paper, which considers the effects of the pitch motion on the incoming wind speed and the expansion of the wake region. The proposed model is then used to predict the wake velocity of floating wind turbines under different operating conditions, and the predictions are compared with the experimental measurements. The results show that 3DM_FP predicts the wake velocity of a floating wind turbine under pitch motion accurately, with an accuracy of not less than 97.6% at distances up to seven rotor diameters downstream. This study can provide a reference for the aerodynamic and the wake model study of floating offshore wind turbine. • A three-dimensional analytical wake model for floating offshore wind turbines is developed. • The effect of momentum difference resulting from fluctuations in incoming wind speed on the accuracy of the wake model is considered. • The effect of the pitch motion on the incoming wind speed and the expansion of the wake region is investigated. • The accuracy of the proposed model is validated by comparing the prediction results with the measured data from the wind farm and the data from the wind tunnel experiments. [ABSTRACT FROM AUTHOR]

Most wind turbines are installed inside wind farms, where they often operate under wake-affected inflow conditions. New methods are required to evaluate the power performance of a wind turbine in wake, as the International Electrotechnical Commission (IEC) standard procedure is applicable only to wake-free turbines. In this work, we investigate the accuracy of a multivariate power curve acquired through a polynomial regression, whose input variables are wind speed and turbulence measurements retrieved upstream of the turbine's rotor. For this purpose, we use measurements from the SpinnerLidar, a continuous-wave, scanning Doppler lidar measuring the turbine inflow. The SpinnerLidar was mounted in the spinner of a Neg Micon 80 wind turbine located within an onshore wind farm in western Denmark. The input variables are selected among the available lidar measurements with a feature-selection algorithm, resulting in seven input variables, distributed in different locations along the rotor area: six wind speed and one turbulence measurements. The multivariate power curve is tested and compared with IEC-similar power curves under both wake-affected and wake-free conditions. Results show that the multivariate power curve estimates the turbine's power output more accurately than the IEC-similar power curves, with error reductions up to 66.5% and 34.2% under wake-affected and wake-free conditions, respectively. Furthermore, the multivariate power curve estimates have an accuracy of the same order under both wake-affected and wake-free conditions. Finally, we show that the multivariate model accurately predicts the power even when a simple measuring geometry is used, such as circular scanning pattern with a diameter equal to 90% of the rotor. • Measurements from a spinner-mounted lidar under wake conditions. • Wind speed and turbulence retrieved from lidar Doppler radial velocity spectra. • Wind turbine power curves modelled as multivariable polynomial regressions. • The multivariate power curves are more accurate than the IEC binning method. • The multivariate power curves are as accurate in wake as in wake-free conditions. [ABSTRACT FROM AUTHOR]

It is well known that a shroud placed around a wind turbine can increase its power coefficient, but it brings complex mechanisms by which the shroud alters the flow passing through the rotor. Such mechanisms impose numerical challenges, as the shrouded turbines present nonlinear behavior in the wake. This paper deals with a comprehensive analysis of tip vortex trajectory in shrouded wind turbines using Reynolds Averaged Navier–Stokes numerical solutions. The analysis includes aerodynamic performance and vortex characteristics of the whole wind turbine. The Multiple Reference Frame is used on a high-order unstructured compressible solver to study both, isolated and shrouded rotor. The NREL Phase VI Unsteady Aerodynamic Experiment rotor is used as a test case. The accuracy of results for wind speeds between 7 and 25 m s − 1 is discussed. Overall, good agreement is achieved between the computed pressure distributions and the experimental reference values. At stalled blade, more efforts are needed to improve numerical solutions, especially for integrated load quantities. The vortex structure is examined, showing that shroud impacts tip vortex trajectory by the increase of the axial induced velocity at the rotor plane. This result, demonstrates that the classical Prandtl tip loss is not accurate for shrouded turbine analysis, and modern finite blade functions are needed. The influence of the flow conditions on the tip vortex trajectory, flow separation and shroud interaction are also discussed. • Tip vortex trajectory of the NREL Phase Vi. • Use of a simplified Multiple Reference Frame model with open-source high-order CFD solver, avoiding conformal interface complexities and enabling efficient shrouded turbine simulation. • Accurate and efficient computation of the integrated loads on the wind turbine. • Unprecedented perspective into profoundly altered tip vortex trajectory behavior within shrouded turbines using validated higher-order framework. • Study of the interaction between the tip vortex and shroud's boundary layer. [ABSTRACT FROM AUTHOR]

Fast and effective numerical models describing the effect of platform motion on the performance of floating offshore wind turbines (FOWTs) are fundamental to assess energy harvesting potential in large offshore wind farms. The purpose of this paper is to implement a CFD-based Computationally-Efficient approach based on an actuator line model (ALM) for FOWTs aerodynamics. Such a tool aims at complementing reasonable accuracy and affordable computational effort while being able to investigate the effects of the platform motions on the wake evolution. The actuator line model for FOWTs is developed by implementing a dedicated C++ library in the OpenFOAM toolbox. In addition, a tip treatment is applied to involve the tip effects induced by the pressure equalization from the suction and pressure sides. Results show that employing ALM decreases computational cost and preprocessing time for producing appropriate computational grids, as just about 400k and 600k grids are necessary for solving two representative test cases of fixed-bottom turbines (NREL Phase VI and NREL 5-MW) with reasonable accuracy. The inclusion of platform motion is then introduced, and the results showed that ALM is capable of capturing vortices trajectory, potential blade-vortex interactions, and vortex pairing and vortex ring state phenomenon in FOWTs. [ABSTRACT FROM AUTHOR]

Summary: The progression of wind turbine technology has led to wind turbines being incredibly optimized machines often approaching their theoretical maximum production capabilities. When placed together in arrays to make wind farms, however, they are subject to wake interference that greatly reduces downstream turbines' power production, increases structural loading and maintenance, reduces their lifetimes, and ultimately increases the levelized cost of energy. Development of techniques to manage wakes and operate larger and larger arrays of turbines more efficiently is now a crucial field of research. Herein, four wake management techniques in various states of development are reviewed. These include axial induction control, wake steering, the latter two combined, and active wake control. Each of these is reviewed in terms of its control strategies and use for power maximization, load reduction, and ancillary services. By evaluating existing research, several directions for future research are suggested. [ABSTRACT FROM AUTHOR]

Turbines in a wind farm dynamically influence each other through wakes. Therefore trade-offs exist between energy output of upstream turbines and the health of downstream turbines. Using both model-based predictive control (MPC) and machine learning techniques, existing works have explored the energy-fatigue trade-off either in a single turbine or only with few turbines due to issues of scalability and complexity. To address this gap, this paper proposes a multi-agent deep reinforcement learning-based coordinated control for wind farms, called FALCON. FALCON addresses the multi-objective optimization problem of maximizing energy while minimizing fatigue damage by jointly controlling pitch and yaw of all turbines. FALCON achieves scale by using multiple reinforcement learning agents; capturing the global state-space efficiently using an auto-encoder; and pruning the action-space using domain knowledge. FALCON is evaluated through a real-world wind-farm case study with 21 turbines; and performs better than the default baseline PID controller and a learning-based distributed control. • One of the first works using multiagent Reinforcement Learning for wind farm control. • Multi-objective optimization of both power and turbine life. • Novel implementation of RL and auto-encoder for solution scalability. • Performance of the approach is demonstrated on real-world wind farm data. • Results show that 6% more energy with 31 years of extra turbine life can be achieved. [ABSTRACT FROM AUTHOR]

A feasibility study of velocity field measurements using the Particle Image Velocimetry (PIV) method in an axial air turbine model is presented. The wakes past the blades of the rotor wheel were observed using the PIV technique. Data acquisition was synchronized with the shaft rotation; thus, the wakes were phase averaged for statistical analysis. The interaction of the rotor blade wakes with the stator ones was investigated by changing the stator wheel's angle. The measurement planes were located just behind the rotor blades, covering approximately 3 cm × 3 cm in axial × tangential directions. The spatial correlation function suggests that the resolution used is sufficient for the large-scale flow-patterns only, but not for the small ones. The scales of fluctuations correspond to the shear layer thickness at the mid-span plane but, close to the end-wall, they contain larger structures caused by the secondary flows. The length-scales of the fluctuations under off-design conditions display a dependence on the area of the stator and rotor wakes cross-sections, which, in turn, depend on their angle. The obtained experimental data are to be used for the validation of mathematical simulation results in the future. [ABSTRACT FROM AUTHOR]

We propose a data-driven model to predict the short-term fatigue Damage Equivalent Loads (DEL) on a wake-affected wind turbine based on wind field inflow sensors and/or loads sensors deployed on an adjacent up-wind wind turbine. Gaussian Process Regression (GPR) with Bayesian hyperparameters calibration is proposed to obtain a surrogate from input random variables to output DELs in the blades and towers of the up-wind and wake-affected wind turbines. A sensitivity analysis based on the hyperparameters of the GPR and Kullback-Leibler divergence is conducted to assess the effect of different input on the obtained DELs. We provide qualitative recommendations for a minimal set of necessary and sufficient input random variables to minimize the error in the DEL predictions on the wake-affected wind turbine. Extensive simulations are performed comprising different random variables, including wind speed, turbulence intensity, shear exponent and inflow horizontal skewness. Furthermore, we include random variables related to the blades lift and drag coefficients with direct impact on the rotor aerodynamic induction, which governs the evolution and transport of the meandering wake. In addition, different spacing between the wind turbines and Wöhler exponents for calculation of DELs are considered. The maximum prediction normalized mean squared error, obtained in the tower base DELs in the fore-aft direction of the wake affected wind turbine, is less than 4%. In the case of the blade root DELs, the overall prediction error is less than 1%. The proposed scheme promotes utilization of sparse structural monitoring (loads) measurements for improving diagnostics on wake-affected turbines. • Predicting loads on unmeasured wake-affected WT from sensors on an adjacent WT. • Gaussian Process Regression model with Bayesian hyperparameters calibration. • Propagation of uncertain inflow, aerodynamic, layout and fatigue random variables. • Recommended minimal set of input to predict the DEL on a wake-affected WT. • The overall prediction error of DELs of the wake-affected WT is less than 4%. [ABSTRACT FROM AUTHOR]

In a large wind farm, the wakes of upstream and downstream wind turbines can interfere with each other, affecting the overall power output of the wind farm. To further improve the numerical accuracy of the turbine wake dynamics under atmosphere turbulence, this work proposes some improvements to the actuator line-large-eddy simulation (AL-LES) method. Based on the dynamic k-equation large-eddy simulation (LES), this method uses a precursor method to generate atmospheric inflow turbulence, models the tower and nacelle wakes, and improves the body force projection method based on an anisotropic Gaussian distribution function. For these three improvements, three wind tunnel experiments are used to validate the numerical accuracy of this method. The results show that the numerical results calculated in the far-wake region can reflect the characteristics of typical onshore and offshore wind conditions compared with the experimental results. After modeling the tower and nacelle wakes, the wake velocity distribution is consistent with the experimental result. The radial migration velocity of the tip vortex calculated by the improved blade body force distribution model is 0.32 m/s, which is about 6% different from the experimental value and improves the prediction accuracy of the tip vortex radial movement. The method proposed in this paper is very helpful for wind turbine wake dynamic analysis and wind farm power prediction. [ABSTRACT FROM AUTHOR]

On a wind farm, the wake has an important impact on the performance of the wind turbines. For example, the wake of an upstream wind turbine affects the blade load and output power of the downstream wind turbine. In this paper, a modified actuator line model with blade tips, root loss, and an airfoil three-dimensional delayed stall was revised. This full-scale modified actuator line model with blades, nacelles, and towers, was combined with a Large Eddy Simulation, and then applied and validated based on an analysis of wind turbine wakes in wind farms. The modified actuator line model was verified using an experimental wind turbine. Subsequently, numerical simulations were conducted on two NREL 5 MW wind turbines with different staggered spacing to study the effect of the staggered spacing on the characteristics of wind turbines. The results show that the output power of the upstream turbine stabilized at 5.9 MW, and the output power of the downstream turbine increased. When the staggered spacing is R and 1.5R, both the power and thrust of the downstream turbine are severely reduced. However, the length of the peaks was significantly longer, which resulted in a long-term unstable power output. As the staggered spacing increased, the velocity in the central near wake of the downstream turbine also increased, and the recovery speed at the threshold of the wake slowed down. The modified actuator line model described herein can be used for the numerical simulation of wakes in wind farms.

Previous research has revealed the need for a validation study that considers several wake quantities and code types so that decisions on the trade‐off between accuracy and computational cost can be well informed and appropriate to the intended application. In addition to guiding code choice and setup, rigorous model validation exercises are needed to identify weaknesses and strengths of specific models and guide future improvements. Here, we consider 13 approaches to simulating wakes observed with a nacelle‐mounted lidar at the Scaled Wind Technology Facility (SWiFT) under varying atmospheric conditions. We find that some of the main challenges in wind turbine wake modeling are related to simulating the inflow. In the neutral benchmark, model performance tracked as expected with model fidelity, with large‐eddy simulations performing the best. In the more challenging stable case, steady‐state Reynolds‐averaged Navier–Stokes simulations were found to outperform other model alternatives because they provide the ability to more easily prescribe noncanonical inflows and their low cost allows for simulations to be repeated as needed. Dynamic measurements were only available for the unstable benchmark at a single downstream distance. These dynamic analyses revealed that differences in the performance of time‐stepping models come largely from differences in wake meandering. This highlights the need for more validation exercises that take into account wake dynamics and are able to identify where these differences come from: mesh setup, inflow, turbulence models, or wake‐meandering parameterizations. In addition to model validation findings, we summarize lessons learned and provide recommendations for future benchmark exercises. [ABSTRACT FROM AUTHOR]

An experimental study of the wake characteristics of a semi-submersible offshore wind turbine model was performed in a wind tunnel and wave flume. The velocity distribution in the vertical direction was obtained with a four-hole pressure probe. First, a self-similarity analysis of the streamwise velocity deficit disclosed the inability of the Gaussian-like wake models for predicting the velocity deficit in regions influenced by the platform. The shelter model can be adopted for the consideration of the platform wake. And the linear self-similarity of the velocity deficit caused by the platform also suggests that a linear model is feasible. Moreover, recently-discovered existence and analytical solution of the Reynolds stress self-similarity in the turbine wake were confirmed experimentally in this study. As the wake develops, the center of the Reynolds stress increment profiles drifts upward while the center of mean velocity deficit profiles remains at the same height. Furthermore, energy transport analysis confirmed the interaction effects between the rotor wake and platform wake. The results of this study will be useful for the design of the whole wind farm with more accuracy, which considers the influence of the platform on the flow field between neighbor wind turbines. • A novel analytical wind turbine wake model including support structure is developed. • Interaction between the platform and rotor wakes is revealed with energy transport analysis. • Self-similarity in Reynolds stress of turbine wake is experimentally confirmed. [ABSTRACT FROM AUTHOR]

An experimental study of the near wake up to four rotor diameters behind a model wind turbine rotor with two different wing tip configurations is performed. A straight‐cut wing tip and a downstream‐facing winglet shape are compared on the same two‐bladed rotor operated at its design tip speed ratio. Phase‐averaged measurements of the velocity vector are synchronized with the rotor position, visualizing the downstream location of tip vortex interaction for the two blade tip configurations. The mean streamwise velocity is found not to be strongly affected by the presence of winglet tip extensions, suggesting an insignificant effect of winglets on the time‐averaged inflow conditions of a possible downstream wind turbine. An analysis of the phase‐averaged vorticity, however, reveals a significantly earlier tip vortex interaction and breakup for the wingletted rotor. In contradistinction, the tip vortices formed behind the reference configuration are assessed to be more stable and start merging into larger turbulent structures significantly further downstream. These results indicate that an optimized winglet design can not only contribute to a higher energy extraction in a rotor's tip region but also can positively affect the wake's mean kinetic energy recovery by stimulating a faster tip vortex interaction. [ABSTRACT FROM AUTHOR]

The growing global interest in sustainable energy has paved the way to the rapid development of large-scale wind farms, consisting of dozens to hundreds of wind turbines. Although these large wind farms can generate enormous amount of power, they are also costly and require large areas of land or water, and thus are not suitable for urban environments. Smaller urban wind turbines have been developed for urban environments, but there are significant challenges to their widespread deployment. One of these challenges are their urban wind flows as they are strongly affected by complex building structures, producing highly turbulent flows. Any urban wind turbine would need to be designed to function efficiently and safely under these flow conditions; however, these unpredictable and turbulent winds can induce undesirable vibrations and cause early failures. Recently, bladeless wind turbines are gaining interest due to their reduced costs compared with conventional wind turbines such as the vertical-axis wind turbine and horizontal-axis wind turbine. These bladeless turbines convert flow wind energy into vibration energy, then converts the vibration energy into electricity. This paper examines the effects of force-induced vibrations on a cantilever beam system through wind tunnel experimentation. When fluid flows around a bluff body, periodic shedding of vortices may occur under the right conditions. The vortex shedding process creates an asymmetric pressure distribution on the body which causes the body to oscillate, known as vortex-induced vibrations. The purpose of the paper is to understand the factors affecting flowinduced vibrations and to improve wind energy harvesting from these vibrations. The first part of the paper focuses on wind tunnel experiments, by utilizing a cantilever beam configuration, conceptualized by previous research. Then, the experimental model was tested in different configurations, to determine the best setup for maximizing vibrations induced on the model. The long-term goal of the project was utilizing the model to optimize the system to improve efficiency of wind energy harvesting. The experimental results showed that the presence of an upstream cylinder will significantly improve the amplitude of vibration for energy harvesting, furthermore, the experiments showed that spacing in different directions also affect the amplitude of the vibrations. A two tandem cylinder system was used in this work, including a fixed rigid upstream cylinder and a downstream cylinder supported by a cantilever beam. Various configurations of these two cylinders in terms of spanwise and streamwise separation distances were studied and their maximum and root mean square displacements are reported for different wind speeds. Results showed that the presence of an upstream cylinder will significantly improve the amplitude of vibrations. This work verified that a wind energy harvester needs to consider the effects of wind speed and separation configuration of the cylinders in order to maximize the harvester's performance in urban environments. [ABSTRACT FROM AUTHOR]

Monin–Obukhov similarity theory (MOST) overestimates the mean vertical velocity gradient in some atmospheric stable conditions, i.e., Richardson number R f < 0.25 . To obtain a given hub-height inflow velocity for a certain roughness length, this overestimated velocity gradient underpredicts the friction wind speed and the turbulence intensity, potentially influencing wake modeling of a wind turbine. This work investigates the side effects of the breakdown of MOST on wake modeling under stable conditions and makes some modifications to the flow similarity functions to eliminate these side effects. Based on a field measurement in a wind farm, we first show that MOST predicts a larger velocity gradient for the atmospheric stability parameter ζ > 0.1 and proposes new flow similarity functions without constraining R f to limit the overestimated velocity gradient. Next, different turbulence models based on MOST and a modified one based on the new similarity functions are investigated through numerical simulations. These turbulence models are combined with the actuator disk model (AD) and Reynolds-averaged Navier–Stokes equations (RANS) to model wind turbine wakes under stable conditions. As compared to measurements, numerical results show that turbulence models based on MOST result in a larger wake deficit and a slower wake recovery rate with a root-mean-squared error (RSME) of wake deficit in the range of 0.07 to 0.20. This overestimated wake effect is improved by applying the new similarity functions, and the RSME of wake deficit is reduced by 0.05 on average. [ABSTRACT FROM AUTHOR]

A procedure for mapping wake‐induced load predictions computed with the dynamic wake meandering model to a computationally efficient surrogate model approximation is defined and demonstrated. Using the mapping function, the load variation can be efficiently estimated for a wind farm with arbitrary layout. The resulting load assessment procedure provides continuous, differentiable output with known analytical derivatives and can be used for applications such as wind turbine layout optimization, estimation of turbine lifetime, and uncertainty analysis. [ABSTRACT FROM AUTHOR]

The wake characteristics of a variable-speed pitch-regulated floating offshore wind turbine (FOWT) under surge motions are studied using a Computational Fluid Dynamics (CFD) method which combines the Actuator Curve Embedding (ACE) method and collective blade pitch control strategy. Verifications and validations show the method is accurate with affordable computational cost. Comparative analysis of a FOWT under specified surge motion at rated wind speed indicates the controller significantly reduces the mean wake deficits, which highlights the necessity of considering controller for FOWT(s) wake analysis and floating offshore wind farm layout optimizations. The mean wake deficits and wake widths are observed to reduce with increasing surge amplitudes for a controlled FOWT. At rated wind speed, the helical tip vortices of a controlled FOWT under surge motions are observed transitioning into well-organized vortex ring structures and the vortex ring distances are found almost independent on surge amplitudes. The total wake vortex transition distance and vortex ring distance are observed increasing linearly with surge periods. These findings contribute to a better understanding of FOWT wake dynamics in real operating conditions. [ABSTRACT FROM AUTHOR]

The interactions between tandem floating offshore wind turbines remain an essential research topic with a view to optimizing future large-scale offshore wind farms. The focus of past floating offshore wind turbine aerodynamic research was centred on platform motion impact on rotor aerodynamics for standalone or in-phase tandem turbines. The question of how the differences in the surge motion phase of tandem turbines influence the performance of the downstream turbine remains open. Recent investigations reveal that these motions might not invariably be in-phase. This study seeks to determine the repercussions of platform motion phase differences on power, thrust, and wake recovery. Notably, it postulates that phase differences could detrimentally affect fatigue. Using a validated Navier–Stokes actuator line model rooted in the OpenFOAM® toolbox, the study aims to address this hypothesis. It was found that the out-of-plane root bending moment amplitude increase by 100% when the rotors are out of phase. A 72% increase in amplitude was also observed for C T. Both of these observations have also been linked to an increase in angle of attack amplitude over the blade span. This work stimulates future research in floating offshore turbine control strategies and provides guidance to minimize turbine failures and downtime. • Phase differences between two tandem FOWTs affect peak-to-peak loads of the downstream turbine. • Increases in root bending moment amplitudes of up to 100% were found on the downstream turbine when the turbine surges asynchronously. • Fatigue loads can be controlled by careful consideration of the motion phase differences. [ABSTRACT FROM AUTHOR]

In this study, the performance, drag, and horizontal midplane wake characteristics of a vertical‐axis Savonius wind turbine are investigated experimentally. The turbine is drag driven and has a helical configuration, with the top rotated 180° relative to the bottom. Both performance and wake measurements were conducted in four different inflow conditions, using Reynolds numbers of ReD≈1.6×105 and ReD≈2.7×105 and turbulence intensities of 0.6% and 5.7%. The efficiency of the turbine was found to be highly dependent on the Reynolds number of the incoming flow. In the high Reynolds number flow case, the efficiency was shown to be considerably higher, compared with the lower Reynolds number case. Increasing the incoming turbulence intensity was found to mitigate the Reynolds number effects. The drag of the turbine was shown to be independent of the turbine's rotational speed over the range tested, and it was slightly lower when the inflow turbulence was increased. The wake was captured for the described inflow conditions in both optimal and suboptimal operating conditions by varying the rotational speed of the turbine. The wake was found to be asymmetrical and deflected to the side where the blade moves opposite to the wind. The largest region of high turbulent kinetic energy was on the side where the blade is moving in the same direction as the wind. Based on the findings from the wake measurements, some recommendations on where to place supplementary turbines are made. [ABSTRACT FROM AUTHOR]

Hydrokinetic turbines, which convert the kinetic energy of flowing water into electrical energy, are increasingly being deployed in arrays, much like multiple wind turbines are commonly organized into a wind farm. Deploying multiple devices in an array can increase the amount of energy produced from a site and tends to make project economics more favourable. Despite growing interest in hydrokinetic turbine arrays, relatively few researchers have studied arrays and limited information exists concerning the character of the turbulent flows within arrays and methods for optimizing array layout, particularly for cross-flow turbines. This paper presents results from a set of experiments conducted in a current flume in which the wakes produced by a single cross-flow turbine and pairs of identical cross-flow turbines were measured in detail. Two different ambient turbulence intensities were studied so that the effect of turbulence level on wake recovery could be quantified. The velocity data has been analyzed to identify the mean velocity and the turbulence intensity in the turbine wake(s). These experiments form one component of a larger study that aims to develop numerical methods and guidelines to help project developers optimize the design of turbine arrays. [ABSTRACT FROM AUTHOR]

Wind turbine farms suffer from wake losses, where the downstream turbines generate less power, and/or the leading turbines are throttled to reduce the downstream power losses. In this paper, we focus on possible external modifications that can enhance the wind turbines' performance when they are operating in a farm environment. In particular, this study is interested in enhancing the performance of the downstream turbines in wind farms. The idea is to move each turbine's wake down and away from subsequent turbines. This goal is achieved by using stationary external airfoils that are placed in proximity to the rotating blades. A number of different designs are tested and the design concepts are tested using Reynolds‐Averaged Navier‐Stokes simulations of an aligned array of 2 wind turbines. The turbines are modeled as actuator disks with axial induction and are placed in a velocity field that is modeled as a turbulent atmospheric boundary layer. It is found that fixed external airfoils can enable partial or full power recovery at turbine separations of as small as 3 rotor diameters downstream. We will also demonstrate that some devices can also improve the performance of the upstream turbine. The physical reasons for these power recovery phenomena are discussed. [ABSTRACT FROM AUTHOR]

With the scale of wind turbine increasing, the obstructing effect of wind turbine to the surrounding flow of local terrain is getting more remarkable. This influence is seldom considered in current micro-siting of complex-terrain wind farms. The present paper investigates the flow around hills with different Height/Width (H/W) ratios, as well as the interactions between the surrounding flow using three dimensional numerical simulations. A 2 MW wind turbine is placed on five different locations. The simulation results show that the power production could be over-predicted by more than 10% if the wind turbine is not modeled in the computational processes. The H/W ratio which has significant influence on the flow structure around the hill. For the hill with a small H/W ratio, the surrounding flow has little variation with and without wind turbine. For the hill with a large H/W ratio, large scale flow separation occurs on the lee side under the wind turbine disturbances. The flow separation on the lee side also occurs when the wind turbine is located upwind in certain distance to the hill. The results indicate that the mutual interactions between wind turbine and terrain must be considered in the micro sitting of complex-terrain wind farms. [ABSTRACT FROM AUTHOR]

We investigate the turbulent dynamics of the coupled atmosphere-ocean-sediment system around a wind turbine. To this end, a coupled two-dimensional idealized numerical model of the ocean and sediment layers, forced by an idealized offshore wind turbine wake is used. The turbine wake impacts the ocean surface and for strong wind and water layer thickness higher than 20 m, large scale eddies of the size comparable to the wake thickness are generated, leading to a turbulent dynamics in the ocean. The turbulence in the ocean is controlled by the shallow wake parameter S. The turbulent ocean dynamics is numerically integrated using time dependent simulations at fine horizontal resolution (1 m). From these simulations, eddy coefficients parametrizating the turbulent fluxes are proposed to be used in larger-scale (RANS) models. The ocean dynamics and the parameter values depend mainly on S. The ocean dynamics is laminar (S > 7.10 −2 ), has a localized (7.10 −2