bluuebunny/crossref_metadata_2025_split huggingface dataset:
Spinner anemometer: wind speed and Spinner Transfer Function seasonal robustness in an offshore wind farm
Authors: Ritter K , Gómez Arranz P 
Published: September 2020

Abstract
This work presents the first, long-term, demonstration case of the use of spinner anemometers for power curve measurements in an offshore wind farm. The stability of the spinner anemometer wind speed and its STF during a 22-month measurement period was quantified, using a nacelle lidar as reference. Power curves and annual energy productions (AEPs) were obtained using this long-term data set, to assess the spinner anemometer wind speed stability. Next, several STFs were obtained at a reference turbine in different periods (four two-months periods) to assess the STF seasonality. Finally, the AEPs of three neighbouring turbines were analysed using the STFs generated at the reference turbine.
Lidar-based Estimation of Turbulence Intensity for Controller Scheduling
Authors: Schlipf David , Guo Feng , Raach Steffen 
Published: September 2020

Abstract
Lidar-assisted wind turbine control is a promising technology and various concepts have been developed. This paper aims to add another concept to the list by describing how turbulence intensity can be estimated and used for controller scheduling to reduce structural loads. The turbulence intensity estimation is applied to lidar data from aero-elastic simulations and good agreement with the turbulence intensity calculation from wind fields is obtained. Further, a controller scheduling scheme is proposed to adjust the power level based on the estimated turbulence intensity. In a first simulation study, the scheduling scheme is able to reduce the power and extreme loads on the tower during severe turbulence conditions while keeping a similar level of power production and fatigue loads for normal turbulent conditions.
On the Use of Cambered Plate Airfoils for Small Wind Turbines
Authors: Kummer Aaron , Dimeo Jack , Hebel Mark , Visser Kenneth 
Published: September 2020

Abstract
The use of constant thickness, cambered plates is being explored as a means to reduce the cost of rotors for small wind turbines. Literature suggests that at low Reynolds numbers there is an advantage over conventional airfoils, and tests conducted on a 2.5 m ducted rotor indicate a constant thickness airfoil to be a viable solution. An operational turbine with a 3 m rotor was developed and is currently being tested. Data supports predictions and shows encouraging results.
Holistic Design of Small-scale Stand-alone Wind Energy Conversion Systems Using Locally Manufactured Small Wind Turbines
Authors: Latoufis Kostas , Serafeim Giannis , Chira Katerina , Riziotis Vasilis , Voutsinas Spyros , Hatziargyriou Nikos 
Published: September 2020

Abstract
A holistic and multi-disciplinary design approach is developed for small-scale stand-alone wind energy conversion systems (WECS) using locally manufactured small wind turbines (LMSWTs), with the aim of reducing capital and maintenance costs while increasing the annual energy production and energy utilization of such systems. Various subsystems are analysed and modelled, using both sequential and integrated design approaches, such as the rotor, including the airfoil and blade geometries, the axial flux permanent magnet generator including economic, thermal and structural aspects of the stator and rotor geometry, the furling system and the electrical system, including the power transmission cables and the battery bank. The holistic design approach is then applied to a 2.4m rotor diameter LMSWT and the complete WECS is dimensioned. Finally, the designed low cost system is compared to a high cost system using a maximum power point converter, with satisfactory results especially for low wind speeds around the mean wind speed of the site. It is thus concluded that a holistic and multi-disciplinary design approach to small-scale stand-alone WECS using LMSWTs, can lower the cost of energy for rural electrification applications by reducing capital costs, while sustaining the system’s efficiency and annual energy production in low wind speed regions.
Wind turbine rotor design under wind farm control laws
Authors: Sartori L , Fidelibus P De , Cacciola S , Croce A 
Published: September 2020

Abstract
In this work, we investigate the impact of different wind farm control techniques on the structural design of a 10MW reference wind turbine. Active wake mixing and wake redirection have been recently proposed as a way to reduce wake-turbine interference in a wind farm and both show potential for improving the overall power production. However, such controllers modify the dynamic behaviour of the individual turbine, so that a thorough assessment of the resulting loads and displacements becomes necessary. In fact, as most wind turbines are designed according to international standards, one or more structural constraints are active on the final design, meaning that an increase of the sizing loads, or deflections, would make necessary to modify the structural layout. To investigate these aspects, we compare three redesigns of the same rotor: the first is equipped with a standard controller, while the second and the third integrate different wind farm controllers. All the solutions are optimized with our in-house design tool so that the three configurations emerge from the same design process. Results are then compared in terms of ultimate and fatigue loads, displacements and blade mass.
Progressive structural scaling of a 20 MW two-bladed offshore wind turbine rotor blade examined by finite element analyses
Authors: Schütt Marcel , Anstock Fabian , Schorbach Vera 
Published: September 2020

Abstract
Two-bladed turbines offer a promising opportunity for rotor cost savings, especially considering the ongoing growth trend in rotor size. An increased chord and airfoil thickness of a two-bladed turbine’s blade results in potential structural improvements caused by a rapidly growing second moment of area. Compared to a three-bladed turbine’s blade, the blade structure would theoretically require less material, while withstanding 50% higher flapwise loads. An analytical method of progressive structural scaling for three-dimensional rotor blade structures, based on equal material stresses, is introduced to calculate the modified structural thickness properties of the two-bladed turbine’s blade. It simplifies the airfoil-shaped structure to a thin-walled rectangle, utilizes a fixed initial flapwise load factor, and scales the edgewise loads proportionally to the required blade mass. To evaluate the validity of this analytical approach, a progressively scaled and an iterated 20 MW two-bladed turbine’s blade are examined with finite element analyses for static loads. The outcomes are then compared to corresponding analyses of a three-bladed turbine’s reference blade. Overall, the static stress comparisons at different blade positions show good agreement with the analytical results. Nevertheless, the buckling analyses performed reveal stability issues, which subsequently will lead to a readjustment of the blade mass.
CFD Investigation of Separation Control on a Vertical Axis Wind Turbine: Steady and Unsteady Suction
Authors: Rezaeiha Abdolrahim , Montazeri Hamid , Blocken Bert 
Published: September 2020

Abstract
High-fidelity two-dimensional unsteady Reynolds-averaged Navier-Stokes (URANS) simulations are employed to investigate the influence of boundary layer suction through a slot located near the leading edge of a vertical axis wind turbine operating in dynamic stall. The analysis includes both steady and unsteady suction with different frequencies. The results shows that: (i) when the suction slot is located within the chordwise extent of the laminar separation bubble, dynamic stall can be avoided with minimal suction amplitude; (ii) the most promising suction location is the most upstream suction location studied, at 8.5%c where c is the blade chord length; (iii) the suction only needs to be applied during the azimuthal angles when dynamic stall occurs; (iv) the oscillation frequency of the suction velocity has insignificant influence on the obtained turbine power gain; (v) applying unsteady suction is interesting as it reduces the energy consumption of the suction system, thus the net power gain.
Best practice for verification of wind turbine numerical models
Authors: Huhn Matthias L. , Popko Wojciech 
Published: September 2020

Abstract
The analysis of wind turbines relies on aero-hydro-servo-elastic simulation tools. OC3, 4, 5, 6 projects showed that much effort is required to obtain consistency in numerical models set up and the agreement in the simulation results. To mitigate the uncertainty in the model setup and to reduce the time spent on its verification, a robust verification procedure is necessary. The presented verification procedure provides a structured and efficient way of checking and comparing aeroelastic wind turbine models. In case a discrepancy between the two models or the model and documentation is found, a procedure for finding the source of this discrepancy is suggested. During several successful applications of the presented verification procedure, it proved to reduce the effort, the number of iteration loops and thus the consumed time that is required to achieve a verified model.
Dynamic stall at high Reynolds numbers due to variant types of airfoil motion
Authors: Kiefer J , Brunner C E , Hultmark M , Hansen MOL 
Published: September 2020

Abstract
Unsteady airfoil experiments were conducted in a high-pressure wind tunnel at chord Reynolds numbers of Rec = 3.0 × 106. A moderately thick NACA0021 airfoil was pitched from rest beyond the static stall angle in six individual ramp tests with increasing and decreasing angles of attack. The variant types of motion of the pitching maneuvers were characterized by constant angular velocity, angular acceleration and angular jerk, respectively. The ramp-up experiments revealed a substantial and time-dependent excess of the aerodynamic forces from static values in all three test cases and exhibited a distinct time delay as a consequence of the variant motion types. Similarly, the ramp-down experiments were largely impacted by the progression of the pitching motion, resulting in pronounced differences in the temporal development of lift and drag. Results are shown as time series of integrated forces and surface pressure distributions.
Dynamic stall on an airfoil undergoing VAWT blade angle of attack variations
Authors: Henne Sabrina , Fouest Sébastien Le , Mulleners Karen 
Published: September 2020

Abstract
Vertical axis wind turbines have several attractive features in the context of energy production in urban areas, but the inherent aerodynamic complexity of the flow around them has challenged their development on a larger scale. They generally operate at low tip speed ratios, where dynamic stall occurs on the blades. The vortex shedding associated with dynamic stall causes highly transient and heavy stress cycles that reduce the aerodynamic performance and increase the risk of fatigue and failure. The flow around an airfoil undergoing VAWT blade angle of attack variations was investigated using particle image velocimetry and force measurements. The formation of vortices and the lift force were studied for different tip speed ratios. A special focus was put on the effect of the asymmetry of the motion. The role of dynamic stall vortices on aerodynamic coeffients was evidenced by comparing experimental data to analytical predictions obtained from Theodorsen’s model. For the lowest equivalent tip speed ratio clockwise and counter clockwise rotating dynamic stall vortices formed on the airfoil with increasing and decreasing angle of attack. The asymmetry in motion lead to an asymmetry in size of the clockwise and counter-clockwise vortices. As the asymmetry in motion has a strong effect on the flow behaviour, the local pitch rate was proposed as a governing parameter. The increase of extrema with increasing pitch rate varies for increasing and decreasing angle of attack, indicating an additional influence of the history of the flow development.
Prediction of an advanced spar’s horizontal motions, validated by full scale observation data
Authors: Yoshimoto Haruki , Wright Christopher , Wada Ryota , Takagi Ken 
Published: September 2020

Abstract
As the offshore wind industry moves into deeper waters, the number of different floating wind platform concepts continues to expand. Accurately predicting the motion characteristics of these novel platforms is crucial to the safe and economic conversion of wind energy into electricity. A challenge in the currently used numerical simulation methods involves successfully predicting the slow drift motions and damping, especially in complex realistic environmental conditions which include multi-directional spectral waves, current and wind loading. Of most uncertainty to these predictions is determining the low frequency viscous damping which dominates the resonant surge and sway motions. In this paper, full-scale motion observations of the Fukushima FORWARD’s floating wind substation are used to determine the platforms viscous drag forces. These drag coefficients are used in a combined potential flow and Morison’s equation model to capture the observed behaviour. This validated numerical model is then used to describe the required fidelity of mooring line models, in order to aid platform developers in the early stages of design.
Full scale rotor blade deformation measurements in comparison with aeroelastic simulations based on measured high-resolution wind fields
Authors: Lehnhoff S , Sekar AP Kidambi , van Dooren M F , Kühn M , Seume J R 
Published: September 2020

Abstract
Aeroelasticity is one of the biggest challenges in wind turbine rotor design, as the length of rotor blades increases which comes along with a slenderer design. The knowledge of the aeroelastic turbine behavior is of great importance. A comparison to field measurements is of huge importance when validating aeroelastic tools. However, the measurement of deformation and torsion in the field is not trivial and the conduction of realistic post-test simulations is a challenge. One crucial factor for these simulations is the wind field, which needs to be captured in a high spatial and temporal resolution. In this paper, the results of deformation measurements conducted in the field with an optical measurement method called Digital Image Correlation (DIC) on one rotor blade will be shown and compared to aeroelastic post-test simulations using highly resolved wind fields measured with a SpinnerLidar.
Study on cyclic pitch control of HAWT by using instantaneous inflow observation
Authors: Kamada Y , Maeda T , Takai K , Mizuno S 
Published: September 2020

Abstract
The power output of a wind turbine depends on both wind speed and turbine control. There is a fluctuation in the natural wind speed, which changes the turbine power output. In this paper blade pitch control based on detailed monitoring of the inflow wind, including vertical velocity distributions, has been experimentally evaluated in a field test wind turbine. The inflow wind speeds were observed by 11 sets of ultrasonic anemometers. In this first test, the blade pitch is changed to keep optimum angle of attack based on the local inflow wind speed in the rotor plane. The blade pitch is independently controlled as a reference radial position of 80% of rotor radius. As a result of the pitch controls, the wind turbine power output was increased and the power output fluctuation was decreased for the cyclic pitch control based on the local wind speed. With the cyclic pitch control, the fluctuation in rotor thrust and average rotor thrust were also reduced compere to collective pitch control.
Stress-based assessment of the lifetime extension for wind turbines
Authors: Saathoff M , Rosemeier M 
Published: September 2020

Abstract
To assess of the lifetime extension (LTE) of wind turbines, the remaining fatigue budget of different turbine components is predicted. The state-of-the-art approach to predicting the LTE uses a comparison of damage-equivalent loads (DEL) under site conditions and design conditions. The DEL-based approach entails several simplifications, i.e., neglecting the load direction and the mean load. An analysis based on stresses can mitigate the above simplifications. In this work, the LTE of a 1.5 MW turbine is assessed on the basis of the DEL-based approach and compared to that of the stress-based approach. The assessment focuses on components which are critical for the lifetime, i.e., the blade bolts, the blade root laminate, and the main shaft. Generic models of these components are implemented to calculate stress histories for the stress-based assessment. In addition, the main assumptions that may have to be based on estimates are outlined. The analysis shows that, depending on the component, the results of the stress-based approach may differ notably from those of the DEL-based approach. The stress-based approach can be used to improve model fidelity in LTE assessments.
Cage Loads of Wind Turbine Blade Bearing
Authors: Buescher M , Leupold S , Schelenz R , Jacobs G 
Published: September 2020

Abstract
Slewing bearings of wind turbine blades pitch system allow the required oscillation while transferring complex dynamical loads from the rotor blades towards the hub. Most common for this application are double rowed four-point contact ball bearings consisting of two rings and two rolling element sets either equipped with a ring cage or with spacers. By equally distributing the rolling elements along the circumference, the bearing cage balances the loads along the raceways and thereby actively prevents damage mechanisms. However, cage stress analyses imply further optimization potentials leading to higher load capacities and the prediction of cage damaging mechanisms via dynamic simulations. This contribution presents a simulation procedure that calculates rolling element dynamics at the presence of a ring cage by taking the perpendicular movement of the contact ellipses due to the elastic deformations of the bearing rings into account. The procedure is carried out exemplary by the load spectra of a 3 MW reference wind turbine and the actual geometry of a 2.4m diameter bearing.
DES vs RANS: The flatback airfoil case
Authors: Papadakis George , Manolesos Marinos , Diakakis Konstantinos , Riziotis Vasilis A. 
Published: September 2020

Abstract
Using flatback airfoils at the root of wind turbine (WT) blades is becoming more popular as the WTs increase in size. The reason is that they provide significant aerodynamic, aeroelastic and structural benefits. However, due to the blunt trailing edge (TE), the wake of such airfoils is highly unsteady and rich in three-dimensional vortical structures. This poses significant challenges on the numerical simulation of the flow around them, given the highly unsteady, three-dimensional turbulent character of their wake. In this work, computational predictions for a flatback airfoil employing both RANS and DES approaches on three successively refined grids up to 25 million cells are compared with available experimental data. Results suggest that even though URANS and DDES are in good agreement in terms of lift and drag, RANS simulations fail to accurately capture the turbulent wake unsteady characteristics.
Quantification of the Dynamic-stall Model Uncertainty in the Performance Prediction of Vertical Axis Wind Turbines
Authors: Persico Giacomo , Sanvito Andrea G. , Dossena Vincenzo 
Published: September 2020

Abstract
Low-fidelity predictions for vertical-axis wind turbines are affected by uncertainty due to the complexity of the rotor aerodynamics. In particular, in the most common operating conditions the blades undergo periodic excursions beyond the static stall limit, activating dynamic-stall effects. In this study we show how advanced dynamic-stall models, implemented in the frame of the Blade-Element-Momentum theory, are able to upgrade significantly the prediction of low-fidelity tools, both in deterministic and probabilistic terms. In particular, an uncertainty quantification is performed to investigate the epistemic uncertainty of the Strickland dynamic-stall model, introducing a large variability on the empirical parameters appearing in the formulation. The resulting variability in the power coefficient and torque exchange, compared to corresponding wind-tunnel and high-fidelity CFD values, remains relatively limited and, in the conditions around peak efficiency, it is comparable with the measurement uncertainty of the experiment. As a further relevant conclusion, the model uncertainty does not alter the general outcome of the deterministic model, thus demonstrating the robustness of the DMST predictions obtained in the present study.
Active Aerodynamic Load Control for Improved Wind Turbine Design
Authors: Griffith D. Todd , Fine Neal E. , Cooney John A. , Rotea Mario A. , Valerio Iungo G. 
Published: September 2020

Abstract
Historically, cost reduction in wind energy has been accomplished by increasing hub heights and rotor diameters to capture more energy per turbine. However, larger wind turbines cannot be expected to lead to lower LCOE without the addition of new technologies. Capital costs grow rapidly with rotor diameter, faster than the rated power, because as rotor diameter increases, the blades get heavier and more costly. The growth in blade mass with blade length is accelerated by the additional structure that must be added to withstand unsteady aerodynamic loads caused by turbulence, gusts, wind shear, misaligned yaw, upwind wakes, and the tower shadow. This paper presents a holistic design solution to integrate active load control via dielectric barrier discharge (DBD) plasma actuators into wind turbine rotors along with initial findings on load reduction, actuator development, and rotor mass reduction.
Effects of yaw misalignment on platform motions and fairlead tensions of the OO-Star Wind Floater Semi 10MW floating wind turbine
Authors: Özinan Umut , Kretschmer Matthias , Lemmer Frank , Cheng Po Wen 
Published: September 2020

Abstract
Floating offshore wind farms (FOWFs) are subject to wake effects similar to onshore and fixed-bottom offshore wind farms. Due to the smaller surface roughness, wake recovery is slower in offshore compared to onshore wind farms. Therefore, wake mitigation methods will become relevant for FOWFs as it is for onshore and fixed-bottom offshore wind farms. A common method is to apply yaw misalignment to steer the wake out of the rotor of the downwind turbine. However, it is necessary to investigate possible effects of yaw misalignment on a single turbine before farm-level investigations. This study focuses on the motion response of the platform and damage equivalent loads (DELs) of the mooring line tensions of the OO-Star Wind Floater Semi 10MW floating wind turbine. In order to satisfy environmental variability, the effect of yaw misalignment is investigated at different wind directions and wind/wave misalignments. DELs of three mooring line tensions at the fairleads were calculated using the rainflow algorithm and were reported as relative, normalized with respect to the aligned nacelle operation. Floater motions were analyzed to understand effects of misaligned operation. The results indicate that yawed inflow leads to a complex coupled response of floating offshore wind turbines (FOWTs). At slightly above rated wind speed, the yaw misalignment generally increases DELs due to the thrust increase. Conversely, yaw misalignment operation at below rated wind speeds leads to different responses on lines depending on environmental conditions. The steady state floater position has a large impact on DELs which may lead to a yet unknown response behavior of FOWFs. DEL trends in the presence of wind/wave misalignment are similar to those of aligned wind and wave.