bluuebunny/crossref_metadata_2025 huggingface dataset:
Co-optimization of the shape, orientation and layout of offshore wind farms
Authors: Feng Ju , Shen Wen Zhong
Published: September 2020

Previous studies on wind farm design optimization usually assume that the wind farm boundary is fixed. However, due to the uneven distribution of energy for different wind directions, the wind farm boundary, which determines the possible shape and orientation of the wind farm, can have large influences on the wind farm’s energy production. Towards an overall design optimization, the shape and orientation of wind farm should also be considered as part of the design variables. To tackle this problem, we formulate a co-optimization problem of the wind farm’s shape, orientation and layout to maximize AEP under several realistic constraints. As an initial effort, we limit the wind farm boundary to parallelogram shapes. A two-stage optimization method is then proposed to solve the co-optimization problem. The case study on the Horns Rev 1 offshore wind farm demonstrates the importance of choosing proper shape and orientation and the effectiveness of the proposed optimization method.
Distributed Aerodynamic Control using Active Trailing-Edge Flaps for Large Wind Turbines
Authors: Feil Roland , Abbas Nikhar , Bortolotti Pietro , Johnson Nick , Mertz Ben
Published: September 2020

This work presents a numerical framework to investigate distributed aerodynamic control devices for application in large wind turbines. Tool capabilities were extended to facilitate multiple aerodynamic polar tables. The airfoil aerodynamics characteristics were automatically determined, and blade-pitch, generator-torque, and trailing-edge-flap controllers were tuned in-the-loop according to a specific blade design. This automated workflow allows analysis and optimization of trailing-edge flaps, enabling codesign studies. Results targeted reductions of root-flap-bending moment derivatives. The applied trailing-edge-flap control reduced the standard deviation of root-flap-bending moments by more than 6% and benefit related parameters, e.g., reduce blade-tip deflections, by up to 8%. Because of varying thrust distributions along the blade span, different flap designs have nonlinear characteristics in terms of the control objective and show best performance when located at the radial position with maximum thrust. In general, larger flaps provide a greater influence to reduce the target control objective.
Optimization of floating wind turbine support structures using frequency-domain analysis and analytical gradients
Authors: Dou Suguang , Pegalajar-Jurado Antonio , Wang Shaofeng , Bredmose Henrik , Stolpe Mathias
Published: September 2020

A framework for conceptual optimal design of floating wind turbine support structures including mooring system is proposed. A four degree-of-freedom frequency-domain model is used for the dynamic response of the floating wind turbine subjected to wind and wave loads. The framework allows for integrated design optimization involving the geometrical properties of the floater and the mooring system and inclusion of long realizations of multiple load cases in the analysis. Analytical design sensitivities of the governing frequency-domain equations and the design requirements are developed. This ensures that modern numerical optimization methods can efficiently be used to solve the design problem. The framework is applied to sizing optimization of a spar-buoy floater including the mooring system. The inclusion of dynamic constraints in the design optimization is demonstrated. The optimization provides designs accurately satisfying optimality conditions in minutes.
IEA Wind Task 32 and Task 37: Optimizing Wind Turbines with Lidar-Assisted Control Using Systems Engineering
Authors: Simley Eric , Bortolotti Pietro , Scholbrock Andrew , Schlipf David , Dykes Katherine
Published: September 2020

Lidar-assisted control is a promising technology for reducing the levelized cost of energy from wind turbines, but quantifying its impact at the overall system level requires sophisticated systems engineering analysis and optimization frameworks. The joint workshop on Optimizing Wind Turbines with Lidar-Assisted Control Using Systems Engineering was held by the International Energy Agency Wind Task 32 (Lidar) and Task 37 (Systems Engineering) in October 2019 to address this challenge. This paper summarizes the outcome of the workshop and presents a road map for further research. The most promising applications of lidar-assisted control identified at the workshop and discussed here include 1) increasing annual energy production, 2) decreasing capital expenditure costs by reducing design loads, 3) extending turbine lifetime by reducing operating loads, and 4) enabling wind turbine class upgrades. For each application, we review the state of the art and highlight remaining research needs. Finally, we discuss strategies for addressing these research needs by conducting high-fidelity systems engineering optimizations.
Design optimization of a wind turbine blade under non-linear transient loads using analytic gradients
Authors: Iori Jenna
Published: September 2020

This work presents and compares two formulations for the co-design optimization of a wind turbine blade under non-linear transient loads: the Nested Analysis and Design (NAND) and the Simultaneous Analysis and Design (SAND) approaches. Analytic sensitivies are used in order to ensure the convergence of the optimization within reasonable computational resources. The two formulations are compared on a mass minimization problem with dynamic constraints, solved with the interior-point method in IPOPT, for a gust input and a turbulent input. Results shows that the NAND and SAND approaches converge towards the same optimum with similar performances. The SAND approach benefits from a simpler design sensitivity analysis and a sparse jacobian of the constraints.
Innovative alternatives for repowering offshore wind farms
Authors: He Wei , Grant Richard J , Baden Victoria , Suntharalingam Jarani
Published: September 2020

To deliver a climate neutral Europe by 2050 there is an unprecedented urgency to decarbonise Europe’s electricity supply. The offshore wind industry is gearing up to this challenge with an increase in the rate at which high generating capacity offshore wind farms (OWFs) are deployed. Innovative repowering integrates early decommissioning and repowering of OWFs by using future large wind turbines (WTs, e.g. 20 MW WT). First-of-its-kind case studies have been presented to quantify the increased power generation capacity and the levelized cost of energy (LCOE) of repowering two OWFs with fixed and floating foundations. The repowering alternatives have been compared with the base case scenario (which involves decommissioning after the design life of 20 years) and the lifetime extension scenario (decommissioning in 25th year). The case studies show that a significant increase in energy output could be coupled with a reduction in the LCOE using the same OWF sites. The capacities of the OWF with fixed and floating foundations have been increased by 2.5 times (317 MW to 800 MW) and 2 times (400 MW to 800 MW) by repowering, respectively. Compared with developing an OWF on a new site, repowering has the potential to significantly accelerate the current installation capacity. Repowering has the potential to provide a competitive alternative to the lifetime extension of OWFs. Furthermore, the OWF with floating foundations has greater LCOE reductions compared with the OWFs with fixed foundations. This study has also provided evidence that enabling technologies and collaboration with other sectors would reduce the environmental impacts and costs of decommissioning of OWFs. This paper has suggested a way forward for research and development to overcome both technological and non-technological barriers to unlock the potential benefits of innovative alternatives of repowering OWFs.
An Unsteady Actuator Line Solver to Enable Adjoint Sensitivity Studies for Wake Steering
Authors: Young Ethan , Allen Jeffery , Bortolotti Pietro , King Ryan , Barter Garrett
Published: September 2020

This study demonstrates the sensitivity of wind turbine wake steering performance to blade design. An actuator line model was implemented within an unsteady adjoint solver that enables efficient execution of gradient-based optimization and sensitivity studies. After first confirming the feasibility of wake steering by controlling actuator line chord profiles and formulating a suitable objective function for wake position, a sensitivity study was conducted to determine the relative importance of chord length as a function of spanwise position on the resulting turbine wake deflection. The results presented here support the idea that blade design choices play a role in wake control. In a larger context, this study demonstrates a computational framework in which turbine and blade designs can be studied at the individual and farm-wide level to enhance wind plant controllability and manage power output.
Uncertainty propagation and sensitivity analysis of an artificial neural network used as wind turbine load surrogate model
Authors: Schröder Laura , Dimitrov Nikolay Krasimirov , Aasted Sørensen John
Published: September 2020

Recent studies have shown the advantage of replacing aeroelastic simulations with regression models based on Artificial Neural Networks (ANNs), which can be used as surrogate models for fast and efficient wind turbine load assessments. Once trained on a high-fidelity load simulation database covering a broad range of conditions, the surrogate model can be applied to predict loads for any site with wind climate falling within the range covered by the database. The aim of this study is to quantify the uncertainty propagation through such an ANN and to analyse how much the selected input variables influence the variance of the fatigue blade load estimations by means of a global sensitivity analysis. Results confirm that the selected ANN architecture seems suitable for this task resulting in small output uncertainties. Furthermore, the sensitivity analysis shows that the turbulence is mainly responsible for the blade load estimation, followed by the wind shear and the wind speed. The contributions of the turbulence length scale, turbulence anisotropy factor and wind veer angle are comparatively low. Comparing three different methods for sensitivity analysis shows that the partial derivative algorithm, Sobol variance decomposition and Shapley effect result in similar sensitivity measures.
Comparison of a Direct Drive Wind Turbine with and without LSS-Coupling Regarding Air Gap Displacement
Authors: Baseer A , He Y , Schelenz R , Kari A , Roscher B , Jacobs G
Published: September 2020

Wind turbines (WT) are highly coupled and complex systems. The dynamic interaction between various components is especially pronounced for multi megawatt wind tur-bines. As a result, the design process is generally split into several phases. [3]

The first step consists of creating a global aero-elastic model that includes essential dynamics of structural components using the minimum possible number of degrees of freedom (DOFs), with the most important simplifications concerning the drivetrain and rotor nacelle assembly. This approach is not suitable to analyse the generator air gap of a direct drive wind turbine. In this paper a more detailed simulation model is depicted, which calculates realistic deformation of the wind turbine and in particular the displacement between the generator-rotor and generator-stator. For these analysis a detailed multibody simulation (MBS) model of a generic direct drive WT is set up and used to analyse the generator air gap [3].

Several concepts of a direct drive WT are available on the market. One of them includes a coupling between the hub and generator-rotor to decouple the input wind loads from the generator and thus to decrease the impact on the generator air gap deflection [5]. The aim of this paper is to compare such a concept with a conventional concept without coupling.
Acceleration of vortex-particle simulations using a pseudo-particle method
Authors: Thönnißen F , Fritz E K , Schröder W
Published: September 2020

Vortex methods like vortex-lattice or vortex-panel methods are particularly promising to enhance the industrial aerodynamic design process of modern wind turbines. However, despite their advantages over low order methods, like the blade-element-momentum theory, vortex methods share an essential disadvantage. Their computational cost rapidly increases, which is due to their n-body problem characteristics. To overcome this issue, a method that neither relies on multipole expansions nor on a multi-grid approach is presented. Based on the aerodynamic simulation of the MEXICO rotor, it is shown that the proposed vortex pseudo-particle method (VPPM) is able to reduce the computational cost related to the n-body problem of vortex methods to O(n × log(n)). However, its application is not only advantageous in terms of the reduction of the computational cost. Since vortex-and pseudo-particles share the same properties, the VPPM’s implementation is quite simple compared to that of fast multipole methods. Furthermore, no method specific boundary-conditions need to be imposed as in the case of multi-grid methods. Therefore, the presented pseudo-particle approach is an attractive alternative to fast multipole or multi-grid methods.
Impact of manufacture-induced blade shape distortion on turbine loads and energy yield
Authors: Rosemeier M , Saathoff M
Published: September 2020

A blade shape distortion develops during the manufacturing process. The distortion is defined as the difference between the target blade shape as per the design and the shape under operational temperatures. This initial distortion varies under operational temperatures due to the different thermal coefficients of expansion of the various blade materials. The resulting manufacture-induced distortion and operational temperatures affect the twist angle, the cross-sectional shape, and the sweep of the blade. Young’s modulus of the blade’s raw materials, i.e., the matrix of the fiber-reinforced polymers and the adhesive material, changes during the manufacturing process, complicating the determination of the distortion. This work calculates the shape distortion for a reference temperature on the basis of a thermal stress analysis using a full 3D finite element blade model. It performs an aero-elastic load simulation for two models: one with the target blade shape and one with the distorted shape. The simulation reveals that the turbine with the distorted shape has an energy yield which is 0.5% lower and a lifetime extension of additional 4. 4 years.
A numerical study of Vertical Axis Wind Turbine performances in twin-rotor configurations
Authors: Guilbot M , Barre S , Balarac G , Bonamy C , Guillaud N
Published: September 2020

Placing two counter-rotating rotors of a Vertical Axis Wind Turbine (VAWT) can lead to a significant power enhancement and a faster wake resorption. This global power output is directly related to the spacing between both rotors permitting a mutual confinement effect. In addition, the relative direction of angular velocity of both rotors can strongly impact the overall performances of the machine. A range of two-dimensional (2D) Unsteady Reynolds Averaged Navier-Stokes (URANS) simulations has been managed in order to study the aerodynamic interactions occurring in a pair of VAWT. By comparing with a single-rotor of VAWT, it has been shown than the global power enhancement of a double-rotor VAWT is linked with an extension of the lift production range in one of the two first quartiles of the upwind path. Moreover, the region of the extra power generation seems to be dependant on the relative rotational directions of counter-rotating rotors. In all cases, the extent of lift generation can be associated with a suppression of the cross-stream velocity induced by the confinement of the neighbouring turbine. This local flow perturbation, closed to the inner region, leads to an augmentation of the incidence experienced by the blades in the upwind path, increasing the global lift and torque recovered by the turbine.
Large wind turbine edge instability field validation
Authors: Volk Dillon M. , Kallesøe Bjarne S. , Johnson Scott , Pirrung Georg R. , Riva Riccardo , Barnaud Félix
Published: September 2020

Wind turbines can exhibit flutter-like edgewise instabilities past a critical rotor speed. These edgewise instabilities are dominated by an edge-torsion coupling due to flapwise blade deflection. This paper experimentally validates the predicted stability limit of a 7 MW machine. State of the art simulation software is used to compare against recorded test data and characterize the observed flutter mechanism. The critical rotor speed at which the edgewise instabilities occur at in field measurements can be predicted by time domain simulations and stability analysis with very good agreement.
Assessing wind turbine energy losses due to blade leading edge erosion cavities with parametric CAD and 3D CFD
Authors: Castorrini A. , Cappugi L. , Bonfiglioli A. , Campobasso M.S.
Published: September 2020

Wind turbine leading edge erosion is a complex installation site-dependent process that spoils the aerodynamic performance of wind turbine rotors. This gradual damage process often starts with the formation of pits and gouges leading ultimately to skin delamination. This study demonstrates the application of open source parametric CAD functionalities for the generation of blade geometries with leading edge erosion damage consisting of pits and gouges. This capability is key to the development of high-fidelity computational aerodynamics frameworks for both advancing knowledge on eroded blade aerodynamics, and quantifying energy losses due to erosion. The considered test case is an offshore 5 MW turbine featuring leading edge pit and gouge damage in the outboard part of its blades. The power and loads of the nominal and damaged turbines are determined by means of a blade element momentum theory code using airfoil force data obtained with 3D Navier-Stokes computational fluid dynamics. An annual energy loss between about 1 and 2.5 percent of the nominal annual energy yield is encountered for the considered leading edge damages. The benefits of adaptive power control strategies for mitigating erosion-induced energy losses are also highlighted.
Estimating a nonparametric data-driven model of the lift on a pitching wing
Authors: Siddiqui M F , Decuyper J , Csurcsia P Z , Ertveldt J , Troyer T De , Schoukens J , Runacres M C
Published: September 2020

In aerodynamics, as in many engineering applications, a parametrised mathematical model is used for design and control. Often, such models are directly estimated from experimental data. However, in some cases, it is better to first identify a so-called nonparametric model, before moving to a parametric model. Especially when nonlinear effects are present, a lot of information can be gained from the nonparametric model and the resulting parametric model will be better. In this article, we estimate a nonparametric model of the lift force acting on a pitching wing, using experimental data. The experiments are done using the Active Aeroelastic Test Bench (AATB) setup, which is capable of imposing a wide variety of motions to a wing. The input is the angle of attack and the output is the lift force acting on the NACA 0018 wing. The model is estimated for two different types of input signal, swept sine and odd-random multisine signals. The experiments are done at two different pitch offset angles (5° and 20°) with a pitch amplitude of 6°, covering both the linear and nonlinear aerodynamic flow regime. In the case of odd-random multisines nonlinearity on the FRF is also estimated. We show that the level and characterisation of the nonlinearity in the output can be resolved through a nonparametric model, and that it serves as a necessary step in estimating parametric models.
Modeling the static and dynamic characteristics of a wind turbine airfoil and validation with experimental data
Authors: Bangga Galih , Kampers Gerrit , Weihing Pascal , Arnold Matthias , Kühn Timo , Hölling Michael , Lutz Thorsten
Published: September 2020

Dynamic stall modeling remains one of the most difficult topic in wind turbine aerodynamics despite its long research effort in the past. In the present studies, comprehensive investigations involving CFD, engineering models and experimental data were performed within the joint collaborative work DSWind. Both static and dynamic characteristics of a pitching wind turbine airfoil under a turbulent boundary layer flow were considered. The results indicate that optimized time-accurate CFD calculations can serve as reference data for verifying the engineering models, which can be potentially used for tuning the models under different air flow conditions.
Application of flow control using plasma actuator to horizontal axis wind turbine
Authors: Matsuda T , Maeda T , Kamada Y , Ushigusa T , Kobayashi H , Tanaka M
Published: September 2020

In this study, the clarification of the effect of plasma flow control on the rotor performance was carried out as a wind turbine control technology. The flow fields around rotor and the loads acting on wind turbine were measured by wind tunnel test using a test wind turbine with plasma actuator. As a result, in the low tip speed ratio, the power coefficient of Plasma On is larger than that of Plasma Off and the thrust coefficient of Plasma On slightly increased. In particular, the remarkable increase in the power coefficient was 24% at the most effective tip speed ratio. The wind turbine thrust increased slightly in the low tip speed ratio regime, but was less affected by the plasma flow control. In addition, when the power coefficient and thrust coefficient were calculated using the velocity components of the flow field around the wind turbine, the same tendency as in the wind turbine performance test was shown.
Validation of noise propagation models against detailed flow and acoustic measurements
Authors: Shen Wen Zhong , Sessarego Matias , Cao Jiufa , Nyborg Camilla M , Hansen Kurt S , Bertagnolio Franck , Madsen Helge A , Hansen Per , Vignaroli Andrea , Sørensen Thomas
Published: September 2020

In this paper, four noise propagation models including the parabolic-equation based WindSTAR model, ray-tracing based Nord2000 model, Danish regulation BEK 135 model and ISO 9613-2 standard model are validated against flow and acoustic measurements of a sound source created from a speaker located at a turbine hub of 109 m height. The flow was measured with a fully instrumented met-mast at 350 m and 218 degrees from the turbine tower base. The sound was measured with 11 microphones: 8 were along a line of 45 degrees and a distance up to 1200 m away from the sound source, 3 were located at IEC positions, and 1 microphone close to the speaker, which was used to measure the source strength. White noise and 1/1 band-limited white noise sound at 2 different wind shears with exponents of 0.12 and 0.23 are used for validation. Results show that an overall agreement between experiment and computation is reached for all the numerical models. Among the 4 numerical models, Nord2000 gives the best prediction for the nearfield microphones of mic 4-mic 6 and WindSTAR gives the best prediction for the far-field microphones of mic 7 and mic 8.
Improvement of the pitch bearing load distribution by shape-optimized stiffening plates
Authors: Loriemi A , Jacobs G , Bosse D , Zweiffel M
Published: September 2020

Due to a small cross-sectional dimension compared to their diameter, pitch bearings have a comparably low stiffness. Therefore, surrounding structures strongly influence the internal load distribution of the bearing. By the use of stiffening plates, the load situation of pitch bearings can be positively affected. With a gradient-based optimization procedure regarding contact force and contact angle distribution, an optimized plate shape is determined. This optimized shape provides at the same time a more equalized load distribution, while minimizing the effect of contact ellipse truncation.
Passive self-aligning of a floating offshore wind turbine
Authors: Netzband S , Schulz CW , Abdel-Maksoud M
Published: September 2020

The development of floating offshore wind turbines opens the way for various new design types, and the platform, tower and turbine can benefit from its floating foundation. Self-aligning platforms, where the entire structure follows the wind direction are a promising concept. A single point mooring with turret system allows for free rotation around the vertical axis. Aerodynamic forces of rotor and tower induce the self-aligning moment. In the present study, the operating principle of a passive platform design with airfoil-shaped tower and downwind rotor is analyzed under steady conditions using a boundary element method (BEM). Rotor cone angle and the tower dimensions have a major influence on the yawing moment. They must be large enough to dominate the hydrodynamic forces induced by seaway and current. The passive self-aligning capability is shown in an integrated simulation for various current velocities and wind-current offset angles.
Assessment of extreme stresses and deflections on wind turbine blades with stochastic material properties using statistical extrapolation methods
Authors: Schinas P N , Serafeim G P , Manolas D I , Riziotis V A , Voutsinas S G , Philippidis T P , Chaviaropoulos P K
Published: September 2020

Since the introduction of the 3rd edition of the IEC Standard 61400-1, designers of wind turbines are required to apply statistical extrapolation techniques, to estimate the extreme (ultimate) load values corresponding to fifty-year return period. In the present paper, the certification procedure is assessed under the uncertainty of the material properties using simulated load time series of the NREL 5MW Reference Wind Turbine. The uncertainty of the material properties is introduced in the elastic properties of the composite blades by using input data from the OptiDAT composite material database. Comparison of the estimated extreme loads and deflections of the blades as well as maximum stresses, also in connection to the Tsai Wu failure criterion, is performed for different material sets. It is found that the variability of the material properties does not affect the estimated ultimate flapwise moments (difference <1.5%) but affects the maximum flapwise deflection (differences ∼8%). It is concluded that for the levels of variation considered in the composite material properties, the coefficient of variation of the extreme stresses and the Tsai Wu failure criterion are in the order of 2% and 8% respectively.
Estimation of roughness effects on wind turbine blades with vortex generators
Authors: Ravishankara A K , Bakhmet I , Özdemir H
Published: September 2020

Surface degradation of the wind turbine blades lead to a reduction while on the other hand blade add-ons like, vortex generators, lead to an increase in the aerodynamic performance. Within this study, both the reduction due to leading edge roughness and the increase due to vortex generators in the aerodynamic performance are quantified individually first, and then it is investigated if the vortex generators would compensate for the losses due to roughness. Roughness models for the Spalart-Allamaras (SA) and k — ω SST turbulence models are implemented in the open source CFD suite SU2 and validated against theoretical predictions and experimental data. The roughness model is then applied to a commonly used airfoil section, DU97-W-300 and steady RANS simulations are carried out with SU2. Four different conditions are considered-no erosion (clean), eroded (rough), clean blade section with VGs and rough blade section with VGs. Numerical simulations are validated with experimental data for the clean airfoil section and airfoil section equipped with vortex generators. Finally, a preliminary analysis is presented for each of the cases considered on the effect of power production.
Impact of Carrier Position and Surrounding Stiffness on Planet Carrier Bearing Loads
Authors: Krause A , Gnauert J , Jacobs G , Bosse D
Published: September 2020

Bearing failures in wind turbine gearboxes still occur which results in downtimes of wind turbines and hence increase the levelized costs of energy. One of these affected bearings is the generator sided planet carrier bearing as it is still subject to unexpected fatigue related failure modes. To reduce the occurrence of these failures a methodology is presented to determine the most influencing wind loads on this planet carrier bearing using the finite element method. Therefore a 1 MW wind turbine drive train model is used and the influence of several parameters influencing the load distribution of the bearing are investigated. These parameters are the surrounding stiffness of the bearing seat, the influence of the individual components of the wind loads and the circumferential position of the planet carrier. The results show that a realistic surrounding of the bearing seat leads to an unequal circumferential load distribution while the bearing itself is less loaded than using the generic surrounding of the bearing seat. The different components of the wind loads do not have much impact on the load distribution while the force acting on the rollers change. The different positions of the planet carrier also do not affect the circumferential load distribution.
The 3D effects of a vertical-axis wind turbine: rotor and wake induction
Authors: Tavernier D. De , Ferreira C. , Paulsen U. , Madsen H.
Published: September 2020

This paper deals with the 3D effects of a vertical-axis wind turbine caused by the tip vortices. In this study, the VAWT rotor is simplified by the infinitely bladed actuator cylinder concept. The loads are prescribed to be uniform and normal to the surface and are distributed between the upwind and downwind half. Depending on the load configuration, the tip vortices are shed at different locations. This causes the wake induction field and the induction at the rotor to be significantly different. The 3D effects cause a power loss that may go up to 15% depending on the load configuration and aspect ratio. Starting from an aspect ratio of 5, the rotor induction and power is approaching 2D results.
Comparison of the free vortex wake and actuator line methods to study the loads of a wind turbine in imposed surge motion
Authors: Corniglion Rèmi , Harris Jeffrey , Peyrard Christophe , Capaldo Matteo
Published: September 2020

To enable the development of floating offshore wind farms, it is important to have a clear understanding of the aerodyamic forces applied on a floating offshore wind turbine. The paper presents comparisons between a lifting line free vortex wake method and an actuator line method in the case of a wind turbine in surge with blade-resolved CFD data as a reference. Each model is compared to a quasi-steady estimation of the loads to understand the variations due to the surge movement. The near-wake flow field is investigated in order to give an insight into the flow features leading to the observed behavior. Both methods predict a higher axial velocity at the position of the rotor and one radius downstream of the rotor in surge conditions compared to the fixed case.
Experimental Comparison of the Wake of a Vertical Axis Wind Turbine and Planar Actuator Surfaces
Authors: Huang M , Ferreira C , Sciacchitano A , Scarano F
Published: September 2020

Wind tunnel experiments on a scaled vertical axis wind turbine (VAWT) and square porous plate with a porosity of 64% are conducted in the W-tunnel of TU-Delft. The VAWT thrusts in axial and lateral directions are measured with an in-house load cell system based on moment conservation. Wake of the VAWT in tip speed ratio of 1.5 and 2.5 and the porous plate is measured with the robotic particle image velocimetry technique, which enables a three-dimensional velocity measurement in a combined volume encompassing from 1 diameter upstream to 3 diameters downstream. Counter-rotating vortex pairs in VAWT wake and the wake shape deformation and deflection are discussed, which are related to the lateral thrust. A square porous plate inducing a similar axial thrust is compared, which has the same shape as the cross-section of the VAWT. Wake of the right porous plate with a yaw angle of 15° is investigated, which produces similar deflection as the VAWT.
Unsteady effects on a pitching airfoil at conditions relevant for large vertical axis wind turbines
Authors: Brunner C E , Kiefer J , Hansen MOL , Hultmark M
Published: September 2020

Interest in offshore vertical axis wind turbines (VAWTs) has created a need to study VAWTs at much higher Reynolds numbers than they have previously been studied at. VAWTs are characterised by unsteady aerodynamics, and high Reynolds numbers have the potential to alter the blade aerodynamics significantly. Here, results are reported on an airfoil that is pitched sinusoidally around zero angle of attack at Rec = 1 × 106, at a reduced frequency k = 0.15 and amplitudes of 5° < α < 20°. Since the static stall angle is not exceeded, no stall effects occur. Nevertheless, lift, drag and moment coefficients show noticeable hysteresis. Despite the high amplitudes of oscillation, lift and moment coefficients show reasonable agreement with unsteady aerodynamic theories by Theodorsen and Motta et al. with regard to the width of the hysteresis loops, but have noticeably steeper slopes.
Numerical Estimation of Anti-icing Heating Power for NREL 5MW Wind Turbine Blades in Cold Climate
Authors: Yassin Khaled , Stoevesandt Bernhard , Peinke Joachim
Published: September 2020

This work aims to give an estimate of to heating power used in the anti-icing system of the NREL 5MW wind turbine rotor operating in a cold climate in the presence of suspended water particles in the air. In this work, the turbine blade was simulated using an in-house code implemented in OpenFOAM framework that uses compressible Lagrange-Euler approach to simulate water particles transport from the atmosphere to the blade surface. On the water film side, Shallow-Water Icing Model (SWIM) is used to simulate the icing and anti-icing process. The results show that at least 3% of the rated power of the rotor should be used just to keep the water film from freezing. It shows also that most of the heating power should be concentrated on the leading edge, lower surface of the blade, and the middle sections of the blade.
Comparative analysis of offshore support structures for two-bladed large wind turbines (10+MW) in deep waters
Authors: Vergassola Marco , Cabboi Alessandro , der Male Pim van
Published: September 2020

The offshore wind market is developing towards exploiting wind resources in deeper water sites. Inevitably, this fuels new research and feasibility studies on alternative solutions for the wind turbine support structures. Within this context, this work aims at comparing three different support structure design concepts for a 14 MW two-bladed downwind wind turbine: an XXL monopile, a hybrid jacket-tower and a lattice tower. To ensure a fair comparability of the three design concepts, a load capacity analysis is first performed to assess the yield strength of each concept and guarantee a similar material utilization. The comparative analysis is then carried out in terms of total mass, dynamic behaviour of the interaction between rotor and support structure, soil-structure interaction and resulting hydrodynamic forces. Based on the given design constraints, the preliminary results of this study favour a lattice tower solution. The study also highlights peculiar dynamic phenomena such as veering, mode hybridization and mode coalescence for the dynamic interaction between the lattice tower and the two-bladed rotor which need to be taken into account during the design phase.
Investigation of flutter for large, highly flexible wind turbine blades
Authors: Kelley C L , Paquette J
Published: September 2020

Improvements to the Sandia blade aeroelastic stability tool have been implemented to predict flutter for large, highly flexible wind turbine blade designs. The aerodynamic lift and moment caused by harmonic edge-wise motion are now included, but did not change the flutter solution, even for highly flexible blades. Flutter analysis of future, large blade designs is presented based on scaling trends. The analysis shows that flutter speed decreases at a rate similar to maximum rotor speed for increasing blade sizes:

Ω

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l
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t
t
e
r

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r
a
t
e
d

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. This indicates the flutter margin is not directly affected by blade length. Rather, it was innovative design technology choices that predicted flutter in previous studies. A 100 m blade, flexible enough to be rail transported, was analyzed and it exhibited soft flutter below rated rotor speed. This indicated that excessive fatigue damage may occur due to limit cycle oscillations for blades that incorporate highly flexible designs.
Uncertainty Quantification of Leading Edge Erosion Impacts on Wind Turbine Performance
Authors: Maniaci David C. , Westergaard Carsten , Hsieh Alan , Paquette Joshua A.
Published: September 2020

Many factors that influence the effect of leading edge erosion on annual energy production are uncertain, such as the time to initiation, damage growth rate, the blade design, operational conditions, and atmospheric conditions. In this work, we explore how the uncertain parameters that drive leading edge erosion impact wind turbine power performance using a combination of uncertainty quantification and wind turbine modelling tools, at both low and medium fidelity. Results will include the predicted effect of erosion on several example wind plant sites for representative ranges of wind turbine designs, with a goal of helping wind plant operators better decide mitigation strategies.
Large Eddy Simulation of Wind Turbine Wakes during the Evening Transition Period
Authors: Chanprasert W. , Sharma R. N. , Cater J. E. , Norris S. E.
Published: September 2020

A transient simulation of a two inline wind turbines during the evening transition period is modelled using Large Eddy Simulation with an actuator disc method. Two inline turbines are placed parallel to the stream-wise direction and the distance between the turbines is seven rotor diameters. The wind profiles and turbulence data are produced by a precursor simulation. The simulation was carried out for four physical hours and the surface heat flux varies with time according to the evening period. The wind profile evolves over the transition period and the atmospheric stability gradually changes from unstable in the late afternoon to a stable condition in the evening. The wake recovers more quickly in the afternoon and it recovers more slowly corresponding to the changes in atmospheric stability. This causes a reduction in the power output and the mean thrust load of the downstream turbine over the period. Additionally, the velocity spectrum displays higher fluctuations at the downstream turbine.
An induction-aware parameterization for wind farms in the WRF mesoscale model
Authors: Mayol M L , Diaz GP Navarro , Saulo AC , Otero AD
Published: September 2020

With the aim of assessing the potential impacts of wind farms on weather and regional climate, in this work an induction-aware modified version of the Wind Farm Parameterization implemented in the WRF model is presented. It uses the undisturbed wind speed, instead of the grid cell velocity, as reference to compute the corresponding momentum sink, source of TKE and power output. The relation between the reference and grid velocity is obtained from a previous calibration process. The modified parameterization is verified by simulating one single wind turbine, showing that the power output becomes independent of the selected horizontal resolution. Finally, the performance of the new parameterization is tested over an utility-scale wind farm. It is applied to compute the wind farm efficiency for different wind directions, under several resolutions and its results compared with former parameterizations.
IEA Wind Task 31: Design of a new comparison metrics simulation challenge for wind resource assessment in complex terrain Stage 1
Authors: Barber S , Buehler M , Nordborg H
Published: September 2020

In wind energy, the accuracy of the estimation of the wind resource has an enormous effect on the expected rate of return of a project. For a given project, the modeller is faced with a difficult choice of a wide range of simulation tools with varying accuracies and costs. In previous work, a new method for estimating the skill and cost scores of different wind modelling tools for a given project has been developed. Although this method worked well, it was shown that further studies are required for a wide range of input conditions and project types in order to develop project-specific transfer functions between the predicted and actual cost and skill scores. In this paper therefore, a new simulation challenge is designed with the goal of collecting comparison metrics data regarding the skill and cost scores of a range of different simulation tools for a complex terrain site, both before and after carrying out the simulations. The complex terrain site Perdigao is chosen for the challenge, due to the volume and quality of available measurement data and the complexity of the terrain. An initial data analysis and WAsP simulations allow mast 29 to be chosen for the input data and masts 7, 10, 20, 22, 25, 27, 34 and 37 for the validation data. The WAsP simulations are compared to previous WRF simulations and are found to capture the main features of the flow over the two ridges. The wind acceleration over the two ridges and the resulting maximum wind speeds at the peak of the ridges as well as the reduced velocity regions in the valley between the ridges are well captured. However, smaller-scale features such as small areas of separated flow seen in the WRF simulations cannot be captured by WAsP. This choice of mast for the challenge will therefore allow the capabilities of different tools for calculating flow in separated regions as well as on top of hills to be assessed.
Wind Tunnel Study of a “Floating” Wind Turbine’s Wake in an Atmospheric Boundary Layer with Imposed Characteristic Surge Motion
Authors: Schliffke Benyamin , Aubrun Sandrine , Conan Boris
Published: September 2020

Floating offshore wind turbines (FOWTs) are a potential source for increased offshore energy production. As the technology is still in a pre-industrial state several questions remain to be addressed where little field data is available. This study uses physical modelling at a reduced scale to investigate the unsteady behaviour and the development of the wake in a simplified FOWT model. A porous disc model is placed in an atmospheric wind tunnel and subjected to a range of different motions. The effects of induced sinusoidal surge motion on the characteristics of the model’s wake at a fixed downstream distance of 4.6D are studied. First results show unchanged mean velocity values but modified turbulence intensity and turbulent kinetic energy profiles. Spectra taken in the wake show shifts to higher frequencies in the longitudinal flow component while the opposite is true for the vertical flow component. Further research into spatial characteristics of the wake are necessary to confirm these results.
POD analysis of the recovery process in wind turbine wakes
Authors: De Cillis G , Cherubini S , Semeraro O , Leonardi S , De Palma P
Published: September 2020

The wake produced by a single three-bladed wind turbine is investigated using the proper orthogonal decomposition (POD) of numerical data obtained by a large eddy simulation. The rotor blades are modeled using the actuator line method, whereas tower and nacelle are simulated through an immersed boundary method. The POD is performed in a three-dimensional subdomain enclosing the wake after conducting a convergence test, which demonstrates that the first ten modes are well converged. Most energetic POD modes identify and isolate different flow features characterising the wake dynamics, such as the tip-vortices spirals, the von Kárman vortices shed by the tower, the Kelvin-Helmholtz instability linked to the wake shear layer. Very low frequency modes are also found, which could be related to the wake meandering phenomenon. Moreover, the wake recovery process is studied by computing the contribution of each POD mode to the mean-kinetic-energy entrainment. This analysis indicates that tip vortices negatively affect the wake recovery, since they provide a negative entrainment. On the contrary, flow structures related to the tower wake are found to be beneficial to wake recovery, demonstrating the importance of including tower and nacelle in numerical simulations.
Increasing turbine dimensions: impact on shear and power
Authors: Barthelmie R.J. , Shepherd T.J. , Pryor S.C.
Published: September 2020

As wind turbine average hub-height (H) and rotor diameter (D) grow, it is assumed that the benefit derived from larger swept areas and higher wind speeds at higher altitudes will outweigh any increase in fatigue loading due to higher shear and manufacturing/installation costs. The impact of increasing wind turbine H and D on power production and the occurrence of extreme positive and negative shear is examined using high-resolution simulations with the Weather Research and Forecasting (WRF) model over Iowa. Three wind turbine scenarios are considered; S#1: H=83m, D=100m; S#2: H=100 m, D=100 m and S#3: H=100 m, D=133 m. Increasing H from 83 m to 100 m while maintaining D=100 m increases power by 16% relative to scenario 1. Increasing D to 133 m from 100 m with H=100 m doubles the power output compared to S#1. Extreme shear across the rotor plane (shear exponent α > 0.2 or α < 0) is frequently observed, but only modestly impacted by the changes in wind turbine dimensions. Thus, the increase in power output from increasing H and D to these levels seems to incur little penalty in terms of increased occurrence of high positive or negative shear.
Evaluation of the WRF model for simulating surface winds and the diurnal cycle of wind speed for the small island state of Fiji
Authors: Dayal K K , Cater J E , Kingan M J , Bellon G D , Sharma R N
Published: September 2020

Evaluation of the performance of the WRF model is carried out for simulating the surface winds and the diurnal cycle of wind speed for the small island developing state of Fiji at a 1.33 km by 1.33 km grid resolution using 1deg gridded data from NCEP-FNL. Simulations are performed for an austral summer (January 2017) and an austral winter (July 2017) month using the dynamical downscaling and the two-way nested approach. A set of physics parameterization schemes together with topo_wind = 1, 2 and ysu_topdown_pblmix = 1 physics settings associated with YSU PBL scheme are used to correct the surface winds and the diurnal cycle of wind speed. The results reveal that the WRF model is able to capture the surface winds and the diurnal cycle of wind speed on the windward side. Surface winds on the leeward side and the outer islands, show positive bias especially at nighttime for January and at both the day and night time for July. The statistical evaluation of all stations for January (July) showed a bias of 1.16 m/s (1.89 m/s), RMSE of 2.40 m/s (3.14 m/s), STDE of 1.88 m/s (2.08 m/s) and diurnal cycle correlation of 0.74 (0.68) using topo_wind = 2 and ysu_topdown_pblmix = 1.