bluuebunny/crossref_metadata_2025 huggingface dataset:
Effects of induction and wake steering control on power and drivetrain responses for 10 MW floating wind turbines in a wind farm
Authors: Binsbergen Dirk Willem van , Wang Shuaishuai , Nejad Amir R.
Published: September 2020

This paper aims to investigate the drivetrain load response caused by induction and wake steering control on two floating wind turbines (FWTs) in a wind farm. In this study, two DTU 10 MW turbines, supported on the nautilus floater, are modelled using FAST.Farm. The downstream turbine is placed at the distance of seven rotor diameters (D) from the upstream turbine in the positive wind direction. Partial wake shading is considered for wake steering control and full wake shading is considered for induction control. An ambient wind speed of 8m/s is used and a representative sea state is selected. The test cases are defined based on different blade pitch and yaw angles of the upstream turbine. Power generation of the offshore wind farm is studied under different test cases. A decouped analysis approach is used to investigate drivetrain response. Global responses are obtained from FAST.Farm. These loads are used as input of the 10 MW wind turbine drivetrain model for the gears and bearings load response analysis. Results show that both induction and wake steering control lead to a limited increase in power generation of the wind farm. Additionally, both control methods affect the drivetrain response statistics, while the features are different. This study facilitates a better understanding on drivetrain dynamic behaviour in a wind farm perspective, which serves as a reference for the wind farm optimizaton in the future.
On wind farm wake mixing strategies using dynamic individual pitch control
Authors: Frederik Joeri , Doekemeijer Bart , Mulders Sebastiaan , van Wingerden Jan-Willem
Published: September 2020

Dynamic wind farm control is a new strategy that aims to apply time-varying, often periodic, control signals on upstream wind turbines to increase the wake mixing behind the turbine. As a result, wake recovery is accelerated, leading to a higher power production of downstream turbines. As the amount of interest in dynamic control strategies for wind turbines in a wind farm is increasing, different approaches are being proposed. One such novel approach is called Dynamic Individual Pitch Control (DIPC). In DIPC, each blade pitch angle of a turbine is controlled independently to dynamically manipulate the direction of the thrust force vector exerted on the wind. Hence, the direction of the wake is varied, inducing wake mixing without significant thrust force magnitude variations on the rotor. In this paper, the effectiveness of different variations in the thrust direction are evaluated and compared using Large Eddy Simulation (LES) experiments.
Model approach for electromagnetically excited mechanical vibrations in direct-drive wind turbines
Authors: Mülder Christoph , Duda Tobias , Jacobs Georg , Hameyer Kay
Published: September 2020

In this paper a multi-physical modelling approach for calculating parasitic magnetic force excitations in wind turbine generators is presented. These parasitic excitations are dependent on the magnetic field distribution in the air gap and the air gap geometric shape, which is dependent on the external loads of the wind turbine. With this methodology mechanical multibody simulation is coupled with electromagnetic field calculation by means of finite-element calculation. First the general methodology and the calculation of the air gap width distribution is presented and subsequently the electromagnetic field and force calculation is explained.
Lightweight optimal rotor design of a 10MW-scale wind turbine using passive load control methods
Authors: Serafeim Giannis P. , Manolas Dimitris I. , Riziotis Vasilis A. , Chaviaropoulos Panagiotis K.
Published: September 2020

The present paper investigates the potential to reduce the mass of the blade of the 10MW DTU Reference Wind Turbine through build-in, material bend-twist coupling (BTC). It is materialized by introducing an offset angle on the plies of the uni-directional material over the spar caps of the blade. Optimum BTC designs are obtained on the basis of an integrated optimization framework combining an aeroelastic solver for the calculation of the structural loads of the blade and a cross-sectional tool that provides beam-like structural properties of the blade and stresses distributions. The derived designs are verified based on a subset of representative fatigue and ultimate design loads cases of IEC 61400-1. Reduction of the combined bending moment at the root of the blade by 5% and reduction of the blade mass by 10% is achieved with a hybrid model consisting of three span-wise segments having different constant ply offset angles.
A control cost criterion for controller tuning of two- and three-bladed 20 MW offshore wind turbines
Authors: Anstock Fabian , Schorbach Vera
Published: September 2020

When wind turbine concepts are modified, the difference in characteristic behavior and the impact on the cost of energy are of great interest. However, before comparing it is mandatory to re-adapt controller gains or weights to altered turbine dynamics in order to guarantee a still suitable overall system design. As an expenditure-based solution, the proposed control cost criterion could help to assess changes of turbine concepts better due to a clear and objective controller tuning. Conflicting design objectives are therefore assessed on their approximated influence on the cost of energy in order to result in the most cost-beneficial trade-off. These objectives are variations of component loads, actuator usage, and energy production. The criterion is applied to three- and two-bladed 20 MW offshore turbines with turbulent wind for full load operation, exemplarily. This provides insight into the benefits and limitations of low controller integral pitch gains and results in potential cost reductions for all turbines.
Development of a digital twin of an onshore wind turbine using monitoring data
Authors: Pimenta F , Pacheco J , Branco C M , Teixeira C M , Magalhães F
Published: September 2020

The present paper has two main objectives: (1) to create a digital twin of an onshore wind turbine to provide to the owner of the wind farm a tool for continuous tracking of accumulated fatigue damage and evaluation of alternative operation strategies; (2) to perform the first tasks for the creation a reliable numerical of a floating wind turbine to simulate experimental data for the testing of monitoring tools on this type of wind turbines. A numerical model of an onshore wind turbine was created with FAST software developed by the US National Renewable Energy Laboratory (NREL). The structural and mechanical properties of the structure were calculated from the geometric properties of the tower and blades. The aerodynamic properties of different sections of the blades were computed with 2D models created with ANSYS Fluent. Having the wind turbine fully characterised, the control mechanisms were calibrated using data from the manufacturer catalogues. Numerous time series of wind excitation for different operation conditions were generated, and the structural response computed. Additionally, the Supervisory Control and Data Acquisition (SCADA) operating in parallel with a series of extensometers and accelerometers located in strategic points along a real wind turbine tower and blades provided a full description of the internal forces in both locations and the estimation of the modal properties for different operating conditions. Both measured and simulated response allowed the identification and validation of structural dynamic properties and static and dynamic internal loads. In spite of several approximations and simplifications, the results obtained with the numerical model were in good agreement with the ones measured in the field.
Loop shaping based robust control for floating offshore wind turbines
Authors: Yu Wei , Lemmer Frank , Schlipf David , Cheng Po Wen
Published: September 2020

In this work, a thorough and complete methodology for the widely used SISO controller is described for floating offshore wind turbines (FOWTs). The motivation is to develop clear, easy implementable and automated design criteria of blade pitch control design, which takes both stability and performance into account for FOWTs without adding new sensors. The primary design criteria is to achieve a similar dynamic step response behaviour, i.e. overshooting, rise time and settling time across the operating points above rated wind speeds. The proposed design procedure can be performed by lower order numerical models with only two degrees of freedom, which can be derived analytically. The minimal required system information eases an early stage controller design, as well as the system engineering and integrated substructure design. The proposed design procedure is evaluated on three state of the art floating wind turbines. The resulting gain scheduling is quite different from the one for onshore turbines. The overall response is satisfying and comparable with an existing stability-oriented robust SISO controller at operation points where stability is critical. An improved performance is found for higher wind speeds.
Modal Analysis of a Quad-Rotor Wind Turbine
Authors: Ferede Etana , Gandhi Farhan
Published: September 2020

Unlike Single-Rotor wind turbines, stability analysis of Multi-Rotor wind turbines is still in its initial stages. This paper presents the modal analysis of a Quad-Rotor wind turbine and identifies the new modes or possible instability modes that are otherwise not present on a Single-Rotor wind turbine. Multi-Blade Coordinate transformation scheme is adapted to a Quad-Rotor wind turbine to write the system’s equation of motion in fixed-reference frame followed by Eigenvalue analysis to determine the natural frequencies and mode shapes of the Quad-Rotor wind turbine. A Campbell diagram of the Quad-Rotor wind turbine is presented. Results indicate that the Quad-Rotor turbine is soft-soft to the first tower modes (fore-aft, side-side, and torsion). Furthermore, the modes with low natural frequency other than the tower modes are a combination of tower, boom, and blade modes. Therefore, due to the presence of blade modes, the modal frequency of these modes increases or decreases with increasing rotor speed due to centrifugal stiffening.
Validation of the quasi-steady performance model for pumping airborne wind energy systems
Authors: Schelbergen Mark , Schmehl Roland
Published: September 2020

The quasi-steady performance model (QSM) has been developed specifically for pumping airborne wind energy systems using flexible membrane wings. In this study, we validate this model using a comprehensive set of flight data that includes 87 consecutive pumping cycles and is acquired with the development platform of Kitepower B.V. The aerodynamic properties of the kite are determined using onboard measurements of the relative flow velocity. We found that neglecting the vertical wind component and straightening and slacking motion of the tether lead to substantial errors in the kite velocity calculated using the system model. A reasonable agreement between the QSM simulations and flight data can be obtained by multiplying the kite’s drag coefficient by a fudge factor and thereby turning the QSM into a grey-box model. The model accuracy is statistically evaluated as opposed to only evaluating a single pumping cycle per system configuration as done in earlier research.
Energy harvesting via co-locating horizontal- and vertical-axis wind turbines
Authors: Hansen M. , Enevoldsen P. , Abkar M.
Published: September 2020

Co-locating horizontal- and vertical-axis wind turbines has been recently proposed as a possible approach to enhance the land-area power density of wind farms. In this work, we aim to study the benefits associated with such a co-location using large-eddy simulation (LES) and analytical wake models. In this regard, small-scale vertical-axis wind turbines (VAWTs) in triangular clusters are deployed within a finite-size wind farm consisting of horizontal-axis wind turbines (HAWTs). Wake flow within the wind farm and the effect of VAWTs on the overall wind-farm efficiency are investigated and quantified. The results show that the optimal deployment of small-scale VAWTs has a negligible impact on the performance of HAWT arrays while increasing the total power production. For the particular cases considered here, the power output of the co-located wind farm increases up to 21% compared to the baseline case in which only the HAWTs are present. Also, by comparing to the LES results, it is shown that the analytical framework proposed here is able to accurately predict the power production of wind farms including both HAWTs and VAWTs. Finally, as a real-world application, potential benefits of deploying small-scale VAWTs inside the Horns Rev 1 wind farm are explored for various wind directions using the calibrated wake model. The results show potential for about an 18% increase in the wind-farm power production, averaged over all wind directions, for a particular VAWT layout investigated in this study. The levelized cost of energy (LCoE) for the co-located wind farm is also assessed. The simulations finds that meanwhile the installation of VAWTs increases the annual energy production (AEP) of the wind farm, it also increases the LCoE, which is caused by a) lack of operational data, and b) a low TRL (Technology Readiness Levels) for VAWTs and floating foundations.
Reynolds-averaged Navier-Stokes simulations of the flow past a leading edge inflatable wing for airborne wind energy applications
Authors: Viré Axelle , Demkowicz Patryk , Folkersma Mikko , Roullier Arthur , Schmehl Roland
Published: September 2020

In this work we present Reynolds-averaged Navier-Stokes (RANS) simulations of the flow past the constant design shape of a leading-edge inflatable (LEI) wing. The simulations are performed with a steady-state solver using a k − ω SST turbulence model, covering a range of Reynolds numbers between 105 ≤ and ≤ 15 × 106 and angles of attack varying between −5° and 24°, which are representative for operating conditions in airborne wind energy applications. The resulting force distributions are used to characterize the aerodynamic performance of the wing. We found that a γ −

R
¯

e
θ

transition model is required to accurately predict the occurrence of stall up to at least Re= 3 × 106. The work highlights similarities with the flow past a two-dimensional LEI airfoil, in particular, with respect to flow transition and its influence on the aerodynamic properties. The computed values of the lift and drag coefficients agree well with in-flight measurements acquired during the traction phase of the LEI wing operation. The simulations show that the three-dimensional flow field exhibits a significant cross flow along the span of the wing.
Scaling of Experimental Buoyancy Vortex Structures with Respect to Power Generation
Authors: Hawkes N.A. , Flay R.G.J. , Cater J.E. , Macdonald M.
Published: September 2020

A scaling or nondimensionalisation of atmospheric buoyancy vortices for power generation (based on the Oberbeck-Boussinesq assumption) is proposed that uses a published formulation from the study of Rotating Rayleigh-Benard Convection. This is combined with assumptions that the vortex flows are pseudo-cyclostrophic and that a radial Richardson number can serve as a predictor of the onset of Kelvin-Helmholtz instability leading to a transition to a turbulent plume, in order to locate the cold reservoir of the vortex when viewed as a heat engine. This permits the prediction of the behaviour of large vortices in atmosphere using data from experiments on small vortices.
Simplified approach for the optimal number of rotors and support structure design of a multi rotor wind turbine system
Authors: Störtenbecker Sven , Anselm Rudolf , Dalhoff Peter
Published: September 2020

In this study different multi rotor wind turbine systems (MRSs) are designed in such a way that the space frame, forming the connection between rotor nacelle assemblies (RNAs) and tower, will be modeled and dimensioned as an ideal truss work. The main focus is on multi digit MRSs (MD-MRS) with a high number of rotors. To dimension the tube diameters and wall thicknesses, a simplified load case is used with an adjusted safety factor for loads. Parameters that are varied are the number of rotors, depth of the space frame, as well as the location of the fixed bearing for the yaw system. As such, the simplified approach is a preliminary step helping to choose a good design parameter combination for a more detailed and comprehensive analysis. Buckling is a main design driver, as well as the thrust forces. For a high number of rotors, a fixed bearing position and therefore tower height, halfway through the space frame height seems favorable regarding the cost.
An engineering model for the induction of crosswind kite power systems
Authors: Gaunaa Mac , Forsting Alexander Meyer , Trevisi Filippo
Published: September 2020

The present paper introduces a new, physically consistent definition of effective induction that should be used in engineering models for power kite performance that use aerodynamic coefficients for the wing. It is argued that in such cases it is physically inconsistent to use disc-based induction models – like momentum models – and thus a new, physically consistent induction model using vortex theory methodology is derived. Simulation results using the new induction model are compared to the previously often used momentum method and Actuator Line (AL) CFD simulations. The comparison shows that the new vortex based model is in much better agreement with the AL results than the momentum method. The new model is as computationally light as the momentum induction method.
Initial performance and load analysis of the LowWind turbine in comparison with a conventional turbine
Authors: Madsen H Aa , Zahle F , Meng F , Barlas T , Rasmussen F , Rudolf R T
Published: September 2020

We present the preliminary design of a high capacity LowWind turbine rotor with a specific power of 100 W/m
2, a rated power of 3.4MW and a 208m diameter rotor. The turbine is designed for optimal system integration and thus with a considerably increased power production at low to medium wind speed and stopped at 13 m/s. The AEP of the turbine, due to the large swept rotor area, is 40-45% higher than the AEP of a conventional on-shore turbine, the IEA Task 37 reference wind turbine with a 130 m diameter rotor and the same rated power of 3.4 MW. Initial aeroelastic simulations show that the loads of the LowWind turbine are generally higher than on the IEA turbine although comparable for some components at steady wind. For Design Load Case 1.2 (DLC1.2) for normal operation in turbulence we see a typical increase of around 20-40% of tower and rotor fatigue loads. However, this moderate increase for such big increase of rotor diameter is only obtained due to the highly flexible light weight rotor designed with a combination of peak shaving and blade bend twist coupling and for a low stop wind speed of 13 m/s.
Steady-state aeroelasticity of a ram-air wing for airborne wind energy applications
Authors: Folkersma Mikko , Schmehl Roland , Viré Axelle
Published: September 2020

In this paper we present a computational approach to simulate the steady-state aeroelastic deformation of a ram-air kite for airborne wind energy applications. The approach is based on a computational fluid dynamics (CFD) solver that is two-way coupled with a finite element (FE) solver. All components of the framework, including the meshing tools and the coupling library, are available in open source. The flow around the wing is described by the steady-state Reynolds-averaged Navier-Stokes (RANS) equations closed by an SST turbulence model. The FE model of the cellular membrane structure includes a wrinkling model and uses dynamic relaxation to find the deformed steady-state shape. Each simulation comprises four distinct steps: (1) generating the FE mesh of the design geometry, (2) pre-inflation of the wing, applying a uniform pressure on the inside, (3) generating the CFD mesh around the pre-inflated wing, and (4) activating the exterior flow and two-way coupling iterations. We first present results for the aerodynamics of the pre-inflated rigid ram-air wing and compare these to similar results for a leading edge inflatable (LEI) tube kite. Both wings are characterized by a high anhedral angle and low aspect ratio which induce spanwise flows that reduce the aerodynamic performance. The comparison shows a better performance for the LEI wing which can be attributed to its higher aspect ratio. The aeroelastic deformation of the ram-air wing further improves the aerodynamic performance, primarily because of the increasing camber which in turn increases the lift force. A competing aeroelastic phenomenon is the formation of bumps near the leading edge which increase the drag.
Tower shadow excitation of a downwind rotor blade of a turbine with a tubular tower
Authors: van der Male P , van Schaik R , Vergassola M , van Dalen K N
Published: September 2020

The downwind configuration of wind turbines offers benefits regarding the blade-tower clearance, as during operation the blade primarily bends away from the tower. Consequently, the blades can be designed with lower stiffness. For tubular towers, however, a significant deficit of the wind speed in the tower wake occurs, resulting in fatigue-inducing vibrations. For this reason, full-height lattice towers are considered the preferred support structures for wind turbines with a downwind rotor. This work estimates the tower shadow excitation of a downwind rotor blade from a tubular tower. To this end, the blade of a commercial 6 MW downwind turbine is modelled with finite-elements. The tower wake is described on the basis of Madsen’s model and for the unsteady aerodynamic interaction Küssner’s function is adopted. At below- and above-rated wind conditions, the tower wake-induced vibrations are compared with the response of a blade of an equivalent upwind rotor, considering both the tip deflections and the root moments, the latter on the basis of damage-equivalent moments, to obtain an indication of the expected difference in fatigue damage. The downwind blade experiences vibrations with considerable larger amplitudes, especially in the out-of-plane direction. From the damage-equivalent moments it can be inferred that the blades of the downwind rotor encounter a much faster accumulation of fatigue damage.
Influence of complex terrain on a flow above a forest with clearing
Authors: Matsfelt Johanna , Davidson Lars , Brennan Kyle
Published: September 2020

Large Eddy simulations of the Lemnhult wind farm in Sweden are compared to LiDAR measurements of the site. Two scans have been performed, one in southerly direction in order to investigate the flow over a forest, another one in westerly direction, in order to investigate the flow over a clearing. A forest model is implemented in STAR-CCM+ using the local forest height data, while assuming flat terrain in one simulation and including the terrain height in another to investigate its effect for each scan. The complex terrain simulations show the best agreement with the measurements. Uncertainty of the stratification of the measurements can be the reason for some of the difference. The vertical wind speed for the clearing scan change considerably when the terrain representation is included, but the magnitude is however still low compared to the horizontal velocity. This shows the importance of including terrain for simulations with variation in terrain height. The normalized turbulent kinetic energy for the vertical profiles show a decrease in the clearing scan compared to the forest scan which should be beneficial for wind turbines.
Experimental study of fluid-structure interaction at a model wind turbine blade using optical measurement techniques
Authors: Kröger L , Wester TTB , Langidis A , Nietiedt S , Göring M , Luhmann T , Peinke J , Hölling M , Gülker G
Published: September 2020

Experimental studies of fluid-structure interaction on a single blade of the model wind turbine MoWiTO 1.8, which has been designed as an aerodynamically downscaled wind tunnel model of the NREL 5 MW turbine, were performed. Tailored inflow generated by means of a 2D active grid is used to induce dynamic stall through periodic angle of inflow changes. The interactions at the model wind turbine blade are investigated using two high-speed and non intrusive optical measurement techniques. The aerodynamics around the turbine blade is measured using temporal highly resolved particle image velocimetry (PIV) measurements at the suction side of the blade. Synchronized measurements with a high-speed photogrammetric system are linking the aerodynamic events to high accuracy measurements of the blade deflections.
Wind speed measurement for absolute power curve determination from induction zone lidar measurements
Authors: Slinger C W , Harris M , Pitter M
Published: September 2020

There is growing interest in turbine performance assessment using nacelle mounted lidars. In the past, this has involved measuring the free stream wind speed, on the periphery of the operating turbine’s induction zone. As turbines increase in size, measuring at correspondingly longer ranges can start to introduce uncertainties in the wind speed, especially in complex terrain or when a turbine is part of an array. One way of reducing these uncertainties is to measure wind speeds inside the turbine’s induction zone, closer to the turbine’s rotor, then use various techniques to adjust these measurements to generate free stream equivalent windspeeds. This paper describes the application of both induction zone model and transfer function approaches. The results from various campaigns are presented and the two methods are compared using correlations. Both yield good results in terms of accuracy (usually gradients of 1.00 ± 0.004) and coefficients of determination (typically 0.99).
Advanced image processing for turbulence wedge detection in thermographic flow visualization on wind turbines in operation
Authors: Gleichauf Daniel , Jacob Dennis , Sorg Michael , Fischer Andreas
Published: September 2020

Environmental conditions like the presence of rainfall or insects can disturb the rotor blade surface of wind turbines in operation, triggering a premature laminar-turbulent flow transition in the boundary layer flow. The local contaminations develop a wedge-shaped surface area of turbulent flow in the area that would otherwise be laminar if the surface would be undisturbed, decreasing the size of the laminar flow regime. This change in the ratio between overall laminar and turbulent flow regime sizes has a negative impact on the aerodynamic performance of the profile, decreasing the efficiency of the wind turbine. While the spatial distribution of the flow regimes can be visualized with thermographic flow visualization, the state-of-the-art image processing method for applications on wind turbines in operation is not robust against localizing the position of the flow transition along these turbulence wedges. Therefore, this work introduces an advancement of the image processing method for localizing the flow transition in thermographic images with a focus on decreasing the localization uncertainty along the turbulence wedges. The state-of-the-art one-dimensional evaluation method is enhanced by a two-dimensional image processing method in order to increase the directional gradients at the turbulence wedges’ flanks. Six out of six previously undetected turbulence wedges are successfully detected in a flow visualization image of a rotor blade of a GE 1.5 sl wind turbine in operation. The new approach yields an improved application of the thermographic flow visualization for locating the flow transition and quantifying the reduction of the laminar flow area on disturbed rotor blade surfaces of wind turbines in operation.
Experimental load measurement on a yawed wind turbine and comparison to FAST
Authors: Samara F , Johnson D A
Published: September 2020

Current interest in unsteady aerodynamics of yawed turbines for wake redirection and cyclic load alleviation requires a clear understanding of turbine load fluctuation with azimuth position. To that end, an experimental 3.5 m diameter wind turbine rig was designed to measure rotor torque, flapwise/edgewise blade root bending moment, and normal force coefficient at r/R=0.66 and 0.82. The turbine was tested inside a large scale wind generation facility with a blockage ratio of 7%. Simultaneously and in time-resolved fashion, measurements were collected to produce phase-averaged performance parameters that are presented versus azimuth while the turbine is operating at different tip speed ratios and yaw angles. The NREL FAST code predictions were then compared to experimental data under the same conditions. It was found that in non-yawed conditions, FAST predictions were accurate but discrepancies start to emerge when the turbine is yawed and operating in dynamic stall conditions. In non-yawed cases, the discrepancy between experimental and FAST data is less than 7% when the flow is attached and increases to 22% when the flow is stalled. One of the main sources of discrepancy found is in the process of correcting the airfoil coefficients to account for 3D flow effects using AirfoilPrep.
Flux-gradient relation and atmospheric wind profiles — an exploration using WRF and lidars
Authors: Santos Pedro , Peña Alfredo , Mann Jakob
Published: September 2020

A common closure for the planetary boundary layer in numerical weather models assumes a direct relation between turbulent fluxes and the mean wind vertical gradient, i.e., the flux-gradient relation or K-theory. This assumption implies that the angle β between the momentum stress vector and the mean gradient of the velocity vector are aligned, i.e., β = 0°. This is not what we observe from measurements. We quantify the misalignment of β in offshore conditions using measurements from a long-range Doppler profiling lidar and numerical simulations from the New European Wind Atlas mesoscale model output. We compare vertical profiles of wind speed, wind direction, momentum fluxes, and β up to 500 m, hence covering the rotor areas of modern offshore wind turbines and beyond. The results show that β ≈ −18° on average, with a lower, but still non-zero, value under stable stability conditions, ≈ −7°. We illustrate that the simulations describe well the mean wind speed and momentum fluxes within the observed levels, but the characterization of wind turning effects could be improved.
Aerodynamic behavior of an airfoil under extreme wind conditions
Authors: Neunaber Ingrid , Braud Caroline
Published: September 2020

Wind turbines operate in the naturally turbulent atmospheric boundary layer. Due to strong flow variations, the aerodynamics at the rotor blades are complex. Therefore, to gain a better understanding of the effect of strong velocity and angle fluctuations on the aerodynamic behavior of an airfoil, we present a new system capable of generating rapid, strong gusts in a wind tunnel, the chopper. It consists of a rotating bar cutting through the inlet of the wind tunnel, thus generating turbulent, strong flow perturbations. Using this system and exposing an airfoil to its flow, we investigate the lift variations caused by the simultaneous, rapid velocity and angle variations. The results show that the lift response of the airfoil is directly correlated with the velocity. The lift response to changes of the angle of attack is determined not only by the change of the angle, but also by the rapidity with which it changes.
Dynamic wake tracking and characteristics estimation using a cost-effective LiDAR
Authors: Lio Wai Hou , Larsen Gunner C. , Poulsen Niels K.
Published: September 2020

Recent developments in Light Detection And Ranging (LiDAR) systems provide the possibility of remote measurement of the upcoming wind speed. Despite significant advances in remote sensing, extracting useful inflow characteristics from a limited number of line-of-sight measurements still requires assumptions of the inflow. Typically, the wind direction is derived based on the assumption of horizontal homogeneous inflow that is well satisfied in flat terrain and over sufficiently large time averages. However, such an assumption is violated if the wake from a neighbouring turbine impinges the inflow, and the velocity deficit in the wake causes a bias on the wind direction, misinterpreting as yaw misalignment by the downstream turbine. The actual yaw misalignment can be recovered by isolating the effect of the wake velocity deficit from the ambient inflow. A scanning LiDAR can easily track and characterise the wake; however, it is non-trivial for a cost-effective LiDAR with only a few fixed laser beams. Therefore, this paper presents a dynamic wake tracking and characteristics estimation algorithm for a cost-effective LiDAR. The proposed algorithm provides estimates of the wake centre location and other wake characteristics by exploiting the nature of wake meandering dynamics and state estimation theory. Assuming neutral stratification of the atmospheric boundary layer, the simulation results show that the wake position and its characteristics estimation is achievable in full and partial wake situations, thus presenting an estimation framework for potential applications, including yaw misalignment control and wake steering control.
A hardware-in-the-loop wave-basin scale-model experiment for the validation of control strategies for floating offshore wind turbines
Authors: Fontanella A. , Liu Y. , Azcona J. , Pires O. , Bayati I. , Gueydon S. , de Ridder E. J. , van Wingerden J. W. , Belloli M.
Published: September 2020

This paper presents a new hardware-in-the-loop methodology for wave-basin scale-model experiments about floating offshore wind turbines and its application as a tool for the validation of control strategies. In the hardware-in-the-loop experiments, the physical Froude-scaled wind turbine model used in conventional scale-model tests is replaced by a numerical model, measurements and a multi-fan actuator. As usual, properly-scaled waves are generated in the wave basin and the floating platform is simulated by means of a scale-model. The hardware-in-the-loop methodology was used to recreate the interaction between the collective pitch controller and the platform pitch mode that, often observed in numerical studies. In addition, the blade-root load measurement available in the numerical model of the rotor was used to implement an individual pitch control strategy. Different from in conventional experiments, the hardware-in-the-loop methodology allows to recreate a realistic three-dimensional wind field that was used to demonstrate the effectiveness of the individual pitch control. The improved emulation of the rotor loads and wind field make the hardware-in-the-loop experimental methodology an effective tool for the development and validation of control strategies for floating offshore wind turbines.
Hardware-in-the-loop Testing of Wind Turbine Nacelles for Electrical Certification on a Dynamometer Test Rig
Authors: Neshati Mohsen , Feja Paul , Zuga Adam , Roettgers Heiko , Mendonca Angelo , Wenske Jan
Published: September 2020

This paper presents the design and implementation of a hardware-in-the-loop testing framework, for electrical certification of wind turbine nacelles on a dynamometer test rig. This is to enable a realistic test environment in the laboratory, comparable to that in the field. Assessment of electrical properties of wind turbines are usually executed in the field for the complete prototype, however, in the laboratory, missing system components have to be emulated and the effect of test rig’s own actuators are to be attenuated. Therefore, implementation of an adequate real-time wind turbine simulation tool, an efficient control system for the test rig drive train, and a real-time automation system are considered and presented here. Furthermore, execution of type certification experiments according to national and international guidelines is described here and illustrative measurements are provided. The measurements in this case are obtained at Dynamic Nacelle Laboratory, while testing with the Enercon E-115E2 turbine. The results achieved demonstrate successful test execution in the laboratory and declare the advantage of test rigs.
On the accuracy of a logarithmic extrapolation of the wind speed measured by horizontal lidar scans
Authors: Theuer F , van Dooren M F , von Bremen L , Kühn M
Published: September 2020

Remote sensing-based wind power forecasts are nowadays being increasingly investigated. Long-range lidar scans are hereby often performed at low heights, causing the need for a wind speed extrapolation to hub height. In this work we analysed the accuracy of the stability corrected logarithmic wind profile and its sensitivity to atmospheric stability, wind speed and extrapolation height by means of a theoretical error estimation using error propagation. Emphasis was given to analyse the contributions of the profile’s individual variables but also considering the measurement campaign framework. We further used lidar measurements at the offshore wind farm Global Tech I to support the theoretical analysis. The logarithmic profile was found to be able to describe profiles during most situations, however, decreasing wind speeds with height cannot be represented. Results showed that due to the nature of the stability correction term extrapolation errors are largest during very stable atmospheric conditions. Here, stability estimation errors were dominant. Under near neutral and neutral atmospheric conditions the wind speed error contributed most to the overall error. We conclude that extrapolation errors can mainly be reduced by optimising the estimation of atmospheric stability using accurate measurement devices. Furthermore, the precise horizontal alignment of the lidar device is important.
Wind Turbine Tip Vortices under the influence of Wind Tunnel Blockage Effects
Authors: Soto-Valle R. , Alber J. , Manolesos M. , Nayeri C. N. , Paschereit C. O.
Published: September 2020

The current paper describes the characteristics of the tip vortex in the near wake of a three-bladed upwind horizontal axis wind turbine with a rotor diameter of 3 m. Phase-locked stereo particle image velocimetry measurements were carried out under the influence of the wind tunnel walls that create a high blockage ratio. The location of the vortex, convection velocity, core radius, and strength were investigated and compared with similar investigations, including different blockages cases. Additionally, the same performance of the wind turbine model was simulated in the open source wind turbine tool QBlade, using the lifting line free vortex wake module in the absence of the walls.

The results showed that the location of the tip vortices was more inboard the tip and more downstream the tunnel compared to the simulations and similar experiments. The convection velocity remained similar in the axial direction and changed in the lateral direction, contributing to the delay of the movement of the tip vortex outboard the tip. The strength, based on the circulation, was found with a difference of 4% between simulation and experiment.
Residual uncertainty in processed line-of-sight returns from nacelle-mounted lidar due to spectral artifacts
Authors: Brown Kenneth , Herges Thomas
Published: September 2020

An uncertainty quantification technique for nacelle-mounted lidar is developed that extends conventional error analyses to precisely account for residual uncertainty due to observed non-ideal features in processed Doppler lidar spectra. The technique is applied after quality assurance/quality control (QAQC) processing to quantify residual error, both bias and random, from solid-body interference, shot noise, and any additional uncertainty introduced to the data from the QAQC process itself. The approach follows from the one-time construction of a high-dimensional parametric database of synthetic lidar spectra and subsequent processing with an existing QAQC technique. A model of the correspondence between the spectral shape and the associated residual errors due to non-ideal features is then developed for quantities of interest (QOIs) including the geometric median and spectral standard deviation of line-of-sight velocity. The model is preliminarily implemented within a neural network framework that is then applied in post-processing to sample returns from a DTU SpinnerLidar. The initial analysis uncovers the effects of specific sources of uncertainty in the context of both individual spectra and full-field maps of the measurement domain. The technique is described in terms of application to continuous wave (CW) lidar, though it is also relevant to pulsed lidar.
Structural Analysis of a Small Wind Turbine Blade Subjected to Gyroscopic Load
Authors: da Costa M S P , Clausen P D
Published: September 2020

A study was conducted to investigate the structural response of a composite constructed small wind turbine blade subjected to gyroscopic load using finite element analysis. An Aerogenesis 5 kW small wind turbine blade was used as a case study. As part of this study, a high-fidelity finite element model of the 2.5 m long composite blade was built, and the accuracy of its predictions validated against experimental data. The blade model was loaded using a comprehensive method for applying gyroscopic load to finite element models of small-scale blades. The predicted results show gyroscopic loading causes the blade to deflect in both the flap-wise and lead-lag directions and induce reasonably high strains in the composite blade structure. The direction of the gyroscopic loading is a function of the direction of turbine yaw as well as the blade position as it rotates, therefore gyroscopic loading can both exacerbate and reduce the effects of aerodynamic loading acting on the blade.
Computational aeroacoustic prediction of trailing edge noise for small wind turbines
Authors: Zilstra Alison , Johnson David A
Published: September 2020

The study of aeroacoustic noise generated by small wind turbines is important to increase acceptance and implementation of the technology. Small wind turbines have unique challenges due to the low Reynolds number (Re) flow the blades experience, which introduces a potential for tonal noise. Computational aeroacoustics can be applied during the design stage of the turbine blades to improve acoustic performance. This work aims to validate a fully analytical aeroacoustic model by analyzing a SD 7037 blade segment at static angles of attack, with comparison to experimental flow and acoustic data. The Ffowcs-Williams and Hawkings (FW-H) acoustic model is used in combination with Large Eddy Simulation (LES). The following simulation parameters were examined: mesh quality, mesh density, inlet turbulence and spanwise boundary condition. These parameters change the boundary layer (BL) transition process and the formation of the laminar separation bubble on the suction side of the blade segment, both of which impact the aeroacoustic noise prediction. It was found that improvement in mesh quality and density on the surface of the blade segment resulted in improved BL simulation and tonal noise prediction. Alteration of the inlet turbulence and spanwise boundary condition did not have as large of an effect.
Performance analysis of a hybrid vertical axis wind turbine through CFD for distributed generation in rural areas of Peru
Authors: Nuñez-Quispe Johan Joseph , Nuñez-Quispe Aaron Alfredo , Cedamanos-Cuenca Roger Omar
Published: September 2020

Currently, in the developing countries of Latin America there is a great deficiency in the transmission of energy. In Peru, being a developing country, there are many rural areas and indigenous and peasant communities that do not yet have access to electricity due to the difficult access or cost of installing transmission lines in their location. Therefore, the present work will briefly evaluate the current situation of the country and, from that, will formulate a solution based on the development of distributed generation systems using small wind turbines. For this purpose, this paper will develop the preliminary performance analysis of a hybrid vertical axis wind turbine (VAWT) by using the computational fluid dynamics (CFD), for this, the data of the wind resource of Cajamarca, a province in Peru, will be used for this analysis because in this province there is a great wind resource. Even a short aerodynamic and efficiency study was carried out in order to characterize its behaviour with different air flows as they are found in many rural sectors of Peru.
An efficient approach to explore the solution space of a wind turbine rotor design process
Authors: Bortolotti Pietro , Dixon Kristian , Gaertner Evan , Rotondo Megan , Barter Garrett
Published: September 2020

This study proposes a novel approach to explore the solution space of a wind turbine rotor design process. The goal is to offer to blade designers the possibility to select an optimal rotor for given market conditions, assessing trade-offs and alternatives in a matter of minutes. The design process consists of sequential aerodynamic-structural optimizations, where the two loops are linked via a blade-loading parameter that can be varied by the user. A design study is performed starting from the IEA Wind Task 37 land-based reference wind turbine. The solution space is characterized for rotor diameters in the range of 130–160 m, rated generator power values in the range of 3.0–6.0 MW, and tip-speed ratios in the range of 7.5–12.5. Results are discussed highlighting optimal design decisions and trade-offs.
Benchmarking different fidelities in wind turbine aerodynamics under yaw
Authors: Muñoz-Simón A , Palacios R , Wynn A
Published: September 2020

This paper analyses the aerodynamics of wind turbines under yaw with different modelling fidelities (BEM, BEM with skewed wake model, UVLM and LES-AL). First of all, models are compared in a zero-yaw case to demonstrate their accuracy in prediction of out-of-plane loads and the discrepancy of UVLM in the in-plane loads due to the lack of viscous drag. Secondly, the yaw aerodynamics are described through the advancing/retreating and skewed wake effects, which are appropriately captured by UVLM and LES-AL and lead to an incorrect prediction of the location of maximum and minimum loading along a revolution by BEM. Further, when a skew-wake model is included in BEM, it predicts the correct locations but exhibits overly large loading variations. These predictions are consistent for all yaw angles studied (γ = 10° − 30°). All solvers predict similar decrease of root-bending moments, rotor power and thrust coefficients up to a yaw angle of 10°. However, at larger yaw angles, BEM overpredicts this decrease of coefficients with the yaw angle due to the unsuccessful performance of yaw corrections as opposed to UVLM that inherently accounts for three-dimensional effects. This study demonstrates the need to use computational models that can account for three-dimensional effects in the computation of aerodynamic loads for yaw angles above 10°.
Design Optimization of 10MW Downwind Turbines with Flexible Blades and Comparison with Upwind Turbines
Authors: Namura Nobuo , Shinozaki Yosuke
Published: September 2020

Downwind turbines can adopt light and flexible blades because of limited risk of tower-strike. We investigate superiority of a downwind configuration in large-scale wind turbines with high extreme wind speed through comparison of optimized 10 MW downwind and upwind turbines. Blades and towers of these turbines are optimized to minimize their levelized cost of energy (LCoE) with steady state aero-structural analyses considering tower shadow and potential flow effects on power production and fatigue damage. Comparison shows that the optimized downwind turbines have an advantage in LCoE due to their lighter and more flexible blades than the upwind turbines with conventional prebend of 6 [m]. This advantage is derived from smaller thrust force, lighter rotor-nacelle-assembly, and a small distance between a tower axis and center of gravity of the rotor-nacelle-assembly. The upwind turbine requires 11 [m] prebend to obtain the comparable LCoE to the downwind turbine.