bluuebunny/crossref_metadata_2025_split huggingface dataset:
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

Abstract
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

AbstractThe 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

Abstract
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

Abstract
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

Abstract
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

Abstract
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

Abstract
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

Abstract
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

Abstract
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

Abstract
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

Abstract
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

Abstract
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

Abstract
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

AbstractThis 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

Abstract
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

AbstractRemote 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

Abstract
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.