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The Science of Making Torque from Wind (TORQUE 2018)

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Room: BL.27 Building
Topic: MET. Poster Session
Form of presentation: Poster
Duration: 75 minutes

Authors:
Jay Prakash Goit, Yifeng Liu, Takeshi Ishihara

Abstract:
The aim of the current work is to investigate wind loading on wind turbine nacelles focusing on nacelle roof which is more prone to structural failure. To this end, wind tunnel experiments are conducted to measure pressure field on the surface of scaled nacelle model. It is found that for DLC 6.1 which only considers yaw misalignment of 15 degree, the measured pressure coefficients is within the limit of GL guideline specified value of -0.6. However, for DLC 6.2 which considers yaw misalignment of 180 degree, the peak value of measured pressure coefficient is as low as -1.2. Furthermore, wind induced peak forces on the nacelle roof are assessed as the function of wind direction. It is observed that maximum values of vertical forces are significantly higher for the turbulent inflow compared to the uniform inflow case. Similar to the pressure coefficient, force coefficient obtained from experiment is well within the limit of GL guideline for DLC 6.1. However, for DLC 6.2, the guideline underestimates the force by more than 30%. Finally, a model for the estimation of peak wind load is proposed. The model agrees well with the measurement.

Authors:
Brandon Arthur Lobo, Koen Boorsma, Alois Peter Schaffarczyk

Abstract:
Boundary layer transition studies have been carried out using the unsteady surface pressure data from the EU project MEXICO and the later New-MEXICO experiment at the DNW wind tunnel, which have been subject of investigation in IEA Wind Task 29 Mexnext. The experiments were conducted on a specially designed 4.5 m diameter wind turbine with and without zig-zag roughness strips applied to the outboard part of the blades. Standstill tests with the MEXICO blades were also conducted at the TU Delft LTT wind tunnel including oil flow and stethoscope tests.Transition is determined by detecting and observing the growth of Tollmien-Schlichting (TS) waves through a Power Spectral Density plot in the frequency domain. The expected frequency range of these waves is determined using a database developed by Van Ingen. The transition results are compared to those determined by RFOIL, a program for the analysis of aerofoils which uses the eN method for transition detection. Generally, a good agreement was observed between measurements and predictions. In addition to that, the effectiveness of the roughness strips was studied. The result is a unique database in controlled conditions to be used for validation and improvement of transition modelling of wind turbine blades.

Authors:
Anna Craig, Mike Optis, Caleb Phillips, Jordan Perr-Sauer, Shuangwen Sheng, Michael Jason Fields, John Meissner

Abstract:
In the present work, we examine the variation introduced in the evaluation of an operating plant’s wind power production as a result of the choices analysts make in the processing of the operational data.  For this study, an idealized power production for individual turbines over an operational period was predicted by fitting power curves to the turbine production data collected during expected operation (that is, without curtailment or availability losses).   A set of 240 possible methods were developed for (a) defining what data represented expected operation and (b) modeling the power curve.  The spread in the idealized power production as predicted by the different methods was on average almost 3% for the 100 turbines considered. Such significant variation places a lower bound on the precision with which analysts may employ such data as benchmarks for calibration of their energy estimation processes and limits the potential for identification of refinements to the energy estimation models for improved accuracy.

Authors:
Rebecca Barthelmie, Sara C. Pryor, Norman Wildmann, Robert Menke

Abstract:
During the intensive period (May-June 2017) of the Perdigao experiment, three sets of Doppler lidar were operated, at least in part, to scan the wake of the wind turbine on the southwest ridge. DTU used a dual Doppler lidar system scanning almost horizontally from the northeast ridge. CU operated a single scanning lidar in the valley bottom approximately 1 km northeast of the WT conducting multiple scans every 10-minutes centered on the WT. Two of the three DLR lidars were in-plane with the WT for the main wind direction, one in the valley and one on the distant mountain ridge. Integrating data resulting from these scanning Doppler lidars, the WT wake can be characterised in 3-D at relatively high temporal/spatial resolution. Here we report on the development of the analysis framework designed to permit integration of data time series from the different instruments, quantification of the temporal and spatial characteristics of the wake, the uncertainty in wake characterization arising from the operation of different types of lidar and their scanning geometry during the campaign, and provide illustrative examples of wake behavior and characteristics based on case studies collected under different atmospheric stability conditions.

Authors:
Farid Samara, David A Johnson

Abstract:
As wind turbines become larger the dynamic loading on the blades also increases. Controlling a section of the trailing edge of the turbine airfoil is found to reduce the loading on wind turbine blades. This experimental work includes a trailing edge flap that covers 20% of an S833 airfoil with a chord of 178 mm. Surface pressure and strain are measured for varying angles of attack and flap angle. Coefficient of lift and moment are obtained from the 54 pressure taps and the lift and drag forces are also obtained from the strain gages. A 3D printed blade section is designed and built to house the actuation and sensing on the airfoil in real time. The trailing edge flap was tested inside a 0.61 m wind tunnel as a baseline case and the results showed how the lift, drag, and moment on the airfoil can change for different flap angles. The coefficient of lift changed by 30% for flap angle of 20 deg. The entire blade with the flap will also be installed on a 3.4 diameter wind turbine to study the influence of a flap on load variation.

Authors:
Ilmas Bayati, Luca Bernini, Alex Zanotti, Marco Belloli, Alberto Zasso

Abstract:
This work presents the results of an extensive experimental campaign aimed atcharacterizing the wake aerodynamics of a oating oshore wind turbines (FOWT) in dynamicconditions. The campaign was carried out in the Politecnico di Milano 14m x 4m atmosphericboundary layer test section, under UNAFLOW project (UNsteady Aerodynamics of FLoiatingWind turbines). Wake measurements were carried out either through hot-wire anemometerswith a traversing system as well as through Particle Image Velocimetry (PIV) technique. A 1/75 scale model of the DTU 10 MW reference wind turbine was adopted, which was mounted on a hydraulically actuated mechanism imposing sinusoidal surge motion atdierence frequencies and amplitudes, and for dierent tip speed ratios . Results are reportedand represent a valuable completion with respect to previous measurements conducted by theauthors and a remarkable support for the understanding of unsteady aerodynamics of FOWT for control and wind farm purposed.

Authors:
Ilmas Bayati, Alan Facchinetti, Alessandro Fontanella, Hermes Giberti, Marco Belloli

Abstract:
This paper presents the technical issues and methodological aspects concerninghybrid/hardware-in-the-loop (HIL) testing of oating oshore wind turbines (FOWTs) insidesthe Politecnico di Milano wind tunnel. A numerical model of platform hydrodynamics andmooring system is used to control a 6-DOF robot reproducing the rigid-body structure motiondue to combined wind and wave loads. An aero-elastic scale model of the DTU 10MW windturbine with individual blade pitch control (IPC) and torque control capabilities makes possibleto investigate, under closely controlled inow conditions, how unsteady aerodynamics anddierent control logics aect the oating system dynamics.

Authors:
Sho Oh, Takeshi Ishihara

Abstract:
Accurate estimation of structural parameters such as natural frequencies, mode shapes and damping ratios is important for both numerical modelling and structural health monitoring, and they are usually evaluated with ambient vibration analysis. Although the Eigensystem Realization Algorithm gives high accuracy, the results are known to depend on the selection of calculation parameters. Since the optimum values of the calculation parameters depends on both the measured vibration amplitude and the ratio of the measurement noise, the information of the sensitivity of the calculation parameters on the estimated results using full-scale measurement is important for the practical application of the method. The aim of this study is to provide information on the sensitivity and the optimum values for these parameters for a MW-size wind turbine. Eigensyetem Realization Algorithm method is applied on the ambient vibration data of a 2.4MW wind turbine which structural parameters are known from previously conducted forced excitation test. Sensitivity of the initial model parameter, the correlation parameter, and data length were evaluated for the estimated natural frequency, mode shape and damping ratio for each levels of wind speeds.

Authors:
Alfredo Peña

Abstract:
We present a preliminary analysis of inflow measurements performed with a SpinnerLidar on a turbine’s nacelle and those from three grounded-based continuous-wave lidars (WindScanners) during the ScanFlow experiment. The SpinnerLidar measurements seem to capture the structure of the inflow and need to be filtered due to blade contamination and hub/nacelle shading of the beam. The WindScanners’ measurements, which in principle can be used to estimate the three wind speed components without assumptions, seem to be a promising reference to evaluate SpinnerLidar reconstructed winds and turbulence measures.

Authors:
Luke Simmons

Abstract:
A comparative exercise for estimating the uncertainty associated with new methods for power performance measurements was coordinated by the International Energy Agency (IEA) Wind Task 32. Both IEA Task 32 and the Power Curve Working Group (PCWG) have identified the application of the new uncertainty guidelines as a problem area. One time series dataset from a wind turbine, hub height mast and vertical profiling lidar was provided to calculate the power curve using three different wind speed definitions. For each wind speed definition, participants had to estimate the wind speed measurement uncertainty based on the guidance provided from the June 2016 Final Draft International Standard (FDIS) of IEC 61400-12-1 Edition 2. The comparative exercise included four iterations over the course of one year to incrementally harmonize the calculations and assumptions. The exercise showed significant variability among participants reflecting difficulty with the interpretation and application of the informative guidance. It also demonstrated that when using current technology and the available calibration techniques the use of a standalone lidar with a short met mast resulted in a significantly higher uncertainty compared to only using a hub height mast (with some measurements of wind shear and wind veer in the lower rotor).

Authors:
Ioanna Karagali, Ebba Dellwik, Jakob Mann

Abstract:
One of the main objectives of the New European Wind Atlas (NEWA) project is to carry out large scale field experiments at a high spatial and temporal resolution, and  provide a significant upgrade to the experimental databases currently available.This study describes the Oesterild balconies experiment, where the purpose was to quantify the effect of patchy forested terrain on the mean wind field. The experiment was performed at the Oesterild test station for large wind turbines in Northern Denmark, April-August 2016. The two 250 m meteorological towers available at the test site were equipped with balconies, first at 50 m above local ground level, later raised to 200 m AGL. Scanning lidars were placed on each of the balconies, performing horizontal scans over 90 degrees arcs with a step of 2 degrees with an East and West orientation depending on the incoming wind direction. The purpose to study the imprint of surface heterogeneity on mean winds and the large scale spatial and temporal statistics of winds.

Authors:
Paula Weidinger

Abstract:
Precise torque measurement in multi-MW nacelle test benches (NTBs) is crucial for the efficiency determination of wind turbines. However, torque transducers of NTBs are not yet traced back to national standards and, hence, their accuracy is not known so far. To this end, a calibration procedure for torque M under rotation using a torque transfer standard was developed. The measurements performed on an NTB and its results for a validation and adjustment of the planned method are presented. Different influences on the calibration measurements, such as static and rotational zero-point determination, rotational speed n, and ambient conditions, are discussed.

Authors:
Tobias Duda

Abstract:
Damage in electrical components during operation of the drive train of wind turbines can lead to highly dynamic load situations, which in turn can lead to further damage or increased wear. In particular, power converter components are affected by frequent damage, which leads to highly dynamic torque reversals. As part of a national project, a research nacelle for a wind turbine was set up at the Center for Wind Power Drives (CWD) and examined with the help of a 4 MW test rig. This test rig allows the drive train of a wind turbine to be intensively investigated so that even the highly dynamic load conditions of a power converter fault could be studied. Among other tests, the fault case of a power converter short circuit was synthetically generated. This paper aims to give an insight into the investigation methodology, test setup and gives an outlook on the results.

Authors:
Maximilian Henkel, Nymfa Noppe, Wout Weijtjens, Christof Devriendt

Abstract:
Measuring on offshore wind turbines is a challenging and cost intensive work especially when aiming for poorly accessible parts of the structure. While it is desirable to replace these measurements by indirect methods uncertainty exists about necessary assumptions of the static and especially dynamic structural and environmental properties. This paper presents preliminary results of a measurement campaign of subsoil strains on three operating offshore wind turbines located in the Belgian North Sea. The measurement data enables to compare assumptions of variables used in design with their corresponding value in situ e.g. soil stiffness and to track their temporal evolution. This can give valuable hints for design optimization. The soil stiffness is highlighted since it influences dynamics and statics of the wind turbine significantly as also indicated by the current research effort of other projects such as PISA or DISSTINCT. The data also allows to improve indirect measurement methods such as virtual sensing by learning from the potentially non-linear behaviour of soil. Furthermore, the sub-soil data can serve as input for validation of indirect methods to assess fatigue and current design of the offshore wind turbine.

Authors:
Roman Rachholz, Johannes Luthe, Andreas Schulze, Christoph Woernle, János Zierath

Abstract:
Investigations of modal parameters on wind turbines are associated with a great technical expense, standstill times and therefore with high costs. This makes experimental research on operating wind turbines, for example testing innovative measurement strategies, uneconomical. Thus, it is desirable to conduct parts of experimental research on laboratory test structures. In this paper the development of a scaled laboratory wind turbine structure is presented. The differential equation describing the dynamics of this test stand is introduced. First results of the mode shapes are presented and the dependency of different dynamic boundary conditions applied to the tower top. Based on these results the topology and structure details are finally described.

Authors:
Martin Hofsäß, Florian Haizmann, Po Wen Cheng

Abstract:
In this study, different methods are used to determine a nacelle-based lidar power curve. The methods used have already been published in other studies. However, these studies differed in environmental conditions, power rating, hub height and rotor diameter and in the evaluation criteria of the power curve. In this study, the published methods for the determination of nacelle-based lidar power curves will be used and evaluated with uniform criteria. The basis for this is provided by measurements of ground- and nacelle-based lidar and met mastmeasurements at an IEC-conform site.

Authors:
Nicoletta Gioia

Abstract:
Wind energy is one of the most promising renewable energy available today. The continuous demand of wind energy production led the interest of the wind industry towards bigger turbines. This upscaling trend has imposed bigger (not quasi-static) loads that are significantly influencing the fatigue life of the wind turbine components and the tonalities generated by the drive train. To tackle noise and vibration problems and validate complex models it is of high interest to continuously track the modal parameters of the machines under operating conditions. This allows a better design of the new prototypes and the reduction of the risk of premature component failures, followed by a possible decrease of the cost of the energy. To do so Operational Modal Analysis represents a powerful tool. One limitation affecting this methodology when applied to rotating machines comes from the presence of harmonics. Their predominance in the spectrum masks the modal content in the signal, making the extraction of the modal parameters impossible. The objective of this research is then to achieve a combination of automatic methodologies for dealing with the harmonics and automatic OMA techniques in order to be able to autonomously process a continuous stream of data.

Authors:
Oliver Bischoff

Abstract:
Assessing uncertainty of floating lidar systems for different wind and wave climates is a difficult task. One possible solution to close this gap is a validated simulation environment for floating lidar systems. A possible approach for such a simulation environment is being developed within the German research project MALIBU and will be presented. This paper shows first results regarding the validation aspects for the moving lidar environment.

Authors:
Faegheh Ghorbanishohrat, David Johnson

Abstract:
A study of airfoil behaviour is undertaken using two independent non-intrusive measurement techniques, surface oil flow visualization (SOFV) and infrared thermography (IT) to quantitatively determine separation and reattachment points on the airfoil. Several airfoil heating methods are described and several airfoil materials are evaluated. Results are presented here for constant Re = 40000, with the angle of attack changed from 0 to 15 degrees. Both methods provide similar results and allow further understanding of these characteristics and the extension to dynamic and unsteady flow situations.

Authors:
Ashim Giyanani, Gerard van Bussel, Feike Savenije

Abstract:
The IEC standards prescribe an inflow wind field based on models with empirical assumptions to perform load calculations. These assumptions employ unnecessarily high safety factors, resulting in higher material usage than required. The use of preview wind speed measurements using nacelle-mounted Lidars allow mitigation of structural loads by suggesting appropriate control action. However, the relationship between the measured wind speed and the dynamic wind turbine loading has not yet been established. This relationship is affected by uncertainty in site conditions and the dynamic control of wind turbine in different operation regimes. Deriving the load statistics based on the inflow wind field is therefore necessary to establish the correct control strategies. We concentrate on two aspects for this study. Firstly, the variability in the inflow conditions due to surface roughness, wind speed fluctuations, turbulence, wind shear and wind direction changes. Secondly, the control aspects related to the correlation between the loads and inflow wind, pitch angle setting, rotor speed and yaw misalignment. By understanding the effect of these two aspects on wind turbine loading, an optimal control strategy can be designed by accessing look-up tables with inflow-loading relationship for each operation regime.

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