Task 20. HAWT Aerodynamics And Models From Wind Tunnel Measurements. 1.0 Introduction

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1 Task 20 4 HAWT Aerodynamics And Models From Wind Tunnel Measurements 1.0 Introduction Wind energy continues to expand worldwide, and wind turbines continue to grow larger. In this environment, sustained technological innovation will require aerodynamics models of greater accuracy and reliability. To achieve these goals, theoretical and computational models must evolve alongside high-quality experimental measurements. Over the past decade, turbine aerodynamics instrumentation and data quality have improved substantially as a result of efforts like IEA Wind Task XIV, Field Rotor Aerodynamics, and Task XVIII, Enhanced Field Rotor Aerodynamics Database. In these efforts, turbine sizes and configurations were comparable to state-of-the-art turbines, and recorded aerodynamic phenomena that were representative of operational machines. Although of high quality, these measurements contained atmospheric inflow fluctuations and anomalies, which precluded clear discernment of complex turbine aerodynamics. Alternatively, wind tunnel experiments offered steady, uniform inflows capable of revealing turbine aerodynamic structures and interactions. However, wind tunnel dimensions generally restricted turbine size, and left doubt as to whether data thus acquired were typical of full-scale turbine aerodynamics. To acquire aerodynamics data representative of full-scale turbines, under conditions of steady uniform inflow, the NREL (National Renewable Energy Laboratory) UAE (Unsteady Aerodynamics Experiment) wind turbine was tested in the NASA Ames 80 foot by 120 foot (24.4 m by 36.6 m) wind tunnel (Figure 1). This test was designed to provide accurate and reliable experimental measurements, having high spatial and temporal resolution, for realistic rotating blade geometry, under closely matched Reynolds number conditions, and in the presence of strictly controlled inflows. Completed in 2000, the test included 22 turbine configurations, and produced over 2,100 data files containing nearly 100 GB (gigabytes) of high-quality data. Shortly after test completion, select data were employed as a reference standard in a blind comparison designed to evaluate wind turbine aerodynamics code fidelity and robustness. In this exercise, participants were given the UAE geometry and structural properties, and then attempted to predict aerodynamic response for a modest number of test cases representing diverse aerodynamic regimes. Code comparison participants did not have access to the experimental aerodynamics data until well after their model predictions were completed and submitted to NREL. Represented in the field of models were blade element momentum models, prescribed wake models, free wake models, and Navier-Stokes codes. Results generally showed unexpectedly large margins of disagreement between the predicted and measured data. Notably, no consistent trends were apparent regarding the magnitudes or the directions of these deviations. The need for improved wind turbine aerodynamics models is clear, and the potential benefits are readily apparent. This research task was established to capitalize on high quality experimental aerodynamics data from the NREL UAE wind tunnel test, as well as comparable data from other sources. Appropriately analyzed, these data will yield unique and unprecedented findings regarding turbine aerodynamics. This information can be exploited to formulate and validate improved wind turbine aerody- IEA Wind Energy 47

2 Implementing Agreement Figure 1 NREL UAE wake flow visualization in NASA Ames 80 foot by 120 foot wind tunnel namics models. More accurate, reliable models will improve wind energy machine design, and continue the trend toward lower cost wind energy. 2.0 Objectives And Strategy 2.1 Objectives Task 20 research objectives and work areas are mutually consistent, and structured to transition aerodynamics data to accurate, robust wind turbine aerodynamics models for machine design and analysis. Acquire accurate, reliable, high-resolution experimental aerodynamic and structural loads data for horizontal axis wind turbines representative of full-scale machines Analyze these data using methodologies designed to reveal the flow physics responsible for phenomena observed on horizontal axis turbines Formalize this understanding in hierarchically structured, physics based model subcomponents, with appropriate consideration for computational efficiency Integrate model subcomponents into comprehensive models in incremental fashion, as a basis for accurate, robust prediction of horizontal axis wind turbine aerodynamics and structural loads. 2.2 Participants In 2006, eleven organizations representing eight Task 20 member countries are participating in Task 20. In addition, three organizations from three other IEA member countries participate in conjunction with Task 11 during Joint Action on Aerodynamics meetings. Center for Renewable Energy Systems (CRES), Greece Centro Nacional de Energias Renovables (CENER), Spain Denmark Technical University, Denmark École de Technologie Supérieure, Canada Energieonderzoek Centrum Nederland (ECN), The Netherlands Gotland University, Sweden Institutt for Energiteknikk, Norway Kyushu University, Japan National Renewable Energy Laboratory (NREL), United States Risø National Laboratory, Denmark Royal Institute of Technology, Sweden Seoul National University, Republic of Korea Technical University of Delft, The Netherlands Kiel University of Applied Sciences, Germany Annual Report

3 Task Resources In the initial stages of Task 20, data acquired during the UAE wind tunnel test were hosted on the Unsteady Aerodynamics Experiment (UAE) Database website ( which was established and continues to be maintained by NREL. Website access can be obtained by requesting a user account through the Operating Agent Representative. Currently, nearly 30 user accounts have been set up, and over 50 users have acquired data for diverse applications. If unique data not available on the website are needed, special arrangements can be made with the Operating Agent Representative. At present, all Task 20 participants have acquired aerodynamic or structural loads data from the Unsteady Aerodynamics Experiment Database website. They also have carried out any data verifications or uncertainty analyses considered necessary in view of the manner in which they intend to use the data. 3.0 Progress In 2006 During 2006, most participants continued research activities previously proposed and initiated under Task 20. In addition, some new activities were initiated as new researchers joined the task. Research results were presented and discussed at the Task 20 Annual Progress Meeting, which was hosted at the Kiel University of Applied Sciences, April As with the previous three Task 20 meetings held in 2003 through 2005, the 2006 Task 20 meeting was conducted in collaboration with the Task 11 Joint Action on Aerodynamics of Wind Turbines meeting. At the April 2006 Task 20 annual meeting, researchers representing their respective countries reported on work carried out during the preceding year. Summarized below are the 13 presentations given at the 2006 meeting, including titles, authors, and affiliations. Aerodynamics of Darrieus Rotors, A. P. Schaffarczyk, Kiel University of Applied Sciences, Germany Previous vertical axis turbine designs were prone to under-performance and early structure failure. As interest in vertical axis machines is renewed, a full spectrum of physics-based, validated design tools, ranging from theoretical approaches to CFD models, will be needed. These design tools will be enabled by fundamental aerodynamic research, and will play a key role in avoiding errors made in early vertical axis turbine designs. Renaissance of Vortex Generators, K. Kaiser, Aero & Structural Dynamics, Germany In the past, vortex generators were used to optimize the power curve of stall regulated turbines running at fixed speed. At present, state of the art turbines use blade pitch control and variable rotor speed. However, some current control algorithms allow blade angle of attack to vary through a broad range in which aerodynamic performance varies substantially. Vortex generators could be used to optimize aerodynamic performance of blades, in combination with pitch control and variable speed. Navier-Stokes Computation of Rotor-Tower Interactions, F. Zahle, Risø National Laboratory, Denmark A newly implemented overset grid method was shown to successfully model the interaction between the tower wake and rotor on a downwind turbine. At certain flow conditions where the tower shedding frequency and the rotor blade passage frequency were sufficiently close to being multiples of each other, vortex lock-in was observed. It was hypothesized that this phenomenon was responsible for unexplained high levels of low frequency noise observed on downwind turbines (Figure 2). Aerodynamic Investigation of Winglets on Wind Turbine Rotors, J. Johansen, Risø National Laboratory, Denmark The aerodynamic benefits of adding a winglet to a wind turbine blade were investigated using computational fluid dynamics. Results showed that adding a winglet increased the force distribution over the outer 0.14R, increasing power production by 0.6% to 1.4% for wind speeds higher than 6 m/s, but increasing thrust by 1.0% to 1.6%. A family of geometry configurations was examined, and suggested that winglets could deliver even greater benefits if properly optimized. Experimental and Computational Fluid Mechanics at Vattenfall Utveckling, J. Westin, Vattenfall Utveckling AB, and S. Ivanell, Gotland University, Sweden The Vattenfall Group is now the fifth largest electricity generator in Europe. It maintains a corporate research and development center, where computation and testing are used to address technical issues in several disciplines, including fluid dynamics. Current fluid dynamics activities do not include wind energy, but a desire exists to expand in this direction. Participation in IEA Task 11/20 activities represents a key step in attaining this goal. Wake Measurements in ECN s Wind Turbine Test Site, G. Schepers, Energy Research Centre of The Netherlands, The Netherlands ECN s Wind Turbine Test Field Wieringermeer (EWTW), test assets, and data are summarized. The north row IEA Wind Energy 49

4 Implementing Agreement Figure 2 Computed iso-vorticity surfaces for a downwind turbine, showing rotor and tower wake structures. (F. Zahle, et al., Risø National Laboratory, Denmark) consists of five Nordex N MW machines. All are equipped with nacelle sonic anemometers and other instrumentation, and one is instrumented to measure blade root and tower base bending moments. A 108 m meteorological mast instrumented at three heights captures wake data from the five N80 s, which are 2.5D to 12.8D upstream of the mast, depending on wind direction. Diverse data for turbine operation have been acquired and analyzed. Wind Turbine Wake Subject to Thermally Stratified Atmospheric Boundary Layer, C. Masson, École de Technologie Supérieure, Canada This work is concerned with the behavior of wind turbine wakes under the influence of various thermal stratifications of the atmospheric boundary layer. Specifically, a numerical model is formulated to simulate turbine aerodynamics, including the wake, in an atmospheric boundary layer under varying thermal stratifications. This model represents the rotor as an actuator disk and exerts blade influences on the flow via blade element theory. Incompressible 3-D RANS is employed to compute the wake flow field, using a modified k-ε model. The Near Wake of a Model Rotor: Measurements and Modeling, W. Haans, Delft Technical University, The Netherlands An experimental campaign produced a comprehensive and consistent set of measured data for a model rotor wake. The near wake was characterized for a range of yawed flow conditions, including the baseline axisymmetric condition. Data included rotor thrust, tip vortex trajectory, phase locked mean velocities, and dynamic stall locations, but did not include blade loads. These data have been modeled with an actuator line code. Initial comparisons are promising, and will improve understanding of near wake aerodynamics under yawed conditions (Figure 3). Actuator Line Computations on Wakes of Wind Turbines in Wind Farms, N. Troldborg, Denmark Technical University, Denmark Wake dynamics of a single wind turbine and three turbines aligned in a row are compared, using a 3-D Navier-Stokes method combined with an actuator line technique. Computations for the single turbine exhibit low frequency wake fluctuations, and show that blade tip vortices may be preserved several ro Annual Report

5 Task 20 tor diameters downstream. Results for the three turbine row demonstrate that tip and root vortices from downstream turbines dissipate near the rotor. Results also show that placing turbines too densely can significantly reduce power. Towards the Optimal Loaded Actuator Disc, R. Mikkelsen, Denmark Technical University, Denmark The optimally loaded actuator disc was considered, including tangential velocities loaded to give constant axial induction. Instead of BEM theory, analyses were done with a vortex model and a Navier-Stokes method. The computed axial loadings were found to increase toward the root section. The analytic solution reveals that the increase is due to the low wake pressure caused by centrifugal acceleration of increasing tangential velocities at inboard radii. Some local Cp levels exceeded the Betz limit, but integration over the disc yielded Cp consistent with Betz. The Steady State Parked Configurations, R. van Rooij, Delft Technical University, The Netherlands NREL UAE lift and drag data at high angles of attack for parked blade conditions were analyzed, with these 3-D data being compared to data that would be obtained under 2-D conditions. Lift and drag characteristics at five span locations (30%, 47%, 63%, 80% and 95%) showed that 3-D loads in parked condition were different than would be produced under 2-D conditions. Detailed analyses of segment lift showed a possible offset in inflow angle at the 47% and 63% span locations, probably caused by flow probe measurements. Identification of Flow Structures on a Rotating Blade Using the NREL UAE Phase VI Data and Frequency Analysis, A. Gonzales and X. Munduate, Centro Nacional de Energías Renovables, Spain Data acquired by the UAE Phase VI wind turbine under parked and zero yaw rotating conditions were compared to ascertain the effects of rotation on blade aerodynamics. Data employed in the comparison consisted of local inflow angles, surface pressure distributions, and force coefficient data, all of which were studied at multiple radial locations. For both parked and rotating conditions, trailing and leading edge separation movement were tracked with respect to incidence angle, and pronounced flow field modifications were observed in response to rotation. Unsteadiness in Rotationally Augmented Blade Flow Fields, S. Schreck, National Renewable Energy Laboratory, United States Means, standard deviations, and spectral decompositions were computed from time records of UAE Phase VI surface pressure coefficient and sectional normal force. These data were correlated with separation/impingement movement data from previous work. While separated flow fields were steady or pseudo-steady, rotationally augmented flow fields were found to be substantially unsteady. Magnitude and spectral content of time variation in rotationally augmented flow fields changed significantly with wind speed and radial location. 4.0 Plans For 2007 The 2007 Annual Progress Meeting will be the fifth and final meeting for Task 20, and will be held at Denmark s Risø National Laboratory on June As in previous years, the Task 20 meeting will be held in collaboration with the Task 11 Joint Action on Aerodynamics of Wind Turbines meeting. Following this final technical progress meeting, the participants will spend six months documenting their Task 20 research. It is anticipated that the Task 20 final report will be delivered to the IEA Wind Secretary near the end of Figure 3 Standard deviation contours of measured near wake velocity. (W. Haans, et al., Delft University of Technology, The Netherlands) Author: Scott Schreck, NREL s National Wind Technology Center, United States IEA Wind Energy 51

6 Implementing Agreement Annual Report