Similar to other renewable energy sources, wind energy is characterized by a low power density. Hence, for wind energy to make considerable contributions to the world's overall energy supply, large wind farms (on- and offshore) consisting of arrays of ever larger wind turbines are being envisioned and built. From a fluid mechanics perspective, wind farms encompass turbulent flow phenomena occurring at many spatial and temporal scales. Of particular interest to understanding mean power extraction and fluctuations in wind farms are the scales ranging from 1 to 10 m that comprise the wakes behind individual wind turbines, to motions reaching 100 m to kilometers in scale, inherently associated with the atmospheric boundary layer. In this review, we summarize current understanding of these flow phenomena (particularly mean and second-order statistics) through field studies, wind tunnel experiments, large-eddy simulations, and analytical modeling, emphasizing the most relevant features for wind farm design and operation.

Flow Structure and Turbulence in Wind Farms

https://ris.utwente.nl/ws/files/30580604/1_s2.0_S0960148117308339_main.pdf

Comparison of wind farm large eddy simulations using actuator disk and actuator line models with wind tunnel experiments

Evaluation of different methods for determining the angle of attack on wind turbine blades with CFD results under axial inflow conditions

. The wind energy industry relies heavily on computational fluid dynamics (CFD)
to analyze new turbine designs. To utilize CFD earlier in the design
process, where lower-fidelity methods such as blade element momentum (BEM)
are more common, requires the development of new tools. Tools that utilize
numerical optimization are particularly valuable because they reduce the
reliance on design by trial and error. We present the first comprehensive 3-D
CFD adjoint-based shape optimization of a modern 10 MW offshore wind
turbine. The optimization problem is aligned with a case study from
International Energy Agency (IEA) Wind Task 37, making it possible to compare
our findings with the BEM results from this case study and therefore allowing
us to determine the value of design optimization based on high-fidelity
models. The comparison shows that the overall design trends suggested by the
two models do agree, and that it is particularly valuable to consult the
high-fidelity model in areas such as root and tip where BEM is inaccurate. In
addition, we compare two different CFD solvers to quantify the effect of
modeling compressibility and to estimate the accuracy of the chosen grid
resolution and order of convergence of the solver. Meshes up to 14×106 cells are used in the optimization whereby flow details are resolved.
The present work shows that it is now possible to successfully optimize
modern wind turbines aerodynamically under normal operating conditions using
Reynolds-averaged Navier–Stokes (RANS) models. The key benefit of a 3-D RANS
approach is that it is possible to optimize the blade planform and
cross-sectional shape simultaneously, thus tailoring the shape to the actual
3-D flow over the rotor. This work does not address evaluation of extreme
loads used for structural sizing, where BEM-based methods have proven very
accurate, and therefore will likely remain the method of choice.

/pdf/multipoint-high-fidelity-cfd-based-aerodynamic-shape-1evtnqkkeq.pdf

Multipoint high-fidelity CFD-based aerodynamic shape optimization of a 10 MW wind turbine

This report describes the results from the third phase of IEA TCP Wind Task 29 Mexnext, denoted as Mexnext-III, in which 22 parties from 10 countries participated. The main impact of the Mexnext-III project lies in the insights offered in the accuracy of design models by which it is possible to assess design calculations in a more thorough way. Moreover improved design models and recommendations on model use are provided which will reduce uncertainties in design calculations. Based on the project results it can be stated that current state of the art design models provide accurate aerodynamic load predictions for design conditions in constant axial and uniform inflow, while more scatter is observed in modeling the turbulent wake state, separated flow, 3d effects and tip effects. A prerequisite for accurate results is a careful consideration of input parameters and output post-processing. This especially holds for CFD simulations, of which the results are highly dependent on the large number of input- and output related choices. Guidelines to reduce the associated uncertainties have been given. Improvements have been made to engineering models describing amongst others yawed and dynamic inflow effects. However, it is shown that still large uncertainties exist in oblique, dynamic and/or nonuniform inflow conditions. Following this result it is recommended to further improve unsteady aerodynamic modeling in the fourth phase of IEA TCP Wind Task 29. Dedicated aerodynamic measurements, mainly from the New MEXICO experiment have been used to validate, improve and understand aerodynamic models. In the New MEXICO experiment pressure and flow field measurements were taken on a 4.5 meter diameter rotor in the large German Dutch Wind Tunnel, DNW. The New MEXICO experiment is a follow-up of the MEXICO experiment which was the subject of analysis in the first phase Mexnext-I. In Mexnext-I significant and unexpected discrepancies were found between simulations and measurements in terms of blade loads and induced velocities. Resolving the reason for these discrepancies and reducing them has been accomplished in Mexnext-III. Thereto, the entire New MEXICO database has been documented and delivered to the Mexnext-III consortium. With this database a large comparison exercise has been performed, featuring six simulation cases in axial and yawed inflow conditions, containing results of over 20 codes ranging from BEM to CFD. More than 10 different variable types ranging from lifting line variables to pressures, loads and velocities have been compared for the different conditions, resulting in over 250 comparison plots. Apart from benchmarking much effort was spent on understanding the the flow physics of various phenomena. The result is a unique insight in the current status and accuracy of rotor aerodynamic modeling. More specifically it can be concluded that most questions from Mexnext-I have been answered and so a much better understanding of the aerodynamics of the MEXICO rotor in terms of blade aerodynamics as well as near wake aerodynamics has been gained. This is illustrated by the better agreement between calculations and measurements compared to the agreement in Mexnext-I. This is true for pragmatic engineering models but also for high fidelity models. Opposite to the situation after the first MEXICO campaign, the relation between loads and velocities now fulfills the momentum balance, not only in axial direction but even in the tangential direction.

Final Report of IEA Wind Task 29 Mexnext (Phase 3)

This report describes the work performed within the first phase of IEA Task 29 Mexnext. In this IEA Task 29 a total of 20 organisations from 11 different countries collaborated in analysing the measurements which have been taken in the EU project ‘Mexico’. Within this Mexico project 9 European institutes carried out a wind tunnel experiment in the Large Low Speed Facility (LLF) of the German Dutch Wind Facilities DNW on a rotor with a diameter of 4.5 m. Pressure distributions were measured at five location along the blade along with detailed flow field measurements around the rotor plane using stereo PIV. The following organisations (and persons) cooperated in the projects: • Canada: École de technologie supérieur, Montréal, ETS (C. Masson, S. Breton, C. Sibuet), and University of Victoria, UVIC (C. Crawford) • Denmark: RISØ-DTU (H. Madsen, N. Sørensen, P. Rethouré) and the Technical University of Denmark DTU-MEK (W. Z. Shen) • Germany: University of Stuttgart, Ustutt (T. Lutz, K. Meister), University of Applied Sciences at Kiel/CEWind EG (P. Schaffarczyk and A. Jeromin), ForWind (B. Stoevesandt and I. Hernandez) • Israel: Technion, Israel Institute of Technology (A. Rosen, V. Ognev. R. Gordon) • Japan: Mie University/National Institute of Advanced Industrial Science (T. Maeda, Y. Kamada, J. Murata) • Korea: Korea Institute of Energy Research KIER, (H. Shin) and Korea Aerospace Research Institute, KARI (C. Kim, T. Cho) • Netherlands: Energy Research Center of the Netherlands, ECN (G. Schepers, K. Boorsma, H. Snel), Delft University of Technology, TUDelft (G. van Bussel, N. Timmer, D. Micallef), Suzlon Blade Technology, SBT (A. Verhoeff), Technical University of Twente, TUTwente (E. van der Weide) • Norway: Institute for Energy Technology/Norwegian University of Science and Technology, IFE (A. Knauer, J. van Rij) • Spain: National Renewable Energy Center, CENER (X. Munduate, S. Gomez-Iradi, A. Gonzalez, A. Irisarri) and National Institute for Aerospace Technology, INTA (C. Redondo Calle) • Sweden: Royal Institute of Technology/University of Gotland, KTH/HGO (S. Ivanell and K. Nilsson) • USA: National Renewable Energy Laboratory, NREL (S. Schreck) The Energy Research Center of the Netherlands, ECN acted as Operating Agent. As a result of the international collaboration within this task a very thorough analysis of the data could be carried out and a large number of codes were validated not only in terms of loads but also in terms of underlying flow field. The detailed pressure measurements along the blade in combination with the detailed flow field measurements gave a unique opportunity to better understand the response of a wind turbine to the incoming flow field. Deficiencies in modelling have been established and directions for model improvement could be given.

Final report of IEA Task 29, Mexnext (Phase 1): Analysis of Mexico wind tunnel measurements

Analysis of Mexico wind tunnel measurements: Final report of IEA Task 29, Mexnext (Phase 1)

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Final report of IEA Task 29, Mexnext (Phase 1): Analysis of Mexico wind tunnel measurements

Analysis of Mexico wind tunnel measurements: Final report of IEA Task 29, Mexnext (Phase 1)