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CONTENTS
Volume 25, Number 6, December 2017
 

Abstract
In the strong wind shutdown state, the blade position significantly affects the streaming behavior and stability performance of wind turbine towers. By selecting the 3M horizontal axis wind turbine independently developed by Nanjing University of Aeronautics and Astronautics as the research object, the CFD method was adopted to simulate the flow field of the tower-blade system at eight shutdown positions within a single rotation period of blades. The effectiveness of the simulation method was validated by comparing the simulation results with standard curves. In addition, the dynamic property, aerostatic response, buckling stability and ultimate bearing capacity of the wind turbine system at different shutdown positions were calculated by using the finite element method. On this basis, the influence regularity of blade shutdown position on the wind-induced response and stability performance of wind turbine systems was derived, with the most unfavorable working conditions of wind-induced buckling failure of this type of wind turbines concluded. The research results implied that within a rotation period of the wind turbine blade, when the blade completely overlaps the tower (Working condition 1), the aerodynamic performance of the system is the poorest while the aerostatic response is relatively small. Since the influence of the structure\'s geometrical nonlinearity on the system wind-induced response is small, the maximum displacement only has a discrepancy of 0.04. With the blade rotating clockwise, its wind-induced stability performance presents a variation tendency of first-increase-then-decrease. Under Working condition 3, the critical instability wind speed reaches its maximum value, while the critical instability wind speed under Working condition 6 is the smallest. At the same time, the coupling effect between tower and blade leads to a reverse effect which can significantly improve the ultimate bearing capacity of the system. With the reduction of the area of tower shielded by blades, this reverse effect becomes more obvious.

Key Words
wind turbine; numerical simulation; shutdown position; ultimate bearing capacity; wind-induced stability

Address
S.T. Ke: Department of Civil Engineering, Nanjing University of Aeronautics and Astronautics, 29 Yudao Road, Nanjing 210016, China;
State Key Laboratory for Disaster Reduction in Civil Engineering, Tongji University, 1239 Siping Road, Shanghai 200092, China


Abstract
In this study, the turbulent flow around a bluff body for different wind velocities was investigated numerically by using its two- and three-dimensional models. These models were tested to verify the validity of the simulation by being compared with experimental results which were taken from the literature. Variations of non-dimensional velocities in different positions according to the bluff body height were analysed and illustrated graphically. When the velocity distributions were examined, it was seen that the results of both two- and three-dimensional models agree with the experimental data. It was also seen that the velocities obtained from two-dimensional model matched up with the experimental data from the ground to the top of the bluff body. Particularly, compared to the front part of the bluff body, results of the upper and back part of the bluff body are better. Moreover, after comparing the results from calculations by using different models with experimental data, the effect of multidimensional models on the obtained results have been analysed for different inlet velocities. The calculation results from the two-dimensional (2D) model are in satisfactory agreement with the calculation results of the three-dimensional model (3D) for various flow situations when comparing with the experimental data from the literature even though the 3D model gives better solutions.

Key Words
two- and three-dimensional model; turbulent flow; bluff body; RNG k-e model; computational fluid dynamics (CFD); wind flow; turbulence modelling

Address
Muhammet Ozdogan, Bilal Sungur, Lutfu Namli and Aydin Durmus: Department of Mechanical Engineering, Ondokuz Mayis University, 55139 Samsun, Turkey

Abstract
Studies of unsymmetrical blade H-Darrieus rotors at low wind speeds in terms of starting time, static torque, and power performances for different blade parameters: thickness-to-chord (t/c), camber position, and solidity are scarce. However these are required for knowing insights of rotor performances to obtain some design guidelines for the selection of these rotors. Here, an attempt is made to quantify the effects of these blade parameters on the performances of three different H-Darrieus rotors at various low wind streams. Different blade profiles, namely S815, EN0005 (both unsymmetrical), and NACA 0018 (symmetrical blade for comparison) are considered. The rotors are investigated rigorously in a centrifugal blower apparatus. Firstly the dynamic and static performances of the rotors are evaluated to determine the best performing rotor and their optimum solidity. Generalised performance equations are developed based on selected blade parameters which are validated for the unsymmetrical rotors. Further, the starting time is quantified with respect to the rotor inertia to determine the suitable range of inertia that helps the unsymmetrical blade rotor to self-start earlier than the symmetrical one. This study can work as a benchmark for the selection of optimum blade parameters while designing an unsymmetrical blade rotor at low wind speeds.

Key Words
blade parameters; H-Darrieus rotor; low wind speed; power coefficient; starting time; torque coefficient

Address
Anal R. Sengupta: Department of Mechanical Engineering, GIMT Guwahati, Guwahati-781017, India
Agnimitra Biswas and Rajat Gupta: Department of Mechanical Engineering, NIT Silchar, Assam-788010, India

Abstract
The aim of this paper is to present a method to obtain the dynamic response of a wind turbine tower in time domain by means of the generation of time series and to estimate the associated fatigue damage by means of a Rainflow counting algorithm. The proposed method is based on assuming the vortex shedding is a bidimensional phenomena and on following a classical modal superposition method to obtain the structure dynamic response. Four di erent wind turbine tower geometric con gurations have been analyzed in a range of usual wind velocities and covering extreme wind velocities. The obtained results have shown that, depending on the turbulence intensity and the mean wind velocity, there are tower geometric con gurations more advantageous from the fatigue load standpoint. Consequently, the presented model can be utilized to de ne assembly strategies oriented to fatigue damage minimization.

Key Words
vortex shedding; fatigue damage; wind turbine towers; assembly process; rainflow

Address
Cristina Nunez-Casado, Oscar Lopez-Garcia, Alvaro Cuerva-Tejero and Cristobal Gallego-Castillo: Departamento de Aeronaves y Vehiculos Espaciales, ETSIAE, Universidad Politecnica de Madrid, Spain
Enrique Gomez de las Heras: Siemens Gamesa Renewable Energy

Abstract
A novel probabilistic approach is presented for estimating the equivalent static wind loads that produce a static response of the structure, which is \"equivalent\" in a probabilistic sense, to the extreme dynamic responses due to the unsteady pressure random field induced by the wind. This approach has especially been developed for complex structures (such as stadium roofs) for which the unsteady pressure field is measured in a boundary layer wind tunnel with a turbulent incident flow. The proposed method deals with the non-Gaussian nature of the unsteady pressure random field and presents a model that yields a good representation of both the quasi-static part and the dynamical part of the structural responses. The proposed approach is experimentally validated with a relatively simple application and is then applied to a stadium roof structure for which experimental measurements of unsteady pressures have been performed in boundary layer wind tunnel.

Key Words
equivalent static wind loads; non-Gaussian unsteady pressure field; polynomial chaos expansion; quasi-static responses; stochastic dynamics; extreme value statistics

Address
Wafaa Kassir: Universite Paris-Est, Modelisation et Simulation Multi-Echelle, MSME UMR 8208 CNRS, 5 Bd
Descartes, 77454 Marne-la-Vallee, France;
Centre Scientifique et Technique du Bˆatiment (CSTB), 11 Rue Henri Picherit, 44300 Nantes, France
Christian Soize: Universite Paris-Est, Modelisation et Simulation Multi-Echelle, MSME UMR 8208 CNRS, 5 Bd
Descartes, 77454 Marne-la-Vallee, France
Jean-Vivien Heck and Fabrice De Oliveira: Centre Scientifique et Technique du Bˆatiment (CSTB), 11 Rue Henri Picherit, 44300 Nantes, France

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