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PREFACE

In recent decades，global warming has worsened owing to the reckless use of fossil fuels.Consequently，as an alternative to fossil fuels，new renewable energy has drawed increasing increased attention and demand across the world.Among new renewables，wind energy，as a clean，sustainable and inexhaustible source of energy，has gained the fastest growth of renewable energy utilization.China has identified wind power as a key growth component of the country's economy.The performance of the Chinese wind power market surprised even optimistic analysts.By the end of 2015，the total installed wind power capacity amounted to 114.6 GW，23.2 GW of which was installed in 2015，leading wind application in the world.It is highly likely that China may reach total installed capacity of 250 GW by 2020.The development of wind energy in China，in terms of scale and rhythm，is absolutely unparalleled in the world.

Wind turbines operate at all times in an extremely complex and unsteady flow due to stochastic wind speed and wind direction.Obviously，aerodynamic loads must be well understood before the structural response can be accurately determined.Moreover，the unsteady aerodynamic forces are always coupled with the wind turbine structure，resulting in complicated dynamic structural responses.Wind turbine failures，reduced machine life，and increased operating maintenance are all directly linked to the unsteady aerodynamic loading and dynamic structural responses.For offshore wind turbines，hydrodynamic forces are exerted to the turbine foundation in addition to the aerodynamic forces and structural responses.Offshore wind farm development trends towards larger turbine sizes.The latest generation of offshore wind turbines has rotor diameters up to 170 m and rated power up to 7 MW，while 10—20 MW machines are currently in the development phase.It is the increasing size of the wind turbine that makes the unsteady and dynamic loads more difficult to be determined and therefore the wind turbine design more challenging.

To meet the national strategic requirements in wind energy，a project of Key Mechanical Issues and Design of Large Scale Wind Turbine was funded by the National Basic Research Program (known as the″973″Program in China) in 2014，undertaken by a team of 30 from six Chinese institutions，led by Prof.Wang Tongguang as the Chief Scientist.This project focuses on the following three key issues in large scale wind turbine mechanics: (1) wind turbine 3D rotational aerodynamics in complicated circumstances，(2) wind turbine aeroelasticity and hydrodynamics and their coupling effects in complicated circumstances，and (3) wind turbinesupport and foundation structural dy-namics in complicated offshore circumstances.A high-performance multi-megawatt wind turbine will be designed based on the studies in this project，where six workpackages are included: (1) Study of unsteady aerodynamic mechanism and high-accuracy numerical simulation for wind turbine; (2) Study of the aeroelastic mechanism and structural dynamics of nonlinear large deformation of wind turbine; (3) Study of offshore floating wind turbine dynamics under Wind，Ocean Wave，and Ocean Current; (4) Study of dynamics and safety for bottom-fixed offshore wind turbine foundation; (5) Experimental study of wind turbine flow structure and aerodynamics-hydrodynamics-Structure Coupling; (6) Study of comprehensive Mechanical Analysis and Integrated Optimization of High-Performance wind turbine.

This special issue of Wind Energy contains a collection of articles documenting recent research mainly supported by the″973″Project.Included in this collection are works that are computationally focused on as well as those that are experimentally oriented，involving wind turbine aerodynamics，hydrodynamics，structural dynamics，structural design，fluid-structure interaction，fatigue assessment，and multi-objective optimization design，etc.Vigorously pursued，these and similar research efforts will provide enhanced analysis and design tools that will，in turn，usher in future wind energy machines of unprecedented size and efficiency.

Wang Tongguang，Gust Editor

Nanjing University of Aeronautics and Astronautics

Nanjing，210016

China

Jan.，2016