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Vertical-axis wind turbines (VAWTs) are a type of wind turbine where the main rotor shaft is set transverse to the wind (but not necessarily vertically) while the main components are located at the base of the turbine. This arrangement allows the generator and gearbox to be located close to the ground, facilitating service and repair. VAWTs do not need to be pointed into the wind,〔Jha, Ph.D., A.R. (2010). Wind turbine technology. Boca Raton, FL: CRC Press〕 which removes the need for wind-sensing and orientation mechanisms. Major drawbacks for the early designs (Savonius, Darrieus and giromill) included the significant torque variation during each revolution, and the huge bending moments on the blades. Later designs solved the torque issue by providing helical twist in the blades. A VAWT tipped sideways, with the axis perpendicular to the wind streamlines, functions similarly. A more general term that includes this option is "transverse axis wind turbine". For example, the original Darrieus patent, US Patent 1835018, includes both options. Drag-type VAWTs such as the Savonius rotor typically operate at lower tipspeed ratios than lift-based VAWTs such as Darrieus rotors and cycloturbines. ==General aerodynamics== The forces and the velocities acting in a Darrieus turbine are depicted in figure 1. The resultant velocity vector, , is the vectorial sum of the undisturbed upstream air velocity, , and the velocity vector of the advancing blade, . Thus the oncoming fluid velocity varies during each cycle. Maximum velocity is found for where is the tip speed ratio parameter. The resultant aerodynamic force is resolved either into lift (F_L) - drag (D) components or normal (N) - tangential (T) components. The forces are considered acting at the quarter-chord point, and the pitching moment is determined to resolve the aerodynamic forces. The aeronautical terms "lift" and "drag" refer to the forces across (lift) and along (drag) the approaching net relative airflow. The tangential force acts along the blade's velocity, pulling the blade around, and the normal force acts radially, pushing against the shaft bearings. The lift and the drag force are useful when dealing with the aerodynamic forces around the blade such as dynamic stall, boundary layer etc.; while when dealing with global performance, fatigue loads, etc., it is more convenient to have a normal-tangential frame. The lift and the drag coefficients are usually normalised by the dynamic pressure of the relative airflow, while the normal and tangential coefficients are usually normalised by the dynamic pressure of undisturbed upstream fluid velocity. Where is the power coefficient, is air density, is the swept area of the turbine, and is the wind speed.〔}〕 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「vertical axis wind turbine」の詳細全文を読む スポンサード リンク
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