Vertical-axis wind turbine
A vertical-axis wind turbine (VAWT) is 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, 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 or "ripple" during each  revolution, and the large bending moments on the blades. Later designs addressed the torque ripple issue by sweeping the blades helically (Gorlov type). Savonius vertical-axis wind turbines (VAWT) are not widespread, but their simplicity and better performance in disturbed flow-fields, compared to small horizontal-axis wind turbines (HAWT) make them a good alternative for distributed generation devices in urban environment. A vertical axis wind turbine has its axis perpendicular to the wind streamlines and vertical to the ground. A more general term that includes this option is "transverse axis wind turbine" or "cross-flow 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. Computer modelling suggests that wind farms constructed using vertical-axis wind turbines are 15% more efficient than conventional horizontal axis wind turbines as they generate less turbulence. General aerodynamics. The forces and the velocities acting in a Darrieus turbine are depicted in figure 1. The resultant velocity vector, formula_1, is the vectorial sum of the undisturbed upstream air velocity, formula_2, and the velocity vector of the  advancing blade, formula_3. formula_4 Thus the oncoming fluid velocity varies during each cycle. Maximum velocity is found for formula_5 and the minimum is found for formula_6, where formula_7 is the azimuthal or orbital blade position. The angle of attack, formula_8, is the angle between the oncoming air speed, W, and the blade's chord. The resultant airflow creates a varying, positive angle of attack to the blade in the upstream zone of the machine, switching sign in the downstream zone of the machine. It follows from geometric considerations of angular velocity as seen in the accompanying figure that: formula_9 and : formula_10 Solving for the relative velocity as the resultant of the tangential and normal components yields: formula_11 Thus, combining the above with the definitions for the tip speed ratio formula_12 yields the following expression for the resultant velocity: formula_13 Angle of attack is solved as: formula_14 Which when substituting the above yields: formula_15 The resultant aerodynamic force is resolved either into lift (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.