![]() A drag-driven turbine typically has a limited efficiency (power coefficient below 0.3) but presents advantages on self-starting and higher power output at low wind speeds. This allows a difference in drag between the leeward and windward parts of the rotation, resulting in a positive work. They typically have rounded paddles with a convex shape. Have been around for centuries in one form or another, including the well-known design of Savonius in 1922. The ancient windmills were made out of clay, straw, and wood and have been used to mill grain for flour. In fact these wind turbines were vertical-axis turbines. The earliest known wind powered grain mills and water pumps were used by the Persians in the years 500–900. Finally, wake aerodynamics and the importance of airfoil design for VAWTs are highlighted. Since VAWTs are inherently unsteady, the main unsteady phenomena that play a crucial role in VAWT aerodynamics are summarized. Further, the state-of-the art aerodynamic modeling techniques, specifically for VAWTs, are discussed. The basics of VAWT aerodynamics and the various rotor simplifications/representations are presented. First, an introduction is provided on the VAWT history and (dis-)advantages. This chapter highlights the main aerodynamic phenomena and challenges of vertical-axis wind turbines. Advancing the understanding and modeling of VAWT’s aerodynamics will be crucial to advance the technology further. The lack of understanding the complex unsteady aerodynamics of VAWTs and the challenge to predict the loads and performance of this kind of turbines accurately, has led to systematic failures and as such variable interest in VAWTs throughout history. Although vertical-axis wind turbines have a long history, the behavior of these turbines and their complex flow field is still not fully understood. Vertical-axis wind turbines (VAWTs) could be more suitable and compatible in these environments, hence, the interest in VAWTs is rekindling. However, wind turbines are moving into new environments, such as floating far-offshore or urban applications, where the operational conditions are significantly different. ![]() Horizontal-axis wind turbines (HAWTs) are widely studied and have proven their technological capabilities.
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