Basics of Wind Turbine Aerodynamics

Jens N. Sørensen

Department of Wind Energy
Technical University of Denmark

The modern development of wind power is a remarkable story of the combined effort of enthusiastic entrepreneurs and skilled engineers and scientists. Today, wind power forms the most rapid advancing renewable energy resource with an annual growth rate of about 30%. Within the last 20 years the size of wind turbines have increased from a rotor diameter of about 30 m to 150 m, corresponding to an increase in power by a factor of more than 25. In the same period of time, the knowledge and scientific level of the aerodynamic research tools to develop optimally loaded rotor blades have increased dramatically. Today, wind turbine aerodynamics forms one of the research frontiers in modern aerodynamics.

The aerodynamics of wind turbines concerns modeling and prediction of the aerodynamic forces on the solid structures of a wind turbine, and in particular on the rotor blades of the turbine. Aerodynamics is the most central discipline for predicting performance and loadings on wind turbines, and it is a prerequisite for design, development and optimization of wind turbines. From an outsider's point of view, aerodynamics of wind turbines may seem simple as compared to aerodynamics of e.g. fixed-wing aircraft or helicopters. However, there are several added complexities. Most prominently, aerodynamic stall is always avoided for aircraft, whereas it is an intrinsic part of the wind turbines operational envelope. Furthermore, wind turbines are subjected to atmospheric turbulence, wind shear from the atmospheric boundary layer, wind directions that change both in time and in space, and effects from the wake of neighboring wind turbines.

In the presentation I will introduce different wind turbine concepts and theories for designing wind turbine blades. In particular, I will give the basics of the momentum theory and the important blade-element momentum theory, which still forms the backbone in the rotor design of wind turbines. Advanced state-of-the-art advanced aerodynamic models will also be presented. This includes vortex models as well as computational fluid dynamics (CFD). Finally, I will give an outlook of important future aerodynamic research topics.