The accurate computation of the aerodynamic forces is a very challenging problem. First of all, the wind environment in which wind turbines generally operate is variable both in space and time and has a strong stochastic content. Because the dimensions of atmospheric turbulence are of the same order as the rotor diameter, individual blades can be engulfed in coherent turbulence bells which lead to severe fatigue loading that significantly can reduce the lifetime of the structure. Secondly, the blade sections can see highly variable, large angle unsteady flows at reduced frequencies, so that non-linear and unsteady aerodynamic effects are significant. Finally, the rotor blades as well as the support structure will in the (near) future become more and more flexible due to the move from relatively small and rigid constant speed wind turbines towards increasingly lightweight and structurally flexible wind turbines operating at variable speed.
Obviously, we need to have models that accurately describes the mutual coupling between the aerodynamics and the structural mechanics in order to make it possible to identify and resolve such (aero-elastic) stability problems already in the design phase of a wind turbine. The starting point of the modeling of the aerodynamics is the Rankine-Froude actuator-disk model which will be discussed below.
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