Advanced Aerodynamic Modeling Requirements

There are a number of comprehensive models that have been developed for the analysis of wind turbines. Many of these models encompass the principles of the BEMT and are coupled to the structural dynamic analysis of the wind turbine and its tower. Such approaches parallel the philosophy of the comprehensive models used for helicopter analy­ses (Section 14.11) – see Manwell et al. (2002) for a discussion of the capabilities of these models. Yet although the fundamentals of wind turbine design are fairly well understood, the success at predicting blade loads and power output does not yet seem as great as would be desired. This is reflected in a recent “blind” prediction of the loads and performance of a comprehensively instrumented wind turbine that was tested under controlled conditions in the 80 x 120 ft (24.4 x 36.6 m) wind tunnel at NASA – see Fingersh et al. (2001). The primary objective of these experiments was to create a definitive set of turbine airloads and performance measurements, over a wide range of operating conditions, that was free of the uncertainties caused by the various atmospheric effects that are always found in field tests
with turbines. These wind tunnel results provided the analyst with an opportunity to really understand better the physics of wind turbine aerodynamics and perhaps gives a definitive data resource for validating predictive methods and resolving outstanding modeling issues. The results from the NREL blind comparisons [Simms et al. (2001)] showed considerable deficiencies between the various predictions for blade loads and turbine power output, even for unyawed, unstalled operating conditions. Such results underline the need for more fun­damental research in aerodynamic subcomponent methodologies and their interdependent coupling if overall predictions are to be improved.

As in helicopter work, advanced computational aerodynamic methods based on numer­ical solutions to the Euler and Navier-Stokes equations (see Chapter 14) have begun to see some use in wind turbine analysis. This class of CFD methods has the potential to provide a consistent and physically realistic simulation of the turbine flow field. The huge compu­tational costs, large memory requirements, and numerous numerical issues associated with such CFD methods (see the detailed discussion in Chapter 14) means that they have not yet seen significant use in wind turbine applications. In particular, problems involving flow separation, such as dynamic stall, have proved extremely challenging for Navier-Stokes based methods, in part because of a need to develop better turbulence models. Also, the prediction of the 3-D vortical wake behind a turbine has proved just as daunting for CFD as for any other type of method. This is because vortical wake formation is a result of complex 3-D, viscous, separated flow effects and also because the numerical methods have difficulty in preserving concentrated vorticity as it is convected downstream.

While it is clear that some have begun to take up the challenge of modeling wind turbine problems using CFD – see Duque et al. (1999,2000,2003) and Iida (2001) – the capabilities of these methods have not yet been validated sufficiently to be assigned the confidence levels that are necessary for wind turbine design purposes. With faster computers and with improved numerical algorithms, CFD methods will ultimately prevail and will become increasingly used in the design of better and more efficient wind turbines.