This chapter has introduced the aerodynamics of the wind turbine, mainly to show both the differences between and similarities to helicopter rotors. The idea that the wind contains energy has been outlined and the ability of a wind turbine to extract that energy has been explained. It is apparent that many strides have been made in the understanding and modeling of wind turbine aerodynamics, especially over the last two decades where environmental concerns have driven more research into renewable energy resources. For the future there is a need to refine and expand existing models and in some cases to develop new aerodynamic models that can encompass a wider range of operating conditions and wind turbine now states. This will allow adverse effects to be properly predicted early in the design cycle and more efficient and reliable wind turbines to be designed for the future.
It has been shown how predictive approaches for wind turbine aerodynamics have paralleled the methods used for helicopter analysis. The fundamental performance of the machine has been examined using momentum theory and blade element momentum theory (BEMT). Despite its relative simplicity and limitations, the BEMT allows a clear examination of the design factors that influence wind turbine performance and design. However, the BEMT is limited in terms of generality to a less than desirable range of operating conditions.
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These limitations are largely eliminated with the use of aerodynamic methods based on vortex theory. Today, most engineering methods used in wind turbine analyses are based on blade element theory combined with either inflow models or vortex models to represent the nonuniform induced velocity associated with the vortical wake trailed from the turbine. One advantage of this class of predictive methods is the flexibility to include a wide range of validated subcomponent models representing various physical effects that are difficult to model from first principles. This approach also allows the subcomponent models to be validated against idealized laboratory experiments, and the flexibility to progressively upgrade the models as a deeper understanding of the physics is obtained. They can also be combined with structural dynamic models of the blades, tower, and power generation systems to produce powerful aeroelastic tools for the detailed structural design of wind turbines. As in the case of all predictive models, however, sustained validation of the approach against experimental measurements is essential, despite the inherent difficulties and lengthy nature of the process.
It is clear, however, that there are two key areas that need continued consideration if the aerodynamic design of wind turbines is to be further improved. These are the modeling of the turbine wake and the modeling of the unsteady aerodynamics of the blade sections. Inflow models have attractive mathematical forms and low computational overheads that will always be useful for certain types of aerodynamic and turbine performance analyses. Vortex wake methods are attractive because of their appealing physical nature and their flexibility to handle a broad range of steady and transient operating conditions. While “prescribed” vortex wake models have seen some use in wind turbine applications, they have limited scope in practical applications, and today they are fast being surpassed by free-vortex wake approaches. It is still up to the wind energy practitioner to use these modeling tools, to explore their capabilities, and to determine the limitations in their use through validation studies with measurements. Only when this is done will the analyst be in a position to decide which models require further development. While it has been emphasized that significant unsteady effects may be produced on wind turbines even with the absence of dynamic stall, the particular problems produced by dynamic stall remain a serious concern for wind turbines, even for pitch-controlled machines. In higher winds when much of a wind turbine blade can be stalled, existing performance methods tend to predict power outputs that are considerably lower than those actually measured. Some of the potential mechanisms contributing to the 3-D aspects of the problem have been discussed, although the exact mechanisms at play still require a much deeper analysis of the flow. While a full understanding and modeling of these problems is the subject of ongoing research, they can, in most cases, be traced to the development of the 3-D boundary layer on the rotating blades.
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