Aerodynamics and aeroelastics of wind turbines - Wind Power Generation and Wind Turbine Design - page 115

large input-output area ratio, whilst being relatively short. A permanent danger is the
early separation of air fl ow through the diffuser as can be seen from Fig. 28 [41,47],
where a relatively short diffuser was designed for an area ratio of approximately 3.
The author does not agree that a fi nal theory has been formulated in which the
specifi c problem has been solved. It may be necessary to extend the work of Loth
and Mc Coy [70] who discussed an array of disks as a model of an actuator volume
to a real continuum theory. This is an important task because a safe upper limit,
such as Betz' law, is important for all investigations. A successful commercial
design has also not been produced so far.
8 Summary and outlook
The history of wind turbine aerodynamics spans approximately 150 years. The
most important infl uence has been given by modelling fl ow properties which then
serve as tools for predicting loads on the structure. As a consequence safe and
competitive wind turbines up to rated power of 6 MW have been constructed so
far. First investigations of even larger blades feasible for rated power up to 10 MW
are under consideration [29].
Further progress can be expected when CFD when taken in the strict sense of the
expression as solving the Navier-Stokes equation without any further assumptions
enters the usual design process in companies. Nevertheless full-scale experiments in
wind tunnels and outdoors are equally as important as analytic theories.
Figure 28 : Sketch of the separation of fl ow through a short diffuser.