Aerodynamics and aeroelastics of wind turbines - Wind Power Generation and Wind Turbine Design - page 104

As a result many problems with consistent interpretation of all the data were
identifi ed. Especially the defi nition of a local angle of attack was very diffi cult.
This of course is also the case when 3D-CFD investigations are discussed. Several
approaches have been tried, one being a correlation between the location of the
stagnation pressure line and AOA. A second is the so-called inversed BEM, where
c N and c T are correlated to the local fl ow angle. One may object that both methods
rely on 2D assumptions for connecting forces and velocities so they may fail when
3D effects are strong. A somewhat detailed review is given by van Rooij [45] and
Shen et al. [ 54 ].
It may be useful to note that a new program subsequent to the above-mentioned
experiments will be started in early 2009 which will be undertaken on the research
wind turbine E30 at the University of Applied Sciences Flensburg. During this
experiment, using an aerodynamic glove, shear stresses which indicate the state
of the boundary layer (laminar, transitional or fully developed turbulent) will be
measured for the fi rst time [ 51 ].

4.2 Chinese-Swedish wind tunnel investigations

In a joint effort of Chinese and Swedish aerodynamicists, a 4.75-m diameter wind
turbine was investigated in the late 1980s [ 21 , 44 ].

4.3 NREL unsteady aerodynamic experiments in the NASA AMES-wind tunnel

As a result of the rather disappointing fi ndings from open air experiments a big
effort was mounted by NREL, the US National Renewable Energy Laboratory.
A 10-m diameter two-bladed rotor was put into the world's largest wind-tunnel,
the NASA-AMES wind tunnellocated in California. The main advantage was a
complete control over all infl ow conditions. Most important was the so-called
blind comparison in which a variety of design and analysis tools were used to
predict the power and forces based on 2D data. Figure 15 gives an overview of
all simulation for the shaft torque, which is proportional to the power, see [56] for
broad discussion. Several important conclusions can be drawn from the results of
this large-scale experiment:

• 10 tested aeroelastic codes (see Section 5) showed extremely large disagree-

ment even in the attached fl ow regime. The measured low-speed shaft torque
being 800 Nm, is scattered in the predictions from 200 up to 1400 Nm (factor
of 7). This situation is only a little bit better at 10 m/s (30%) but became worse
at the same level as before in the deep-stalled case.
So-called performance codes (only three, being descended from the famous PROP •

Code) showed strong variations in the deep-stalled condition with one remarkable
exception which gives almost the same result as one CFD investigation.
Two wake codes give also non-uniform results. •

• Most impressive were the CFD results of Sørensen [59]. There, without any

2D-profi le data, results with the same degree of accuracy of the best BEM