Aerodynamics and aeroelastics of wind turbines - Wind Power Generation and Wind Turbine Design - page 107

Some of the tasks are to:

• ensure safety against aeroelastic instabilities such as divergence/fl utter,

• investigate loads from extreme events expected only a few times within the

estimated lifetime,
accumulate the effects of loads from rapidly changing operating conditions •

during normal or electricity generation.

A turbine may be called optimized if it can resist equally against extreme winds and
fatigue loads. Because a detailed description of recent procedures is rather exhausting
to the beginner, he or she may start with a somewhat out-dated but classical text: Wind
turbine engineering design by Eggleston and Stoddard [76]. There the reader will
fi nd a clear description of how the basic physics together with engineering require-
ments may be fi t together in a computer code. A recent review more closely to pure
aeroelastics was given by Hansen et al. [25]. On the other hand, it is worth noting
how classical aeroelastics (usually coming from airplane design) now re-enters
into recent wind turbine design for offshore applications [19].

5.3 Instructive example: the Baltic Thunder

Presenting a whole aeroelastic case study is far beyond the scope of this short intro-
duction. Here it is tried to exemplify a description in a somewhat different way. In
2008 a Dutch organization called for a competition for a wind driven car for the
fi rst time. Six teams from four countries presented their design, one of them was the
Baltic Thunder (see Fig. 16). Because one goal was to reduce parasitic drag as much
as possible, the weight had to be reduced as far as possible. A light but fl exible struc-
ture was the result. For other reasons a vertical axis rotor was chosen (see Section
7.1) This rotor is known to be particularly prone to aeroelastic instabilities. Therefore
a Campbell diagram was produced by Vollan's Code GAROS. Due to the soft blade
suspension a vertical bending mode of about 2.5 Hz was not to be avoided. This
gives rise to a fl utter type instability at 500 RPMs. Therefore the fi nal design had
carbon reinforced fi ber (CRF) tubes as suspension giving much higher fi rst blade
eigenfrequencies. One of the various safety proofs included safety against a 18 m/s
gust when operating in normal mode. Figure 18 shows the response of the main
rotor tower. A static (constant) force of about 700 N superimposed on the dynamic
response of the RPM excursion from about 200 to 300 RPM only. It has to be noted
that a 18 to 12 m/s increase of wind speed is equivalent to an increase of power by
factor of 3.4 if c P remains constant. In this case maximum force excursion is only up
to 1200 N so that at least the central column could be regarded as safe.

6 I mpact on commercial systems

6.1 Small wind turbines

In general it is hard to see what effect the impact of scientifi c work will have on a
specifi c commercial product. On the one hand, there is a huge number of engineering