## 3 Results and Discussion
Regarding a possible distinction between the three regulation schemes, it can be noted that not only the stall regulation showed again the highest peak at the fundamental rotor frequency, but also was the first harmonics was stronger than those in the signals obtained from pitch regulation. Interestingly, the fourth harmonics seemed to be stronger in the pitch-regulated signal, but its overall contribution was small. Having collected these initial insights, some of the results of the rainflow analysis will now be inspected. In Figure 9, the results obtained with the rainflow counting analysis of the stress time series obtained with each of the regulation schemes are shown (upper row), as well as short segments of the corresponding time series. Additionally, the inflection points as identified by the rainflow algorithm are marked for clarity. Prior to generating these diagrams, the stress time series were subjected to some moving-average filtering in order to eliminate the large number of very small-amplitude cycles, which do not contribute to fatigue damage, but obscure the direct comparison between the bivariate histograms. As shown in Figure 9, the higher load level in the stall-regulation scheme, with vanishing differences between the two pitch-regulated schemes, become apparent. Not only is the average stress level higher (as evidenced by both the bivariate histogram and the time series segment), but also are the stress cycles deeper. The situation changed quite drastically when a higher average wind speed, specifically 18 m/s, was considered. As shown in Figure 10, at this wind speed, the stall-regulated signal now showed a significantly smaller average stress and also a smaller depth of the stress cycles. As opposed to the 9-m/s case, the two pitch regulation schemes now showed some differentiation, with the variable rpm pitch variant exhibiting a lower average stress, very similar to the one experienced by the stall regulated turbine, albeit with a higher alternating component. After these preparations, a fatigue life diagram can now be constructed as a function of the wind speed class. Figure 11 shows such a diagram for low (Figure 11a) and high levels of the turbulence intensity (Figure 11b). A number of things can be observed: first of all, and not unexpectedly, fatigue life decayed rapidly with wind speed; note the logarithmic scale of the figure. For each increase in wind speed by about 2 m/s, a reduction in fatigue life by one order of magnitude was observed, so evidently higher wind speeds put a higher strain on the rotor. Secondly, higher turbulence intensity generally led to higher fatigue damage, also an expected finding. This effect was also substantial: the higher turbulence cases showed a fatigue life about half an order of magnitude smaller than their  
Figure 8: Results of the aeroelastic simulation for an average wind speed of 9 m/s and high turbulence intensity: power spectra. Figure 9: Results of the rainflow analysis for 9 m/s. Upper row (**a**–**c**): results of the rainflow analysis for the three regulation schemes. Lower row (**d**–**f**): corresponding time series with inflection points marked. low-turbulence counterparts, except for very high wind speeds (16 m/s and higher). This is clear evidence of the fact that the stress cycle amplitude (rather than the average stress) is the main driver behind fatigue degradation, at least for most of the wind speeds encountered in a normal wind climate. Thirdly, and most importantly in the context of the present study, the type of power/rotor speed regulation makes a difference, and a great one. For medium wind speeds (7 m/s-10 m/s), the stall-regulated turbine experiences a fatigue damage about an order of magnitude larger than the pitch-regulated wind turbines. This differentiation was, again, expected since stall-regulated wind turbines experience a far more turbulent environment in their regulation regime, as compared to pitch-controlled turbines where flow remains attached most of the time.