other turbine modes. Longitudinal and lateral tower modes have very close frequencies and damping ratios when the turbine is at parked condition, but as the turbine starts rotating the damping of the longitudinal tower mode observed is significantly greater than the damping of lateral tower mode. Figure 9 PSD of laser measurements taken on the tower (longitudinal direction) The tower frequencies were also identified by using the data obtained from the strain gauges. The turbine was tested at 3 different pitch angle configurations. At parked conditions, the controller of the turbine was keeping the pitch angles at 90, 110 or 180 degrees. The 180 degree orientation represents the case where the blade flapwise vibration corresponds to out of plane direction motion. Similarly, at the 90 degree orientation edgewise blade modes represent out of plane vibration. Besides the 3 different pitch angle configuration, since the turbine was kept at a fixed angle, the relative direction between the effective wind direction and turbine orientation was changing which resulted at different aerodynamic coupling between the wind loads and the structure. The identified damping parameters were observed to be dependent on the pitch angle and the relative angle between the turbine and wind direction which supports the idea that the aerodynamic coupling has a strong influence on the identified results. The measurements performed on the parked turbine by using the tower strain gauges and the acquired results are summarized in Table 2. The abbreviations TF, TD and PTMB1 stand for Tower Frequency, Tower Damping and Parked Turbine Measurement Block1, respectively. Similarly, LON and LAT are used to refer the Longitudinal and Lateral direction strain gauge signals. The results displayed in the table are calculated by using 3 different signal groups. The signal used in the analysis is shown in parenthesis. (LON) or (LAT) groups were calculated by using one strain gauge measurements whereas (LON-LAT) group contains two signals together. It can easily be seen that tower frequencies change slightly depending on the pitch angle Measurement blocks 1, 2, 3 and 7 having very close pitch angles produce very close longitudinal tower frequencies varying between 0.9991 and 1.0006. The slight difference can be attributed to measurement noise, physical factors changing during the long measurement blocks or round off errors. The only frequency which is thought to be related to lateral tower mode is 1.0119. A 268
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