Frequency (normalized with respect to the tower frequency) SO Si L 1.0 X X X 2.6 X X X 2.8 X x X 3.2 X x X 5.3 X X 5.4 X X 6.7 X X X 7.1 X X X 8.7 X X 9.1 X X 17.9 X X Table 3 frequencies identified for parked turbine blade vibrations data (normalized wrt. tower frequency) As performed in tower measurements, the obtained data were analyzed in different signal combinations. The abbreviations SO, Si refer to the 6 strain gauges placed out of plane direction and in plane directions, respectively. Similarly L represents the laser measurements. As can be seen from Table 3, some of the frequencies are detectable either from in-plane strains or from out of plane strains only but not from both of them at the same time. However, some frequencies are detectable both from in-plane and out of plane strain signals. Laser signals are able to detect all the frequencies displayed in Table 3. Therefore it can be concluded that laser signals can be used together with both in-plane and out of plane signals. Table 4 shows the system parameters calculated from the signals explained above. Signals 2, 4, 6 are used as out of plane direction strain gauges whereas signals 1, 3, 5 represent in-plane direction strain gauges. Signal 7 is used for laser measurements. It can be noticed from Table 4 that even if the estimated damping ratios slightly change depending on the time series analyzed, the value obtained by using LDV and one strain gauge is always very close to those calculated from other signal combinations. This proves that LDV measurements are always consistent with strain gauge measurements and the results found are almost the same. Although more measurements and analyses are required for the final conclusion the normalized frequencies of 2.6, 2.8 and 3.2 may be related to the first yaw, first tilt and first symmetric flapwise modes, respectively. The normalized frequencies of 5.3 and 5.4 are mainly related to the first vertical edgewise and first horizontal edgewise modes, respectively. The identified frequencies and damping values are consistent with those reported for the edgewise modes in the literature. The normalized frequencies of 6.7 and 7.1 are believed to correspond to the second tilt and the second yawing modes. However the identified damping values are again lower than those reported in the literature. 270
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