frequency appearing in lateral vibration direction does not need to be caused by lateral tower mode. Since the vibration paths of longitudinal and lateral tower modes are not exactly vertical to each other, high amplitude longitudinal vibration can also be dominant in lateral direction. As the pitch angle decreases to 110 degrees, the longitudinal tower mode slightly increases to 1.0017 (PTMB5) and 1.0023 (PTMB6). A further decrease to 93 degrees results in a slightly larger longitudinal frequency of 1.0049 for PTMB4. The same tendency was also observed in LDV measurements; the longitudinal tower frequency changed from 1.0009 to 1.0040 when the pitch angle was changed from 180 to 90. On the other hand damping is observed to be affected from aerodynamic coupling and excitation direction. The drag phenomenon mentioned several times in the article is thought to cause an unexpected damping increase in the lateral tower mode. 0.0086 is approximately 3 times larger than the damping ratio expected for this wind speed. The results presented in Table 2 conflicts with what is reported in literature because longitudinal tower mode are usually believed to have higher frequencies than the lateral tower mode at parked condition. However, there is not a unique pitch angle configuration for a parked turbine. For a rotating turbine, a certain wind speed approximately corresponds to a specific pitch angle; besides this, the relative angle between the turbine and wind direction is always small due to active yawing. However, there is not a unique configuration of relative angle, pitch angle or wind speed for a parked turbine. Therefore, more measurements and data analyses are needed to validate this observation. Name Duration Relative Angle Pitch Angle Wind Speed TF(LON) TD(LON) TF(LAT) TD (LAT) TF(LON-LAT) TD(LON-LAT) PTMB1 2800 -110 178 5.30 1.000 0.0046 1.0119 0.0086 - - PTMB2 2100 -100/-110 178 4.50 1.0003 0.0030 - - 1.0000 0.003 PTMB3 4650 -10 / -20 178 5.10 0.9991 0.0030 - - 0.9985 0.001 PTMB4 3700 -110 93 3.75 1.0049 0.0011 1.0035 0.0050 - - PTMB5 2808 changes a lot 110 1.50 - - - - 1.0017 0.002 PTMB6 4860 changes a lot 110 1.55 1.0023 0.0020 1.0009 0.0030 1.0009 0.003 PTMB7 468 -110 180 5.20 1.0006 0.0032 0.9959 0.0044 1.0009 0.003 Table 2 Longitudinal and lateral tower frequencies and damping ratios extracted from tower strain gauges LDV measurements were also taken at several locations on the blade and the obtained results were compared with those acquired from the strain gauges installed in the blades. Table 3 shows the frequencies identified from blade vibration data measured on a parked turbine. 269
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