106 J. Baqersad et al. 0 0.5 1 1.5 2 -200 -100 0 100 200 S33 time (s) Measured Strain Predicted Strain 0 0.5 1 1.5 2 -200 -100 0 100 200 S32 time (s) Measured Strain Predicted Strain 0 0.5 1 1.5 2 -200 -100 0 100 200 S31 time (s) Measured Strain Predicted Strain 0 0.5 1 1.5 2 -40 -20 0 20 40 S22 time (s) Measured Strain Predicted Strain 0 0.5 1 1.5 2 -40 -20 0 20 40 S21 time (s) Measured Strain Predicted Strain 0 0.5 1 1.5 2 -200 -100 0 100 200 S13 time (s) Measured Strain Predicted Strain 0 0.5 1 1.5 2 -200 -100 0 100 200 S11 time (s) strain (με) strain (με) strain (με) strain (με) strain (με) strain (με) strain (με) Measured Strain Predicted Strain TRAC=99.9 TRAC=99.9 0 0.5 1 1.5 2 -200 -100 0 100 200 S12 time (s) Measured Strain Predicted Strain strain (με) TRAC=99.8 TRAC=99.9 TRAC=99.3 TRAC=99.0 TRAC=99.0 TRAC=99.7 Fig. 10.5 Comparison of the strain measured with mounted strain-gages (red boxes) and the predicted strain using the proposed approach when the wind turbine is excited by a shaker with a sinusoidal input at the frequency of the first mode (5.3 Hz). The photo shows the location of the strain-gages where the comparison is performed. Sensors S12, S22, and S32 were mounted to the underside surface of the blades where no optical targets were placed or measured. The contour plot (top) shows the full-field predicted strain at one sample in time 10.5.4 Random Excitation with a Mechanical Shaker In this case, the wind turbine was excited using a mechanical shaker with a random input that could excite all the modes below approximately 60 Hz. A comparison between the measured and predicted strain for the random force excitation with the shaker is shown in Fig. 10.10. The results show very good agreement between the results for most of the strain-gages. It should be mentioned that for the strain-gages that do not have high TRAC values, the strain levels are very low and are below the noise floor of the proposed technique. In these cases there are slight variations and the predicted strain may differ from the measured strain. However, the amplitude of the predicted strain still correlates well with the measured strain.
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