Experimental Studies Experimental Modal Analysis in Dry Dock Modal testing of the azimuthing thruster was carried out in dry dock. The thruster was excited by a pendulum made by mass of 50 kg, see Fig. 4. The steering mechanism was locked during testing. Excitation force was measured by an accelerometer installed on the end of the mass and response of the azimuthing thruster was measured by 10 triaxial accelerometers. Sampling frequency used was 512 Hz. Signal processing and FRF calculation was carried out by using LMS Scadas III front-end and LMS Cada-X software. Modal parameters (frequencies, modal damping ratios and mode shapes) were estimated using LMS TestLab and LMS Cada-X softwares [19, 20]. The parameters were estimated by using PolyMAX algorithm [21]. Frequency range of interest was from zero to the most significant excitation frequencies during normal operation. One of the most significant excitation was blade frequency. Identified modes are presented in Table 1. For reasons of confidentiality all frequencies and modal damping factors in this paper are presented as scaled values. The reference frequency fref used in scaling is the natural frequency of first longitudinal mode of the azimuthing thruster measured in dry dock. Also the reference modal damping factor ȗref is the damping factor of first longitudinal mode of the azimuthing thruster measured in dry dock. Lowest transversal mode shape is shown in Fig. 6. In order to quantify orthogonality or independency of measured modes, AutoMAC criterion [22, 23] was applied, see Fig. 5. MAC takes values between 0 and 1; the closer to 1 means high correlation, while the closer to 0 means that those modes are orthogonal or independent. As can be seen, between modes 1 and 2 there is quite high MAC value. These modes are global mode of the ship hull and lowest transversal mode of the thruster. Also there is quite high off-diagonal term in AutoMAC matrix between modes 6 and 7. These modes relates to vertical bending modes of the rotor. High off-diagonal values originate from the fact that the number of measuring points is too small to identify these modes (spatial aliasing). In order to identify global modes of the ship there should be more measuring points on the ship hull. Also there should be more measuring points on the rotor and propeller blades in order to identify more accurately rotor bending modes. However, used measuring points was noticed to be sufficient for identifying satisfactory the lowest modes. Fig. 4 Impact testing of thruster by using pendulum 7
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