The transmission of vibration energy from the location of excitation, i.e. the shell plating above the propeller, to the tank bulkheads was investigated by performing an operational modal analysis of the aft peak tank structure using Stochastic Subspace Identification [5, 6, 7]. A stability diagram of the aft peak tank structure, produced with OMA, is shown in Figure 6. There are a number of natural frequencies located between 28 Hz and 85 Hz and these represent modes of the aft peak tank and local modes of the stiffener to which the sensors were attached. These modes will resonate when energy is input at and close to the frequencies at which they are present. This phenomenon is seen in Figure 7 which shows the waterfall plot related to a sensor in the aft peak tank. There are two distinct areas of high energy: below 10 Hz and between 28 Hz and 60 Hz. Vibration energy is present at twice and three times propeller blade passing frequencies, between approximately 11 Hz and 26 Hz, but this is less than the energy measured between 28 Hz and 60 Hz. This is despite the fact that Figure 4 shows that the second and third blade rates have higher amplitudes of input pressure than the higher blade rates. The greater vibration energy in the 28 Hz to 60 Hz band is due to the modal properties of this stiffener. The largest peaks in the 28 Hz to 60 Hz spectrum are located at the stiffener’s natural frequencies. Vibration energy from propeller excitation must pass through the aft peak tank structure on its way to the water tank bulkheads. It may therefore be concluded that pressure energy input between 28 Hz and 85 Hz will be passed through the aft peak tank to the connecting tank bulkhead above it with little loss in energy. A large number of modes between 28 and 85 Hz Order Vibration energy (mm/s/Hz) Shaft speed (rpm) Fig. 6 Stability diagram of the aft peak tank structure. Red circles indicate stable poles while green and blue crosses indicate unstable pole Fig. 7 Frequency distribution of vibration energy of the aft peak tank structure during normal operating. The modal analysis of a bulkhead of an empty and a filled water tank is shown in Figure 8. For the empty tank a large number of modes were present between 40Hz and 80Hz. During the test there was machinery running with shaft speeds of 1170rpm and 1800rpm and hence, the modes identified at 20Hz and 30Hz are attributed to harmonic excitation. For the bulkhead of the filled tank a large number of modes were present between 25Hz and 80Hz. The reduction of frequencies when compared to the empty tank was due to the added effective mass of the water. The added mass lowers the natural frequencies of the tank bulkhead. The effect tends to be greater for lower modes as higher modes engage less of the structural mass. 284
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