Trans RINA, Vol 153, Part A4, Intl J Maritime Eng, Oct-Dec 2011


To further evaluate the hull pressure improvement due to the new VG design, the peak-to-peak hull pressure magnitudes for the bare hull (without VG), as well as the original VG design and the new VG design, all under the same cooling water discharging jet effect, are plotted in Figure 17. As seen in the figure, comparing to “the without VG case” (the green bars in the figure), large pressure reductions of more than 50% are obtained for the 1st BPF pressure in the two VG “add-on” cases. However, it should be noted that the original VG design (the red bars) causes higher hull pressures at high blade passing frequency components (2nd, 3rd, 4th and 5th blade rate pressure). For the new VG design (the blue bars in the figure), it is found that


pressure can be achieved for all BPF pressures compared to the original VG and the “without VG” cases. Table 1 summarizes the % reduction of the hull pressure, based on the model test results for the original and the new VG design cases. As seen from the table, the hull pressure reduction due to the new VG design is in the range of 7.5% - 34%. On average, the hull pressure pulsation can be reduced by about 23% due to the new optimised VG design. This shows that experience does not always help when it comes to empirically positioning Vortex Generators in order to cure a propeller cavitation problem. Instead, only comprehensive CFD analysis can assist in improving the wake quality.


It is eventually noted that the BPF pressure patterns observed during these measurements reflect the Fast Fourier Transform


(FFT) pattern of the vibration


measurements analyzed earlier, in terms of frequency content. However, the two patterns (FFT pressure and FFT vibration) differ in that the higher order vibration BPF components (above 1 x BPF) excite certain natural frequencies of structures and get magnified at relatively high amplitude magnitudes. This becomes the cause of the intense vibration. Consequently, the corresponding frequency responses could be additionally used as a diagnostic tool to detect onsets or even advanced cases of propeller cavitation.


From the vibration measurements (see Figure 18), one can see a frequency response overlay between the original (without VG’s) situation and the last VG (optimised) design. It can be seen that the vibration is substantially reduced with the successful installation of the new VG’s.


4 CONLUSIONS


Analysis of the vibration signal at locations in the aft area of the vessel through frequency spectrum plots and spectrograms shows a dominant blade passing frequency component and its multiples, occasionally at higher amplitudes than the main blade passing frequency. This is a diagnostic signature of a propeller hydrodynamic problem and, in particular, a problem of


propeller


cavitation. The frequency content of the signal can cause excitation of natural frequencies of structures that were


7. 5. 6.


not designed to be excited at more than two or three times the propeller blade passing frequency. Thus, the existence of


these higher BPF multiples can cause further reduction of the


undesirable, unpredicted vibration on board the vessel. Assuming it is too costly or impractical to replace the propeller during vessel’s operational service with a better propeller design with reduced cavitation, interventions on the wakefield through flow-improving devices such as vortex generators can reduce the cavitation intensity and hence the induced vibration on the hull. Comprehensive CFD analysis, accompanied by model tank tests where possible, rather than pure experience, is necessary for a successful and effective VG installation. Where, after wakefield flow improvements, vibration problems are not completely eliminated, then stiffening of


the subject


structures should be attempted to further reduce the vibration. The stiffening process, in order to be effective, should involve both measurements


through vibration


monitoring and/or modal tests, always correlated with results from FE analysis.


5 ACKNOWLEDGMENTS


The authors would like to acknowledge the invaluable assistance of AK Seah (ABS Vice President, Corporate Environmental Solutions),


Peter Tang-Jensen (ABS


Senior Vice President, Corporate Technology) and Dr Jer-Fang Wu (ABS Singapore Offshore Technology) for their contribution and for facilitating the coordination among the authors. Last but not least, the authors would like to acknowledge the invaluable contribution of Davor Sverko (Senior Principal Engineer, Corporate Marine Technology) with respect to the technical content and the measurements, as well as Wärtsilä Danmark A/S.


6 1. 2. 3. 4. REFERENCES


Some Experiments Related To Ship Hull Vibration And Pressure Fluctuation Above The Propeller, Tran., RINA, Part B, Vol. 132, 1990


Vrijdag A., Stapersma D. and van Terwisga T., Control of propeller cavitation in operational conditions. IMarEST Journal of Marine Engineering and Technology, No.A16, 2010 Lee, S.K., Liao, M., Wang, S.,


Propeller-


Induced Hull Vibration – Analytical Methods, ABS Technical Papers, 2006.


Li, J.Y., Lee, S.K., Propeller weight reduction and efficiency


enhancement – design


verification for a large containership, ABS Technical Papers, 2006.


Breslin, J.P., Skaar, K.T., and Raestad, A.E., the Relative Importance of Ship Vibration Excitation Forces, Symp. On Propeller Induced Ship Vibration, Trans, RINA, 1979.


Breslin, J.P., Tsakonas, S., Marine Propeller Pressure Field Due To Loading And Thickness Effects, Trans. SNAME, 67, 1959


Carlton, J.S., Marine Propellers and Propulsion, Butterworth-Heinemann, 1994


A-272


©2011: The Royal Institution of Naval Architects


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