Trans RINA, Vol 153, Part A4, Intl J Maritime Eng, Oct-Dec 2011
found to be bad wake quality due to bad stern shape design, in this instance it is found from the model tank test wake measurements that the port side cooling water discharging jet at the upstream of propeller (see Figure 10), is the potential root cause of the adverse wake quality. To demonstrate the effect of the cooling water jet on the wake quality, measured wakes with and without cooling water jet at port side are CFD computed and plotted in Figure 11. As seen, the cooling water jet (with a velocity of 5 m/s) causes a higher extent of slow velocity area at 12 o’clock position and a stronger swirling flow at port side. Hence, it is expected that more cavitation on propeller blade surface will occur and higher hull pressure impulses will be created compared to the original wake field without the cooling water jet scenario.
10. To evaluate the effectiveness of the experience-based design,
sea-trial measurements for hull pressure were
performed using eight pressure sensors installed in the stern areas, see Figure 12. During the sea-trials, the propeller operated in the range of 80-84 rpm with ship speed around 21 knots. To have a general idea of the hull pressure impulse intensity, the measured pressures at 84 rpm are plotted in Figure 13. From the measurement, it is understood that the maximum pressure of 5.5 kPa is above the usual levels found in similar sized LNG carriers.
Figure 14: Typical tip vortex cavitation bursting from the actual video recording.
Figure 13: Measured pressure amplitudes from eight pressure sensors – 84 rpm at a speed of 21 knots (full scale measurements).
To improve the wake flow for reducing the propeller- induced excitation, a pair of VGs (Vortex Generators) were designed based on experience and were installed at the port and starboard sides of the actual hull, see Figure
Furthermore, from the video records based on boroscopic camera for propeller cavitation observations, it is found that strong bursting of tip vortex cavitation, Figure 14, does occur causing strong pressure pulses transmitted to the hull, especially for higher blade passing frequency (BPF) pressure components of the 2nd, 3rd and 4th order, as measured by sensor E, see Figure 13.
Figure 15: Location of port side VG – original location versus. new location, CFD study. ©2011: The Royal Institution of Naval Architects A-269
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