Trans RINA, Vol 153, Part A4, Intl J Maritime Eng, Oct-Dec 2011
some guidance notes notifying clients about the potential risks and the effects of propeller cavitation. The only intervention from the ship owner/operator side can take place during the model tank tests, see Figure 1, whereby a report is established, where the blade area percentage of cavitation can be assessed together with a table of normalized pressure forces or pressure patterns from the propeller to the hull. The latter (pressure patterns) appear as amplitude versus propeller blade passing frequencies and its multiples. Assuming these frequency components are strong,
vibration transmission to the hull and
subsequent structures can cause resonances at the various structures’ natural frequencies.
2
PROPELLER CAVITATION AND INDUCED VIBRATION MECHANISM
When cavitation bubbles collapse, broadband noise is generated. The noise is
radiated to the surrounding
marine environment and transferred onto the ship structure. The cavitation extent varies throughout one cycle or revolution of the propeller blade. Typically, the cavitation volume reaches its peak when the propeller blade is close to the hull, see Figure 2a. The fluctuating volume of the cavity then causes a pressure pulse, as the blades pass close to the surface of the hull, acting on the hull plating in the aft ship. Generally, the lower the volume rise as the propeller blade rotates, the lower the generated pressure
differential across the blade. The
cavitating blade contribution to the hull pressure field is therefore considered to derive from the suction side sheet and tip vortex cavities. Hence, these types of cavitation
may collapse either on or off the blade. When cavitation bubbles collapse on the blade, these are related to the generation of the blade passing frequencies, while when “off the blade”, they cause mainly white frequency noise. Thus, the frequency content of propeller cavitation contains a combination of impact and harmonic characteristics. See Figure 2b.
To estimate the effects of cavity volume variations on the cavitating pressures, modelling of the cavity volume on the blade as the propeller rotates in the wakefield is done through CFD. Numerical formulation of the magnitude of the pressure vector has been discussed in Breslin, JP et al [5] and an expression of the following form has been developed:
PVe R
cqz Zq
2
() Re[ P
qz 32 iq z ]
From this expression, it can be seen that the pressure magnitude of a specific harmonic order
is
rudder, etc.) in a specific manner (PR2.5 toPR1
cqz p p inversely
proportional to the distance Rp and demonstrates that the higher the clearance around the propeller the less the pressure pulse acting on the adjacent structure (hull or
compared cqz ). Comparing this with a similarly derived
expression for a non-cavitating propeller, as derived by Breslin and Tsakonas [6], indicates that the pressure pulse from a non-cavitating propeller decays far more rapidly with distance, see Carlton, JS, [7].
Figure 1: Cavitation Tunnel Tests and Typical Corresponding Report.
A-262
©2011: The Royal Institution of Naval Architects
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