Propeller Cavitation Induced Hull Pressure Reduction by Air-Bubble Layer

Abstract

Cavitation generated by a marine propeller is a primary concern among the possible vibration- and noise- sources in commercial ships. By exploiting the compressibility of air, there have been many attempts to form an air-bubble layer underneath the stern-hull surface above the propeller, and consequently to isolate the cavity-induced pressure wave across the layer. However, it could not be popularly used because the cost was so expensive to deliver a huge amount of air for a sufficient isolation performance. In this work, full-scale ship measurements reveal that a significant reduction of pressure-amplitude is possible at the outside of an air-bubble layer, where the isolation effect is not involved. Moreover a hull-vibration reduction of approximately 75% was found to be achievable. Instead of excessive consumption of air, considerably small amount is necessary for a reduction of cavity-induced pressure amplitude, which can make the constitution of relevant system simple. Hence the purpose of this study is to provide a physical proof for such a beneficial phenomenon. By approximating the solution of acoustic scattering from a bubble, we find that phase-reversal reflection provoking a destructive interference is the main reason for a pressure reduction outside the layer.