Boundary element modelling of fractal and other enhanced bandwith schroeder diffusers

The aim of this paper is to explore techniques that are being developed by the authors and their colleagues to estimate underwater noise radiated by marine propellers. Some fundamental aspects of propeller noise prediction are considered, including the nature of broadband noise due to flow over propeller blades and the effects of rotating and fluctuating forces that lead to tonal components in the underwater noise spectrum. It is shown how research in aeroacoustics can be applied at the much lower Mach numbers associated with marine propellers. A major challenge in surface ship design is prediction of the onset of cavitation, occurring when the pressure in the flow over the rotating propeller drops below the vapour pressure of water, and of its effect on the spectrum of underwater noise as ship speed increases. Acoustic sources change from higher order sources such as dipoles and quadrupoles, to monopole sources that have much greater acoustic efficiency. Furthermore, these cavitation sources are sensitive to the properties of water, including temperature and air content, as well as sea state and ship heading. Overall propeller noise increases progressively with speed, with changes in the spectral shape that bias the content to lower frequencies. The nature of the spectral changes, and the combination of experimental and numerical methods that can be used to estimate cavitation noise are discussed.