Hydroacoustic analysis of a marine propeller through the Ffowcs Williams-Hawkings equation

In the last decade, the ship underwater noise has become a key theme to shipbuilding industry. The increasing concern regarding the sea acoustic pollution and its negative impact on marine wildlife, in fact, urged the governments to finance different, ad hoc research projects on many issues related to hydroacoustics, while several classification societies have set new, more stringent regulations about ship noise emission. Not to mention the relevance of this topic for military purposes, concerning the ship identification and traceability. Within this context, it is well-known that the propeller represents one of the most relevant noise sources and the availability of some reliable numerical tool, able to provide an assessment of the acoustic field at a design stage, appears as a need of paramount importance. From a theoreticalpoint of view, the prediction of propeller noise can be achieved through the Acoustic Analogy and the solution of the Ffowcs Williams-Hawkings equation, which governs the sound generated by any body moving in a fluid. This approach is widely used by aeronautical industry and its potentiality is enormous, since it is able not only to achieve a reliable assessment of noise, but, in principle, also to identify the different source mechanisms occurring in the flow and related to body motion. The paper deals with the use of the Acoustic Analogy for marine propellers (in no-cavitating condition): this technique is rapidly becoming very popular in the hydroacoustic community, but is somehow affected by more than 40 years of research activities strictly limited to aeroacoustics. In particular, our analysis is focused on the behavior of the surface integrals of the linear solving formula: it reveals that, contrarily to what happens for analogous aeronautical devices, the noise generated by a marine propeller is not a strong function of the blades shape and/or the body surface loads, but the nonlinear flow phenomena occurring in the downstream region is the primary source on noise. Such a (quite unexpected) behavior represents an intrinsic feature of a rotating body and is due to both the range of the rotational Mach number occurring underwater and the usual multi-bladed configuration of the device. Then, in spite of a rather common popular belief and just because of the low rotational speed, the assessment of noise from a marine propeller represents an essentially nonlinear problem and always requires an accurate description of the entire flow field.