A method to predict underwater noise from cavitating propellers

Submitted for the special session Oceans of Tomorrow: AQUO and SONIC activities towards the ship noise characterisation, invited by Ruggero Dambra (ruggero.dambra@cetena.it). The potential impact that all types of vessel emissions, including underwater noise, could have on marine fauna has become an important issue during past decade. Surface vessels radiate underwater noise mainly due to machinery on board, propulsion mechanism and hydrodynamic flow around the ship hull ad appendages. Growing attention of the international community on environmental issues and, in particular, the requirements of the European Union on the underwater noise has recently led to a research effort to better understand and predict propeller cavitation noise. In fact, a cavitating propeller is one of the main sources on board ships, since it produces both structure-borne and radiated noise with a spectrum that usually covers a wide range of frequencies, with both tonal (blade passage related) and broadband components. The EU SONIC project is aimed to develop tools to investigate and mitigate the effects of underwater noise generated by shipping, mainly focussed on understanding and modeling propeller cavitation noise. In the frame of the SONIC project, CETENA has implemented a full numerical procedure for the prediction of propeller cavitation noise in terms of tonal pressure fluctuations and broadband noise, based on the combination of hydrodynamic and acoustic boundary element methods, and has applied it to the case of a container vessel. As far as the tonals are concerned, the associated source levels are estimated based on the computation of the hull pressure levels at the blade passing frequencies (bpf); from the hull pressures, equivalent noise sources are identified and used to compute far field noise levels. As far as the broadband noise is concerned, two main contributions are modelled, respectively due to the propeller tip vortex and to the broadband effects of sheet cavitation. The former is estimated based on a semi-empirical model, the latter using an analytical model of cavitation dynamics. In particular the prediction of hull flow and wake at propeller disk is carried out with the RANSE solver ANSYS-CFX, the analysis of propeller hydrodynamic behaviour and cavitation prediction is performed with CRS PROCAL, the identification of propeller equivalent noise sources is carried out with CRS EXCALIBUR, the computation of the broadband part due to propeller tip vortex of the hull pressure is carried out with the CRS ETV model, the prediction of the broadband effects of sheet cavitation is performed with CRS MATusiakLAB and the computation of the radiated noise is carried out with LMS SYSNOISE. This paper presents CETENA's procedure, describes and discusses its application to the test case. For the purposes of extensive validation, propeller cavitation pattern, hull pressures and far field noise spectra are compared with experimental data achieved by HSVA within the SONIC project. The results of the validation study indicate that the adopted methodology is able to capture the relevant features of the propeller acoustic spectrum. Further research and development is needed concerning numerical methods to predict hull pressures that account for interaction between sheet and tip vortex cavitation. The methodology involves modelling/computational efforts that are compatible with the typical propeller/ship design process; it is therefore suitable for design applications. Strengths and weaknesses of the methodology are further discussed in the paper, with focus on design applications; areas for further development and validation are highlighted too. The research leading to these results has received funding from the European Union Seventh Framework Programme (FP7/2007-2013) under grant agreement n°314394.