Julian Kimmerl M. Sc., SCHOTTEL GmbH, Dörth
The significance of the acoustical realm, from structural transmission into the hull and subsequent airborne-noise, to underwater noise emissions in the world oceans, lead to a new found focus of research and regulation in the field of ship acoustics in recent years, as a result of the rise of civilian attention to environmental issues and increased use in military applications. Due to the diversity of the utilization of auditory perception of marine biology on the one hand and technological advances in warfare sensor equipment on the other, it is imperative to achieve accurate prediction capabilities for the complete spectral range of underwater radiated noise of propulsion devices. In response to market demands manifested by implemented class guidelines, a rapid and significant evolution of the simulation methods for URN in the marine industry has proceeded over the past decade, which is summarized in this work with an outlook at future developments.
Starting from simple empirical approaches, over BEM and single-phase RANS simulations, to hybrid acoustic-hydrodynamic high-fidelity CFD simulation methods, the trend towards more detailed analysis in both space and frequency domain and accompanying larger resource demand is unstoppable. Now two-phase Volume-of-Fluid CFD methods with LES based turbulence modelling and Schnerr-Sauer cavitation model in combination with acoustic analogies such as the permeable surface FWH method are the new industry standard for evaluation of emitted noise of a propulsor-hull combinations. Additionally, the spectral post-processing of the results is not limited to local single point observers and frequencies, such as the propeller blade harmonics, anymore, but instead the acoustic information is investigated in a more integrated approach to judge the underwater radiated noise emissions from propulsion machinery into the vessel hull and the fluid domain.
Simple hydrofoils and propeller test cases in model scale, as well as hull-propeller combinations in full scale are analyzed by classical flow examinations of vortices and cavitation behavior and compared to reference measurements in the field to assess the respective methods capabilities for underwater noise prediction of these geometries and their iterative improvements with higher modelling quality. Further detailed local investigations, as well as advanced spectral and data driven methods are explored to achieve a better overview of the advances of post-processing of noise regarding source and propagation over the last years.
The development of the single tools for simulation of hydrodynamic flows and their associated noise generation and emissions are identified and critically measured against suitability and quality criteria, their degree of innovation and most importantly the practicality in a future-proof engineering environment. The suitability of a methods results for exchanging information between stakeholders and different fields of engineering is assessed. Different disciplines such as ship building or marine science may be involved in the utilization of the final produced data as well as specialists in fundamentally different but ultimately interconnected physical fields, e.g. hydrodynamics, structural mechanics, and acoustics. Finally, also the exchange of information between different software suites may play an important role in the success of a method and its produced resulting data.