Much, if not most, of the underwater noise is caused by propeller cavitation
– IMO, Guidelines for the reduction of underwater noise from commercial shipping to address adverse impacts on marine life, 2014
The radiated underwater noise due to shipping activities is considered a harmful form of pollution for marine ecosystem. In the frequency range from 10 to 300Hz the natural background noise level is estimated to be raised by 20 to 30 dB due to shipping. In the last 30 years the increase amounted 10 dB, i.e. over 3 dB per decade. Low frequency noise covering the 63 Hz to the 125 Hz 1/3 octave bands is often dominated by the ship noise related to propeller cavitation. This frequency band of emission coincides with important frequencies of perception of baleen whales and fish, and it may therefore have negative impact on their natural activities. These concerns are reflected in the EU Commission Decisions (EU, 2008; EU, 2010; EU, 2017) and IMO Guidelines (IMO, 2014).The noise emissions in the same, and higher frequency range may also interfere with acoustic sensors used by naval, research and oceanographic vessels and underwater monitoring systems.
On the other hand, the ship on-board noise also receives close attention these days, due to the increasing awareness of health hazards caused by the long-term exposure of the crew to high noise and vibration levels, and due to the considerations of safety and comfort for crew and passengers, as reflected in the IMO Resolution (IMO, 2012). The requirements are particularly high for cruise liners, yachts and ferries. The analysis of full-scale measurements conducted on a variety of merchant ship types indicates that the highest levels of on-board noise are frequently noted in the same lower-frequency bands as mentioned above, and are thus associated with propeller cavitation.
Notwithstanding the importance of other sources in the overall picture of ship induced noise, the propeller cavitation noise is herefore identified as the dominating noise source, and it needs to be addressed in the development of noise mitigation measures regarding both the radiated and on-board noise levels.
In its turn, it requires reliable tools for the prediction of propeller cavitation of different types, its dynamic behaviour during propeller operation behind ship hull, hydroacoustic models and noise propagation models. The said tools need to include the high-fidelity flow simulation methods to gain insight into the physical mechanisms behind cavitation noise and thus suggest effective noise mitigation measures, on the one hand, and engineering methods which can efficiently be employed for product optimization in the design phase, on the other hand.
The overall objective of the present project is to improve the numerical and experimental methods for the prediction of noise and
vibration induced by a propeller operating behind ship hull in full scale conditions, and to elaborate practical recommendations for the
reduction of noise and vibration levels for specific design applications.
The achievements of the previous research projects have provided a significant contribution to the state of the art and marked the route to address the problem of noise and vibration emissions from ships and their activities. Nevertheless, the complex, multidisciplinary nature of the problem, as well as many open methodological and physical issues that still remain challenging, leave a large room for further research and development efforts. In this picture, the strategy to split the general problem into specific sub-problems and address them through dedicated and focused research may be a valid solution to make appreciable steps forward in addressing the problem as a whole.
In order to achieve the project objectives, the following presently unsolved research tasks will therefore be addressed:
Development of more reliable methods to increase the accuracy of experimental predictions in low frequency range, accounting for the influence of facility size and test procedures, sound reflection on walls and/or free surface, and filtering of facility background noise
Collection of reliable and accurate experimental data for the validation of CFD models and design tools for noise predictions, in model and full scale conditions
Improvement of numerical methods for the simulation of multiscale flow turbulence, induced vorticity, cavitation dynamics and acoustic propagation
Extension of the applicability of the numerical tools to accommodate the effects of hull and appendages, and to improve the accuracy of predictions in higher frequency range
Filling gaps in understanding of fundamental physical mechanisms underlying propeller noise emission related to turbulence, cavitation and bubble dynamics
Evaluation and enhancement of previously outlined noise mitigation measures in application to specific types of ships and propulsion systems
Thus, the main scientific goal of the present project is to make progress beyond state-of-the-art in theoretical understanding and practical simulation of the complex and multi-disciplinary problem of ship and propeller hydroacoustics.
The primary technological aims are seen in further development, elaboration and validation of the advanced experimental and numerical modelling techniques regarding propeller cavitation and noise,
development of practical tools for propeller noise prediction, and application of these tools to selected cases of ships and propulsion systems with the focus on noise mitigation measures.
The tools developed in the project will be used by the researchers and ship and propeller designers, while findings and recommendations are expected to contribute into the development of noise mitigation guidelines by the classification societies.
Cavitating Tip-Vortex with Two-Phase Calculation
In ProNoVi three target cases are investigated corresponding to the three industry partners.
Twin screw mega yacht with fixed pitch propellers
Single screw container ship with controllable pitch propeller
Twin screw catamaran for servicing wind farms