Suppression Of underwater Noise Induced by Cavitation
With the steady growth of shipping activities over the last decades, underwater noise is increasing. It may be hindering sea mammals, or may even be harmful to them. Underwater noise is a European concern and is identified as a qualitative descriptor for determining the Good Environmental Status under the Marine Strategy Framework Directive. The main sources of noise are the ships' propellers, especially if the flow around the propellers is cavitating. Most propellers are cavitating when the ship is sailing at its designed speed. The surge for more fuel-efficient propellers is increasing cavitation. Therefore the dilemma is the following: less CO2 emissions into the air yields more noise emissions under water.
The first goal of the project is to enhance the understanding of radiated noise from ships. From earlier studies, it is known that the kind of noise generated far away from the ship can vary according to the form of cavitation. Based on this knowledge, the project aims to improve computational and measurement techniques used in the design of a ship. Numerical tools are extended and tuned to correctly predict the noise level; measurement techniques in model basins and cavitation tunnels are calibrated to correct noise measurement for the reflection
of noise from the walls of the facilities. Both the numerical and experimental tools need to be validated for the frequency ranges that are most harmful to sea mammals. Once a ship is designed, it is critical to establish the actual noise level with a trial. Such trials are usually conducted at ballast conditions, for which cavitation is absent. Noise measurements, therefore, need to be performed during service of the ship with the minimum possible hindrance to the economical operation of the ship. New techniques will be tested to measure the noise
generated by the propeller on-board the ship. Another technique is to measure the noise offboard with buoys or sound arrays. Both techniques will be compared in order to obtain a reliable and cheap measurement procedure. With data available on the noise level of ships, it becomes possible to estimate underwater noise given the actual shipping activity. Using AIS (Automatic Identification System) data, the project will develop a technique for mapping underwater noise in the North Sea. This map can be used by authorities to manage shipping or to study the influence of mitigation measures.
The project will use model basins to study the noise emission from ships in design stage. Fullscale measurements at sea will be performed to establish accurately the noise of several ships, but also to determine long-term noise profiles of seas. All measurements will be fed into a database which will be made available to ship designers outside the project. The project will finally use computational tools to predict the noise in a given sea for a longer period, showing the hinder to sea mammals in a noise map. SONIC will closely cooperate with the AQUO project, which will adopt a complementary approach to study cavitation noise.