Numerical Investigation of Scale Effects on Cavitation and Underwater Radiated Noise

Licentiate thesis, 2025

Recent research efforts to reduce shipping noise emissions have been motivated by the growing awareness of its adverse effect on marine life. At this moment, there is a need for reliable noise assessment methods to support in designing more silent vessels. While sea trial measurements and model scale tests provide valuable data, they cannot be used during the early stages of ship design. Numerical methods, on the other hand, offer the tools for the assessment of noise emissions during these early stages. However, there is currently no well-established, consistent, and reliable approach for the prediction methods. In addition, the lack of research on scale effects on cavitation limits the ability to make informed design decisions.

In this research work, the objectives focus on the assessment of scale effects on cavitation and the development of reliable tools for predicting underwater radiated noise from ships. Studied cases include a cavitating hydrofoil and a propeller operating in-behind condition. Results show that scale effects are more significant for the propeller case due to the interaction with the wakefield. At full scale, cavitation is less pronounced, which leads to significant differences in both pressure pulse levels and underwater radiated noise. In addition to scale effects, the impact of the domain/tunnel size on cavitation is investigated. Results indicate that blockage influences the wakefield upstream of the propeller, leading to less cavitation when using a larger computational domain.

For the noise assessment, it is demonstrated that using the Ffowcs Williams–Hawkings acoustic analogy for hydroacoustic applications can produce inconsistent results. An alternative noise prediction methodology is proposed which models cavitation as a monopole source. Results are compared with sea trial measurements and model scale tests. While good agreement is obtained for pressure pulses, discrepancies occur for the noise level predictions. At low frequencies, sea trial measurements are dominated by engine noise, a source not accounted for in the numerical predictions. Better agreement is obtained when compared with model tests, but the numerical predictions still significantly underpredict the broadband noise levels. The results highlight challenges in obtaining reliable noise predictions and the need for more research in this area.