Abstract
Comfort on board has become an important aspect in the design of ships over the last decades, especially comfort for passengers on cruise vessels and for owners of yachts. Therefore, cavitation on propellers for these ships should be minimized, leaving often only a tip-vortex cavity. However, tip-vortex cavitation is known to be a cause of broadband pressure fluctuations on the hull above the propeller, typically in a frequency range from 30 to 100 Hz, which has led to vibration problems for some ships. Broadband noise of cavitation is also of relevance for the underwater radiated noise emitted by the ship, not only from the perspective of the acoustic signature of military ships, but also because of the impact of noise on fish and marine mammals.
The objective of the present PhD thesis was to develop prediction methods for broadband noise, including hull-pressure fluctuations, by developed propeller tip-vortex cavitation. Fundamental aspects of a cavitating vortex were investigated for this purpose, aiming at understanding the mechanisms involved in the generation of broadband noise and the effect of viscosity. This has been achieved by theoretical and computational studies of the kinematics, dynamics, and acoustics of vortex cavitation and by the analysis of experimental data. An analytical as well as a semi-empirical formulation for the azimuthal velocity distribution of a cavitating vortex has been developed. The semi-empirical formulation describes available experimental data well. A dispersion relation has been derived that describes disturbances on the cavity interface of a columnar vortex cavity of infinite length. The relation is derived for potential flow but contains an ad-hoc correction for viscous effects. The wavenumber-frequency diagram of this relation shows acceptable agreement with available experimental data.
The knowledge and formulations obtained were used for the development of a semi-empirical prediction method for the broadband noise by tip vortex cavitation of a propeller operating in a ship wake field. The formulations were also used to develop a novel methodology for model tests to correct for the Reynolds-number scale effect on the radius of the vortex-cavity and its broadband noise.
The objective of the present PhD thesis was to develop prediction methods for broadband noise, including hull-pressure fluctuations, by developed propeller tip-vortex cavitation. Fundamental aspects of a cavitating vortex were investigated for this purpose, aiming at understanding the mechanisms involved in the generation of broadband noise and the effect of viscosity. This has been achieved by theoretical and computational studies of the kinematics, dynamics, and acoustics of vortex cavitation and by the analysis of experimental data. An analytical as well as a semi-empirical formulation for the azimuthal velocity distribution of a cavitating vortex has been developed. The semi-empirical formulation describes available experimental data well. A dispersion relation has been derived that describes disturbances on the cavity interface of a columnar vortex cavity of infinite length. The relation is derived for potential flow but contains an ad-hoc correction for viscous effects. The wavenumber-frequency diagram of this relation shows acceptable agreement with available experimental data.
The knowledge and formulations obtained were used for the development of a semi-empirical prediction method for the broadband noise by tip vortex cavitation of a propeller operating in a ship wake field. The formulations were also used to develop a novel methodology for model tests to correct for the Reynolds-number scale effect on the radius of the vortex-cavity and its broadband noise.
| Original language | English |
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| Qualification | Doctor of Philosophy |
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| Award date | 21 Sept 2018 |
| Place of Publication | Enschede |
| Publisher | |
| Print ISBNs | 978-94-92679-52-9 |
| DOIs | |
| Publication status | Published - 21 Sept 2018 |