A Semi-Empirical Prediction Method for Broadband Hull-Pressure Fluctuations and Underwater Radiated Noise by Propeller Tip Vortex Cavitation †

Johan Bosschers

doi:10.3390/jmse6020049

Abstract

A semi-empirical method is presented that predicts broadband hull-pressure fluctuations and underwater radiated noise due to propeller tip vortex cavitation. The method uses a hump-shaped pattern for the spectrum and predicts the centre frequency and level of this hump. The principal parameter is the vortex cavity size, which is predicted by a combination of a boundary element method and a semi-empirical vortex model. It is shown that such a model is capable of representing the variation of cavity size with cavitation number well. Using a database of model- and full-scale measured hull-pressure data, an empirical formulation for the maximum level and centre frequency has been developed that is a function of, among other parameters, the cavity size. Acceptable results are obtained when comparing predicted and measured hull-pressure and radiated noise spectra for various cases. The comparison also shows differences that require adjustments of parameters that need to be further investigated.

Highlights

Noise and vibration on board ships are important design considerations for the comfort of crew and passengers
The reduction of cavitation-induced noise and vibration is usually achieved by unloading the propeller tip and decreasing sheet cavitation as much as possible, often leaving only tip vortex cavitation on the propeller
The relation between cavitation number and cavity size for a vortex cavity can be obtained from the radial distribution of the azimuthal velocity vθ

Summary

Introduction

Noise and vibration on board ships are important design considerations for the comfort of crew and passengers. The reduction of cavitation-induced noise and vibration is usually achieved by unloading the propeller tip and decreasing sheet cavitation as much as possible, often leaving only tip vortex cavitation on the propeller This type of cavitation generates low-frequency broadband hull-pressure fluctuations that may lead to ship vibration issues [1,2]. The spectrum of the broadband hull-pressure fluctuations is characterized by a hump, the maximum level of which increases with ship speed, while the centre of the hump simultaneously moves to lower frequencies This centre frequency is typically located between 30 Hz and 200 Hz. A possible explanation of the hump in the spectrum is that it is caused by a resonance frequency of the cavitating vortex [3], experimental evidence of this is missing. Tlehviesl pofrothceebdruoardebiasnpdrheusmenpt.eTdhiisnprSoeccetdiuorne i2s.p3r.eAsenftleodwinchart of the method isSeschtioown 2n.3i.nAFfliogwurceha1rt. oTf htheesmpeetchtordalissshhaopwen oinf Ftihgeurseo1u. rTchee lsepvecetlraslpsehcaptreuomf thaensdoucrocenlveevresl ion to hull pressure ssppeeccttrruummaannddcoUnvReNrsiosnpetocthruulml pirsespsureresesnpteectdruimn SanecdtiUoRnN3.spCeoctmrupmarisispornesoenftpedreidn iScetcetdionan3d. measured spectra is CaproremedspiesacnruitsseosdendionifnpSSreeeccdttiiicootnend54.a.nTd hmeearseusrueldtsspoefcttrhaeismperetsheondtedarine Sdeicstciouns4s.eTdhienreSseuclttsioonf t5h.e method

Vortex Models

Prediction of Cavity Size

Findings

Conclusions