The resonance frequency of tip-vortex cavitation

Abstract

Over time much research has been performed on the subject of cavitation. A recent development in this field is the investigation into the behaviour of the cavity within a propeller's tip-vortex. It is found that this cavity can produce a very distinct broadband sound-spectrum which can cause trouble on-board ships either by the sound itself or due to the coincidence with the natural frequencies of a ships structure, causing resonance problems. Recent research by Bosschers and Pennings have resulted in an engineering approach to the calculation of a center frequency of this specific type of cavitation sound. To validate this engineering method, measurements were performed by Pennings[2016] on a 2-bladed propeller in the cavitation tunnel at the Delft University of Technology. The purpose of these measurements was finding the resonance frequency of the tip-vortex cavity. From early measurements it was found that the self-exciting behaviour of the cavity, as it was found by Maines & Arndt[1997], could not be reproduced. To get the cavity to generate sound, an artificial wake was introduced which resulted in the distinct broadband sound-spectrum associated with tip-vortex cavitation. To investigate whether the velocity gradient introduced by the wake is sufficiently steep to excite the natural frequencies of the vortex cavity, velocity measurements have been performed from which a steep dip in the flow velocity was found near and behind the wake generator. Also a highly fluctuating behaviour of the flow, with large deviations from the mean flow, was found. These large spreads are associated with the broadband sound spectrum that comes with tip-vortex cavitation. From this it is concluded that the wake generator, as it is used in the measurements by Pennings[2016], gives a good approximation of a real wake. To be able to use the engineering method as a tool to predict the center frequency, a method is needed to accurately calculate the cavity radius and the associated circumferential velocity without the need of full scale tests. When such a method is available the resonance frequency associated with tip-vortex cavitation for a specific propeller can be calculated in the design stage of the ship and/or propeller. With this information available an educated trade-off can be made between the alteration of the propeller and the construction of the ship in order to prevent coinciding natural frequencies. To obtain the necessary cavitation information a numerical simulation is performed of which the results show good agreement with measurement data. With this simulation method a relatively fast and accurate prediction can be made of the center frequency of tip-vortex cavitation with the use of the engineering method. However, to grow the confidence in the results produced, further research should be performed. The current numerical simulations have been performed for a very small propeller. It would be interesting to see whether the method, as it is described and tested in this thesis, also produces good results for a regular scale propeller. Next to that, because the cavity shows no chordwise region over which the parameters of interest are constant, a further investigation into the decay rate of the cavity is advised in order to increase confidence in the choice of location at which the cavitation parameters should be evaluated.