Cavitation volume behaviour derived from full-scale pressure fluctuations

Abstract

This study uses measurements of pressure fluctuations of a full-scale ship for the calculation of the cavitation volume based on an acoustical model. The sound pressure signal had been recorded by pressure sensors mounted above the propeller of a 3600 TEU container vessel, combined with a synchronous tachometer signal from the propeller shaft. An acoustic monopole is used to model the unsteady sheet cavitation as a sound source. The analysis comprises intermediate high-pass filtering and integration followed by post-processing the resulting volume time signal. The cavitation volume evolution produced by consecutive blade passages shows well-recognisable steady growth and rapid collapse phases. By means of shape-dependend and time-synchronous sequencing and averaging methods, a representative volume signal of one blade passage has been deduced and several influential parameters have been identified. This allows modelling and manipulating the volume evolution and thus enables further investigations of the associated noise generation by propellers.

On the Influence of Cavitation Volume Variations on Propeller Broadband Noise

“…The present study follows the approach to calculate the cavitation volume from a measured sound pressure signal, as first proposed by [7] and recently applied by [13]. This approach is based on the monopole characteristic of sheet cavitation noise as described by [5,14] and confirmed in recent experimental and numerical studies [13,15,16].…”

“…The present study follows the approach to calculate the cavitation volume from a measured sound pressure signal, as first proposed by [7] and recently applied by [13]. This approach is based on the monopole characteristic of sheet cavitation noise as described by [5,14] and confirmed in recent experimental and numerical studies [13,15,16]. Noise measurements (both on-board and experimental) are commonly analysed in the frequency domain, lacking connections to the physical processes, while many studies focusing on the physics of cavitation are performed without noise measurements.…”

“…Analysing the cavitation volume behaviour and its resultant noise in the time domain provides the opportunity to study the physical processes involved in the noise generation as recommended in [17]. The core of the novel strategy was introduced by [13] and continues in the present study; it involves the analysis of the cavitation volume and sound pressure signals of propeller blade passages from a full-scale propeller. The previous work of [13] focussed on the development of methods for signal integration and sequencing based on onboard measurement data from a 3600 TEU container vessel provided by [8].…”

“…The core of the novel strategy was introduced by [13] and continues in the present study; it involves the analysis of the cavitation volume and sound pressure signals of propeller blade passages from a full-scale propeller. The previous work of [13] focussed on the development of methods for signal integration and sequencing based on onboard measurement data from a 3600 TEU container vessel provided by [8]. It culminated in the extraction of representative signals of volume and sound pressure for a propeller blade passage and the identification of two variation causes.…”

“…It identifies characteristic shape parameters of the cavitation volume evolution that contribute to the high-pressure peaks on the one hand and create the above-mentioned broadband hump at low frequencies on the other. These shape parameters are set up in modelling the mean signals from [13] with a fitted high-degree polynomial function, tested systematically for their effect on the resultant noise spectrum, and quantified by evaluating the blade passage sequences from the onboard measurements. The application of their mean and standard deviation values within a Monte Carlo simulation of artificially varied noise produces a spectrum similar to the original measurement.…”