A novel approach for the isolation of the sound and pseudo-sound contributions from near-field pressure fluctuation measurements: analysis of the hydroacoustic and hydrodynamic perturbation in a propeller-rudder system

Abstract

The main scope of the present work is to investigate the mechanisms underlying the hydroacoustic and hydrodynamic perturbations in a rudder operating in the wake of a free running marine propeller. The study consisted of detailed near-field pressure fluctuation measurements which were acquired on the face and back surfaces of the rudder, at different deflection angles. To this aim, a novel wavelet-filtering procedure was applied to separate and analyze distinctly the acoustic and hydrodynamic components of the recorded near-field pressure signals. The filtering procedure undertakes the separation of intermittent pressure peaks induced by the passage of eddy structures, interpreted as pseudo-sound, from homogenous background fluctuations, interpreted as sound. The use of wavelet in the filtering procedure allows to overcome the limitations of the earlier attempts based on frequency (wave number) band-pass filtering, retrieving the overall frequency content of both the acoustic and the hydrodynamic components and returning them as independent signals in the time domain. Acoustic and hydrodynamic pressure distributions were decomposed harmonically and compared to the corresponding topologies of the vorticity field, derived from earlier LDV measurements performed by Felli and Falchi (Exp Fluids 51(5):1385-1402, 2011. The study highlighted that the acoustic perturbation is mainly correlated with the unsteady load variations of the rudder and to the shear layer fluctuations of the propeller streamtube. Conversely, the dynamics of the propeller tip and hub vortices underlies the hydrodynamic perturbation.

Experimental approaches for the diagnostics of hydroacoustic problems in naval propulsion

“…If these issues are not appropriately dealt with, such measurements can be easily misinterpreted. Recently, this problem has been successfully overcome by Felli et al (2014) which proposed a wavelet-based filtering procedure capable to return both the acoustic and the hydrodynamic components of the near field pressure fluctuation signals as independent signals in the time domain. Moreover, pressure fluctuation measurements can be adopted for the identification of the noise sources in cavitating flows where the application of the other variants, based on the use of optical techniques, is not appropriate.…”

“…Moving away from the noise source, the hydrodynamic contribution disappears and pressure field reduces only to sound (Howe, 1960). Recently, Felli et al (2014) presented and successfully applied an effective wavelet-based procedure to separate out the two contributions. This procedure allows to extend the choice of the in-flow quantity to the acoustic pressure distribution in proximity of the noise source.…”

“…The use of two pressure transducers to record the distribution of the pressure fluctuations in the nearfield was needed to apply the filtering technique mentioned in Section 3.3and, thus, to isolate the sound and pseudo-sound contributions. This topic is thoroughly addressed in Felli et al (2014).…”

“…Near-field pressure signals were filtered by the wavelet procedure documented in Felli et al (2014); the extracted acoustic and hydrodynamic contributions were then correlated to the far-field acoustic one as acquired by hydrophone H1 (see Fig. 10).…”

“…The perturbation induced by the passage of eddy structures involves the pressure signals to be pre-processed through a signal decomposition technique that is able to filter out the hydrodynamic contribution from the near-field pressure fluctuation signals (Felli et al, 2014). Usually, hydrophone measurements are performed in combination with detailed flow measurements in the source region by means of Hot Film, Laser Doppler Velocimetry (LDV) or Particle Image Velocimetry (PIV).…”