"-"An experimental study of the pressure field generated by a subsonic, single stream,\ud round jet is presented. The investigation is conducted in the near-field region at\ud subsonic Mach numbers (up to 0.9) and Reynolds numbers Re > 105. The main\ud task of the present work is the analysis of the near-field acoustic pressure and\ud the characterization of its spectral properties. To this aim, a novel post-processing\ud technique based on the application of wavelet transforms is presented. The method\ud accomplishes the separation of nearly Gaussian background fluctuations, interpreted\ud as acoustic pressure, from intermittent pressure peaks induced by the hydrodynamic\ud components. With respect to more standard approaches based on Fourier filtering,\ud the new technique permits one to recover the whole frequency content of both the\ud acoustic and the hydrodynamic contributions and to reconstruct them as independent\ud signals in the time domain. The near-field acoustic pressure is characterized in\ud terms of spectral content, sound pressure level and directivity. The effects of\ud both the Mach number and the distance from the jet axis are analysed and the\ud results are compared with published far-field observations and theoretical predictions.\ud Simultaneous velocity/pressure measurements have been also performed using a hotwire\ud probe and a microphone pair in the near field. It is shown that the crosscorrelation\ud between the near-field acoustic pressure and the axial velocity is large (of\ud the order of 0.2) in the potential core region whereas large velocity/hydrodynamic\ud pressure correlations are located at the nozzle exit and downstream of the potential\ud core

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.

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