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Grizzi S.,Insean Italian Ship Model Basin | Camussi R.,Third University of Rome
Journal of Fluid Mechanics | Year: 2012

An experimental study of the pressure field generated by a subsonic, single stream, round jet is presented. The investigation is conducted in the near-field region at subsonic Mach numbers (up to 0.9) and Reynolds numbers Re\gt 10 5. The main task of the present work is the analysis of the near-field acoustic pressure and the characterization of its spectral properties. To this aim, a novel post-processing technique based on the application of wavelet transforms is presented. The method accomplishes the separation of nearly Gaussian background fluctuations, interpreted as acoustic pressure, from intermittent pressure peaks induced by the hydrodynamic components. With respect to more standard approaches based on Fourier filtering, the new technique permits one to recover the whole frequency content of both the acoustic and the hydrodynamic contributions and to reconstruct them as independent signals in the time domain. The near-field acoustic pressure is characterized in terms of spectral content, sound pressure level and directivity. The effects of both the Mach number and the distance from the jet axis are analysed and the results are compared with published far-field observations and theoretical predictions. Simultaneous velocity/pressure measurements have been also performed using a hot-wire probe and a microphone pair in the near field. It is shown that the cross-correlation between the near-field acoustic pressure and the axial velocity is large (of the order of 0.2) in the potential core region whereas large velocity/hydrodynamic pressure correlations are located at the nozzle exit and downstream of the potential core. © 2012 Cambridge University Press.

Antuono M.,Insean Italian Ship Model Basin
Journal of Fluid Mechanics | Year: 2010

A global shock solution for the nonlinear shallow water equations (NSWEs) is found by assigning proper seaward boundary data that preserve a constant incoming Riemann invariant during the shock wave evolution. The correct shock relations, entropy conditions and asymptotic behaviour near the shoreline are provided along with an in-depth analysis of the main quantities along and behind the bore. The theoretical analysis is then applied to the specific case in which the water at the front of the shock wave is still. A comparison with the Shen & Meyer (J. Fluid Mech., vol. 16, 1963, p. 113) solution reveals that such a solution can be regarded as a specific case of the more general solution proposed here. The results obtained can be regarded as a useful benchmark for numerical solvers based on the NSWEs. © 2010 Cambridge University Press.

Gennaretti M.,Third University of Rome | Testa C.,Insean Italian Ship Model Basin | Bernardini G.,Third University of Rome
Journal of Sound and Vibration | Year: 2012

A novel frequency-domain formulation for the prediction of the tonal noise emitted by rotors in arbitrary steady motion is presented. It is derived from Farassats 'Formulation 1A', that is a time-domain boundary integral representation for the solution of the Ffowcs-Williams and Hawkings equation, and represents noise as harmonic response to body kinematics and aerodynamic loads via frequency-response-function matrices. The proposed frequency-domain solver is applicable to rotor configurations for which sound pressure levels of discrete tones are much higher than those of broadband noise. The numerical investigation concerns the analysis of noise produced by an advancing helicopter rotor in bladevortex interaction conditions, as well as the examination of pressure disturbances radiated by the interaction of a marine propeller with a non-uniform inflow. © 2012 Elsevier Ltd.

Antuono M.,Insean Italian Ship Model Basin | Brocchini M.,Marche Polytechnic University
Physics of Fluids | Year: 2013

A novel approach for the description of both wave propagation and flow circulation in the nearshore zone has been defined. This is based on an integro-differential system which, at the leading-order, coincides with classical depth-averaged models (e.g., Boussinesq-type models) and, in addition, describes flow deviations from the depth-averaged values. Thanks to this feature, the proposed system enables exact calculation of the linear dispersion relation, of the linear shoaling coefficient, and of second-order nonlinear solutions for monochromatic waves. A simplified version of the original system has also been proposed. This latter model is exact up to the first order and predicts a linear shoaling coefficient which is comparable with the most advanced, fully nonlinear Boussinesq-type models. The general approach, which can be exploited to obtain a family of models, has clear computational advantages over those which solve the flow over the vertical and improves the flow description accuracy of typical depth-averaged models. © 2013 American Institute of Physics.

Iafrati A.,Insean Italian Ship Model Basin | Babanin A.,Swinburne University of Technology | Onorato M.,University of Turin | Onorato M.,National Institute of Nuclear Physics, Italy
Physical Review Letters | Year: 2013

We use direct numerical simulation of the Navier-Stokes equations for a two-phase flow (water and air) to study the dynamics of the modulational instability of free surface waves and its contribution to the interaction between the ocean and atmosphere. If the steepness of the initial wave exceeds a threshold value, we observe wave-breaking events and the formation of large-scale dipole structures in the air. Because of the multiple steepening and breaking of the waves under unstable wave packets, a train of dipoles is released in the atmosphere; those dipoles propagate at a height comparable with the wavelength. The amount of energy dissipated by the breaker in water and air is considered, and contrary to expectations, we observe that the energy dissipation in air is greater than that in water. The possible consequences on the wave modeling and on the exchange of aerosols and gases between air and water are discussed. © 2013 American Physical Society.

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