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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.

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.

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 | Colagrossi A.,Insean Italian Ship Model Basin | Colagrossi A.,Norwegian University of Science and Technology | Marrone S.,Insean Italian Ship Model Basin
Computer Physics Communications | Year: 2012

A discussion on the use of numerical diffusive terms in SPH models is proposed. Such terms are, generally, added in the continuity equation, in order to reduce the spurious numerical noise that affects the density and pressure fields in weakly-compressible SPH schemes. Specific focus has been given to the theoretical analysis of the diffusive term structure, highlighting the main benefits and drawbacks of the most widespread formulations. Finally, specific test cases have been used to compare such formulations and to confirm the theoretical findings. © 2012 Elsevier B.V. All rights reserved.

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

The boundary value problem for the nonlinear shallow-water equations with a beach source term is solved by direct use of physical variables, so that solutions are more easily inspected than those obtained by means of hodograph transformations. Beyond an overall description of the near-shoreline flows in terms of the nonlinear shallow-water equations, significant results are provided by means of a perturbation approach which enables much of the information on the flow to be retained. For sample waves of interest (periodic and solitary), first-order solutions of the shoreline motion and of the near-shoreline flows are computed, illustrated and successfully compared with the equivalent ones obtained through a hodograph transformation method previously developed by the authors. Wavewave interaction, both at the seaward boundary and within the domain, is also accurately described. Analytical conditions for wave breaking within the domain are provided. These, compared with the authors' hodograph model, show that the first-order condition of the present model is comparable to the second-order condition of that model. © 2009 Cambridge University Press.

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.

Diez M.,Insean Italian Ship Model Basin | Peri D.,Insean Italian Ship Model Basin
Ocean Engineering | Year: 2010

The paper presents an approach for the robust optimization of a bulk carrier conceptual design, subject to uncertain operating and environmental conditions. The uncertainties involved in the optimization process are addressed and a general formulation for robust design is given. Specifically, the uncertainties involved in the decision making process are taken into account by means of their probabilistic distributions. The expected values and the standard deviations of the relevant quantities are assessed and included in the optimization objectives, whereas the constraints are evaluated in the worst case. This leads to a robust design able to keep a good performance in the whole probabilistic operating scenario. A particle swarm optimization algorithm is used for the global minimization process, minimizing the expectation and the standard deviation of the unit transportation cost. © 2010 Elsevier Ltd. All rights reserved.

Felli M.,Insean Italian Ship Model Basin | Falchi M.,Insean Italian Ship Model Basin
Experiments in Fluids | Year: 2011

In the present paper, the interaction mechanisms of the vortices shed by a single-screw propeller with a rudder installed in its wake are addressed; in particular, following the works by Felli et al. (Exp Fluids 6(1):1-11, 2006a, Exp Fluids 46(1):147-1641, 2009a, Proceedings of the 8th international symposium on particle image velocimetry: Piv09, Melbourne, 2009b), the attention is focused on the analysis of the evolution, instability, breakdown and recovering mechanisms of the propeller tip and hub vortices during the interaction with the rudder. To investigate these mechanisms in detail, a wide experimental activity consisting in time-resolved visualizations, velocity measurements by particle image velocimetry (PIV) and laser Doppler velocimetry (LDV) along horizontal chordwise, vertical chordwise and transversal sections of the wake have been performed in the Cavitation Tunnel of the Italian Navy. Collected data allows to investigate the major flow features that distinguish the flow field around a rudder operating in the wake of a propeller, as, for example, the spiral breakdown of the vortex filaments, the rejoining mechanism of the tip vortices behind the rudder and the mechanisms governing the different spanwise misalignment of the vortex filaments in the pressure and suction sides of the appendage. © 2011 Springer-Verlag.

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.

Ianniello S.,Insean Italian Ship Model Basin | Di Mascio A.,Insean Italian Ship Model Basin
Journal of Computational Physics | Year: 2010

A new method for tracking evolving interfaces by lagrangian particles in conjunction with a Level-Set approach is introduced. This numerical technique is based on the use of time evolution equations for fundamental vector and tensor quantities defined on the front and represents a new and convenient way to couple the advantages of the Eulerian description given by a Level-Set function φ{symbol} to the use of Lagrangian massless particles. The term oriented points out that the information advected by the particles not only concern the spatial location, but also the local (outward) normal vector n to the interface Γ and the second fundamental tensor (the shape operator) ∇ n. The particles are exactly located upon Γ and provide all the requested information for tracking the interface on their own. In addition, a self-adaptive mechanism suitably modifies, at each time step, the markers distribution in the numerical domain: each particle behaves both as a potential seeder of new markers on Γ (so as to guarantee an accurate reconstruction of the interface) and a de-seeder (to avoid any useless gathering of markers and to limit the computational effort). The algorithm is conceived to avoid any transport equation for φ{symbol} and to confine the Level-Set function to the role of a mere post-processing tool; thus, all the numerical diffusion problems usually affecting the Level-Set methodology are removed. The method has been tested both on 2D and 3D configurations; it carries out a fast reconstruction of the interface and its accuracy is only limited by the spatial resolution of the mesh. © 2009 Elsevier Inc. All rights reserved.

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