Archivio della ricerca di Triestehttps://arts.units.itIl sistema di repository digitale IRIS acquisisce, archivia, indicizza, conserva e rende accessibili prodotti digitali della ricerca.Tue, 20 Apr 2021 17:25:39 GMT2021-04-20T17:25:39Z1061Hydrodynamic noise from a propeller in open sea conditionhttp://hdl.handle.net/11368/2936905Titolo: Hydrodynamic noise from a propeller in open sea condition
Abstract: In the present work a hybrid methodology is used to evaluate the hydrodynamic noise generated by a marine propeller in open sea condition. The hydrodynamic field is computed using Large Eddy simulation under the assumption of incompressible flow field; the acoustic field is reconstructed by applying the advec-tive Ffowcs Williams and Hawkings equation. For the hydrodynamics, we use the dynamic Lagrangian model for the closure of the subgrid-scale stresses and a wall-layer model to skip the resolution of the viscous sub-layer. We consider a propeller well studied in literature for a single value of the advance ratio. A grid of about 6x106 cells is used for reproducing accurately both the stresses over the propeller and the wake, the latter responsible of quadrupole noise. The equations are solved in a fixed-to-the-body frame of reference. The different noise generation mechanisms are investigated separately. Thickness and loading terms related to the propeller shape and velocity, provide significant pressure disturbance in the near field. The quadrupole noise component is obtained by integrating over an external permeable surface. Its contribution is investigated in relation to the presence of vortex persisting in the wake
Mon, 01 Jan 2018 00:00:00 GMThttp://hdl.handle.net/11368/29369052018-01-01T00:00:00ZNumerical prediction of the far field noise generated by a ship propellerhttp://hdl.handle.net/11368/2978891Titolo: Numerical prediction of the far field noise generated by a ship propeller
Abstract: Our research focuses on assessment and use of mathematical and numerical models for the characterization of the noise source, when the noise is generated by a fluid in motion around a body. In particular, within the framework of the acoustic analogy, we first evaluate the fluid dynamic field using Large-Eddy Simulation (LES) and then we apply the Ffowcs-Williams and Hawkings (FWH) equation to reconstruct the acoustic field. First we show the conditions under which the non-linear terms of the FWH equation can be computed by direct integration neglecting the compressible delay in the volume integral terms. Successively we evaluate the noise generated by simple, still significant, geometries, and finally we consider noise propagation from a ship propeller in uniform flow. For the latter we observe the tonal signal given by the blade rotational frequency together with a high amplitude broadband noise given by the wake.
Tue, 01 Jan 2019 00:00:00 GMThttp://hdl.handle.net/11368/29788912019-01-01T00:00:00ZHydroacoustic noise from different geometrieshttp://hdl.handle.net/11368/2917576Titolo: Hydroacoustic noise from different geometries
Abstract: Turbulent flow around bluff bodies generates pressure fluctuations which propagate as acoustic waves.
Differences in the shape of a body can affect frequencies and amplitudes of the propagating pressure signals. In
the present work three elementary geometries (sphere, cube and prolate spheroid), immersed in a uniform water
flow, are examined in order to analyze the differences of the resulting hydroacoustic fields. The turbulent flow at
ReA = 4430 (based on the cross-sectional area of the bodies) is reproduced through wall-resolving Large-Eddy
Simulation and the hydroacoustic far-field is analyzed by adopting the Ffowcs Williams and Hawkings analogy.
The quadrupole term of the acoustic equation is first reformulated in the convective form and then solved
through direct computation of the volume integrals. This procedure is found possible in hydrodynamics where
the speed of sound is very large and the flow velocities are small. In spite of the fact that the frontal section of the
bodies has the same area, the analysis shows that a streamlined body is able to produce a pressure signal one
order of magnitude lower than that generated by a bluff geometry. The separate analysis of the loading noise and
of the quadrupole one has shown that the former is larger than the latter in case of 3D-shaped bluff body (sphere
and cube), whereas the opposite is true in case of a streamlined body. A preliminary analysis between the case of
an elongated square cylinder and a cube, shows that the persistence of a two-dimensionally shaped wake when
compared to a three-dimensional one contributes to increase the quadrupole part of the radiated noise.
Sun, 01 Jan 2017 00:00:00 GMThttp://hdl.handle.net/11368/29175762017-01-01T00:00:00ZAcoustic Analogies and Large-Eddy Simulations of Incompressible and Cavitating Flows Around Bluff Bodieshttp://hdl.handle.net/11368/2922550Titolo: Acoustic Analogies and Large-Eddy Simulations of Incompressible and Cavitating Flows Around Bluff Bodies
Fri, 23 Mar 2018 00:00:00 GMThttp://hdl.handle.net/11368/29225502018-03-23T00:00:00ZNon-linear noise from a ship propeller in open sea conditionhttp://hdl.handle.net/11368/2978899Titolo: Non-linear noise from a ship propeller in open sea condition
Abstract: In the present paper, we study the hydrodynamic noise generated by a ship propeller in open sea conditions. We use Large Eddy Simulation for the hydrodynamic field whereas the acoustic field is reconstructed by applying the advective form of the Ffowcs-Williams and Hawkings equation. A dynamic Lagrangian model is adopted for the closure of the subgrid-scale stresses and a wall-layer model allows to skip the resolution of the viscous sub-layer. The acoustic equation is formulated in the integral form and solved through direct integration of the volume terms. The propeller herein considered is a benchmark case, whose fluid dynamic data are available in the literature. A grid of about 3 ×106 cells is able to reproduce accurately both integral quantities like thrust and torque over the propeller, and turbulence propagating downstream in the wake. Different noise generation mechanisms are investigated separately. The linear terms give rise to a narrow-band noise spectrum, with a mean peak at the blade frequency and other peaks at frequencies multiple of the rotational one. The non-linear quadrupole term reveals a broad band noise spectrum; the shaft vortex provides the largest contribution to the non-linear part of the noise propagated in the far field.
Tue, 01 Jan 2019 00:00:00 GMThttp://hdl.handle.net/11368/29788992019-01-01T00:00:00ZAssessment of methodologies for the solution of the Ffowcs Williams and Hawkings equation using LES of incompressible single-phase flow around a finite-size square cylinderhttp://hdl.handle.net/11368/2955934Titolo: Assessment of methodologies for the solution of the Ffowcs Williams and Hawkings equation using LES of incompressible single-phase flow around a finite-size square cylinder
Abstract: The acoustic analogy represents a powerful tool for the prediction of noise generated by the
interaction between the flow and a moving body. It is based on decoupling the acoustic problem
from the fluid dynamic one: the velocity and pressure fields, obtained through a separate numerical
simulation, are used as source terms in an inhomogeneous wave equation whose solution reconstructs
the noise in the far field. When the method is based on the fundamental Ffowcs Williams and
Hawkings (FW-H) equation, different solving methodologies may be adopted.
The present work considers the original FW-H equation and gives the advective formulation of
the volume integral terms. The results are compared with those obtained with the Curle and porous
formulations.
To account for volume integrals, the assumption of compact noise source is needed. This assumption
is common in literature, however, in the present work, a dimensional analysis is proposed, in
order to indicate in a rigorous way the cases in which the compressibility delays can be avoided.
The dimensional analysis is tested in the case of an acoustic monopole field. Successively, the FW-H
porous formulation is compared with the original FW-H equation in the case of an irrotational advected
vortex. This example puts in evidence the different response of the two methods in the case
of a vortex crossing the acoustic domain.
Then, different solution strategies of the FW-H are evaluated using a fluid dynamic dataset
obtained through large eddy simulation of a turbulent flow around a finite-length cylinder with square
section. The analysis allows to point out the strengths and drawback of the different techniques and
to achieve, through the comparison of the different solutions, an accurate understanding of the noise
source mechanisms taking place in the flow. Finally, a mixed procedure, merging the advantages of
the porous formulation with the direct evaluation of the volume integral terms is proposed. It may
be used in presence of significant time delays. Overall, the present study is oriented to the analysis
of very low Mach number flows, although the complete porous method might be applicable in a more
general framework. This aspect will require additional research in the future.
Tue, 01 Jan 2019 00:00:00 GMThttp://hdl.handle.net/11368/29559342019-01-01T00:00:00Z