Christian Audoly

A condensed transfer function method as a tool for solving vibroacoustic problems

Substructuring approaches are nowadays widely used to predict numerically the vibroacoustic behavior of complex mechanical systems. Some of these methods are based on admittance or mobility frequency transfer functions at the coupling interfaces. They have already been used intensively to couple subsystems linked by point contacts and enable to solve problems at higher frequency while saving computation costs. In the case of subsystems coupled along lines, a Condensed Transfer Function method is developed in the present paper. The admittances on the coupling line are condensed in order to reduce the number of coupling forces evaluated. Three variants are presented, where the transfer functions are condensed using three different functions. After describing the principle of the CTF method, simple structures will be given as test cases for validation.

Discussion About Different Methods for Introducing the Turbulent Boundary Layer Excitation in Vibroacoustic Models

For controlling the noise radiated from vibrating structures excited by turbulent boundary layer (TBL) it is relevant to develop numerical tools for understanding how the structure reacts to TBL excitation. Usually, the wall pressure fluctuations of the TBL are described through statistical quantities (i.e. space-frequency or wavenumber-frequency spectra) which depend on the TBL parameters. On the other hand, the vibro-acoustic models (i.e. Finite Elements, Boundary Elements, Transfer Matrix Methods, Analytical models, etc.) evaluate deterministic transfer functions which characterise the response of the considered structures. The first part of this paper focuses on the coupling between the stochastic TBL and the deterministic vibro-acoustic models. Five techniques are presented. Numerical applications on an academic marine test case are proposed in order to discuss the calculation parameters and the interests/drawbacks of each technique. In the second part of the paper, the high frequency modelling with the Statistical Energy Analysis (SEA) method is considered. The focus is placed on the estimation of an important input of this method: the injected power by the TBL into the structure for each third octave band.

Mitigation of Underwater Radiated Noise Related to Shipping and Its Impact on Marine Life: A Practical Approach Developed in the Scope of AQUO Project

There is increasing concern within the scientific community about the underwater noise due to anthropogenic activity and its impact on marine life, with negative consequences on biodiversity and sea resources. In that context, the European Marine Strategy Framework Directive stated in 2008 that the anthropogenic noise due to shipping was to be mitigated. To address this issue, the European Union (EU) project “Achieve QUieter Oceans by shipping noise footprint reduction” (AQUO) (www.aquo.eu) started in October 2012 with a duration of four years. The project brought together experts from shipbuilding, underwater acoustics, and bioacoustics, with a multidisciplinary approach. In this paper, after giving a brief overview of the project structure, the methodology proposed by the AQUO project to set guidelines for controlling the underwater noise from commercial shipping is presented in more detail. Such a methodology is aimed at identifying the most promising strategies for the mitigation of the impact of shipping noise on marine fauna. Different technical as well as operational solutions are evaluated by taking into account the impact on marine life, the feasibility in terms of ship design, and the cost effectiveness, also considering fuel efficiency. While technical solutions are usually more effective at the design stage both in terms of costs and performance, operational solutions can potentially be adopted without any modification to the existing fleet. Furthermore, operational prescriptions can be set by national/local authorities who cannot directly intervene on ship configurations. The different solutions have been evaluated by means of numerical modeling carried out by using a Noise Footprint Assessment Model derived from the Quonops tool.

Influence of stiffeners and internal structures on the vibroacoustic behavior of submerged cylindrical shells under random excitations

The vibroacoustic behavior of structures excited by random pressure fields such as turbulent boundary layers or diffuse sound fields is of great interest for industrial applications. Many works have been carried out for periodically stiffened plates. In particular, the influence of Bloch-Floquet waves on the panel radiation has been highlighted. However, few studies have investigated more complex structures under random excitations. The present work studies the influence of internal structures on the vibro-acoustic behavior of submerged cylindrical shells. The geometric complexity is successively increased by including periodic, non-periodic stiffeners and various internal frames. The numerical prediction is based on the combination of two methods developed by the authors. The first one is the wavenumber-point (k,M) reciprocity technique. This method estimates the response of the system at point M from the shell velocity in the wavenumber space under a point excitation at M. The velocity field is estimate...

A substructuring approach for modeling the acoustic scattering from stiffened submerged shells coupled to non-axisymmetric internal structures

The scattered pressure from a stiffened axisymmetric submerged shell impinged by acoustic plane waves has been investigated experimentally, analytically and through numerical models. In the case where the shell is periodically stiffened, it is shown that helical, Bragg, and Bloch-Floquet waves can propagate. The influence of non-axisymmetric internal frames on these scattering phenomena is nevertheless not well known, as it can considerably increase the computational cost. To overcome this issue, the condensed transfer function (CTF) method, which has been developed to couple subsystems along linear junctions in the case of a mechanical excitation, is extended to acoustical excitations. It consists in approximating transfer functions on the junctions and deducing the behavior of the coupled system using the superposition principle and the continuity equations at the junctions. In particular, the CTF method can be used to couple a dedicated model of an axisymmetric stiffened submerged shell with non-axisymmetric internal structures modeled by the finite element method. Incident plane waves are introduced in the formulation and far-field reradiated pressure is estimated. An application consisting of a stiffened shell with curved plates connecting the ribs is considered. Supplementary Bloch-Floquet trajectories are observed in the frequency-angle spectrum and are explained using a simplified interference model.

Noise footprint: A proposal within the framework of FP7 AQUO project to define a goal based approach towards the reduction of underwater radiated noise from shipping

The Marine Strategy Framework Directive has officially stated as soon as 2008 the anthropogenic noise due to shipping were to be mitigated. The policy makers, the yards and the ship owners still strongly rely on the expert studies and guidelines to find the appropriate methodology to assess and then mitigate the acoustic pollution impact shipping on the marine biota. To address this issue, the project AQUO

Simulations of the influence of sound speed profile and sensor configuration in the measurement of radiated noise from ships in deep and shallow waters

The ISO TC43/ SC3 standardization committee for underwater acoustics was launched a few years ago, one of the priorities being the availability of internationally agreed procedures to measure radiated noise from ships. A first step was achieved by the publication of a first standard applicable for deep waters. However, in order to study the impact of shipping noise on marine fauna in wide maritime areas, it is necessary to input sound source levels in the form of equivalent monopoles, instead of a “radiated noise level,” which is affected by the interaction with sea surface and sea floor. Therefore, the committee is currently working on correction terms to remove these effects. In that context, the objective of the present study is to determine the influence of sound speed profile and sensor configuration on the sound source level estimation, using numerical simulator. The first case study is the correction term in deep waters: here, we look at the influence of sound speed profile, whereas in previous stu...

Forward acoustic scattering analysis from solid objects immersed in the water

Forward acoustic scattering of an immersed solid LINE (cylinder bounded by hemispherical endcaps) in water is investigated in our study. The object is made of stainless steel and its L/2a ratio is equal to 2 (L: Length of the cylinder part and a: the radius is equal to 60 mm). An impulse measurement method is used in the experimentation. Most of results are obtained experimentally, in bistatic configuration. Mobile receiver transducer is located in a distinct position from the emitter. The polar diagram patterns of the scattered pressure shows an important amplitude of pressure in the shadow side of this object. Analysis of this phenomenon is based on theoretical and experimental results obtained for a sphere with the help of elasticity theory. Moreover, this study relies on the grey-level representation of the angular position of the receiver in function of recorded time signals. Thus on forward acoustic time signals, it is possible to identify echoes due to propagation paths of waves on this object.

Turbulent Flow-Induced Self Noise and Radiated Noise in Naval Systems—An Industry Point of View

In naval warfare strategy, different scenarios are considered according to vessels involved and most of the time their acoustic performances determine the advantage of one over the other. To succeed, the vessel needs to be the most silent while being the most efficient in detecting the others. That is why naval reducing both the ship far field radiated noise and the self noise affecting sonar array efficiency is a permanent matter of concern for the naval industry. Several phenomena have to be considered to describe far field radiated noise and self noise. This study focuses on contributions generated by flow noise phenomena induced by Turbulent Boundary Layer (TBL) along the hull. In order to fulfil contractual commitments and design requirements, naval industry has to know how to model these phenomena to predict acoustic performances as accurately as possible. Nowadays, several models enable to describe TBL excitation from which it is possible to calculate vibro-acoustic response by different ways according to the phenomenon and the frequency range considered. In this study, flow noise phenomena induced by TBL in the hydro-acoustic studies of naval vessels and Chase model [3] are first described in general. Then, different modelling techniques of noise prediction are especially introduced for TBL excitation generating self noise on the sonar array located inside a cavity [1, 7] and far field radiated noise through external structure response. To illustrate the problems the naval industry has to deal with, some practical examples are discussed.

Propagation number in periodic structures considering losses

The development of absorbing or reflecting panels is of interest for underwater acoustic stealth. Most of the time, losses are not considered in the structures studied, specifically in the propagation number determination, despite the fact that most materials used, such as polymer matrices, have non negligible viscous losses. So, for a better understanding of the acoustic properties of these structures and to allow their optimization, simulations should consider the material properties, including losses. In order to obtain more realistic results from simulation, two numerical tools based on the finite element method (FEM) are proposed, with the help of the ATILA software. One is based on a differential method, the other on the transfer matrix. The two methods are first validated in the lossless case, then when losses are taken into account. Both methods give results in good agreement and give the propagation number where losses are taken into account.