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C. AUDOLY - RESUME de PUBLICATIONS PRINCIPALES C. AUDOLY – ABSTRACTS of MAIN PUBLICATIONS Diffraction acoustique par des écrans élastiques minces C. AUDOLY, Electromagnetic and acoustic scattering: detection and inverse problem. 31 Mai-3 Juin 1988, Marseille Luminy, p.69-81, World Scientific. The calculation of scattering of an acoustic wave by rigid screens can be done classicaly using integral equation methods. With a simplified modeling of the structure, the calculation is extended here in the case of screens made with one thin layer of elastic or viscoelastic material. Theoretical and experimental resuits on steel and soft material plane panels placed in water show the importance of edge effects and diffraction phenomena. These effects must be taken into account when the purpose is to characterize materials using standard panel measurements in near field. SPAL, a facility to characterize the machinery noise sensitivity of flank arrays C. AUDOLY, E. LANGLOIS, J.-F. ROSSETTO. UDT 1994 Conference (Undersea Defence Technology), Wembley. Proceedings Nexus business communications, pp. 102-106. S.P.A.L. (“Structure Porteuse d’Antennes Laterales”) is a facility designed to characterize the behavior of flank array elements versus machinery self noise, operated at Castillon lake. It consists of a 20 tons steel structure representative of a submarine hull, excited by internal dashpots and loudspeakers. The instrumentation has been improved recently, and includes in particular 64 accelerometers on a one meter square on the internal side of the hull. With this facility, it is possible to investigate the influence of the array technology, decoupling mountings and materials on the machinery noise on the hydrophones or after beamforming. Quantities such as average impedance along frequency, array noise gain for different steering directions, can be computed. Examples of results obtained on different test flank array panels are given. Introducing decoupling coatings in SEA interpretation and results on simple models models -Physical C. AUDOLY, NOISE-CON 97, Penn State University 15-17 June 1997, Proceedings pp. 21-26. To reduce the acoustic radiation from ship hulls excited by internal machinery, one solution consists in using external decoupling coatings. Some analytical models are available to predict the radiation of a structure completely covered by a decoupling coating. However, these models are limited to structures of simple shape (plates, circular cylinders). An adequate method to predict the vibro-acoustic response of a complex structure at “high” frequencies, like the structure of a ship, is the Statistical Energy Analysis. The same approach can then be used to predict the radiation of a structure partially or totally covered by a decoupling coating. However, some difficulties may arise to introduce some of the parameters required in the SEA model (acoustic efficiency of the coating, added mass of the plate...). After a presentation of the behaviour of the decoupling coatings used in underwater acoustics, the purpose of this paper is to look for physical interpretations of the introduction of the coating efficiency in a SEA model, and to put forward the effect of some modelling parameters. Reference is made to the basic SEA equations and comparison is made to an analytical model for an infinite plate. The method is also applied on bare and stiffened cylindrical shells, and the results compared with other predictive models. Prediction of the efficiency of a decoupling coating partially covering a cylindrical hull using the SEA method C. AUDOLY, Proc. Undersea Defence Technology Conference 1997, Hambourg, Publ. Nexus Information Technology pp. 293-297. To reduce the acoustic radiation from ship hulls excited by internal machinery, one solution consists in using external decoupling coatings. Because of cost and other constraints, it is often not possible to cover completely the hull with the coating. In this paper, the efficiency of a decoupling coating partially covering a cylindrical hull excited by a point force is predicted using the Statistical Energy Analysis approach. First, some results are compared with an analytical mode!, and the validity of the method checked. Then, the results of a parametric study, with coating configurations in strips and rings on a realistic stiffened shell are given. It is shown that the damping loss factor of the structure has a strong effect on the efficiency of a partial covering by comparison to the full coating. Prediction of turbulent flow noise inside an acoustic sounder cavity C. AUDOLY, S. BERETTI, Proc. Euromech Colloquium Vibro-Acoustics and Flow-Structure Interactions, Cargèse 19-23 April 1999, pp. 245-253 The detection performances of an underwater acoustic system are strongly dependent from the self noise level on the receiving array (for example the array of an echo sounder system placed in a cavity located in the keel), In most cases, turbulent flow noise along the hull is the dominant component in the self noise. Then, it is important to quantify it at the design stage. In this paper, three theoretical models are presented. The first one uses a decomposition of the excitation in the frequency/wavenumber domain, and assumes that the plate representing the acoustic window is infinite. The others use for the excitation a pressure fluctuation spectrum and correlation distances obtained from experiments. The vibro-acoustic response of the plate and of the fluid cavity uses a « statistical energy analysis» approach for the second model, and a finite element representation for the last one. First, the numerical models are compared to an experiment carried out in a water tunnel facility, The best results are obtained with the finite element model, but if three dimensional geometry is taken into account, large computational effort is required. The statistical energy analysis model gives fair results at medium/high frequencies, however with a slight over-estimate. The noise levels predicted by the infinite plate model are too low, indicating that this model is not adapted to the problem under consideration. A prediction of self noise in a cavity housing the array of an echo sounder system is then computed, and compared to ambient sea state noise levels. Feasibility of experimental determination of SEA damping loss factors of submarine structures C. AUDOLY, G. BORELLO, Internoise, Nice, 27-30 Août 2000. Statistical Energy Analysis is a useful technique to predict at the design stage average noise and vibration levels on ships or submarines, in particular self noise levels on sonar arrays due to machinery on-board noise or flow noise. In order to achieve accurate predictions, it is necessary to introduce realistic damping loss factors in the models. For that purpose, an experiment was conducted on a specific facility whose structure is representative of a piece of submarine hull. Damping loss factors and other parameters of portions of the hull and of some connected structures have been obtained along frequency up to several kHz, using excitation either with a shaker or a hammer, the second method being quicker to operate. The results have shown consistency of the data and confirm the feasibility of the technique, even for thick structures as those used on ships or submarines. Medium/high frequency vibro-acoustic problems in combat ship design C. AUDOLY, AutoSEA Users Conference, San Diego, 27-28 Juin 2000. Proceedings p. 86-91. The prediction of mediurn/high frequency noise and vibration levels is of primary interest in the design of combat ship. The main benefits appear during the feasibility or preliminary design stage, and after trials to define corrective actions if necessary. For a combat ship, three main problems are under consideration: - farfield underwater radiated noise, in order to avoid detection by adverse sonar systems; - noise in rooms and compartments, to ensure fair environmental conditions to the crew; - self noise, i.e. the disturbances of the ship itself on the sonar equipment it carries. After a brief description of these problems, some examples of the use of SEA Technology will be given. The main conclusions from the experience acquired at DCN in this field shows the benefit of this method, provided a sufficient knowledge of experimental data such as damping loss factors of realistic structures and of the excitation inputs (boundary layer pressure fluctuations, forces acting on the hull due to internal machinery...). Experimental determination of the damping loss factor of submarine structures C. AUDOLY, J.C. POULAIN, Undersea Defence Technology 26-30 Juin 2001, Hambourg. In order to predict noise and vibration response of structures submitted to different types of excitations (mainly related to internal machinery and to external turbulent flow), numerical techniques such as finite element analysis and statistical energy analysis are often used. However, the accuracy of the results depends on the quality of the input parameters, one of these being the damping loss factor of structures. This paper presents experimental results obtained with a method based on the statistical energy analysis, with a decomposition of a complex structure into several subsystems. The first step was to assess the feasibility of the technique for realistic submarine structures, using a special facility. The results have shown that the best practical method is to use the reciprocity principle and to excite the structures using a hammer. Then, the technique has been used to characterize real structures. Prediction of flow-induced sound and vibration : on different methods for introducing the TBL excitation in the vibroacoustic model. M. BERTON, L. MAXIT, D. JUVE, C. AUDOLY : ACOUSTICS 2013 NEW DELHI, New Delhi, India, November 10-15, 2013, p. 1281 The modeling of the vibroacoustic response of a heavy fluid loaded panel excited by a turbulent boundary layer is investigated. The pressure fluctuations due to the Turbulent Boundary Layer (TBL) at the surface of the structure are characterized by cross-spectra which can either be expressed in the physical space or in the wavenumber space. Various models have been proposed in the literature to obtain closed form expressions of these spectra starting from global TBL characteristics. A difficulty to estimate the vibroacoustic response of the panel excited by TBL relies on the coupling of the deterministic vibroacoustic model with the statistical wall pressure fluctuations model. In the present paper, we study different techniques to achieve this coupling: (a), in the physical space; (b), considering an uncorrelated wall plane waves field; (c),using a reciprocity technique; (d), using realizations of the stochastic uncorrelated plane waves field. These techniques require calculating different transfer functions with a vibroacoustic model. For that, we use the Patch Transfer Function formalism (PTF). This approach allows us to couple the structure problem with the heavy fluid problem. The added mass effect of the fluid on the panel (i.e. fluid reactance) is taken into account in the modal frequencies of the panel whereas the acoustic radiation from the panel into the fluid (i.e. fluid resistance) is modelled using the real part of the patch acoustic impedances of the fluid domain. For validation purpose, the results of these vibroacoustic calculations are compared with a result from the literature for a test case composed of a simply supported rectangular thin plate immersed in water. Afterwards, the response of the panel excited by the turbulent boundary layer is predictedin the low and medium frequency domains using the four techniques described above. The response is characterized here by the velocity spectrum at one point on the plate. Convergence of the four techniques is studied. The results with different values of calculation parameters are presented and analyzed in order to find the optimal ones. We observe that the four techniques give very similar results for these optimal parameters. Finally, we discuss the interest of each technique. Turbulent flow-induced self noise and radiated noise in naval systems - an industry point of view. F. CHEVALIER, C. AUDOLY : Flinovia (flow induced noise and vibration issues and aspects), 11-13 November 2013, Roma, Italy. 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 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, 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. Analyse du comportement vibro-acoustique d’une structure immergée excitée par une source transitoire R. SCHERRER, L. MAXIT, J.L. GUYADER, C. AUDOLY, T. LEISSING, C. GRANGER : Congrès Français d’Acoustique, Poitiers, 22-25 Avril 2014. De nos jours, les systèmes de détection acoustique utilisés dans la lutte sous-marine sont capables d’analyser les bruits transitoires. Il est donc nécessaire pour l’industriel de maitriser la discrétion acoustique dans ce domaine. Afin de définir une méthodologie qui permettra de spécifier des exigences sur les matériels sources de bruit transitoire, on étudie les vibrations et le rayonnement acoustique d’une structure immergée excitée par une source de bruit transitoire. Un modèle numérique est mis en place pour l’étude d’une structure simplifiée : plaque infinie excitée par une force impulsionnelle ponctuelle, en contact sur l’une des faces avec un milieu fluide infini. Le modèle permet de déterminer la réponse du système, en termes de vibrations et de pression rayonnée en champ proche et en champ lointain, à la fois dans les domaines fréquentiels et temporels. L’étude des vibrations met en évidence l’effet du fluide, la dispersion des ondes de plaque et les ondes d’interface fluide/solide. L’étude de la pression rayonnée met en évidence une directivité du rayonnement et la propagation des ondes dans la plaque avant d’être rayonnées. Une expérimentation sur une plaque en air et semi-immergée en cuve acoustique est effectuée afin de retrouver certains phénomènes mis en évidence par les simulations. Réponse vibro-acoustique d’une structure excitée par une Couche Limite Turbulente : Vers la prise en compte de l’évolution spatiale de la CLT M. BERTON, L. MAXIT, D. JUVE, C. AUDOLY : Congrès Français d’Acoustique, Poitiers, 22-25 Avril 2014. Cette étude s’inscrit dans le cadre de la modélisation du comportement vibro-acoustique d’une structure immergée dans un fluide lourd excitée par une couche limite turbulente (CLT). Plus particulièrement, on s’intéresse à la prise en compte des variations spatiales des paramètres d’excitation de la structure qui peuvent être induites par le développement de la CLT ou par un gradient de pression statique. Dans un premier temps, l’influence de ces variations sur les spectres des fluctuations de pression en paroi est présentée sur un exemple académique d’écoulement en eau sur un massif de sous-marin. On met en évidence des variations significatives de niveaux des spectres de pression en paroi de la structure vibrante. Ceci va à l’encontre du champ de pression généralement considéré dans les modèles vibro-acoustiques qui est supposé homogène spatialement. Dans le but de prendre en compte ces variations dans le calcul de la réponse vibro-acoustique, différentes méthodes pour introduire cette excitation variable spectralement sont développées et comparées. On met ainsi en évidence l’influence du gradient de pression dans l’écoulement sur la réponse vibratoire de la structure dans le cas d’un massif. Virtual time-reversal technique for localization of transient vibratory sources in complex immersed structures T. LEISSING, C. AUDOLY, J.L. GUYADER, G. GUYADER, Q. BUISSON, J.L. MORANGE,. Forum Acusticum, Krakow, 7-12 September 2014. Transient sonar detection systems have been developed over the last decades, and hence transient noise emissions from ships have become a matter of concern for acoustic discretion. A step towards the mitigation of transient noise emissions consists in the accurate localization and identification of the noise sources. We propose in this paper to apply the virtual time-reversal method to an immersed structure in order to localize vibratory transient noise sources. The principle of the virtual time-reversal technique consists in reemitting recorded signals at specific locations in a virtual structure, which should accurately describe the real structure in order to allow focalisation of reemitted signals at the source location. The virtual structure can be of different kinds: analytical model for simple structures, numerical models (e.g. finite element model) or experimental models. In previous studies, the method has been validated on a small structure in air. The test structure considered here is a section of a hull, which is stiffened, partly immersed, and equipped with some machinery items (pumps, engines, etc.). Due to its large dimensions and its complexity, it is decided to construct the virtual structure based on transfer function measurements, measured on an accelerometer grid along the hull. This transfer function database is then used to reemit reversed signal and to identify the transient sources locations. The method is applied to the localization of impact hammer excitations. The analysis of results allows drawing conclusions on the feasibility of the method on complex marine structures. A condensed transfer function method as a tool for solving vibroacoustic problems. V. MEYER, L. MAXIT, J.-L. GUYADER, T. LEISSING, C. AUDOLY, NOVEM 2015, Dubrovnik, 13-15 April 2015. Numerical methods are nowadays widely used to predict the vibroacoustic behavior of mechanical systems. Nevertheless, when the system is complex as for instance in the aerospace or naval industry, it is practically impossible to solve the problem at once, more particularly at medium or high frequencies. That is why substructuring approaches have been developed to tackle complex systems and save computation costs. Some of these methods consist in dividing the system in several subsystems that can be studied separately and to deduce the response of the whole system by calculating the coupling forces at the interfaces between the subsystems. It is based on admittance, impedance or mobility frequency transfer functions at the coupling interfaces. Such methods have already been used intensively to couple subsystems linked by point contacts. In the case of subsystems coupled along lines, a Condensed Transfer Function (CTF) 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 of the method are presented, where the transfer functions are condensed using 3 different ways: (1) averaged on segments, (2) projected on exponential functions along the line and (3) projected on Chebyshev polynomials. After describing the principle of the CTF method, the case of a plate will be given as a test case for validation, before applying it to an industrial system. In comparison with a classical Craig-Bampton method, which is used to reduce the size of finite element models, the CTF method has the advantage to be useable with several subsystems which vibrational behavior has not necessarily been described by the finite element method (FEM). For instance, it may be used to couple a submerged non-periodically stiffened shell described using the circumferential admittance approach (CAA), with internal substructures described by FEM that offers greater flexibility on their design. Way ahead to take into account non-axisymmetric internal substructures in an underwater structure will be given.