Geometrical theory of diffraction for sound radiation and structural response

International audienceThis study focuses on geometrical theory of diffraction (GTD) applied to vibroacoustics with a particular emphasis to fluid-structure interaction. Six types of hybrid rays are identified travelling either from structure-to-fluid (sound radiation) or from fluid-to-structure (structural response). Three rays correspond to sound radiation by the surface, edge, or corner of a structure while the three others are the reciprocal paths corresponding to sound-to-vibration conversion by the surface, edge, or corner. We present the calculation of geometrical properties of wavefronts (principal directions and curvatures) and their laws of transformation during an interaction process. Furthermore, some simple explicit relationships for diffraction coefficients are given under the light fluid assumption. Finally, two examples are discussed to illustrate the concepts. The first one is a pure radiation problem while the second one involves transmission through walls, structural response and sound radiation

Relaxation Tribometry: A Generic Method to Identify the Nature of Contact Forces

Recent years have witnessed the development of so-called relaxation tribometers, the free oscillation of which is altered by the presence of frictional stresses within the contact. So far, analysis of such oscillations has been restricted to the shape of their decaying envelope, to identify in particular solid or viscous friction components. Here, we present a more general expression of the forces possibly acting within the contact , and retain six possible, physically relevant terms. Two of them, which had never been proposed in the context of relaxation tribometry, only affect the oscillation frequency, not the amplitude of the signal. We demonstrate that each of those six terms has a unique signature in the time-evolution of the oscillation, which allows efficient identification of their respective weights in any experimental signal. We illustrate our methodology on a PDMS sphere/glass plate torsional contact

High frequency vibroacoustics: A radiative transfer equation and radiosity based approach

International audienceThis paper is a review of the theoretical framework for the application of radiative transfer equations to structural dynamics and acoustics. It is shown that under the assumption of geometrical acoustics and when the phase of rays is neglected, a representation of a sound field in terms of incoherent fictitious sources provides a sufficiently large framework to embody diffuse as well as specular reflection, steady-state or transient phenomena and even diffraction. Two special cases are mentioned. The so-called radiosity method in acoustics corresponds to a purely diffusing boundary and statistical energy analysis when the sound field is diffuse

Ergodic billiard and statistical energy analysis

International audienceThis paper highlights the importance of ergodicity of billiards in Statistical energy analysis (SEA), a statistical theory of sound and vibration. We show that the main relationship of statistical energy analysis, the so-called coupling power proportionality, is intimately linked with the establishment of a diffuse vibration field in subsystems. In particular, we show that when subsystems have ergodic geometries or when the nature of excitation enforces a diffuse field, the energy exchange between two weakly coupled subsystems is proportional to the difference of vibrational energies. But when the field is not diffuse (either non isotropic or non homogeneous), the exchange of energy does not generally follow this proportionality. Numerical simulations are provided to support the discussion

Dynamique des interfaces multicontact

Le bruit de frottement de deux surfaces rugueuses est dû à la vibration verticale engendrée par les impacts inter-aspérités de deux solides glissants. Il relève de la physique des interfaces multicontact dont les propriétés sont encore largement méconnues. L'objet de cette thèse est de comprendre les mécanismes de transfert d'énergie et de génération des vibrations à l'œuvre à l'interface entre deux surfaces rugueuses en glissement relatif. Ces interfaces présentent des spots de contact qui se renouvellent très rapidement mais dont la physique statistique reste à découvrir. Un outil numérique est spécialement développé pour étudier efficacement ce phénomène aux échelles microscopique et macroscopique. Les simulations sont effectuées à l'aide de centres de calcul haute performance à Lyon. Elles ont mené aux conclusions suivantes. Le niveau de la vibration Lv (dB) est une fonction croissante du logarithme de la rugosité de surface Ra et de la vitesse de glissement V, ce qui est en accord avec les résultats expérimentaux issus de la littérature. De plus, grâce à cet outil numérique, on a pu analyser précisément les chocs entre surfaces définis à partir de l'évolution temporelle de la force de contact. Leur durée est de l'ordre de 0.1 ms, la force maximale de contact peut atteindre 100 fois le poids propre du solide glissant, et le nombre de chocs est de l'ordre de 10000 par seconde pour une surface de l'ordre de 4 cm2. Les chocs sont donc des excitations transitoires brèves mais nombreuses et intenses. Ces chocs se comportent comme les sources d'énergie vibratoire qui sont responsables d'un transfert d'énergie à l'interface. C'est en effet la transformation de l'énergie cinétique du mouvement solide glissant en énergie vibratoire qui est responsable du bruit de frottement.The friction noise between two rough surfaces is caused by the vertical vibration generated by inter-asperity impacts of sliding solids. This phenomenon involves the physics of multicontact interfaces, a field which is largely unknown. The purpose of this thesis is to understand the mechanisms of noise generation and the energy transfer process between two rough surfaces in sliding contact. The contact spots in the interface are rapidly renewed during the movement in a random fashion but their statistical properties remain to be discovered. A numerical tool is developed in order to efficiently study this phenomenon at both macroscopic and microscopie scales. The simulations are carried out thanks to the high performance computing centre in Lyon. This study leads to the following conclusions. The vibration level Lv (dB) is an increasing logarithm function of surface roughness Ra and sliding velocity V. This statement is consistent with experimental results available in the literature. Moreover, we can analyze precisely the asperity shocks which are defined from the time evolution of the contact force. The shock duration is of the order of 0.1 ms, the maximal contact force can reach to 100 times the weight of sliding solid, and the shock rate is of the order of 10000 for a surface of 4 cm2 . The asperity shocks are transient excitations, brief but abundant and intensive. These shocks behave like vibrational energy sources and are responsible of the energy transfer in the interface. This is the transformation process of kinetic energy to vibrational energy which is responsible of friction noises.LYON-Ecole Centrale (690812301) / SudocSudocFranceF

Simulation numérique directe en dynamique du contact rugueux

Le bruit de frottement entre deux surfaces rugueuses est dû à la vibration verticale engendrée par les impacts inter-aspérités de deux solides frottés. Les expérimentations ne donnent pas l?accès aux grandeurs mécaniques locales comme l?intensité de choc, la déformation d?aspérité, la pression locale.?Afin de révéler la relation entre les derniers et les paramètres macroscopiques, on propose une approche numérique 1D basée sur la méthode de décomposition modale, les formulations de Lagrange et pénalité, et certains schémas d'intégration

Caractérisation expérimentale d'instabilités de frottement pour un contact élastomère/verre lubrifié à l'eau

Ce papier traite du frottement élastomère/verre avec lubrification à l'eau. Sous certaines conditions de lubrification et de vitesses relatives, un bruit de crissement peut être généré. Des mesures expérimentales ont été effectuées sur un tribomètre conçu pour l'étude des phénomènes transitoires. Cela a consisté à corréler des mesures stationnaires de frottement à des mesures vibratoires et de variation d'effort dynamique de frottement, permettant de comprendre l'apparition de ces instabilités

Wear generated by sliding impacts

Vibrations of the steam generator tubes in nuclear power plants induce stochastic impacts between the tubes and their supports. As a consequence, wear is generated. A test rig is designed and used to perform impacts between two metal crossed cylinders with various incidence angles and impact velocities. The normal and tangential components of the contact load are measured during the tests. Rate and duration of impacts, instantaneous ratio between normal and tangential loads for each impact are deduced. Influence of incidence angle and impact velocity on impact duration, ratio between tangential and normal loads during impact and wear volume is highlighted

Modèle géométrique de frottement entre matériaux composites

Dans cette étude, on propose un modèle géométrique de frottement sous faible charge normale entre deux matériaux composites. Le contact est présenté comme une multitude de microcontacts dont la nature peut être de trois types : fibre-fibre, fibre-matrice ou matrice-matrice. La force de frottement interfaciale est la somme des forces locales de frottement en ces microcontacts. Le calcul montre que le coefficient de frottement hybride ne dépend pas de l'orientation de fibres mais seulement de leur fraction surfacique. Ce modèle est testé expérimentalement