Effect of coupling with internal and external fluids on the mechanical behavior of aerostats

International audienceIn the context of the simulation of aerostats in flight, we are interested here in the coupling between a deformable structure, the fluid contained inside and the fluid flow outside. To study the dynamic stability of such systems, the fluid-structure coupled equations are linearized around an equilibrium position and, by assuming that the fluid flow perturbations are potential, the loads exerted by the fluids on the moving structure can be decomposed in terms proportional, respectively, to the displacement, velocity and acceleration fields of the structure, representing what are generally called the added stiffness, damping and mass effects of the fluid on the structure. In this work, a focus is made on the added mass because, for such lightweight structures, its effect is of prime importance. A Boundary Element Method (BEM) is proposed to compute the fluid added mass operators, for external and internal fluids, and for any structure deformation field. Numerical and experimental validations are conducted on an axisymmetric ellipsoid mockup immerged in water and subject to rigid motions. Variations of the imposed movement amplitude and velocity have also helped to evaluate the validity domain of this model

Sur le mer comme au ciel... Aperçu de l'histoire de la mission de la Mer

El nacimiento de la Misión del Mar en 1945 marca un giro en la historia de las relaciones entre la Iglesia católica y el mundo marítimo en Francia en el siglo 20. Sin embargo, cristianos se van de misiones hacia el país de la gente del mar desde el final del s. 19. Tres grandes periodos en le curso de la historia marcan el apostolado marítimo. La época de las Parroquias, obras del Mar y de las casas de los marinos, la época de los Movimientos y de la Acción Católica, y por fin, la época de la Presencia y de la Oración se yuxtaponen progresivamente.Itsas Misioaren sorrera, 1945ean, bihurgune bat izan zen Frantziako Eliza katolikoaren eta itsas munduaren arteko harremanetan, XX. mendeari dagokionez. Nolanahi ere, XIX. mendearen amaieratik, kristauak misioetara joaten ziren itsas jendearen eskualdeetara. Historian, hiru garai bereizten dira itsas apostolutzan. Elkarren ondozka datoz: Parrokien aldia, Itsas obren eta itsasgizonen egoitzen aldia, Ekintza Katolikoaren mugimenduen aldia eta, azkenik, Presentziaren eta otoitzaren aldia.La naissance de la Mission de la Mer en 1945 marque un tournant dans l'histoire des relations entre l'Eglise catholique et le monde maritime en France au 20ème siècle. Toute fois, depuis la fin du 19ème S. des chrétiens vont en mission vers le pays des gens de mer. Trois grandes périodes au cours de l'histoire marquent l'apostolat maritime. Se juxtaposent progressivement, le temps des Paroisses, des oeuvres de Mer et des demeures de marins, le temps des Mouvements et de l'Action catholique, en fin le temps de la Présence et de la prière.The founding of the Mission of the Sea in 1945 marks a change in the history of the relationships between the Catholic Church and the maritime world in 20th century France. However, Christians start to go from missions toward the countries of the people of the sea from the end of the 19th century. Three long periods in the course of history mark the maritime apostolate. The era of the Parishes, the Works of the Sea and Sailors' Homes, the era of Movement and of Catholic Action, and finally, the era of Presence and Prayer are progressively juxtaposed

Etude du contact corde / frette

Les contacts entre une corde vibrante et un obstacle rigide sont fréquemment rencontrés dans divers instruments de musique (basse électrique, contrebasse, sitar, tampoura...), ce qui donne lieu à des sonorités riches et variées. Alors qu'un certain nombre d'études analytiques et numériques ont été menées pour modéliser ces contacts, on ne trouve que peu d'études expérimentales dans la littérature. Cette étude a pour objectif de mettre en évidence expérimentalement le comportement d'une corde vibrante en présence d'un contact unilatéral, comme cela peut se produire lors d'un contact corde/frette. Pour cela, on s'intéresse plus particulièrement au contact entre une corde de basse électrique et un obstacle ponctuel, dont des observations expérimentales sont comparées avec les résultats analytiques pour une corde idéale, et avec des résultats numériques basés sur un schéma conservatif en différences finies, pour une corde avec raideur. Les résultats montrent une bonne concordance entre les signaux obtenus analytiquement, expérimentalement et numériquement

A modal approach to the numerical simulation of a string vibrating against an obstacle:Applications to sound synthesis

International audienceA number of musical instruments (electric basses, tanpuras, si-tars...) have a particular timbre due to the contact between a vibrating string and an obstacle. In order to simulate the motion of such a string with the purpose of sound synthesis, various technical issues have to be resolved. First, the contact phenomenon, inherently nonlinear and producing high frequency components, must be described in a numerical manner that ensures stability. Second, as a key ingredient for sound perception, a fine-grained frequency-dependent description of losses is necessary. In this study, a new conservative scheme based on a modal representation of the displacement is presented, allowing the simulation of a stiff, damped string vibrating against an obstacle with an arbitrary geometry. In this context, damping parameters together with eigenfrequencies of the system can be adjusted individually, allowing for complete control over loss characteristics. Two cases are then numerically investigated: a point obstacle located in the vicinity of the boundary , mimicking the sound of the tanpura, and then a parabolic obstacle for the sound synthesis of the sitar

Self-oscillation

Physicists are very familiar with forced and parametric resonance, but usually not with self-oscillation, a property of certain dynamical systems that gives rise to a great variety of vibrations, both useful and destructive. In a self-oscillator, the driving force is controlled by the oscillation itself so that it acts in phase with the velocity, causing a negative damping that feeds energy into the vibration: no external rate needs to be adjusted to the resonant frequency. The famous collapse of the Tacoma Narrows bridge in 1940, often attributed by introductory physics texts to forced resonance, was actually a self-oscillation, as was the swaying of the London Millennium Footbridge in 2000. Clocks are self-oscillators, as are bowed and wind musical instruments. The heart is a "relaxation oscillator," i.e., a non-sinusoidal self-oscillator whose period is determined by sudden, nonlinear switching at thresholds. We review the general criterion that determines whether a linear system can self-oscillate. We then describe the limiting cycles of the simplest nonlinear self-oscillators, as well as the ability of two or more coupled self-oscillators to become spontaneously synchronized ("entrained"). We characterize the operation of motors as self-oscillation and prove a theorem about their limit efficiency, of which Carnot's theorem for heat engines appears as a special case. We briefly discuss how self-oscillation applies to servomechanisms, Cepheid variable stars, lasers, and the macroeconomic business cycle, among other applications. Our emphasis throughout is on the energetics of self-oscillation, often neglected by the literature on nonlinear dynamical systems.Comment: 68 pages, 33 figures. v4: Typos fixed and other minor adjustments. To appear in Physics Report

Colloque

Le Doaré Hélène. Colloque. In: Cahiers du Genre, N°29, 2000. Variations sur le corps. pp. 159-161

Présentation

Le Doaré Hélène. Présentation. In: Cahiers du GEDISST (Groupe d'étude sur la division sociale et sexuelle du travail), N°9-10, 1994. Travail, politique et genre. Perspective internationale. pp. 1-2

George L. Mosse - L'image de l'homme. L'invention de la virilité moderne. Paris. Ed. Abbeville. 1997

Le Doaré Hélène. George L. Mosse - L'image de l'homme. L'invention de la virilité moderne. Paris. Ed. Abbeville. 1997. In: Cahiers du GEDISST (Groupe d'étude sur la division sociale et sexuelle du travail), N°22, 1998. La petite enfance : pratiques et politiques. pp. 157-158

Équateur : Le mouvement des femmes face aux éléctions

Le Doaré Hélène. Équateur : Le mouvement des femmes face aux éléctions. In: Cahiers du féminisme, n°77, 1996. Du Nord au Sud, le libéralisme contre les femmes (été 1996) p. 41