Instabilités de frottement : Approches temporelle et fréquentielle

International audienceThe vibrations generated at the interface between the two bodies in friction are responsible for various noises such as squealing, juddering, hammering, hooting, etc... In order to model and understand friction-induced vibration phenomenon, two approaches are compared in this article: temporal approach and modal approach. This analysis has been made on a simplified system composed of two beams in contact. The two different approaches have been programmed using Finite Element method. Assumptions on the contact calculation are different for the two approaches. Modal approach consists in calculating eigenvalues of the friction coupled system. Instabilities appear when a pair of modes merges. Eigenvalues with positive real parts are identified as potentially unstable modes. Temporal approach calculates the evolution of displacements, velocities, accelerations, forces ... One speaks about instabilities when stick or separation zones appear in the contact surfaces. With this approach frequencies which are excited during instability are obtained. Results have been compared and both methods give coherent and complementary results

Experimental and numerical investigation of friction-induced vibration of a beam-on-beam in contact with friction

International audienceThe vibrations generated by friction are responsible for various noises such as squealing, squeaking and chatter. Although these phenomena have been studied for a long time, it is not well-understood. In this study, an experimental and numerical study of friction-induced vibrations of a system composed of two beams in contact is proposed. The experimental system exhibits periodic steady state vibrations of different types. To model and understand this experimental vibratory phenomenon, complex eigenvalue and dynamic transient analyses are performed. In the linear complex eigenvalue analysis, flutter instability occurs via the coalescence of two eigenmodes of the system. This linear study provides an accurate value of the experimental frequency of vibration. To understand what happens physically during friction-induced instability, a dynamic transient analysis that takes account of the non-linear aspect of a frictional contact is performed. In this analysis, friction-induced instability is characterized by self-sustained vibrations and by stick, slip and separation zones occurring at the surface of the contact. The results stemming from this analysis show that good correlation between numerical and experimental vibrations can be obtained (in time and frequency domains). Moreover, time domain simulations permit understanding the physical phenomena involved in two different vibratory behaviours observed experimentally. (C) 2010 Elsevier Ltd. All rights reserved

Apports des méthodes fréquentielle et temporelle dans l'étude des instabilités de frottement responsables du crissement

International audienceIn order to model and understand friction-induced vibration phenomenon, two approaches are compared in this article: temporal approach and modal approach. This analysis has been made on a simplified system composed of two beams in contact. Modal approach consists in calculating eigenvalues of the friction coupled system. Instabilities appear when a pair of modes merges. Eigenvalues with positive real parts are identified as potentially unstable modes. Temporal approach calculates the evolution of displacements, velocities, accelerations, forces ... One speaks about instabilities when stick or separation zones appear in the contact surfaces. With this approach frequencies which are excited during instability are obtained. Results have been compared and both methods give coherent and complementary results

Friction-induced instabilities: modal, transient analysis and experimental validation

International audienceThe vibrations generated at the interface between the two bodies in friction are responsible for various noises such as squealing, juddering, hammering, hooting, etc. In order to model and understand friction-induced vibration phenomenon, two types of analysis, modal analysis and transient analysis, are compared in this article. This study has been made on a simplified system composed of two beams in contact. In modal analysis, instabilities appear when a pair of modes merges. Eigenvalues with positive real parts are identified as potentially unstable modes. In transient analysis, one speaks about instabilities when stick or separation zones appear in the contact surfaces. Results have been compared and both analysis give coherent and complementary results. An experimental validation has been made and shows a good correlation between experimental and numerical results

Sensitive photonic system to measure oxidative potential of airborne nanoparticles and ROS levels in exhaled air

A photonic system has been developed that enables sensitive quantitative determination of reactive oxygen species (ROS) - mainly hydrogen peroxide (H2O2) - in aerosol samples such as airborne nanoparticles and exhaled air from patients. The detection principle relies on the amplification of the absorbance under multiple scattering conditions due to optical path lengthening [1] and [2]. In this study, the presence of cellulose membrane that acts as random medium into the glass optical cell considerably improved the sensitivity of the detection based on colorimetric FOX assay (FeII/orange xylenol). Despite the loss of assay volume (cellulose occupies 75% of cell volume) the limit of detection is enhanced by one order of magnitude reaching the value of 9 nM (H2O2 equivalents). Spectral analysis is performed automatically with a periodicity of 5 to 15 s, giving rise to real-time ROS measurements. Moreover, the elution of air sample into the collection chamber via a micro-diffuser (impinger) enables quantitative determination of ROS contained in or generated from airborne samples. As proof-of-concept the photonic ROS detection system was used in the determination of both ROS generated from traffic pollution and ROS contained in the exhaled breath as lung inflammation biomarkers

Caractérisation Vibro-Acoustique d'une table d'harmonie de Piano droit – Comparaison Mesures /Modèle Analytique

Un modèle vibroacoustique analytique de table d'harmonie de piano est développé au LVA dans le cadre d'un travail de doctorat [1],. De nombreux paramètres entrent en jeu dans la conception d'une table (géométrie, position et nombre de raidisseurs et chevalets, direction d'orthotropie, matériaux…), rendant une étude paramétrique basée sur l'expérimentation ou une approche éléments finis peu envisageable. L'objectif de ce modèle analytique est dès lors de créer un outil numérique simplifié d'aide à la facture instrumentale. Cet outil permettrait alors d'évaluer l'impact des modifications géométriques effectuées sur la table sur le son rayonné par le piano. Dans le but de valider ce modèle et les hypothèses de simplification associées, une étude expérimentale a lieu en parallèle. Ce papier présente la démarche de validation expérimentale, effectuée sur une table de piano droit Pleyel P131, représentative d'un piano milieu de gamme. Après une courte présentation du modèle, le papier expose les informations comparées, puis les méthodes de mesures. Enfin, les résultats sont discutés. Des perspectives concernant la puissance rayonnée seront présentées