Dynamique non linéaire et acoustique de structures à interfaces frottantes

Cette étude propose d'étudier le comportement dynamique et l'acoustique des structures comportant des interfaces. Notre attention se porte tout particulièrement sur les vibrations et le bruit induits par frottement lors d'un évènement de crissement. Le premier point développé dans ces travaux de recherche concerne la mise en place d'une méthodologie globale d'estimation des émissions sonores induites par frottement. Cette démarche s'accompagne du développement d'outils numériques spécifiques à l'étude du rayonnement acoustique induit par frottement permettant notamment d'étudier l'influence de certains paramètres sur le comportement dynamique et le rayonnement acoustique. De plus, on cherche à développer des méthodes numériques performantes permettant l'estimation rapide et contrôlée des niveaux sonores

Hyperoxemia and excess oxygen use in early acute respiratory distress syndrome : Insights from the LUNG SAFE study

Publisher Copyright: © 2020 The Author(s). Copyright: Copyright 2020 Elsevier B.V., All rights reserved.Background: Concerns exist regarding the prevalence and impact of unnecessary oxygen use in patients with acute respiratory distress syndrome (ARDS). We examined this issue in patients with ARDS enrolled in the Large observational study to UNderstand the Global impact of Severe Acute respiratory FailurE (LUNG SAFE) study. Methods: In this secondary analysis of the LUNG SAFE study, we wished to determine the prevalence and the outcomes associated with hyperoxemia on day 1, sustained hyperoxemia, and excessive oxygen use in patients with early ARDS. Patients who fulfilled criteria of ARDS on day 1 and day 2 of acute hypoxemic respiratory failure were categorized based on the presence of hyperoxemia (PaO2 > 100 mmHg) on day 1, sustained (i.e., present on day 1 and day 2) hyperoxemia, or excessive oxygen use (FIO2 ≥ 0.60 during hyperoxemia). Results: Of 2005 patients that met the inclusion criteria, 131 (6.5%) were hypoxemic (PaO2 < 55 mmHg), 607 (30%) had hyperoxemia on day 1, and 250 (12%) had sustained hyperoxemia. Excess FIO2 use occurred in 400 (66%) out of 607 patients with hyperoxemia. Excess FIO2 use decreased from day 1 to day 2 of ARDS, with most hyperoxemic patients on day 2 receiving relatively low FIO2. Multivariate analyses found no independent relationship between day 1 hyperoxemia, sustained hyperoxemia, or excess FIO2 use and adverse clinical outcomes. Mortality was 42% in patients with excess FIO2 use, compared to 39% in a propensity-matched sample of normoxemic (PaO2 55-100 mmHg) patients (P = 0.47). Conclusions: Hyperoxemia and excess oxygen use are both prevalent in early ARDS but are most often non-sustained. No relationship was found between hyperoxemia or excessive oxygen use and patient outcome in this cohort. Trial registration: LUNG-SAFE is registered with ClinicalTrials.gov, NCT02010073publishersversionPeer reviewe

Pechers : ameliorer la production

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Incidence du porte-greffe sur la croissance et la fructification du pêcher (Prunus persica (L) Batsch)

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Pechers : ameliorer la production

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Numerical study of friction-induced instability and acoustic radiation - Effect of ramp loading on the squeal propensity for a simplified brake model

International audienceThis paper presents a numerical study of the influence of loading conditions on the vibrational and acoustic responses of a disc brake system subjected to squeal. A simplified model composed of a circular disc and a pad is proposed. Nonlinear effects of contact and friction over the frictional interface are modelled with a cubic law and a classical Coulomb's law with a constant friction coefficient. The stability analysis of this system shows the presence of two instabilities with one and two unstable modes that lead to friction-induced nonlinear vibrations and squeal noise. Nonlinear time analysis by temporal integration is conducted for two cases of loadings and initial conditions: a static load near the associated sliding equilibrium and a slow and a fast ramp loading. The analysis of the time responses show that a sufficiently fast ramp loading can destabilize a stable configuration and generate nonlinear vibrations. Moreover, the fast ramp loading applied for the two unstable cases generates higher amplitudes of velocity than for the static load cases. The frequency analysis shows that the fast ramp loading generates a more complex spectrum than for the static load with the appearance of new resonance peaks. The acoustic responses for these cases are estimated by applying the multi-frequency acoustic calculation method based on the Fourier series decomposition of the velocity and the Boundary Element Method. Squeal noise emissions for the fast ramp loading present lower or higher levels than for the static load due to the different amplitudes of velocities. Moreover, the directivity is more complex for the fast ramp loading due to the appearance of new harmonic components in the velocity spectrum. Finally, the sound pressure convergence study shows that only the first harmonic components are sufficient to well describe the acoustic response

Assimilate allocation to vegetative and fruit growth

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Influence of physical parameters and operating conditions for structural integrity of mechanical system subjected to squeal noise

International audienceThis work proposes to study the effects of physical parameters and loading conditions on both dynamic and acoustic responses of a brake system subjected to squeal. A simplified brake system model composed of a disc and a pad is investigated. The friction interface is modeled by introducing linear and non-linear stiffnesses at several local nodes to model contact. The classical Coulomb law is applied to model friction and the friction coefficient is assumed to be constant. A stability analysis of this system is performed with respect to the friction coefficient and the hydraulic brake pressure. The system presents up to four instabilities and the focus is on a single instability case with a fundamental frequency of 930 Hz. For this case, self-excited vibrations are investigated for two loading conditions: static and ramp loadings. Time responses for these cases are significantly different: the amplitude of the case with ramp loading is 130 times higher than the amplitude of the static load case. Spectrum analysis are performed by the Continuous Wavelet Transform and the response associated with the static load is composed of the fundamental frequency and its harmonic components. Time response with the progressive load is composed of two fundamental frequencies, their harmonic components and linear combinations which are not expected according to the stability analysis. Noise emissions for these two loading conditions present significantly different features in terms of level and directivity. It is noted that levels in the near field are about 127 dB for the static load and 193 dB for the progressive load. Moreover, the directivity patterns in the near and far fields are composed of lobes for the static load and circular wave front lines for the ramp loading

A simplified approach for the calculation of acoustic emission in the case of friction-induced noise and vibration

International audienceThe acoustic response associated with squeal noise radiations is a hard issue due to the need to consider non-linearities of contact and friction, to solve the associated nonlinear dynamic problem and to calculate the noise emissions due to self-excited vibrations. In this work, the focus is on the calculation of the sound pressure in free space generated during squeal events. The calculation of the sound pressure can be performed by the Boundary Element Method (BEM). The inputs of this method are a boundary element model, a field of normal velocity characterized by a unique frequency. However, the field of velocity associated with friction-induced vibrations is composed of several harmonic components. So, the BEM equation has to be solved for each frequencies and in most cases, the number of harmonic component is significant. Therefore, the computation time can be prohibitive. The reduction of the number of harmonic component is a key point for the quick estimation of the squeal noise. The proposed approach is based on the detection and the selection of the predominant harmonic components in the mean square velocity. It is applied on two cases of squeal and allows us to consider only few frequencies. In this study, a new method will be proposed in order to quickly well estimate the noise emission in free space. This approach will be based on an approximated acoustic power of brake system which is assumed to be a punctual source, an interpolated directivity and the decrease of the acoustic power levels. This method is applied on two classical cases of squeal with one and two unstable modes. It allows us to well reconstruct the acoustic power levels map. Several error estimators are introduced and show that the reconstructed field is close to the reference calculated with a complete BEM

Relationships between shoots and the rest of the tree : effect on fruit growth

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