Simulations and Experiments of a Nonlinear Aircraft Braking System With Physical Dispersion

International audienceThis paper deals with the simulation of nonlinear vibration induced by friction in an aircraft braking system. Experimental tests reveal that in similar experimental conditions the mechanism can generate vibrations of various amplitudes. The aim of this study is to simulate the behavior of the brake by taking into account the dispersion of parameters, which produce the variability of the response. A nonlinear model of the brake is presented. The time-history response is obtained by integration of the full set of nonlinear dynamic equations. Based on experimental results, the dispersions of the coefficient of friction and of the damping configuration are introduced. Monte Carlo simulations are performed and show a very good agreement with the experimental results

Une formulation mixte MES / BEM pour le calcul en acoustique interne

La Méthode Énergétique Simplifiée (MES) développée au laboratoire LTDS se prête bien à la description énergétique de phénomènes acoustiques dans une cavité. Considérée sous sa forme différentielle discrétisée, elle est proche d’une méthode de tirs de rayons énergétique ; en cela, elle est relativement sensible à la formulation des réflexions aux parois, qui sont le plus souvent représentées au moyen d’une directivité lambertienne. Si cette directivité donne de bons résultats pour l’expression du champ réverbéré, elle occasionne en revanche des approximations importantes pour le champ direct. La méthode mixte proposée ici permet d’exprimer le champ direct au moyen d’une formulation par éléments de frontière (BEM), tout en gardant une formulation énergétique pour le champ réverbéré. On atteint ainsi un bon compromis entre le temps de calcul et la précision requise

Analyse des instabilités de couplage en présence d'amortissement et d'actions gyroscopiques

Ce papier traite d'un phénomène acoustique, le grincement d'embrayage automobile, en tant qu'instabilité de couplage modal engendré par du frottement. Un modèle de couplage est d'abord proposé, qui présente un effet circulatoire non-conservatif dû aux forces de frottement. La stabilité est ensuite analysée au travers des valeurs propres des équations linéarisées du système. Les effets des actions circulatoire et gyroscopique sont pris en compte afin de déterminer leur influence sur le domaine de stabilité, analytiquement et numériquement. Leurs effets respectifs et combinés sont analysés avec et sans amortissement; d'importants résultats sur le rôle et l'interaction de chaque paramètre sur la stabilité globale sont établis. Il ressort que l'amortissement structurel est un facteur essentiel; en outre, une relation particulière avec les effets gyroscopiques est mise en évidence. L'optimisation du domaine de stabilité et de sa robustesse vis-à-vis des incertitudes du système est discutée

Digital expression profiling of novel diatom transcripts provides insight into their biological functions

Background: Diatoms represent the predominant group of eukaryotic phytoplankton in the oceans and are responsible for around 20% of global photosynthesis. Two whole genome sequences are now available. Notwithstanding, our knowledge of diatom biology remains limited because only around half of their genes can be ascribed a function based onhomology-based methods. High throughput tools are needed, therefore, to associate functions with diatom-specific genes. Results: We have performed a systematic analysis of 130,000 ESTs derived from Phaeodactylum tricornutum cells grown in 16 different conditions. These include different sources of nitrogen, different concentrations of carbon dioxide, silicate and iron, and abiotic stresses such as low temperature and low salinity. Based on unbiased statistical methods, we have catalogued transcripts with similar expression profiles and identified transcripts differentially expressed in response to specific treatments. Functional annotation of these transcripts provides insights into expression patterns of genes involved in various metabolic and regulatory pathways and into the roles of novel genes with unknown functions. Specific growth conditions could be associated with enhanced gene diversity, known gene product functions, and over-representation of novel transcripts. Comparative analysis of data from the other sequenced diatom, Thalassiosira pseudonana, helped identify several unique diatom genes that are specifically regulated under particular conditions, thus facilitating studies of gene function, genome annotation and the molecular basis of species diversity. Conclusions: The digital gene expression database represents a new resource for identifying candidate diatom-specific genes involved in processes of major ecological relevance

Performance of a large scale prototype of the ATLAS accordion electromagnetic calorimeter

A 2 m long prototype of a lead-liquid argon electromagnetic calorimeter with accordion-shaped electrodes, conceived as a sector of the barrel calorimeter of the future ATLAS experiment at the LHC, has been tested with electron and pion beams in the energy range 10 to 287 GeV. A sampling term of 10%/root E(GeV) was obtained for electrons in the rapidity range 0 < eta < 1, while the constant term measured over an area of about 1 m(2) is 0.69%. With a cell size of 2.7 cm the position resolution is. about 4 mm/root E(GeV)

Performance of an endcap prototype of the ATLAS accordion electromagnetic calorimeter

The design and construction of a lead-liquid argon endcap calorimeter prototype using an accordion geometry and conceived as a sector of the inner wheel of the endcap calorimeter of the future ATLAS experiment at the LHC is described. The performance obtained using electron beam data is presented. The main results are an energy resolution with a sampling term below $11\%/\sqrt{E(\rm GeV)}$ and a small local constant term, a good linearity of the response with the incident energy and a global constant term of 0.8\% over an extended area in the rapidity range of $2.2 < \eta <2.9$. These properties make the design suitable for the ATLAS electromagnetic endcap calorimeter

Test beam results of a stereo preshower integrated in the liquid argon accordion calorimeter

This paper describes the construction of an integrated preshower within the RD3 liquid argon accordion calorimeter. It has a stereo view which enables the measurement of two transverse coordinates. The prototype was tested at CERN with electrons, photons and muons to validate its capability to work at LHC ( Energy resolution, impact point resolution, angular resolution, $\pi^o$/$\gamma$ rejection )

Construction and test of a fine-grained liquid argon preshower prototype

A separate liquid argon preshower detector consisting of two layers featuring a fine granularity of 2.5~10$^{\mathrm{-3}}$ was studied by the RD3 collaboration. A prototype covering approximately 0.8 in pseudo-rapidity and 9 degrees in azimuth was built and tested at CERN in July 94. CMOS and GaAs VLSI preamplifiers were designed and tested for this occasion. The combined response of this detector and an accordion electromagnetic calorimeter prototype to muons, electrons and photons is presented. For minimum ionizing tracks a signal-to-noise ratio of 4.5 per preshower layer was measured. Above 150~GeV the space resolution for electrons is better than 250~$\mu$m in both directions. The precision on the electromagnetic shower direction, determined together with the calorimeter, is better than 4 mrad above 50~GeV. It is concluded that the preshower detector would adequately fulfil its role for future operation at CERN Large Hadron Collider

Search for dark matter produced in association with bottom or top quarks in √s = 13 TeV pp collisions with the ATLAS detector

A search for weakly interacting massive particle dark matter produced in association with bottom or top quarks is presented. Final states containing third-generation quarks and miss- ing transverse momentum are considered. The analysis uses 36.1 fb−1 of proton–proton collision data recorded by the ATLAS experiment at √s = 13 TeV in 2015 and 2016. No significant excess of events above the estimated backgrounds is observed. The results are in- terpreted in the framework of simplified models of spin-0 dark-matter mediators. For colour- neutral spin-0 mediators produced in association with top quarks and decaying into a pair of dark-matter particles, mediator masses below 50 GeV are excluded assuming a dark-matter candidate mass of 1 GeV and unitary couplings. For scalar and pseudoscalar mediators produced in association with bottom quarks, the search sets limits on the production cross- section of 300 times the predicted rate for mediators with masses between 10 and 50 GeV and assuming a dark-matter mass of 1 GeV and unitary coupling. Constraints on colour- charged scalar simplified models are also presented. Assuming a dark-matter particle mass of 35 GeV, mediator particles with mass below 1.1 TeV are excluded for couplings yielding a dark-matter relic density consistent with measurements