HAL Portal Univ-Poitiers
Not a member yet
51771 research outputs found
Sort by
Dans les Alpes, pouvoir et contre-pouvoir face au contrôle migratoire
No full textInternational audienc
Comparison of the acute effects of breaking up prolonged sitting time with single-task or a dual-task walking on cognitive function and cerebral blood flow in older adults: a randomized crossover trial
No full textInternational audienceThis randomized cross-over study examined the acute effects of single-task or dual-task walking physical activity (PA) breaks of prolonged sitting among healthy older adults on cognitive performance and cerebral blood flow velocity (CBFv). Twenty-seven healthy older adults (69.4 ± 5.6 years, 25 female) were randomly assigned to engage in three experimental conditions (i.e., control, single-task, and dual-task walking), each one on a different day. All conditions included 3 h of sitting time, with a break every 30 min involving either 2 min of single-task or dual-task walking in the intervention conditions. In the control condition, the participants remained seated for 3 h without any breaks. Cognitive functioning (Trail Making Tests [TMT] A and B, Stroop test, and verbal fluency test) and CBFv were assessed before and 10 min after the cessation of the conditions, and their changes were analyzed with a Generalized Estimated Equation (GEE) model. A significant interaction effect was observed in Verbal Fluency phonological and semantic, and TMA (p < 0.01). It was found an effect size of 0.63 and 0.79 in phonological conditions for dual-task and single-task breaks, respectively. In addition, an effect size of 0.67 and 0.71 in semantic condition for dual-task and walking breaks, respectively, and for the TMA, it was found an effect size of - 0.22 for both break types. Furthermore, a significant improvement in CBFv was observed in the dual-task condition (ES = 0.91) and walking condition (ES = 0.55) (p < 0.05) compared to uninterrupted sitting. PA breaks of prolonged sitting by two different types of breaks are effective in improving verbal fluency and set-shifting performance and increasing the CBFv in older adults
[110] tensile testing of single crystalline gold thin films with nanotwins: In situ TEM and XRD studies
No full textInternational audienceTensile tests of 50-nm-thick single-crystalline (001) gold thin films containing nanotwins are monitored in situ with x-ray diffraction (XRD) and transmission electron microscopy (TEM). It is argued that for uniaxial tensile tests along the [110] direction, the induced average stress in the films during loading hardly depends on whether they are deposited onto a polymeric substrate (for XRD) or free-standing (for TEM). When straining along this direction, XRD experiments show an impressive ≈500% increase in the twin volume for two favorably twin orientations at an applied strain of only 4.0%. Furthermore, the twin volume increase is accompanied by a twin rotation. Submicron scale TEM observations show that the increase in volume is correlated with the glide of partial dislocations, either along the coherent twin boundaries (TB) or at the incoherent TBs, or close to microstructure imperfections. In the last case, twins coalesce and acquire a micrometer size. An intense dislocation activity occurs in these large twins, still increasing the strain concentration and resulting in crack expansion. The multiplicity of plasticity mechanisms suggests that the twinning quantification may significantly depend on the initial defect type (twins, dislocations, nanometric holes) and density. The twin rotation is also observed postmortem in TEM. However, the XRD data analysis shows that twinning is not the only plasticity mechanism: first, the yield point of the film supported on a substrate is ≈0.5%, but the twinning starts only at 1.0%, and, second, at an applied strain of 4.0%, the plastic strain induced by the twin is less than 0.8%
Couplage fort partitionné et méthode de pénalisation : application à l'étude de l'érosion de cavitation
No full textInternational audienceA novel fluid-structure coupling strategy is developed, based on a strong partitioned coupling and on the Brinkman penalization method to model the deformable fluid-solid interface. This numerical model is applied to the study of cavitation-induced damage at the microscopic scale, focusing on the collapse of a bubble near an elastoplastic material. The size of the microscopic crater (i.e., cavitation pit) increases rapidly as the bubble-wall distance and the material yield strength decrease. While a closer bubble generates a deep, circular pit, a more distant bubble can produce a shallower, annular pit. Wall deformation leads to a damping of the wall pressure, resulting in significant differences between weakly and strongly coupled simulations. The damping of wall pressure is initially governed by the ratio of the acoustic impedances of the fluid and solid media. It is then further amplified locally by plasticity. A slight damping of the wall pressure results in a much more significant reduction in pit size.Une nouvelle stratégie de couplage fluide-structure est développée, basée sur un couplage fort de type partitionné et sur la méthode de pénalisation de Brinkman pour modéliser l’interface fluide-solide déformable. Ce modèle numérique est appliqué à l’étude des dommages induits par la cavitation à l’échelle microscopique, en se concentrant sur le collapse d’une bulle au voisinage d’un matériau élastoplastique. La taille du cratère microscopique (i.e., pit de cavitation) augmente rapidement lorsque la distance bulle-paroi et la limite d’élasticité du matériau diminuent. Tandis qu’une bulle rapprochée génère un pit circulaire et profond, une bulle éloignée peut produire un pit annulaire et moins profond. La déformation de la paroi entraîne un amortissement de la pression pariétale, conduisant à des différences importantes entre des simulations faiblement et fortement couplées. L’amortissement de la pression pariétale est d’abord gouverné par le rapport des impédances acoustiques des milieux fluide et solide. Il est ensuite amplifié localement par la plasticité. Un faible amortissement de la pression pariétale conduit à une atténuation bien plus importante de la taille du pi
Synthesis, microstructural characterization and transport properties of a new (Ti2Nb)AlC2 ternary nanolaminate carbide solid solution.
No full textInternational audienceA new (Ti2Nb)AlC2 MAX phase solid solution is synthesized by hot isostatic pressing a powder mixture of sub-stoichiometric titanium carbide, niobium and aluminum. It is demonstrated that the synthesis temperature and Al content are key parameters to obtain a large volume fraction of 312 MAX phase solid solution. Rietveld refinement allows determining the cell parameters which are shown to vary from one sample to another. Thus, (Ti2Nb)AlC2 is not a line compound but a disordered solid solution. The 312 MAX phase solid solution formation mechanism is discussed considering intermediate phases observed by scanning electron microscopy and analysed using energy dispersive X-ray spectroscopy. Finally, transport properties are characterized. It is demonstrated that electron-phonon coupling is more efficient than in Ti3AlC2 and Ti3SiC2
Antennerie microphonique tridimensionnelle en soufflerie pour l’analyse du bruit aéroacoustique généré par des obstacles en écoulement
No full textThis study focuses on the aerodynamic noise generated by obstacles within a flow. The identification of these aeroacoustic sources is generally carried out in a plane using a planar array. In this thesis, to analyze these sources in a volume, a three-dimensional array is used, composed of 1024 microphones forming a tunnel around the test section of an anechoic wind tunnel. The aeroacoustic sources from acoustically compact objects are dipolar, and for some, their orientation varies in spaces (e.g., for curved obstacles or combinations of obstacles). An inverse method is thus implemented in this thesis, which does not require any prior assumptions about the orientation of the sources, and allows to determine the orientation of dipoles and their level in space. This method is adapted to the three-dimensional domain, but it is more effective when the focus area is restricted to the contours of the studied obstacles. Specifically, in this study, arrangements of cylinders with various cross-sections, as well as a ring, are experimentally investigated. The noise sources are then placed along the cylinders and have a dipolar directivity orthogonal to the flow. The orientation of the dipoles, as well as the coherence length of the aeroacoustic sources along the axis of the cylinder, are estimated using the inverse method and compared to the literature. Finally, identification tools are used to study the direction of the acoustic dipole generated by cylinders with square and rectangular cross-sections, depending on the angle of incidence of the flow. This study confirms numerical predictions on the directivity of the radiation of these cylinders, based on Curle's solutions.Cette étude porte sur le bruit aérodynamique généré par des obstacles au sein d’un écoulement. L’identification de ces sources aéroacoustiques se fait généralement dans un plan, au moyen d’une antenne plane. Dans cette thèse, afin d’analyser ces sources dans un volume, une antenne tridimensionnelle est utilisée, composée de 1024 microphones formant un tunnel autour de la veine d’essai d’une soufflerie anéchoïque. Les sources aéroacoustiques issues d'objets acoustiquement compacts sont dipolaires, et pour certaines, leur orientation varie dans l’espace (par exemple pour des obstacles courbés ou des associations d'obstacles). Une méthode inverse est ainsi mise en œuvre dans cette thèse, qui ne nécessite aucune hypothèse préalable sur l’orientation des sources, et permet de retrouver l’orientation de dipôles et leur niveau dans l’espace. Cette méthode est adaptée au domaine tridimensionnel, mais est plus efficace lorsque la zone de focalisation est restreinte aux contours des obstacles étudiés. Plus particulièrement, dans cette étude, sont étudiés expérimentalement des agencements de cylindres de sections diverses, ainsi qu’un anneau. Les sources de bruit sont alors disposées le long des cylindres, et ont une directivité dipolaire orthogonale à l’écoulement. Cette orientation des dipôles, ainsi que la longueur de cohérence des sources aéroacoustiques dans la direction de l'axe des cylindres, sont estimés grâce à la méthode inverse, et comparées à la littérature. Enfin, des outils d'identification développés sont utilisés pour étudier la direction du dipôle acoustique généré par des cylindres de sections carrée et rectangulaire, en fonction de l'angle d'incidence de l'écoulement. Cette étude vient confirmer des prédictions numériques sur la directivité du rayonnement de ces cylindres, basées sur les solutions de Curle
« Gare les poches! » Comment se protéger des pickpockets dans la France du XIXe siècle
No full textISBN : 979-10-95518-35-8International audienc
Drivers of contaminated sediment dynamics over 80 years at a basin scale (Loire river basin, France): a multifactorial approach
No full textInternational audienceUnderstanding the long-term dynamics of sediment contamination requires integrating multiple drivers, including anthropogenic activities, river management and sediment transport processes. This study investigates legacy trace element (TE) contamination (Ag, Bi, Cd, Cu, Hg, Pb, Sb, Sn, Zn) in sediments from the Loire river and its main tributaries, highlighting contamination hot periods and hotspots over eight decades. Prior to the 1950s, sediment composition was stable but limited localized contamination was already identified. During the 1950s–1970s, TE enrichments spread basin-wide, driven by industrial and mining activities plus active sediment transport. River modifications such as dam construction and intensive sand mining started to intensify in the 1970s. From the 1980s onward, TE concentrations globally declined due to regulatory measures, industrial closures and sediment connectivity disruptions caused by sand mining, dams and weirs, confining contamination hotspots mostly within sub-basins. Since the 2000s, sediment quality has significantly improved. However, legacy contaminants can still be remobilized during specific events (major flood events and/or river management operations). A conceptual proxy, the Dynamic of the Recovery Period index, expressed as a percentage of remaining contamination over the period 1970s–2010s, enables comparative analysis of recovery trends, independently of the time frame, the river station or maximum of contamination. It emphasizes the influence of spatially heterogeneous legacy contamination and river management practices on sediment quality. Despite overall recovery, some areas remain impacted, notably by mercury. These findings highlight the importance of integrating riverbed and bank sediment quality and dynamics monitoring into river management to support the resilience of fluvial ecosystems
