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Anisotropic shrinkage and finite strains in confined frictional contacts
No full textInternational audienceWe report on an experimental investigation of the interplay between friction, contact geometry and finite strains for smooth frictional contacts between rigid spherical glass probes and flat silicone substrates. Using both bulk and layered substrates under various loading conditions (normal force, radius of the probe), we show that shear-induced anisotropic shrinkage of the adhesive contact area under steady-state sliding is an effect of finite-elasticity conditions and is drastically affected by the level of geometric confinement. The resulting non-linear coupling between the normal and lateral directions is also investigated by measuring the changes in the indentation depth (conv. normal load) during the stiction of the adhesive contacts under imposed normal load (conv. indentation depth) conditions, with strong effects of contact confinement. From a comparison with adhesiveless linear contact mechanics calculations, we show that the experimental observations can only be accounted for by the occurrence of finite strains/displacements conditions. Accordingly, measurements of the in-plane surface displacements at the surface of the rubber substrates confirm that strain levels well in the neo-Hookean range are experienced during steady-state frictional sliding
Euclid: Methodology for derivation of IPC-corrected conversion gain of nonlinear CMOS APS
No full textInternational audienceWe introduce a fast method to measure the conversion gain in Complementary Metal-Oxide-Semiconductors (CMOS) Active Pixel Sensors (APS), which accounts for nonlinearity and interpixel capacitance (IPC). The standard 'mean-variance' method is biased because it assumes pixel values depend linearly on signal, and existing methods to correct for nonlinearity are still introducing significant biases. While current IPC correction methods are prohibitively slow for a per-pixel application, our new method uses separate measurements of the IPC kernel to make an almost instantaneous calculation of gain. Validated using test data from a flight detector from the ESA Euclid mission, the IPC correction recovers the results of slower methods within 0.1% accuracy. Meanwhile the nonlinearity correction ensures an estimation of the gain that is independent of signal, correcting a bias of more than 2.5% on gain estimation
A gradient flow on control space with rough initial condition
No full textInternational audienceWe consider the (sub-Riemannian type) control problem of finding a path going from an initial point to a target point , by only moving in certain admissible directions. We assume that the corresponding vector fields satisfy the bracket-generating (Hörmander) condition, so that the classical Chow-Rashevskii theorem guarantees the existence of such a path. One natural way to try to solve this problem is via a gradient flow on control space. However, since the corresponding dynamics may have saddle points, any convergence result must rely on suitable (e.g. random) initialisation. We consider the case when this initialisation is irregular, which is conveniently formulated via Lyons' rough path theory. We show that one advantage of this initialisation is that the saddle points are moved to infinity, while minima remain at a finite distance from the starting point. In the step -nilpotent case, we further manage to prove that the gradient flow converges to a solution, if the initial condition is the path of a Brownian motion (or rougher). The proof is based on combining ideas from Malliavin calculus with Łojasiewicz inequalities. A possible motivation for our study comes from the training of deep Residual Neural Nets, in the regime when the number of trainable parameters per layer is smaller than the dimension of the data vector
Matrix Elements and Characters of the Discrete Series ("Massive") Unitary Irreducible Representations of Sp(4,R)
No full textInternational audienceThis paper obtains the matrix elements and characters of the discrete series unitary irreducible representations (UIRs) of the Sp group. With an isomorphic relationship to the two-fold covering of SO (Sp SO), this group holds particular importance as the kinematical/relativity group within the framework of (-dimensional) anti-de Sitter spacetime
Integration of physical bound constraints to alleviate shortcomings of statistical models for extreme temperatures
No full textInternational audienceHeatwaves have devastating impacts on societies and ecosystems. Their frequencies and intensities are increasing globally with anthropogenic climate change. Statistical models using Extreme Value Theory (EVT) have been used for quantifying risks of extreme temperatures but recent very intense events have cast doubt on their ability to represent the tail probabilities of temperatures. Using outputs from a large ensemble of a climate model, we show that physics-based estimates of the upper-bound of temperatures in the mid-latitudes are 3–8°C higher than suggested by EVT-based models. We propose a new method to bridge the gap between the physical and statistical estimates by forcing the EVT-based models to have an upper bound coherent with the bound provided by the instability of the air column. We show that our method reduces the underestimation of tail risks while not deteriorating the performance of the statistical models on the core of the distribution of extreme temperatures
Vers une normalisation du contentieux disciplinaire des universitaires
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L’avenir de la ressource en eau face aux changements climatiques dans les Pays de la Loire: Rapport spécial Eau
No full textEn Pays de la Loire, les évolutions climatiques transforment profondément le cycle de l’eau. La hausse des températures, l’augmentation de l’évapotranspiration et la modification du régime des précipitations accentuent les déficits estivaux, fragilisent la recharge des nappes et renforcent les tensions sur les cours d’eau. Dans le même temps, les risques d’inondation et de submersion demeurent, tandis que la qualité de l’eau reste un défi majeur pour les milieux et l’alimentation en eau potable.Ces effets ne s’expriment pas de manière homogène sur le territoire régional. Selon les bassins versants, les caractéristiques géologiques, l’occupation des sols et l’intensité des usages, la disponibilité de la ressource et les marges d’adaptation varient fortement. Derrière une apparente abondance, des déséquilibres structurels s’installent, rendant certains territoires plus exposés aux pénuries, aux conflits d’usages et aux dégradations écologiques. Une question centrale s’impose alors : comment adapter la gestion de l’eau sans aggraver les vulnérabilités existantes ?Ce rapport du GIEC des Pays de la Loire propose une analyse scientifique de l’avenir de la ressource en eau dans la région. Il éclaire les mécanismes à l’oeuvre, documente les vulnérabilités territoriales et identifie les leviers d’action mobilisables. Il rappelle que l’adaptation ne peut se limiter à des réponses ponctuelles, mais qu’elle suppose une transformation durable des pratiques, fondée sur la sobriété, la protection des milieux et une gouvernance capable d’anticiper les évolutions à venir
Numerical simulations of oscillating and differentially rotating neutron stars
No full textThe remnants of binary neutron star mergers are expected to be massive, rapidly rotating stars whose oscillations produce gravitational waves in the kilohertz band. The degree of differential rotation and the rotation profiles strongly influence their structure, stability and oscillation spectrum, and must therefore be taken into account when modeling their dynamics. We extend the pseudospectral code ROXAS (Relativistic Oscillations of non-aXisymmetric neutron stArS) to enable the dynamical evolution of oscillating, differentially rotating neutron stars. Using the updated code, we aim to study the star's oscillation frequencies. We extend the previous formalism, based on primitive variables and the conformal flatness approximation, to differential rotation. Within this framework, we run a series of axisymmetric and non-axisymmetric simulations of perturbed, differentially rotating neutron stars with different rotation rates, and extract their oscillation frequencies. Axisymmetric modes, as well as those under the Cowling approximation, show excellent agreement with published results. We show that the secondary fundamental mode in the Cowling approximation is an artifact that does not appear in dynamical spacetimes. In addition, we provide, for the first time, frequency values for non-axisymmetric modes in differentially rotating configurations evolved in conformal flatness. This extension broadens the range of physical scenarios that can be studied with ROXAS, and represents a step toward more realistic modeling of post-merger remnants and their gravitational-wave emission
Explainable AI for Marine Ecological Quality Prediction: Integrating Microbiome Data, Metadata, and Diversity
No full textAssessing ecological quality (EQ) is crucial for marine biodiversity monitoring. With the advent of High-Throughput Sequencing technologies, metabarcoding has enabled large-scale microbial community analysis through Operational Taxonomic Unit (OTU) tables, providing an alternative for EQ assessment. Machine learning (ML) models have been successfully applied for this task, but they often treat microbial abundance as the sole predictor, overlooking environmental meta-data (e.g., pH, salinity, temperature) and diversity indices (alpha and beta diversity). This study integrates metadata and diversity indices into an explainable ML framework for EQ prediction. Using SHapley Additive Explanations (SHAP), we assess the contribution of these features to model predictions across five genetic markers (V1V2, V3V4, V4, 37F, and V9). Our results highlight marker-dependent feature importance, demonstrating that while OTU-based models remain dominant, incorporating metadata improves accuracy for certain markers. This work enhances interpretability in AI-driven biomonitoring, fostering more reliable marine ecosystem assessments.</div