HAL-Université de Bretagne Occidentale
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Triazole iron(II) coordination polymer showing cooperative spin cross‐over tuned by crystal packing flexibility
No full textInternational audienceA new 1D iron(II) spin crossover (SCO) coordination polymer, [Fe(but‐trz) 3 ][Pt(CN) 4 ]•H 2 O (1), based on 4‐butyl‐1,2,4‐triazole (but‐trz), has been synthesized and structurally and magnetically characterized. Compound 1 exhibits an abrupt and reversible high‐spin ( HS ) to low‐spin ( LS ) transition at T 1/2 = 272 K without significant hysteresis, as confirmed by magnetic susceptibility and differential scanning calorimetry measurements. Single crystal X‐ray diffraction studies at 296 K ( HS ) and 150 K ( LS ) reveal significant anisotropic structural changes and a reversible symmetry‐breaking transition involving a shift of the cationic chains. These changes are accommodated without crystal degradation, indicating exceptional mechanical resilience upon cycling. In contrast to its parent compound [Fe(bn‐trz) 3 ][Pt(CN) 4 ]•H 2 O, compound 1 features additional direct interchain interactions at room temperature, likely arising from the flexible butyl side groups. These interactions may enhance elastic cooperativity within the lattice and could also have a limited influence on the higher transition temperature, offering new insights into structure–property relationships in triazole‐based SCO systems
Lecture Notes on a One-Equation Turbulence Closure Model: Modeling and Analysis
No full textThese lecture notes are devoted to the modeling and the analysis of turbulent incompressible flows through Reynolds-averaged Navier-Stokes (RANS) models. After recalling the fundamental limitations of direct numerical simulation of turbulence, we focus on a one-equation closure model coupling a Navier-Stokes-type system for the mean velocity and pressure with an evolution equation for the turbulent kinetic energy (TKE). This system, referred to as the NS-TKE model, is widely used in engineering and geophysical applications.The first part of the notes is dedicated to the derivation of the model, starting from the Navier-Stokes equations and introducing the Reynolds decomposition, the Boussinesq hypothesis, and the concept of eddy viscosity. A formal derivation of the TKE equation is presented in the framework of isotropic and homogeneous turbulence.The second part is concerned with the mathematical analysis of the NS-TKE model with homogeneous boundary conditions. Due to the low regularity of the source term in the TKE equation and the presence of eddy viscosities, the system exhibits significant analytical difficulties. To address these issues, a truncated and regularized version of the model is introduced. The existence of weak solutions is established via the Galerkin method, and the passage to the limit is carried out using energy methods and compactness arguments, with a distinction between the two-and threedimensional cases.These notes are intended for graduate students and researchers in mathematical fluid dynamics and aim to provide a self-contained introduction to both the modeling and analysis of RANS turbulence models.</div
Constructions graphiques de surfaces d'onde et de paramètres du col d'un faisceau gaussien
No full textWe provide several diagrams for the graphical determination of certain elements of a Gaussian beam based on prior knowledge of other elements. For example, these diagrams allow us to determine the plane of the beam waist and the Rayleigh range from knowledge of two wavefronts constituting the beam, or to determine the size of the light spot on a given wavefront. We also present a simple method for determining the waist position and the Rayleigh range of the image of a Gaussian beam formed by a lens.Se presentan varias construcciones gráficas para determinar parámetros característicos de un haz gaussiano a partir del conocimiento previo de otros de sus parámetros. Estas construcciones permiten determinar, por ejemplo, el plano de la cintura y la longitud de Rayleigh a partir de dos frentes de onda, así como la dimensión transversal del haz en un frente de onda arbitrario. Asimismo, se describe un método sencillo para determinar la posición de la cintura y la longitud de Rayleigh de la imagen de un haz gaussiano formada por un objetivo.Nous présentons plusieurs constructions pour déterminer graphiquement certains éléments d'un faisceau gaussien, fondées sur la connaissance a priori d'autres éléments de ce faisceau. Ces constructions permettent par exemple de déterminer le plan du col et la longueur de Rayleigh à partir de deux surfaces d'onde, ou bien encore de déterminer la dimension transversale du faisceau sur une surface d'onde quelconque. Nous indiquons une méthode simple permettant de trouver la position du col et la longueur de Rayleigh de l'image d'un faisceau gaussien formée par un objectif
Uncovering the relative movements of ecological trajectories
No full textMovement analogies are often employed by ecologists to describe how ecological dynamics relate to one another. For instance, two communities whose similarity increases in time may be said to converge. Here we argue that the movement analogies used by ecologists to compare ecological dynamics could be enriched with other notions such as "pursuit" or "parallel" movements, if accompanied by appropriate statistical testing. By building on the framework of Ecological Trajectory Analysis, we present here Relative Trajectory Movement Assessment (RTMA), a statistical method to detect and qualify the existence of relative movements in ecological dynamics defined as trajectories in multivariate space. Using synthetic trajectory data, we illustrate how RTMA can reveal a diverse range of relative movements beyond the convergence and divergence patterns already recognized in ecology. We exemplify the use of RTMA on real ecological datasets describing 1) old field successional dynamics in eastern North America and 2) temporal patterns in tree size structure of a New Zealand forest. RTMA provides ecologists with a new way of describing and comparing ecological dynamics that could be widely applied, from plot-scale dynamics to the effects of global change
Analyse du développement cérébral précoce par recalage d'IRM longitudinales basée sur un champ de vélocité stationnaire
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Double bandgap formation in a locally resonant metamaterial micromorphic beam
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Radial elastic heterogeneity in a high-modulus carbon fibre assessed by nano-indentation and micro-pillar compression
No full textInternational audienceUnderstanding the radial heterogeneity of carbon fibres is critical to optimising their performance in structural composites. In this study, we investigate the mechanical structure of a high-modulus PAN-based carbon fibre by combining instrumented nano-indentation (IIT) and micro-pillar compression (MCP). Nano-indentation mapping with 50 nm lateral resolution reveals a distinct layered skin–core structure. By combining IIT and MCP, we resolve for the first time the compressive modulus of individual radial layers within a single carbon fibre: a compliant core (110GPa at 0.45% compressive strain), a transitional middle layer, and a stiff outer skin (520 ± 100GPa, at 0.45% compressive strain). Our results confirm earlier indirect estimates and show that the skin modulus approaches that of highly oriented crystallites, whereas the core remains significantly softer due to misorientation. This integrated approach provides a robust framework for characterising anisotropic heterogeneous fibres and offers new insight into fibre-scale mechanical testing
Shifts in Fossil Benthic Foraminiferal Community Trajectories During the Last Deglaciation Along the European Margin
No full textInternational audienceConstraining how faunal communities vary over time and space in response to environmental change has long been a major goal of paleoecologists. Among fossilized organisms, few yield a better and greater detailed record than foraminifera to understand past marine conditions. The analytical frameworks used to build palaeoenvironmental reconstructions have progressed over the years, providing new perspectives within the field. One of these proposed frameworks, Ecological Trajectory Analysis (ETA), offers the possibility to compare geometric properties of trajectories to understand temporal changes within communities. We provide here an application of ETA to the fossil record by comparing benthic foraminiferal data sets from three previously studied cores from the French Atlantic margin. The three cores are each separated by approximately 350 km and cover the last deglaciation, thus allowing a comparison of community dynamics at spatial and temporal scales. A major perturbation in fossil benthic communities was identified in ETA metrics at the onset of Heinrich Stadial 1, with a twofold departing and recovering trend of communities. The three sites along the French Atlantic margin also show spatial differences during the deglaciation, testimony to a contrasting response of benthic foraminifera to ice sheet dynamics and fluctuating fluvial discharges. In this instance, the application of ETA allows to compare multiple fossil foraminiferal records. At a broader scale, the use of trajectory metrics within one unique multivariate space of community resemblance stands out as a powerful tool to compare many other fossil records within the field of paleoecology
