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L'objectif de cette étude était d'examiner l'effet de l'irradiation UV sur la durée de vie en fatigue d'un polymère semi-cristallin en vrac. Des échantillons de polyéthylène basse densité exposés à différentes doses d'irradiation UV ont été testés en fatigue. Un indicateur de fatigue basé sur l'énergie dissipée par cycle s'est avéré présenter la meilleure corrélation avec les résultats expérimentaux de fatigue. Une courbe maîtresse unifiant les résultats expérimentaux de fatigue pour les matériaux tels que reçus et vieillis par les UV a été obtenue en soustrayant le seuil d'énergie dissipée de l'énergie totale dissipée. Enfin, l'évolution des dommages avec un chargement cyclique a été analysée et une modélisation préliminaire a été tentée. The objective of this study was to investigate the effect of UV irradiation on the fatigue life of a bulk semi-crystalline polymer. Low-density polyethylene samples exposed to different UV irradiation doses were fatigue tested. Fatigue indicator based on dissipated energy per cycle was found to present the best correlation with the experimental fatigue results. A master curve unifying the experimental fatigue results for as-received and UV-aged materials was obtained when subtracting the dissipated energy threshold from the total dissipated energy. Finally, the evolution of the damage with cyclic loading was analyzed and preliminary modeling was attempted. 142

In this paper, multi-scale numerical modelling analysis is proposed to study the ballistic responses of a Kevlar KM2 woven fabric. Three meso-microscopic multi-scale numerical models are conducted by varying the number of microscopic primary yarns in the impact zone from 2 to 10. In each model, we analyse the evolution of the contact force, the projectile velocity and the fabric energies. An analysis of the failure modes of each configuration is also performed. Results of the three multi-scale models and a mesoscopic model are compared in order to find the optimal number of microscopic primary yarns to predict the fabric’s ballistic behaviour. It is highlighted that the multi-scale model consumes a reasonable computation cost. Furthermore, with the primary yarns modelled in the fibre scale, this model can fully describe the complex physical phenomena, such as fibre failure and the interaction between fibres, fibres and yarns, that occur during the impact.

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The dynamics of heat transfer in a model system of Rayleigh-B\'enard (RB) convection reduced to its essential, here dubbed Burgers-Rayleigh-B\'enard (BRB), is studied. The system is spatially one-dimensional, the flow field is compressible and its evolution is described by the Burgers equation forced by an active temperature field. The BRB dynamics shares some remarkable similarities with realistic RB thermal convection in higher spatial dimensions: i) it has a supercritical pitchfork instability for the onset of convection which solely depends on the Rayleigh number $(Ra)$ and not on Prandlt $(Pr)$, occurring at the critical value $Ra_c = (2\pi)^4$ ii) the convective regime is spatially organized in distinct boundary-layers and bulk regions, iii) the asymptotic high $Ra$ limit displays the Nusselt and Reynolds numbers scaling regime $Nu = \sqrt{RaPr}/4$ for $Pr\ll 1$, $Nu=\sqrt{Ra}/(4\sqrt{\pi})$ for $Pr\gg1$ and $Re = \sqrt{Ra/Pr}/\sqrt{12}$, thus making BRB the simplest wall-bounded convective system exhibiting the so called ultimate regime of convection. These scaling laws, derived analytically through a matched asymptotic analysis are fully supported by the results of the accompanying numerical simulations. A major difference with realistic natural convection is the absence of turbulence. The BRB dynamics is stationary at any $Ra$ number above the onset of convection. This feature results from a nonlinear saturation mechanism whose existence is grasped by means of a two-mode truncated equation system and via a stability analysis of the convective regime., Comment: 17 pages, 9 figures

International audience; We study, by means of extensive direct numerical simulations, the turbulent flow produced by a two-dimensional cellular forcing in a cubic box with periodic boundary conditions. We find that, in spite of the developed turbulence that recovers isotropic features at small scales, the signature of forcing remains in the mean flow (averaged over time and over the homogeneous direction). This allows us to introduce a friction factor, whose dependence on the Reynolds number is investigated. We further show that the flow is characterized by large temporal fluctuations of the total energy, as a consequence of the exchange between the forced mean flow at large scales and turbulent fluctuations at small scales. Such temporal fluctuations produce a correction to the energy spectrum that can be explained by a simple dimensional argument.

A uniform framework for the dynamic behavior of a rod composed of a linearized anisotropic material is constructed based on an asymptotic reduction method. Taylor–Young series expansions of the displacement vector and stress tensor are adopted at the beginning. By substituting the series expansions into the three-dimensional (3D) equilibrium equations and the lateral traction condition followed by proper manipulations, four scalar rod equations are obtained for the leading-order displacement vector and the twist angle of the central line. The associated one-dimensional (1D) boundary conditions are obtained through the virtual work principle. One key feature of the present theory is the establishment of recursive relations so that most of the unknowns from the expansion can be eliminated. Also, all the remainders are retained, so the pointwise error estimate of the equations is established. One important advantage of the present theory is that no ad hoc assumption on the forms of displacement and external loading is adopted. Therefore, the rod theory provides a uniform framework to study dynamic behaviors. As an illustrative example, the natural vibration of a linearized isotropic rod with fixed-free boundary conditions is studied. The natural frequency is calculated by applying this uniform framework for the bending, torsional, and longitudinal modes. The obtained results are compared with those of the 3D simulations, Euler–Bernoulli, and Timoshenko beam theories. It turns out that the present theory is accurate enough for moderate and long rods in the bending vibration, and provides explicit results with high accuracy for either long or short rods in torsional and longitudinal vibrations.

A novel reduced model is constructed for a linearized anisotropic rod with doubly symmetric cross-section. The derivation starts from the Taylor expansion of the displacement vector and the stress tensor. The goal is to establish rod equations for the leading order displacement and the twist angle of the mean line of the rod in an asymptotically consistent way. Fifteen vector differential equations are derived from the 3D (three-dimensional) governing system, and elaborate manipulations between these equations (including the Fourier series expansion of the lateral traction condition) lead to four scalar rod equations: two bending equations, one twisting equation, and one stretching equation. Also, recursive relations are established between the higher order coefficients and the lower order ones, which eliminate most of the unknowns. Six boundary conditions at each edge are obtained from the 3D virtual work principle, and 1D (one-dimensional) virtual work principle is also developed. The rod model has three features: it adopts no ad hoc assumptions for the displacement form and the scalings of the external loadings; it incorporates the bending, twisting, and stretching effects in one uniform framework; and it satisfies the 3D governing system in a point-wise manner.

Sea ice is crucial in many natural processes and human activities. Understanding the dynamical couplings between the inception, growth and equilibrium of sea ice and the rich fluid mechanical processes occurring at its interface and interior is of relevance in many domains ranging from geophysics to marine engineering. Here we experimentally investigate the complete freezing process of water with dissolved salt in a standard natural convection system, i.e., the prototypical Rayleigh-B\'enard cell. Due to the presence of a mushy phase, the studied system is considerably more complex than the freezing of freshwater in the same conditions (Wang et al. 2021c). We measure the ice thickness and porosity at the dynamical equilibrium state for different initial salinities of the solution and temperature gaps across the cell. These observables are non-trivially related to the controlling parameters of the system as they depend on the heat transport mode across the cell. We identify in the experiments 5 out of the 6 possible modes of heat transport. We highlight the occurrence of brine convection through the mushy ice and of penetrative convection in stably stratified liquid underlying the ice. A one-dimensional multi-layer heat flux model built on the known scaling relations of global heat transport in natural convection systems in liquids and porous media is proposed. It allows, given the measured porosity of the ice, to predict the corresponding ice thickness, in a unified framework., Comment: 20 pages, 6 figures

In this study, nano-structural and nano-constraints behavior of two silica aggregate mortars are investigated. The first silica aggregate (A) is almost pure silica and the second one (B) contains silica, calcite and dolomite phases. The relationship between the durability and nano-structural changes is evidenced using Transmission Electron Microscope, X-ray diffraction (XRD) and TGA/DTA. The nano-structural results show that the macroscopic properties of mortars greatly depend on physico-chemical properties of aggregates. Higher the water absorption, higher the consumption of the Portlandite; the consumption of Portlandite, after 28 days of curing, is more in Bm sample than in Am sample. A relationship between the Portlandite content and the water absorption is evidenced. The relationship between reactivity and nano-constraints deduced from XRD modeling is highlighted.

In this work, a method is proposed for modeling RTM process and the associated mechanical behavior of composites filled with mono-sized spherical Alumina particles. This method combines (i) a numerical model (RTM model) that allows the simulation of the RTM process during the injection of particle filled resins, and (ii) a computational strategy of mechanical properties based on the homogenization methods. These proposed models have already been validated with experimental results. The RTM model is based on 3 sub-models: the first one to describe the suspension flow, the second one to simulate the advance of the flow front, and the last one to model the particles filtration by the fibrous medium. The distribution result of the concentration of particles in the fibrous medium obtained at the end of the simulation of the injection is used as input data for mechanical models of homogenization. The homogenization numerical model was constructed from a representative volume element of the microstructures using the Poisson process. The idea here is to couple these two steps (RTM simulation + mechanical properties computation) in a complete model which allows at the same time and in a single operation: to simulate the process of the manufactured composites loaded with particles and to deduce their induced mechanical properties. The pertinence of the proposed method is confirmed by the simulation of nine elastic properties of composites with the finite element method. The influence of post-filling on the induced mechanical properties has been studied.

Plankton dynamics depend in a complex manner on a variety of physical phenomena, according to both experimental and numerical data. In particular, experimental field studies have highlighted the relation between phytoplankton survival and turbulent upwelling and downwelling from thermal convection. Recent numerical works have also shown the importance of accounting for advective transport by persistent structures in simulation models. In nutrient-rich polar marine environments phytoplankton blooms are critically limited by light availability under ice-covered waters. Such heterogeneity of the light intensity distribution, in association with a large-scale coherent fluid flow, can give rise to nontrivial growth dynamics. In this work we extend a previous advection-reaction-diffusion model of phytoplankton light-limited vertical dynamics in the presence of convective transport. Specifically, we consider horizontally heterogeneous light conditions through the use of two regions with different production regimes, modelling the absence (presence) of light under (in between) obstacles. Such a model is intended as an idealized representation of nonuniformly ice-covered polar waters. By means of numerical simulations, we find that the main role of advective transport is to hinder phytoplankton growth, but also that such effect depends on the positions of the obstacles with respect to the upwelling and downwelling flow regions. Furthermore, we show that the sinking speed due to the density difference between phytoplankton organisms and water, while small, plays an important role, which depends on how it adds to the flow. These results indicate that advective transport can have a crucial impact on the survival conditions of sinking phytoplankton species in polar environments., Comment: 10 pages, 5 figures

We investigate numerically the Soret effect on the linear instability properties in convection due to viscous dissipation in a horizontal channel filled with a binary fluid mixture. Two sets of boundary conditions of experimental interest are considered. Both have no-slip boundaries for the velocity and no mass flux through them. The lower boundary is considered adiabatic, while the upper boundary is isothermal for case A and inversely for case B. As no external temperature or concentration difference is imposed on the layer, the cause of thermal instability is the flow rate through the volumetric heating induced by the viscous dissipation and the Soret effect inherent to binary mixtures. It is found that longitudinal rolls (LR) represent the preferred mode for the onset of convection. For case A, both oscillatory and steady-state LR may develop depending on the value of the separation ratio ψ, which represents the ratio between the mass contribution and the temperature contribution to buoyancy forces. The dependence of the instability thresholds on the separation ratio is discussed near and far from the codimension-two bifurcation point. For case B, the basic state remains stable for positive separation ratios, while it loses its stability via a stationary bifurcation with zero wave number for negative values of the separation ratio. The relevance of the theoretical results for the observability of such instability in real systems is discussed. Finally, we suggest a protocol to determine Soret coefficients by using the stability diagrams obtained in the current paper.

International audience; The paper presents the sound absorption coefficient of acoustic absorbers fabricated from natural durian husk fibers, which are currently still considered as agricultural wastes, especially in Malaysia. Samples were fabricated with different fiber densities and thicknesses and the sound absorption performance was measured using the impedance tube method. The results reveal that the durian husk fibers can have absorption coefficient of more than 0.5 above 1 kHz for a minimum thick sample of 20 mm and with minimum density of 160 kg/m3. The optimised macroscopic parameters for various densities were calculated using the inverse method employing the well-known Johnson-Champoux-Allard (JCA) model for porous material.

Granular materials can be used as protection to stop ballistic projectiles. Nevertheless, very few studies discuss the mechanisms of kinetic energy dissipation into granulates. An experimental setup using ballistic screens was developed to evaluate the deceleration of rigid spherical projectiles penetrating CRUSHMAT® targets. The method was validated by varying the thickness of the target, measuring residual velocities and checking whether the ballistic screens and samples end surfaces did influence the target's response or not. Based on numerical simulations and a suitable constitutive model for CRUSHMAT®, the experimental method was further validated, the projectile kinetic energy dissipation assessed and the validity of the proposed constitutive model discussed. The use of ballistic screens looked suitable to perform such experimental investigations and the proposed constitutive model for CRUSHMAT® satisfying to simulate its ballistic impact response. Additional analyses should be performed to acquire a better understanding of the CRUSHMAT® structure on its mechanical behaviour.

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International audience; In this work, wood–cement composites (WCC) based on poplar sawdust were developed and fabricated by the extrusion process. The volume fraction of wood particles in the mixes was varied from 23% to 46%. The mechanical properties of these WCC were characterized in compression to determine the maximum compressive strength and the Young’s modulus. In the second part, these Young’s modulus values were estimated in compression using a 3D numerical homogenization which takes into account the variability in wood particle lengths and the random distribution in the mixes. The obtained results show a good agreement between the experimental data and numerical calculations up to a 35% volume fraction. The model’s poor estimation for large volume fractions (over 35%) could be attributed to the experimental sample size, which is not representative for large volume fractions, the percolation of the wood particles into the mixes and the inhibition of the cement setting.

International audience; Particle pair statistics from synthetic drifter trajectories reconstructed from realistic, high-resolution numerical simulations (SYMPHONIE model) and HF radar velocity measurements are used to investigate the dispersion properties in the Gulf of Tonkin (GoT). This study takes an approach based on two-particles statistics providing the relative dispersion, relative diffusivity and Finite Size Lyapunov Exponent (FSLE) estimates. In the GoT, the relative dispersion follows the predictions from the theory of two-dimensional turbulence with two inertial subranges identified in the kinetic energy spectrum with the spectral slopes − 5/3 and − 3. The time evolution of dispersion shows an exponential growth during 5-8 days, followed by a power law regime during the next 6-20 days. Fixed-length indicators from the relative diffusivity and the FSLE reveal a local dispersion at large and intermediate scales (above Rossby radius of deformation) and non-local dispersion in sub-mesoscale range (below Rossby radius of deformation). The effect of river runoff on the local hydrodynamics and dispersion processes is assessed using the numerical model simulations without river discharge. The results show that in the model, the river plume, when present, highly impacts the Lagrangian statistics. High gradients of buoyancy reinforce the sub-mesoscale circulation in a large region along the Vietnamese coast and modify the scales and intensity of turbulent dispersion. However, a clear change of dispersion regime in the sub-mesoscale range is not identified, suggesting that the mesoscale circulation in the GoT largely governs particle spreading even at small scale.

We study the dispersion of tiny molecular clouds in turbulence by writing patterns in turbulent air and following their deformation in time. The writing is done by fusing O_{2} and N_{2} molecules into NO in the focus of a strong ultraviolet laser beam. By crossing several of these laser beams, patterns that have both small and large scales can be painted. The patterns are visualized a while later by inducing fluorescence of the NO molecules with a second UV laser and registering the image. The width of the lines that make the pattern is approximately 50 μm, a few times the Kolmogorov length η, the smallest length scale in turbulence, while the overall size of the patterns (≈4 mm) is inside the inertial range of the used turbulent jet flow. At small scales molecular clouds disperse under the joint action of molecular diffusion and turbulence. The experiments reveal for the first time this subtle, yet very important interaction. At macroscales (≈200 η) we verify the Batchelor dispersion of objects whose size is inside the inertial range; however, the expected influence of molecular diffusion is smaller than the accuracy of the experiments.

International audience; Analyses in terms of GVCs generally consider that the sharing of value is more favorable to leading firms than to subordinate firms, which may in turn lead the latter to adopt upgrading strategies with the aim of capturing a larger share of the value. However, by means of a dynamic model, we show that multi-annual supply contracts in the champagne industry, by favoring downstream investments of the leading firms (the houses), make it possible to create a higher value which, if it is partially transferred to the subordinate firms (the growers) via a higher price of grapes, contributes to stabilizing the organization of the industry by limiting the upgrading strategies of the latter. We thus contribute to the developing literature on GVCs and, in particular, to the question of value creation and distribution between leading and subordinate firms.; Les analyses en termes de CGV estiment généralement que le partage de la valeur est plus favorable aux firmes leaders qu’aux firmes subordonnées, ce qui peut en retour conduire ces dernières à adopter des stratégies d’upgrading afin notamment de capter une part plus grande de la valeur. Au moyen d’un modèle dynamique, nous montrons, au contraire, que les contrats pluriannuels d’approvisionnement dans la filière du champagne, en favorisant les investissements avals des firmes leaders (les maisons), permettent de créer une valeur supérieure qui, si elle est partiellement reversée aux firmes subordonnées (les vignerons) via un prix du raisin élevé, contribue à stabiliser l’organisation de la filière en limitant les stratégies d’upgrading de ces derniers. Nous contribuons ainsi à la littérature sur les CGV en traitant du sujet particulier de la création et de la répartition de la valeur entre firmes leaders et firmes subordonnées.

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International audience; An asymptotic reduction method is introduced to construct a rod theory for a linearized general anisotropic elastic material for space deformation. The starting point is Taylor expansions about the central line in rectangular coordinates, and the goal is to eliminate the two cross-section spatial variables in order to obtain a closed system for displacement coefficients. This is first achieved, in an ‘asymptotically inconsistent’ way, by deducing the relations between stress coefficients from a Fourier series for the lateral traction condition and the three-dimensional (3D) field equation in a pointwise manner. The closed system consists of 10 vector unknowns, and further refinements through elaborated calculations are performed to extract bending and torsion terms and to obtain recursive relations for the first- and second-order displacement coefficients. Eventually, a system of four asymptotically consistent rod equations for four unknowns (the three components of the central-line displacement and the twist angle) are obtained. Six physically meaningful boundary conditions at each edge are obtained from the edge term in the 3D virtual work principle, and a one-dimensional rod virtual work principle is also deduced from the weak forms of the rod equations.

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International audience; In this paper, new 1D nano-fibrous SiO2 with functionalized surfaces is prepared. First, the effect of dispersion on the morphology and the surface properties of the silica SiO2 compounds are investigated. Second, energy dispersive spectrometer (EDS) and variable pressure scanning electron microscope (VP-SEM) show typically pure and fibrous texture on the surface of SiO2. Third, the presence of the bridging oxygen stretching vibration Si-O-Si, as well as the increase in the intensity ratio between Si-OH band and Si-O-Si are revealed by (FTIR) spectroscopy. Furthermore, X-ray diffraction (XRD) validates the conservation of the SiO lattice after chemical treatment through the KOH for both dispersed and non-dispersed samples. In addition, the shift of the XRD main peak (101) is in good agreement with the FTIR results showing the shift of Si-O-Si peak and the increase in the intensity ratio of Si-OH/Si-O-Si. The dispersed SiO2 sample exhibits a promising functionalized surface with satisfactory results in terms of silica nanofibers crystallinity and chemical composition. As a result, gigh resolution transmission electron microscopy (HR-TEM) data corroborate the claim of the presence of SiO2 nanofibers on the surface from 20 nm to 250 nm. New nano-fibrous SiO2 surfaces will be used to improve interfacial bonding strength between SiO2 compounds and polymer (or organic materials).

International audience; We have derived a rod theory by an asymptotic reduction method for a straight and circular rod composed of linearized anisotropic material in part I of this series. In the current work, we first verify the derived rod theory through five benchmark Saint-Venant’s problems. Then, under a specific loading condition (line force at the lateral surface with two clamped ends), we apply the rod theory to conduct a parametric study of the effects of elastic moduli on the deformation of a rod composed of four types of anisotropic materials including cubic, transversely isotropic, orthotropic, and monoclinic materials. Analytical solutions for the displacement, axial twist angle, stress, and principal stress have been obtained and a systematic investigation of the effects of elastic moduli on these quantities is conducted, which is the main feature of this paper. It is found that these elastic moduli arise in a certain form and in a certain order in the solutions, which gives information about how to appropriately choose moduli to adjust the deformation. Among the four anisotropic materials, it turns out that the monoclinic material presents the most remarkable mechanical behavior owing to the existence of a coupling coefficient: it yields coupled leading-order rod equations, non-trivial axial twist angle, non-negligible transverse shear deformation, and a more adjustable principal stress along the axis.

International audience; We study the statistical properties of scalar fields undergoing reversible chemical reactions in a turbulent environment by means of numerical simulations. To produce strong chemical fluctuations in a wide region of the domain, an original flow configuration has been proposed, where the species are supplied from buffer boundaries with adjustable thickness, while the flow is developed homogeneous and isotropic turbulence in a periodic domain. With the presence of the mean scalar gradient in the bulk region, the strength of turbulent advection is comparable with the chemical source, which is quantified by the Damköhler number. Our analysis focuses on the global and spatial properties of the reactive scalars in their statistically steady regime. We show how for the case of a second-order reaction such features can be connected to the properties of a non-reactive scalar field advected in the same system. Analytical predictions of the scalar moments in the fast reaction regime agree satisfactorily with the direct numerical simulation results. In comparison with the existing results of the isotropic turbulence case, we conclude that the scalar correlation is jointly determined by both the chemical source and the flow configuration. Moreover, the chemical reaction also plays an important role in determining the scalar energy spectra.

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For both economic and reliability concerns, sustainability monitoring and checking of the synthetic insulated electrical cables operating in nuclear power plants, has been a reserach topic at EDF for many years. Under normal operating conditions, these cables are subjected to maximum thermal constraints of about 50°C and are exposed to gamma radiations. The cumulative dose absorbed by the material can reach 45kGy after 40 years of operating time. Radio-thermal-oxidation mechanisms of EPDM materials has been investigated and reported in the literature. The general aim was to evaluate the respective impacts of these mechanisms on the mechanical fracture properties, especially the nominal strain at fracture for which a threshold value is internationally admitted as an end-of-life criterion. When the material is subjected to physical-chemical aging, the macromolecular network is deeply modified by two main mechanisms: cross-linking and chain scission. These degradation mechanisms alter both their mechanical behavior and fracture properties. Building a predictive mechanical tool, able to capture the evolution of the mechanical fracture properties during aging, whatever the degradation mechanism undergone by the material, is the main issue to be addressed in this work. To reach this goal, the influence of radio-thermo-oxidative aging on the mechanical properties at failure of EPDM materials, corresponding to the insulating part of the cables operating in the reactor building of the nuclear power plant, is examined. Two industrial formulations of EPDM materials were studied in this work. The samples underwent accelerated radiative aging (at different dose rates) and thermal aging (at different temperatures). The main physical and mechanical properties were measured using a set of experimental set-up including DSC, IRTF, ATG and mechanical tension tests. The analysis of the obtained results allowed to identify the density of elastically active chains as the most relevant damage parameter to describe the degradation of the network, whatever the predominant damage mechanism. Two hyperelastic models, one phenomenological and the second one based upon a physical approach, were respectively coupled to the damage parameter in order to describe the overall mechanical behavior of the aged materials beyond failure. These approaches allow obtaining estimates of stress and strain at failure. The predictive values given by each model have shown a good agreement with the experimental results, demonstrating the capability and the usefullness of the two approaches.Finally, based upon previous works on EPDM materials aging, a kinetic model, describing the chemical reactions taking place during the oxidative process, was developed to describe the molecular evolutions of the different chemical species. The model allows to quantify the number of crosslinks and chain scissions, and to deduce the concentration of elastically active chains in the macromolecular network. The two chemical and mechanical models were then coupled in order to build a chemo-mechanical model, which can be considered as a global tool for predicting the lifetime of EPDM insulators subjected to thermo-oxidation. As a perspective, this modeling must be assessed by analysing more data and EPDM materials. The implementation of such a model in a finite element code would allow the analysis of structures aging as cables, but not only, on which engineers could rely., Pour des raisons aussi bien économiques que de fiabilité, le suivi et l’évaluation de la durabilité des câbles électriques à isolation synthétique installés dans les centrales nucléaires, est un sujet de recherche d’EDF depuis de nombreuses années. En conditions normales de service, ces câbles subissent des contraintes thermiques maximales d’environ 50°C et sont exposés à des rayonnements gamma où les doses cumulées peuvent atteindre 45kGy au bout de 40ans de service. Depuis de nombreuses années, différents travaux ont traité de l’analyse des mécanismes de radio-thermo-oxydation des EPDM, dans le but d’évaluer leurs impacts respectifs sur les propriétés mécaniques, notamment les propriétés de rupture, grandeur sur laquelle repose le critère de fin de vie internationalement admis pour ce type d’application. Lorsque le matériau est soumis à un vieillissement physico-chimique, la structure du réseau macromoléculaire est profondément modifiée par deux principaux mécanismes : la réticulation et la scission de chaîne. Ces mécanismes de dégradation agissent à la fois sur le comportement mécanique et sur les propriétés de rupture.Le but de ce travail est de proposer un outil mécanique prédictif, capable d’estimer l’évolution des propriétés mécaniques au cours du vieillissement, et ce quel que soit le mécanisme de dégradation subi par le matériau. Pour atteindre l’objectif fixé, l'influence du vieillissement sur les propriétés mécaniques à rupture des matériaux élastomères est examinée. Les matériaux, objets de notre étude, correspondent à la partie isolante en EPDM (Ethylène-Propylène-Diène-Monomère) des câbles installés dans l’enceinte du bâtiment réacteur de la centrale nucléaire. Deux formulations industrielles de matériaux EPDM ont été étudiées. Les échantillons ont subi des vieillissements radiatifs (à différents débits de dose) et thermiques (à différentes températures) accélérés, puis ont été caractérisé via des mesures physico-chimiques et mécaniques. Cette étape du travail a permis d’avoir un ensemble de données expérimentales sur lequel s’appuyer. L’analyse des résultats a permis d’identifier la concentration en chaînes élastiquement actives comme étant le paramètre d’endommagement le plus pertinent pour décrire la dégradation du réseau, et ce quel que soit le mécanisme prédominant. Deux modèles de comportement hyperélastique, l’un basé sur une approche phénoménologique et le second sur une approche physique ont été respectivement couplés au paramètre d’endommagement afin de décrire l'ensemble du comportement mécanique des matériaux vieillis au-delà de la rupture. Ces approches permettent ainsi d’obtenir des estimations de contrainte et de déformation à rupture. Les prédictions données par les modèles ont montré une bonne adéquation avec les résultats expérimentaux, démontrant l’intérêt des approches adoptés.Enfin, sur la base de précédents travaux sur le vieillissement des EPDM, un modèle cinétique a été développé pour décrire les évolutions moléculaires opérant au cours du vieillissement. Le modèle permet notamment de quantifier le nombre de réticulations et de scissions de chaînes, et d’en déduire la concentration en chaînes élastiquement actives du réseau macromoléculaire (paramètre d’endommagement). Un couplage des deux modélisations chimique et mécanique a ensuite été réalisé, permettant de construire un modèle chemo-mécanique, qui peut être considéré comme étant un outil global de prédiction de la durée de vie des isolants EPDM soumis à la thermo-oxydation. En termes de perspectives, cet outil, qui constitue une première approche, nécessite d’être améliorée, notamment sur la base d’un ensemble plus important de résultats expérimentaux et d’autres matériaux EPDM. L’implantation d’un tel modèle dans un code éléments finis permettrait d’analyser le vieillissement des structures tels que les câbles sur lequel pourrait s’appuyer les ingénieurs.

International audience; To take advantage of the data generated in buildings, this document proposes a methodology based on a machine learning model to improve thermal comfort and energy efficiency. This methodology uses measured data (e.g., indoor/outdoor temperature, relative humidity, etc.) and forecast data (e.g., meteorological data) to train a multiple linear regression model to forecast the indoor temperature of the space under study. Using the genetic algorithm optimization method, this model is then used to evaluate the different heating strategies generated. For each strategy, a score is assigned according to user-defined criteria in order to prioritize them and select the best one. By studying an office building simulated under the TRNSYS software, a multiple linear regression model was implemented with errors less than 1% and an adjusted R2 coefficient close to 0.9. Compared to a conventional heating strategy, this methodology can improve thermal comfort by up to 43%.

International audience

International audience; In this work, the influence of aging on the mechanical properties at break of rubber materials are examined. When subjected to physical-chemical aging, the structure of the rubber network is deeply modified through two main mechanisms: crosslinking and chain scission. These aging mechanisms act both on the mechanical behaviour and on the fracture properties of the rubber materials. The goal of this work is to propose a predictive mechanical tool able to give estimates of these mechanical properties. To address this issue, an hyperelastic constitutive model based upon the energy limiter approach, was coupled with physical-chemical parameters in order to capture the whole mechanical behaviour of the damaged materials beyond the fracture. That allows fracture stress and strain estimates. A wide set of materials and experimental data extracted from the literature or obtained in our team were selected based upon the aging mechanism they can exhibit. We found that the elastically active chains concentrations and the swelling rate can be used as relevant indicators of damage either for crosslinking or chain scission process. These parameters are then introduced as damage parameters in the mechanical modelling. The proposed approach leads to very satisfactory predictions, either to capture the whole mechanical behaviour in uniaxial tension or in terms of stress and strain at break estimates.

We study the translational and rotational dynamics of neutrally-buoyant finite-size spheroids in hydrodynamic turbulence by means of fully resolved numerical simulations. We examine axisymmetric shapes, from oblate to prolate, and the particle volume dependences. We show that the accelerations and rotations experienced by non-spherical inertial-scale particles result from volume filtered fluid forces and torques, similar to spherical particles. However, the particle orientations carry signatures of preferential alignments with the surrounding flow structures, which is reflected in distinct axial and lateral fluctuations for accelerations and rotation rates. The randomization of orientations does not occur even for particles with volume equivalent diameter size in the inertial range, here up to 60 $\eta$ at $Re_{\lambda}=120$. Additionally, we demonstrate that the role of fluid boundary layers around the particles cannot be neglected to reach a quantitative understanding of particle statistical dynamics, as they affect the intensities of angular velocities, and the relative importance of tumbling with respect to spinning rotations. This study brings to the fore the importance of inertial-scale flow structures in homogeneous and isotropic turbulence and their impacts on the transport of neutrally-buoyant bodies with size in the inertial range., Comment: 21 pages, 11 figures

Reaction transport systems emerge in many areas of research and applications as, e.g.: chemical reaction kinetics in fluids, plankton dynamics, infection spreading. While the dynamics of reacting species have been widely studied in homogeneous media, their understanding in heterogeneous environments, particularly relevant for ecological problems, is still limited. In this project we propose to elucidate, by means of numerical simulations of advection-reaction-diffusion systems, the complex interplay between environmental heterogeneity (due to resource limitations, such as light availability in the water column in the sea or in a lake) and transport by a fluid flow, which controls the appearance of phytoplankton blooms in stirred aquatic environments. The focus is on the role of spatially structured flows and turbulent mixing on growth and persistence of algal populations, as well as on the resulting detailed vertical/horizontal population distribution. To investigate these issues, turbulent flows arising from Large-Eddy-Simulation (LES) models of Navier-Stokes equation or kinematic models will be considered. Such an approach is expected to allow access to the details at different scales of the interacting mechanisms that determine phytoplankton dynamics. Due to the wide range of spatial and temporal scales involved, this represents a highly challenging task and it constitutes the main original aspect of the project. For the purpose of enhancing biological and physical realism, model complexity will be gradually increased. By analysing the temporal behaviour of the adopted model systems it will eventually be possible to explore the mechanisms controlling the onset of environmental conditions acting as precursors of algal blooms. This study is expected to provide insights on how to better account for the complexity of fluid motions, with respect to the population dynamics models typically used in fundamental and applied studies at scales larger than the spatial (turbulent) structures of the flow. In particular, we aim at characterizing the main effects due to turbulent fluctuations and, hence, at the possibility to account for a richer phenomenology. Results will then be useful to establish new constraints for improving food web models in marine ecology. Ameliorating the possibility to predict and, potentially, control phytoplankton blooms also has a major societal impact, e.g. for the management of the anthropogenically induced eutrophication (enhanced algal growth due to excess nutrients) of freshwater and coastal marine ecosystems., Les systèmes de transport de réaction apparaissent dans de nombreux domaines de recherche et applications, par exemple: cinétique de réaction chimique dans les fluides, dynamique du plancton, diffusion des infections. Alors que la dynamique de réaction des espèces à avoir été largement étudiée en milieux homogènes, leur compréhension dans des environnements hétérogènes particulièrement pertinents pour les problèmes écologiques, est à nouveau limite. Dans ce projet que nous proposons à élucider, au moyen de simulations numériques de systèmes d'advection - réaction - diffusion, la réciproque complexe entre dans l'hétérogénéité de l'environnement (des limites de ressources telles que la disponibilité de la lumière dans la colonne d'eau dans la mer ou dans un lac) et transport par un écoulement de fluide, qui contrôle l'apparition des fleurs de phytoplancton dans les milieux aquatiques shakes. L'accent est mis sur le rôle des écoulements structurés dans l'espace et le mélange turbulent et la persistance des populations d'algues, ainsi que sur la répartition verticale / horizontale détaillée de la population. Pour étudier ces problèmes, les écoulements turbulents résultant des modèles de simulation à grandes échelles (LES) de l'équation de Navier-Stokes ou des modèles cinématiques seront pris en compte. Une telle approche devrait permettre d'accéder aux détails à différentes échelles des mécanismes d'interaction qui déterminent la dynamique du phytoplancton. En raison du large éventail d'échelles patiales et temporelles impliquées, cela représente une tâche très difficile et constitue le principal aspect original du projet. Dans le but d'améliorer le réalisme biologique et physique, la complexité du modèle augmentera progressivement. En analysant le comportement temporel des systèmes modèles adoptés, il sera finalement possible d’explorer les mécanismes contrôlant l’apparition de conditions environnementales agissant comme précurseurs de proliférations d’algues. Cette étude devrait fournir des indications sur la manière de mieux prendre en compte la complexité des mouvements de fluides, par rapport aux modèles de dynamique des populations généralement utilisés dans les études fondamentales et appliquées à des échelles supérieures aux structures spatiales (turbulentes) du flux. En particulier, nous visons à caractériser les principaux effets dus aux fluctuations des turbulences et donc à la possibilité de rendre compte d’une phénoménologie plus riche. Les résultats seront alors utiles pour établir de nouvelles contraintes pour améliorer les modèles de réseau alimentaire en écologie marine. L'amélioration de la possibilité de prédire et, potentiellement, de contrôler les proliférations de phytoplancton a également un impact sociétal majeur, par ex. pour la gestion de l'eutrophisation induite par l'homme (augmentation de la croissance des algues due aux excès d'utrients) des écosystèmes marins d'eau douce et côtiers.

International audience

In the current study the mechanical properties of jute/polypropylene commingled composite made through compression hot press were investigated and compared with non-commingled composites. The effect of instant and gradual loading during thermoforming process was also investigated to check the effect of pressure on the physical and mechanical properties of the composites. To make the commingled composites, the reinforcement used for the fabrication was made with the jute/polypropylene co-twisted yarn. The jute yarn was signed on low flame to remove the protruding fibers, then jute/polypropylene yarn were co-twisted on the simplex frame to make commingled yarn. The UD, 2D and 3D reinforcement was prepared using the commingled yarn. UD and 2D reinforcements were also made with simple jute yarn to develop non-commingled composite to compare with commingled composites. All type of composites were made using compression hot press. For the commingled composites the readymade prepreg made with commingled yarn was used while for non-commingled composites the jute woven fabric and polypropylene matrix sheet were placed alternatively. To evaluate the mechanical performance tensile, flexural, short beam shear and Charpy impact test were conducted as per standard test methods. The microscopic evaluation was also done of tested samples to check the failure pattern in different composites.The results revealed that the commingled composites were dominant over the non-commingled counterparts. The fabrication of composites using commingled yarns technique resulted in impregnation of reinforcement at fiber level, which leads to good adhesion of matrix with fibers, which resulted in improved mechanical properties such as, the tensile strength and the flexural strength of commingled composites as compared to the non-commingled composites. In addition, the Charpy impact energy and Fmax value of commingled composites were also enhanced, which confirms the high quality and better fiber-matrix adhesion in commingled composites as compared to conventional non-commingled ones. It was observed that the fabrication of thermoplastic composites using the commingled technique is a viable solution resulting in the development of high-quality composites with improved matrix distribution around the reinforcement along with enhanced properties. Both UD and 2D commingled composites showed similar kind of behavior as compared to non-commingled composites. The 3D commingled composites were developed for the first time and tested at 0° and 90° orientation. The results showed that the highest mechanical properties at 90° direction due to higher number of reinforcing yarns, whereas in 0° the fewer number of reinforcing yarn result into poor mechanical performance. The cross-section of samples shows that reinforcing yarn are placed in layers which through binding yarn with even fiber-matrix distribution.The results of composites made through instant and gradual loading were evaluated through microstructure study and mechanical performance. The microstructure study was carried out up to fibril level in order to observe the physical effect of instant and gradual loading on the cellulosic structure of jute. The microstructure of fibers of composites made using instant loading showed significant damage as compared to samples which were made with gradual loading. The fibers of composite made using gradual loading had a very healthy fibers with even polygonal boundary. The tensile, flexural, SBS and Charpy impact results showed superior mechanical performance of gradual loaded samples as compared to instant loaded samples.; Cette thèse porte sur une étude expérimentale visant à analyser le couplage entre les proécdés d’élaboration et les propriétés mécanique des bio-composites à base de fibres en jute et du Polypropylène (Jute/PP). Une nouvelle technique a été développée dans le cadre de ce travail. Il s’agit d’un processus basé sur la réalisation des fils co-torsadés Jute/PP qui servent à la fabrication de tissus 2D et 3D. Ainsi, les composites sont fabriqués à l'aide d'une presse à chaud par compression. Cette méthode contribue à améliorer la qualité de fabrication des biocomposites à base de fibres naturelles en minimisant les défauts d’imprégnation. Pour évaluer cet apport, plusieurs essais mécaniques standardisés ont été rélaisés : des essais monotones de traction, de flexion et de cisaillement et des essais dynamiques du type Charpy. Les résultats on été compares à ceux obtenus par une technique alternant un tissu de jute et une feuille de matrice en polypropylene. Tous les résultats montrent que les biocomposités réalisés par la nouvelle technique des fils co-torsadés présentent une tenue mécanique meilleure par rapport à la seconde méthode.En effet, la fabrication de composites utilisant la technique des fils co-torasdés jute/PP entraîne une bonne imprégnation du renfort au niveau des fibres, ce qui conduit à une meilleure adhérence entre la matrice et les fibres. On note que les niveaux de l'énergie d'impact Charpy et de la valeur maxiamle de la charge obtenues pour les biocomposites fabriqués avec des fils co-torsadés sont nettement supérieures et démontrent bien que cette technique améliore notablement la qualité de l’adhrérence Fibres de jute/PP. Aussi, des observations microscopiques ont été effectuées sur des échantillons testés pour vérifier et analyser les modes de dommage pour les différents biocomposites analysés dans cette étude. Cette nouvelle technique de fabrication des biocomposites à base de fibres naturelles a été améliorée en ajustant les paramètres opératoires à savoir la pression et la température.

The onset of thermal instabilities in the plane Poiseuille flow of weakly elastic fluids is examined through a linear stability analysis by taking into account the effects of viscous dissipation. The destabilizing thermal gradients may come from the different temperatures imposed on the external boundaries and/or from the volumetric heating induced by viscous dissipation. The rheological properties of the viscoelastic fluid are modeled using the Oldroyd-B constitutive equation. As in the Newtonian fluid case, the most unstable structures are found to be stationary longitudinal rolls (modes with axes aligned along the streamwise direction). For such structures, it is shown that the viscoelastic contribution to viscous dissipation may be reduced to one unique parameter: γ=λ1(1−Γ), where λ1 and Γ represent the relaxation time and the viscosity ratio of the viscoelastic fluid, respectively. It is found that the influence of the elasticity parameter γ on the linear stability characteristics is non-monotonic. The fluid elasticity stabilizes (destabilizes) the basic Poiseuille flow if γ<γ* (γ>γ*) where γ* is a particular value of γ that we have determined. It is also shown that when the temperature gradient imposed on the external boundaries is zero, the critical Reynolds number for the onset of such viscous dissipation/viscoelastic-induced instability may be well below the one needed to trigger the pure hydrodynamic instability in weakly elastic solutions.

International audience; We study a predator–prey model of plankton dynamics in the two and three-dimensional wakes of turbulent flows behind a cylinder, focusing on the impact of the three-dimensional character of the carrying velocity field on population variance spectra and spatial distributions. By means of direct numerical simulations, we find that the qualitative behavior of the biological dynamics is mostly independent of the space dimensionality, which suggests that only the relation between the typical flow and biological timescales is crucial to observe persistent blooms. Similarly, in both cases, we find that the spectral properties of the planktonic populations are essentially indistinguishable from those of an inert tracer. The main difference arising from the comparison of the two- and three-dimensional configurations concerns the local spatial distribution of plankton density fields. Indeed, the three-dimensional turbulent dynamics tend to destroy the localized coherent structures characterizing the two-dimensional flow, in which the planktonic species are mostly concentrated, thus reducing the phytoplankton biomass in the system.

We study the conductive and convective states of phase-change of pure water in a rectangular container where two opposite walls are kept respectively at temperatures below and above the freezing point and all the other boundaries are thermally insulating. The global ice content at the equilibrium and the corresponding shape of the ice-water interface are examined, extending the available experimental measurements and numerical simulations. We first address the effect of the initial condition, either fully liquid or fully frozen, on the system evolution. Secondly, we explore the influence of the aspect ratio of the cell, both in the configurations where the background thermal-gradient is antiparallel to the gravity, namely the Rayleigh-B\'enard (RB) setting, and when they are perpendicular, i.e., vertical convection (VC). We find that for a set of well-identified conditions the system in the RB configuration displays multiple equilibrium states, either conductive rather than convective, or convective but with different ice front patterns. The shape of the ice front appears to be always determined by the large scale circulation in the system. In RB, the precise shape depends on the degree of lateral confinement. In the VC case the ice front morphology is more robust, due to the presence of two vertically stacked counter-rotating convective rolls for all the studied cell aspect-ratios., Comment: 7 pages, 4 figures + supplementary material

Hybrid composites based on tin chloride and the conductive polymers, polyaniline (PAni) and poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS), were integrated into high-performance hydrogen sulphide (H2S) gas sensors working at room temperature. The morphology and chemical properties were studied by scanning and transmission electron microscopy (SEM, TEM), energy dispersive spectroscopy (EDS) and Fourier-transform infrared (FTIR). The composites demonstrated a slightly porous nanostructure and strong interactions between the polymers and the metal salt, which slightly dopes PAni. The hybrid sensors exhibited a very low detection limit (&lt, 85 ppb), fast response, repeatability, reproducibility and stability over one month. Moreover, this work presents how calibration based on the derivative of the signal can give hybrid sensors the ability to quantify the concentration of targeted gas, even during continuous variation of the analyte concentration. Finally, the effect of interfering species, such as water and ammonia, is discussed.

A predator-prey model of plankton dynamics in a turbulent flow past an idealized island is studied through fully-resolved numerical simulations to understand the complex interplay between biological excitable behavior and flow transport. We provide evidence that for a certain relation between advective and biological time scales, plankton accumulates in localized filamentary regions where velocity gradients compete with reaction-diffusion spreading, favoring persistence and primary production. The impact of small turbulent scales and of the geometrical details of the obstacle are investigated as well, quantifying their effects on plankton dynamics and patchiness.

Turbulence has been recognized as a factor of paramount importance for the survival or extinction of sinking phytoplankton species. However, dealing with its multiscale nature in models of coupled fluid and biological dynamics is a formidable challenge. Advection by coherent structures, as those related to winter convection and Langmuir circulation, is also recognized to play a role in the survival and localization of phytoplankton. In this work we revisit a theoretically appealing model for phytoplankton vertical dynamics, and numerically investigate how large-scale fluid motions affect the survival conditions and the spatial distribution of the biological population. For this purpose, and to work with realistic parameter values, we adopt a kinematic flow field to account for the different spatial and temporal scales of turbulent motions. The dynamics of the population density are described by an advection-reaction-diffusion model with a spatially heterogeneous growth term proportional to sunlight availability. We explore the role of fluid transport by progressively increasing the complexity of the flow in terms of spatial and temporal scales. We find that, due to the large-scale circulation, phytoplankton accumulates in downwelling regions and its growth is reduced, confirming previous indications in slightly different conditions. We then explain the observed phenomenology in terms of a plankton filament model. Moreover, by contrasting the results in our different flow cases, we show that the large-scale coherent structures have an overwhelming importance. Indeed, we find that smaller-scale motions only quite weakly affect the dynamics, without altering the general mechanism identified. Such results are relevant for parameterizations in numerical models of phytoplankton life cycles in realistic oceanic flow conditions., Comment: 12 pages, 9 figures

International audience; In the last two decades, an increasing interest has been observed for the use of natural fibers such as jute fibers in different applications. These fibers are characterized by their low cost and their availability. They are mainly used in fabric bag manufacturing. The objective of this paper is to study the effect of temperature on the mechanical properties of jute yarns. An experimental study was conducted at different temperatures (22 °C; 80 °C; 105 °C and 150 °C) for 24 h. Each degree was followed by tensile testing of the specimen. Besides, the thermo-gravimetric analysis (TGA) was used to investigate the effect of the temperature on the thermal stability and the thermal degradation of the jute fibers. In addition, the statistical analysis was performed using the method of two and three-parameter Weibull distribution to determine the spatial distribution of the properties. The results showed that there was a degradation of the mechanical properties of the jute yarns heated to high temperature compared to the raw yarns (considered as a specimen exposed to 22 °C). The tensile strain and the Young modulus failed by 36% and 13% respectively for the specimens heated at 150 °C and the failure kinematic of the jute fiber was affected by the temperature of heating. The TGA showed that the thermal stability of the jute yarns was affected by two factors: fibers drying and their changing temperatures.

A linear stability analysis for the onset of mixed convection in a saturated porous medium through an absolute instability of both two- and three-dimensional disturbances is performed. Relevant control parameters associated with the inclined temperature gradient and the vertical throughflow are the vertical and horizontal Rayleigh numbers, $$R_{\mathrm{v}}$$ and $$R_{\mathrm{h}}$$, and the vertical Peclet number, $$Q_{\mathrm{v}}$$, respectively. This work extends previous studies on the very same problem in two fronts. For two-dimensional disturbances, the present results do not agree with the literature for a few of the parametric conditions reported. This is caused by the collision of the convectively unstable downstream propagating branch with multiple upstream propagating branches, which generates several saddle points and, hence, makes the identification of the correct pinching point more difficult. In other words, literature results are all saddle points but not always pinching points. For three-dimensional disturbances, this issue is not present and the current results agree with the literature. On the other hand, due to the inherent difficulties associated with a three-dimensional absolute instability analysis, literature results have only been able to report the group velocities at the onset of convective instability. When their real parts are zero, transition occurs directly from stable to absolutely unstable. Otherwise, transition occurs from stable to convectively unstable first and nothing can be said about the onset of absolute instability. In this work, a novel technique recently developed by the authors allowed the identification of the onset of absolute instability under all parametric conditions investigated in the literature, extending earlier results. Doing so confirmed the dichotomy already observed in these earlier studies, i.e., the onset of absolute instability for two- and three-dimensional longitudinal modes indeed differs.

International audience; Using cracked pipes design standards and finite element analysis, the limit load analysis for pipes containing surface cracks was determined. The study was performed on five pipes of different diameters with a constant crack length and depth. The crack geometry is a semi-elliptical surface crack. The cracked pipes are subjected to internal pressures which are obtained from formulas of cracked pipes design standards. Due to the ductile behavior of polyethylene pipes, the failure occurs when the critical stress reaches a value equal to the ultimate tensile strength multiplied by a constraint factor. In this work, the constraint factor was calculated and its evolution with the pipe diameter was analyzed. Three different definitions of a constraint factor based on global or local approaches were also compared, so that a new failure criterion can be obtained. The new failure criterion makes the prediction of the pipe residual life possible which, in turn, facilitates a systematic approach to maintenance and replacement of pipes.

This paper deals with a new analytical method, the exponential matrix method EMM, for calculating the T-stress and the stress intensity factors SIFs under mixed mode loading. The linear, elastic, two-dimensional and stationary equations of the crack problem are transformed into a Hamiltonian system. This is solved by the proposed method involving eigensolutions which satisfy the adjoint symplectic orthogonality by means of performing an angular stress variation with respect to the radial stress. A good description of the expected solution of the studied problem is then obtained as a linear combination of these eigensolutions. To illustrate the validity of the present method, some numerical examples are used and the results obtained are compared with those of literature.

The dynamic behavior of crystals in convecting fluids determines how magma bodies solidify. In particular, it is often important to estimate how long crystals stay in suspension in the host liquid before being deposited at its bottom (or top, for light particles). We perform a systematic 3D numerical study of particle-laden Rayleigh-Benard convection, and derive a robust model for the particle residence time. For Rayleigh numbers higher than 10^7, inertial particles' trajectories exhibit a monotonic transition from fluid tracer-like to free-fall dynamics, the control parameter being the ratio between particle Stokes velocity and the fluid velocity. The average settling rate is proportional to the particle Stokes velocity in both the end-member regimes, but the distribution of the residence times differs markedly from one to the other. For lower Rayleigh numbers (, To be submitted to Earth and Planetary Science Letters

This paper focuses on the microstructural investigation of ultra-high performance concrete (UHPC) mixtures featuring various replacement levels for blast furnace slag (BFS), in the aim of becoming more environmentally-friendly. Three levels of slag are applied (namely 30%, 50% and 80%) per unit volume of cement. The microstructural characteristics examined included X-ray diffraction (XRD) analysis, transmission electron microscopy (TEM), and mercury intrusion porosimetry (MIP). These characteristics are assessed along with the mechanical strengths at 3 and 90 days. Similarly, the effects of chemical and thermal activation methods on incorporating BFS into UHPC mixtures at high volumes were studied from a microstructural point of view, and a comparison is drawn between the non-activated and activated mixtures. The results of this investigation show that the beneficial physical impact of slag replacement lies at the 30% level, due to its filler property in stimulating hydration to quickly generate portlandite. For this reason, the portlandite and C-S-H peaks are quite similar to those in the reference mixture (i.e. 0% BFS) at 3 and 90 days, as demonstrated in the XRD analysis. Moreover, the pore size distribution could be refined, the microstructure densified (as indicated on the TEM images), and compressive strengths at both ages improved. In contrast, for slag replacement levels of 50% and 80%, the portlandite and C-S-H peaks are significantly lowered, as observed in the XRD analysis, thus causing a magnification of the capillary pores and a drop in compressive strength at 3 and 90 days. Chemical activation through a potassium hydroxide, with a dosage of 10.17 kg/m3, at the 80% BFS replacement level is able to accelerate hydration and portlandite consumption, as exhibited in the XRD analysis through the absence of portlandite peaks and limited quantities of C-S-H at 3 and 90 days. However, this step results in smaller pores and a higher compressive strength. As for the thermal activation, the portlandite in the blended mixtures with 50% and 80% BFS replacement decreases in quantity, as confirmed by XRD analysis, and the reaction is perfectly activated by means of the solubility acceleration of the alkalis; consequently, the pore structure is compacted, as revealed on the TEM image and the compressive strength rises considerably.