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Fifty shades of yield stress fluids: rheological challenges and engineering perspectives
No full textInternational audienceWhile they were still marginal around thirty years ago and even the very existence of the yield stress was still under debate, research work involving yield stress fluids has exploded over the last 20 years in rheology and physics, even to the point of sometimes appearing hackneyed. Yield stress fluids are now fully recognized as a specific state of matter by physicists and widely studied for this reason. They are also used for their remarkable mechanical behavior in a rapidly growing range of applications, notably in additive manufacturing or 3D printing in bioengineering, civil engineering, food processing, etc. This review first discusses the areas in which a sufficient knowledge might be considered acquired to be used in yield stress fluid engineering. This in particular includes the characterization of materials, through practical tests or sophisticated approaches, the use of simplistic constitutive equations or more complex models including various subtleties of behavior in view of flow simulations, a basic rheophysical framework for predicting the behavior of yielding dispersions or aggregated systems, but also for the widespread practical case of suspensions in yield stress fluids. However, there also appear large areas of major impact for which a comprehensive knowledge seems still lacking. This is in particular the case of: a relevant 3D formulation of the constitutive equation to describe the complex flows encountered in numerous applications, the physical and mechanical characteristics of the solid-liquid transition, the characterization and description of thixotropy, the transition to pasty materials, at the very frontier of “pure” solids
Representing improved tropospheric ozone distribution over the Northern Hemisphere by including lightning NO x emissions in CHIMERE
No full textInternational audienceAbstract. Estimating nitrogen oxide emissions from lightning (LNOx) in models is highly uncertain, affecting the accuracy of atmospheric composition and air quality assessments. Still, it is essential to include these emissions in models to increase the realism of representing the gases and aerosols. LNOx emissions have recently been incorporated into the updated version of the CHIMERE model (v2023r2). In this study, we evaluate the present state of modelling the lightning flashes over the Northern Hemisphere (NH), using a classical scheme based on cloud-top height (CTH) and an updated ice-flux-based scheme (ICEFLUX). We conduct a comprehensive 3D comparison of model outputs, including in situ measurements and satellite data, to rigorously assess the robustness and applicability of these parameterizations. The comparative analysis reveals that the CTH scheme provides a more accurate spatial variability of lightning flashes over lands and tropical oceans. Both parameterizations accurately capture the magnitude of lightning flashes over the tropics, while the ICEFLUX scheme is more effective in representing mid-latitudinal flashes. However, both schemes perform well in capturing the seasonal variation of lightning flashes. The estimated flash frequencies over the NH from the experiments closely align with satellite observations, and the LNOx emissions fall within the range reported in previous modelling studies. There is an overall increase in ozone (O3) concentration due to inclusion of LNOx, which substantially improves the tropospheric O3 distribution, specifically in the tropical free troposphere. The LNOx emissions hence critically influence the O3 burden as well as the hydroxyl radicals, which further impact the atmospheric lifetime of trace gas methane
Effects of hyperalkaline solutions on the water retention properties and microstructure of the opalinus clay from the lower sandy facies of mont terri site, Switzerland
No full textInternational audienceIn Switzerland, the Opalinus Clay has been selected as a potential host rock for the deep geological disposal of radioactive waste due to its low hydraulic conductivity and favorable swelling properties. During the operational phase of the repository, the host rock will be exposed to pH values as high as 13.5 due to concrete degradation, which will certainly affect its hydraulic properties. This study investigates the effect of pH increase on the water retention properties of Opalinus Clay. A series of samples from the lower sandy facies of the Mont Terri site in Switzerland, at initial dry state, were exposed to a hyperalkaline solution of pH = 13.5 and to the synthetic water of pH = 7.5 at different water contents. After equilibrium, the total suction was measured with a dew point potentiometer and microstructural analyses were conducted via mercury intrusion porosimetry (MIP) and nitrogen adsorption-desorption technique. It was found that the total suction decreased with hydration and pH increase. Since the two investigated solutions have the same osmotic suction, the decrease in total suction was attributed to the decrease in matric suction. Indeed, the total porosity increased with saturation and pH increase. This was confirmed by MIP data that evidenced an increase in the proportion of macropores, and by Barret–Joyner–Halenda (BJH) data that showed mesopore generation. The specific surface area (SSA) also increased. The previous results were due to mineral hydration and, exclusively in the case of alkaline solution, to (1) the dissolution of quartz and calcite and (2) the acid-base reactions, which were concentrated at the edges of the clay particles, leading to an increase in negatively charged groups and thus to a face-to-face association of the clay particles (dispersion), causing an increase in the repulsive forces between the clay particles. In addition, the weakening of covalent bonds led to the primary dissolution of clay minerals, i.e. silicon and aluminum detachment
Foam-based microbially-induced calcite precipitation
No full textInternational audienceBuilding on the development history of the Microbially Induced Calcite Precipitation (MICP) method and recent insights into the organization of liquid and bubbles within pore spaces filled with liquid foam, we hypothesize a potential synergy between MICP and liquid foam for enhancing soil properties. We assess the respiration and calcification capabilities of the bacterium Sporosarcina pasteurii in various foaming solutions formulated with bio-based surfactants. The most promising formulations are then used for in-situ observations of bacteria-induced calcification within foam-embedded 2D granular packings. We observe that maltoside-type AlkylPolyGlucosides with relatively short chains, in particular, enable optimal calcification in less than an hour. In the granular packing, the foam spontaneously creates liquid zones enriched with bacteria at the inter-grain contacts. As a result, instead of being distributed across the entire porous space, calcification is localized in these zones, promoting the formation of solid bridges (CaCO_3 ). This suggests that the foam-based MICP method could represent a relevant variation of the original approach, though this remains to be confirmed at larger scales through mechanical testing on representative soil samples
What is the impact of local taxation on the production of affordable housing? An assessment of the case of France
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Identification of climatic extremes by multi-fractal analysis of long climate data series
No full textInternational audienceShrinking and swelling of clays (SSC), occur as a result of water content fluctuations in expansive clayey soils, governed by seasonal cycles of precipitation and drought. This hazard causes ground movement, which can affect foundations and infrastructures. In France, where 54% of constructions are exposed to this hazard, SSC is the second largest category for natural disaster compensation.With climate change, modification in the intensity and frequency of droughts, heat waves and precipitation are likely to exacerbate this phenomenon. In this context, further research is needed to anticipate the influence of climatic changes on the evolution of the SSC hazard and its impact on constructions in the next decades.In particular, it is crucial to understand soil-atmosphere interactions on some appropriate spatial and temporal scales, but also through scales. Climate impact studies use hydrological or agricultural models, fed by global climate data adapted locally by statistical adjustments or downscaling. These methods improve local accuracy but increase bias and uncertainty, as they are often based on stationarity assumptions, which are not always valid in the context of climate change. The modeling of extreme values, essential for risk management, thus becomes more complex.In response to the difficulties of climate models in representing extreme events at high spatio- temporal resolutions, and in understanding hydro-climatic interactions with clay soil, several geostatistical approaches are proposed.An in-depth study of the existing literature has enabled us to compare the various downscaling methods. This state of the art is complemented by the study of data (extreme meteorological phenomena, humidity, soil displacements, etc.) acquired by various organizations concerned by the SSC problem (sources: BRGM, INRAE, SNCF, Météo-France, etc.).This presentation will include the results of geostatistical analyses based on (multi)fractals conducted on this data (spatiotemporal variability, scale breaks, estimation of extreme values, spectral analysis, etc.). The data analyzed will cover the main parameters influencing soil moisture, i.e., precipitation and temperature.These analyses may reveal the statistical signatures of climatic extremes. By identifying them, it will then be possible to research the different climate scenarios, and thus represent the extremes with precision. This step is essential to understanding SSC phenomena.The final objective of this research work is to propose a soil-atmosphere interaction model, capable of generating the input data required for a numerical SSC behavior model. This model will take into account the various hydro-climatic parameters mentioned above, focusing mainly on evaporation and infiltration processes, as well as soil heterogeneity
-reconstruction of piecewise polynomial fields with application to -a posteriori nonconforming error analysis for Maxwell's equations
No full textWe devise and analyse a novel -reconstruction operator for piecewise polynomial fields on shape-regular simplicial meshes. The (non-polynomial) reconstruction is devised over the mesh vertex patches using the partition of unity induced by hat basis functions in combination with local Helmholtz decompositions. Our main focus is on homogeneous tangential boundary conditions. We prove that the difference between the reconstructed -field and the original, piecewise polynomial field, measured in the broken curl norm and in the -norm, can be bounded in terms of suitable jump norms of the original field. The bounds are always -optimal, and -suboptimal by -order for the broken curl norm and by -order for the -norm. An auxiliary result of independent interest is a novel broken-curl, divergence-preserving Poincar\'{e} inequality on vertex patches. Moreover, the -norm estimate can be improved to -order suboptimality under a (reasonable) assumption on the uniform elliptic regularity pickup for a Poisson problem with Neumann conditions over the vertex patches. We also discuss extensions of the -reconstruction operator to the prescription of mixed boundary conditions, to agglomerated polytopal meshes, and to convex domains. Finally, we showcase an important application of the -reconstruction operator to the -a posteriori nonconforming error analysis of Maxwell's equations. We focus on the (symmetric) interior penalty discontinuous Galerkin (dG) approximation of some simplified forms of Maxwell's equations
Rainfall Dynamics in Wind Energy Scenarios
No full textInternational audienceThe presence of rain in wind farms involves several modeling challenges, as the momentum exchanges between turbulent wakes and the particle phase present subtle phenomena. For instance, rain droplets are typically large enough to exhibit inertia relative to the air carrier phase. Under these conditions, it has been found that the gravitational settling of particles in turbulent flows may be either enhanced or hindered compared to stagnant conditions. While this has significant implications for rainfall transport, ash pollutants, and pollen dispersion, very few studies have been conducted in field conditions. Moreover, the scaling laws and non-dimensional parameters governing this phenomenon have not yet been properly identified, and determining which configurations result in the enhancement or hindrance of settling velocity remains an open question.We propose a hybrid experimental/numerical approach. Field data from a meteorological mast located at a wind farm in Pays d’Othe, 110 km South-East of Paris, France, were used to characterize the background turbulent flow through a set of sonic anemometers. Additionally, disdrometers were employed to characterize the settling velocity of raindrops, discriminating by particle size. Numerical simulations complement this data analysis. Specifically, 3D space and time vector fields that realistically reproduce the observed spatial and temporal variability of wind fields are generated using multifractal tools. Then, 3D trajectories of non-spherical particles are simulated and their settling velocity derived.Our findings indicate that the presence of turbulence significantly hinders the settling velocity of raindrops in turbulent environments. Our study covers several distinct rainfall events, allowing us to analyze the influence of turbulent flow properties on this phenomenon
Est-on entré dans un âge de la maintenance des infrastructures ?
No full textInternational audienceFace à la fin de l’ère de l’équipement, un nouveau défi émerge : gérer l’existant. L’investissement massif dans de nouvelles infrastructures cède la place à une approche sur la maintenance et la durabilité. Ce virage, perceptible chez les décideurs, invite à repenser les politiques publiques et à privilégier le soin des infrastructures existantes
Average-Tree Phylogenetic Diversity of Networks
No full textInternational audiencePhylogenetic diversity is a measure used to quantify the biodiversity of a set of species. Here, we introduce the "average-tree" phylogenetic diversity score in rooted binary phylogenetic networks and consider algorithms for computing and maximizing the score on a given network. Basically, the score is the weighted average of the phylogenetic diversity scores in all trees displayed by the network, with the weights determined by the inheritance probabilities on the reticulation edges used in the embeddings. We show that computing the score of a given set of taxa in a given network is #P-hard, directly implying #P-hardness of finding a subset of k taxa achieving maximum diversity score and, thereby, ruling out polynomial-time algorithms for these problems unless the polynomial hierarchy collapses. However, we show that both problems can be solved efficiently if the input network is close to being a tree in the sense that its reticulation number is small. More precisely, we prove that we can solve the optimization problem in networks with n leaves and r reticulations in 2^O(r) • n • k time.Using experiments on data produced by simulating a reticulate-evolution process, we show that our algorithm runs efficiently on networks with hundreds of taxa and tens of reticulations.</div