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Multi-scale in situ mechanical investigation of the superelastic behavior of a Cu–Al–Be polycrystalline shape memory alloy
Full text linkThe superelastic behavior of a Cu–Al–Be alloy was studied in situ during tensile tests combining two high-energy synchrotron techniques. The initial microstructure was reconstructed using diffraction contrast tomography; the elastic strain and stress tensors of each individual grain were then determined using a 3D X-ray diffraction microscopy technique. The alloy was heat-treated until coarse grains formed, and the probed volume fraction was limited to ∼ 200 grains. The mean grain size, as estimated from both techniques, agreed well with that determined by optical microscopy, i.e., approximately 130 µm. During the early stage of the martensitic transformation (MT), 187 grains with strong stress heterogeneities were detected in the elastic domain; in particular, the stress values along the tensile direction varied by a factor of three between different grains. The reconstructed 3D microstructure served as input data for finite element modeling, wherein a micromechanical approach factoring in the martensitic transformation was used to simulate the in situ tensile tests. The coupled numerical and experimental tensors of the 187 grains confirmed strong stress heterogeneities between them. The influence of the position and crystallographic orientation of the neighboring grains were also examined
Detachment mechanism and reduced evaporation of an evaporative NaCl salt crust
Full text linkSalt crusts forming at the surface of a porous medium are commonly observed in nature as well as on building materials and pieces of our cultural heritage where they represent a risk for the supporting substrate integrity. Previous research indicates that the salt crust can detach from the porous substrate and severely reduces the evaporation. However, the current understanding of the detachment mechanisms and the reduced evaporation is very limited. In the present experiment, we evidence dissolution-precipitation processes as key mechanisms in the detachment process. We also show that the crust remains wet and the observed reduced evaporation is explained by the formation of tiny pores in the nanometer range and the Kelvin effect. The resulting crust permeability is very low. Combined with previous results, this shows that the crust permeability is highly dependent on the crust formation conditions. More generally, salt structures in a water vapor concentration gradient are shown to be self-propelled systems capable to carry small objects such as, for instance, soil particles. Our study has significance for understanding the impact of salt crusts on evaporation and the associated important phenomena, such as soil salinization and porous material degradation inherent to salt crystallization
Performances des protocoles de transport dans les constellations de satellites
Full text linkLes constellations de satellites ont pris un nouvel essor ces dernières années avec des projets particulièrement ambitieux qui ont suffi à raviver la flamme de la communauté de recherche. Un des problèmes cruciaux est alors l'étude de l'adéquation des protocoles principalement conçus et utilisés dans un contexte terrestre vis-à-vis de ces communications par satellite. Par le passé, par exemple, des versions de TCP ont été spécialement conçues pour les réseaux de satellites. Néanmoins, ces travaux pourraient se révéler obsolètes, en raison des évolutions récentes de TCP et en particulier de la refonte complète du contrôle de congestion. La question à laquelle nous nous sommes attaqués dans cette thèse est la suivante: les versions récentes de TCP e.g. CUBIC et BBR sont-elles ou seront-elles capables de répondre aux besoins dans un tel environnement? Nous avons évalué les différences entre les piles TCP du passé et celles qui sont actuellement déployées. Nous décrivons en particulier l'évolution de l'utilisation des protocoles du point de vue de la couche transport de CUBIC et de TCP BBRv1, un algorithme hybride qui repose sur l'estimation du délai et de la capacité du réseau. Nous avons identifié les sources de variation de délai dans les constellations de satellites pour en étudier l'impact et la fréquence. Les variantes modernes de TCP s'en accommodent en particulier pour les flux longs. Nous nous sommes ensuite intéressés à l'équité entre les flux utilisant ou non des variantes différentes. Nous avons mis en évidence certains niveaux d'iniquité dans les contextes hétérogènes assez uniformes à ceux trouvés dans le contexte terrestre. L'ensemble de ces études a été mené au travers de simulations à événements discrets mais aussi d'émulations permettant d'obtenir des résultats plus réalistes
Impact of gravity and inertia on stable displacements of DNAPL in highly permeable porous media
No full textThe flow of Dense Non-Aqueous Phase Liquid (DNAPL) in highly permeable porous media is characterized by a complex interplay between surface tension, viscous, gravity, and inertia forces. Gravitational effects in these systems have been particularly studied in the context of displacement instability, but little work has focused on the impact of gravitational and inertia forces on the stable displacement of DNAPL in highly permeable but non-fractured porous media. Here, we study the impact of the gravity and Forchheimer numbers on the stable displacement of DNAPL fronts in porous media. We first performed DNAPL injection experiments in bead packings of different sizes for different inlet flow rates and initial saturation. These experiments were accurately modeled using a Darcy–Forchheimer model combined with an Arbitrary Lagrangian–Eulerian tracking of the DNAPL front. Once validated against stable injections in glass beads, the model was then used to perform a broader parametric study than available with our experimental setup. We explored different injection and pumping scenarios over a range of dimensionless numbers. We found that gravity can significantly alter the fluid front in flows commonly found in contaminant hydrology. We estimate, however, that inertia will have a non-negligible impact upon the displacement of DNAPL only during active remediation techniques (e.g., free product pumping method) or pipe rupture events involving low-viscous DNAPL in highly-permeable porous media
Characterisation of boundary layer transition over a low Reynolds number rotor
Full text linkAn experimental investigation on the flow topology over a rotor operating atlow Reynolds numbers is presented. The feasibility of laminar to turbulenttransition experiments over small rotors is demonstrated. Phase-locked infraredthermography coupled with simultaneous force and torque measurements wereused to study a three bladed rotor with a NACA0012 aerofoil section set at auniform angle of incidence of 10◦. Boundary layer transition was forced usingtwo-dimensional (2D) and three-dimensional (3D) roughness elements, placedat approximately 5% and 28% of the rotor blades chord. In the 3D roughnessconfigurations once the critical roughness Reynolds was attained the streaksdownstream developed into fully turbulent wedges. For the current rotor con-figuration, it was found that the state of the boundary layer can significantlyaffect its performance, with the non-optimal forcing of laminar to turbulenttransition generally resulting in a loss of performance when compared to thesmooth reference rotor case.Keywords:Infrared Thermography; Boundary layer transition; Low Reynoldsnumber; roto
On the effect of roughness parameters on adherence using the three-point bending test (ISO 14679:1997)
Full text linkIn the present work, a specific three-point bending test is applied to evaluate how the roughness can impact the bond strength (adherence) and the mechanism of interfacial failure initiation. The study is conducted using an aluminum alloy 2024-T3 as substrate and the DGEBA (polyepoxide bisphenol A diglycidyl ether) /DETA (diethylenetriamine) as adhesive, considering different abrasive surface treatments. An optimal roughness is reached to maximize the critical force during failure initiation; besides, the roughness impacted the failure propagation mechanism and the failure initiation area for each abrasive treatment. A power-law regression is considered to correlate the critical force and the failure initiation area, considering different average roughness. Local assessment using a mechanical profilometer and Scanning Electron Microscope (SEM) with Focused Ion Beam (FIB) are applied to measure the residual adhesive thickness at failure initiation and failure propagation zones as well as the initiation-to-propagation transitions. It is constated that the residual adhesive thickness is dependent on the average roughness of the substrate. Finally, Weibull’s analysis is undertaken to demonstrate that the mechanism of failure initiation for all substrate groups is essentially the same, although the failure propagation mechanism can be different
Aggregate production planning considering organizational learning with case based analysis
Full text linkResponding rapidly to customer needs is one of the main targets of industrial organizations that want to survive in the current market competition. This objective can be attained through robust planning. Workforce productivity is considered one of the important entities in production planning. However, it has a dynamic nature, i.e. the productivity growths thanks to on-job training or learning phenomenon. Considering this fact in manufacturing planning enhances the robustness of the developed plans. The present paper presents a mathematical model for medium-range production planning that is used to find the optimal aggregate production plan. The model aims to optimize the total production costs while respecting most of the operational constraints and considering the process of organizational learning. The presented model is constructed relying on the real industrial practices; the outcome is a mixed-integer linear program. The model was validated and checked using real data collected from an Egyptian factory that produces electric motors for home appliances. The proposed mathematical model was optimally solved using “ILOG-CPLEX 12.6”. By comparing the results obtained versus that of the method adopted in the factory, a cost reduction of 6.3% is achieved for the presented data set. A set of managerial aspects are concluded after the model analysis. Moreover, the impact of using detailed learning rates on the production cost is discussed
On the properties of high-order least-squares finite-volume schemes
Full text linkHigh-order finite-volume schemes based on polynomial least-squares methods are an active research topic for the discretization of hyperbolic equations as they allow to obtain high-order spatial discretization schemes in arbitrary meshes. However, few studies have analyzed their performance in good-quality/near-to-uniform meshes, which are commonly used as a meshing strategy in zones where turbulent effects are important. In this paper, the theoretical numerical properties of commonly used least-squares (LSQ) k-exact high-order finite volume schemes are studied in one-dimensional and in several two- dimensional meshes (with some remarks regarding their properties in three-dimensional meshes). These results are compared to those obtained using fully-constrained polynomial reconstructions only compatible with structured meshes. The numerical properties of the schemes are investigated through the von Neumann analysis methodology applied to the one-dimensional and two-dimensional finite volume formulation, including temporal discretization errors. This analysis is also extended to non-uniform and unstructured two- dimensional meshes. At last, the schemes are tested with several numerical experiments using the linear advection, the Euler equations and the Navier-Stokes equations. The analysis of both studies yields similar conclusions regarding the numerical errors and stability of the different studied schemes showing that the high-order least-squares finite volume schemes yield stable and robust results across different uniform and non-uniform unstructured meshes. However, their performance is heavily degraded in simulations with low mesh resolution compared to schemes specially catered to structured meshes. On the other hand, the latter schemes lack stability and robustness in general structured meshes and its formulation cannot be straightforwardly extended to unstructured meshes. Moreover, this work shows that the use of weighted-LSQ can drastically improve the results of LSQ schemes in under-resolved simulations
Rethinking LEO Constellations Routing with the Unsplittable Multi-Commodity Flows Problem
Full text linkThis study investigates the performance of an innovative routing protocol inspired by the Unsplittable Multi-Commodity Flow (UMCF) problem. LEO routing schemes are often based on Shortest Path (SP) algorithms, the Floyd-Warshall algorithm is usually chosen to compute these network paths within the constellation and their end-to-end latency. Instead of considering latency as a criterion, we seek to optimize the overall amount of IP traffic crossing the constellation. This criterion can be optimized by considering the Unsplittable Multi Commodity Flow problem associated with the system. To solve this problem, we use a heuristic algorithm based on randomized rounding that was shown to return solutions of good quality of the Unsplittable Multi Commodity Flow problem in the optimization literature. Using network simulation over Telesat constellation, we show this proposal significantly reduces the overall congestion level compared to the standard SP routing schemes
Effets de la complémentarité des habitats semi-naturels dans les paysages agricoles sur les communautés d'abeilles sauvages. : Quelles contributions des habitats arborés ?
Full text linkLa pollinisation entomophile tient une place centrale parmi les différents services écosystémiques contribuant à la production agricole, et en Europe les abeilles sauvages en sont des actrices majeures. Dans les paysages agricoles, ces espèces mobiles dépendent de ressources fournies par les cultures et par les habitats semi-naturels (HSN) herbacés (prairies) ou arborés (bois, lisières, haies). L’objectif de cette thèse est de comprendre dans quelles mesures les HSN arborés fournissent des ressources florales complémentaires à celles des HSN herbacés et soutiennent une diversité d’abeilles dans les paysages agricoles. Notre étude se base principalement sur plusieurs jeux de données récoltées entre 2013 et 2019 dans le site d’étude à long-terme des Vallées et Coteaux de Gascogne. Les résultats obtenus confirment l’importance de la présence d’HSN arborés dans les paysages agricoles pour soutenir une diversité d’abeilles sauvages. Nous mettons en évidence que, localement, les HSN arborés abritent des communautés d’abeilles riches, dont des espèces qui ne sont pas présentes dans les autres HSN. Ils fournissent des pollens préférentiellement consommés par les abeilles sauvages et permettent ainsi des interactions plantes-abeilles complémentaires à celles favorisées par les prairies. Nous montrons que les HSN arborés fournissent des ressources florales de manière stable dans le temps, et notamment à des périodes où les autres HSN et cultures ne fournissent pas de ressources florales, favorisant ainsi la présence d’espèces d’abeilles ayant une longue période de recherche de nourriture. Enfin, nous montrons que la présence d’HSN arborés dans le paysage a un effet positif sur la diversité et l’abondance d’abeilles en bordure de cultures. Nous mettons par ailleurs en lumière le rôle positif que jouent les prairies pour fournir des ressources florales aux abeilles sauvages et contrebalancer l’effet négatif de l’intensité des pratiques agricoles dans les cultures de céréales d’hiver. Cette thèse met ainsi en évidence la complémentarité entre les prairies et les HSN arborés, et montre que le maintien d'une quantité mais également d'une diversité d’HSN dans les paysages agricoles est nécessaire à la conservation d’une diversité d’abeilles sauvages dans les agroécosystèmes