Publications

2024

• Nanostructured S@VACNTs Cathode with Lithium Sulfate Barrier Layer for Exceptionally Stable Cycling in Lithium-Sulfur Batteries
  
  • Ezzedine Mariam
  • Jardali Fatme
  • Florea Ileana
  • Cojocaru Costel-Sorin
  Journal of The Electrochemical Society, Electrochemical Society / IOPscience, 2024, 171 (5), pp.050531. Lithium-sulfur technology garners significant interest due to sulfur’s higher specific capacity, cost-effectiveness, and environmentally friendly aspects. However, sulfur’s insulating nature and poor cycle life hinder practical application. To address this, a simple modification to the traditional sulfur electrode configuration is implemented, aiming to achieve high capacity, long cycle life, and rapid charge rates. Binder-free sulfur cathode materials are developed using vertically aligned carbon nanotubes (CNTs) decorated with sulfur and a lithium sulfate barrier layer. The aligned CNT framework provides high conductivity for electron transportation and short lithium-ion pathways. Simultaneously, the sulfate barrier layer significantly suppresses the shuttle of polysulfides. The S@VACNTs with Li 2 SO 4 coating exhibit an extremely stable reversible areal capacity of 0.9 mAh cm −2 after 1600 cycles at 1 C with a capacity retention of 80% after 1200 cycles, over three times higher than lithium iron phosphate cathodes cycled at the same rate. Considering safety concerns related to the formation of lithium dendrite, a full cell Si-Li-S is assembled, displaying good electrochemical performances for up to 100 cycles. The combination of advanced electrode architecture using 1D conductive scaffold with high-specific-capacity active material and the implementation of a novel strategy to suppress polysulfides drastically improves the stability and the performance of Li-S batteries. (10.1149/1945-7111/ad47d5)
  DOI : 10.1149/1945-7111/ad47d5
• A Symmetry-Aware Exploration of Bayesian Neural Network Posteriors
  
  • Laurent Olivier
  • Aldea Emanuel
  • Franchi Gianni
  , 2024. The distribution of modern deep neural networks (DNNs) weights - crucial for uncertainty quantification and robustness - is an eminently complex object due to its extremely high dimensionality. This paper proposes one of the first large-scale explorations of the posterior distribution of deep Bayesian Neural Networks (BNNs), expanding its study to real-world vision tasks and architectures. Specifically, we investigate the optimal approach for approximating the posterior, analyze the connection between posterior quality and uncertainty quantification, delve into the impact of modes on the posterior, and explore methods for visualizing the posterior. Moreover, we uncover weight-space symmetries as a critical aspect for understanding the posterior. To this extent, we develop an in-depth assessment of the impact of both permutation and scaling symmetries that tend to obfuscate the Bayesian posterior. While the first type of transformation is known for duplicating modes, we explore the relationship between the latter and L2 regularization, challenging previous misconceptions. Finally, to help the community improve our understanding of the Bayesian posterior, we release the \href{https://huggingface.co/datasets/torch-uncertainty/Checkpoints}{first large-scale checkpoint dataset}, including thousands of real-world models, along with our \href{https://github.com/ENSTA-U2IS-AI/torch-uncertainty}{codes}.
• Application of a generic path-following method to phase-field fracture
  
  • Loiseau Flavien
  • Lazarus Veronique
  , 2024. The phase-field approach to fracture has emerged as a powerful tool to simulate the nucleation and growth of cracks in a structure. In the past two decades, it has been extensively applied to fracture problems as it captures crack initiation, propagation, and interaction without explicitly tracking the crack path. One of the most popular algorithms to solve phase-field problems is alternate minimization. However, it can suffer from slow convergence, especially when dealing with unstable crack propagation. Moreover, force-controlled loading often leads to unstable crack propagation and the lack of equilibrium solution after the crack propagation, preventing their use. Path-following methods offer a promising solution to those limitations, enabling the tracking of unstable crack propagation while preserving the equilibrium during the whole loading (Rastiello et al., 2022). Based on various control strategies, these methods also improve the solver stability. Singh et al. (2016) and May et al. (2016) proposed path-following approaches specifically tailored to the resolution scheme of Miehe et al. (2010) based on crack surface and fracture dissipation. Additionally, Wu (2018) adapted the nodal displacement control (Borst, 1987) and the fracture surface control (Singh et al., 2016) to the alternate minimization. Nevertheless, the first approach is problem-dependent, and the second approach may fail under force loading (Rastiello et al., 2022). This work proposes a generic path-following method applicable to various fracture problems, regardless of geometry, boundary conditions, or fracture model complexity, by leveraging the maximum strain increment control (Chen & Schreyer, 1990). This method is model-independent, as it relies solely on the displacement field, and problem-independent, it does not rely on a specific choice of control DOF. After presenting the modified alternate minimization solver, we demonstrate its effectiveness through simulations of crack propagation in the SENT test. The results are compared to a semi-analytical solution based on LEFM and to the alternate minimization solution. Notably, the classic alternate minimization fails to capture the snap-back (instability under displacement control) observed in the semi-analytical method. The proposed approach correctly capturesthis phenomenon, which converges towards the semi-analytical solution. Then, this method is also applied to the simulation of Compact Tension (CT) experiments, in which the selection of numerical boundary conditions at the pinhole significantly influences the fracture behavior (Triclot et al., 2023). The proposed solver renders the application of force boundary conditions possible, better representing the experimental conditions. References Borst, R. de. (1987). Computation of post-bifurcation and post-failure behavior of strain-softening solids. Computers & Structures, 25(2), 211–224. https://doi.org/10.1016/0045-7949(87)90144-1 Chen, Z., & Schreyer, H. L. (1990). A numerical solution scheme for softening problems involving total strain control. Computers & Structures, 37 (6), 1043–1050. https://doi.org/10.1016/0045-7949(90)90016-U May, S., Vignollet, J., & Borst, R. de. (2016). A new arc-length control method based on the rates of the internal and the dissipated energy. Engineering Computations, 33(1), 100–115. https://doi.org/10.1108/EC-02-2015-0044 Miehe, C., Hofacker, M., & Welschinger, F. (2010). A phase field model for rate-independent crack propagation: Robust algorithmic implementation based on operator splits. Computer Methods in Applied Mechanics and Engineering, 199(45), 2765–2778. https://doi.org/10.1016/j.cma.2010.04.011 Rastiello, G., Oliveira, H. L., & Millard, A. (2022). Path-following methods for unstable structural responses induced by strain softening: A critical review. Comptes Rendus. Mécanique, 350, 205–236. https://doi.org/10.5802/crmeca.112 Singh, N., Verhoosel, C. V., Borst, R. de, & Brummelen, E. H. van. (2016). A fracture-controlled path-following technique for phase-field modeling of brittle fracture. Finite Elements in Analysis and Design, 113, 14–29. https://doi.org/10.1016/j.finel.2015.12.005 Triclot, J., Corre, T., Gravouil, A., & Lazarus, V. (2023). Key role of boundary conditions for the 2D modeling of crack propagation in linear elastic compact tension tests. Engineering Fracture Mechanics, 277, 109012. https://doi.org/10.1016/j.engfracmech.2022.109012 Wu, J.-Y. (2018). Robust numerical implementation of non-standard phase-field damage models for failure in solids. Computer Methods in Applied Mechanics and Engineering, 340, 767–797. https://doi.org/10.1016/j.cma.2018.06.007
• Magnetospheric Time‐History in Storm‐Time Magnetic Flux Dynamics: A Global Simulation Campaign
  
  • Atilaw T.
  • Akhavan-Tafti M.
  • Al Shidi Q.
  • Pulkkinen T.
  • Fontaine D.
  • Le Contel Olivier
  • Slavin J.
  • Le G.
  • Chen L.‐j.
  • Reiff P.
  • Alqeeq S.
  Journal of Geophysical Research Space Physics, American Geophysical Union/Wiley, 2024, 129 (5). Abstract This paper aims to quantify the magnetospheric magnetic flux contents under moderate to intense space weather conditions using global simulations. This study is a companion to Akhavan‐Tafti, Atilaw, et al. (2023, https://doi.org/10.1029/2023JA031832 ) where magnetic flux evolution is presented for a catalog of storm events, using Heliophysics System Observatory (HSO) observations. For this study, we used the Space Weather Modeling Framework (SWMF) in Geospace configuration to study magnetic flux dynamics for a subset of their storm events (15 events). Simulations reliably resolve the storm‐time magnetic flux Bz and current density |J| asymmetries across the different storm phases. It is revealed that: relative to the quiet period, flux content is enhanced during the storm sudden commencement (SSC) phase in the dayside by ΔBz/Bz, quiet = +17%, and reduced in the nightside magnetosphere (r[R E ] < −6 R E ) by −15%. At the same time, the cross‐tail current is found to enhance (|J| = 2 nA/m 2 ), which suggests the storm impact in the nightside magnetosphere is much earlier in the storm cycle than previously shown. Concurring with previous studies, a significant depletion of magnetic flux by up to −40%, with day‐night and dawn‐dusk asymmetries, can be seen during the main and recovery phases. This corresponds to the enhanced current density (|J| = 5–8 nA/m 2 ) at ∼6 R E further confirming the role of ring current in driving magnetospheric dynamics during the main and recovery phases. This is in contrast with the SSC phase wherein the Chapman‐Ferraro and cross‐tail currents are the dominant current systems. (10.1029/2023JA031997)
  DOI : 10.1029/2023JA031997
• Nitrogen atoms ps-TALIF in atmospheric pressure nanosecond volume DBD plasma
  
  • Kreyder G
  • Stefas D
  • Invernizzi L
  • Lombardi G
  • Gazeli K
  • Prasanna S
  • Starikovskaia S M
  , 2024.
• Transient gradient nanosecond plasmas for the initiation of a detonation wave: plasma characterisation with OES and O-TALIF
  
  • Lafaurie V
  • Shu Z
  • Sadauskaite M
  • Vidal Pierre
  • Starikovkaia S M
  , 2024.
• Path differences between quasistatic and fatigue cracks in anisotropic media
  
  • Zhai Xinyuan
  • Corre Thomas
  • Mesgarnejad Ataollah
  • Karma Alain
  • Lazarus Véronique
  Physical Review E, American Physical Society (APS), 2024, 110 (6), pp.L063001. The propagation path of quasistatic cracks under monotonic loading is known to be stronglyinfluenced by the anisotropy of the fracture energy in crystalline solids or engineered materials witha regular microstructure. Such cracks generally follow directions close to minima of the fractureenergy. Here we demonstrate both experimentally and computationally that fatigue cracks undercyclic loading follow dramatically different paths that are predominantly dictated by the symmetryof the loading with the microstructure playing a negligible or subdominant role. (10.1103/PhysRevE.110.L063001)
  DOI : 10.1103/PhysRevE.110.L063001
• Online Locality Meets Distributed Quantum Computing
  
  • Akbari Amirreza
  • Coiteux-Roy Xavier
  • d'Amore Francesco
  • Le Gall François
  • Lievonen Henrik
  • Melnyk Darya
  • Modanese Augusto
  • Pai Shreyas
  • Renou Marc-Olivier
  • Rozhoň Václav
  • Suomela Jukka
  , 2024. We extend the theory of locally checkable labeling problems (LCLs) from the classical LOCAL model to a number of other models that have been studied recently, including the quantum-LOCAL model, finitely-dependent processes, non-signaling model, dynamic-LOCAL model, and online-LOCAL model [e.g. STOC 2024, ICALP 2023]. First, we demonstrate the advantage that finitely-dependent processes have over the classical LOCAL model. We show that all LCL problems solvable with locality $O(\log^\star n)$ in the LOCAL model admit a finitely-dependent distribution (with constant locality). In particular, this gives a finitely-dependent coloring for regular trees, answering an open question by Holroyd [2023]. This also introduces a new formal barrier for understanding the distributed quantum advantage: it is not possible to exclude quantum advantage for any LCL in the $\Theta(\log^\star n)$ complexity class by using non-signaling arguments. Second, we put limits on the capabilities of all of these models. To this end, we introduce a model called randomized online-LOCAL, which is strong enough to simulate e.g. SLOCAL and dynamic-LOCAL, and we show that it is also strong enough to simulate any non-signaling distribution and hence any quantum-LOCAL algorithm. We prove the following result for rooted trees: if we can solve an LCL problem with locality $o(\log \log n)$ in the randomized online-LOCAL model, we can solve it with locality $O(\log^\star n)$ in the classical deterministic LOCAL model. Put together, these results show that in rooted trees the set of LCLs that can be solved with locality $O(\log^\star n)$ is the same across all these models: classical deterministic and randomized LOCAL, quantum-LOCAL, non-signaling model, dynamic-LOCAL, and deterministic and randomized online-LOCAL.
• High harmonic generation in Solids driven by high energy fiber laser source
  
  • Boukhaoui Djamila
  • Idlahcen Saïd
  • Houard Jonathan
  • Blum Ivan
  • Godin Thomas
  • Amrani Foued
  • Gérôme Frédéric
  • Benabid Fetah
  • Gauthier David
  • Boutu Willem
  • Merdji Hamed
  • Hideur Ammar
  • Vella Angela
  , 2024, pp.1299208. We present findings on High Harmonic Generation (HHG) in solids utilizing a high-energy fiber laser system operating at 1550 nm. The driving laser source comprises an Erbium-Doped Fiber chirped pulse Amplifier (EDFA) combined with a post-compression stage employing a hollow-core photonic crystal fiber (HC-PCF) filled with noble gases. Nonlinear self-compression in the HC-PCF enables the generation of ultrashort pulses with a duration of 50 fs and energy of 0.91 μJ at a repetition rate of 660 kHz. In a first step, harmonics up to H7 were observed when focusing the laser into small bandgap materials such as Zinc Oxide (ZnO). Subsequently, the system was enhanced to measure high harmonics in the extreme ultraviolet (XUV) range, with harmonics up to H25 observed using a large bandgap material, magnesium oxide (MgO). To the best of our knowledge, this represents the first solid-state HHG source driven by a high-energy few-cycle fiber laser in the telecom region
• Maîtriser l'art de la narration académique À propos de How to use storytelling in your academic writings de Timothy Pollock
  
  • Colin Laure
  Le Libellio d'AEGIS, Libellio d'AEGIS, 2024, 20 (1), pp.37-42. <div><p>L e travail des chercheurs vise à identifier et réfléchir à des questions jusqu'alors non résolues, à trouver des moyens de les résoudre et d'y répondre. La dernière étape de ce travail, et non des moindres, consiste à narrer l'histoire. Le récit ponctue chaque article, chaque chapitre de livre, et même chaque présentation ou cours que nous dispensons. En effet, les données ne parlent pas d'elles-mêmes : nous devons les inscrire dans un contexte, nous assurer que le lecteur en comprend et accepte l'importance, tant de la question de recherche que des cadres conceptuels mobilisés, tout en interprétant les implications des résultats de manière à donner une signification aux données. Comment éviter, cependant, l'écueil d'écrire une histoire peu engageante ? L'art du récit est souvent négligé lors des séminaires de recherche ou formations doctorales. Pourtant, posséder des compétences avérées en écriture et maîtriser les outils narratifs sont des critères essentiels pour exposer ses idées avec succès. Les articles clairement rédigés sont non seulement les plus cités, mais ils ont également le potentiel de devenir des travaux à fort impact. Timothy G. Pollock, l'auteur de cet ouvrage et ancien rédacteur en chef de l' Academy of Management Journal, a pu constater comment des écrits mal rédigés pouvaient entraver des idées prometteuses, les empêchant d'atteindre le public après avoir échoué au processus de révision. L'objectif de ce livre est donc d'accompagner le lecteur, qu'il soit doctorant ou chercheur plus expérimenté, afin d'améliorer ses chances que ses idées soient non seulement lues, mais également pleinement comprises.</p><p>Dans cet ouvrage, l'auteur explique comment les chercheurs utilisent la structure et les outils du récit pour rendre l'écriture académique plus accessible, influente et facile à lire, sans pour autant simplifier ni réduire la rigueur de l'argumentation. En reprenant la structure classique d'un article de recherche, il montre qu'il est possible d'être créatif tout en respectant les normes attendues pour la publication. En ce sens, les chapitres 2 à 4 mettent en lumière l'ossature des articles, en mettant en avant tous les outils permettant à chacun d'intégrer le récit dans ses articles. Ensuite, les chapitres 5 à 8 se penchent respectivement sur l'écriture de l'introduction, de la revue</p></div>
• Encoding the Latent Posterior of Bayesian Neural Networks for Uncertainty Quantification
  
  • Franchi Gianni
  • Bursuc Andrei
  • Aldea Emanuel
  • Dubuisson Séverine
  • Bloch Isabelle
  IEEE Transactions on Pattern Analysis and Machine Intelligence, Institute of Electrical and Electronics Engineers, 2024, 46 (4), pp.2027-2040. Bayesian Neural Networks (BNNs) have long been considered an ideal, yet unscalable solution for improving the robustness and the predictive uncertainty of deep neural networks. While they could capture more accurately the posterior distribution of the network parameters, most BNN approaches are either limited to small networks or rely on constraining assumptions, e.g., parameter independence. These drawbacks have enabled prominence of simple, but computationally heavy approaches such as Deep Ensembles, whose training and testing costs increase linearly with the number of networks. In this work we aim for efficient deep BNNs amenable to complex computer vision architectures, e.g., ResNet-50 DeepLabv3+, and tasks, e.g., semantic segmentation and image classification, with fewer assumptions on the parameters. We achieve this by leveraging variational autoencoders (VAEs) to learn the interaction and the latent distribution of the parameters at each network layer. Our approach, called Latent-Posterior BNN (LP-BNN), is compatible with the recent BatchEnsemble method, leading to highly efficient (in terms of computation and memory during both training and testing) ensembles. LP-BNNs attain competitive results across multiple metrics in several challenging benchmarks for image classification, semantic segmentation, and out-of-distribution detection. (10.1109/TPAMI.2023.3328829)
  DOI : 10.1109/TPAMI.2023.3328829
• Unified two-scale Eulerian multi-fluid modeling of separated and dispersed two-phase flows
  
  • Loison Arthur
  , 2024. Liquid-gas two-phase flows are present in numerous industrial applications such as aerospace propulsion, nuclear hydraulics or bubble column reactors in the chemical industry.The simulation of such flows is of primary interest for their understanding and optimization.However, the dynamics of the interface separating the gas from the liquid can present a multiscale dynamics and thus makes simulations of industrial processes computationally too expensive.Some modelling efforts have been conducted on the development of cheaper multi-fluid models adapted to particular interface dynamics regime, e.g. in the separated regime where the fluids are separated by a single smooth surface or in the disperse regime where there are inclusions of one fluid carried by the other.Attempts of coupling between these models have showed some progress to simulate multiscale flows like atomization, but usually have physical or mathematical drawbacks.This thesis then pursues the goal of proposing a unified two-scale modelling framework with appropriate numerical methods adapted to this multiscale interface dynamics which goes from a separated to a disperse regime.The main contributions related to this modelling effort are :1- The combination of compressible multi-fluid models of the literature adapted to either the separated or the disperse regime into a unified two-scale multi-fluid model relying on Hamilton’s Stationary Action Principle;2- The local coupling of the models with an inter-scale mass transfer both regularizing the large-scale inter face and modelling mixed regime phenomena such as in primary break-up;3- Enhancing the small-scale models for the disperse regimes by adding the dynamics of geometrical quantities for oscillating droplets and pulsating bubbles, built as moments of a kinetic description.From the numerical perspective, finite-volume schemes and relaxation methods are used to solve the system of conservative laws of the models.Eventually, simulations with the open-source finite solver Josiepy demonstrates the regularization properties of the model on a set of well-chosen numerical setups leading to multi-scale interface dynamics. (10.70675/d226f1ffz5109z4052z8990z5fa063db66ed)
  DOI : 10.70675/d226f1ffz5109z4052z8990z5fa063db66ed
• Gradient pulsed transient plasma for initiation of detonation
  
  • Lafaurie Victor
  • Shu Zhan
  • Vidal Pierre
  • Starikovskaia Svetlana
  Combustion and Flame, Elsevier, 2024, 261, pp.113311. The formation of a gradient of atomic oxygen is demonstrated by means of a nanosecond non-equilibrium plasma for a varying gap size plane-to-plane electrode. Using a flat high-voltage electrode in front of a rounded triangle a&nbsp;2.9 to 5&nbsp;cm gap is formed over a 9.8&nbsp;cm span. ICCD imaging determined an adequate ground electrode shape and slope to create a gradient. The plasma is formed by three consecutive high voltage pulses of&nbsp;-30, -40 and -50&nbsp;kV in 100&nbsp;mbar of air. O-TALIF measurements confirm that atomic oxygen production changed with gap size within the same plasma. This setup will be used to test detonation initiation by Zel’dovich gradient mechanisms in stoichiometric H₂:O₂ mixtures. Novelty and Significance: A novel configuration of a nanosecond non-equilibrium discharge was developed to create a controllable gradient of atomic oxygen. This was achieved by using varying gap plane-to-plane electrodes to generate an electric field of varying strength along the length of the gap. This setup will be tested in combustible mixtures to initiate a detonation wave using a gradient mechanism of Zel’dovich. (10.1016/j.combustflame.2024.113311)
  DOI : 10.1016/j.combustflame.2024.113311
• La création de l’Agence de l’innovation de défense : une innovation institutionnelle
  
  • Colin Laure
  • Dumez Hervé
  Gérer et Comprendre. Annales des Mines, Eska, 2024, 1 (155), pp.24-34. L’Agence de l’innovation de défense (AID) est créée en 2018. Il s’agit d’une innovation institutionnelle dans un domaine, celui de la défense, structuré et stable. L’agence créée a pour objectif de regrouper au sein d’une même structure les anciens dispositifs de gestion épars au sein du ministère en matière d’innovation, ainsi que les nouveaux dispositifs d’innovation « ouverte ». Ces derniers visent à capturer et exploiter rapidement les innovations issues d’acteurs du domaine civil. Comment analyser le processus ayant conduit à une telle innovation ? Dans cet article, nous nous proposons, à partir d’une narration de la création de l’AID et d’une série d’entretiens menés auprès d’acteurs du milieu de la défense, de montrer, d’une part, que cette innovation est le résultat d’une cristallisation, et, d’autre part, de mettre en évidence trois dilemmes, en écho au titre du livre de Christensen (Christensen, 1997), propres à une innovation institutionnelle : celui de la création subtile ou restructurante ; celui de l’adaptation ou de la création ; celui, enfin, de l’attèlement. L’étude de cas permet d’enrichir la théorie de l’innovation institutionnelle formulée par Van de Ven et Hargrave (Van de Ven et Hargrave, 2004 ; Hargrave et Van de Ven, 2006), qui n’identifiait qu’un de ces dilemmes (adaptation versus création) repensé à partir de la notion d’innovation subtile.
• Study of the breathing mode development in Hall thrusters using hybrid simulations
  
  • Petronio Federico
  • Alvarez Laguna Alejandro
  • Bourdon Anne
  • Chabert Pascal
  Journal of Applied Physics, American Institute of Physics, 2024, 135 (7). We use a 2.5D hybrid simulation to study the breathing mode (BM) dynamics in Hall thrusters (HTs). This involves a 1D Euler fluid simulation for neutral dynamics in the axial direction, coupled with a 2D axial–azimuthal Particle-in-Cell (PIC) simulation for charged species. The simulation also includes an out-of-plane virtual dimension for wall losses. This setup allows us to replicate the BM’s macroscopic features observed in experiments. A comprehensive analysis of plasma parameters in BM’s phases divides it into two growth and two decay sub-phases. Examining 1D axial profiles of electron temperature, gas and plasma densities, and particle creation rate shows that an increase in electron temperature alone cannot sustain ionization. Ionization seems to be influenced by the spatial correlation between electron and gas densities and the ionization rate coefficient. Investigating ion back-flow reveals its impact on modulating neutral flux entering the ionization region. The hybrid simulation’s outcomes let us assess the usual 0D predator–prey model’s validity and identify its limitations. The ionization and ion convection term approximations hold, but the gas convective term approximation does not. Introducing an alternative gas convective term approximation involving constant density ejection from the ionization region constructs an unstable BM model consistent with simulation results. In addition, this paper explores how varying the imposed voltage and mass flow rate impacts the BM. The BM frequency increases with imposed voltage, aligning with theoretical predictions. The mass flow rate variation has a limited effect on BM frequency, following the theoretical model’s trend. (10.1063/5.0188859)
  DOI : 10.1063/5.0188859
• Pinning of crack fronts by hard and soft inclusions: A phase field study
  
  • Henry Hervé
  Physical Review E, American Physical Society (APS), 2024, 109 (2), pp.025002. Through tridimensonal numerical simulations of cracks propagating in material with an elastic moduli heterogeneity, it is shown that the presence of a simple inclusion can dramatically affect the propagation of the crack. Both the presence of soft and hard inclusions can lead to the arrest of a crack front. Here the mechanism leading to the arrest of the crack are described and shown to depend on the nature of the inclusion. This is also the case in regimes where the presence of the inclusion leads to a slowdown of the crack. (10.1103/PhysRevE.109.025002)
  DOI : 10.1103/PhysRevE.109.025002
• A coupled VOF/embedded boundary method to model two-phase flows on arbitrary solid surfaces
  
  • Tavares Mathilde
  • Josserand Christophe
  • Limare Alexandre
  • Lopez-Herrera José-Maria
  • Popinet Stéphane
  Computers and Fluids, Elsevier, 2024. We present an hybrid VOF/embedded boundary method allowing to model two-phase flows in presence of solids with arbitrary shapes. The method relies on the coupling of existing methods: a geometric Volume of fluid (VOF) method to tackle the two-phase flow and an embedded boundary method to sharply resolve arbitrary solid geometries. Coupling these approaches consistently is not trivial and we present in detail a quad/octree spatial discretization for solving the corresponding partial differential equations. Modelling contact angle dynamics is a complex physical and numerical problem. We present a Navier-slip boundary condition compatible with the present cut cell method, validated through a Taylor-Couette test case. To impose the boundary condition when the fluid-fluid interface intersects a solid surface, a geometrical contact angle approach is developed. Our method is validated for several test cases including the spreading of a droplet on a cylinder, and the equilibrium shape of a droplet on a flat or tilted plane in 2D and 3D. The temporal evolution and convergence of the droplet spreading on a flat plane is also discussed for the moving contact line given the boundary condition (Dirichlet or Navier) used. The ability of our numerical methodology to resolve contact line statics and dynamics for different solid geometries is thus demonstrated.
• PHARE: Parallel hybrid particle-in-cell code with patch-based adaptive mesh refinement
  
  • Aunai Nicolas
  • Smets Roch
  • Ciardi Andrea
  • Deegan Philip
  • Jeandet Alexis
  • Payet Thibault
  • Guyot Nathan
  • Darrieumerlou Loic
  Computer Physics Communications, Elsevier, 2024, 295, pp.108966. Modeling multi-scale collisionless magnetized processes constitutes an important numerical challenge. By treating electrons as a fluid and ions kinetically, the so-called hybrid Particle-In-Cell (PIC) codes represent a promising intermediary between fully kinetic codes, limited to model small scales and short durations, and magnetohydrodynamic codes used large scale. However, simulating processes at scales significantly larger than typical ion particle dynamics while resolving sub-ion dissipative current sheets remain extremely difficult. This paper presents a new hybrid PIC code with patch-based adaptive mesh refinement. Here, hybrid PIC equations are solved on a hierarchy of an arbitrary number of Cartesian meshes of incrementally finer resolution dynamically mapping regions of interest, and with a refined time stepping. This paper presents how the hybrid PIC algorithm is adapted to evolve such mesh hierarchy and the validation of the code on a uniform mesh, fixed refined mesh and dynamically refined mesh. (10.1016/j.cpc.2023.108966)
  DOI : 10.1016/j.cpc.2023.108966
• Acceleration of an interplanetary shock through the magnetosheath: a global hybrid simulation
  
  • Moissard C.
  • Savoini P.
  • Fontaine D.
  • Modolo Ronan
  Frontiers in Astronomy and Space Sciences, Frontiers Media, 2024, 11, pp.1330397. According to most observations and simulations, interplanetary shocks slow down when they propagate through the magnetosheath. In this article, we present results from a self-consistent global hybrid PIC simulation of an interplanetary shock which, by contrast, accelerates as it propagates through the magnetosheath. In this simulation, the solar wind upstream of the interplanetary shock is set up with an Alfvén Mach number M A = 4.5 and the interplanetary magnetic field (IMF) is set up to be almost parallel to the y direction in GSE coordinate system. The ‘planet’ is modelled as a magnetic dipole with no tilt: the dipole is in the GSE’s z direction. In the ecliptic plane (Oxy), which contains the interplanetary magnetic field (IMF), the magnetic field lines are piling up against the magnetopause, and the velocity of the interplanetary shock decreases from 779 ± 48 km/s in the solar wind down to 607 ± 48 km/s in the magnetosheath. By contrast, in the noon-meridian plane (Oxz), which is perpendicular to the IMF, the velocity of the interplanetary shock in the magnetosheath can reach values up to 904 ± 48 km/s. This study suggests that interplanetary shocks can accelerate as they propagate through the magnetosheath. This finding, reported here for the first time, could have important implications for space weather, as it corresponds to the case where an interplanetary shock catches up with a low Alfvén Mach number solar transient such as an interplanetary coronal mass ejection. (10.3389/fspas.2024.1330397)
  DOI : 10.3389/fspas.2024.1330397
• Abundance of G-Quadruplex Forming Sequences in the Hepatitis Delta Virus Genomes
  
  • Brázda Václav
  • Valková Natália
  • Dobrovolná Michaela
  • Mergny Jean-Louis
  ACS Omega, ACS Publications, 2024, 9, pp.4096-4101. Hepatitis Delta virus (HDV) is a unique and highly unusual RNA satellite virus that depends on the presence of the hepatitis B virus (HBV) to be infectious. Its single-stranded RNA genome is very compact and variable, consisting of eight major genotypes distributed unequally on various continents. The significance of noncanonical secondary structures in DNA or RNA, such as G-quadruplexes (G4s), is becoming more evident especially for transcription, replication, and translation. G4s are formed from guanine-rich sequences and have been found in most eukaryotic and prokaryotic genomes, as well as viruses. In this study, we analyzed the G-quadruplex propensity of HDV genomes using G4Hunter. In contrast to the HBV virus, which has a G4 density similar to that of the human genome, the HDV virus possesses a significantly higher number of potential quadruplex-forming sequences (PQS) with a density more than four times higher than that of the human genome. This observation suggests a crucial role for G-quadruplexes in HDV, particularly because the tracks with the potential to form G-quadruplexes are well conserved. Furthermore, the high prevalence of the G-quadruplex-forming sequence could represent a promising therapeutic target to control HDV replication. (10.1021/acsomega.3c09288)
  DOI : 10.1021/acsomega.3c09288
• InfraParis: A multi-modal and multi-task autonomous driving dataset
  
  • Franchi Gianni
  • Hariat Marwane
  • Yu Xuanlong
  • Belkhir Nacim
  • Manzanera Antoine
  • Filliat David
  , 2024. Current deep neural networks (DNNs) for autonomous driving computer vision are typically trained on specific datasets that only involve a single type of data and urban scenes. Consequently, these models struggle to handle new objects, noise, nighttime conditions, and diverse scenarios, which is essential for safety-critical applications. Despite ongoing efforts to enhance the resilience of computer vision DNNs, progress has been sluggish, partly due to the absence of benchmarks featuring multiple modalities. We introduce a novel and versatile dataset named InfraParis that supports multiple tasks across three modalities: RGB, depth, and infrared. We assess various state-of-the-art baseline techniques, encompassing models for the tasks of semantic segmentation, object detection, and depth estimation. More visualizations and the download link for InfraParis are available at https://ensta-u2is.github.io/infraParis (10.1109/WACV57701.2024.00295)
  DOI : 10.1109/WACV57701.2024.00295
• Learning to Generate Training Datasets for Robust Semantic Segmentation
  
  • Hariat Marwane
  • Laurent Olivier
  • Kazmierczak Rémi
  • Bursuc Andrei
  • Yao Angela
  • Franchi Gianni
  , 2024. Semantic segmentation techniques have shown significant progress in recent years, but their robustness to real-world perturbations and data samples not seen during training remains a challenge, particularly in safety-critical applications. In this paper, we propose a novel approach to improve the robustness of semantic segmentation techniques by leveraging the synergy between label-to-image generators and image-to-label segmentation models. Specifically, we design and train Robusta, a novel robust conditional generative adversarial network to generate realistic and plausible perturbed or outlier images that can be used to train reliable segmentation models. We conduct in-depth studies of the proposed generative model, assess the performance and robustness of the downstream segmentation network, and demonstrate that our approach can significantly enhance the robustness of semantic segmentation techniques in the face of real-world perturbations, distribution shifts, and out-of-distribution samples. Our results suggest that this approach could be valuable in safety-critical applications, where the reliability of semantic segmentation techniques is of utmost importance and comes with a limited computational budget in inference. We will release our code shortly. (10.1109/wacv57701.2024.00385)
  DOI : 10.1109/wacv57701.2024.00385
• Analytical model of a Hall thruster
  
  • Lafleur Trevor
  • Chabert Pascal
  Physics of Plasmas, American Institute of Physics, 2024, 31 (9), pp.093507. Hall thrusters are one of the most successful and prevalent electric propulsion systems for spacecraft in use today. However, they are also complex devices and their unique E×B configuration makes modeling of the underlying plasma discharge challenging. In this work, a steady-state model of a Hall thruster is developed and a complete analytical solution presented that is shown to be in reasonable agreement with experimental measurements. A characterization of the discharge shows that the peak plasma density and ionization rate nearly coincide and both occur upstream of the peak electric field. The peak locations also shift as the thruster operating conditions are varied. Three key similarity parameters emerge that govern the plasma discharge and which are connected via a thruster current–voltage relation: a normalized discharge current, a normalized discharge voltage, and an amalgamated parameter, α¯, that contains all system geometric and magnetic field information. For a given normalized discharge voltage, the similarity parameter α¯ must lie within a certain range to enable high thruster performance. When applied to a krypton thruster, the model shows that both the propellant mass flow rate and the magnetic field strength must be simultaneously adjusted to achieve similar efficiency to a xenon thruster (for the same thruster geometry, discharge voltage, and power level). (10.1063/5.0220130)
  DOI : 10.1063/5.0220130
• Long-term performance of innovation systems in East Asian developing countries: A structural analysis
  
  • Lebert Didier
  , 2024. To measure the performance of East Asian developing countries’ innovation systems, we adopt a product space approach by considering the import and export flows linking each country to its trading partners over the long term (1980-2018). We apply economic dominance theory to construct and analyze this space. Using an extension of Pavitt’s (1984) taxonomy proposed by Castellacci (2008), we classify manufactured goods into categories distinguished by the technological intensity required for their development, production, and diffusion. We propose indices of economic complexity to position countries in relation to each other, to identify significant developments, and to infer the capacity of a national innovation system to deepen a country’s insertion into the global value chain based on the mastery of complex technologies. In this way, we identify three groups of countries: those whose current innovation system favors moving up the value chain, those caught in the middle-income trap and struggling to renew their innovation system, and those without a clearly structured innovation system.
• Construction of transparent conditions for electromagnetic waveguides
  
  • Bonnet-Ben Dhia Anne-Sophie
  • Chesnel Lucas
  • Fliss Sonia
  • Parigaux Aurélien
  , 2024. We are interested in the numerical resolution of diffraction problems in closed electromagnetic waveguides by means of finite elements methods. To proceed, we need to truncate the domain and design adapted transparent conditions on the artificial boundary to avoid spurious reflections. When the guide is homogeneous in the transverse section, this can be done by writing an Electric-to-Magnetic condition based on a modal decomposition of the field. The latter takes a rather simple form thanks to the orthogonality of transverse modes. For guides that are heterogeneous in the transverse section, the transverse modes are no longer orthogonal but satisfy bi-orthogonality relations linked to the Poynting energy flux. Modal decompositions are more delicate to derive and it may happen that certain modes have phase and group velocities of different sign, which prevents the use of Perfectly Matched Layers. Adapting techniques already developed in elasticity, we derive a new transparent condition based on a Poynting-to-Magnetic operator with overlap. To illustrate the method, we present numerical results obtained with Nédélec finite elements using the XLiFE++ library.