Publications

2024

• ω-regular energy problems
  
  • Dziadek Sven
  • Fahrenberg Uli
  • Schlehuber Philipp
  Formal Aspects of Computing, Springer Verlag, 2024, 37 (1), pp.1-30. We show how to efficiently solve problems involving a quantitative measure, here called energy, as well as a qualitative acceptance condition, expressed as a Büchi or Parity objective, in finite weighted automata and in one-clock weighted timed automata. Solving the former problem and extracting the corresponding witness is our main contribution and is handled by a modified version of the Bellman-Ford algorithm interleaved with Couvreur’s algorithm. The latter problem is handled via a reduction to the former relying on the corner-point abstraction. All our algorithms are freely available and implemented in a tool based on the open-source platforms TChecker and Spot. (10.1145/3678265)
  DOI : 10.1145/3678265
• Development and analysis of efficient multi-scale numerical methods, with applications to plasma discharge simulations relying on multi-fluid models
  
  • Reboul Louis
  , 2024. Our main focus is the design and analysis of multi-scale numerical schemes for the simulation of multi-fluid models applied to low-temperature low-pressure plasmas. Our typical configuration of interest includes the onset of instabilities and sheaths, i.e. micrometric charged boundary layers that form at the plasma chamber walls. Our prototypical plasma model is the isothermal Euler-Poisson system of equations, but we also consider simpler models, the hyperbolic heat equations and the isothermal Euler-friction Equations, for the development and analysis of numerical methods. In a first axis, we develop and analyze a uniformly asymptotic-preserving second-order time-space coupling implicit-explicit method for the hyperbolic heat equations (linear case). We provide theoretical results on flux limiters for asymptotic-preserving methods, and a new well-balanced strategy. In a second axis, we propose several methods for the Euler-Poisson system of equations, to improve the accuracy of simulations of configurations featuring sheaths. In a third axis, we use these methods to conduct a parametric study of a 2D (rectangular) isothermal non-magnetized plasma discharge with sheaths, at various collisional regimes and aspect-ratios. We compare our result to PIC simulations and reference solutions. We show that simulating a fluid model with a tailored numerical method substantially reduces the time of simulation and improves the accuracy of the obtained solution. A discussion on the extensions of the multi-scale methods for the full non-isothermal Euler equations and to highly-magnetized cases is provided in the perspectives of our work. (10.70675/860f553az7ef2z4bb0z9118ze834fdcc9368)
  DOI : 10.70675/860f553az7ef2z4bb0z9118ze834fdcc9368
• Freezing receding contact lines
  
  • Grivet Rodolphe
  • Huerre Axel
  • Séon Thomas
  • Duchemin Laurent
  • Josserand Christophe
  , 2024. We investigate experimentally the receding of a contact line when a Peltier module is pulled out of a water bath at constant speed, while a ice layer is also growing at constant speed on the Peltier module. A steady regime is therefore reached for all the parameters used in this studied, corresponding to a dynamical stationnary meniscus. We show that the height of the meniscus provides most of the properties of the flow. For high pulling rate, it is related to the amount of liquid of the equivalent Landau-Levich (LL) film that would be extracted from the bath, which is eventually freezing as the plate is lifted upward. For smaller velocity, so that no LL film would be formed without freezing, the meniscus height is directly linked to the contact angle of water on ice in these conditions. Solving numerically the meniscus equation taking into account the solidifcation of water, our results suggest that the contact angle of water on ice should be around $6^\circ$. (10.48550/arXiv.2409.00385)
  DOI : 10.48550/arXiv.2409.00385
• ARCH-COMP24 Category Report: Continuous and Hybrid Systems with Nonlinear Dynamics
  
  • Geretti Luca
  • Dit Sandretto Julien Alexandre
  • Althoff Matthias
  • Benet Luis
  • Collins Pieter
  • Forets Marcelo
  • Mitsch Stefan
  • Tillet Joris
  • Schilling Christian
  • Wetzlinger Mark
  , 2024, pp.39-13. We present the results of a friendly competition for formal verification of continuous and hybrid systems with nonlinear continuous dynamics. The friendly competition took place as part of the workshop Applied Verification for Continuous and Hybrid Systems (ARCH) in 2024. This year, 5 tools participated: Ariadne, CORA, DynIbex, JuliaReach and KeYmaera X (in alphabetic order). These tools are applied to solve reachability analysis problems on six benchmark problems, two of them featuring hybrid dynamics. We do not rank the tools based on the results, but show the current status and discover the potential advantages of different tools. (10.29007/21ch)
  DOI : 10.29007/21ch
• Role of melting and solidification in the spreading of an impacting water drop
  
  • Sarlin Wladimir
  • Grivet Rodolphe
  • Xu Julien
  • Huerre Axel
  • Séon Thomas
  • Josserand Christophe
  Journal of Fluid Mechanics, Cambridge University Press (CUP), 2024, 996, pp.A14. The present study reports experiments of water droplet impacting on ice or on a cold metallic substrate, with the aim of understanding the effect of phase change on the impingement process. Both liquid and substrate temperatures are varied, as well as the height of fall. The dimensionless maximum spreading diameter, $\beta_m$, is found to increase with both temperatures as well as with the impact velocity. Furthermore, $\beta_m$ is reduced when solidification, which enhances dissipation, is present, whereas fusion favours the liquid film spreading. These observations are rationalized by extending an existing model of effective viscosity, in which phase change alters the size and shape of the developing viscous boundary layer, thereby modifying the value of $\beta_m$. The use of this correction allows to adapt a scaling law existing for isothermal drop impacts to propose a universal law giving the maximum diameter of an impacting water droplet in the presence of melting or solidification. (10.1017/jfm.2024.817)
  DOI : 10.1017/jfm.2024.817
• Guaranteed Satisfaction of a Temporal Logic Formula with a Confidence Parameter
  
  • Tillet Joris
  • Vanneaux Elena
  • Alexandre Dit Sandretto Julien
  , 2024.
• Plasma-enhanced detonability: experimental and calculated reduction of the detonation cell size
  
  • Cherif Mhedine Ali
  • Masuda Ryu
  • Claverie Alain
  • Vidal Pierre
  • Starikovskaia Svetlana
  Combustion and Flame, Elsevier, 2024, 268, pp.113639. This work analyzes the interaction between non-equilibrium plasma and detonation. The aim is to enhance the detonability of gaseous mixtures by reducing the detonation cell width through dissociation of a fresh gas mixture by plasma action. The experiments were performed in a square-section detonation tube, and the diagnostic tools used were ICCD chemiluminescence imaging, soot-plate recording, dynamic pressure sensors, and back current shunt technique. The results show that the application of a nanosecond plasma ahead of a self-sustained detonation reduces the cell width by a factor of about 2 in H₂:O₂:Ar, H₂:O₂, CH₄:H₂:O₂:Ar and CH₄:O₂:Ar mixtures for initial pressures between 100 and 200 mbar. A parametric study of plasma properties focused on the effect of the initial pressure on the deposited energy and homogeneity. A kinetic mechanism was proposed to estimate the dissociation effect of plasma chemistry on the fresh combustible mixture. The obtained densities of atoms produced in the plasma were used as input parameters to calculate the thermicity and temperature profiles of the detonation reaction zone according to the Zel’dovich-von Neumann-D¨oring model. The reduction factor of the ZND characteristic chemical length is about the same as the experimental cell widths, i.e. 2. This combination of experiments and calculations substantiates the relationship between plasma parameters, ZND chemical lengths, and detonation cell widths and, thus, demonstrates the possibility of controlling detonability using a nanosecond discharge. (10.1016/j.combustflame.2024.113639)
  DOI : 10.1016/j.combustflame.2024.113639
• Frequency-Fresnel Domain Equalization for Underwater Acoustic OCDM Systems
  
  • Arbi Tarak
  • Geller Benoit
  , 2024. <div><p>Orthogonal Chirp Division Multiplexing (OCDM) is a promising multi-carrier modulation for Underwater Acoustic (UWA) systems, as several low complexity OCDM receivers capable of exploiting the channel diversity have already been proposed. However, OCDM receivers are often designed and studied over time-invariant channels. This paper proposes a hybrid Frequency-Fresnel domain equalizer to enhance the system performance over the doubly selective UWA channel. Our method first estimates the Channel Impulse Response (CIR) using pilot blocks and the Least Square (LS) criterion. The CIR is then sparsified by removing coefficients below a threshold, and the receiver performs equalization in the frequency domain. To address residual InterSymbol Interference (ISI) in doubly selective UWA channels, we propose to equalize again the obtained symbols in the Fresnel domain and perform jointly phase synchronization. Simulations both on synthetic channels and a real underwater channel show that the proposed algorithm improves significantly the BER performance compared to existing methods.</p></div>
• Global Compression of the Plasma Sheet and Magnetotail During Intense Storms From THEMIS Observations
  
  • Alqeeq S.
  • Fontaine D.
  • Le Contel O.
  • Akhavan Tafti M.
  • Cazzola E.
  • Atilaw T.
  • Angelopoulos V.
  • Auster H.
  Journal of Geophysical Research Space Physics, American Geophysical Union/Wiley, 2024, 129 (9). Abstract We estimate the global impact of storms on the global structure and dynamics of the night side plasma sheet from observations by the NASA mission Time History of Events and Macroscale Interactions during Substorms (THEMIS). We focus on an intense storm occurring in December 2015 triggered by interplanetary coronal mass ejections (ICMEs). It starts with a storm sudden commencement (SSC) phase (SYM‐H +50 nT) followed by a growth phase (SYM‐H −188 nT at the minimum) and then a long recovery phase lasting several days. We investigate THEMIS observations when the spacecraft were located in the midnight sector of the plasma sheet at distances typically between 8 and 13 Earth's radii. It is found that the plasma sheet has been globally compressed up to a value of about 4 nPa during the SSC and main phases, that is, 8 times larger than its value during the quiet phase before the event. This compression occurs during periods of high dynamic pressure in the ICME (20 nPa) about one order of magnitude larger than its value in the pristine solar wind. We infer a global increase of the lobe magnetic field from 30 to 100 nT, confirmed by THEMIS data just outside the plasma sheet. During the SSC and main phases, the plasma sheet is found thinner by a factor of 2 relative to its thickness at quiet times, while the Tsyganenko T96 magnetic field model shows very stretched magnetic field lines from inner magnetospheric regions toward the night side. During the recovery phase, whereas the interplanetary pressure has dropped off, the plasma sheet tends to gradually recover its quiet phase characteristics (pressure, thickness, magnetic configuration, etc.) during a long recovery phase of several days. (10.1029/2024JA032888)
  DOI : 10.1029/2024JA032888
• Unsupervised radiometric change detection from synthetic aperture radar images
  
  • Bultingaire Thomas
  • Meraoumia Inès
  • Kervazo Christophe
  • Denis Loïc
  • Tupin Florence
  , 2024. Change detection is an important data processing task in remote sensing, with applications such as deforestation monitoring or natural disaster assessment. Synthetic Aperture Radar (SAR) imaging offers key advantages for change detection, in particular due to its robustness to weather condition and cloud coverage. Because of the speckle phenomenon, the intensity of SAR images suffer from strong fluctuations, making the detection of radiometric changes challenging. Our method builds on a recently introduced self-supervised despeckling technique. It estimates despeckling uncertainty to better identify meaningful differences between two despeckled images. Conformal prediction permits to approach the change detection problem from the angle of anomaly detection. Thus, we develop a fully unsupervised change detection approach with a controlled probability of false alarm. Experimental results on TerraSAR-X satellite images with metric resolution show the capability of our method to detect changes without any supervision.
• THREE DIMENSIONAL FREEZING OF A RIVULET
  
  • de Miramon Hélie
  • Josserand Christophe
  • Huerre Axel
  • Séon Thomas
  , 2024.
• Far-field sound field estimation using robotized measurements and the boundary elements method
  
  • Pascal Caroline
  • Marchand Pierre
  • Chapoutot Alexandre
  • Doaré Olivier
  , 2024, 270 (11), pp.816-827. Sound Field Estimation (SFE) is a numerical technique widely used to identify and reconstruct the acoustic fields radiated by unknown structures. In particular, SFE proves to be useful when data is only available close to the source, but information in the whole space is required. However, the practical implementation of this method is still hindered by two major drawbacks: the lack of efficient implementation of existing numerical methodologies, and the time-consuming and tedious roll-out of acoustic measurements. This paper aims to provide a solution to both issues. First, the measurements step is fully automated by using a robotic arm, able to accurately gather geometric and acoustic data without any human assistance. In this matter, a particular attention has been paid to the impact of the robot on the acoustic pressure measurements. The sound field prediction is then tackled using the Boundary Element Method (BEM), and implemented using the FreeFEM++ BEM library. Numerically simulated measurements have allowed us to assess the method accuracy, and the overall solution has been successfully tested using actual robotized measurements of an unknown loudspeaker (10.3397/IN_2024_2661)
  DOI : 10.3397/IN_2024_2661
• ON THE FREEZING OF RECEDING CONTACT LINES
  
  • Josserand Christophe
  • Séon Thomas
  • Huerre Axel
  • Duchemin Laurent
  • Grivet Rodolphe
  , 2024.
• The genuinely multipartite nonlocality of graph states is model-dependent
  
  • Coiteux-Roy Xavier
  • Makuta Owidiusz
  • Curran Fionnuala
  • Augusiak Remigiusz
  • Renou Marc-Olivier
  , 2024. Bell's theorem proves that some quantum state correlations can only be explained by bipartite non-classical resources. The notion of genuinely multipartite nonlocality (GMNL) was later introduced to conceptualize the fact that nonclassical resources involving more than two parties in a nontrivial way may be needed to account for some quantum correlations. In this letter, we first recall the contradictions inherent to the historical definition of GMNL. Second, we turn to one of its redefinitions, called Local-Operations-and-Shared-Randomness GMNL (LOSR-GMNL), proving that all caterpillar graph states (including cluster states) have this second property. Finally, we conceptualize a third, alternative definition, which we call Local-Operations-and-Neighbour-Communication GMNL (LONC-GMNL), that is adapted to situations in which short-range communication between some parties might occur. We show that cluster states do not have this third property, while GHZ states do. Beyond its technical content, our letter illustrates that rigorous conceptual work is needed before applying the concepts of genuinely multipartite nonlocality, genuine multipartite entanglement or entanglement depth to benchmark the nonclassicality of some experimentally-produced quantum system. We note that most experimental works still use witnesses based on the historical definitions of these notions, which fail to reject models based on bipartite resources. (10.48550/arXiv.2404.15861)
  DOI : 10.48550/arXiv.2404.15861
• A model-based approach for assessing the security of cyber-physical systems
  
  • Teixeira de Castro Hugo
  • Hussain Ahmed
  • Blanc Gregory
  • El Hachem Jamal
  • Blouin Dominique
  • Leneutre Jean
  • Papadimitratos Panos
  , 2024 (121), pp.1-10. Cyber-Physical Systems (CPSs) complexity has been continuously increasing to support new life-impacting applications, such as Internet of Things (IoT) devices or Industrial Control Systems (ICSs). These characteristics introduce new critical security challenges to both industrial practitioners and academics. This work investigates how Model-Based System Engineering (MBSE) and attack graph approaches could be leveraged to model secure Cyber-Physical System solutions and identify high-impact attacks early in the system development life cycle. To achieve this, we propose a new framework that comprises (1) an easily adoptable modeling paradigm for Cyber-Physical System representation, (2) an attack-graph-based solution for Cyber-Physical System automatic quantitative security analysis, based on the MulVAL security tool, (3) a set of Model-To-Text (MTT) transformation rules to bridge the gap between SysML and MulVAL. We illustrated the validity of our proposed framework through an autonomous ventilation system example. A Denial of Service (DoS) attack targeting an industrial communication protocol was identified and displayed as attack graphs. In future work, we intend to connect the approach to dynamic security databases for automatic countermeasure selection. (10.1145/3664476.3670470)
  DOI : 10.1145/3664476.3670470
• Mealy verifier : an automated, exhaustive, and explainable methodology for analyzing state machines in protocol implementations
  
  • Tran Van Arthur
  • Levillain Olivier
  • Debar Herve
  , 2024 (16), pp.1-10. Many network protocol specifications are long and lack clarity, which paves the way to implementation errors. Such errors have led to vulnerabilities for secure protocols such as SSH and TLS. Active automata learning, a black-box method, is an efficient method to discover discrepancies between a specification and its implementation. It consists in extracting state machines by interacting with a network stack. It can be (and has been) combined with model checking to analyze the obtained state machines. Model checking is designed for exhibiting a single model violation instead of all model violations and thus leads to a limited understanding of implementation errors. As far as we are aware, there is only one specialized exhaustive method available for analyzing the outcomes of active automata learning applied to network protocols,Fiterau-Brostean’s method. We propose an alternative method, to improve the discovery of new bugs and vulnerabilities and enhance the exhaustiveness of model verification processes. In this article, we apply our method to two use cases: SSH, where we focus on the analysis of existing state machines and OPC UA, for which we present a full workflow from state machine inference to state machine analysis. (10.1145/3664476.3664506)
  DOI : 10.1145/3664476.3664506
• OAM driven nucleation of sub-50 nm compact antiferromagnetic skyrmions
  
  • Mallick Sougata
  • Ye Peng
  • Boutu Willem
  • Gauthier David
  • Merdji Hamed
  • Bibes Manuel
  • Viret Michel
  • Bouzehouane Karim
  • Cros Vincent
  Advanced Functional Materials, Wiley, 2024, pp.2409528. Abstract Owing to their high mobility and immunity to topological deflection, skyrmions in antiferromagnetic (AFM) systems are gaining attention as a potential solution for next‐generation magnetic data storage. Synthetic antiferromagnets (SAFs) offer a promising avenue to tune the properties of the individual magnetic layers, facilitating the conditions necessary for skyrmions to be used in practical devices. Despite recent advancements achieving fast skyrmion mobility, the nucleation of small and rigid circular skyrmions without an external field remains challenging in SAFs. Theoretical predictions suggest that optical vortex (OAM) beams can stabilize skyrmionic spin textures by transferring their spin and orbital angular momentum to the magnetic material. Here, this intriguing proposal is delved into and the creation of sub‐50 nm compact skyrmions in SAFs using OAM beams is successfully demonstrated, eliminating the need for external magnetic fields. Additionally, the results underscore the importance of beam energy and the number of pulses, as both factors play critical roles in the stabilization of these AFM skyrmionic textures. This breakthrough is significant as it paves the way for stabilizing true zero‐field skyrmions in AFM systems, where magnetization is minimally affected by external magnetic fields. This work will open a potential avenue for stabilizing small, compact skyrmions in antiferroic systems, facilitating their implementation in logic and memory devices. (10.1002/adfm.202409528)
  DOI : 10.1002/adfm.202409528
• An analysis of the mutual information upper bound for sensor-subset selection
  
  • Leroy Idyano
  • Saucan Augustin
  • Saucan Augustin Alexandru
  • Petetin Yohan
  • Clark Daniel
  , 2024, pp.1-8. The ability to rapidly select an optimal subset of sensors is of critical importance in massive multi-sensor target tracking. Various information metrics exist for selecting the subset of sensors that is most informative with respect to the target being tracked. Moreover, information bounds were proposed as approximate metrics in order to speed up the selection algorithms. In this paper, we provide an analysis on the information loss and its impact on the subset selection problem when employing an information upper bound instead of the exact mutual information metric. We design several greedy sensor-selection algorithms that sequentially evaluate the exact mutual information between a set of sensors and the target. Subsequently, we compare these algorithms with a sensor-selection method that employs an information upper bound and highlight situations where the latter finds sub-optimal solutions. (10.23919/FUSION59988.2024.10706439)
  DOI : 10.23919/FUSION59988.2024.10706439
• 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.
• No distributed quantum advantage for approximate graph coloring
  
  • Coiteux-Roy Xavier
  • d'Amore Francesco
  • Gajjala Rishikesh
  • Kuhn Fabian
  • Le Gall François
  • Lievonen Henrik
  • Modanese Augusto
  • Renou Marc-Olivier
  • Schmid Gustav
  • Suomela Jukka
  , 2024, pp.1901-1910. We give an almost complete characterization of the hardness of $c$-coloring $\chi$-chromatic graphs with distributed algorithms, for a wide range of models of distributed computing. In particular, we show that these problems do not admit any distributed quantum advantage. To do that: 1. We give a new distributed algorithm that finds a $c$-coloring in $\chi$-chromatic graphs in $\tilde{\mathcal{O}}(n^{\frac{1}{\alpha}})$ rounds, with $\alpha = \bigl\lfloor\frac{c-1}{\chi - 1}\bigr\rfloor$. 2. We prove that any distributed algorithm for this problem requires $\Omega(n^{\frac{1}{\alpha}})$ rounds. Our upper bound holds in the classical, deterministic LOCAL model, while the near-matching lower bound holds in the non-signaling model. This model, introduced by Arfaoui and Fraigniaud in 2014, captures all models of distributed graph algorithms that obey physical causality; this includes not only classical deterministic LOCAL and randomized LOCAL but also quantum-LOCAL, even with a pre-shared quantum state. We also show that similar arguments can be used to prove that, e.g., 3-coloring 2-dimensional grids or $c$-coloring trees remain hard problems even for the non-signaling model, and in particular do not admit any quantum advantage. Our lower-bound arguments are purely graph-theoretic at heart; no background on quantum information theory is needed to establish the proofs. (10.1145/3618260.3649679)
  DOI : 10.1145/3618260.3649679
• A Study of Deep Perceptual Metrics for Image Quality Assessment
  
  • Kazmierczak Rémi
  • Franchi Gianni
  • Belkhir Nacim
  • Manzanera Antoine
  • Filliat David
  , 2022. Several metrics exist to quantify the similarity between images, but they are inefficient when it comes to measure the similarity of highly distorted images. In this work, we propose to empirically investigate perceptual metrics based on deep neural networks for tackling the Image Quality Assessment (IQA) task. We study deep perceptual metrics according to different hyperparameters like the network's architecture or training procedure. Finally, we propose our multi-resolution perceptual metric (MR-Perceptual), that allows us to aggregate perceptual information at different resolutions and outperforms standard perceptual metrics on IQA tasks with varying image deformations.
• Interval Methods applied to Signal Temporal Logic - Overview and Extension on Tubes
  
  • Tillet Joris
  • Vanneaux Elena
  • Alexandre Dit Sandretto Julien
  , 2024.
• Numerical methods and relaxation techniques for diffuse interface models in high-velocity two-phase flow simulations
  
  • Haegeman Ward
  • Dupays Joël
  • Le Touze Clement
  • Massot Marc
  , 2024. Compressible multiphase flows are at the heart of a great number of engineering applications in several domains. Some examples include the aerospace industry, since many rocket propulsion systems rely on the injection of a liquid reactant into the combustion chamber and the efficiency of the combustion is directly related to the atomization process. Other applications include the civil nuclear industry safety analysis but also the naval industry for which underwater solid propulsion systems are of great interest. The design and optimization requirements of these systems lead to an increasing need for predictive numerical simulations. Diffuse interface models are widely used for these tasks as they provide a good trade-off between accuracy and robustness. We consider the class of Baer-Nunziato type of models, in which the most general one allows for full disequilibrium between the two-phases. Reduced order models are obtained by assuming some local equilibrium (velocity, pressure, temperature or chemical potential equilibrium). Among this hierarchy of models, the pressure and velocity equilibrium model of Kapila et al is of particular interest. It allows to recover the classical Wood sound velocity for two-phase mixtures while still allowing thermal disequilibrium which is paramount for many applications such as high-temperature jets impinging on liquid surfaces and for proper modelling of phase changes. Several strategies to solve this model rely on a 6-equation model endowed with stiff pressure relaxation terms. For cavitating flows, an accurate computation of the pressure equilibrium is particularly important to compute the mass transfer fluxes. The purpose of the present contribution is to study the assumptions on the thermodynamics of the two phases and its impact on the mathematical structure of the resulting system of PDEs (where potentially several relaxation processes are involved either relying on finite-rate or instantaneous relaxation source terms). We propose an analysis of the pressure relaxation process in terms of thermodynamically admissible paths and propose a robust numerical scheme, which preserves the set of admissible states of the system. The robustness and accuracy of the proposed numerical scheme involving convection and sources is then assessed on several challenging configurations including shock-interface interactions and cavitating flows, such as shock-droplet interaction or Richtmyer–Meshkov instability. (10.23967/eccomas.2024.071)
  DOI : 10.23967/eccomas.2024.071
• Rhétorique émotionnelle et montée du populisme au Parlement européen
  
  • Subtil Hugo
  • Verger Vincent
  , 2024. Les progrès vraisemblables des partis dits populistes a lors des élections européennes du 9 juin prochain sont susceptibles de transformer en profondeur les dynamiques politiques au sein du Parlement européen b. Cette transformation ne se limitera pas aux seuls équilibres numériques entre groupes politiques : elle affectera aussi la forme des débats et les comportements de communication des parlementaires européens, notamment par une adaptation des partis non populistes à la rhétorique populiste, qui mobilise largement les émotions. Cette note s’appuie sur les transcriptions des débats tenus lors des séances plénières du Parlement européen entre 1999 et 2022. Des méthodes de traitement automatique du langage naturel y sont utilisées pour étudier la tonalité des interventions des députés européens. Il apparaît que la tonalité des débats au Parlement européen évolue. Leur niveau d’émotion a en effet nettement augmenté depuis 1999. Les députés européens s’expriment de façons différentes selon les sujets discutés, leur État membre d’origine et leur positionnement idéologique. Ceux des partis dits populistes se distinguent par davantage d’émotion dans leurs prises de parole. Toutefois, en réponse à la progression de ces partis, les autres élus du Parlement européen adaptent leur rhétorique en intensifiant la tonalité de leurs discours dans le même sens. Si cette étude ne permet pas d’identifier les conséquences d’une telle évolution sur la qualité des travaux parlementaires et la perception du Parlement européen par les citoyens européens, elle invite à y réfléchir davantage.
• A unified two-scale gas-liquid multi-fluid model with capillarity and interface regularization through a mass transfer between scales
  
  • Loison Arthur
  • Kokh Samuel
  • Pichard Teddy
  • Massot Marc
  International Journal of Multiphase Flow, Elsevier, 2024. In this contribution, we derive a gas-liquid two-scale multi-fluid model with capillarity effects to enable a novel interface regularization approach for multi-fluid models. As this unified modelling is capable of switching from the interface representation of a separated to a disperse regime it lays a new way of modelling regime transitions as it occurs in atomization processes. Above a preset length threshold at large scale, a multi-fluid diffuse interface model resolves the dynamics of the interface while, at small-scale, a set of geometric variables is used to characterize the interface geometry. These variables result from a reduced-order modelling of the small-scale kinetic equation that describes a collection of liquid inclusions. The flow model can be viewed as a two-phase two-scale mixture, and the equations of motion are obtained thanks to the Hamilton’s Stationary Action Principle, which requires to specify the kinetic and potential energies at play. We particularly focus on modelling the effects of capillarity on the mixture’s energy by including dependencies on additional variables accounting for the interface’s geometry at both scales. The regularization of the large-scale interface is then introduced as a local and dissipative process. The local curvature is limited via a relaxation toward a modified Laplace equilibrium such that an inter-scale mass transfer is triggered when the mean curvature is too high. We propose an original numerical method and assess the properties and potential of the modelling strategy on the relevant test-case of a two-dimensional liquid column in a compressible gas flow. (10.1016/j.ijmultiphaseflow.2024.104857)
  DOI : 10.1016/j.ijmultiphaseflow.2024.104857