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Harnessing swarms for directed migration of interacting active particles via optimal global control
No full textThis study investigates the use of global control strategies to enhance the directed migration of swarms of interacting self-propelled particles confined in a channel. Uncontrolled dynamics naturally leads to wall accumulation, clogging, and band formation due to the interplay between self-organization and confinement. This work explores whether a uniform global control, such as magnetic field acting on all particles, can optimize collective transport. Using a discrete Vicsek-like model, it is found that simple global alignment controls, optimized via reinforcement learning, efficiently suppress unfavorable configurations and significantly increase the net particle flux along a prescribed channel direction. These results highlight that coarse, system-level observations are sufficient to achieve near-optimal control, even in regimes with strong fluctuations or partial ordering
Non-unique self-similar blowups in shell models: insights from dynamical systems and machine-learning
No full textInternational audienceStrong numerical hints exist in favor of a universal blowup scenario in the Sabra shell model, a popular cascade model of 3D turbulence, which features complex velocity variables on a geometric progression of scales ℓ n ∝ λ − n . The blowup is thought to be of self-similar type and characterized by the finite-time convergence towards a universal profile with non-Kolmogorov (anomalous) small-scale scaling ∝ ℓ n x . Solving the underlying nonlinear eigenvalue problem has however proven challenging, and prior insights mainly used the Dombre–Gilson renormalization scheme, transforming self-similar solutions into solitons propagating over infinite rescaled time horizon. Here, we further characterize Sabra blowups by implementing two strategies targeting the eigenvalue problem. The first involves formal expansion in terms of the bookkeeping parameter δ = ( 1 − x ) log λ , and interpretes the self-similar solution as a (degenerate) homoclinic bifurcation. Using standard bifurcation toolkits, we show that the homoclinic bifurcations identified under finite-truncation of the series converge to the observed Sabra solution. The second strategy uses machine-learning optimization to solve directly for the Sabra eigenvalue. It reveals an intricate phase space, with the presence of a continuous family of non-universal blowup profiles, characterized by various number N of pulses and exponents x N ⩾ x
ADAPTIVE CROSS APPROXIMATION WITH A GEOMETRICAL PIVOT CHOICE: ACA-GP METHOD
No full textThe Adaptive Cross Approximation (ACA) method is widely used to approximate admissible blocks of hierarchical matrices, or H -matrices, from discretized operators in the boundary integral method. These matrices are fully populated, making their storage and manipulation resource-intensive. ACA constructs a low-rank approximation by evaluating only a few rows and columns of the original operator, significantly reducing computational costs. A key aspect of ACA's effectiveness is the selection of pivots, which are entries common to the evaluated row and column of the original matrix. This paper proposes combining the classical, purely algebraic ACA method with a geometrical pivot selection based on the central subsets and extreme property subsets. The method is named ACA-GP, GP stands for Geometrical Pivots. The superiority of the ACA-GP compared to the classical ACA is demonstrated using a classical Green operator for two clouds of interacting points
Directional solidification of eutectic alloys with complex composition
No full textInternational audienc
Dealing with Indistinguishability in Gluing KKL Observer Design for Hybrid Systems with Unknown Jump Times
No full textInternational audienceIn previous work, we presented a novel systematic observer design for hybrid systems with unknown jump times by employing the Kravaris-Kazantzis/ Luenberger (KKL) observer paradigm and the concept of "gluing" the jumps. Specifically, we demonstrated the existence of a transformation that converts a hybrid system with continuous output at jumps into a continuous-time stable filter of this output, for which an observer can always be designed without requiring jump detection. However, the inherent indistinguishability appearing at jumps (i) prevents the convergence of the Euclidean estimation error to zero, thus rendering the definition and analysis of convergence/stability/robustness non-straightforward; and (ii) prevents the injectivity of the change of coordinates, thus raising issues in the implementation of the observer. In this paper, we deal with those two problems by (i) showing asymptotic convergence and robust stability of the gluing KKL observer in the sense of a distance to an appropriate indistinguishability-aware set; and (ii) proposing a systematic implementation scheme that involves learning numerical models of two inverse maps based on neural networks that are trained on data generated by offline simulations. Our contributions are academically illustrated on a bouncing ball and then applied for state and parameter estimation in a friction oscillator exhibiting stick-slip
Nouvelles modalités d'inclusion par les métiers. Récit du projet 100% inclusion des Compagnons Du Devoir
No full textInternational audienceLes Compagnons du Devoir, reconnus pour leur transmission d’excellence dans les métiers manuels, ont lancé en 2019 le programme « 100 % inclusion ». Ce dispositif cible des jeunes de 16 à 21 ans, très éloignés de l’em- ploi et de la formation, pour les préparer à l’entrée en apprentissage. Déployé dans quatre régions, ce projet constitue un défi pour les Compagnons. En interne, il implique d’inclure des profils atypiques, auxquels l’organisation est peu habituée ; en externe, il contribue aux politiques publiques envers les NEET, Not in Education, Employment or Training, expérimentant de nou- velles formes d’inclusion « par et dans » le métier. À travers le récit de cette expérimentation, analysé par une méthodologie longitudinale et qualitative, nous examinons comment la formation aux métiers « manuels » peut servir de point d’appui pour insérer socialement et professionnellement des jeunes en situation de décrochage. Ce chapitre explore le rôle de la communauté occupationnelle dans l’inclusion. Théoriquement, le projet révèle un pro- cessus d’inclusion original centré sur le métier, combinant compétences techniques et comportementales, tout en conciliant unicité et acculturation.Pratiquement, il démontre le potentiel de la formation professionnelle aux métiers « manuels » pour intégrer des publics en rupture
Probing the Phenomenology of Dark Matter from Decoupled Freeze-Out
No full textInternational audienceWe consider a model of dark matter where the mediator corresponds to a superposition of a scalar and pseudoscalar, and the scenario where, after reheating, the number densities of the dark sector particles, i.e. the dark matter and the mediators, are negligible. If the coupling of the mediators to the Standard Model is feeble, but the coupling to the dark matter is large enough, the dark sector may reach equilibrium at a temperature distinct from that of the thermal bath. The relic density is then said to be obtained via decoupled freeze out (DFO). We focus on the -wave annihilation scenario, which particularly benefits from the DFO mechanism by evading standard CMB limits while still yielding indirect detection signals. We calculate the relic density by solving a set of four coupled Boltzmann equations for the number densities of the dark sector particles and the energy transfer from the light to dark sector. We finally perform a thorough analysis of experimental bounds on this scenario, namely from indirect detection and the CMB, as well as from BBN, and find that, while there are considerable constraints on the parameter space where the correct relic density is obtained, a viable region remains to be explored
Sources and fate of CO2 along the soil–aquifer–stream–atmosphere continuum (the Orgeval headwater catchment, France)
No full textInternational audienceFew studies have examined CO 2 sources and pathways along the soil-aquifer-stream-atmosphere continuum. We measured the water concentration of dissolved CO 2 , HCO 3 -, and dissolved organic carbon (DOC), together with ancillary variables (pH, total alkalinity [TA], and major ions [Ca 2+ , Mg 2+ , SO 4 2-, NO 3 -]), in piezometer transects and adjacent streams at a monthly interval over a 5-year period (2019-2023). Such a continuum was studied within a small catchment underlain by carbonate geology and subject to intensive agriculture. On the basis of the values of Ca 2+ , Mg 2+ , SO 4 2-, and alkalinity in the upslope and downslope piezometers, we assumed that the more mineralized downslope water would characterize the slow groundwater flow component of the stream, while the less mineralized upslope water would reflect a fast sub-surface flow (surface runoff + agricultural drainage flow). These water flows components were calculated using the HYPE calibrated hydrological model.Accounting for weathering reactions and using a mass balance of dissolved C fluxes, we found that the total alkalinity flux (as HCO 3 -) reaching the river (amounting to 29 Mmol yr -1 , i.e., 79 kgC ha -1 yr -1 ) derived for about one third from soil CO 2 respiration, one third from carbonate dissolution induced by the reaction with CO 2 , and one third from carbonate dissolution induced by the acidity generated by nitrification of reduced N fertilizers.The groundwater CO 2 fluxes to the stream (13 Mmol y -1 , i.e., 36 kgC ha -1 yr -1 ) were 2 orders of magnitude lower than the total direct soil CO 2 flux emitted to the atmosphere. Most (at least 95 %).of the groundwater CO 2 fluxes was degassed once reaching the stream.Dissolved organic carbon flux to the stream represented only ~ 2 % of the total dissolved carbon flux to the stream; the input of dissolved CO 2 in rainfall to the soil-aquifer system amounted to less than 1 % of this total dissolved carbon flux to the stream.The carbonate weathering induced by soil respired CO 2 amounted to ~ 26 kgC ha -1 yr -1 in the Avenelles subcatchment, representing ~ 0.5 ‰ yr -1 of the total soil organic carbon stock. The alkalinity flux generated by this process might hence act as significant soil transient CO 2 sink, depending on the prevalence of downstream processes affecting the stream carbonate buffering and speciation.</p
Analyse du potentiel des systèmes de fabrication en réseau pour saisir de nouvelles opportunités de production
No full textIn today’s volatile, uncertain, complex, and ambiguous manufacturing context, firms must adapt their production systems to stay competitive. Traditional risk management based on static data often fails to address unforeseen production requirements. Reconfiguration, defined as adjusting system parameters without new resources, offers a strategic response but requires collaboration across networks. This thesis proposes a framework with two contributions: an ontology-based model to identify configuration alternatives using available capabilities, and an optimization model to evaluate them through cost, feasibility, and logistics criteria. Applied to folding bicycle and hydroalcoholic gel production, the framework shows how firms can leverage shared resources to seize new production opportunities efficiently.Dans le contexte actuel de fabrication volatile, incertain, complexe et ambigu, les entreprises doivent adapter leurs systèmes de production pour rester compétitives. La gestion traditionnelle des risques basée sur des données statiques ne permet souvent pas de répondre à des exigences de production imprévues. La reconfiguration, définie comme l'ajustement des paramètres du système sans nouvelles ressources, offre une réponse stratégique mais nécessite une collaboration entre les réseaux. Cette thèse propose un cadre comportant deux contributions : un modèle ontologique permettant d'identifier les alternatives de configuration à partir des capacités disponibles, et un modèle d'optimisation permettant de les évaluer selon des critères de coût, de faisabilité et de logistique. Appliqué à la production de vélos pliants et de gel hydroalcoolique, ce cadre montre comment les entreprises peuvent tirer parti des ressources partagées pour saisir efficacement de nouvelles opportunités de production
