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Enhancing damping performance below the bandgap in metamaterial beams with geometric nonlinearity and bistable attachments via nonlinear energy transfer
No full textInternational audienceNonlinear metamaterials embedded with bistable resonators enable efficient energy exchange between acoustic and optical branches via 1 : n internal resonances. Recognising that the low-frequency acoustic branch is typically low-damping while the optical one near the bandgap exhibits higher losses, tuning their frequency relationship promotes nonlinear coupling, offering substantial and robust passive damping below the bandgap. To validate the feasibility of this pathway, a theoretical investigation is carried out on a clamped–clamped metamaterial beam with bistable attachment, which is realised by employing a dual pre-curved beam structure to design an easily achievable micro-bistable resonator, then periodically embedding it into the host beam. The geometric nonlinearity of the host structure, modelled using the von Kármán beam theory, is considered to capture the coupling effects accurately. The dynamics of the nonlinear beam are reduced using nonlinear normal modes, computed via the direct parametrisation method for invariant manifolds, which are then assembled with the dynamic equations governing the internal resonators. This strategy provides an accurate reduced-order model of the coupled system, achieving a significant speed-up in the computing time. The nonlinear vibrations of the coupled system are then analysed through linear dispersion spectrum and nonlinear frequency response curves, demonstrating significant damping improvements below the bandgap. Reductions of up to 30 dB and 10 dB are observed in the first and second resonant peaks, respectively, benefiting from the rich nonlinear energy transfer phenomena, enabled by the combination of geometric nonlinearity and the bistable characteristics of the resonators. The parameter analysis provides optimisation guidelines for the parameters related to the bistable configurations of the dual pre-curved beam, where the linear frequency of the bistable resonators, which governs the coupling between the low-damping and the high-damping modes, should remain between one and three times the frequency of the target mode to achieve effective damping enhancement
High-fidelity Simulation of Developing Mixed Convection in a Vertical Tube Bundle Configuration
No full textInternational audienceThe upward and spatially developing flow in an asymmetrically heated tube bundle configuration is investigated by means of wall-resolved Large-Eddy Simulation in a scenario for which forced and natural convection effects are comparable (mixed convection). Analyses of wall friction, first- and second-order turbulence statistics, streamline visualisations and periodograms are carried out to highlight the various physical phenomena that coexist in this flow configuration where the buoyancy force strongly influences the spatial development of turbulence and heat transfer characteristics
NILMFormer: Non-Intrusive Load Monitoring that Accounts for Non-Stationarity
No full textInternational audienceMillions of smart meters have been deployed worldwide, collecting the total power consumed by individual households. Based on these data, electricity suppliers offer their clients energy monitoring solutions to provide feedback on the consumption of their individual appliances. Historically, such estimates have relied on statistical methods that use coarse-grained total monthly consumption and static customer data, such as appliance ownership. Non-Intrusive Load Monitoring (NILM) is the problem of disaggregating a household's collected total power consumption to retrieve the consumed power for individual appliances. Current state-of-the-art (SotA) solutions for NILM are based on deep-learning (DL) and operate on subsequences of an entire household consumption reading. However, the non-stationary nature of real-world smart meter data leads to a drift in the data distribution within each segmented window, which significantly affects model performance. This paper introduces NILMFormer, a Transformer-based architecture that incorporates a new subsequence stationarization/de-stationarization scheme to mitigate the distribution drift and that uses a novel positional encoding that relies only on the subsequence's timestamp information. Experiments with 4 real-world datasets show that NILMFormer significantly outperforms the SotA approaches. Our solution has been deployed as the backbone algorithm for EDF's (Electricité De France) consumption monitoring service, delivering detailed insights to millions of customers about their individual appliances' power consumption
Using niche based-approach to predict the recovery of aquatic communities in restored stream
No full textInternational audienceHydro-morphological restoration is a common practice to rehabilitate perturbed streams and promote recovery of sensitive biodiversity. However, improving habitat quality does not always result in the recovery of resilient and functional community assemblages. This has been highlighted by using the substantial panel of bio-indicators over the last two decades to assess restoration outcomes, which has shown highly variable results, including negative ones. These results are primarily attributed to the rapid colonization by the surrounding species pool, considered as the « depauperate community », during the early stages of community’s establishment. These species, more tolerant, tend to compete for resources and prevent the establishment of a community more sensitive and in tune with the habitat generated by restoration. Although the concept of priority effect is widely recognized in terrestrial restoration, the role of competition during colonization in restored streams is poorly understood. Reasons of this lack of knowledge were primarily due to the fact that most post-restoration monitoring have a low temporal resolution, which prevents the study of community dynamics during community’s establishment. For the first time, we attempted to identify the potential role of priority effects in the community observed after river restoration by applying the concept of ecological niche occupancy. We used a decade of biannual monitoring of the macroinvertebrate community in a 200 m reach within a newly created channel of 5 Hydro-morphological restoration is a common practice to rehabilitate perturbed streams and promote recovery of sensitive biodiversity. However, improving habitat quality does not always result in the recovery of resilient and functional community assemblages. This has been highlighted by using the substantial panel of bio-indicators over the last two decades to assess restoration outcomes, which has shown highly variable results, including negative ones. These results are primarily attributed to the rapid colonization by the surrounding species pool, considered as the « depauperate community », during the early stages of community’s establishment. These species, more tolerant, tend to compete for resources and prevent the establishment of a community more sensitive and in tune with the habitat generated by restoration. Although the concept of priority effect is widely recognized in terrestrial restoration, the role of competition during colonization in restored streams is poorly understood. Reasons of this lack of knowledge were primarily due to the fact that most post-restoration monitoring have a low temporal resolution, which prevents the study of community dynamics during community’s establishment. For the first time, we attempted to identify the potential role of priority effects in the community observed after river restoration by applying the concept of ecological niche occupancy. We used a decade of biannual monitoring of the macroinvertebrate community in a 200 m reach within a newly created channel of 5 km long located in a degraded and heavily invaded river floodplain (upper Rhine, France). We specifically focused on possible negative resistance of the local depauperate community arising from the nearby un-restored section on the establishment of a sensitive community. We observed an important colonization of opportunist sensitive taxa during the primary succession (0-3 years), which were rapidly outcompeted by a community of tolerant taxa, largely constituted by exotic-invasive ones. We observed that establishment success decreased over time for all taxa and was higher in invasive taxa compared to native ones. Niche approaches based on multivariate analyses of functional traits revealed that the competition for feeding resources and habitat use can be related to taxa establishment success. However, it accounted for a minor part of the establishment success, suggesting that other traits or factors played a key role in the priority effect we observed. Although the extent of functional niches appeared stabilized after a decade, population dynamic analysis indicated that secondary succession was not fully completed. This was due to a slow recruitment of native community with competitive abilities (e.g., specialists in feeding) that in turn exclude invasive taxa in more recent years. Overall, these results suggest that duration of ecological successions in restored sections may considerably exceed the average time used for assessing community response to restoration (5 years). This could explain the large variability of responses to restoration observed in aquatic communities, and should encourage longer monitoring to assess an equilibrium status of communities. We argue that integrating functional approaches based on niche models could help to predict restoration effectiveness depending on the local species pool, particularly in a local context of invasive taxa occurrences
Analyse paléosismologique des failles de la terminaison NE cévenole après le séisme du Teil : qu’avons-nous appris ?
No full textNational audienceLe séisme du Teil en 2019, dont l’épicentre est situé le long de la faille de la Rouvière à proximité des centrales nucléaires de Cruas et de Tricastin, est apparu comme un séisme sans précédent en France métropolitaine. Il a généré une rupture de surface inédite (4.5 km de long, 10 cm de déplacement du sol en moyenne avec un mouvement inverse), de fortesaccélérations du sol (1g le long de la faille) et est situé à une profondeur superficielle (~1 km).Cet événement a soulevé un ensemble d’interrogations auxquelles la communauté scientifique tente de répondre par le biais de nombreuses observations et recherches, notamment dans le cadre de l'axe Failles actives France de l'Action transverse sismicité d'Epos-France. Début 2025, toutes les failles de la terminaison NE cévenole n’ont pu être étudiées. Il est donc fondamental de poursuivre ces études paléosismologiques pour mieux contraindre l’aléa sismique associé aux réseau de failles situé dans une partie de la vallée du Rhône, fortement peuplée et industrialisée
Characterization of Atmospheric Visibility through Extinction Coefficient and the Influence of Lower Threshold on Assessment of Multifractal Parameters
No full textInternational audienceScaling analysis of subjectively defined fields, which are often expressed as a range of values based on the field of application, is not straightforward. To avoid potential biases in statistical analysis, extracting the actual underlying field and understanding the effect of ranged values are important. One such example is atmospheric visibility which is often estimated as a range in meteorological context, as meteorological optical range (MOR). This is estimated from the extinction coefficient σ e , an objective measurement of light attenuation by constituent gases and aerosols in the atmosphere, and expressed as a range depending on the application needs. Accurate estimation of visibility and its variability is required for the safe functioning of various domains such as transport sectors and free optic communication or for understanding regional variations in air quality and climate. Since MOR is a subjective range, here we attempt to characterize it using the objectively measured σ e . In this context, we identify and illustrate the effect of a lower threshold in the data, which is not exclusive to the current problem, and examine its consequences in the multifractal characterization of the field. Here, σ e was extracted from visibility data by a present weather sensor located in the Paris region (France). This was then compared with σ e extracted from MOR measured at Paris Charles de Gaulle airport during the same period. Variability in σ e was investigated under the framework of universal multifractals (UMs), which is widely used to characterize geophysical fields that exhibit extreme variability across scales. With direct data analysis and numerical simulations mimicking the behavior, it was found that the multifractal properties exhibited by σ e are influenced by the upper limit of visibility range in the data. The biases are identified within the theoretical framework of UM, thus expanding the general understanding on the retrieval of the underlying unbiased stochastic field. Significance Statement Data obtained from measurement campaigns are often influenced by instrumental limitations such as upper and lower detection limits. One such example is atmospheric visibility which is usually represented as a range, from objective measurement of light attenuation by constituent particles. Here, we examine the consequences of characterizing visibility using the extinction coefficient, by considering the presence of a lower limiting value, using two datasets in the Paris region—forward scattering sensor and airport visibility measurement. Through data analysis, numerical simulation, and theoretical formulation, the biases are identified and the underlying unbiased stochastic field is retrieved. This enables a more accurate analysis and simulation of visibility, as well as other geophysical fields with similar instrumental or application limitations
Variabilité des écarts de température induits par les éclusées et impacts écologiques associés : Revue bibliographique et typologie des altérations potentielles selon les schémas d’aménagements
No full textInternational audienceThe rapid flow variations induced downstream of hydropower plants by the release of turbined water can be associated with temperature variations. Given the multiplicity of plant configurations and the diversity of structure characteristics, these temperature variations linked to hydropeaking are potentially highly variable from one site to another. The aim of our work was therefore firstly to review the state of the art of thermal alterations linked to hydropeaking and their impact on biological communities. Secondly, using our knowledge of the characteristics of the French hydropower fleet and representative case studies, we sought to define the different types of alteration possible, depending on the characteristics of the facilities. For most facilities, warming in winter and cooling in summer are generally observed, with widely varying amplitudes.. The most likely impacts on organisms are those linked to temperature gradients and the frequency of events. Changes in temperature regimes induced by hydropeaking are highly dependent on site configuration. Nevertheless, whatever the type of configuration, the temperature deviations induced are not systematic and concern a limited number of hydropeaking events. These results highlight the need to acquire knowledge about the effects of high temperature gradients on the physiology and behavior of individuals, and their indirect impacts at population or community level.Les variations rapides de débit induites à l’aval des centrales hydroélectriques par les lâchers d’eau turbinées peuvent être accompagnées de variations de température. Etant donné la multiplicité de configurations d’aménagements et la diversité de caractéristiques des ouvrages, ces écarts de température liées aux éclusées sont potentiellement très variables d’un site à un autre. Aussi, l’objectif de nos travaux a été d’une part de faire un état de l’art des altérations thermiques liées aux éclusées et de leurs impacts sur les communautés biologiques. D’autre part, à l’aide de la connaissance des caractéristiques du parc hydroélectrique français et de cas d’étude représentatifs, nous avons cherché à définir les différents types d’altérations possibles en fonction des caractéristiques des aménagements. Pour la plupart des aménagements, un réchauffement en hiver et un refroidissement en été sont généralement observés, avec des amplitudes très variées. Les incidences les plus probables sur les organismes sont celles liées aux gradients de baisse ou de hausse de température et la fréquence des évènements. Les écarts de température induits par les éclusées dépendent fortement de la configuration du site. Néanmoins, quel que soit, le type de configuration, les écarts de température induits ne sont pas systématiques et concernent un nombre restreint d’éclusées. Ces résultats mettent en avant le besoin d’acquérir des connaissances sur les effets de gradients de température élevés sur la physiologie et le comportement des individus et leurs impacts indirects à l’échelle des populations ou des communautés
Multidimensional simulations of incompressible turbulent flows using aGodunov-type scheme for second-order moment equations
No full textSecond moment closure for thermal fluxes for forced and mixed convection
No full textInternational audienceAccurate modelling of natural convection is essential for nuclear safety applications, particularly in the passive cooling systems of Small Modular Reactors (SMRs). Although Reynolds-Averaged Navier–Stokes (RANS) models are widely employed in industry, they frequently fail to capture key buoyancy-driven effects, thereby limiting their predictive accuracy. This study introduces a new transport equation model for the dissipation rate of temperature variance, eps_theta . The objective is to improve the estimation of εθ which is essential for the modelling of thermal turbulence. The proposed model is evaluated a priori using Direct Numerical Simulation (DNS) data fromAlcántara-Ávila et al. [1] and Kasagi and Nishimura [2], corresponding to forced and mixed convection regimes, respectively. The budget terms of the εθ equation obtained with the new model are compared with those obtained using the formulation of Mangeon et al.[3]. The new model not only provides improved results for the forced convection case on which Mangeon et al. calibrated their model, but also performs well in the mixed convection regime, which was not considered in Mangeon’s originalwork
