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Flamelet modelling of turbulent reactive flows with non-premixed reactants and multiple inlets
No full textInternational audienceThe present work describes an extension of the standard (i.e., two-inlet) mixture-fraction-based flamelet model, that is aimed at accommodating an arbitrary number of inlets. The corresponding framework relies on the consideration of inlet tracers, which allow to retrieve the fresh mixture composition, used in conjunction with a single mixture fraction variable that discriminates oxidizer inlets from fuel inlets contributions. This mixture fraction variable is also retained as a mapping variable to parameterize a chemical manifold generated from diluted one-dimensional non-premixed flamelets, with corresponding levels of dilution set from the knowledge of inlet tracers values. The resulting turbulent combustion model is presented in detail and subsequently applied to the Reynolds-averaged Navier–Stokes (RANS) and large-eddy simulation of an experimental benchmark featuring three distinct inlets. The detailed comparisons between computational results and experimental data together with ternary plots in the inlet tracers space confirm the relevance of the proposed modelling framework
Anticipatory muscle activations to coordinate balance and movement during motor transitions: A narrative review
No full textInternational audienceBackground: Maintaining balance while moving is vital for day-to-day activities. A key challenge in the comprehension of human movement is to determine how muscles contribute to balance-movement coordination. Motor transitions, defined as movements executed between two steady balance states, are particularly interesting phases to study balance-movement coordination because a large, discrete change in whole-body momentum may disturb balance. During voluntarily-initiated motor transitions, anticipatory muscle patterns provide the biomechanical conditions that are favourable to both maintaining balance and executing the movement.Research question: What are the mechanical consequences of anticipatory muscle activations for balance-movement coordination during voluntarily-initiated motor transitions?Methods: We review the biomechanical contributions of the anticipatory muscle activations identified in the literature during four types of voluntarily-initiated motor transitions, through the prism of three balance mechanisms (‘moving the centre of pressure (CoP)’, ‘counter-rotating segments’, and ‘applying new external force(s)’). In particular, we investigate how anticipatory muscle activations modulate whole-body centre of mass acceleration.Results:We show that the mechanical consequences of anticipatory muscle activations have been extensively described, but mainly using the ‘moving the CoP’ mechanism. Unlike their role during steady balance states, both ‘moving the CoP’ and ‘applying new external force(s)’ mechanisms create a required mechanical instability during the anticipatory phase of motor transitions. The ‘counter-rotating’ mechanism may act as a stabiliser during motor transitions, but additional research is needed to clarify this assumption.Significance: This review establishes that muscle activation processes have different mechanical consequences for balance-movement coordination during the anticipatory phases of motor transitions, compared to steady balance states. Because the mechanical instability that is created can lead to falls, a better understanding of the mechanisms underlying motor transitions is needed to enable the design of more effective fall prevention programs and/or devices for population with balance deficits
Modélisation de la géométrie du canal spinal et des réseaux neuronaux intramédullaires et analyse de la diffusion des champs électriques générés par la stimulation médullaire épidurale
No full textPain is defined by the International Association for the Study of Pain as “An unpleasant sensory and emotional experience associated with, or resembling that associated with, actual or potential tissue damage.” Chronic pain significantly affects quality of life and imposes a substantial economic burden on society. Chronic pain (lasting for more than 6 months) is often treated by pharmacotherapy, which can cause side effects and addictions. Some patients might also be refractory to such treatments. The analgesic properties of electrical fields on the nervous system have been empirically known for centuries. This understanding evolved into epidural Spinal Cord Stimulation (SCS), a therapy requiring the implantation of a lead in the epidural space, facing the spinal cord, and targeting it to achieve inhibition of pain as described in Melzack and Wall’s theory of Gate Control. Despite the proven efficacy of SCS, its success in each patient remains unpredictable. This can be attributed to the complex anatomy in which SCS evolves, comprising of many tissues and liquids whose properties are unknown, as well as targeting complex microscopic anatomy. There exists a multitude of stimulation parameters (frequency, waveform, number and arrangement of anode/cathodes) available to clinicians, but no gold standard exists in the choosing of those parameters. To this end, computational modelling, particularly through the Finite-Element Method (FEM), can be of help in gaining insight into the electrical field generated by epidural SCS in the complex space in which it is situated as well as its effects on the intricate structures of the spinal cord. To this effect, we aimed at reproducing the two-step simulations that have been developed since the works of Coburn and Sin as well as Holsheimer’s team. First the electrical field generated by SCS is computed by the FEM, that field is then applied to models of the electrical behavior of neural tissue relevant to SCS. To reproduce these works, we aimed at building a reproducible, reusable and scripted workflow. First, a geometrical representation of the spinal cord and its surroundings, as well as implanted leads, is generated using Ansys SpaceClaim. This representation was progressively refined, notably by the use of imaging resources such as the PAM50 template of the spinal cord. The electrical field is then computed using Ansys MAPDL with boundary conditions resembling the stimulation applied by SCS. The effects of the computed electrical field on axons of the dorsal columns of the spinal cord were then assessed using the NEURON solver. These methods were tested for stability and convergence and allowed for the reproduction of some known results of the literature. The influence of tissue conductivities on computed impedance was also investigated to gain insight into the effects of material properties of tissues surrounding the implanted leads. The developed methods were also able to produce results comparable to measurable data in the form of paresthesia maps. These methods were then adapted in their geometry to fit to patient specific imaging to correlate to patient-reported paresthesia. While semi-automatic model generation was achieved, model prediction showed relatively moderate agreement with patient-reported outcomes. The methods developed were also used in the context of simulation of the mechanical behavior of the implanted lead and surrounding tissues. These methods were also briefly used in the context of high frequency stimulation, simulation of Evoked Compound Action Potentials (ECAPs). Overall, this study demonstrates the potential of computational modeling in improving the predictability of SCS outcomes and provides a framework for future refinements in stimulation strategies.La douleur est définie par l’International Association for the Study of Pain comme « une expérience sensorielle et émotionnelle désagréable associée, ou ressemblant à celle associée, à un dommage tissulaire réel ou potentiel. » La douleur chronique affecte significativement la qualité de vie et et pèse économiquement sur la société. La douleur chronique est souvent traitée par la pharmacothérapie, qui peut entraîner des effets secondaires et des addictions. Certains patients peuvent également être réfractaires à ces traitements. Les propriétés antalgiques des champs électriques sur le système nerveux sont connues empiriquement depuis des siècles. Cette compréhension a conduit au développement de la Stimulation Médullaire Épidurale (SME), une thérapie nécessitant l’implantation d’une électrode dans l’espace épidural, face à la moelle épinière, et la ciblant afin d’inhiber la douleur, comme décrit dans la théorie du Gate Control de Melzack et Wall. Malgré l’efficacité prouvée de la SME, son succès chez chaque patient reste imprévisible. Cela peut être attribué à l’anatomie complexe dans laquelle la SME agit, composée de nombreux tissus et liquides dont les propriétés sont inconnues, ainsi qu’à la nécessité de cibler une anatomie microscopique complexe. Une multitude de paramètres de stimulation sont disponibles pour les cliniciens, mais aucun standard n’existe pour leur sélection. La modélisation numérique, en particulier par la méthode des éléments finis (MEF), peut être utile pour mieux comprendre le champ électrique généré par la SME dans l’environnement complexe où elle est située, ainsi que ses effets sur les tissus neuronaux de la moelle épinière. Dans cette optique, nous avons cherché à reproduire les simulations en deux étapes développées depuis les travaux de Coburn et Sin ainsi que ceux de l’équipe de Holsheimer. Tout d’abord, le champ électrique généré par la SME est calculé par la MEF, puis ce champ est appliqué à des modèles du comportement électrique des tissus neuronaux pertinents pour la SME. Afin de reproduire ces travaux, nous avons développé un processus reproductible et réutilisable. Tout d’abord, une représentation géométrique de la moelle épinière et de son environnement, ainsi que des électrodes implantées, a été générée à l’aide d’Ansys SpaceClaim. Cette représentation a été progressivement affinée. Le champ électrique généré par la stimulation est ensuite calculé à l’aide d’Ansys MAPDL. L’effet du champ électrique calculé sur les axones de la moelle épinière ont ensuite été évalués à l’aide du logiciel NEURON. Ces méthodes ont été testées pour leur stabilité et leur convergence et ont permis de reproduire des résultats connus de la littérature. L’influence des conductivités des tissus sur l’impédance calculée a également été étudiée. Les méthodes développées ont également permis de produire des résultats comparables aux données mesurables sous forme de cartes de paresthésie. Ces méthodes ont ensuite été adaptées à l’imagerie spécifique des patients afin de corréler les résultats avec les paresthésies rapportées par les patients. Bien qu’une génération semi-automatique des modèles ait été atteinte, la prédiction du modèle a montré une concordance relativement modérée avec les résultats rapportés par les patients. Les méthodes développées ont également été utilisées dans le cadre de la simulation du comportement mécanique de l’électrode implantée et des tissus environnants, ainsi que dans le cadre de nouvelles modalités de stimulation. Dans l’ensemble, ce travail montre le potentiel de la modélisation pour améliorer la prévisibilité des résultats de la SCS et propose un cadre pour de futures améliorations des stratégies de stimulation
NiO whiskers growth driven by the presence of Pt nanoparticles during Ni-base superalloy oxidation
No full textInternational audienceThe formation and growth of NiO whiskers on a platinum-containing nickel-base superalloy was monitored continuously up to 113 hours, at 920°C, under 50 Pa of air, using High-Temperature Environmental Scanning Electron Microscopy. The images recorded revealed that two types of whiskers form and grow at two different stages of the oxidation process. The first type of whisker forms as soon as the isothermal plateau at 920°C is reached. The growth rate is greater than 500 nm/min. After a few hours of isothermal oxidation, these whiskers have grown and are integrated into the oxide layer covering the surface of the sample. The second type of NiO whiskers forms after 20 hours of isothermal stabilization. They grow at a rate of around 1 to 10 nm/min for several tens of hours. The formation and growth of these whiskers is directly linked to the presence of platinum nanoparticles on the surface of the alloy. The mechanisms of whiskers formation and growth are discussed
PWP measurement of charge distribution in dielectric and conductive liquids under DC electric field
No full textInternational audienceAll liquids exhibit non-zero electrical conductivity, revealing the presence of electric charges. Though often considered uniformly distributed, these charges are influenced by interfaces (electrical double layer) or external electric fields. Their local accumulation can affect the performance of power equipment. Few precise methods exist to measure charge distribution in liquids. The PPRIME Institute has been adapting solid-state techniques, such as the Pressure Wave Propagation (PWP) method, for use in liquids. These have enabled the observation of how charges spatially and temporally reorganize under a DC electric field in dielectric and conductive liquids
Method for manufacturing a part made of a monocrystalline superalloy
No full textThe invention concerns a method for manufacturing an aircraft part, the part comprising a monocrystalline nickel-based superalloy substrate, the method consecutively implementing the steps of moulding the part at a moulding temperature greater than the melting temperature of the superalloy, and cooling the part, such that the monocrystalline superalloy has a γ phase and a γ phase, solution heat treatment of the part at a first temperature between the solves temperature of the γ′ phase and the melting temperature of the superalloy, homogenising the crystalline structure or the part, cooling the part to ambient temperature, first tempering and second tempering
Effect of reflection wall distance and supply conditions on reflective shuttling detonation combustor
No full textInternational audienceIn a reflective shuttling detonation combustor (RSDC), detonation waves propagate between the two reflection walls of a thin combustion chamber. As the chamber is two-dimensional and does not have any curvature, optical and pressure measurements can be conducted simultaneously. In this study, pressure measurements as well as self-luminous and schlieren visualizations were performed using a chamber with a reflection wall distance of 90 mm with ethylene and oxygen. Consequently, the detonation wave number increased from 2 to 5 following the total mass flux, indicating that the wave number depended on the reflection wall distance because the maximum wave number increased with increasing distance. In addition, although the reflection wall distance was varied, the mode transition, in which the wave number increased from 1 to 2, 2 to 3, and 3 to 4, occurred at similar values to the critical value at which the maximum mixture fill height was non-dimensionalized by the cell size. The dimensionless detonation wave propagation distance, that is, the reflection wall distance divided by the wave number and maximum mixture fill height, was 3.0 ± 0.3, which is almost the same as that of the previous 45 mm combustor. Additionally, when the ratio of the reflective wall distance to the wave number is the same, the dimensionless quantities match, except in a few cases. Comparing the model results with the schlieren results, different trends were caused by non-ideal phenomena arising from the supply conditions, such as the equivalence ratio and the relationship between the plenum pressure and combustion pressure
Linear reactive control of jet installation noise
No full textInternational audienceThis paper presents an experimental application of reactive control to jet installation noise based on destructive interference. The work is motivated by the success of previous studies in applying this control approach to mixing layers (Sasaki et al. Theor. 2018 b Comput. Fluid Dyn . 32 , 765–788), boundary layers (Brito et al. 2021 Exp. Fluids 62 , 1–13; Audiffred et al. 2023 Phys. Rev. Fluids 8 , 073902), flow over a backward-facing step (Martini et al. 2022 J. Fluid Mech . 937 , A19) and, more recently, to turbulent jets (Maia et al. 2021 Phys. Rev. Fluids 6 , 123901; Maia et al. 2022 Phys. Rev. Fluids 7 , 033903; Audiffred et al. 2024b J. Fluid Mech . 994 , A15). We exploit the fact that jet–surface interaction noise is underpinned by wavepackets that can be modelled in a linear framework and develop a linear control strategy where piezoelectric actuators situated at the edge of a scattering surface are driven in real time by sensor measurements in the near field of the jet, the objective being to reduce noise radiated in the acoustic field. The control mechanism involves imposition of an anti-dipole at the trailing edge to cancel the scattering dipole that arises due to an incident wavepacket perturbation. We explore two different control strategies: (i) the inverse feed-forward approach, where causality is imposed by truncating the control kernel, and (ii) the Wiener–Hopf approach, where causality is optimally enforced in building the control kernel. We show that the Wiener–Hopf approach has better performance than that obtained using the truncated inverse feed-forward kernel. We also explore different positions of the near-field sensors and show that control performance is better for sensors installed for streamwise positions downstream in the jet plume, where the signature of hydrodynamic wavepacket is better captured by the sensors. Broadband noise reductions of up to 50 % are achieved
Electrostatic Separation Of Particulate Matter Current Challenges And Innovative Solutions
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Formation of skin-core structure in filled room-temperature vulcanized polydimethylsiloxane after long exposure to moderate temperatures
No full textInternational audienceThis study highlights the formation of a skin-core effect inside a filled silicone rubber exposed to air for durations up to 600 days at temperatures between 30 • C and 70 • C. Coupling mechanical and physico-chemical characterisations reveal that the volatile components, such as crosslinking by-products and oligomers, evaporate from the elastomeric network during the exposure, resulting in the shrinkage of the sample. This phenomenon gives rise to the formation of a composition gradient from the sample's surface towards its core. All the samples become more brittle with ageing, but the tensile modulus increase only slightly at the longest durations and highest temperatures. More fillers are found on the outer surface while the infrared spectra in the core of the sample remain unaffected by neither the temperature nor the duration. The nanoindentation highlights gradients of elastic modulus on the cross-section and provides an estimation of the skin thickness. By wrapping the samples in aluminum foil, the evaporation of volatile components is prevented. The measured properties remain constant with the duration of thermal exposure even at the highest temperature