Portail HAL Ensta
Not a member yet
11080 research outputs found
Sort by
A mathematical study of a hyperbolic metamaterial in free space
No full textInternational audienceWave propagation in hyperbolic metamaterials is described by the Maxwell equations with a frequency dependent tensor of dielectric permittivity, whose eigenvalues are of different signs. In this case the problem becomes hyperbolic (Klein-Gordon equation) for a certain range of frequencies. The principal theoretical and numerical difficulty comes from the fact that this hyperbolic equation is posed in a free space, without initial conditions provided. The subject of the work is the theoretical justification of this problem. In particular, this includes the construction of a radiation condition, a well-posedness result, a limiting absorption principle and regularity estimates on the solution
Utilisation des technologies mobiles auprès des enfants avec TSA.
No full textInternational audienceLes technologies numériques sont de plus en plus plébiscitées pour proposer des interventions psycho-éducatives aux enfants avec trouble du spectre autistique (TSA), et sont aujourd’hui con-sidérées comme des supports d’interventions prometteurs. Une brève synthèse de la littérature sur ce type d’interventions permet de constater la prédominance des interventions en remédia-tion clinique et cognitive, en lien avec les principaux symptômes du TSA. D’autres domaines de besoins comme l’assistance en vie réelle et les apprentissages scolaires sont en revanche bien moins investis et pourraient compléter efficacement l’offre de soutien et d’accompagnement pour les individus avec TSA. En lien avec ces constats, ce chapitre propose de discuter des ap-ports spécifiques des technologies mobiles, mais aussi d’examiner l’état des recherches dans ce domaine. Ce chapitre dresse enfin plusieurs pistes de recherche, en lien avec la conception et l’évaluation d’interventions numériques pour les enfants avec TSA
Towards a balancing safety against performance approach in human–robot co-manipulation for door-closing emergencies
No full textInternational audienceTelemanipulation in power stations commonly require robots first to open doors and then gain access to a new workspace. However, the opened doors can easily close by disturbances, interrupt the operations, and potentially lead to collision damages. Although existing telemanipulation is a highly efficient master–slave work pattern due to human-in-the-loop control, it is not trivial for a user to specify the optimal measures to guarantee safety. This paper investigates the safety-critical motion planning and control problem to balance robotic safety against manipulation performance during work emergencies. Based on a dynamic workspace released by door-closing, the interactions between the workspace and robot are analyzed using a partially observable Markov decision process, thereby making the balance mechanism executed as belief tree planning. To act the planning, apart from telemanipulation actions, we clarify other three safety-guaranteed actions: on guard, defense and escape for self-protection by estimating collision risk levels to trigger them. Besides, our experiments show that the proposed method is capable of determining multiple solutions for balancing robotic safety and work efficiency during telemanipulation tasks
Corrosion and Electrochemical Properties of Laser-Shock-Peening-Treated Stainless Steel AISI 304L in VVER Primary Water Environment
No full textInternational audienceLaser Shock Peening (LSP) is a surface treatment technique for metallic materials. It induces plastic deformation at the surface of up to around 1 mm in depth. This process introduces residual stresses that lead to strain hardening, and potentially improvements in fatigue, stress corrosion cracking (SCC) and general corrosion behaviour in many, but not all, corrosive media. In this paper, two specimens made of AISI 304L stainless steel, one LSP-treated and one un-treated, were tested at 280 °C and 8 MPa in VVER (or PWR) primary circuit water chemistry using in situ Electrochemical Impedance Spectroscopy (EIS). This experiment serves to qualify the influence of LSP on the changes in corrosion behaviour in high-temperature, high-density water. The residual stress (RS) measurement of the surface showed a compression RS. Before LSP treatment, RS at the surface was 52.2 MPa in the rolling direction 0°RD and 10.42 MPa in the transverse rolling direction 90°RD. After the treatment, surface RS was −175.27 MPa and −183.51 MPa for Scan and TScan directions, respectively. The effect of compressive RS at the surface was studied and showed an increase in corrosion rate. The analysis of oxide layer by SEM revealed differences between LSP-treated and untreated AISI 304L specimens and their connection to corrosion rates
Controlled acceleration of GeV electron beams in an all-optical plasma waveguide
No full textInternational audienceLaser-plasma accelerators produce electric fields of the order of 100 GV/m, more than 1000 times larger than radio-frequency accelerators. Thanks to this unique field strength, they appear as a promising path to generate electron beams beyond the TeV, for high-energy physics. Yet, large electric fields are of little benefit if they are not maintained over a long distance. It is therefore of the utmost importance to guide the ultra-intense laser pulse that drives the accelerator. Reaching very high energies is equally useless if the properties of the electron beam change completely shot to shot. While present state-of-the-art laser-plasma accelerators can already separately address guiding and control challenges by tweaking the plasma structures, the production of beams combining high quality and high energy is yet to be demonstrated. Here we use a new approach for guiding the laser, and combined it with a controlled injection technique to demonstrate the reliable and efficient acceleration of high-quality electron beams up to 1.1 GeV, from a 50 TW-class laser
Arbitrary-rate relaxation techniques for the numerical modeling of compressible two-phase flows with heat and mass transfer
No full textInternational audienc
Correction to: Numerical certification of Pareto optimality for biobjective nonlinear problems
No full textThe top-right part of Fig. 9 is missing from the published version of [1]. Figure 1 is the correct one, with the small portion of the curve corresponding to the non-equilateral hexagons at P ≈ 3.1 and A ≈ 0.674.International audienceThe top-right part of Fig. 9 is missing from the published version of [1]. Figure 1 is the correctone, with the small portion of the curve corresponding to the non-equilateral hexagons atP ≈ 3.1 and A ≈ 0.674
Military-industrial complex vs innovation ? An institutional perspective
No full textInternational audienceThe war in Ukraine has been demonstrating that Russian military forces are less powerful than one could expect. Despite being equipped with very advanced systems, not only Putin's expected Blitzkrieg was a failure; in addition, Ukrainian forces have been able to counter and even challenge their adversaries with less advanced but quite able capabilities like Bayraktar TB2 drones. However, this situation takes place after a major modernisation process of Russian armed forces over the past decade and an impressive display of military power in Syria.Beyond the instance of Russia, this situation raises many question on the ability of the defence technological and industrial base (DTIB) to deliver appropriate capabilities. Indeed, major arms-producing countries use to invest massively in defence research and development. Even though resulting technological progress has true military value, conflicts over the past two decades have underlined that the legacy DTIB does not cover the while spectrum of required or possible military capabilities.It is therefore useful to revisit the scientific literature on the military-industrial complex, dating mainly from the 1970s and 1980s. Indeed, what can explain the gap between what capabilities armed forces may expect and what industry is delivering in major arms-producing countries? Does this inertia result from the influence of a "military-industrial complex" that is more able to defend vested interests rather than fulfil capability needs of armed forces?The proposed paper will explore this question by focusing on innovation stakes. Indeed, reforms of defence procurement and industrial strategy are aimed at delivering the best value for money, but innovation is also a structuring stake with much longer objectives. In the field of innovation, such reforms seem to have particularly under delivered. This is therefore a good means to question whether causes come from market dynamics or institutional issues.We could wonder whether such tension between potential innovation and procurement process results from a lock-in effect in the armament market. It is therefore relevant to revisit the literature on military-industrial complex and wonder if the way arms-producing countries have built their defence technological and industrial base is contradictory to their desire to nurture innovations in the field of military capabilities
Analyzing defects and their effects on the strength of a three-layer FSW joint by using X-ray microtomography, localized spectrum analysis, and acoustic emission
No full textInternational audienceThe present work aims at proposing an experimental strategy involving three measurement techniques, namely X-ray micro-computed tomography, Localized Spectrum Analysis, and Acoustic Emission (AE) in order to detect the initiation of damage mechanisms within an FSW joint and to track its evolution during a mechanical test. The specimen was manufactured by superimposing and welding together three aluminum alloy sheets named AA6061, AA7075, and AA2024. First, the defects within the internal structure of the joint were identified by using X-ray micro-computed tomography. The joint was then subjected to a tensile test. The evolution of the defects as a function of the tensile stress was monitored by using acoustic emission coupled with non-contact strain fields measurements on two perpendicular faces of the specimen. The findings highlight a good correlation between the strain-concentration zones and those characterized by a high density of weld defects, as identified from the analysis of the microtomography results. The comparison between AE results with those obtained from the computed microtomography analysis enabled us to state that the first crack sources were caused by the weakness zones detected within the initial state of the welded joint. The findings from an unsupervised classification of the AE activity are that three main mechanisms governed the damage evolution of the studied FSW joint. The acoustic signature of each cracking mechanism is defined by a pair of values (peak frequency, amplitude), each within a specific range. A deep analysis of the experimental results highlights a good correlation between the AE results with those from the strain analysis
The Complex-Scaled Half-Space Matching Method
No full textInternational audienceThe Half-Space Matching (HSM) method has recently been developed as a new method for the solution of 2D scattering problems with complex backgrounds, providing an alternative to Perfectly Matched Layers (PML) or other artificial boundary conditions. Based on half-plane representations for the solution, the scattering problem is rewritten as a system of integral equations in which the unknowns are restrictions of the solution to the boundaries of a finite number of overlapping half-planes contained in the domain: this integral equation system is coupled to a standard finite element discretisation localised around the scatterer. While satisfactory numerical results have been obtained for real wavenumbers, wellposedness and equivalence to the original scattering problem have been established only for complex wavenumbers. In the present paper, by combining the HSM framework with a complex-scaling technique, we provide a new formulation for real wavenumbers which is provably well-posed and has the attraction for computation that the complex-scaled solutions of the integral equation system decay exponentially at infinity. The analysis requires the study of double-layer potential integral operators on intersecting infinite lines, and their analytic continuations. The effectiveness of the method is validated by preliminary numerical results