Paris, France
Paris, France

The ENSTA ParisTech, École nationale supérieure de techniques avancées , is one of the most prestigious and selective French grande école in engineering. It is a member of ParisTech .It is located in Palaiseau , next to École polytechnique.Every year about 150 engineers graduate from it.ENSTA offers its students general engineering training with the aim of enabling them to design, produce and oversee complex industrial systems, while meeting strict economic constraints and dealing with an international environment. To do this, the School provides high-level scientific and technological training, which is frequently updated to keep pace with changes in the leading edge technologies and supplemented by language, general culture, law and economics teaching.The teaching is given by research professors at ENSTA with the participation of numerous auxiliary teachers from the economic and industrial world familiar with the latest technical developments in a wide variety of fields.Research, which is one of the School's primary missions, makes a dynamic contribution in both the fundamental and applied fields, to the School's pedagogical project and to meeting the needs of business. Half is the responsibility of the School's research professors, and the other half is carried out by researchers from the CNRS, the INSERM and the École polytechnique working in ENSTA's premises.The general nature of the training given enables ENSTA graduates to find a career in a large number of sectors such as the automotive or naval industry, networks and telecommunications, space propulsion, robotics, oceanology and the environment. Most of the ENSTA graduate engineers are much sought-after by companies and generally find their first job in R&D departments and design offices, rapidly moving towards supervisory and project management posts.ENSTA is a public teaching and research establishment operating under the supervision of the Ministry of Defence. It is headed by a general officer of the Corps of Ordnance Ingineers . Some former graduates of École polytechnique attend ENSTA before joining the military Corps of Ordnance Ingineers, which staffs the DGA. Wikipedia.

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Pelanti M.,ENSTA ParisTech | Shyue K.-M.,National Taiwan University
Journal of Computational Physics | Year: 2014

We model liquid-gas flows with cavitation by a variant of the six-equation single-velocity two-phase model with stiff mechanical relaxation of Saurel-Petitpas-Berry (Saurel et al., 2009) [9]. In our approach we employ phasic total energy equations instead of the phasic internal energy equations of the classical six-equation system. This alternative formulation allows us to easily design a simple numerical method that ensures consistency with mixture total energy conservation at the discrete level and agreement of the relaxed pressure at equilibrium with the correct mixture equation of state. Temperature and Gibbs free energy exchange terms are included in the equations as relaxation terms to model heat and mass transfer and hence liquid-vapor transition. The algorithm uses a high-resolution wave propagation method for the numerical approximation of the homogeneous hyperbolic portion of the model. In two dimensions a fully-discretized scheme based on a hybrid HLLC/Roe Riemann solver is employed. Thermo-chemical terms are handled numerically via a stiff relaxation solver that forces thermodynamic equilibrium at liquid-vapor interfaces under metastable conditions. We present numerical results of sample tests in one and two space dimensions that show the ability of the proposed model to describe cavitation mechanisms and evaporation wave dynamics. © 2013 Elsevier Inc.

Corde S.,ENSTA ParisTech | Ta Phuoc K.,ENSTA ParisTech | Lambert G.,ENSTA ParisTech | Fitour R.,ENSTA ParisTech | And 4 more authors.
Reviews of Modern Physics | Year: 2013

Relativistic interaction of short-pulse lasers with underdense plasmas has recently led to the emergence of a novel generation of femtosecond x-ray sources. Based on radiation from electrons accelerated in plasma, these sources have the common properties to be compact and to deliver collimated, incoherent, and femtosecond radiation. In this article, within a unified formalism, the betatron radiation of trapped and accelerated electrons in the so-called bubble regime, the synchrotron radiation of laser-accelerated electrons in usual meter-scale undulators, the nonlinear Thomson scattering from relativistic electrons oscillating in an intense laser field, and the Thomson backscattered radiation of a laser beam by laser-accelerated electrons are reviewed. The underlying physics is presented using ideal models, the relevant parameters are defined, and analytical expressions providing the features of the sources are given. Numerical simulations and a summary of recent experimental results on the different mechanisms are also presented. Each section ends with the foreseen development of each scheme. Finally, one of the most promising applications of laser-plasma accelerators is discussed: the realization of a compact free-electron laser in the x-ray range of the spectrum. In the conclusion, the relevant parameters characterizing each sources are summarized. Considering typical laser-plasma interaction parameters obtained with currently available lasers, examples of the source features are given. The sources are then compared to each other in order to define their field of applications. © 2013 American Physical Society.

Doare O.,ENSTA ParisTech | Michelin S.,Ecole Polytechnique - Palaiseau
Journal of Fluids and Structures | Year: 2011

This paper investigates the energy harvested from the flutter of a plate in an axial flow by making use of piezoelectric materials. The equations for fully coupled linear dynamics of the fluid-solid and electrical systems are derived. The continuous limit is then considered, when the characteristic length of the plate's deformations is large compared to the piezoelectric patches' length. The linear stability analysis of the coupled system is addressed from both a local and global point of view. Piezoelectric energy harvesting adds rigidity and damping on the motion of the flexible plate, and destabilization by dissipation is observed for negative energy waves propagating in the medium. This result is confirmed in the global analysis of fluttering modes of a finite-length plate. It is finally observed that waves or modes destabilized by piezoelectric coupling maximize the energy conversion efficiency. © 2011 Elsevier Ltd.

Depresseux A.,ENSTA ParisTech
Nature Photonics | Year: 2015

The advent of X-ray free-electron lasers has granted researchers an unprecedented access to the ultrafast dynamics of matter on the nanometre scale. Aside from being compact, seeded plasma-based soft X-ray lasers (SXRLs) turn out to be enticing as photon-rich sources (up to 1015 per pulse) that display high-quality optical properties. Hitherto, the duration of these sources was limited to the picosecond range, which consequently restricts the field of applications. This bottleneck was overcome by gating the gain through ultrafast collisional ionization in a high-density plasma generated by an ultraintense infrared pulse (a few 1018 W cm-2) guided in an optically pre-formed plasma waveguide. For electron densities that ranged from 3 × 1018 cm-3 to 1.2 × 1020 cm-3, the gain duration was measured to drop from 7 ps to an unprecedented value of about 450 fs, which paves the way to compact and ultrafast SXRL beams with performances previously only accessible in large-scale facilities. © 2015 Nature Publishing Group

A thermodynamic approach is proposed to determine the dissociation conditions of salt hydrates and semiclathrate hydrates. The thermodynamic properties of the liquid phase are described with the SAFT-VRE equation of state, and the solid-liquid equilibria are solved by applying the Gibbs energy minimization criterion under stoichiometric constraints. The methodology is applied to water + halide salt systems, and an excellent description of the solid-liquid coexistence curves is obtained. The approach is extended to the water + tetra-n-butylammonium bromide (TBAB) binary mixture, and an accurate representation of the solid-liquid coexistence curves and dissociation enthalpies is obtained. The van der Waals-Platteeuw (vdW-P) theory combined with the new model for salt hydrates is used to determine the dissociation temperatures of semiclathrate hydrates of TBAB + carbon dioxide. A good description of the dissociation pressures of CO2 semiclathrate hydrates is obtained over wide temperature, pressure, and TBAB composition ranges (AAD = 10.5%). For high TBAB weight fractions the new model predicts a change of hydrate structure from type A to type B as the partial pressure of CO2 is increased. The model can also capture a change of behavior with respect to TBAB concentration, which has been observed experimentally: an increase of the TBAB weight fraction leads to a stabilization of the gas semiclathrate hydrate at low initial TBAB concentrations below the stoichiometric composition but leads to a destabilization of the hydrate at TBAB concentrations above the stoichiometric composition. © 2010 American Chemical Society.

Malka V.,ENSTA ParisTech
Physics of Plasmas | Year: 2012

This review article highlights the tremendous evolution of the research on laser plasma accelerators which has, in record time, led to the production of high quality electron beams at the GeV level, using compact laser systems. I will describe the path we followed to explore different injection schemes and I will present the most significant breakthrough which allowed us to generate stable, high peak current and high quality electron beams, with control of the charge, of the relative energy spread and of the electron energy. © 2012 American Institute of Physics.

Nedjar B.,ENSTA ParisTech
International Journal of Solids and Structures | Year: 2016

The idea in this paper is to build a class of constitutive equations for highly compressible isotropic materials that, among others, are capable to describe a zero apparent Poisson's ratio in the whole finite strain range, not only for moderate straining. This remarkable property is, for instance, observed in many soft materials with micro-structures such as sponges and polymeric foams with high porosities. It would then be suitable to describe their behavior within a macroscopic modeling framework. More specifically, herein by means of elementary considerations, we deduce adequate forms of strain-energy functions that are a priori decomposed into purely volumetric and volume-preserving parts. A class of compressible hyperelastic materials of the general Odgen type is obtained. It can consequently be specialized, for instance, to neo-Hookean, Mooney-Rivlin, and Varga's model types as well. Furthermore, for the elastic parameters, a connection with the limiting case of linear elasticity is made whenever possible, in particular with the classical Poisson's ratio, and with the bulk to shear moduli ratio. © 2016.

Ciarlet Jr. P.,ENSTA ParisTech
Computers and Mathematics with Applications | Year: 2012

To solve variational indefinite problems, a celebrated tool is the Banach-Nečas-Babuška theory, which relies on the inf-sup condition. Here, we choose an alternate theory, T-coercivity. This theory relies on explicit inf-sup operators, both at the continuous and discrete levels. It is applied to solve Helmholtz-like problems in acoustics and electromagnetics. We provide simple proofs to solve the exact and discrete problems, and to show convergence under fairly general assumptions. We also establish sharp estimates on the convergence rates. © 2012 Elsevier Ltd. All rights reserved.

Baranes A.,ENSTA ParisTech | Oudeyer P.-Y.,ENSTA ParisTech
Robotics and Autonomous Systems | Year: 2013

We introduce the Self-Adaptive Goal Generation Robust Intelligent Adaptive Curiosity (SAGG-RIAC) architecture as an intrinsically motivated goal exploration mechanism which allows active learning of inverse models in high-dimensional redundant robots. This allows a robot to efficiently and actively learn distributions of parameterized motor skills/policies that solve a corresponding distribution of parameterized tasks/goals. The architecture makes the robot sample actively novel parameterized tasks in the task space, based on a measure of competence progress, each of which triggers low-level goal-directed learning of the motor policy parameters that allow to solve it. For both learning and generalization, the system leverages regression techniques which allow to infer the motor policy parameters corresponding to a given novel parameterized task, and based on the previously learnt correspondences between policy and task parameters. We present experiments with high-dimensional continuous sensorimotor spaces in three different robotic setups: (1) learning the inverse kinematics in a highly-redundant robotic arm, (2) learning omnidirectional locomotion with motor primitives in a quadruped robot, and (3) an arm learning to control a fishing rod with a flexible wire. We show that (1) exploration in the task space can be a lot faster than exploration in the actuator space for learning inverse models in redundant robots; (2) selecting goals maximizing competence progress creates developmental trajectories driving the robot to progressively focus on tasks of increasing complexity and is statistically significantly more efficient than selecting tasks randomly, as well as more efficient than different standard active motor babbling methods; (3) this architecture allows the robot to actively discover which parts of its task space it can learn to reach and which part it cannot. © 2012 Elsevier B.V. All rights reserved.

Nedjar B.,ENSTA ParisTech
International Journal of Solids and Structures | Year: 2014

Delayed fracture due to debonding can be observed in many unidirectional fibre-reinforced composites when the fibre/matrix interface experiences creep. The aim of this work is to describe such a phenomenon within the recently proposed modeling framework of transverse isotropy that allows for a neat decomposition of the mechanical behavior into fibre-directional, transverse, and pure shear parts. Specifically, debonding is here chosen to be governed by the tension transverse to the fibres. One can then speak of a mode-I debonding if use is made of the terminology adopted in fracture mechanics. On another hand, the time-dependent response is attributed to the matrix constituent. As the role of this latter is to deform and support stresses primarily in shear, a viscoelastic behavior is introduced that affects solely the pure shear part of the behavior. We show that both characteristics can be easily embedded into the aforementioned formulation. Among others, the occurrence of tertiary creep is made possible to predict. It is otherwise found that the predicted debonding path always propagates along the direction of the fibres in agreement with many experimental observations found in the literature. On the numerical side, the algorithmic treatment of debonding is independent of the one for viscoelasticity. This renders the implementation within the context of the finite element method very easy. © 2014 Elsevier Ltd. All rights reserved.

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