Showing papers by "Mines ParisTech published in 1998"

Cormas: Common-Pool Resources and Multi-agent Systems

Abstract: This paper describes a simulation environment, called Cormas, that relies on multi-agent systems and has been achieved in Smalffalk, using VisualWorks software Such a simulation tool may prove useful to better understand the complex interactions between natural and social dynamics when studying renewable resource management The general principles of the Cormas platform are first presented, then the implementation is described Two models built with Cormas allow to illustrate the use and the genericity of this tool

From the World of Science to the World of Research

Abstract: In the last century and a half, scientific development has been breathtaking, but the understanding of this progress has dramatically changed It is characterized by the transition from the culture of "science" to the culture of "research" Science is certainty; research is uncertainty Science is supposed to be cold, straight, and detached; research is warm, involving, and risky Science puts an end to the vagaries of human disputes; research creates controversies Science produces objectivity by escaping as much as possible from the shackles of ideology, passions, and emotions; research feeds on all of those to render objects of inquiry familiar

A global computer software for polymer flows in corotating twin screw extruders

Abstract: A global computation model for self-wiping corotating twin screw extruders is proposed. Based on a 1D approximated approach, it has been validated by comparison with experimentation and more sophisticated numerical models. It allows one to obtain, for any screw profile including left-and right-handed screw elements and kneading discs, the profile along the screws of the main flow variables, such as pressure, mean temperature, residence time, and filling ratio. Owing to the approximations made, this model can be easily and rapidly run on a personal computer or a workstation. Important applications may be found in screw profile design, scaleup, compounding or reactive extrusion.

Geochemical Evidence for a Comet Shower in the Late Eocene

Abstract: Analyses of pelagic limestones indicate that the flux of extraterrestrial helium-3 to Earth was increased for a 2.5-million year (My) period in the late Eocene. The enhancement began ∼1 My before and ended ∼1.5 My after the major impact events that produced the large Popigai and Chesapeake Bay craters ∼36 million years ago. The correlation between increased concentrations of helium-3, a tracer of fine-grained interplanetary dust, and large impacts indicates that the abundance of Earth-crossing objects and dustiness in the inner solar system were simultaneously but only briefly enhanced. These observations provide evidence for a comet shower triggered by an impulsive perturbation of the Oort cloud.

Connectivity on Complete Lattices

Abstract: Classically, connectivity is a topological notion for sets, often introduced by means of arcs. A nontopological axiomatics has been proposed by Matheron and Serra. The present paper extends it to complete sup-generated lattices. A connection turns out to be characterized by a family of openings labelled by the sup-generators, which partition each element of the lattice into maximal terms, of zero infima. When combined with partition closings, these openings generate strong sequential alternating filters. Starting from a first connection several others may be designed by acting on some dilations or symmetrical operators. When applying this theory to function lattices, one interprets the so-called connected operators in terms of actual connections, as well as the watershed mappings. But the theory encompasses the numerical functions and extends, among others, to multivariate lattices.

Disappearance of Elliptic Flow: A New Probe for the Nuclear Equation of State

Abstract: Using a relativistic hadron transport model, we investigate the utility of the elliptic flow excitation function as a probe for the stiffness of nuclear matter and for the onset of a possible quark-gluon-plasma (QGP) phasetransition at AGS energies 1 < EBeam < 11 AGeV. The excitation function shows a strong dependence on the nuclear equation of state, and exhibits characteristic signatures which could signal the onset of a phase transition to the QGP.

Stabilization of a rotating body beam without damping

Abstract: This paper deals with the stabilization of a rotating body-beam system with torque control. The system we consider is the one studied by Baillieul and Levi (1987). Xu and Baillieul proved (1993) that, for any constant angular velocity smaller than a critical one, this system can be stabilized by means of a feedback torque control law if there is damping. We prove that this result also holds if there is no damping.

Mechanics of generalized continua: construction by homogenizaton

Abstract: La mecanique des milieux continus generalises fournit des outils pour decrire les effets d'echelle que presentent certains materiaux heterogenes. Cependant les liens entre les nouveaux degres de liberte que certains de ces milieux introduisent et la microstructure ne sont clairement etablis que pour quelques cas extremes (cristaux liquides...). Dans ce travail, on essaie de construire un milieu de Cosserat homogene equivalent a un milieu classique micro-heterogene, par des methodes d'homogeneisation generalisees. Ces techniques permettent alors de rendre compte des champs de contraintes et de deformation dans un materiau heterogene sollicite de telle sorte que la taille de l'element de volume representatif ne soit pas negligeable devant la longueur d'onde des sollicitations.

Low-pressure, high-density plasma nitriding: mechanisms, technology and results

Abstract: This paper reviews the low-pressure (<10 Pa), high-density plasma-assisted nitriding processes recently developed for metallurgical surface modification to improve wear, hardness and fatigue resistance of ferrous and non-ferrous materials. For that purpose, plasma generation is most frequently ensured by d.c. glow discharges at relatively high pressure (100–1000 Pa) with the underlying limitations associated with this technology. Nevertheless, more flexibility and control are required for plasma nitriding of promising non-ferrous materials such as titanium, aluminium and their alloys. These requirements are fulfilled by the recently developed enhanced or intensified plasma nitriding processes that operate at lower pressures (<10 Pa) such as: thermionically assisted d.c. triode arrangements (TAT), plasma immersion ion implantation (PIII) or plasma source ion implantation (PSII), electron cyclotron resonance (ECR) systems and thermionic arc discharges (TAD). The purpose of this paper is to review these new nitriding processes from both technological and fundamental points of view. Plasma parameters and plasma–surface interactions are considered for these processes.

Numerical simulation of macrosegregation: a comparison between finite volume method and finite element method predictions and a confrontation with experiments

Abstract: Micro-macrosegregation calculations have been performed for a rectangular cavity containing either a Pb-48 wt pct Sn alloy or a Sn-5 wt pct Pb alloy. The numerical results calculated with a finite volume method (FVM) and a finite element method (FEM) are compared with experimental results previously obtained by Hebditch and Hunt. The two methods are based on the same average conservation equations governing heat and mass transfer and the same assumptions: lever rule, equal and constant density of the solid and liquid phases (except in the buoyancy term), permeability of the mushy zone given by the Carman-Kozeny relation, and no transport of the solid phase. Although the same parameters are used in both calculations, small differences are observed as a result of the different formulations. In particular, the instabilities appearing in the mushy zone (channels) of the Sn-5 wt pct Pb alloy are more pronounced with the FVM formulation as compared with FEM, whereas the opposite trend is observed for the Pb-48 wt pct Sn alloy. Nevertheless, the final segregation maps at the end of solidification compare fairly well with the experimental findings.

General models for the distributions of electric field gradients in disordered solids

Abstract: Hyperfine studies of disordered materials often yield the distribution of the electric field gradient (EFG) or related quadrupole splitting (QS). The question of the structural information that may be extracted from such distributions has been considered for more than fifteen years. Experimentally most studies have been performed using Mossbauer spectroscopy, especially on . However, NMR, NQR, EPR and PAC methods have also received some attention. The EFG distribution for a random distribution of electric charges was for instance first investigated by Czjzek et al [1] and a general functional form was derived for the joint (bivariate) distribution of the principal EFG tensor component and the asymmetry parameter . The importance of the Gauss distribution for such rotationally invariant structural models was thus evidenced. Extensions of that model which are based on degenerate multivariate Gauss distributions for the elements of the EFG tensor were proposed by Czjzek. The latter extensions have been used since that time, more particularly in Mossbauer spectroscopy, under the name `shell models'. The mathematical foundations of all the previous models are presented and critically discussed as they are evidenced by simple calculations in the case of the EFG tensor. The present article only focuses on those aspects of the EFG distribution in disordered solids which can be discussed without explicitly looking at particular physical mechanisms. We present studies of three different model systems. A reference model directly related to the first model of Czjzek, called the Gaussian isotropic model (GIM), is shown to be the limiting case for many different models with a large number of independent contributions to the EFG tensor and not restricted to a point-charge model. The extended validity of the marginal distribution of in the GIM model is discussed. It is also shown that the second model based on degenerate multivariate normal distributions for the EFG components yields questionable results and has been exaggeratedly used in experimental studies. The latter models are further discussed in the light of new results. The problems raised by these extensions are due to the fact that the consequences of the statistical invariance by rotation of the EFG tensor have not been sufficiently taken into account. Further difficulties arise because the structural degrees of freedom of the disordered solid under consideration have been confused with the degrees of freedom of QS distributions. The relations which are derived and discussed are further illustrated by the case of the EFG tensor distribution created at the centre of a sphere by m charges randomly distributed on its surface. The third model, a simple extension of the GIM, considers the case of an EFG tensor which is the sum of a fixed part and of a random part with variable weights. The bivariate distribution is calculated exactly in the most symmetric case and the effect of the random part is investigated as a function of its weight. The various models are more particularly discussed in connection with short-range order in disordered solids. An ambiguity problem which arises in the evaluation of bivariate distributions of centre lineshift (isomer shift) and quadrupole splitting from Mossbauer spectra is finally quantitatively considered.

Object-Oriented Programming Applied to the Finite Element Method Part II. Application to Material Behaviors

Abstract: This paper examines the application of C++ design patterns to the development of material constitutives equations to be used in finite element simulation softwares. All material behaviors use the same generic interface. Accordingly, the same governing principles can be applied to simple elasto-(visco)-plastic materials as well as single crystals, polycrystals and multiphased materials. As these models use numerous parameters, a generic optimization tools was also developed to adjust these parameters. Finally, a specific pre-processor can be used to quickly implement and test new material behaviors. The result (i.e., interface + optimizer + pre—processor) is an integrated approach to the development of new constitutive equations for structural computations.

Integrator backstepping for rounded controls and control rates

Abstract: We present a backstepping procedure for the design of globally stabilizing state feedback control laws such that the magnitudes of the control signals and their derivatives are bounded by constants which do not depend on the initial conditions. We accomplish this by propagating such boundedness properties through each step of the recursive design.

Stationary and stability analysis of the film casting process

Abstract: Film casting process is widely used to produce polymer film: a molten polymer is extruded through a flat die, then stretched in air and cooled on a chill roll. This study is devoted to the extensional flow between the die and the chill roll. The film shows a lateral neck-in as well as a inhomogeneous decrease of the thickness. Thickness as well as width instabilities may be observed above a critical draw ratio. An isothermal and time dependent two-dimensional (2D) membrane model is proposed and compared to a non-constant width 1D model. Newtonian and viscoelastic constitutive equations have been tested. The influence of the processing parameters (draw ratio and aspect ratio) and of the rheology of the polymer (Deborah number) on the film geometry is first determined. The onset of the draw resonance instability is finally studied by linear stability analysis and through the dynamic response to small perturbations. A critical curve splitting the processing conditions into a stable and an unstable zone is derived. It is shown that an increase of the air-gap between the die and the roll improves the stability of the process. Numerical results concerning periodic fluctuations of the flow in unstable conditions are compared with previous experimental results.