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Search Results: 1 - 10 of 8297 matches for " Grandclément Philippe "
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Spectral Methods for Numerical Relativity
Grandclément Philippe,Novak Jér?me
Living Reviews in Relativity , 2009,
Abstract: Equations arising in general relativity are usually too complicated to be solved analytically and one must rely on numerical methods to solve sets of coupled partial differential equations. Among the possible choices, this paper focuses on a class called spectral methods in which, typically, the various functions are expanded in sets of orthogonal polynomials or functions. First, a theoretical introduction of spectral expansion is given with a particular emphasis on the fast convergence of the spectral approximation. We then present different approaches to solving partial differential equations, first limiting ourselves to the one-dimensional case, with one or more domains. Generalization to more dimensions is then discussed. In particular, the case of time evolutions is carefully studied and the stability of such evolutions investigated. We then present results obtained by various groups in the field of general relativity by means of spectral methods. Work, which does not involve explicit time-evolutions, is discussed, going from rapidly-rotating strange stars to the computation of black-hole–binary initial data. Finally, the evolution of various systems of astrophysical interest are presented, from supernovae core collapse to black-hole–binary mergers.
Binary black holes in circular orbits. II. Numerical methods and first results
Philippe Grandclément,Eric Gourgoulhon,Silvano Bonazzola
Physics , 2001, DOI: 10.1103/PhysRevD.65.044021
Abstract: We present the first results from a new method for computing spacetimes representing corotating binary black holes in circular orbits. The method is based on the assumption of exact equilibrium. It uses the standard 3+1 decomposition of Einstein equations and conformal flatness approximation for the 3-metric. Contrary to previous numerical approaches to this problem, we do not solve only the constraint equations but rather a set of five equations for the lapse function, the conformal factor and the shift vector. The orbital velocity is unambiguously determined by imposing that, at infinity, the metric behaves like the Schwarzschild one, a requirement which is equivalent to the virial theorem. The numerical scheme has been implemented using multi-domain spectral methods and passed numerous tests. A sequence of corotating black holes of equal mass is calculated. Defining the sequence by requiring that the ADM mass decrease is equal to the angular momentum decrease multiplied by the orbital angular velocity, it is found that the area of the apparent horizons is constant along the sequence. We also find a turning point in the ADM mass and angular momentum curves, which may be interpreted as an innermost stable circular orbit (ISCO). The values of the global quantities at the ISCO, especially the orbital velocity, are in much better agreement with those from third post-Newtonian calculations than with those resulting from previous numerical approaches.
Mergers of Irrotational Neutron Star Binaries in Conformally Flat Gravity
Joshua A. Faber,Philippe Grandclément,Frederic A. Rasio
Physics , 2003, DOI: 10.1103/PhysRevD.69.124036
Abstract: We present the first results from our new general relativistic, Lagrangian hydrodynamics code, which treats gravity in the conformally flat (CF) limit. The evolution of fluid configurations is described using smoothed particle hydrodynamics (SPH), and the elliptic field equations of the CF formalism are solved using spectral methodsin spherical coordinates. The code was tested on models for which the CF limit is exact, finding good agreement with the classical Oppenheimer-Volkov solution for a relativistic static spherical star as well as the exact semi-analytic solution for a collapsing spherical dust cloud. By computing the evolution of quasi-equilibrium neutron star binary configurations in the absence of gravitational radiation backreaction, we have confirmed that these configurations can remain dynamically stable all the way to the development of a cusp. With an approximate treatment of radiation reaction, we have calculated the complete merger of an irrotational binary configuration from the innermost point on an equilibrium sequence through merger and remnant formation and ringdown, finding good agreement withprevious relativistic calculations. In particular, we find that mass loss is highly suppressed by relativistic effects, but that, for a reasonably stiff neutron star equation of state, the remnant is initially stable against gravitational collapse because of its strong differential rotation. The gravity wave signal derived from our numerical calculation has an energy spectrum which matches extremely well with estimates based solely on quasi-equilibrium results, deviating from the Newtonian power-law form at frequencies below 1 kHz, i.e., within the reach of advanced interferometric detectors.
Oscillons and Quasi-breathers in the φ^4 Klein-Gordon model
Gyula Fodor,Péter Forgács,Philippe Grandclément,István Rácz
Physics , 2006, DOI: 10.1103/PhysRevD.74.124003
Abstract: Strong numerical evidence is presented for the existence of a continuous family of time-periodic solutions with ``weak'' spatial localization of the spherically symmetric non-linear Klein-Gordon equation in 3+1 dimensions. These solutions are ``weakly'' localized in space in that they have slowly decaying oscillatory tails and can be interpreted as localized standing waves (quasi-breathers). By a detailed analysis of long-lived metastable states (oscillons) formed during the time evolution it is demonstrated that the oscillon states can be quantitatively described by the weakly localized quasi-breathers.It is found that the quasi-breathers and their oscillon counterparts exist for a whole continuum of frequencies.
Circular geodesics and thick tori around rotating boson stars
Z. Meliani,F. H. Vincent,P. Grandclément,E. Gourgoulhon,R. Monceau-Baroux,O. Straub
Physics , 2015, DOI: 10.1088/0264-9381/32/23/235022
Abstract: Accretion disks play an important role in the evolution of their relativistic inner compact objects. The emergence of a new generation of interferometers will allow to resolve these accretion disks and provide more information about the properties of the central gravitating object. Due to this instrumental leap forward it is crucial to investigate the accretion disk physics near various types of inner compact objects now to deduce later constraints on the central objects from observations. A possible candidate for the inner object is the boson star. Here, we will try to analyze the differences between accretion structures surrounding boson stars and black holes. We aim at analysing the physics of circular geodesics around boson stars and study simple thick accretion tori (so-called Polish doughnuts) in the vicinity of these stars. We realize a detailed study of the properties of circular geodesics around boson stars. We then perform a parameter study of thick tori with constant angular momentum surrounding boson stars. This is done using the boson star models computed by a code constructed with the spectral solver library KADATH. We demonstrate that all the circular stable orbits are bound. In the case of a constant angular momentum torus, a cusp in the torus surface exists only for boson stars with a strong gravitational scalar field. Moreover, for each inner radius of the disk, the allowed specific angular momentum values lie within a constrained range which depends on the boson star considered. We show that the accretion tori around boson stars have different characteristics than in the vicinity of a black hole. With future instruments it could be possible to use these differences to constrain the nature of compact objects.
A Trotter product formula for gradient flows in metric spaces
Philippe Clément,Jan Maas
Mathematics , 2010,
Abstract: We prove a Trotter product formula for gradient flows in metric spaces. This result is applied to establish convergence in the L^2-Wasserstein metric of the splitting method for some Fokker-Planck equations and porous medium type equations perturbed by a potential.
A Comparative Study of Parallel Kinematic Architectures for Machining Applications
Philippe Wenger,Clément Gosselin,Damien Chablat
Computer Science , 2007,
Abstract: Parallel kinematic mechanisms are interesting alternative designs for machining applications. Three 2-DOF parallel mechanism architectures dedicated to machining applications are studied in this paper. The three mechanisms have two constant length struts gliding along fixed linear actuated joints with different relative orientation. The comparative study is conducted on the basis of a same prescribed Cartesian workspace for the three mechanisms. The common desired workspace properties are a rectangular shape and given kinetostatic performances. The machine size of each resulting design is used as a comparative criterion. The 2-DOF machine mechanisms analyzed in this paper can be extended to 3-axis machines by adding a third joint.
Modulation of Phytoalexin Biosynthesis in Engineered Plants for Disease Resistance
Philippe Jeandet,Christophe Clément,Eric Courot,Sylvain Cordelier
International Journal of Molecular Sciences , 2013, DOI: 10.3390/ijms140714136
Abstract: Phytoalexins are antimicrobial substances of low molecular weight produced by plants in response to infection or stress, which form part of their active defense mechanisms. Starting in the 1950’s, research on phytoalexins has begun with biochemistry and bio-organic chemistry, resulting in the determination of their structure, their biological activity as well as mechanisms of their synthesis and their catabolism by microorganisms. Elucidation of the biosynthesis of numerous phytoalexins has permitted the use of molecular biology tools for the exploration of the genes encoding enzymes of their synthesis pathways and their regulators. Genetic manipulation of phytoalexins has been investigated to increase the disease resistance of plants. The first example of a disease resistance resulting from foreign phytoalexin expression in a novel plant has concerned a phytoalexin from grapevine which was transferred to tobacco. Transformations were then operated to investigate the potential of other phytoalexin biosynthetic genes to confer resistance to pathogens. Unexpectedly, engineering phytoalexins for disease resistance in plants seem to have been limited to exploiting only a few phytoalexin biosynthetic genes, especially those encoding stilbenes and some isoflavonoids. Research has rather focused on indirect approaches which allow modulation of the accumulation of phytoalexin employing transcriptional regulators or components of upstream regulatory pathways. Genetic approaches using gain- or less-of functions in phytoalexin engineering together with modulation of phytoalexin accumulation through molecular engineering of plant hormones and defense-related marker and elicitor genes have been reviewed.
Parametric Stiffness Analysis of the Orthoglide
Félix Majou,Clément Gosselin,Philippe Wenger,Damien Chablat
Computer Science , 2007,
Abstract: This paper presents a parametric stiffness analysis of the Orthoglide, a 3-DOF translational Parallel Kinematic Machine. First, a compliant modeling of the Orthoglide is conducted based on an existing method. Then stiffness matrix is symbolically computed. This allows one to easily study the influence of the geometric design parameters on the matrix elements. Critical links are displayed. Cutting forces are then modeled so that static displacements of the Orthoglide tool during slot milling are symbolically computed. Influence of the geometric design parameters on the static displacements is checked as well. Other machining operations can be modeled. This parametric stiffness analysis can be applied to any parallel manipulator for which stiffness is a critical issue.
Parametric stiffness analysis of the Orthoglide
Félix Majou,Clément Gosselin,Philippe Wenger,Damien Chablat
Computer Science , 2007,
Abstract: This paper presents a parametric stiffness analysis of the Orthoglide. A compliant modeling and a symbolic expression of the stiffness matrix are conducted. This allows a simple systematic analysis of the influence of the geometric design parameters and to quickly identify the critical link parameters. Our symbolic model is used to display the stiffest areas of the workspace for a specific machining task. Our approach can be applied to any parallel manipulator for which stiffness is a critical issue.
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