Action (physique)

Résumé

L’action est une grandeur fondamentale de la physique théorique, ayant la dimension d'une énergie multipliée par une durée, ou d'une quantité de mouvement multipliée par une distance. Elle est notée habituellement \mathcal{S} et plus rarement \mathcal{A}. Cette grandeur a été définie par Leibniz en 1690. Elle s'est avérée d'une grande importance lors de la mise en évidence du principe de moindre action par Maupertuis en 1744, et plus tard lors de la découverte par Planck en 1900 de la constante universelle qui porte son nom, nommée par lui « quantum élémentaire d'action ». À la différence de l'énergie, qui est relative à la vitesse, l'action est une unité universelle et un invariant relativiste. Caractérisant globalement l'état d'un système et son évolution, c'est une grandeur fonctionnelle, qui prend en argument la trajectoire du système et la décrit globalement par un scalaire. L'évolution du système obéit au principe de moindre action, ce qui permet de

Source officielle

https://fr.wikipedia.org/wiki/Action_(physique)

À propos de ce résultat

Cette page est générée automatiquement et peut contenir des informations qui ne sont pas correctes, complètes, à jour ou pertinentes par rapport à votre recherche. Il en va de même pour toutes les autres pages de ce site. Veillez à vérifier les informations auprès des sources officielles de l'EPFL.

Publications associées (5)

Publications associées

Chargement

Personnes associées

Chargement

Unités associées

Chargement

Concepts associés (76)

La mécanique hamiltonienne est une reformulation de la mécanique newtonienne. Son formalisme a facilité l'élaboration théorique de la mécanique quantique. Elle a été formulée par William Rowan Hamil

vignette|296x296px|Ce diagramme de Feynman représente l'annihilation d'un électron et d'un positron, qui produit un photon (représenté par une ligne ondulée bleue). Ce photon se décompose en une paire

vignette|Joseph-Louis Lagrange Les équations de Lagrange, découvertes en 1788 par le mathématicien Joseph-Louis Lagrange, sont une reformulation de la mécanique classique. Équations de première

Afficher plus

Concepts associés

Chargement

Cours associés (10)

Cours associés

Chargement

Séances de cours associées (22)

Séances de cours associées

Chargement

, ,

Using an isolated Milky Way-mass galaxy simulation, we compare results from nine state-of-the-art gravitohydrodynamics codes widely used in the numerical community. We utilize the infrastructure we have built for the AGORA High-resolution Galaxy Simulations Comparison Project. This includes the common disk initial conditions, common physics models (e. g., radiative cooling and UV background by the standardized package GRACKLE) and common analysis toolkit yt, all of which are publicly available. Subgrid physics models such as Jeans pressure floor, star formation, supernova feedback energy, and metal production are carefully constrained across code platforms. With numerical accuracy that resolves the disk scale height, we find that the codes overall agree well with one another in many dimensions including: gas and stellar surface densities, rotation curves, velocity dispersions, density and temperature distribution functions, disk vertical heights, stellar clumps, star formation rates, and Kennicutt-Schmidt relations. Quantities such as velocity dispersions are very robust (agreement within a few tens of percent at all radii) while measures like newly formed stellar clump mass functions show more significant variation (difference by up to a factor of similar to 3). Systematic differences exist, for example, between mesh-based and particle-based codes in the low-density region, and between more diffusive and less diffusive schemes in the high-density tail of the density distribution. Yet intrinsic code differences are generally small compared to the variations in numerical implementations of the common subgrid physics such as supernova feedback. Our experiment reassures that, if adequately designed in accordance with our proposed common parameters, results of a modern high-resolution galaxy formation simulation are more sensitive to input physics than to intrinsic differences in numerical schemes.

Iop Publishing Ltd2016

Chargement

Semiclassical analysis of the quantum instanton approximation

Christophe Louis Joseph Jean-Luc Vaillant, Jiri Vanicek

We explore the relation between the quantum and semiclassical instanton approximations for the reaction rate constant. From the quantum instanton expression, we analyze the contributions to the rate constant in terms of minimum-action paths and find that two such paths dominate the expression. For symmetric barriers, these two paths join together to describe the semiclassical instanton periodic orbit. However, for asymmetric barriers, one of the two paths takes an unphysically low energy and dominates the expression, leading to order-of-magnitude errors in the rate predictions. Nevertheless, semiclassical instanton theory remains accurate. We conclude that semiclassical instanton theory can be obtained directly from the semiclassical limit of the quantum instanton for symmetric systems. We suggest a modification of the quantum instanton approach which avoids sampling the spurious path and thus has a stronger connection to semiclassical instanton theory, giving numerically accurate predictions even for very asymmetric systems in the low temperature limit.

2019

Chargement

A Coarse-mesh Methodology for the Analysis of One and Two-phase Nuclear Reactor Thermal-hydraulics in a Multi-physics Context

Stefan Radman

The analysis of nuclear reactors for performance and safety assessment benefits from the use of computational tools. In this context, this work aims at the development and application of a thermal-hydraulics methodology and related software that respond to emerging needs in the computational field: 1) greater geometric and physics modelling flexibility; 2) streamlined coupling with other single-physics to enable multi-physics capabilities; 3) parallel scalability on High Performance Computing clusters; 4) adoption of modern programming practices. A coarse-mesh approach is proposed to offer a reasonable balance between computational accuracy, comparable to that of sub-channel codes, and computational burdens. The developed approach can make use of general 3-D geometries with unstructured meshes, which are beneficial to the aforementioned geometric flexibility needs. Additionally, in a multi-physics context, field transfer operations between the different physics are simplified both by the adoption of a coarse-mesh approach and by the use of standardized mesh formats. The employed programming framework consists of the Finite Volume Method-based OpenFOAM library, which offers the desired features of massive parallel scalability and of a modern object-oriented programming paradigm. The coarse-mesh methodology is presented alongside a thorough theoretical derivation of the governing equations for a generic multi-phase system. Based on this, a computer code is developed for the modelling of one-phase and two-phase flows, with a focus on the simulation of Sodium-cooled Fast Reactors (SFRs), which represent the nearest-term deployable fast reactor technology. These were also chosen as the simulation of phase change in sodium represents a challenging case for the numerical stability of two-phase solution algorithms. The main achievements of this development effort consist of: 1) a novel solution algorithm for two-phase pressure-velocity coupling that enhances stability and performances compared to existing algorithms; 2) implementation of the code based on object-oriented programming practices, which allow for a seamless implementation of different working fluids and structure models; 3) code verification via an ad-hoc implementation of the Method of Manufactured Solutions; 4) demonstrated good parallel scaling of the code up to thousands of computer cores. In terms of applications: 1) preliminary validation based on sodium boiling experiments; 2) detailed investigation of existing and novel features for SFR fuel elements. Furthermore, the multi-physics capabilities of the developed methodology are demonstrated by integrating it within the GeN-Foam multi-physics environment. As a test case, the resulting software is applied to the simulation of a Loss Of Flow Without SCRAM test performed at the Fast Flux Test Facility. This benchmark re-analysis takes place within the framework of a coordinated research project by the International Atomic Energy Agency and is set to provide valuable feedback in terms of code-to-code comparison data.

EPFL2021

Chargement

A Coarse-mesh Methodology for the Analysis of One and Two-phase Nuclear Reactor Thermal-hydraulics in a Multi-physics Context

Stefan Radman

EPFL2021