Comportement d'un assemblage de billes non frottantes dans la limite géométrique

8 PAGESNational audienceNous étudions numériquement le comportement d'un matériau granulaire modèle constitué de billes sphériques élastiques identiques non frottantes dans la limite géométrique macroscopique (à savoir la triple limite où les sollicitations extérieures sont très lentes et les grains extrêmement rigides et très nombreux). Nous montrons que les coefficients de frottement macroscopique statique et dynamique coïncident, que l'assemblage ne présente aucune dilatance, que le matériau satisfait un critère de rupture de Lade-Duncan et que ces résultats ne sont pas affectés par la nature du contrôle (en contrainte ou en volume) imposé. La résistance au cisaillement s'explique par l'anisotropie géométrique et mécanique que développe le matériau sous sollicitation

Shear flow of sphere packings in the geometric limit

3 pagesInternational audienceWe investigate the behavior of a model granular material made of frictionless, nearly rigid equal-sized beads, in the quasistatic limit, by numerical simulation. In the macroscopic geometric limit (that is the macroscopic, rigid and quasistatic limits), with either volume or normal stress controlled simulations, static and dynamic macroscopic friction coefficients coincide, dilatancy vanishes and the material satisfies a Lade-Duncan failure criterion. The macroscopic shear strength stems from both contact network and force anisotropy

A tributação dos residentes não habituais: Análise comparada

Neste trabalho pretende-se mostrar quais os fatores que foram cruciais para a implementação do benefício fiscal concedido a estrangeiros que alteram sua residência fiscal para Portugal para se beneficiarem do Regime do Residente Não Habitual, criado em 2009 para atrair investimento estrangeiro para Portugal. Analisar-se-ão, também, os requisitos para adesão do regime, com ênfase à diferenciação entre residência e domicílio fiscal. Em sequência, será traçado um estudo comparativo entre países com maior adesão ao regime, bem como, aqueles que possuem regimes fiscais similares, a fim de compreender o fluxo migratório derivado de razões fiscais, principalmente por cidadãos do Reino Unido e concluir se Portugal realmente é, de facto, a melhor opção quando o objetivo é pagar menos impostos.The objective of this essay is to show what factors were crucial for the implementation of the tax benefit granted to foreigners who change their tax residence to Portugal to benefit from the Non Habitual Resident Regime, created in 2009 to attract foreign investment to Portugal. We will also analyze the requirements for joining the regime, with emphasis on the differentiation between residence and tax domicile. In sequence, a comparative study between countries with larger adhesion to the regime, as well as, which have similar tax regimes, will be outlined in order to understand the migratory flow derived from tax reasons, mainly by United Kingdom citizens and to conclude if Portugal really is, in fact, the best option when the objective is to pay less taxes

Rotation Of The Second Cervical Vertebra In Pediatric Patient

14457557

Frictionless bead packs have macroscopic friction, but no dilatancy

The statement of the title is shown by numerical simulation of homogeneously sheared packings of frictionless, nearly rigid beads in the quasistatic limit. Results coincide for steady flows at constant shear rate γ in the limit of small γ and static approaches, in which packings are equilibrated under growing deviator stresses. The internal friction angle ϕ, equal to 5.76 $\pm$ 0.22 degrees in simple shear, is independent on the average pressure P in the rigid limit. It is shown to stem from the ability of stable frictionless contact networks to form stress-induced anisotropic fabrics. No enduring strain localization is observed. Dissipation at the macroscopic level results from repeated network rearrangements, like the effective friction of a frictionless slider on a bumpy surface. Solid fraction Φ remains equal to the random close packing value ≃ 0.64 in slowly or statically sheared systems. Fluctuations of stresses and volume are observed to regress in the large system limit, and we conclude that the same friction law for simple shear applies in the large psystem limit if normal stress or density is externally controlled. Defining the inertia number as I = γ m/(aP), with m the grain mass and a its diameter, both internal friction coefficient $\mu$∗ = tan ϕ and volume 1/Φ increase as powers of I in the quasistatic limit of vanishing I, in which all mechanical properties are determined by contact network geometry. The microstructure of the sheared material is characterized with a suitable parametrization of the fabric tensor and measurements of connectivity and coordination numbers associated with contacts and near neighbors.Comment: 19 pages. Additional technical details may be found in v

Simulations numériques du comportement de pâtes granulaires concentrées

Les suspensions non browniennes très concentrées en régime de Stokes sont étudiées par simulation numérique, en ne gardant que les interactions hydrodynamiques dominantes (lubrification) entre particules voisines. Le contact solide est possible en raison de leur rugosité de surface. En écoulement stationnaire on trouve des lois de comportement analogues à celles des matériaux granulaires, dont on met en évidence la sensibilité à la micromécanique du contact (rôle du frottement, effets non-newtoniens liés aux forces à courte portée) et les liens avec la microstructure

Phase diagram of the random frequency oscillator: The case of Ornstein-Uhlenbeck noise

We study the stability of a stochastic oscillator whose frequency is a random process with finite time memory represented by an Ornstein-Uhlenbeck noise. This system undergoes a noise-induced bifurcation when the amplitude of the noise grows. The critical curve, that separates the absorbing phase from an extended non-equilibrium steady state, corresponds to the vanishing of the Lyapunov exponent that measures the asymptotic logarithmic growth rate of the energy. We derive various expressions for this Lyapunov exponent by using different approximation schemes. This allows us to determine quantitatively the phase diagram of the random parametric oscillator.Comment: to appear in Physica

Solid behavior of anisotropic rigid frictionless bead assemblies

We investigate the structure and mechanical behavior of assemblies of frictionless, nearly rigid equal-sized beads, in the quasistatic limit, by numerical simulation. Three different loading paths are explored: triaxial compression, triaxial extension and simple shear. Generalizing recent results [1], we show that the material, despite rather strong finite sample size effects, is able to sustain a finite deviator stress in the macroscopic limit, along all three paths, without dilatancy. The shape of the yield surface is adequately described by a Lade-Duncan (rather than Mohr-Coulomb) criterion. While scalar state variables keep the same values as in isotropic systems, fabric and force anisotropies are each characterized by one parameter and are in one-to-one correspondence with principal stress ratio along all three loading paths.The anisotropy of the pair correlation function extends to a distance between bead surfaces on the order of 10% of the diameter. The tensor of elastic moduli is shown to possess a nearly singular, uniaxial structure related to stress anisotropy. Possible stress-strain relations in monotonic loading paths are also discussed

Mixed formulation for an easy and robust numerical computation of sorptivity

Sorptivity is one of the most important parameters for the quantification of water infiltration into soils. proposed a specific formulation to derive sorptivity as a function of the soil water retention and hydraulic conductivity functions, as well as initial and final soil water contents. However, this formulation requires the integration of a function involving hydraulic diffusivity, which may be undefined or present numerical difficulties that cause numerical misestimations. In this study, we propose a mixed formulation that scales sorptivity and splits the integrals into two parts: the first term involves the scaled degree of saturation, while the second involves the scaled water pressure head. The new mixed formulation is shown to be robust and well-suited to any type of hydraulic function - even with infinite hydraulic diffusivity or positive air-entry water pressure heads - and any boundary condition, including infinite initial water pressure head, h→-∞. Lastly, we show the benefits of using the proposed formulation for modeling water into soil with analytical models that use sorptivity. Copyright