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Water vapor of the tropical atmosphere from the SAPHIR microwave sounder
No full textInternational audienc
Erratum: Solution to the many-body problem in one point
No full textInternational audienceIn this work we determine the one-body Greenʼs function as solution of a set of functional integro-differential equations, which relate the one-particle Greenʼs function to its functional derivative with respect to an external potential. In the same spirit as Lani et al (2012 New J. Phys. 14 013056), we do this in a one-point model, where the equations become ordinary differential equations (DEs) and, hence, solvable with standard techniques. This allows us to analyze several aspects of these DEs as well as of standard methods for determining the one-body Greenʼs function that are important for real systems. In particular: (i) we present a strategy to determine the physical solution among the many mathematical solutions; (ii) we assess the accuracy of an approximate DE related to the +cumulant method by comparing it to the exact physical solution and to standard approximations such as ; (iii) we show that the solution of the approximate DE can be improved by combining it with a screened interaction in the random-phase approximation. (iv) We demonstrate that by iterating the Dyson equation one does not always converge to a solution and we discuss which iterative scheme is the most suitable to avoid such errors
The Mechanisms of Deformation and Damage of Mudstones: A Micro-scale Study Combining ESEM and DIC
No full textInternational audienceCombining environmental scanning electron microscopy (ESEM) and digital image correlation techniques, the mechanical behaviour of mudstones is studied at the scale of their composite microstructure (that is, grains of carbonate and quartz embedded in a clay matrix). A specially designed apparatus is developed to allow in-situ uniaxial compression tests on samples with controlled humidity states in the ESEM chamber. As the mechanical behavior of mudstones is sensitive to water content, two tests on samples with contrasting water contents (3.8 and 7.4 %) are performed to identify the unified mechanisms of deformation and damage. We illustrate heterogeneous local strain fields that well correlate with the microstructure of mudstones. Three types of deformation bands involving different mechanisms have been classified: orthogonal (compaction of macro-pores and closure of pre-existing cracks), parallel (micro-cracking) and inclined (shear deformation) to the uniaxial compression direction. These deformation modes are activated at different stress levels, and they strongly interact: for instance, a high-strained shear band may result in tensile micro-cracks at its tip. We also illustrate damage phenomena, particularly at the inclusion-matrix interface, which is found to be a hazardous position for nucleation of micro-cracks
Effects of an axial flow on the centrifugal, elliptic and hyperbolic instabilities in Stuart vortices
No full textInternational audienceLinear stability of the Stuart vortices in the presence of an axial flow is studied. The local stability equations derived by Lifschitz & Hameiri (Phys. Fluids A, vol. 3 (11), 1991, pp. 2644–2651) are rewritten for a three-component (3C) two-dimensional (2D) base flow represented by a 2D streamfunction and an axial velocity that is a function of the streamfunction. We show that the local perturbations that describe an eigenmode of the flow should have wavevectors that are periodic upon their evolution around helical flow trajectories that are themselves periodic once projected on a plane perpendicular to the axial direction. Integrating the amplitude equations around periodic trajectories for wavevectors that are also periodic, it is found that the elliptic and hyperbolic instabilities, which are present without the axial velocity, disappear beyond a threshold value for the axial velocity strength. Furthermore, a threshold axial velocity strength, above which a new centrifugal instability branch is present, is identified. A heuristic criterion, which reduces to the Leibovich & Stewartson criterion in the limit of an axisymmetric vortex, for centrifugal instability in a non-axisymmetric vortex with an axial flow is then proposed. The new criterion, upon comparison with the numerical solutions of the local stability equations, is shown to describe the onset of centrifugal instability (and the corresponding growth rate) very accurately
Irreversible deformation and damage in argillaceous rocks induced by wetting/drying
No full textInternational audienceSome irreversible phenomena of argillaceous rocks during wetting and drying processes are experimentally investigated at the micrometric scale, by combining environmental scanning electron microscope (ESEM) imaging and digital image correlation (DIC) techniques. According to previous macroscopic experimental results, argillaceous rocks under a free wetting–drying cycle usually exhibit a reversible deformation; however, the microscopic observation of this work evidences several irreversible phenomena. Some irreversible deformations located in clay matrix are observed and their mechanisms are identified. Some local damage phenomena, in the form of a network of micro-cracks typically with ~ 1 μm openings, are also observed in both the wetting and drying cases: the former locates in the bulk of clay matrix and/or at inclusion-matrix interfaces, whereas the latter is mostly found in the bulk of clay matrix. The two types of micro-cracks are activated by different mechanisms, and their morphology and extension have been evidenced to be strongly dependent on the wetting or drying rate. All these irreversible phenomena are localized and sometimes counteracted each other, leading the macroscopic deformation to be apparently reversible. Nevertheless, they are of crucial importance for the reliability of long-term storage
Studies of Transverse-Momentum-Dependent distributions with A Fixed-Target ExpeRiment using the LHC beams (AFTER@LHC)
No full textSymposium SPIN2014, BeijingInternational audienceWe report on the studies of Transverse-Momentum-Dependent distributions (TMDs) at a future fixed-target experiment --AFTER@LHC-- using the or Pb ion LHC beams, which would be the most energetic fixed-target experiment ever performed. AFTER@LHC opens new domains of particle and nuclear physics by complementing collider-mode experiments, in particular those of RHIC and the EIC projects. Both with an extracted beam by a bent crystal or with an internal gas target, the luminosity achieved by AFTER@LHC surpasses that of RHIC by up to 3 orders of magnitude. With an unpolarised target, it allows for measurements of TMDs such as the Boer-Mulders quark distributions and the distribution of unpolarised and linearly polarised gluons in unpolarised protons. Using polarised targets, one can access the quark and gluon Sivers TMDs through single transverse-spin asymmetries in Drell-Yan and quarkonium production. In terms of kinematics, the fixed-target mode combined with a detector covering allows one to measure these asymmetries at large in the polarised nucleon
Fixed point of payment-free Shapley operators and structural properties of mean payoff games
No full textPosterNational audienc
Fixed Point Sets of Payment-Free Shapley Operators and Structural Properties of Mean Payoff Games
No full textInternational audienc
A max-plus based randomized algorithm for solving a class of HJB PDEs
No full textInternational audienceMcEneaney introduced the curse of dimensionalityfree method for the special class of infinite horizon optimalcontrol problems where the Hamiltonian is represented asa maximum of quadratic affine functions. This method isfeatured by its cubic complexity with respect to the state spacedimension, but the number of basis functions is multiplied bythe number of switches at each iteration, referred to as the’curse of complexity’. In previous works, an SDP-based pruningtechnique was incorporated into the method in order to reducethe curse of complexity. Its efficiency was proved on manyexamples.In this paper we develop a new max-plus based randomizedalgorithm to solve the same class of infinite horizon optimalcontrol problems. The major difference between the newalgorithm and the previous SDP-based curse of dimensionalityfree method is that, instead of adding a large number offunctions and then pruning the less useful ones, the newalgorithm finds in cheap computation time (linear in the currentnumber of basis functions), by a randomized procedure, usefulquadratic functions and adds only those functions to the setof basis functions. Experimental results show that the max-plus randomized algorithm can reach the same precision orderobtained by the SDP-based method with a speedup varyingfrom 10 up to 100 and that the maximal precision orderattainable by the new algorithm is much better than what canbe done by the SDP-based algorithm in reasonable computationtime. Besides, with the randomized algorithm we are now ableto tackle switched problems with more number of switches,which will allow us to extend the algorithm to more generalclasses of optimal control problems
Contraction of Riccati flows applied to the convergence analysis of a max-plus curse of dimensionality free method
No full textAlso arXiv:1301.4777International audienceMax-plus based methods have been recently explored for solution of first-order Hamilton--Jacobi--Bellman equations by several authors. Among several max-plus numerical methods, McEneaney's curse-of-dimensionality--free method applies to the equations where the Hamiltonian takes the form of a (pointwise) maximum of linear/quadratic forms. In previous works of McEneaney and Kluberg, the approximation error of the method was shown to be +, where is the time discretization step and is the number of iterations. Here we use a recently established contraction result for the indefinite Riccati flow in Thompson's part metric to show that under different technical assumptions, still covering an important class of problems, the error is only of order for some . This also allows us to obtain improved estimates of the execution time and to tune the precision of the pruning procedure, which in practice is a critical element of the method