Face-centred cubic lattices and particle redistribution in vortex methods

In vortex particle methods one is concerned with the problem of clustering and depletion of particles in different regions of the flow. The overlap of the vortex blobs is indeed of primary importance for the convergence of the method. In this paper we consider face-centred cubic (FCC) lattices for particle redistribution in three dimensions. This lattice is in fact the most natural way to pack spheres (the FCC is also known as a closest-sphere packing lattice). As a consequence, a point has 12 equidistant close neighbours rather than six for the cubic lattice. The FCC lattice thus offers some symmetry properties that should prove useful for a number of reasons, e.g., the core overlap issue. A few results for this scheme are presented. The problem of two colliding vortex rings at Re = 250 and 500 is studied with both the FCC and cubic lattice schemes. This problem subjects the vortex tubes to a quite strong stretching field and can amply test the quality of the lattice and the remeshing

Reconnection of Colliding Vortex Rings

We investigate numerically the Navier-Stokes dynamics of reconnecting vortex rings at small Reynolds number for a variety of configurations. We find that reconnections are dissipative due to the smoothing of vorticity gradients at reconnection kinks and to the formation of secondary structures of stretched antiparallel vorticity which transfer kinetic energy to small scales where it is subsequently dissipated efficiently. In addition, the relaxation of the reconnection kinks excites Kelvin waves which due to strong damping are of low wave number and affect directly only large scale properties of the flow

Simulations of propelling and energy harvesting articulated bodies via vortex particle-mesh methods

The emergence and understanding of new design paradigms that exploit flow induced mechanical instabilities for propulsion or energy harvesting demands robust and accurate flow structure interaction numerical models. In this context, we develop a novel two dimensional algorithm that combines a Vortex Particle-Mesh (VPM) method and a Multi-Body System (MBS) solver for the simulation of passive and actuated structures in fluids. The hydrodynamic forces and torques are recovered through an innovative approach which crucially complements and extends the projection and penalization approach of Coquerelle et al. and Gazzola et al. The resulting method avoids time consuming computation of the stresses at the wall to recover the force distribution on the surface of complex deforming shapes. This feature distinguishes the proposed approach from other VPM formulations. The methodology was verified against a number of benchmark results ranging from the sedimentation of a 2D cylinder to a passive three segmented structure in the wake of a cylinder. We then showcase the capabilities of this method through the study of an energy harvesting structure where the stocking process is modeled by the use of damping elements

Masses des lois multinomiales négatives. Application au traitement d'images polarimétriques

International audienceCet article est dérivé d'une nouvelle expression des masses des lois multinomiales négatives multivariées. Cette expression des masses peut être utilisée pour déterminer les estimateurs du maximum de vraisemblance de ses paramètres inconnus. Une application au traitement d'images polarimétriques est étudiée. Plus précisément, les estimateurs du degré de polarisation utilisant la méthode du maximum de vraisemblance avec différentes combinaisons d'images sont comparés

Masses of negative multinomial distributions: Application to Polarimetric Image Processing

International audienceThis paper derives new closed-form expressions for the masses of negative multinomial distributions. These masses can be maximized to determine the maximum likelihood estimator of its unknown parameters. An application to polarimetric image processing is investigated. We study the maximum likelihood estimators of the polarization degree of polarimetric images using different combinations of images

Earth curvature effects on subduction morphology: Modeling subduction in a spherical setting

We present the first application in geodynamics of a (Fast Multipole) Accelerated Boundary Element Method (Accelerated-BEM) for Stokes flow. The approach offers the advantages of a reduced number of computational elements and linear scaling with the problem size. We show that this numerical method can be fruitfully applied for the simulation of several geodynamic systems at the planetary scale in spherical coordinates, and we suggest a general approach for modeling combined mantle convection and plate tectonics. The first part of the paper is devoted to the technical exposition of the new approach, while the second part focuses on the effect played by Earth curvature on the subduction of a very wide oceanic lithosphere (W=6,000km and W=9,000km), comparing the effects of two different planetary radii (ER=6,371km, 2ER=2×6,371km), corresponding to an "Earth-like" model (ER) and to a "flat Earth" one (2ER). The results show a distinct difference between the two models: while the slab on a "flat Earth" shows a slight undulation, the same subducting plate on the "Earth-like" setting presents a dual behavior characterized by concave curvature at the edges and by a folding with wavelength of the order of magnitude of 1,000km at the center of the sla

On the emergence and molecular characteristics of LRR-RLKs and LRR-RLPs in plant immunity

Pflanzenimmunität wird von mustererkennenden Rezeptoren (PRRs) vermittelt, die auf der Zelloberfläche lokalisiert sind und verschiedene "pathogen-associated molecular patterns" (PAMPs) erkennen können.Plant immunity is commonly mediated by pattern-recognition receptors (PRRs), located at the cell-surface, which can recognise a broad spectrum of pathogen-associated molecular patterns (PAMPs)

Quel modèle de gouvernance pour les associations ?

Avec un budget cumulé de 59 milliards d'euros en 2007, un volume d'emploi de plus d'un million d'emploi en équivalent temps plein, 14 millions de bénévoles, les associations (dont le nombre dépasse le million) ont un poids considérable dans l'économie française. Par ailleurs, elles ont de plus en plus tendance à se professionnaliser, à adopter des méthodes de management et à généraliser leur recours aux outils de gestion. Ce phénomène, observable dans de nombreux pays, s'accompagne de l'apparition sur ce secteur de consultants spécialisés et de formations de gestion. Dès lors, les questions de gouvernance qu'elles rencontrent prennent une importance particulière. L'enjeu est maintenant de savoir si cette question du mode de gouvernance associatif peut se résoudre par une simple transposition du modèle de l'entreprise ou du nouveau management public

Ascent of Bubbles in Magma Conduits Using Boundary Elements and Particles

AbstractWe investigate the use of the Multipole-accelerated Boundary Element Method (BEM) and of the Singularity Method for studying the interaction of many bubbles rising in a volcanic conduit. Observation shows that the expression of volcanic eruption is extremely variable, from slow release of magma to catastrophic explosive manifestation. We investigate the application of the Fast Multipole Method to the solution of (i) the Boundary Element Formulation of the Stokes flow and of (ii) the particle formulation using the Stokeslets, the Green Function of the Stokes flow law, as a particle kernel. We show how these implementations allow for the first time to numerically model in a dynamic setting a very large number of bubbles, i.e few thousands with the BEM models, allowing investigating the feedback between the single bubble deformation and their collective evolution, and few hundred of thousands of bubbles with the particle approach. We illustrate how this method can be used to investigate the intense interaction of a large number of bubbles and suggest a framework for studying the feedback between many bubbles and a complex thermal nonlinear magmati