Shape minimization of the dissipated energy in dyadic trees

In this paper, we study the role of boundary conditions on the optimal shape of a dyadic tree in which flows a Newtonian fluid. Our optimization problem consists in finding the shape of the tree that minimizes the viscous energy dissipated by the fluid with a constrained volume, under the assumption that the total flow of the fluid is conserved throughout the structure. These hypotheses model situations where a fluid is transported from a source towards a 3D domain into which the transport network also spans. Such situations could be encountered in organs like for instance the lungs and the vascular networks. Two fluid regimes are studied: (i) low flow regime (Poiseuille) in trees with an arbitrary number of generations using a matricial approach and (ii) non linear flow regime (Navier-Stokes, moderate regime with a Reynolds number 100) in trees of two generations using shape derivatives in an augmented Lagrangian algorithm coupled with a 2D/3D finite elements code to solve Navier-Stokes equations. It relies on the study of a finite dimensional optimization problem in the case (i) and on a standard shape optimization problem in the case (ii). We show that the behaviours of both regimes are very similar and that the optimal shape is highly dependent on the boundary conditions of the fluid applied at the leaves of the tree.Comment: \`a para\^itre dans Discrete Contin. Dyn. Syst. (B

Transport properties of 2D graphene containing structural defects

We propose an extensive report on the simulation of electronic transport in 2D graphene in presence of structural defects. Amongst the large variety of such defects in sp$^2$ carbon-based materials, we focus on the Stone-Wales defect and on two divacancy-type reconstructed defects. First, based on ab initio calculations, a tight-binding model is derived to describe the electronic structure of these defects. Then, semiclassical transport properties including the elastic mean free paths, mobilities and conductivities are computed using an order-N real-space Kubo-Greenwood method. A plateau of minimum conductivity ($\sigma^{min}_{sc}= 4e^2/\pi h$) is progressively observed as the density of defects increases. This saturation of the decay of conductivity to $\sigma^{min}_{sc}$ is associated with defect-dependent resonant energies. Finally, localization phenomena are captured beyond the semiclassical regime. An Anderson transition is predicted with localization lengths of the order of tens of nanometers for defect densities around 1%.Comment: 17 pages, 17 figures, submitted to Phys. Rev.

Une étude synthétique des phénomènes d’adaptabilité en période de crise

La crise sardinière va imposer aux pêcheurs morbihannais un certain nombre de contraintes nouvelles, dont ils vont devoir tenir compte pour assurer la pérennité de leurs activités professionnelles. Les principaux acteurs de cette économie halieutique sont dès lors confrontés à un certain nombre de choix. En effet, les armateurs traditionnels, qui jusqu’alors assumaient à eux seuls la plus grande partie du pouvoir décisionnel, se désengagent de l’activité. Les pêcheurs eux-mêmes, qui ne peuvent pas toujours s’engager dans la voie de la reconversion – qui dépend des capacités d’évolution des modes de financement et d’armement – vont donc devoir adopter ou subir certaines évolutions structurelles et techniques pour s’accommoder de la crise. Mais les capacités d’adaptabilité des marins morbihannais, qui peuvent également dépendre de facteurs extérieurs, se heurtent à la rigidité des méthodes de travail traditionnelles et des mentalités, issue en partie d’une incompréhension face aux mécanismes de la crise.The crisis of the french sardine fishing lays dows to the South Brittany fishermen some new constraints, which they have to take into consideration to assure the perenniality of their professionnal activities. The actors of this halieutic economy, from that time onwards, are compelled to do some choices. Indeed, the traditional ship-owners – who assume until then the decisions abilities in most cases- take their activities out of pawn. The fishermen, who may not ever turn over to other employments, will so have to carry or to submit to some structural or technical evolutions to put up to the crisis. But the South Brittany seamen capacities of adaptability – which may depends on external factors – have to come up against the inflexibility of the mental habits and the traditional methods of working, partly resulting from a lack of understanding in the crisis gear

On geometric equations and duality for free higher spins

We provide a general scheme for dualizing higher-spin gauge fields in arbitrary irreducible representations of GL(D,R). We also give a recipe for constructing Fronsdal-like field equations and Lagrangians for such exotic fields.Comment: 12 pages, no figur

Sensitivity of Main Polarimetric Parameters of Multifrequency Polarimetric SAR Data to Soil Moisture and Surface Roughness Over Bare Agricultural Soils

International audienceThe potential of polarimetric synthetic aperture radar data for the soil surface characterization of bare agricultural soils was investigated by using air- and spaceborne data acquired by Radar Aéroporté Multi-Spectral d'Etude des Signatures (RAMSES), Système Expérimental de Télédétection Hyperfréquence Imageur (SETHI), and RADARSAT-2 sensors over several study sites in France. Fully polarimetric data at ultrahigh frequency, X-, C-, L-, and P-bands were compared. The results show that the main polarimetric parameters studied (entropy, α angle, and anisotropy) are not very sensitive to the variation of the soil surface parameters. Low correlations are observed between the polarimetric and soil parameters (moisture content and surface roughness). Thus, the polarimetric parameters are not very relevant to the characterization of the soil surface over bare agricultural areas