The Jungle Universe

In this paper, we exploit the fact that the dynamics of homogeneous and isotropic Friedmann-Lemaitre universes is a special case of generalized Lotka-Volterra system where the competitive species are the barotropic fluids filling the Universe. Without coupling between those fluids, Lotka-Volterra formulation offers a pedagogical and simple way to interpret usual Friedmann-Lemaitre cosmological dynamics. A natural and physical coupling between cosmological fluids is proposed which preserve the structure of the dynamical equations. Using the standard tools of Lotka-Volterra dynamics, we obtain the general Lyapunov function of the system when one of the fluids is coupled to dark energy. This provides in a rigorous form a generic asymptotic behavior for cosmic expansion in presence of coupled species, beyond the standard de Sitter, Einstein-de Sitter and Milne cosmologies. Finally, we conjecture that chaos can appear for at least four interacting fluids.Comment: 26 pages, 4 figure

On Algorithms Based on Joint Estimation of Currents and Contrast in Microwave Tomography

This paper deals with improvements to the contrast source inversion method which is widely used in microwave tomography. First, the method is reviewed and weaknesses of both the criterion form and the optimization strategy are underlined. Then, two new algorithms are proposed. Both of them are based on the same criterion, similar but more robust than the one used in contrast source inversion. The first technique keeps the main characteristics of the contrast source inversion optimization scheme but is based on a better exploitation of the conjugate gradient algorithm. The second technique is based on a preconditioned conjugate gradient algorithm and performs simultaneous updates of sets of unknowns that are normally processed sequentially. Both techniques are shown to be more efficient than original contrast source inversion.Comment: 12 pages, 12 figures, 5 table

On the assimilation of SWOT type data into 2D shallow-water models

In river hydraulics, assimilation of water level measurements at gauging stations is well controlled, while assimilation of images is still delicate. In the present talk, we address the richness of satellite mapped information to constrain a 2D shallow-water model, but also related difficulties. 2D shallow models may be necessary for small scale modelling in particular for low-water and flood plain flows. Since in both cases, the dynamics of the wet dry front is essential, one has to elaborate robust and accurate solvers. In this contribution we introduce robust second order, stable finite volume scheme [CoMaMoViDaLa]. Comparisons of real like tests cases with more classical solvers highlight the importance of an accurate flood plain modelling. A preliminary inverse study is presented in a flood plain flow case, [LaMo] [HoLaMoPu]. As a first step, a 0th order data processing model improves observation operator and produces more reliable water level derived from rough measurements [PuRa]. Then, both model and flow behaviours can be better understood thanks to variational sensitivities based on a gradient computation and adjoint equations. It can reveal several difficulties that a model designer has to tackle. Next, a 4D-Var data assimilation algorithm used with spatialized data leads to improved model calibration and potentially leads to identify river discharges. All the algorithms are implemented into DassFlow software (Fortran, MPI, adjoint) [Da]. All these results and experiments (accurate wet-dry front dynamics, sensitivities analysis, identification of discharges and calibration of model) are currently performed in view to use data from the future SWOT mission

Inferring river properties with SWOT like data

Inverse problems in hydraulics are still open questions such as the estimation of river discharges. Remotely sensed measurements of hydrosystems can provide valuable information but adequate methods are still required to exploit it. The future Surface Water and Ocean Topography (SWOT) mission would provide new cartographic measurements of inland water surfaces. The highlight of SWOT will be its almost global coverage and temporal revisits on the order of 1 to 4 times per 22 days repeat cycle [1]. Lots of studies have shown the possibility of retrieving discharge given the river bathymetry or roughness and/or in situ time series. The new challenge is to use SWOT type data to inverse the triplet formed by the roughness, the bathymetry and the discharge. The method presented here is composed of two steps: following an inverse formulation from [2], the first step consists in retrieving an equivalent bathymetry profile of a river given one in situ depth measurement and SWOT like data of the water surface, that is to say water elevation, free surface slope and width. From this equivalent bathymetry, the second step consists in solving mass and Manning equation in the least square sense [3]. Nevertheless, for cases where no in situ measurement of water depth is available, it is still possible to solve a system formed by mass and Manning equations in the least square sense (or with other methods such as Bayesian ones, see e.g. [4]). We show that a good a priori knowledge of bathymetry and roughness is compulsory for such methods. Depending on this a priori knowledge, the inversion of the triplet (roughness, bathymetry, discharge) in SWOT context was evaluated on the Garonne River [5, 6]. The results are presented on 80 km of the Garonne River downstream of Toulouse in France [7]. An equivalent bathymetry is retrieved with less than 10% relative error with SWOT like observations. After that, encouraging results are obtained with less than 10% relative error on the identified discharge

Les sons des Cyclopes

The aim of this paper is to study the soundscape of the Cyclops’ Island as it is built by the poet in the ninth song of the Odyssey (v. 166-506). I propose to list all descriptions or sound actions of this excerpt and to try to understand which soundscape they are forming. I will also examine their relationship with other types of descriptions - visual or sensory - in order to show how they strongly participate in the general picture painted by the poet. I will concentrate particularly on the sounds and cries of the Cyclops and especially on the term φθόγγος. In parallel, I will study some specific terms used in the acoustic description of the blinding of Polyphemus to emphasize the importance of expressive vocalisms and to show that the use of one or another term is never insignificant. Finally, this will give food for thought about the uses and especially the meanings of these different expressive terms in the Homeric poetry. φθόγγος, ὀρυμαγδός, ῥοῖζος, σίζω, οἰμώζω

Underwater acoustic wave generation by filamentation of terawatt ultrashort laser pulses

Acoustic signals generated by filamentation of ultrashort TW laser pulses in water are characterized experimentally. Measurements reveal a strong influence of input pulse duration on the shape and intensity of the acoustic wave. Numerical simulations of the laser pulse nonlinear propagation and the subsequent water hydrodynamics and acoustic wave generation show that the strong acoustic emission is related to the mechanism of superfilamention in water. The elongated shape of the plasma volume where energy is deposited drives the far-field profile of the acoustic signal, which takes the form of a radially directed pressure wave with a single oscillation and a very broad spectrum.Comment: 9 pages, 12 figure

Improved error estimates for Hybrid High-Order discretizations of Leray-Lions problems

We derive novel error estimates for Hybrid High-Order (HHO) discretizations of Leray-Lions problems set in W^(1,p) with p in (1,2]. Specifically, we prove that, depending on the degeneracy of the problem, the convergence rate may vary between (k+1)(p-1) and (k+1), with k denoting the degree of the HHO approximation. These regime-dependent error estimates are illustrated by a complete panel of numerical experiments.Comment: 20 pages, 4 figures, 4 table

Une Nouvelle Pédagogie pour des Travaux Pratiques en Mécanique des Fluides

National audienceThe Fluid Mechanics Teaching Group of Ecole Centrale de Lyon introduced new teaching methods for the lab works of 2nd year students (eq. Master 1). Instead of a classical linear experiment on a dedicated rig, the students choose a scientific ant technical theme they illustrate on a panel of rigs and numerical simulations. During the first session, the students choose a theme and define objectives and concepts to illustrate, in conjunction with the theoretical classes. They also select appropriate experimental rigs and numerical simulations. During the next two sessions, they have to carry out the experiments and simulations, over the same configurations. Data are analyzed according to the objectives, combining experimental and numerical approaches. Finally, the work is synthesized through a report and a presentation during the last session. This talk is also a way to present the results to the other groups that studied different themes.L'Equipe d'Enseignement de Mécanique des Fluides de l'Ecole Centrale de Lyon pratique une nouvelle pédagogie pour les Travaux Pratiques dédiés aux élèves de 2ème année. Plutôt que de réaliser classiquement une expérience formatée sur une installation dédiée, les étudiants définissent eux-mêmes un thème scientifique et technique qu'ils illustrent grâce à un panel d'installations expérimentales et de simulations numériques. Lors d'une première séance, les étudiants choisissent leur thème d'étude et définissent des objectifs et des concepts à illustrer, en lien avec les enseignements théoriques dispensés. Ils identifient les installations expérimentales et les simulations utiles. Lors des deux séances intermédiaires, ils réalisent leurs expériences et des simulations numériques, menées sur les mêmes configurations. Les données traitées sont analysées en cohérence avec les objectifs prédéfinis, et en confrontant les approches numériques et expérimentales. La dernière séance est consacrée à la restitution. Un rapport écrit et un exposé oral constituent un exercice de synthèse. Par ailleurs, l'exposé oral est un vecteur de transmission des connaissances acquises, à l'adresse des étudiants ayant choisi un autre thème d'étude

A procedure for identifying the plastic behaviour of single crystals from the local response of polycrystals

The overall and local tensile responses of an α iron multicrystalline sample are investigated in order to derive the plastic constitutive equations for the constituent single crystals. The macroscopic stress–strain curve and some statistical characteristics of the strain field measured on the sample surface are compared with their simulated counterparts. The optimal values of the material parameters of four types of hardening laws are derived by a minimization procedure. The best results are obtained with a nonlinear anisotropic law which uses the dislocation densities on the slip systems. This procedure is then validated on a fine-grained polycrystalline sample of a similar material by using the measured displacement field on the edge of a selected area as boundary conditions for finite element method (FEM) computation. The resulting optimal material parameters for the single crystal are found to be consistent with the values available in the literature, and the whole simulated strain fields as well as the evolution of the crystallographic texture, is compared satisfactorily with the experimental data