Saturation of a large scale instability and non linear stuctures in a rotating stratified flow

In this letter, we present non linear structures resulting from the saturation of a large scale instability in rotating stratified fluids with small scale forced turbulence which we found in \cite{[1]}. These structures are of helical kinks type. The different solutions that we plotted correspond to solutions linking two different stationnary points for different values of the Rayleigh and the Taylor numbers

What do we really know about emerging risks ? A literature review and some comments

International audienceThe issue of emerging risks is today recognized as an important concern for risk governance. Several organisations have developed their own definitions of this concept and deduced from that a framework to deal with them. This paper aims to compare the various definitions associated today with this term and tries to infer a conceptualization of the concept of emergence in risk management

Resonant triad interactions in one and two-layer systems

This dissertation is a study of the weakly nonlinear resonant interactions of a triad of gravity-capillary waves in systems of one and two fluid layers of arbitrary depth, in one and two-dimentions. For one-layer systems, resonant triad interactions of gravity-capillary waves are considered and a region where resonant triads can be always found is identified, in the two-dimensional wavevector angles-space. Then a description of the variations of resonant wavenumbers and wave frequencies over the resonance region is given. The amplitude equations correct to second order in wave slope are used to investigate special resonant triads that, providing their initial amplitude and relative phase satisfy appropriate conditions, exchange no energy during their interactions, which implies that the wave amplitudes remain constant in time. From the fact that the steadiness of the wave amplitudes is a necessary condition for resonant triads to form traveling waves, a transversely modulated two-dimensional wave field of permanent form is found and can be considered as a generalization of Wilton ripples. For two-layer systems, resonant triad interactions between surface and interfacial gravity waves propagating in two horizontal dimensions are considered. As the system supports both surface and internal wave modes, two different types of resonant triad interactions are possible: one with two surface and one internal wave modes and the other with one surface and two internal wave modes. Presented are the spectral domains, where, for given physical parameters, the two resonance scenarios can be found. It is shown that one-dimensional triads occur on the boundary of the spectral domain of resonance. Using a set of amplitude equations recently derived by Choi et al, [10], the necessary and sufficient conditions to form traveling waves are found when the three wave trains travel in the same directions (class-III and class-IV resonance). In addition, a set of physical parameters for one-dimensional triads, for which these traveling waves are possible, is presented

Le secteur prive des transports urbains de voyageurs, quelles logiques de fonctionnement ? Une enquête auprès des opérateurs privés à alger

C’est dans un climat de crise économique où les pouvoirs publics perdent la capacité de financement et de développement des transports en commun et devant la déficience financière des entreprises publiques qu’est intervenue la levée du monopole de l’Etat sur l’activité des transports collectifs et son ouverture à l’investissement privé en 1988. Passant par une phase d’ouverture totale, voire débridée, du marché des transports urbains de voyageurs on assiste actuellement à une étape de re-réglementation suivant les recommandations de la Banque Mondiale qui préconisent une régulation du marché par une concurrence encadrée par le biais des appels d’offre pour la concession des services de transport urbain de voyageurs. C’est précisément dans cette phase de transition que nous cherchons à mieux connaitre la situation et le fonctionnement actuel des opérateurs privés du transport urbain de voyageurs exerçant dans la ville d’Alger, tout comme il s’agit de cerner leurs positions vis-à-vis des réformes du secteur envisagées par les autorités publiques.Mots clés : Transport en commun, Opérateurs privés, Réformes, Regroupement, Autorité organisatrice

About the relevance of the concept of risk acceptability in the risk management process : a decisional approach in the French national context

International audienceA rapid view to the evolution of the legal context at the national European and international level shows a need to introduce both more "visibility" and more "legibility" on the way the decisions in risk analysis and risk management are taken. This can be introduced by: i) giving an image of what the scientific and the experts agree to be the technical "state of the Art" in there respective discipline to reduce and control hazardous activities; (ii) improving the way the population and the other stakeholders are involved and participate to the risk management process. In France, the Toulouse disaster has revealed a real need to improve the way decisions are taken in the risk prevention processes. In this paper, we will show how the establish concept of "risks acceptability" can induce bias on the way risk analysis are performed in the context of the hazard induced by industrial activities. We will show that there is a need to distinguish the acceptability known as "technical" from the one known as "social". We will then propose a new enlightenment on the way risk analysis are performed in Safety Studies and then discuss about the issue of land-use planning in France using "Risk prevention plans" (TRPP) around SEVESO sites

Comparison between two state estimation techniqueds for linear systems

20th World Congress of the International Federation of Automatic Control, Jul 2017, Toulouse, FranceThis paper presents a comparison in terms of accuracy and complexity between two approaches used for state estimation of linear systems: a classic Kalman filter and a guaranteed set-membership state estimation technique. The main goal of this paper is to analyze the advantages of these techniques and to combine them in the future in a new accurate and simple extension that handles system uncertainties and chance constraints. Two academic examples illustrate the main differences between the compared techniques

A new approach for Guaranteed ellipsoidal state estimation

The 19th World Congress of the International Federation of Automatic Control 2014. Cape Town, SudáfricaThis paper proposes a new ellipsoid-based guaranteed state estimation approach for linear discrete-time systems with bounded perturbations and bounded measurement noise. This approach is based on the minimization of the radius of the ellipsoidal state estimation set. Firstly, the ellipsoidal state estimation is computed by off-line solving a Linear Matrix Inequality optimization problem. Secondly, a new online method is developed in order to improve the accuracy of the estimation but it leads to an increase of the online computation load. A new scaling technique is proposed to reduce the computation time, while keeping a good accuracy of the state estimation. An illustrative example is analyzed in order to show the advantages of the proposed approach