Study of passive control study of the cavitation instability on a venturi profile

As a source of vibrations, noise, erosion and structure damages, cavitation is a major handicap for many industrial flows used in different domains from propeller or pump studies, to the analysis of hydrodynamic flows around a profile. This work presents results of the effect of a passive control method, based on surface roughness, on a venturi profile in order to determine how to reduce or even suppress cloud cavitation developed on its suction side. Visualizations and velocity measurements permit to detect the influence of roughness on flow development. So as to have a better understanding of flow dynamics of sheet and cloud cavitation, robust mathematical methods of imaging post-processing have been used like Proper Orthogonal Decomposition

D-STAG: a Formalism for Discourse Analysis based on SDRT and using Synchronous TAG

We propose D-STAG, a new formalism for the automatic analysis of discourse. The analyses computed by d-stag are hierarchical discourse structures annotated with discourse relations, which are compatible with discourse structures computed in sdrt. A discursive STAG grammar pairs up trees anchored by discourse connectives with trees anchored by (functors associated with) discourse relations.D-STAG est un nouveau formalisme pour l'analyse automatique de discours

Learning Recursive Segments for Discourse Parsing

Automatically detecting discourse segments is an important preliminary step towards full discourse parsing. Previous research on discourse segmentation have relied on the assumption that elementary discourse units (EDUs) in a document always form a linear sequence (i.e., they can never be nested). Unfortunately, this assumption turns out to be too strong, for some theories of discourse like SDRT allows for nested discourse units. In this paper, we present a simple approach to discourse segmentation that is able to produce nested EDUs. Our approach builds on standard multi-class classification techniques combined with a simple repairing heuristic that enforces global coherence. Our system was developed and evaluated on the first round of annotations provided by the French Annodis project (an ongoing effort to create a discourse bank for French). Cross-validated on only 47 documents (1,445 EDUs), our system achieves encouraging performance results with an F-score of 73% for finding EDUs.Comment: published at LREC 201

Experimental study of the instationary flow between two ducted Counter-rotating rotors

The purpose of this work is to study experimentally the aerodynamic characteristics of a subsonic counter-rotating axial-flow fans system operating in a ducted configuration. The fans of diameter D = 375 mm were designed to match the specification point using an original iterative method: the front rotor blade cascade is designed with a conventional inverse method, setting the radial distribution of the Euler work. The through-flow is then computed using an axisymmetric and radial equilibrium asumption, with empirical models of losses. The rear rotor is not conventional but is designed to straighten the radial profile of the tangential velocity. The design of the front rotor is then modified until the stage meets the requirements. The experimental setup is arranged such that the rotation rate of each fan is independently controlled and that the axial distance between the rotors can be varied from 17% to 310% of the mid-span chord length. Systematic measurements of the global performances and local measurements of the velocity field and of the wall pressure fluctuations are performed, in order to first validate the design method, and to explore the effects of the two specific free parameters of the system: the axial spacing and the ratio of rotation rates. The results show that the efficiency is strongly increased compared to a conventional rotor or to a rotor-stator stage. The developed design method slightly over-predicts the pressure rise and slightly under-predicts the best ratio of rotation rates. Flow angle measurements downstream of the stage show that the outflow is not completely straightened at the design point. Finally, the system is highly efficient on a wide range of flow-rates and pressure rises: this system has thus a very flexible use, with a large patch of high efficient operating points in the parameter space

Cavitation regime detection through Proper Orthogonal Decomposition: dynamics analysis of the sheet cavity on a grooved convergent-divergent nozzle

The unsteady character of the sheet cavity dynamics on the suction side of hydrofoils, on convergent–divergent nozzles or on blades in turbines and propellers is responsible for many issues like erosion, noise and vibrations. This two-phase flow dynamics is investigated using a robust method based on Proper Orthogonal Decomposition (POD). This method is applied to sequences of sheet cavity images, in order to identify the cavitation regimes (sheet cavity or cloud cavitation regimes). Once this method is validated on a reference case, POD calculation is used to evaluate the efficiency of a passive control method. Different longitudinal grooved surfaces are machined on the diverging wall of a Venturi. The grooves geometry allows to change the cavitation regime for a fixed cavitation number, and even to avoid the cloud cavitation shedding, which may damage structures

Linguistique informatique: Traduction automatique

Dans le célèbre film "2001, L'Odyssée de l'espace" de S. Kubrik, l'ordinateurHAL comprend l'homme, dialogue avec lui dans sa langue, exécute sescommandes, et ressent des émotions. En 1968, Marvin Minsky, conseillerscientifique du film en tant que spécialiste de L'Intelligence Artificielle,pensait qu'on pourrait effectivement réaliser un ordinateur tel que HAL en2001. En l'an 2000, est-on prêt de la réalisation de HAL ? Cet articleessayera de montrer et d'expliquer pourquoi on en est loin

D-STAG : un formalisme d'analyse automatique de discours basé sur les TAG synchrones

International audienceNous proposons D-STAG, un nouveau formalisme pour l'analyse automatique de la structure discursive des textes. Les analyses produites par D-STAG sont des structures de discours hiérarchiques annotées de relations de discours, qui sont compatibles avec les structures de discours produites en SDRT. L'analyse discursive prolonge l'analyse phrastique, sans modifier celle-ci, ce qui rend envisageable la mise en oeuvre d'un analyseur de discours

Study of the cavitating instability on a grooved Venturi profile

Cavitation is a limiting phenomenon in many domains of fluid mechanics. Instabilities of a partial cavity developed on an hydrofoil, a converging-diverging step or in an inter-blade channel in turbomachinery, have already been investigated and described in many previous works. The aim of this study is to evaluate a passive control method of the sheet cavity. According to operating conditions, cavitation can be described by two different regimes: an unstable regime with a cloud cavitation shedding and a stable regime with only a pulsating sheet cavity. Avoiding cloud cavitation can limit structure damages since a pulsating sheet cavity is less agressive. The surface condition of a converging-diverging step, like a Venturi-type obstacle, is here studied as a solution for a passive control of the cavitation. This study discusses the effect of an organized roughness, in the shape of longitudinal grooves, on the developed sheet cavity. Analyzes conducted with Laser Doppler Velocimetry, visualisations and pressure measurements show that the grooves geometry, and especially the groove depth, acts on the sheet cavity dynamics. Results show that modifying the surface condition, by varying the grooves geometry, can reduce cavity sheet length and even suppress the cloud cavitation shedding.Comment: Submitted to Journal of Fluids Engineerin