3,848 research outputs found
Content-access QoS in peer-to-peer networks using a fast MDS erasure code
This paper describes an enhancement of content access Quality of Service in peer to peer (P2P) networks. The main idea is to use an erasure code to distribute the information over the peers. This distribution increases the usersâ choice on disseminated encoded data and therefore statistically enhances the overall throughput of the transfer. A performance evaluation based on an original model using the results of a measurement campaign of sequential and parallel downloads in a real P2P network over Internet is presented. Based on a bandwidth distribution, statistical content-access QoS are guaranteed in function of both the content replication level in the network and the file dissemination strategies. A simple application in the context of media streaming is proposed. Finally, the constraints on the erasure code related to the proposed system are analysed and a new fast MDS erasure code is proposed, implemented and evaluated
The stability of the variable-density Kelvin-Helmholtz billow
We perform a three-dimensional stability analysis of the Kelvin-Helmholtz billow, developing in a shear-layer between two fluids with different density. We begin with two-dimensional simulations of the temporally evolving mixing-layer yielding the unsteady base flow fields. The Reynolds number is 1500 while the Schmidt and Froude numbers are infinite. Then exponentially unstable modes are extracted from a linear stability analysis performed at the saturation of the primary mode kinetic energy. The spectrum of the least stable modes exhibits two main classes. The first class comprises three-dimensional core-centred and braid-centred modes already present in the homogeneous case. The baroclinic vorticity concentration in the braid lying on the light side of the KH-billow turns the flow into a sharp vorticity ridge holding high shear levels. The hyperbolic modes benefit from the enhanced level of shear in the braid while elliptic ones remain quite insensitive to the modifications of the base flow. In the second class, we found typical two-dimensional modes resulting from a shear instability of the curved vorticity-enhanced braid. For a density contrast of 0.5, the wavelength of the two-dimensional instability is about ten times shorter than the one of the primary wave. Its amplification rate competes well against the ones of the hyperbolic three-dimensional modes. The vorticity-enhanced braid thus becomes the preferred location for the development of secondary instabilities. This stands as the key feature of the transition of the variable-density mixing layer. We carry out a fully resolved numerical continuation of the nonlinear development of the two-dimensional braid-mode. Secondary roll-ups due to a small-scale Kelvin-Helmholtz mechanism are promoted by the underlying strain field and develop rapidly in the compression part of the braid. Originally analysed in Reinaud et al. (2000) from two-dimensional non-viscous numerical simulations, this instability is shown to substantially increase the mixing
Waves and instabilities in rotating free surface ïŹows
The stability properties of the rotating free surface ïŹow in a cylindrical container is studied using a global stability approach, considering succesively three models. For the case of solid body rotation (Newtonâs bucket), all eigenmodes are found to be stable, and are classiïŹed into three families : gravity waves, singular inertial modes, and Rossby waves. For the case of a potential ïŹow, an instability is found. The mechanism is explained as a resonance between gravity waves and centrifugal waves, and is thought to be at the origin of the ârotating polygon instabilityâ observed in experiments where the ïŹow is driven by rotation of the bottom plate (see [9]). Finally, we give some preliminary results concerning a third model : the Rankine vortex
Fractal Kelvin-Helmholtz breakups
The KelvinâHelmholtz billow developing in an infinite-
Schmidt number mixing layer at Re=1500 between two density-contrasted fluids experiences a two-dimensional shear instability. Secondary KelvinâHelmholtz billows are seen to emerge on the light side of the primary structure, and then are advected towards the core of the main billow as the wave overturns. Due to the inertial baroclinic vorticity production, the braid region turns into a sharp vorticity ridge holding high shear levels and is thus sensitized to the KelvinâHelmholtz instability. We carry out numerical simulations of the temporal development of the secondary mode when the flow is seeded at t=18 with the perturbation obtained from a linear stability analysis of the primary billow
A Model-based transformation process to validate and implement high-integrity systems
Despite numerous advances, building High-Integrity Embedded systems remains a complex task. They come with strong requirements to ensure safety, schedulability or security properties; one needs to combine multiple analysis to validate each of them. Model-Based Engineering is an accepted solution to address such complexity: analytical models are derived from an abstraction of the system to be built. Yet, ensuring that all abstractions are semantically consistent, remains an issue, e.g. when performing model checking for assessing safety, and then for schedulability using timed automata, and then when generating code. Complexity stems from the high-level view of the model compared to the low-level mechanisms used. In this paper, we present our approach based on AADL and its behavioral annex to refine iteratively an architecture description. Both application and runtime components are transformed into basic AADL constructs which have a strict counterpart in classical programming languages or patterns for verification. We detail the benefits of this process to enhance analysis and code generation. This work has been integrated to the AADL-tool support OSATE2
The RayleighâTaylor instability of two-dimensional high-density vortices
We investigate the stability of variable-density two-dimensional isolated vortices in the frame of incompressible mixing under negligible gravity. The focus on a single vortex flow stands as a first step towards vortex interactions and turbulent mixing. From heuristic arguments developed on a perturbed barotropic vortex, we find that highdensity vortices are subject to a RayleighâTaylor instability. The basic mechanism relies on baroclinic vorticity generation when the density gradient is misaligned with the centripetal acceleration field. For Gaussian radial distributions of vorticity and density, the intensity of the baroclinic torque due to isopycnic deformation is shown to increase with the ratio ÎŽ/ÎŽÏ of the vorticity radius to the density radius. Concentration of mass near the vortex core is confirmed to promote the instability by the use of an inviscid linear stability analysis. We measure the amplification rate for the favoured azimuthal wavenumbers m=2, 3 on the whole range of positive density contrasts between the core and the surroundings. The separate influence of the density-contrast and the radius ratio is detailed for modes up to m=6. For growing azimuthal wavenumbers, the two-dimensional structure of the eigenmode concentrates on a ring of narrowing radial extent centred on the radius of maximum density gradient. The instability of the isolated high-density vortex is then explored beyond the linear stage based on high-Reynolds-number numerical simulations for modes m=2,3 and a moderate density contrast CÏ =0.5. Secondary roll-ups are seen to emerge from the nonlinear evolution of the vorticity and density fields. The transition towards m smaller vortices involves vorticity exchange between initially-rotating dense fluid particles and the irrotational less-dense medium. It is shown that baroclinic enstrophy production is associated with the centrifugal mass ejection away from the vortex centre
« Faire (re)vivre lâIndien au coeur de lâenfant » : Rituels de la premiĂšre fois chez les Atikamekw Nehirowisiwok
En se basant sur son expĂ©rience de terrain dans diffĂ©rentes communautĂ©s autochtones du QuĂ©bec, et plus particuliĂšrement dans la communautĂ© atikamekw de Wemotaci (Haute-Mauricie), lâauteur propose dans cet article de livrer un regard sur les rituels de la premiĂšre fois qui cĂ©lĂšbrent diffĂ©rentes Ă©tapes dans la vie de jeunes atikamekw. Il sâattardera particuliĂšrement sur un rituel peu abordĂ© dans la littĂ©rature portant sur les groupes algonquiens du Canada : la cĂ©rĂ©monie des premiers pas (ou de la premiĂšre sortie, walking out ceremony). Plus quâun rite de passage, la cĂ©rĂ©monie des premiers pas valorise et renforce un ensemble de relations : avec les personnes, avec le territoire et avec le monde non humain.On the basis of his fieldwork experiences in aboriginal communities of QuĂ©bec, especially in the Atikamekw community of Wemotaci (Haute-Mauricie), the author considers the methodological, theoretical and ethnographical dimensions of the rituals of the first time that marks different transitions in the life of a young Atikamekw. In particular, the paper reflects on a ritual little tackled in the literature on the Algonquian Peoples of Canada, the Walking Out Ceremony. More than a rite of passage, the ceremony enhances and reinforces a cluster of relations: with the people, the territory and the non-human world
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