Dynamics of a two-dimensional upflowing mixing layer seeded with bubbles : bubble dispersion and effect of two-way coupling

The evolution and structure of a spatially evolving two-dimensional mixing layer seeded with small bubbles are numerically investigated. The one-way coupling approach is first employed to show that characteristics of bubble dispersion are dominated by the possibility for sufficiently small bubbles to be captured in the core of the vortices. A stability analysis of the ODE system governing bubble trajectories reveals that this entrapment process is governed by the presence of stable fixed points advected by the mean flow. Two-way coupling simulations are then carried out to study how the global features of a two-dimensional flow are affected by bubble-induced disturbances. The local interaction mechanism between the two phases is first analyzed using detailed simulations of a single bubbly vortex. The stability of the corresponding fixed point is found to be altered by the collective motion of bubbles. For trapped bubbles, the interphase momentum transfer yields periodic sequences of entrapment, local reduction of velocity gradients, and eventually escape of bubbles. Similar mechanisms are found to take place in the spatially-evolving mixing layer. The presence of bubbles is also found to enhance the destabilization of the inlet velocity profile and to shorten the time required for the roll-up phenomenon to occur. The most spectacular effects of small bubbles on the large-scale flow are a global tilting of the mixing layer centerline towards the low-velocity side and a strong increase of its spreading rate. In contrast, no significant modification of the flow is observed when the bubbles are not captured in the large-scale vortices, which occurs when the bubble characteristics are such that the drift parameter defined in the text exceeds a critical value. These two contrasted behaviors agree with available experimental results

Preferential accumulation of bubbles in Couette-Taylor flow patterns

We investigate the migration of bubbles in several flow patterns occurring within the gap between a rotating inner cylinder and a concentric fixed outer cylinder. The time-dependent evolution of the two-phase flow is predicted through three-dimensional Euler-Lagrange simulations. Lagrangian tracking of spherical bubbles is coupled with direct numerical simulation of the Navier-Stokes equations. We assume that bubbles do not influence the background flow (one-way coupling simulations). The force balance on each bubble takes into account buoyancy, added-mass, viscous drag and shear-induced lift forces. For increasing velocities of the rotating inner cylinder, the flow in the fluid gap evolves from the purely azimuthal steady Couette flow to Taylor toroidal vortices and eventually a wavy vortex flow. The migration of bubbles is highly dependent on the balance between buoyancy and centripetal forces (mostly due to the centripetal pressure gradient) directed toward the inner cylinder and the vortex cores. Depending on the rotation rate of the inner cylinder, bubbles tend to accumulate alternatively along the inner wall, inside the core of Taylor vortices or at particular locations within the wavy vortices. A stability analysis of the fixed points associated with bubble trajectories provides a clear understanding of their migration and preferential accumulation. The location of the accumulation points is parameterized by two dimensionless parameters expressing the balance of buoyancy, centripetal attraction toward the inner rotating cylinder, and entrapment in Taylor vortices. A complete phase diagram summarizing the various regimes of bubble migration is built. Several experimental conditions considered by Djéridi et al.1 are reproduced; the numerical results reveal a very good agreement with the experiments. When the rotation rate is further increased, the numerical results indicate the formation of oscillating bubble strings, as observed experimentally by Djéridi et al.2. After a transient state, bubbles collect at the crests or troughs of the wavy vortices. An analysis of the flow characteristics clearly indicates that bubbles accumulate in the low-pressure regions of the flow field

De la redondance vers la cohérence…

Dans l’esprit de l’approche programme, nous avons fusionné deux cours de Techniques d’éducation spécialisée dont les compétences visées sont complémentaires afin d’en présenter les contenus de façon imbriquée, logique et sans redondances. Nous avons également élaboré une stratégie d’évaluation commune, permettant une évaluation différenciée des compétences. Cette initiative d’enseignement en collaboration a donné lieu à une belle synergie entre nous tant dans la planification et la prestation de l’enseignement que dans l’évaluation des apprentissages

Versatile Kernels for Aspect-Oriented Programming

Aspect-Oriented Programming (AOP) is a promising approach to modularizing software in presence of crosscutting concerns. Numerous proposals for AOP have been formulated, some of them generic, others specific to particular concerns. There are commonalities and variabilities among these approaches, which are worth exploring. Unfortunately, in practice, these various approaches are hard to combine and to extend. This results from the fact that the corresponding tools, such as aspect weavers, have not been designed to be used along with others, although they usually perform very similar low-level tasks. In this report, we suggest to include common functionality into a versatile kernel for AOP. Such a kernel alleviates the task of implementing an aspect-oriented approach by taking care of basic program alterations. It also lets several approaches coexist without breaking each other by automatically detecting interactions among aspects and offering expressive composition means. From a review of the main features of Aspect-Oriented Programming, we present the main issues that the design of such an AOP kernel should address: open support for aspect languages taking care of both behavior and structure, base language compliance, and aspect composition. As this suggests that partial reflection is an appropriate general framework for AOP, we concretize these ideas with a Java AOP kernel based on partial reflection, reconciling in this way reflection and aspect orientation. A case study based on an implementation of the Sequential Object Monitors illustrates the benefits of the approach

Les medias et les guerres en ex-Yougoslavie

Pourquoi revenir sur le traitement médiatique des conflits en ex‑Yougoslavie après les livraisons que lui ont consacrées les revues Mots et Les Cahiers de médiologie ? Il est patent que ces revues étaient tributaires de l’actualité. Non parce qu’il s’agissait d’être dans l’air du temps, mais bien parce que ces conflits questionnaient nos consciences citoyennes, nos modalités d’intervention, nos capacités à tenir un discours se voulant scientifique. Ceux qui s’intéressent au fonctionnement des..

Sensitivity of chloride ingress modelling in concrete to input parameter variability

The Rosenblueth point estimator method is used to propagate uncertainty on stochastic parameters in a multi-ionic reactive transport model for concrete degradation. The degradation mechanism that was analysed is the ingress of chlorides into concrete. This point estimator method is used because it requires a minimum number of simulations to compute means, standard deviations and skewness for the solution of the transport model. Two exposure cases are considered in this article: the first is a saturated case, and the second is an unsaturated case with wetting (4 days) and drying (3 days at 50 % relative humidity) cycles. The sodium chloride exposure during the wetting cycle of both cases was 500 mmol/L. In both cases, predictions from the model were compared to experimental results. The Rosenblueth approach yielded results comparable to Monte Carlo simulations for both saturated and unsaturated cases. The relative sensitivity of model parameters on prediction results are investigated through a sensitivity analysis. The sensitivity results show the high importance of ionic diffusion parameters and of exposure conditions, while the calculated initial mineral phases in the hydrated paste have lower importance

Le territoire, un levier complémentaire pour l’atteinte du facteur 4

Les exercices de prospective économique ou technico-économique intègrent de plus en plus fortement les contraintes énergétiques et climatiques afin de satisfaire aux exigences du facteur 4. Cependant, ces exercices réalisés dans les cadres conceptuels et méthodologiques habituels, qu’ils soient techniques ou économiques, permettent surtout de réaliser la difficulté d’atteindre les objectifs climatiques sans changer de paradigme. Dès lors, l’intégration de la problématique territoriale peut apparaître comme un moyen de mobiliser des leviers complémentaires à la prospective technico-économique. Le territoire peut en outre permettre de traiter ces problèmes de manière transversale tout en traitant des questions de gouvernance à cette échelle. La Mission Prospective du Commissariat Général au Développement Durable et l’ADEME ont donc lancé et codirigent le programme « Repenser les villes dans une société post carbone » afin de donner à l’approche territoriale toute la place qu’elle peut tenir dans la transition énergétique.Energy and climate-related issues are getting more and more tackled in economic and techno-economic strategic foresight exercises in order to meet the 75% reduction of greenhouse gas emissions challenge by 2050 in France. However, when performed within the traditional conceptual and methodological frameworks, these exercises show how difficult it is to reach climate change objectives without shifting paradigm. Considering territorial aspects can provide complementary tools to traditional techno-economic strategic foresight exercises. This approach can also help to deal with these issues across-the-board, as well as to tackle governance-related issues at the local level. The Commissariat Général au Développement Durable (the French Sustainable Development General Commission) and the ADEME (the French Environment and Energy Management Agency) have jointly launched a research program called “Reconsidering cities in a post-carbon society” in order to emphasize the role of territorial aspects in the energy transition