Mass or heat transfer inside a spherical gas bubble at low to moderate Reynolds number

Mass (or heat) transfer inside a spherical gas bubble rising through a stationary liquid is investigated by direct numerical simulation. Simulations were carried out for bubble Reynolds number ranging from 0.1 to 100 and for Péclet numbers ranging from 1 to 2000. The study focuses on the effect of the bubble Reynolds number on both the interfacial transfer and the saturation time of the concentration inside the bubble. We show that the maximum velocity Umax at the bubble interface is the pertinent velocity to describe both internal and external transfers. The corresponding Sherwood (or Nusselt) numbers and the saturation time can be described by a sigmoid function depending on the Péclet number Pemax = Umaxdb/D (db and D being the bubble diameter and the corresponding diffusion coefficient)

Experiments and modelling of a draft tube airlift reactor operated at high gas throughputs

One-dimensional modelling of global hydrodynamics and mass transfer is developed for an annulus sparged draft tube airlift reactor operating at high gas throughputs. In a first part, a specific closure law for the mean slip velocity of bubbles in the riser is proposed according for, in one hand, the collective effects on bubble rise velocity and, in the other hand, the size of the liquid recirculation in the airlift riser. This global hydrodynamics model is found towel explain the global gas volume fraction measurements in the airlift riser for a wide range of superficial gas velocity (0.6 ≤ Jg ≥10 cm sˉ¹). In a second part, mass transfer in the airlift has been studied by using the gassing-out method and a dual-tip optical probe to measure the bubble size distributions. As for bubble columns, in such airlift, the volumetric mass transfer coefficient appears to be quite proportional to the gas superficial velocity. Finally, as in Colombet et al. (2011), mass transfer at the bubble scale seems to be weakly influenced by an increase of gas volume fraction

Experiments and modelling of a draft tube airlift reactor operated at high gas throughputs

One-dimensional modelling of global hydrodynamics and mass transfer is developed for an annulus sparged draft tube airlift reactor operating at high gas throughputs. In a first part, a specific closure law for the mean slip velocity of bubbles in the riser is proposed according for, in one hand, the collective effects on bubble rise velocity and, in the other hand, the size of the liquid recirculation in the airlift riser. This global hydrodynamics model is found towel explain the global gas volume fraction measurements in the airlift riser for a wide range of superficial gas velocity (0.6 ≤ Jg ≥10 cm sˉ¹). In a second part, mass transfer in the airlift has been studied by using the gassing-out method and a dual-tip optical probe to measure the bubble size distributions. As for bubble columns, in such airlift, the volumetric mass transfer coefficient appears to be quite proportional to the gas superficial velocity. Finally, as in Colombet et al. (2011), mass transfer at the bubble scale seems to be weakly influenced by an increase of gas volume fraction

Mettre en œuvre pour la première fois la démarche d'investigation avec des élèves de CE2 : une séquence sur les conditions de germination d'une graine

Au cours d'un stage en pratique accompagnée, nous avons mis en œuvre la démarche d'investigation avec une classe de CE2, lors d'une séquence sur les conditions de germination d'une graine. Il s'agissait d'une découverte, tant pour les élèves que pour nous. Notre objectif était, au-delà de l'acquisition de connaissances, de permettre aux élèves de comprendre cette démarche scientifique et de se l'approprier. Nous pensions que par sa seule mise en œuvre nous atteindrions ce but. Il ressort de l'analyse des résultats et de notre pratique qu'il n'est pas suffisant de faire vivre la démarche d'investigation aux élèves pour qu'ils la comprennent et se l'approprient, et a fortiori la première fois. Un fort étayage est nécessaire, afin de baliser chacune des étapes et les rendre porteuses de sens pour les élèves

Parallel Copy Elimination on Data Dependence Graphs

Register allocation regained much interest in recent years due to the development of decoupled strategies that split the problem into separate phases: spilling, register assignment, and copy elimination. Traditional approaches to copy elimination during register allocation are based on interference graphs and register coalescing. Variables are represented as nodes in a graph, which are coalesced, if they can be assigned the same register. However, decoupled approaches strive to avoid interference graphs and thus often resort to local recoloring. A common assumption of existing coalescing and recoloring approaches is that the original ordering of the instructions in the program is not changed. This work presents an extension of a local recoloring technique called Parallel Copy Motion. We perform code motion on data dependence graphs in order to eliminate useless copies and reorder instructions, while at the same time a valid register assignment is preserved. Our results show that even after traditional register allocation with coalescing our technique is able to eliminate an additional 3% (up to 9%) of the remaining copies and reduce the weighted costs of register copies by up to 25% for the SPECINT 2000 benchmarks. In comparison to Parallel Copy Motion, our technique removes 11% (up to 20%) more copies and up to 39% more of the copy costs

Seasonal Depth-Related Gradients in Virioplankton: Lytic Activity and Comparison with Protistan Grazing Potential in Lake Pavin (France)

International audienceThis study presents an original depth-related survey of virioplankton lytic activity in relation to prokaryotic production and potential protistan bacterivory in the deep (Zmax092 m) meromictic volcanic Lake Pavin (Massif Central, France). The sampling strategy was designed to be representative of the physico-chemical gradients of the water column of the lake, and of the seasonal variability as well, i.e. 12 different depths sampled in triplicates from April to December 2005. In the space, viral lytic activity estimated from the frequency of visibly infected prokaryotic cells and from burst size over the study period generally decreased with depth. This was viewed as a paradox compared to the abundances of viruses and prokaryotes and to the prokaryotic production which increased with depth. The seasonal variability in viral lytic activity was correlated with prokaryotic variables (abundance and production) in the deepest waters, i.e. from the hypolimnion downwards. Compared to previous studies known from the mixolimnion, we conclude that the deep waters in Lake Pavin represent an exclusive environment for heterotrophic prokaryotes whose seasonal activity offers an optimal and unique resource for thriving viral communities, some of which may be typical, endemic to the ambient dark, cold and stable deep water masses. Overall, the main findings in the present study get well around a previous statement that the ecology of the deepest waters of Lake Pavin is essentially driven by the dark viral loop (dissolved organic matter-prokaryotes- viruses) processes, which can sequester organic matters and nutrients for a long-lived turnover time. This is in agreement with recent demonstrations from marine systems that meso- and bathypelagic waters are optimal environments for viral survival and proliferation

SPAWN: An Iterative, Potentials-Based, Dynamic Scheduling and Partitioning Tool

International audienceMany applications of physics modeling use regular meshes on which computations of highly variable cost can occur. Distributing the underlying cells over manycore architec-tures is a critical load balancing step that should increase the period until another step is required. Graph partitioning tools are known to be very effective for such problems, but they exhibit scalability problems as the number of cores and the number of cells increases. We introduce a dynamic task scheduling approach inspired by physical particles interactions. Our method allows cores to virtually move over a 2D/3D mesh of tasks and uses a Voronoi domain decomposition to balance workload among cores. Displacements of cores are the result of force computations using a carefully chosen pair potential. We evaluate our method against graph partitioning tools and existing task schedulers with a representative physical application, and demonstrate the relevance of our approach

Impact of geometric field of view on speed perception

This paper deals with changes of the geometric field of view on speed perception. This study has been carried out using the SAAM dynamic driving simulator (Arts et Métiers ParisTech). SAAM provides motion cues thanks to a 6 DOF electromechanical platform and is equipped with a cylindrical screen of 150°. 20 subjects have reproduced 2 speeds (50 km/h and 90 km/h) without knowing the numerical values of these consigns, and with 5 different visual scale factors: 0.70, 0.85, 1.00, 1.15 and 1.30. This visual scale factor correspond to the ratio between the driver’s field of view covered by the screen (constant) and the geometric field of view. This study shows that this visual scale factor has a significant impact on the speed reached by the subjects and thus shows that perceived speed increases with this visual scale factor. A 0.15 modification of this factor is enough to obtain a significant effect. The modification of the geometric field of view remained unnoticed by all the subjects, which implies that this technique can be easily used to make drivers reduce their speed in driving simulation conditions. However, this technique may also modify perception of distances.Cet article présente l’effet du changement du champ de vision géométrique sur la perception de la vitesse. Cette étude a été réalisée sur le simulateur de conduite dynamique SAAM (Arts et Métiers ParisTech). SAAM utilise une plate-forme électromécanique à 6 DDL et est équipé d’un écran cylindrique de 150° pour restituer la sensation de mouvement. 20 sujets ont reproduit 2 vitesses (50 km/h et 90 km/h), sans connaître les valeurs de ces vitesses, et avec 5 facteurs d’échelle visuelle différents : 0.70, 0.85, 1.00, 1.15 et 1.30. Ces facteurs d’échelle correspondent aux rapports entre le champ de vision du conducteur couvert par l’image (constant) et le champ de vision géométrique. Cette étude montre que ce changement visuel a un impact significatif sur la vitesse qu’atteignent les sujets et montre donc que la vitesse perçue augmente avec ce facteur d’échelle visuelle. Un changement de 0.15 de ce facteur suffit pour obtenir un effet significatif. Les changements de champ de vision géométrique n’ont été détectés par aucun des sujets, ce qui implique que cette technique peut facilement être utilisée pour amener les conducteurs à réduire leur vitesse en conditions de simulation de conduite. Cependant, cette technique pourrait aussi modifier la perception des distances.Le Grand Chalo

Agregado de semillas de Lotus tenuis, en la estepa de halófitas de la Pampa Deprimida

En la misma se ensayaron dos tipos de semillas (una población local y una variedad comercial), en diferentes situaciones (laboratorio, invernáculo y campo), bajo diferentes valores de pH. Se evaluó germinación, emergencia, y establecimiento de plántulas de Lotus tenuis.Facultad de Ciencias Agrarias y Forestale

On numerical simulation of cavitating flows under thermal regime

International audienceIn this work, we investigate closure laws for the description of interfacial mass transfer in cavitating flowsunder thermal regime. In a first part, we show that, if bubble resident time in the low pressure area of theflow is larger than the inertial/thermal regime transition time, bubble expansion are no longer monitoredby Rayleigh equation, but by heat transfer in the liquid phase at bubbles surfaces. The modelling of inter-facial heat transfer depends thus on a Nusselt number that is a function of the Jakob number and of thebubble thermal Péclet number. This original approach has the advantage to include the kinetic of phasechange in the description of cavitating flow and thus to link interfacial heat flux to interfacial mass fluxduring vapour production. The behaviour of such a model is evaluated for the case of inviscid cavitatingflow in expansion tubes for water and refrigerant R114 using a four equations mixture model. Comparedwith inertial regime (Rayleigh equation), results obtained considering thermal regime seem to predictlower local gas volume fraction maxima as well as lower gradients of velocity and gas volume fraction.It is observed that global vapour production is closely monitored by volumetric interfacial area (bubblesize and gas volume fraction) and mainly by the Jakob number variations. It is found that, in contrast withphase change occurring in common boiling flow, Jakob number variation is influenced by phasic temper-ature difference but also by density ratio variation with pressure and temperature