Global Diffusion in a Realistic Three-Dimensional Time-Dependent Nonturbulent Fluid Flow

We introduce and study the first model of an experimentally realizable three-dimensional time-dependent nonturbulent fluid flow to display the phenomenon of global diffusion of passive-scalar particles at arbitrarily small values of the nonintegrable perturbation. This type of chaotic advection, termed {\it resonance-induced diffusion\/}, is generic for a large class of flows.Comment: 4 pages, uuencoded compressed postscript file, to appear in Phys. Rev. Lett. Also available on the WWW from http://formentor.uib.es/~julyan/, or on paper by reques

Experimental study of chaotic advection regime in a twisted duct flow

The generation of Lagrangian chaos has been studied experimentally in a twisted duct flow, a configuration representing a three-dimensional steady open flow in which various signatures of Lagrangian chaos are documented. The twisted duct consists of four 90 degrees bends of square cross-section; the plane of curvature of each bend is at 90 degrees to that of its neighbors. Dean roll-cells, generated by centrifugal forces and the geometrical perturbation due to the change in curvature plane, are the source of the irregular trajectories of the fluid particles. The Eulerian nature of the flow was investigated using a laser Doppler velocimeter (LDV). From the Eulerian point of view, the flow is completely regular. We show by laser-induced fluorescence (LIF) visualization that many characteristics of a Lagrangian chaotic system are present in this flow: - strong stretching and folding of material lines and surfaces; - sensitivity to initial conditions; - exponential growth of stretching in some flow regions. It is also shown that in some regions of the flow stretchings grow linearly with space, indicating non-chaotic behavior. Due to the chaotic nature of the flow, an equalization of the fly-rime of fluid particles was observed during their passage through the twisted duct. (C) 2001 Editions scientifiques et medicales Elsevier SAS

RESUME

Nous présentons une nouvelle méthode qui permet de visualiser les structures compliquées d’écoulements turbulents ou tourbillonnaires. Celle-ci consiste à suivre en écoulement les évolutions d’un mince filament de traceur qui combine des propriétés rhéologiques épaississantes et fortement élongationnelles. Le traceur viscoélastique est composé d’un mélange de solutions de polymère et de surfactant. Les visualisations sont réalisées au sein d’une maquette d’oscillateur fluidique installée sur une boucle hydrodynamique. Mots-clés: visualisation d’écoulement, traceur viscoélastique, tourbillons. We present a new method to visualize complex flow patterns of turbulent or vortex flows. It consists to follow the evolution of a thin thread tracer which combines shear-thickening and high extensional rheologic properties. The experiments were performed on a fluidic oscillator moke-up installed in a water tunnel. The viscoelastic tracer is made of mixed solutions of polymer and surfactant

Institution of Chemical Engineers www.ingentaselect.com=titles=02638762.htm Trans IChemE, Vol 81, Part A, September 2003 EXPERIMENTAL STUDY OF A BUOYANT PARTICLE

INTRODUCTION Solid--liquid dispersions are widely encountered in many industrial processes, especially in chemical engineering (thermal energy transportation, process plant transfer pipelines, catalytic reactors, fluidized beds, etc.). Different cases may be distinguished, according to many factors such as: particle type (colloidal or not), monodisperse or polydisperse particle distributions, particle concentrations, density higher or lower than that of the carrier fluid, viscosity of the liquid, laminar or turbulent flow regime (Shook and Roco, 1991). The flow patterns associated give rise to highly heterogeneous flows for which the study is far from complete. The substantial lack of experimental data does not facilitate the elaboration of models to predict flow patterns or pressure drops. Satisfactory modelling involves correct predictions of solid--liquid (particle--fluid or wall-- fluid) and solid--solid (particle--particle or wall--particle) interactions in the flow mixture. Th

Timescales of water accumulation in magmas and implications for short warning times of explosive eruptions

Volatiles such as water play a key role in magma ascent and ultimately triggering explosive eruptions. Here, the authors show that water-rich melts with water concentrations of 6–9 wt.% can ascend rapidly to the surface over the timescales of hours to days with very short warning times

Accurate numerical modeling of convective heat transfer coefficient for a high power PLA-core toroidal electromagnetic coupler subject to cooling

International audienceThe toroidal electromagnetic coupler has many advantages over the classical transformer. It is widely used in low-power applications. However, there are some limits that hamper its use in modern and high-power applications, such as the cost, the difficulty of winding and, especially, the limited number of studies carried out on the thermal aspect. In this article, we look at the problem of cooling a high-power and very compact toroidal electromagnetic coupler used in smart grid systems. The cooling is modeled thanks to heat transfer coefficients considering the windings as a surface heat source. A mathematical model which is quick and useful for a first approach is developed, and the results obtained show the temperature distribution for convective boundary conditions inside and outside the hollow torus. In addition, a parametric study is carried out. It aims to determine the optimal internal and external convective heat transfer coefficients for which the temperature of the coupler remains below the material’s destruction threshold. Additionally, a 3D numerical model taking into account the wires of the toroidal coupler is studied and compared with the mathematical model. The good consistency between the two methods ensures that this simplified thermal modeling can be trusted for the study of thermal management during the design of future electromagnetic couplers