Non-linear oscillatory rheological properties of a generic continuum foam model: comparison with experiments and shear-banding predictions

The occurence of shear bands in a complex fluid is generally understood as resulting from a structural evolution of the material under shear, which leads (from a theoretical perspective) to a non-monotonic stationnary flow curve related to the coexistence of different states of the material under shear. In this paper we present a scenario for shear-banding in a particular class of complex fluids, namely foams and concentrated emulsions, which differs from other scenarii in two important ways. First, the appearance of shear bands is shown to be possible both without any intrinsic physical evolution of the material (e.g. via a parameter coupled to the flow such as concentration or entanglements) and without any finite critical shear rate below which the flow does not remain stationary and homogeneous. Secondly, the appearance of shear bands depends on the initial conditions, i.e., the preparation of the material. In other words, it is history dependent. This behaviour relies on the tensorial character of the underlying model (2D or 3D) and is triggered by an initially inhomogeneous strain distribution in the material. The shear rate displays a discontinuity at the band boundary, whose amplitude is history dependent and thus depends on the sample preparation.Comment: 18 pages - 17 figure

Stabilité d'écoulements bifluides dans un microcanal

L'instabilité de Plateau-Rayleigh est une instabilité classique, liée aux effets interfaciaux, qui concerne les écoulements de cylindres de fluides. Afin d'abaisser son énergie de surface, le cylindre va se fragmenter et former des gouttes. En microfluidique, les phénomènes de tension de surface prédominent. Par conséquent, lorsque l'on considère l'écoulement de deux fluides dans un microcanal cylindrique, suivant les conditions de la manipulation, on peut observer le morcellement du fluide interne. Les expériences montrent qu'il y a trois types de configurations: la formation de gouttes, bouchons ou la persistance du jet. Ces arrangements résultent de la compétition entre le confinement, la vitesse du jet et le gain d'énergie à faire des gouttes. Partant des constatations expérimentales, le travail consiste à réaliser numériquement une étude paramétrique afin de comprendre le rôle des variables physiques en présence (vitesses d'injections des fluides, viscosité, tension de surfac

Pore-scale numerical simulation of two phase flow of newtonian and viscoelastic fluids

This work is motivated by the need for better understanding the Polymer Enhanced Oil Recovery technique at the pore-scale. We consider two phase immiscible and incompressible fluids in a microchannel network. The newtonian fluid is governed by incompressible Stokes equations. The Oldroyd-B rheological model is used to capture viscoelastic behavior. To trace the interface between the two fluids, we use the Level-Set method. We will present simulations in a two and three dimensional microfluidic pore network, then we will compare numerical results with experimental results

Enablers for robust POD models

International audienceThis paper focuses on improving the stability as well as the approximation properties of Reduced Order Models (ROM) based on Proper Orthogonal Decomposition (POD). The ROM is obtained by seeking a solution belonging to the POD subspace and that at the same time minimizes the Navier-Stokes residuals. We propose a modified ROM that directly incorporates the pressure term in the model. The ROM is then stabilized making use of a method based on the fine scale equations. An improvement of the POD solution subspace is performed thanks to an hybrid method that couples direct numerical simulations and reduced order model simulations. The methods proposed are tested on the two-dimensional confined square cylinder wake flow in laminar regime

A numerical study of two dimensional flows past a bluff body for dilute polymer solutions

International audienceIn this paper, we use a simple Oldroyd B constitutive model to study the role of the viscoelasticity of dilute polymer solutions in two-dimensional flows past a bluff body using numerical simulations. This investigation is motivated by the numerous experimental results obtained in quasi two dimensional systems such as soap film channels. The numerical modeling is novel for this case and therefore a comprehensive comparison is carried out to validate the present penalization method and artificial boundary conditions. In particular we focus on flow past a circular object for various values of the Reynolds number, Weissenberg number, and polymer viscosity ratio. Drag enhancement and drag reduction regimes are discussed in detail along with their flow features such as the pattern of vortex-shedding, the variation of lift as well as changes in pressure, elongational rates, and polymer stress profiles. A comprehensive study of the flow behavior and energy balance are carefully carried out for high Reynolds numbers. Flow instabilities in both numerical and experimental results are discussed for high Weissenberg number

Bandes de cisaillement dans un modèle continu de mousse ou d'émulsion concentrée : comment un matériau homogène à 3D peut sembler inhomogène à 2D

13 pages - 16 figures - publié (Eur. Phys. J. E) en anglais : hal-00530995v3Nous décrivons les conditions d'apparition d'un écoulement inhomogène en régime stationnaire (bande cisaillée et bande bloquée) dans un modèle mécanique continu de matériau visco-plastique mou tel qu'une mousse ou une émulsion concentrée en géométrie de cisaillement imposé. Habituellement, la possibilité d'apparition de bandes de cisaillement dans un fluide complexe résulte d'une évolution structurale du matériau sous écoulement qui se traduit, d'un point de vue théorique, par l'existence d'une courbe d'écoulement stationnaire non monotone. Notre modèle mécanique prédit une courbe d'écoulement non monotone mais a priori pas d'évolution physique intrinsèque via un paramètre couplé à l'écoulement tel que la concentration ou l'enchevêtrement. Pourtant nous prédisons l'apparition de bandes de cisaillement. Dans notre cas, courbe d'écoulement non monotone et apparition de bandes résultent spécifiquement du caractère tensoriel du modèle (2D ou 3D). Pour un écoulement en géométrie de Couette, l'introduction d'un formalisme tensoriel 3D permet d'incorporer une liberté supplémentaire de relaxation du système dans la troisième direction qui modifie la fluidité du matériau

Bandes de cisaillement dans un modèle continu de mousse ou d'émulsion concentrée : comment un matériau homogène à 3D peut sembler inhomogène à 2D

13 pages - 16 figures - publié (Eur. Phys. J. E) en anglais : hal-00530995v3Nous décrivons les conditions d'apparition d'un écoulement inhomogène en régime stationnaire (bande cisaillée et bande bloquée) dans un modèle mécanique continu de matériau visco-plastique mou tel qu'une mousse ou une émulsion concentrée en géométrie de cisaillement imposé. Habituellement, la possibilité d'apparition de bandes de cisaillement dans un fluide complexe résulte d'une évolution structurale du matériau sous écoulement qui se traduit, d'un point de vue théorique, par l'existence d'une courbe d'écoulement stationnaire non monotone. Notre modèle mécanique prédit une courbe d'écoulement non monotone mais a priori pas d'évolution physique intrinsèque via un paramètre couplé à l'écoulement tel que la concentration ou l'enchevêtrement. Pourtant nous prédisons l'apparition de bandes de cisaillement. Dans notre cas, courbe d'écoulement non monotone et apparition de bandes résultent spécifiquement du caractère tensoriel du modèle (2D ou 3D). Pour un écoulement en géométrie de Couette, l'introduction d'un formalisme tensoriel 3D permet d'incorporer une liberté supplémentaire de relaxation du système dans la troisième direction qui modifie la fluidité du matériau

Thermal Flows

Flows of thermal origin and heat transfer problems are central in a variety of disciplines and industrial applications. The present book entitled Thermal Flows consists of a collection of studies by distinct investigators and research groups dealing with different types of flows relevant to both natural and technological contexts. Both reviews of the state-of-the-art and new theoretical, numerical and experimental investigations are presented, which illustrate the structure of these flows, their stability behavior, and the possible bifurcations to different patterns of symmetry and/or spatiotemporal regimes. Moreover, different categories of fluids are considered (liquid metals, gases, common fluids such as water and silicone oils, organic and inorganic transparent liquids, and nano-fluids). This information is presented under the hope that it will serve as a new important resource for physicists, engineers and advanced students interested in the physics of non-isothermal fluid systems; fluid mechanics; environmental phenomena; meteorology; geophysics; and thermal, mechanical and materials engineering

Extracorporeal Membrane Oxygenation for Severe Acute Respiratory Distress Syndrome associated with COVID-19: An Emulated Target Trial Analysis.

RATIONALE: Whether COVID patients may benefit from extracorporeal membrane oxygenation (ECMO) compared with conventional invasive mechanical ventilation (IMV) remains unknown. OBJECTIVES: To estimate the effect of ECMO on 90-Day mortality vs IMV only Methods: Among 4,244 critically ill adult patients with COVID-19 included in a multicenter cohort study, we emulated a target trial comparing the treatment strategies of initiating ECMO vs. no ECMO within 7 days of IMV in patients with severe acute respiratory distress syndrome (PaO2/FiO2 <80 or PaCO2 ≥60 mmHg). We controlled for confounding using a multivariable Cox model based on predefined variables. MAIN RESULTS: 1,235 patients met the full eligibility criteria for the emulated trial, among whom 164 patients initiated ECMO. The ECMO strategy had a higher survival probability at Day-7 from the onset of eligibility criteria (87% vs 83%, risk difference: 4%, 95% CI 0;9%) which decreased during follow-up (survival at Day-90: 63% vs 65%, risk difference: -2%, 95% CI -10;5%). However, ECMO was associated with higher survival when performed in high-volume ECMO centers or in regions where a specific ECMO network organization was set up to handle high demand, and when initiated within the first 4 days of MV and in profoundly hypoxemic patients. CONCLUSIONS: In an emulated trial based on a nationwide COVID-19 cohort, we found differential survival over time of an ECMO compared with a no-ECMO strategy. However, ECMO was consistently associated with better outcomes when performed in high-volume centers and in regions with ECMO capacities specifically organized to handle high demand. This article is open access and distributed under the terms of the Creative Commons Attribution Non-Commercial No Derivatives License 4.0 (http://creativecommons.org/licenses/by-nc-nd/4.0/)