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Dive into the research topics where Frédéric Risso is active.

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Featured researches published by Frédéric Risso.


Journal of Fluid Mechanics | 2001

On the rise of an ellipsoidal bubble in water: oscillatory paths and liquid-induced velocity

Kjetil Ellingsen; Frédéric Risso

This work is an experimental study of the rise of an air bubble in still water. For the bubble diameter considered, path oscillations develop in the absence of shape oscillations and the effect of surfactants is shown to be negligible. Both the three-dimensional motion of the bubble and the velocity induced in the liquid are investigated. After the initial acceleration stage, the bubble shape remains constant and similar to an oblate ellipsoid with its symmetry axis parallel to the bubble-centre velocity, and with constant velocity magnitude. The bubble motion combines path oscillations with slow trajectory displacements. (These displacements, which consist of horizontal drift and rotation about a vertical axis, are shown to have no influence on the oscillations). The bubble dynamics involve two unstable modes which have the same frequency and are π/2 out of phase. The primary mode develops first, leading to a plane zigzag trajectory. The secondary mode then grows, causing the trajectory to progressively change into a circular helix. Liquid-velocity measurements are taken up to 150 radii behind the bubble. The nature of the liquid flow field is analysed from systematic comparisons with potential theory and direct numerical simulations. The flow is potential in front of the bubble and a long wake develops behind. The wake structure is controlled by two mechanisms: the development of a quasi-steady wake that spreads around the non-rectilinear bubble trajectory; and the wake instability that generates unsteady vortices at the bubble rear. The velocities induced by the wake vortices are small compared to the bubble velocity and, except in the near wake, the flow is controlled by the quasi-steady wake.


Journal of Fluid Mechanics | 1998

Oscillations and breakup of a bubble immersed in a turbulent field

Frédéric Risso; Jean Fabre

A special facility was designed to obtain intense turbulence without significant mean flow. The experiments were performed under microgravity conditions to ensure that turbulence was the only cause of bubble deformation. A scalar parameter, characteristic of this deformation, was obtained by video processing of high-speed movies. The time evolution and spectral representation of this scalar parameter showed the dynamical characteristics of bubble deformation. The signatures of the eigenmodes of oscillation predicted by the linear theory were clearly observed and the predominance of the second mode was proved. In addition, numerical simulations were performed by computing the response of a damped oscillator to the measured turbulence forcing. Simulations and experiments were found to be in good agreement both qualitatively, from visual inspections of the signals, and quantitatively, from a statistical analysis. The role of bubble dynamics in the deformation process has been clarified. On the one hand, the time response of the bubble controls the maximum amount of energy which can be extracted from each turbulent eddy. On the other hand, the viscous damping limits the energy that the bubble can accumulate during its fluctuating deformation


Journal of Fluid Mechanics | 2010

Experimental characterization of the agitation generated by bubbles rising at high Reynolds number

Guillaume Riboux; Frédéric Risso; Dominique Legendre

An experimental investigation of the flow generated by a homogeneous population of bubbles rising in water is reported for three different bubble diameters ( d = 1.6, 2.1 and 2.5 mm) and moderate gas volume fractions (0.005 ≤ α ≤ 0.1). The Reynolds numbers, Re = V 0 d /ν, based on the rise velocity V 0 of a single bubble range between 500 and 800. Velocity statistics of both the bubbles and the liquid phase are determined within the homogeneous bubble swarm by means of optical probes and laser Doppler anemometry. Also, the decaying agitation that takes place in the liquid just after the passage of the bubble swarm is investigated from high-speed particle image velocimetry measurements. Concerning the bubbles, the average velocity is found to evolve as V 0 α −0.1 whereas the velocity fluctuations are observed to be almost independent of α. Concerning the liquid fluctuations, the probability density functions adopt a self-similar behaviour when the gas volume fraction is varied, the characteristic velocity scaling as V 0 α 0.4 . The spectra of horizontal and vertical liquid velocity fluctuations are obtained with a resolution of 0.6 mm. The integral length scale Λ is found to be proportional to V 0 2 / g or equivalently to d / C d 0 , where g is the gravity acceleration and C d 0 the drag coefficient of a single rising bubble. Normalized by using Λ, the spectra are independent on both the bubble diameter and the volume fraction. At large scales, the spectral energy density evolves as the power −3 of the wavenumber. This range starts approximately from Λ and is followed for scales smaller than Λ/4 by a classic −5/3 power law. Although the Kolmogorov microscale is smaller than the measurement resolution, the dissipation rate is however obtained from the decay of the kinetic energy after the passage of the bubbles. It is found to scale as α 0.9 V 0 3 /Λ. The major characteristics of the agitation are thus expressed as functions of the characteristics of a single rising bubble. Altogether, these results provide a rather complete description of the bubble-induced turbulence.


Journal of Fluid Mechanics | 2007

Oscillatory motion and wake instability of freely rising axisymmetric bodies

Pedro C. Fernandes; Frédéric Risso; Patricia Ern; Jacques Magnaudet

This paper reports on an experimental study of the motion of freely rising axisym- metric rigid bodies in a low-viscosity fluid. We consider flat cylinders with height h smaller than the diameter d and density ρ b close to the density ρ f of the fluid. We have investigated the role of the Reynolds number based on the mean rise velocity u m in the range 80 ≤ Re = u md /ν ≤ 330 and that of the aspect ratio in the range 1.5 ≤ χ = d / h ≤ 20. Beyond a critical Reynolds number, Re c , which depends on the aspect ratio, both the body velocity and the orientation start to oscillate periodically. The body motion is observed to be essentially two-dimensional. Its description is particularly simple in the coordinate system rotating with the body and having its origin fixed in the laboratory; the axial velocity is then found to be constant whereas the rotation and the lateral velocity are described well by two harmonic functions of time having the same angular frequency, ω. In parallel, direct numerical simulations of the flow around fixed bodies were carried out. They allowed us to determine (i) the threshold, Re cf 1 (χ), of the primary regular bifurcation that causes the breaking of the axial symmetry of the wake as well as (ii) the threshold, Re cf 2 (χ), and frequency, ω f , of the secondary Hopf bifurcation leading to wake oscillations. As χ increases, i.e. the body becomes thinner, the critical Reynolds numbers, Re cf 1 and Re cf 2 , decrease. Introducing a Reynolds number Re * based on the velocity in the recirculating wake makes it possible to obtain thresholds and that are independent of χ. Comparison with fixed bodies allowed us to clarify the role of the body shape. The oscillations of thick moving bodies (χ Re c (χ) is equal to Re cf 1 (χ) and ω is close to ω f . However, in the range 6 ≤ χ ≤ 10 the flow corrections induced by the translation and rotation of freely moving bodies are found to be able to delay the onset of wake oscillations, causing Re c to increase strongly with χ. An analysis of the evolution of the parameters characterizing the motion in the rotating frame reveals that the constant axial velocity scales with the gravitational velocity based on the body thickness, , while the relevant length and velocity scales for the oscillations are the body diameter d and the gravitational velocity based on d , , respectively. Using this scaling, the dimensionless amplitudes and frequency of the bodys oscillations are found to depend only on the modified Reynolds number, Re *; they no longer depend on the body shape.


Journal of Fluid Mechanics | 2006

Experimental investigation of a bioartificial capsule flowing in a narrow tube

Frédéric Risso; Fabienne Collé-Paillot; Mokhtar Zagzoule

This work is an experimental study of the motion and deformation of a bioartificial capsule flowing in a tube of 4 mm diameter. The capsules, initially designed for medical applications, are droplets of salt water surrounded by a thin polymeric membrane. They are immersed in a very viscous Newtonian silicone oil that flows through a tube in the Stokes regime. The properties of the capsules were carefully determined. Two previous experimental papers were devoted to their characterization by osmotic swelling and compression between two plates. The present work also provides a series of tests that allows an accurate definition of the experimental model under investigation. The capsules are buoyant and initially quasi-spherical. Nevertheless, buoyancy and small departures from sphericity are shown to have no significant effects, provided the flowing velocity is large enough for the viscous stress to become predominant. The capsules are also initially slightly over-inflated, but there is no mass transfer through the membrane during the present experiments. Their volume therefore remains constant. The membrane can be described as an elastic two-dimensional material, the elastic moduli of which are independent of the deformation. Far from the tube ends, the capsule reaches a steady state that depends on two parameters: the capillary number, Ca; and the ratio of the radius of the capsule to that of the tube, a/R. The capillary number, which compares the hydrodynamic stresses to the elastic tensions in the membrane, was varied between 0 and 0.125. The radius ratio, which measures the magnitude of the confinement, was varied from 0.75 to 0.95. In the range investigated, the membrane material always remains in the elastic domain. At fixed a/R, the capsule is stretched in the axial direction when Ca is increased. The process of deformation involves two main stages. At small to moderate Ca, the lateral dimension of the capsule decreases whereas its axial length increases. The capsule is rounded at both ends, but the curvature of its rear decreases as Ca increases. At large Ca, the rear buckles inward. Then, the negative rear curvature goes on decreasing whereas the lateral dimension of the capsule reaches a constant value. On the other hand, increasing a/R promotes the deformation: the process remains qualitatively the same, but the different stages are attained for smaller values of Ca. Comparisons with available numerical simulations show that the results are strongly dependent on the properties of the capsules.


Journal of Fluid Mechanics | 2002

Velocity fluctuations in a homogeneous dilute dispersion of high-Reynolds-number rising bubbles

Frédéric Risso; Kjetil Ellingsen

An experimental investigation of a homogeneous swarm of rising bubbles is presented. The experimental arrangement ensures that all the bubbles have the same equivalent radius, a = 1.25 mm. This particular size corresponds to high-Reynolds-number ellipsoidal rising bubbles. The gas volume fractions α is small, ranging from 0.5 to 1.05%. The results are compared with the reference situation of a single rising bubble, which was investigated in a previous work. From the use of conditional statistics, the existence of two regions in which the liquid velocity fluctuations are of a different nature are distinguished. In the vicinity of the bubbles, the liquid fluctuations are the same as those measured close to a single rising bubble. They therefore do not depend on α. Far from the bubble, the liquid fluctuations are controlled by the nonlinear interactions between the wakes of all the bubbles. Their probability density function scales as α 0.4 , exhibiting a self-similar behaviour. The total fluctuation combines the contributions of these two regions weighted by the fraction of the liquid volume they occupy. The contribution of the bubble vicinity is thus shown to vary linearly with α while the wake contribution does not. Both are non-isotropic since strong upward vertical fluctuations are more probable.


Philosophical Transactions of the Royal Society A | 2008

Wake attenuation in large Reynolds number dispersed two-phase flows

Frédéric Risso; Véronique Roig; Zouhir Amoura; Guillaume Riboux; Anne-Marie Billet

The dynamics of high Reynolds number-dispersed two-phase flow strongly depends on the wakes generated behind the moving bodies that constitute the dispersed phase. The length of these wakes is considerably reduced compared with those developing behind isolated bodies. In this paper, this wake attenuation is studied from several complementary experimental investigations with the aim of determining how it depends on the body Reynolds number and the volume fraction α. It is first shown that the wakes inside a homogeneous swarm of rising bubbles decay exponentially with a characteristic length that scales as the ratio of the bubble diameter d to the drag coefficient Cd, and surprisingly does not depend on α for 10−2≤α≤10−1. The attenuation of the wakes in a fixed array of spheres randomly distributed in space (α=2×10−2) is observed to be stronger than that of the wake of an isolated sphere in a turbulent incident flow, but similar to that of bubbles within a homogeneous swarm. It thus appears that the wakes in dispersed two-phase flows are controlled by multi-body interactions, which cause a much faster decay than turbulent fluctuations having the same energy and integral length scale. Decomposition of velocity fluctuations into a contribution related to temporal variations and that associated to the random character of the body positions is proposed as a perspective for studying the mechanisms responsible for multi-body interactions.


Journal of Fluid Mechanics | 1997

Diffusive turbulence in a confined jet experiment

Frédéric Risso; Jean Fabre

An experimental analysis of the turbulence in an axisymmetrical jet within a closed tube is presented. At some distance from the nozzle, a turbulent region develops, where the kinetic energy of the mean flow almost vanishes. In this region, the turbulence is transported by turbulent diffusion and its energy decreases with the distance from the inlet. A complete description of the flow field has been achieved using laser Doppler anemometry. Some unusual features are highlighted: the statistical moments of the velocity decay exponentially, the integral length scales remain constant, the radial profiles are self-similar and the Reynolds stress tensor is isotropic and homogeneous in the radial direction. These results highlight the roles of pressure fluctuations and any mean residual motion in the return to isotropy.


Journal of Fluid Mechanics | 2007

Dynamics of drop breakup in inhomogeneous turbulence at various volume fractions

Sophie Galinat; Frédéric Risso; O. Masbernat; Pascal Guiraud

We report experimental and numerical determinations of the breakup probability of a drop travelling through inhomogeneous turbulent flow generated in a pipe downstream of a restriction. The model couples the Rayleigh-Lamb theory of drop oscillations with the Kolmogorov-Hinze theory of turbulent breakup. The interface deformation is modelled by a linear oscillator forced by the Lagrangian turbulent Weber number measured in experiments. The interface is assumed to rupture when either (i) the instantaneous Weber number exceeds a critical value or (ii) the predicted deformation exceeds a given threshold. Seven flow configurations have been tested, corresponding to various Reynolds numbers, damping coefficients and drop volume fractions. The history of the drop deformation proves to play an important role, and simulations assuming a critical Weber number fail to reproduce the experiments. Simulations assuming a critical deformation predict well the main features observed in the experiments. The linear oscillator appears able to describe the main feature of the dynamics of the drop deformation in inhomogeneous turbulence. Provided the oscillation frequency and the damping rate are known, the model can be used to compute the breakup probability in concentrated dispersed two-phase flows.


Journal of Computational Physics | 2015

On the computation of viscous terms for incompressible two-phase flows with Level Set/Ghost Fluid Method

Benjamin Lalanne; Lucia Rueda Villegas; Sébastien Tanguy; Frédéric Risso

In this paper, we present a detailed analysis of the computation of the viscous terms for the simulation of incompressible two-phase flows in the framework of Level Set/Ghost Fluid Method when viscosity is discontinuous across the interface. Two pioneering papers on the topic, Kang et al. 10 and Sussman et al. 26, proposed two different approaches to deal with viscous terms. However, a definitive assessment of their respective efficiency is currently not available. In this paper, we demonstrate from theoretical arguments and confirm from numerical simulations that these two approaches are equivalent from a continuous point of view and we compare their accuracies in relevant test-cases. We also propose a new intermediate method which uses the properties of the two previous methods. This new method enables a simple implementation for an implicit temporal discretization of the viscous terms. In addition, the efficiency of the Delta Function method 24 is also assessed and compared to the three previous ones, which allow us to propose a general overview of the accuracy of all available methods. The selected test-cases involve configurations wherein viscosity plays a major role and for which either theoretical results or experimental data are available as reference solutions: simulations of spherical rising bubbles, shape-oscillating bubbles and deformed rising bubbles at low Reynolds numbers.

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Patricia Ern

Centre national de la recherche scientifique

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Pedro C. Fernandes

Centre national de la recherche scientifique

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