Hydrodynamique d’un essaim de bulles en ascension

Dans de nombreuses applications, des bulles sont utilisées pour agiter un liquide afin de favoriser le mélange et les transferts. Ce travail est consacré à l’étude de l’hydrodynamique d’une colonne à bulles. Expérimentalement, nous avons déterminé les propriétés des fluctuations de vitesses à l’intérieur et derrière un essaim de bulles homogène en ascension pour différentes tailles de bulles et fractions volumiques α : auto-similarité en α0,4, spectre en k−3 et échelle intégrale de longueur imposée par la force de flottabilité. Numériquement, nous avons reproduit ces propriétés par simulation des grandes échelles en modélisant les bulles par des volumes-forces. Nous avons ainsi prouvé que la dynamique est maîtrisée par les intéractions des sillages. ABSTRACT : In many applications, bubbles are used to agitate a liquid in order to enhance mixing and transfer. This work is devoted to the study of the hydrodynamics in a stable bubble column. Experimentally, we have determined the properties of the velocity fluctuations inside and behind a homogeneous swarm of rising bubbles for different bubble sizes and gas volume fractions α : selfsimilarity in α0,4, spectrum in k−3 and integral length scale controlled by buoyancy. Numerically, we have reproduced these properties by means of large-scale simulations, the bubbles being modeled by volume-forces. This confirms that the dynamics is controlled by wake interactions

Whipping Instabilities in Electrified Liquid Jets

A liquid jet may develop different types of instabilities, like the so-called Rayleigh-Plateau instability, which breaks the jet into droplets. However, another type of instabilities may appear when we electrify a liquid jet and induce some charge at his surface. Among them, the most common is the so-called Whipping Instability, which is characterized by violent and fast lashes of the jet. In the submitted fluid dynamic video(see http://hdl.handle.net/1813/11422), we will show an unstable charged glycerine jet in a dielectric liquid bath, which permits an enhanced visualization of the instability. For this reason, it is probably the first time that these phenomena are visualized with enough clarity to analyze features as the effect of the feeding liquid flow rate through the jet or as the surprising spontaneous stabilization at some critical distance to the ground electrode.Comment: 3 pages, no figures, links to videos, Submission to the 26th Gallery of Fluid Motion (2009

A model of bubble-induced turbulence based on large-scale wake interactions

Navier–Stokes simulations of the agitation generated by a homogeneous swarm of high-Reynolds-number rising bubbles are performed. The bubbles are modelled by fixed momentum sources of finite size randomly distributed in a uniform flow. The mesh grid is regular with a spacing close to the bubble size. This allows us to simulate a swarm of a few thousand bubbles in a computational domain of a hundred bubble diameters, which corresponds to a gas volume fraction α from 0.6% to 4%. The small-scale disturbances close to the bubbles are not resolved but the wakes are correctly described from a distance of a few diameters. This simple model reproduces well all the statistical properties of the vertical velocity fluctuations measured in previous experiments: scaling as α0.4, self-similar probability density functions and power spectral density including a subrange evolving as the power −3 of the wavenumber k. It can therefore be concluded that bubble-induced agitation mainly results from wake interactions. Considering the flow in a frame that is fixed relative to the bubbles, the combined use of both time and spatial averaging makes it possible to distinguish two contributions to the liquid fluctuations. The first is the spatial fluctuations that are the consequence of the bubble mean wakes. The second corresponds to the temporal fluctuations that are the result of the development of a flow instability. Note that the latter is not due to the destabilization of individual bubble wakes, since a computation with a single bubble leads to a steady flow. It is a collective instability of the randomly distributed bubble wakes. The spectrum of the time fluctuations shows a peak around a frequency fcwi, which is independent of α. From the present results it is possible to determine the origin of the overall properties of the total fluctuations observed in the experiments. The scaling of the velocity fluctuation as α^0.4 is a combination of the scalings of the spatial and temporal fluctuations, which are different from each other. As the time fluctuations are symmetric in the vertical direction, the asymmetry of the probability density function of the vertical velocity comes from that of the spatial fluctuations. Both contributions exhibit a k−3 spectral behaviour around the same range of wavenumbers, which explains why it is observed regardless of the nature of the dominant contribution

The initial impact of drops cushioned by an air or vapour layer with applications to the dynamic Leidenfrost regime

This work is devoted to the study of the conditions under which a drop directed normally towards a superheated or isothermal smooth substrate prevents the initial contact with the solid by skating over a micrometre-sized vapour or air layer. The results have been obtained analysing the gas flow at the spatio-temporal region where the maximum liquid pressure is attained, which is also where and when the minimum values of the film thickness are reached. For the common case in which WeSt−1/6≳1, where We=ρlU2R/γ and St=ρlUR/ηa denote, respectively, the Weber and Stokes numbers, we find that capillary effects are negligible and the ratio between the minimum film thickness and the local drop radius of curvature is hm/R∝St−7/6, with ρl, γ, ηa, U and R indicating the liquid density, interfacial tension coefficient, gas viscosity, impact velocity and drop radius, respectively. In contrast, when WeSt−1/6≲1, capillary effects can no longer be neglected and hm/R∝We−1/3St−10/9. The predicted values of the minimum film thickness are compared with published experimental data, finding good agreement between predictions and measurements for the cases of both isothermal and superheated substrates. In addition, using mass conservation, we have also deduced an equation providing the minimum value of the substrate temperature for which a cylindrical central vapour bubble of constant height hd/R∝St−2/3, with hd≫hm, grows radially at the wetting velocity deduced in Riboux & Gordillo (Phys. Rev. Lett., vol. 113, 2014, 024507). The predicted values are in good agreement with the dynamic Leidenfrost temperatures reported by Shirota et al. (Phys. Rev. Lett., vol. 116, 2016, 064501).Ministerio de Ciencia e Innovación PID2020-115655

Large impact velocities suppress the splashing of micron-sized droplets

Article number 023605Here we investigate the transition from spreading to splashing of drops with radii R varying from millimeters to tens of microns impacting onto a smooth and dry partially wetting substrate at normal atmospheric conditions. Experiments show that the smaller R is, the larger the impact velocity V for the drop to splash needs to be but also that splash is inhibited if Weλ = ρV 2λ/σ 0.5, with σ, ρ, and λ indicating the interfacial tension coefficient, the liquid density, and the mean free path of gas molecules. This result has been validated for two different values of the Ohnesorge number Ohλ = μ/√ρλσ, with μ indicating the liquid viscosity, defined using only the material properties of the liquid and of the surrounding gaseous atmosphere. The underlying reason for this a priori unexpected finding results from the fact that the thin liquid film ejected after the drop touches the substrate is, under many practical conditions, Ht σ/(ρV 2 ) Riboux and Gordillo [Phys. Rev. Lett. 113, 024507 (2014)]. Then, for sufficiently large values of V , the thickness of the lamella becomes similar to the mean free path of gas molecules, i.e., Ht ≈ λ, and, under these conditions, the splash of the drop is inhibited because the lift force causing the liquid to dewet the partially wetting solid is negligible. The spreading to splashing and the splashing to spreading transitions observed experimentally as the impact velocity is increased and the radii of the droplets is above a certain threshold value are very well predicted by the theory in G. Riboux and J. M. Gordillo [Phys. Rev. Lett. 113, 024507 (2014)] and J. M. Gordillo and G. Riboux [J. Fluid Mech. 871, R3 (2019)] once the aerodynamic lift force is set to zero for Ht /λ 2, i.e., when Weλ 0.5Ministerio de Economía y Competitividad (MINECO) DPI2017-88201-C3-1-RMinisterio de Economía y Competitividad (MINECO) RED2018-102829-TJapan Society for the Promotion of Science 20H00222Japan Society for the Promotion of Science 20H0022

Wake attenuation in large Reynolds number dispersed two-phase flows

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 a. 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 a for 10K2%a%10K1. The attenuation of the wakes in a fixed array of spheres randomly distributed in space (aZ2!10K2) 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

Sound generation on bubble coalescence following detachment

A system in which bubbles coalesced on formation was used to probe one mechanism by which bubbles create sound. The aim was to determine in which situations sound is produced and to predict its amplitude. A set of carefully co-ordinated high-speed video and acoustic timeseries showed that needle-formed bubbles generated loud bubble-acoustic emissions at the instant of coalescence of secondary bubbles with the primary bubble. As the air flow rate increased, the size and number of secondary bubbles increased, and the sound amplitude also increased. On coalescence, the sound pressure always rose initially. A dimensionless scaling found that the sound amplitude emitted scaled with the volume of the secondary bubble. This scaling was shown to be consistent with the sound-emission mechanism being the equalization of pressures in the coalescing bubbles. The trend in amplitude with bubble production rate was well predicted by the scaling

Hydrodynamique d'un essaim de bulles en ascension

In many applications, bubbles are used to agitate a liquid in order to enhance mixing and transfer. This work is devoted to the study of the hydrodynamics in a stable bubble column. Experimentally, we have determined the properties of the velocity fluctuations inside and behind a homogeneous swarm of rising bubbles for different bubble sizes and gas volume fractions [alpha] : selfsimilarity in [alpha] to the power 0,4, wave number power law in -3 and integral lenght scale controlled by buoyancy. Numerically, we have reproduced these properties by means of large-scale simulations, the bubbles being modeled by volume-forces. This confirms that the dynamics is controlled by wake interactions.Dans de nombreuses applications, des bulles sont utilisées pour agiter un liquide afin de favoriser le mélange et les transferts. Ce travail est consacré à l'étude de l'hydrodynamique d'une colonne à bulles. Expérimentalement, nous avons déterminé les propriétés des fluctuations de vitesses à l'intérieur et derrière un essaim de bulles homogène en ascension pour différentes tailles de bulles et fractions volumiques [alpha] : auto-similarité en [alpha] à la puissance 0,4, spectre en nombre d'onde à la puissance -3 et échelle intégrale de longueur imposée par la force de flottabilité. Numériquement, nous avons reproduit ces propriétés par simulation des grandes échelles en modélisant les bulles par des volumes-forces. Nous avons ainsi prouvé que la dynamique est maîtrisée par les interactions des sillages.TOULOUSE-ENSEEIHT (315552331) / SudocSudocFranceF