Effect of rising motion on the damped shape oscillations of drops and bubbles

The objective of this work is to determine the effect of the rising motion on the dynamics of inertial shape oscillations of drops and bubbles. We have carried out axisymmetric direct numerical simulations of an ascending drop (or bubble) using a level-set method. The drop is initially elongated in the vertical direction and therefore performs shape oscillations. The analysis is based on the decomposition of the inter- face into spherical harmonics, the time evolutions of which are processed to obtain the frequency and the damping rate of the oscillations. As the drop accelerates, its shape flattens and oscillations no longer take place around a spherical equilibrium shape. This causes the eigenmode of oscillations to change, which results in the ap- pearance of spherical harmonics of high order that all oscillate at the same frequency. For both drops and bubbles, the frequency, which remains controlled by the potential flow, slightly decreases with the rising velocity. The damping rate of drops, which is controlled by the dissipation within boundary layers at the interface, strongly in- creases with the rising velocity. At terminal velocity, the damping rate of bubbles, which results from the dissipation by the potential flow associated with the oscillating motion, remains close to that of a non-rising bubble. During the transient, the rate of deformation of the equilibrium shape of bubbles can be comparable to the oscillation frequency, which causes complex evolutions of the shape. These results extend the description of shape oscillations to common situations where gravity plays a role. In particular, the present conclusions are useful to interpret experimental results where the effect of the rising motion is often combined with that of surfactant

Drop breakup modelling in turbulent flows

This paper deals with drop and bubble break-up modelling in turbulent flows. We consider the case where the drop/bubble slip velocity is smaller than or of the order of the turbulent velocity scales, or when the drop/bubble deformation is mainly caused by the turbulent stress (atomisation is not addressed here). The deformation of a drop is caused by continuous interactions with turbulent vortices; the drop responds to these interactions by performing shape-oscillations and breaks up when its deformation reaches a critical value. Following these observations, we use a model of forced oscillator that describes the drop deformation dynamics in the flow to predict its break-up probability. Such a model requires a characterization of the shape- oscillation dynamics of the drop. As this dynamics is theoretically known only under restrictive conditions (without gravity, surfactants), CFD two-phase flow simulations, based on the Level-Set and Ghost Fluid methods, are used to determine the interface dynamics in more complex situations: deformation of a drop in the presence of gravity, bubble-vortex interactions. Results are compared with experimental data. The perspectives to apply this model to breakup in emulsification processes are also discussed

Benchmarks and numerical methods for the simulation of boiling flows

Comparisons of different numerical methods suited to the simulations of phasechanges are presented in the framework of interface capturing computaions on structured fixed computational grids. Due to analytical solutions, we define some reference test- cases that every numerical technique devoted to phase change should succeed. Realistic physical properties imply some drastic interface jump conditions on the normal velocity or on the thermal flux.The efficiencies of Ghost Fluid and Delta Function Methods are compared to compute the normal velocity jump condition. Next, we demonstrate that high order extrapolation methods on the thermal field allow performing accurate and robust simulations for a thermally controlled bubble growth. Finally, some simulations of the growth of a rising bubble are presented, both for a spherical bubble and a deformed bubble

Unsteady rising of clean bubble in low viscosity liquid

When a submerged bubble is initially at rest in a stagnant low viscosity liquid such as water, buoyancy forces accelerate the bubble upwards. The increasing relative velocity of the bubble with the surrounding liquid provokes deformations on the bubble shape that affect its vertical acceleration and also induce surface tension driven oscillations. Our theoretical model, which is compared with full Navier–Stokes simulations predicts, with a reasonable accuracy, both the position of the bubble centre of mass, as well as the time varying bubble shape under those conditions for which the Reynolds number is large, the amplitude of the deformation is small, the bubble interface is free of surfactants and the bubble rises following a straight vertical path. The model can be used as a first approximation to describe the initial instants of the unsteady buoyancy driven rising of millimetre sized bubbles typically generated in water aerators

Oscillations de forme d'une goutte ou d'une bulle en ascension

Les oscillations de forme d'une goutte en ascension sont étudiées par simulation numérique directe. La forme de la goutte est décomposée en harmoniques sphériques afin de déterminer les fréquences et les amortissements de ses modes propres. En particulier, nous montrons que les mécanismes de dissipation de l'énergie par les couches limites et les régions d'écoulement potentiel sont différents selon que l'on considère une bulle ou une goutte

Towards a monolithic optical cavity for atom detection and manipulation

We study a Fabry-Perot cavity formed from a ridge waveguide on a AlGaAs substrate. We experimentally determined the propagation losses in the waveguide at 780 nm, the wavelength of Rb atoms. We have also made a numerical and analytical estimate of the losses induced by the presence of the gap which would allow the interaction of cold atoms with the cavity field. We found that the intrinsic finesse of the gapped cavity can be on the order of F ~ 30, which, when one takes into account the losses due to mirror transmission, corresponds to a cooperativity parameter for our system C ~ 1

Fluids-membrane interaction with a full Eulerian approach based on the level set method

A fully Eulerian approach to predict fluids-membrane behaviours is presented in this paper. Based on the numerical model proposed by Ii et al. (2012), we present a sharp methodology to account for the jump conditions due to hyperelastic membranes. The membrane is considered infinitely thin and is represented by the level set method. Its deformations are obtained from the transport of the components of the left Cauchy-Green tensor throughout time. Considering the linear or a hyperelastic material law, the surface stress tensor is computed and gives the force exerted by the membrane on the surrounding fluids. The membrane force is taken into account in the Navier-Stokes equations as jump conditions on the pressure and on the velocity derivatives by imposing suitable singular source terms in cells crossed by the interface. To prevent stability issues, an extension algorithm has been developed to remove the normal derivatives of the scalar fields specific to the membrane. In particular, a subcell resolution at the interface of the extrapolated variable is proposed for increasing the accuracy of the extension algorithm. These improvements are validated by comparing our numerical results with benchmarks from the literature. Moreover, a new benchmark is proposed for fluids with both different viscosities and different densities to target applications where a gas and a liquid phase are separated by a membrane

Climate driven life histories: the case of the Mediterranean Storm petrel

Seabirds are affected by changes in the marine ecosystem. The influence of climatic factors on marine food webs can be reflected in long-term seabird population changes. We modelled the survival and recruitment of the Mediterranean storm petrel (Hydrobates pelagicus melitensis) using a 21-year mark-recapture dataset involving almost 5000 birds. We demonstrated a strong influence of prebreeding climatic conditions on recruitment age and of rainfall and breeding period conditions on juvenile survival. The results suggest that the juvenile survival rate of the Mediterranean subspecies may not be negatively affected by the predicted features of climate change, i.e., warmer summers and lower rainfall. Based on considerations of winter conditions in different parts of the Mediterranean, we were able to draw inferences about the wintering areas of the species for the first time

Variability factors of beta-lactam concentration-effect relationship in infective endocarditis

Le suivi thérapeutique pharmacologique (STP) des bêta-lactamines permet l’individualisation de la prise en charge d'infections profondes comme l’endocardite infectieuse (EI). La faible diffusion supposée au site infectieux, à savoir les valves cardiaques, est un des éléments qui justifie leur administration à forte dose par voie intra-veineuse. Ainsi, les patients atteints d’EI se voient donc exposés à de fortes concentrations bêta-lactamines pendant 2 à 6 semaines. Dans ce contexte, la survenue d’une neurotoxicité ou d’une néphrotoxicité aux bêta-lactamines peut être lourde de conséquences. Or il existe peu de données de diffusion des bêta-lactamines dans les valves cardiaques et le lien concentration-neurotoxicité des bêta-lactamines est mal documenté en particulier pour l’amoxicilline. Enfin, dans un contexte d’augmentation de l’incidence des EI staphylococciques, la réponse microbiologique aux antibiotiques dans la prise en charge de l’EI à S. aureus est elle-même mal décrite et nécessite d’être évaluée en condition biofilm. Ainsi, la première partie de ce travail a permis d’identifier un seuil de concentrations neurotoxiques en amoxicilline par l’exploitation d’un entrepôt de données intra-hospitalières. La deuxième partie du travail a permis de caractériser au sein d’un essai prospectif une diffusion conséquente des bêta-lactamines au sein des valves cardiaques, en particulier de l’amoxicilline avec un ratio diffusion médian plasma:valve de 62%. Enfin la mise au point d’un modèle ex vivo d’EI à S. aureus sur valves humaines a permis la comparaison de l’activité anti-biofilm de la vancomycine,et d’un lipoglycopeptide; la dalbavancine et d’identifier une activité éradicatrice du biofilm marquée pour cette dernière, fournissant un élément supplémentaire de l’acitivé de la dalbavancine dans l’EI. L’ensemble de ces données a permis d’affiner la fenêtre thérapeutique des anti-infectieux indiqués dans la prise en charge de l’endocardite.Beta-lactam therapeutic drug monitoring (TdM) is routinely performed, particularly in the management of deep infections such as infective endocarditis (IE). Diffusion to the site of infection, i.e. the heart valves, is supposed to be low, justifying high-doses intravenous administration. Thus, patients are exposed to high concentrations for 2-6 weeks. In this context, the onset of beta-lactam neurotoxicity or nephrotoxicity may have serious consequences. However, there is poor knowledge about the diffusion of beta-lactams in the heart valves and their concentration-neurotoxicity relationship, especially for amoxicillin. Finally, in a context of increasing incidence of staphylococcal IE, the microbiological response to antibiotics in S. aureus IE is itself poorly described and needs to be assessed in biofilm growing conditions. Thus, the first part of this work identifed a threshold of neurotoxic concentrations of amoxicillin using our hospital data warehouse. The second part of this work characterised in a prospective trial a consequent diffusion rate of beta-lactams within the heart valves, in particular for amoxicillin with a median plasma:valve diffusion ratio of 62%. Finally, the implementation of an ex vivo model of S. aureus IE on human valves confirmed the antibiofilm activity of penicillin M currently used on methicillin-sensitive strains. The comparison of the anti-biofilm activity of vancomycin, currently recommended for methicillin-resistant S. aureus strains, and a new lipoglycopeptide, dalbavancin, identified a marked biofilm-eradicating activity for the latter, provinding an additional evidence of dalbavancin activity in IE

Time perception and impulsivity: A proposed relationship in addictive disorders

International audienceAddictive disorders are characterized by impulsive behavior that leads to difficulties in preventing certain behaviors despite negative consequences. This symptom predisposes to a vulnerability in developing addictive disorders and is also aggravated by the addiction process itself. As such, understanding the underlying mechanisms of impulsivity is a challenge for understanding the pathophysiology of addiction. One common link between impulsivity and addiction is that both involve actions and decisions that are realized faster than they should be in time. Impulsive traits increase the tendency to choose immediate gratification instead of delayed gratification even when preferred. This observation suggests a relationship between impulsivity and time processing. To better understand this relationship, we reviewed the literature that describes the relationship between time processing and impulsivity in addictive disorders in humans. Despite a lack of literature concerning this question in alcohol, cannabis and gambling disorders, we highlight that addictive behaviors are a good model for understanding the pathophysiology of impulsivity, and could help us to better understand the relationship between time perception and impulsivity