Programme de la Réunion Exceptionnelle 2020 (Covid19)

Dans cet exposé nous rappelerons les principes de la modélisation stochastique Lagrangienne des écoulements turbulents dispersés, incluant des particules inertielles ou des traçeurs. Ces modèles résultent importants pour des écoulements complexes, notamment non-homogènes, lorsque une approche de type DNS se trouve à être pratiquement impossibles. Les derniers développements ainsi que les limitations actuelles seront présentées.

The transport of particles by fluids is an important aspect for the formation of clouds, spreading of aerosols or dust by wind and mixing of suspensions in industrial applications. The complexity of these systems is indicated by the large number of dimensionless parameters that control their behaviour: the particle inertia (Stokes number), the particle volume fraction, the flow turbulence (Reynolds number based on the Taylor microscale) as well as the influence of buoyancy and drag forces on the particles (Rouse number and Froude number). To disentangle their overlapping influence on important physical aspects, such as preferential concentration or settling velocity alteration, we are performing an experimental study in a water tank. By using different particle populations with diameters of 20 to 200 micrometer (on the scale of or below the Kolmogorov scale) and particle-to-fluid density ratios of 2.5 to 15.6 we are able to cover a large range of the parameter space, accessing regimes which are challenging to investigate and have been rarely considered. Turbulence is generated by two oscillating grids which have been shown to be able to create conditions close to isotropy and homogeneity. The particle motion is measured using particle tracking velocimetry (PTV) in a vertical laser sheet. The flow itself is visualised by rhodamine-coated tracers and evaluated using particle image velocimetry (PIV). This simultaneous double measurement with tracers and particles of similar size is another challenging aspect of our work: two cameras record the experiment, with one only receiving fluorescent light emitted by the tracers and the other being used for the tracking of the inertial particles. The resulting experimental database will be a step forward in quantifying the respective influence of each control parameter while a help in improving theoretical models and accompanying numerical simulations [Monchaux and Dejoan, Phys. Rev. Fluids 2, 104302 (2017)].

A Lagrangian study of an axisymmetric turbulent water jet is performed to investigate the highly anisotropic and inhomogeneous flow field. The measurements were conducted at the École Normale Supérieure de Lyon, within a Lagrangian Exploration Module, an icosahedron apparatus, to facilitate optical access of three cameras. The stereoscopic particle tracking velocimetry results in three component tracks of position, velocity and acceleration of the tracer particles within the vertically-oriented jet with a Taylor-based Reynolds number Reλ = 260. Analysis is performed at seven locations from near the jet exit at 15 diameters downstream up to 45 diameters downstream. Eulerian analysis is first carried out to obtain critical parameters of the jet and relevant scales, namely the Kolmogorov and large turnover (integral) scales as well as the energy dissipation rate. Lagrangian statistical analysis is then executed on velocity components stationarised following methods inspired by Batchelor (Batchelor 1957 J. Fluid Mech.) which aim to extending Taylor's stationary Lagrangian theory of diffusion to the case of self-similar flows. The evolution of typical Lagrangian scaling parameters as a function of the developing jet is explored and results show validation of the proposed stationarisation. The universal scaling constant C0 (for the Lagrangian second-order structure function), as well as Eulerian and Lagrangian integral time scales are discussed in this context. C0 is found to converge to a constant value (of the order of C0 = 3) within 30 diameters downstream of the nozzle. Finally, the existence of finite particle size effects are investigated through consideration of acceleration dependent quantities.

The understanding of two-particle dispersion in turbulence is crucial for the estimation of aerosol transport and airborne virus exposure. At long times, the famous Richardson prediction states that the particle dispersion behaves as a t^3 power law. At short times, the two-particle dispersion quantifies the flow irreversibility as the time-symmetry breaking shows that ‘particle separate slower backwards than forward [Jucha 2014]’. Furthermore, it is now understood that in the limit of large Reynolds number, the deterministic particle trajectories are expected to become non-unique. This phenomenon termed spontaneous stochasticity [Eyink 2014-20] states that in a turbulent flow, two particles separate independently of their initial separation length. The experimental study of such phenomena is extremely difficult since it requires long-time highly dense Lagrangian tracking. New 4D-PTV methods [Schanz 2016] seem to enable us to access such fields. However, the particle trajectories are strongly affected by measurement noise that deteriorate the measurement of short time velocity evolution as well as high order statistics. The subject of this talk is twofold : - First, we study a Lagrangian trajectory denoising method based on regularized B-spline [Gesemann 2016]. The aim is to find systematic criteria for optimization of algorithms used in 4D-PTV in order to optimize the quality of 4D-PTV measurements of turbulent flows as well as high-order of turbulence statistics. We introduce and adapt to this context two innovative tuning strategies which are commonly used in the Tikhonov regularization of inverse problems based on L-curve shape and Normalized Cumulative Periodogram (NCP). - Secondly, thanks to the denoising technique, we will show a first experimental exploration of both Eulerian and Lagrangian Irreversibility in a Von Karman flow at a resol

Les Variations Physiques sont un ensemble de créations artistiques inspirées de la recherche scientifique, réalisées entre 2018 et 2020. Ce site web présente deux expériences de réalité virtuelle, portées en format film plat ainsi qu’un ensemble de tableaux d’art algorithmique.Site Web : http://variationsphysiques.fr - Full Video : https://vimeo.com/477577934

While marine plastic pollution is ubiquitous and represents a global environmental threat, fundamental questions related to the fate of plastic debris in oceans remain poorly understood. In particular, the fragmentation process of marine plastic items, at the origin of the dispersion of microplastic debris across the globe, remains qualitatively described in the literature and a quantitative description is therefore needed. For instance, the fragmentation model currently used considers impact between solids, which is not relevant for a turbulent fluid environment. Here, I will present a physical model for the fragmentation of plastic debris in the turbulent upper layer of the ocean. Using laboratory experiments on the fragmentation of brittle fibres in a turbulent flow complemented by numerical simulations and theoretical analyses, our results demonstrate that the fragmentation process is limited at small scales by a physical cut-off length. This length originates from the fluid-structure interactions between the debris and the surrounding turbulent flow field, and is shown to be independent of the brittleness of the fibres. Such limitation mechanism of the fragmentation process at small scales is able to reproduce the size distribution of floating plastic debris measured in the ocean and therefore paves the way for a thorough understanding of marine plastic fragmentation.

Nous présentons la notion de production de vorticité le long d'une interface séparant deux fluides immiscibles et incompressibles. Ce concept est ensuite appliqué aux cas de l'interaction entre un vortex pré-existant et une interface. Suivant les mécanismes (barocline, tension de surface, gravité,…) et les nombres de Weber et de Reynolds, des patterns se forment engendrant des structures non-linéaires de diverses tailles. Les résultats issus de ce travail permettent de comprendre en partie la physique des écoulements turbulents en présence d'interfaces.

Je propose de présenter une étude approfondie des propriétés de la turbulences à petite échelle en utilisant les données numériques et expérimentales d'un écoulement dans la même géométrie (von Kármán). Cette étude cherche à comprendre quel rôle jouent les transferts d'énergie locaux dans le phénomène d'intermittence. Nous avons calculé à l'aide d'ondelettes les fonctions de structure du champ de vitesse , ainsi que des transferts inter-échelle d'énergie cinétique locaux. On constate que ces derniers obéissent une loi d'échelle étendue généralisée, similaire à celle déjà observée pour les fonctions de structures calculées sur le champ de vitesse. Le calcul des spectres multi-fractals de toutes les fonctions de structure donne des résultats similaires dans les deux cas, ce qui témoigne de comportement liés. Nous constatons que les zones de forte vorticité et de fort transfert local d'énergie sont très intermittentes et sont corrélées. Dans la plupart des cas, la localisation du maximum local de transfert d'énergie est décalé par rapport à l'emplacement du maximum local de vorticité. On observe cependant une une corrélation beaucoup plus forte entre la vorticité et le transfert local d'énergie dans la couche de cisaillement, qui peut être un indice d'une structure quasi-singulière auto-similaire qui peut dominer les propriétés d'échelle des fonctions de structure de grand ordre, et expliquer l'intermittence.

It is expected that a high cell volumetric concentration of human mesenchymal stromal cells (hMSCs) for cell therapy can be achieved by using scalable cell culture platforms as bioreactors. In this paper, the Taylor vortex flow bioreactor (TVFB) was evaluated for hMSCs expansion due to its potential to produce low and homogeneous hydrodynamic forces, which is important for the expansion of these cells. Thus, parameters such as volumetric coefficient of oxygen transfer and shear stress were estimated for the TVFB. Additionally, hMSCs cell expansion on microcarriers was evaluated in TVFB and compared with the traditional spinner flasks. Maximum shear stress and kLa were in the range of 0.027−0.065 Pa and 0.9-11.53 h−1, respectively, at rotation speed between 60−120 rpm. Similar cell concentrations were obtained in both bioreactors, ∼1.7 × 105 cells/mL, representing a fold increase of ∼10 times. However, cell-microcarrier aggregates formed in the TVFB were smaller, with diameter of 0.36 ± 0.01 mm compared to 0.75 ± 0.31 mm obtained in the spinner flask, at the end of the culture. The results obtained in this study demonstrate the potential of TVFB to support manufacturing of anchorage dependent cell cultures for clinical applications.