Chute d'un nuage de particules dans une turbulence diffusive. Etude des couplages entre phases par diagnostics optiques

Ce travail expérimental s'inscrit dans le cadre des études sur les écoulements diphasiques et porte plus particulièrement sur la caractérisation des interactions entre une phase porteuse gazeuse et une phase dispersée composée de particules solides : i.e. la dispersion des particules, la formation d'amas de particules, et leur effet sur la modification de l'écoulement porteur. La plupart des résultats expérimentaux, obtenus dans des configurations complexes, ne peuvent être comparés que difficilement à la théorie et aux résultats numériques. Le premier travail a ainsi consisté à réaliser un dispositif expérimental présentant un écoulement monophasique avec un régime proche de la THI (Turbulence Homogène Isotrope) et permettant de se placer dans des régimes d'écoulement propices à l'apparition de concentration préférentielle. L'écoulement retenu est celui engendré par un jet d'air confiné dans un tube fermé hermétiquement en son sommet. Cet écoulement présente bien une zone de turbulence diffusive. Les grandeurs caractéristiques de cet écoulement (vitesse moyenne, fluctuations de vitesse, cisaillement, moments d'ordre 3 et 4) sont comparables à celles rapportées dans la littérature. L'écoulement diphasique est obtenu en utilisant un système d'ensemencement de particules assurant une injection régulière et spatialement homogène des particules par le haut du tube. L'originalité de l'expérience consiste à mesurer simultanément les champs de vitesse des particules et du fluide par une méthode optique non intrusive afin d'analyser le couplage entre les deux phases. Ces résultats ont été obtenus à l'aide d'une méthode de diagnostic optique, couplant la technique de PIV 2D2C (2 dimensions 2 composantes) classique et une méthode de PTV mise au point dans le cadre de cette étude. Cette méthode reste robuste pour des écoulements présentant des zones de fortes surconcentrations en particules, dans le cas de cette étude où la fraction moyenne volumique en particules est de l'ordre de 10-5. L'obtention de mesures fiables et simultanées sur les deux phases par couplage PIV/PTV a fait l'objet d'un soin particulier. L'analyse des statistiques des champs de vitesse fluide diphasique dans la partie haute du tube met en évidence que la présence des particules provoque une augmentation de l'énergie cinétique turbulente par rapport à l'écoulement monophasique et fait perdre le caractère isotrope de l'écoulement. Une analyse de la position instantanée des particules dans différentes régions du tube met en évidence un régime de concentration préférentielle caractérisé par la formation d'amas de particules au sein de l'écoulement. Nous proposons un modèle de production d'énergie cinétique turbulente par les fluctuations de concentration des particules associées aux amas en présence de gravité. Malgré le caractère dilué en particules de notre écoulement, la formation d'amas modifie fortement la turbulence du fluide. Cette même dynamique collective des particules, regroupées en amas, pourrait être une piste intéressante afin d'expliquer aussi la modification du mouvement relatif moyen des particules. ABSTRACT : This experimental work lies within the scope of the studies on the two-phase flows and concerns more particularly the characterization of the interactions between a carrier gas phase and a dispersed phase consisting in solid particles : i.e. the dispersion of the particles, the clusters generation and their effect on the continuous phase. The majority of the experimental results, obtained in complex configurations, can be compared only with difficulty with existing theory and numerical results. Thus, the first work consisted in carrying out an experimental apparatus presenting a single-phase flow with a region as similar as possible to homogeneous and isotropic turbulence, and in making possible the apparition of preferential concentration in two-phase flow. The single-phase flow consisted in a vertical air jet confined in a tube closed hermetically at the top section, which presents a zone of diffusive turbulence. The characteristic sizes of this flow (mean velocity, velocity fluctuations, shearing, moments of order 3 and 4) are the same with those reported in the literature. The two-phase flow is obtained by using a stationary and spatially homogeneous injection of particles from the top of the tube. The originality of the experiment consists in simultaneous measurements of the velocity fields of the particles and of the fluid by a non-intrusive optical method in order to analyse the coupling between the two phases. These results are obtained using a method of optical diagnostic, coupling the traditional technique of PIV 2D2C (2 dimensions 2 components) and a method of PTV developed in this study. This method remains robust for flows with zones of strong over-concentrations in particles, in this study where the average volume fraction in particles is of about a 10-5. The simultaneous measurements on the two phases by coupling PIV and PTV were the particular careful aim of this work. The analysis of the statistics of the velocity fields of the two-phase flow in the high part of the tube highlights that the particles cause an increase of the turbulent kinetic energy compared to the single-phase flow and a loss of the isotropic character of the flow. An analysis of the instantaneous position of the particles in various areas of the tube demonstrates that a region of preferential concentration exists. It is characterised by the presence of clusters of particles. We suggest a model of turbulent kinetic energy production by the fluctuations of concentration of the particles associated with clusters in the presence of gravity. In spite of low volume fraction of particles, the formation of cluster strongly modifies the turbulence of the fluid. This same collective dynamics of the particles could be an interesting track in order to analyse the modification of the average relative movement of the particles

Lagrangian acceleration statistics in a turbulent channel flow

Lagrangian acceleration statistics in a fully developed turbulent channel flow at $Re_\tau = 1440$ are investigated, based on tracer particle tracking in experiments and direct numerical simulations. The evolution with wall distance of the Lagrangian velocity and acceleration time scales is analyzed. Dependency between acceleration components in the near-wall region is described using cross-correlations and joint probability density functions. The strong streamwise coherent vortices typical of wall-bounded turbulent flows are shown to have a significant impact on the dynamics. This results in a strong anisotropy at small scales in the near-wall region that remains present in most of the channel. Such statistical properties may be used as constraints in building advanced Lagrangian stochastic models to predict the dispersion and mixing of chemical components for combustion or environmental studies.Comment: accepted for publication in Physical Review Fluid

Analyse par imagerie laser des interactions entre une turbulence diffusive et un nuage de particules

Cette étude expérimentale porte sur la caractérisation des interactions entre un nuage de particules solides et un écoulement de jet d’air confiné, présentant une région de dynamique proche d’une turbulence homogène isotrope. L’originalité de l’expérience consiste à mesurer simultanément les champs de vitesse des particules et du fluide afin d’analyser le couplage entre les deux phases. Ces résultats sont obtenus en couplant la technique de PIV 2D2C (2 dimensions 2 composantes) classique et une méthode de PTV développée afin de permettre une mesure de vitesse des particules pour des écoulements globalement dilués mais présentant des zones de fortes surconcentrations en particules. Une analyse de la position instantanée des particules a mis en évidence un régime de concentration préférentielle qui modifie fortement la turbulence du fluide. La mesure simultanée des deux champs de vitesse a permis aussi de définir les statistiques conditionnelles des vitesses du fluide près des particules

Etude de couches limites oscillantes par vélocimétrie laser Doppler

International audienceLe transport sédimentaire induit par les vagues à l'approche de la côte est piloté par des processus non linéaires et turbulents. Les non-linéarités des vagues se caractérisent par une dissymétrie de vitesse (les crêtes hautes des vagues sont de courte durée et les creux peu profonds de longue durée) et une asymétrie de vitesse (ou dissymétrie d'accélération, caractérisant la raideur des fronts). Des études récentes indiquent que des fronts raides (vagues asymétriques) produisent des vitesses dissymétriques dans la couche limite. Ainsi, pour développer des formules de prédiction de transport des sédiments, la compréhension détaillée de la dynamique de la couche limite de fond et des contraintes de cisaillement sous les ondes de surface apparaît essentielle. Ceci justifie de chercher à réaliser des mesures de vitesse dans les tous premiers millimètres au-dessus du lit.Par ailleurs, la caractérisation de la turbulence sous les vagues déferlantes reste une question ouverte, en particulier pour chercher à évaluer la part provenant de la vague déferlée de celle produite par frottement au fond.Actuellement, l’essentiel de notre connaissance des couches limites oscillantes est issu de mesures réalisées sur des fonds fixes horizontaux. En laboratoire, l’évolution des non-linéarités des vagues, lors de leur propagation et de leur déferlement, a été principalement étudiée pour des plages de pentes relativement fortes (> 1:40). Cependant, des études de terrain récentes sur des plages réelles de pentes moins raides (1:80) ont montré que certains processus non-linéaires sont différents par rapport aux cas des plages de pentes relativement raides (> 1:40). Cette constatation a motivé le lancement d’une série d'expériences de laboratoire dans le cadre du projet européen GLOBEX sur une plage à fond fixe de pente 1:80

Efficacy and Safety of Intravitreal Gene Therapy for Leber Hereditary Optic Neuropathy Treated within 6 Months of Disease Onset.

PURPOSE: To evaluate the efficacy of a single intravitreal injection of rAAV2/2-ND4 in subjects with visual loss from Leber hereditary optic neuropathy (LHON). DESIGN: RESCUE is a multicenter, randomized, double-masked, sham-controlled, phase 3 clinical trial. PARTICIPANTS: Subjects with the m.11778G>A mitochondrial DNA mutation and vision loss ≤6 months from onset in 1 or both eyes were included. METHODS: Each subject's right eye was randomly assigned (1:1) to treatment with rAAV2/2-ND4 (single injection of 9 × 1010 viral genomes in 90 μl) or to sham injection. The left eye received the treatment not allocated to the right eye. MAIN OUTCOME MEASURES: The primary end point was the difference of the change from baseline in best-corrected visual acuity (BCVA) between rAAV2/2-ND4-treated and sham-treated eyes at week 48. Other outcome measures included contrast sensitivity, Humphrey visual field perimetry, retinal anatomic measures, and quality of life. Follow-up extended to week 96. RESULTS: Efficacy analysis included 38 subjects. Mean age was 36.8 years, and 82% were male. Mean duration of vision loss at time of treatment was 3.6 months and 3.9 months in the rAAV2/2-ND4-treated eyes and sham-treated eyes, respectively. Mean baseline logarithm of the minimum angle of resolution (logMAR) BCVA (standard deviation) was 1.31 (0.52) in rAAV2/2-ND4-treated eyes and 1.26 (0.62) in sham-treated eyes, with a range from -0.20 to 2.51. At week 48, the difference of the change in BCVA from baseline between rAAV2/2-ND4-treated and sham-treated eyes was -0.01 logMAR (P = 0.89); the primary end point of a -0.3 logMAR (15-letter) difference was not met. The mean BCVA for both groups deteriorated over the initial weeks, reaching the worst levels at week 24, followed by a plateau phase until week 48, and then an improvement of +10 and +9 Early Treatment Diabetic Retinopathy Study letters equivalent from the plateau level in the rAAV2/2-ND4-treated and sham-treated eyes, respectively. CONCLUSIONS: At 96 weeks after unilateral injection of rAAV2/2-ND4, LHON subjects carrying the m.11778G>A mutation treated within 6 months after vision loss achieved comparable visual outcomes in the injected and uninjected eyes

Bilateral Visual Improvement with Unilateral Gene Therapy Injection for Leber Hereditary Optic Neuropathy

REVERSE is a randomized, double-masked, sham-controlled, multicenter, phase III clinical trial that evaluated the efficacy of a single intravitreal injection of rAAV2/2 ND4 in subjects with visual loss from Leber hereditary optic neuropathy (LHON). A total of 37 subjects carrying the m.11778G>A (MT-ND4) mutation and with duration of vision loss between 6 to 12 months were treated. Each subject’s right eye was randomly assigned in a 1:1 ratio to treatment with rAAV2/2 ND4 (GS010) or sham injection. The left eye received the treatment not allocated to the right eye. Unexpectedly, sustained visual improvement was observed in both eyes over the 96-week follow-up period. At Week 96, rAAV2/2 ND4-treated eyes showed a mean improvement in best-corrected visual acuity (BCVA) of -0.308 LogMAR (+15 ETDRS letters). A mean improvement of 0.259 (0.068) LogMAR (+13 ETDRS letters) was observed in the sham treated eyes. Consequently, the primary endpoint, defined as the difference in the change in BCVA from baseline to Week 48 between the two treatment groups, was not met (p = 0.894, ANCOVA). At Week 96, 25 subjects (68%) had a clinically relevant recovery in BCVA from baseline in at least one eye and 29 subjects (78%) had an improvement in vision in both eyes. A non-human primate study was conducted to investigate this bilateral improvement. Evidence of transfer of viral vector DNA from the injected eye to the anterior segment, retina and optic nerve of the contralateral non-injected eye supports a plausible mechanistic explanation for the unexpected bilateral improvement in visual function after unilateral injection

Mycobacterial PIMs Inhibit Host Inflammatory Responses through CD14-Dependent and CD14-Independent Mechanisms

Mycobacteria develop strategies to evade the host immune system. Among them, mycobacterial LAM or PIMs inhibit the expression of pro-inflammatory cytokines by activated macrophages. Here, using synthetic PIM analogues, we analyzed the mode of action of PIM anti-inflammatory effects. Synthetic PIM1 isomer and PIM2 mimetic potently inhibit TNF and IL-12 p40 expression induced by TLR2 or TLR4 pathways, but not by TLR9, in murine macrophages. We show inhibition of LPS binding to TLR4/MD2/CD14 expressing HEK cells by PIM1 and PIM2 analogues. More specifically, the binding of LPS to CD14 was inhibited by PIM1 and PIM2 analogues. CD14 was dispensable for PIM1 and PIM2 analogues functional inhibition of TLR2 agonists induced TNF, as shown in CD14-deficient macrophages. The use of rough-LPS, that stimulates TLR4 pathway independently of CD14, allowed to discriminate between CD14-dependent and CD14-independent anti-inflammatory effects of PIMs on LPS-induced macrophage responses. PIM1 and PIM2 analogues inhibited LPS-induced TNF release by a CD14-dependent pathway, while IL-12 p40 inhibition was CD14-independent, suggesting that PIMs have multifold inhibitory effects on the TLR4 signalling pathway

Chute d'un nuage de particules dans une turbulence diffusive (étude des couplages entre phases par diagnostics optiques)

Ce travail expérimental porte sur la caractérisation des interactions turbulence-particules solides : dispersion, formation et effet des amas de particules sur l'écoulement porteur. L'écoulement monophasique gazeux est créé par un jet d'air confiné dans un tube fermé en son sommet. Une zone proche de la turbulence homogène isotrope s'y développe. L'écoulement diphasique est obtenu en injectant des particules uniformément par le haut du tube. Nous mesurons simultanément les champs de vitesse des particules et du fluide par une méthode optique couplant PIV 2D2C et une méthode de PTV développée dans cette étude. Un régime de concentration préférentielle (avec formation d'amas) apparaît dans l'écoulement. Malgré le faible chargement solide, la dynamique collective des particules entraîne une forte modification de la turbulence du fluide et du mouvement relatif moyen des particules. Un modèle qui rend compte de ces effets est proposé.This experimental study concerns the characterization of the turbulence-particles interactions : dispersion, clusters generation and effects on the carrier flow. The experimental apparatus consists in a vertical jet confined in a tube closed at the top section. The single-phase flow presents a region near homogeneous and isotropic turbulence. In the two-phase flow, we inject particles uniformly at the top of tube. We have developed a PIV/PTV 2D2C methodology to measure simultaneously the velocity fields of both phases. We notice a generation of clusters of particles by preferential concentration. In spite of two-phase flow with low volume fraction of particles, the clusters dynamics leads to a strong modification of the settling velocity of the particles and of the carrier flow turbulence. A model that represents these effects is discussed.TOULOUSE-ENSEEIHT (315552331) / SudocSudocFranceF

Darrieus turbine modellings - Comparisons with performance measurements and PIV fields

International audienceDarrieus marine turbines are subjected to large variations of relative velocity on the blades both in amplitude and direction during rotation. As a result, the incidence of the relative speed of the incoming flow on the blade can exceed by far the static stall angle, in which case a major unsteady phenomenon, known as "dynamic stall", may occur. It is characterized by the shedding of large vortical structures impacting the mean and dynamic load as well as the turbine efficiency. In this paper, the ability of a RANS modelling to address the complex flow field of a Darrieus turbine is examined. The considered turbine is the A-12 small scale marine turbine model developed at LEGI. It is a three bladed model inspired by the Darrieus turbine (Fig. 1). The specificity of the turbine is a special design of the arms linking the blades to the rotor which limits the induced drag and the hydrodynamic interactions with the blade. The turbine diameter and height are 175 mm. The chord of the blades defined as the length of the curved midline is c = 32.08 mm, leading to a solidity σ = 1.1 which corresponds to the best efficiency according to Shiono et al. [1]. The blade profile is a NACA0018 foil section projected on the rotation circle. The corresponding blade camber is 4.6%. The incident flow velocity U∞ is 2.8 m.s-1 resulting in a Reynolds number based on the diameter of the turbine ReD = 4.9 x 105. The blade Reynolds number ReC, based on the chord length, depends on the operating condition investigated in this paper and is on the order of 2 x 105. The LEGI water tunnel [2] used for testing the turbine model. It is a closed loop with a water capacity of 36 m3. A motor pump of 165 kW with variable speed allows a flow rate up to 500 L.s-1. The flow rate is measured by a turbine flow meter with an accuracy of ± 0.5%. The test section used for marine turbine experiments is shown in Fig. 2. It has a cross section of 0.70 x 0.25 m with a length of 1 m. Maximum speed in the test section is 2.8 m.s-1. A force balance mounted on the top of the test section allows measuring the instantaneous torque and the forces exerting on the turbine. Three transparent PMMA (Polymethyl Methacrylate) windows on the bottom and on the two sides allow ample optical access for visualization and PIV measurements [3]. The turbulence level has been measured in the water tunnel equipped with another test section by Franc and Michel [4] and is very low (typically smaller than 2‰) thanks to honeycombs and elbows equipped with guiding blades. A fully turbulent model and a transitioning model are used to study the possible occurrence of a partially laminar boundary layer developing along the blades at moderate Reynolds number aforementioned. The wall grid refinement sensitivity (y+) has been precisely studied in a previous study [5]. The performances obtained with the 2 models are compared to those obtained experimentally with the balance. In addition the ability of computations to capture the dynamic stall is investigated using two dimensions-two components (2D-2C) and two dimensions-three components (2D-3C) Particle Image Velocimetry (PIV) (fig.3). The vortex shedding mechanism is described in detail on the basis of measured velocity and vorticity fields for different values of the tip speed ratio which strongly influences dynamic stall

2D‐2C Particle Image Velocimetry measurements of the velocity field in a Darrieus turbine – comparison with CFD results

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