Étude de jets sous-détendus axisymétriques d'air et d'hélium par la méthode BOS

L'objectif est d'identifier et de quantifier les principaux paramètres qui influencent la dispersion d'une fuite gazeuse d'hydrogène à haute pression. Dans cette approche, la fuite est représentée par un jet axisymétrique sous-détendu, qui s'échappe d'un réservoir dont les niveaux de pression sont compris entre 1 et 30 bars. Un gaz non réactif (air ou hélium) remplace l'hydrogène pour étudier le processus de mélange, dans une configuration sécurisée. Pour apporter des éléments de réponse, ce travail propose une investigation des effets de compressibilité dans le champ proche des jets libres axisymétriques d'air et d'hélium. Les résultats montrent que la strioscopie orientée sur l'arrière-plan (BOS) est bien adaptée pour mettre en évidence les variations de densité dues aux structures compressibles et aux variations de concentration. La BOS est utilisée pour détecter les cellules de choc du champ proche (X/De < 5) d'un jet libre sous-détendu (air ou hélium) de 1, 2, ou 3 mm de diamètre. Pour conclure, la strioscopie orientée sur l'arrière-plan (BOS) fournit de bons résultats comparés aux calculs analytiques et aux données de la littérature, avec des détails quantitatifs aux petites échelles de la structure du jet

Turbulent flows interacting with varying density canopies

In the case of dense and homogeneous canopies, it is well-known that canopy flows are similar to mixing layer flows (so-called mixing layer analogy). When the canopy becomes sparser, a transition between the mixing layer and the boundary layer perturbed by interactions between element wakes occurs. This transition has still to be fully understood and characterized. An experimental work has been developed in order to study this transition for various density canopies

Méthode LSE appliquée à la résonance acoustique d'un tuyau corrugué sous écoulement

International audienceLa dynamique de l'écoulement dans un tuyau corrugué est étudiée par des mesures par PIV couplées à des mesures de niveau de pression acoustique par microphones et de vitesse par fils chauds. L'objectif est d'utiliser la méthode LSE (Linear Stochastic Estimation) pour combiner la haute résolution spatiale des mesures par PIV à la haute résolution temporelle des mesures ponctuelles de vitesse et de pression par capteurs. On caractérise ainsi le lien entre l'aérodynamique et l'acoustique qui conduit à un sifflement intense sous certaines conditions d'écoulement dans le tuyau corrugué

Flow characteristics and turbulence analysis of a large-scale pressure-atomized spray

International audienceA typical water round-nozzle jet for agricultural applications is presented in this study. The dispersion of a liquid for irrigation or pesticides spraying is a key subject to reduce both water consumption and air pollution. A simplified study case is constructed to tackle both scenarios, where a round dn = 1.2mm nozzle of a length Ln = 50dn is considered. The water injection bulk velocity is equal to Uj = 35m/s, aligned with gravity, placing the liquid jet in a turbulent atomization regime. Experimental and numerical approaches are considered. LDV and DTV optical techniques are used to gather statistical information from both the liquid and the gas phases of the spray. The experimental campaign is carried out from x/dn = 0 to 800. Concerning the LDV, small (∼ 1µm) olive-oil tracers are used to capture the gas phase, where a distinction between the liquid droplets and tracers is achieved by a specific setup of the laser power source and the burst Doppler setting (BP-Filter and SNR). On the dispersed zone, DTV measurements are carried out to determine velocities and sizes of droplets. Special attention to the depth-of-field (DOF) estimation is taken in order to obtain a less biased droplet's size-velocity correlation. Finally, an optical probe (OP) was also used to determine the liquid volume fraction í µí±Œ ̅ , liquid mass fraction í µí±Œ ̃ , and mixture density í µí¼Œ̅ , which are important features for such flows. These are key quantities for the determination of the mixture mean velocities and Reynolds stresses, and evaluation of the terms in their balance equations. Combining OP, LDV and DTV data allows to determine quantities such as the mixture mean density, í µí¼Œ̅ = í µí±Œ ̅ í µí¼Œ í µí°¿ + (1 − í µí±Œ ̅)í µí¼Œ í µí°º , mixture mean velocity along the i direction, í µí±¢ ̃ í µí±– = í µí±Œ ̃ í µí±¢ ̃ í µí±–,í µí°¿ + (1 − í µí±Œ ̃)í µí±¢ ̃ í µí±–,í µí°º , or mean slip velocity, í µí±¢ ̅ í µí±–,í µí±† = í µí±¢ ̅ í µí±–,í µí°¿ − í µí±¢ ̅ í µí±–,í µí°º = í µí±¢ í µí±– ′′ í µí±Œ ′′ ̃ í µí±Œ ̃ (1−í µí±Œ ̃) , where the notation '' denotes fluctuations with respect to the Favre averaged mean values. Similar relations hold for the Reynolds stresses. For such a flow, three dimensionless quantities can be constructed as a function of the forces that intervene in the atomization process. First, the nozzle Reynolds number, í µí±í µí±’ = í µí±ˆ í µí±— í µí±‘ í µí±› í µí¼ˆ í µí°¿ , allows to identify if the liquid flow inside the injector is turbulent. Then, the liquid Weber number, í µí±Ší µí±’ í µí°¿ = í µí¼Œ í µí°¿ í µí±ˆ í µí±—í µí±‘ í µí±› 2 í µí¼Ž , and the gas Weber number, í µí±Ší µí±’ í µí°º = í µí¼Œ í µí°º í µí±ˆ í µí±—í µí±‘ í µí±› 2 í µí¼Ž , which weights the importance of surface tension once the flow is in contact with the surrounding air. Finally, the Ohnesorge number, í µí±‚ ℎ = í µí¼Œ í µí°¿ í µí¼ˆ í µí°¿ √í µí¼Œ í µí°¿ í µí¼Ží µí±‘ í µí±› , characterizes the form of the liquid packets or droplets in the atomization process. Choosing Re = 41833 and Ln/dn = 50 makes the internal flow fully turbulent and ensures that the boundary layer inside the nozzle is fully developed for any upstream conditions. With WeL = 20158, WeG = 24.3 and Oh = 0.0034, the liquid phase turbulent kinetic energy should be the main responsible of the liquid-jet primary break-up, these flow conditions lying within the second wind-induced atomization regime

An improved image processing method for particle characterization by shadowgraphy

[EN] Shadowgraphy is one of the most popular imaging techniques to characterize moving particles by their size, geometry as well as velocity, due to its simplicity. However, it requires advanced image processing to handle various image defects such as non-uniform illumination, overlapped particles, etc., which are normally only solved for individual applications. This study proposes a robust image processing method for particle shadowgraphy, aiming to process imperfect particle shadow images. The proposed method first detects qualified particles from particle shadow images, and then processes detected particles individually. Therefore different defects from different particles can be handled separately and locally. An overlapped particles detection and separation algorithm is also implemented to improve the accuracy of size and geometry characterization. The proposed method is first proved by synthetic generated particle shadow images, followed by a proof test with shadow images from a transparent dot pattern target. Finally this method is successfully applied to a shadow image acquired from a water spray and proved to be able to handle various issues of shadowgraphy.This work is supported by a collaboration project among IRSTEA, IRPHE and Dantec Dynamics A/S, aiming to improve characterization of moving irregular particles by shadowgraphy measurements.Wang, H.; Felis, F.; Tomas, S.; Anselmet, F.; Amielh, M. (2017). An improved image processing method for particle characterization by shadowgraphy. En Ilass Europe. 28th european conference on Liquid Atomization and Spray Systems. Editorial Universitat Politècnica de València. 962-967. https://doi.org/10.4995/ILASS2017.2017.4614OCS96296

Experimental and numerical investigations of the aeroacoustics in a corrugated pipe flow

International audienceOur study is focused on the singing risers phenomenon which is encountered in corrugated channels under flow. Internal corrugations are responsible for flow instabilities that synchronize with longitudinal acoustic modes of the channel giving powerful pure tones. Experiments are performed in a specifically designed facility. Numerical simulations of the flow based on a lattice Boltzmann method (LBM) are faced to the experimental results. They aimed at investigating the ability of a LBM based simulation to predict the aeroacoustics of corrugated channels. Acoustic modes and turbulence in the corrugated channel are quite well predicted except the sound pressure levels that need better description of the acoustic boundary conditions

EXPERIMENTAL AND NUMERICAL COMPARISONS OF THE AEROACOUSTICS IN A CORRUGATED PIPE FLOW

International audienceOur study is focused on the singing risers phenomenon which is encountered in corrugated channels under flow. Internal corrugations are responsible for flow instabilities that synchronize with longitudinal acoustic modes of the channel giving powerful pure tones.Experiments are performed in a specifically designed facility. Numerical simulations of the flow based on a lattice Boltzmann method (LBM) are faced to the experimental results. They aimed at investigating the ability of a LBM based simulation to predict the aeroacoustics of corrugated channels.Acoustic modes and turbulence in the corrugated channel are quite well predicted except the sound pressure levels that need better description of the acoustic boundary conditions at the open ends

Anisotropie de la turbulence dans une couche limite sur couvert végétal

L'analogie de couche de mélange est un résultat classique des écoulements sur des couverts végétaux denses et homogènes. Quand le couvert devient épars, on passe d'un écoulement de couche de mélange à celui d'une couche limite perturbée. Afin de caractériser finement cette transition, des mesures des trois composantes de la vitesse sont réalisées au sein de couverts de différentes densités. Le but du travail est de déterminer toutes les composantes du tenseur de Reynolds en écoulement de couvert et d'analyser comment cette transition se traduit sur le diagramme de Lumley