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

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

Etude de jets turbulents à masse volumique variable

Des mesures de vitesse et concentration par PIV et PLIF sur un jet turbulent à masse volumique variable sont réalisées et ont pour objectif, dans un premier temps, d'apporter une meilleure compréhension des modifications induites par la présence de gradients de masse volumique sur les propriétés générales de la turbulence. On s'intéresse, plus particulièrement, aux propriétés du mélange à petite échelle (micro-mélange). Dans un second temps, l'analyse des résultats de ces mesures visera à apporter des informations complémentaires vis-à-vis d'hypothèses qui sont couramment utilisées dans les modèles de modélisation numérique de la combustion turbulente

Sodium Selenide Toxicity Is Mediated by O2-Dependent DNA Breaks

Hydrogen selenide is a recurrent metabolite of selenium compounds. However, few experiments studied the direct link between this toxic agent and cell death. To address this question, we first screened a systematic collection of Saccharomyces cerevisiae haploid knockout strains for sensitivity to sodium selenide, a donor for hydrogen selenide (H2Se/HSe−/Se2−). Among the genes whose deletion caused hypresensitivity, homologous recombination and DNA damage checkpoint genes were over-represented, suggesting that DNA double-strand breaks are a dominant cause of hydrogen selenide toxicity. Consistent with this hypothesis, treatment of S. cerevisiae cells with sodium selenide triggered G2/M checkpoint activation and induced in vivo chromosome fragmentation. In vitro, sodium selenide directly induced DNA phosphodiester-bond breaks via an O2-dependent reaction. The reaction was inhibited by mannitol, a hydroxyl radical quencher, but not by superoxide dismutase or catalase, strongly suggesting the involvement of hydroxyl radicals and ruling out participations of superoxide anions or hydrogen peroxide. The •OH signature could indeed be detected by electron spin resonance upon exposure of a solution of sodium selenide to O2. Finally we showed that, in vivo, toxicity strictly depended on the presence of O2. Therefore, by combining genome-wide and biochemical approaches, we demonstrated that, in yeast cells, hydrogen selenide induces toxic DNA breaks through an O2-dependent radical-based mechanism

Recueil des actes du 14e Congrès Francophone de Techniques Laser

National audienc

Effect of the vegetation density on the turbulence properties in a canopy flow.

International audienc

Simultaneous measurements of temperature and velocity fluctuations in a slightly heated jet combining a cold wire and laser doppler anemometry

International audienceSimultaneous measurements of two velocity components and temperature are performed combining Laser Doppler Anemometry (LDA) and cold wire thermometry. LDA is a common technique suitable for velocity measurements in turbulent jets where strong turbulence intensities and reverse flows may exist, but temperature measurements in association with LDA are difficult because the fine wire response is altered by the seeding deposit, so that the wire must be regularly cleaned. Results reported herein concern velocity–temperature correlations, as well as velocity and temperature marginal probability density functions and temperature (or velocity) probability density functions conditioned by the sign of the velocity (or temperature) fluctuation. The evolution of these various quantities is analysed in order to better understand the mixing properties in the near-field of a turbulent jet where the initial conditions still have a strong influence. It is shown that, while the velocity field tends to relax rather quickly (within a few nozzle diameters from the exit) to almost gaussian statistics, the temperature properties are still significantly skewed towards the hot jet exit temperature until x/Dj about 7–8. On the contrary, the signature of the cold ambient temperature vanishes rather quickly

Visualisation de l'écoulement turbulent autour d'un couvert végétal

Session : Combustion - Aérodynamique extern

Turbulent mixed convection of heat and water vapor transfers in a two-dimensional vegetation canopy

The present study consists in a numerical investigation of turbulent mixed-convection of heat and water vapor transfers inside two-dimensional (2-D) vegetation canopy, in the surrounding atmosphere and in a wet underground. The time-averaged Navier-Stokes equations are used to characterize the flow field surrounding the canopy and within it. Reynolds shear stresses are calculated using the eddy turbulence model and the Prandtl mixing length. The governing equations are solved numerically using an implicit finite difference method and Thomas algorithm. The present model is used for the determination of the micro climatic profiles such as streamlines, isotherms and iso-concentration. Special emphasis is laid on the systematic analysis of the total evaporation rate (evapotranspiration), the local and average heat fluxes, the Nusselt and Sherwood numbers. The effects of Leaf Area Density distribution, the canopy stomata regulation, as well as the atmospheric forcing conditions on the transfers, are presented and analysed. The results show that buoyancy force caused by properties variation reduces the local heat and mass transfer coefficients, and that this reduction increases at lower wind velocities