Ecoulements diphasiques liquide-gaz en régime à bulles dans les lits fixes : étude expérimentale et modélisations mono- et bi-dimensionnelles

National audienceCet article présente une nouvelle modélisation 1D des écoulements diphasiques en lits fixes granulaire sbasé sur une approche mécaniste de type bi-fluide. La structure des lois de fermetures du modèle, déduite des mécanismes à micro-échelle, est validée par des expériences. Enfin, sur la base d’observations expérimentales, une proposition d’extension du modèle à des configurations 2D ou 3D est présentée

Simultaneous eradications of Black Rat (Rattus rattus) and Ice plants (Carpobrotus spp.) on Bagaud Island (Port-Cros National Park, Provence, France): preliminary results on their impacts on Athropods communities

Les invasions biologiques sont reconnues comme l’une des principales menaces pour la conservation de la biodiversité, et de nombreux travaux ont été conduits à travers la planète pour restaurer la biodiversité insulaire par des programmes d’éradication d’espèces invasives. Cependant, peu d’études scientifiques post-éradication ont été conduites notamment pour l’évaluation de la biodiversité des communautés d’Arthropodes. L’île de Bagaud, située dans l’archipel des îles d’Hyères (Var, France), est une réserve intégrale du Parc national de Port-Cros (PNPC) qui a notamment été soumise à deux perturbations majeures d’origine anthropique au cours des derniers siècles : l’invasion du Rat noir (Rattus rattus) et celle des Griffes de sorcière (Carpobrotus spp.). Ces espèces allochtones engendrent des effets particulièrement néfastes sur la flore et la faune des écosystèmes insulaires méditerranéens, notamment sur les Arthropodes. Le PNPC a ainsi lancé un programme décennal de restauration écologique qui implique l’éradication de ces deux taxons invasifs. La communauté des Arthropodes de l’île a fait l’objet d’un état initial au printemps 2011, les éradications ont été effectuées entre septembre 2011 et janvier 2013, et la première campagne d’échantillonnage post-éradication a été réalisée au printemps 2013. L’échantillonnage de la faune épigée a été réalisé au moyen de quatre transects fixes de pièges Barber répartis dans différents milieux écologiques de l’île. Cette première étude post-éradication révèle une augmentation de l’abondance des Arthropodes piégés entre 2011 (4868 individus dans 74 pièges) et 2013 (6892, n = 60). Les effectifs moyens par piège augmentent significativement dans les zones où les Carpobrotus spp. ont été arrachés, mais chutent dans la zone de maquis qui abritait une forte densité de R. rattus. La richesse spécifique reste globalement stable (220 morpho-espèces en 2011, 216 en 2013), cependant les communautés de décomposeurs explosent a contrario des populations de prédateurs. Les suivis doivent être poursuivis afin de mieux déterminer les impacts directs et indirects à long terme de ces éradications s’exerçant sur la biodiversité des communautés d’Arthropodes de l’île.Biological invasions are recognized as one of the major threats to island biodiversity conservation, and numerous studies have been conducted around the world to restore the biodiversity of islands submitted to invasive species impacts. However, few scientific post-eradication studies were piloted notably for the biodiversity assessment of Arthropod communities. Bagaud Island is an integral reserve of the Port-Cros National Park (PNPC), located in the îles d’Hyères Archipelago (Var, France). In the last centuries, it has undergone two major anthropogenic disturbances: the invasion of the Black Rat (Rattus rattus) and the Ice plants (Carpobrotus spp.), two alien taxa known for their particularly negative effects on the flora and fauna of the Mediterranean island ecosystems, including Arthropods. PNPC has launched a ten-year program of ecological restoration that involves the eradication of these two invasive taxa. The Arthropod communities of the island have been analysed in their initial state in spring 2011, eradications were conducted between September 2011 and January 2013, and the first post-eradication campaign began in spring 2013. The ground fauna was sampled using four transects of Barber traps distributed in different ecological contexts of the island. This first post-eradication study reveals an increase in the abundance of arthropods trapped between 2011 (4868 individuals in 74 traps) and 2013 (6892, n = 60). The average number of trapped Arthropods was significantly higher in areas where Carpobrotus spp. have been eradicated, but declined in the bush area that housed a high density of R. rattus. The communities of decomposers explode, in contrast to predator populations. However, the global specific richness remains stable (220 morphospecies in 2011, 216 in 2013). Scientific monitoring by the PNC must be continued to better determine the direct and indirect long-term impacts of these eradications upon the structure and the functioning of the Arthropods communities of the Bagaud Island

Bursting phenomenon and incipient motion of solid particles in bed-load transport

International audienceIn this paper, we present the results of an experimental investigation aiming at understanding the interaction between near-wall coherent structures (or burst) and bedload transport in an open channel flow. This experiment associates Laser Doppler Anemometry measurements of the instantaneous velocity near the wall with real time measurement of sand particle trajectories on the smooth bed of a hydraulic flume. We will first give a description of the bursting phenomenon and we will present the experimental process. Then, we will give some details about the specific signal processing used in this work to detect coherent structures in the velocity signal. To conclude we will show that some characteristic scales of the particle motion are commensurate with those of the bursts

Cours de remise à niveau sous forme personnalisée et asynchrone

Cours de remise à niveau sous forme personnalisée et asynchron

The rôle of near wall turbulent structures on sediment transport

International audienceIn this paper, we present the results of an experimental investigation designed to analyze the interaction between near wall coherent structures (or burst) and bedload transport in an open channel flow. Laser doppler velocimetry (LDV) measurements of the instantaneous velocity near the wall were coupled with real time measurements of sand particle trajectories on the bottom of an hydraulic plume. We will first give a description of the bursting phenomenon and we will present the experimental design. Then, we will give some details about the specific signal processing techniques we used in this work to detect coherent structures on the velocity signal. The third part of this paper will present our results. We will show that the period between two consecutive displacements of a solid particle can be compared to the mean period between two consecutive ejections. This would suggest that these structures are involved in bedload transport. For small displacements, the time distribution between two consecutive movements shows two modes. One would be associated to particles whose mean deposit time corresponds to the ejection period, the second would be associated to particles whose mean deposit time is close to the sweep period. We made the assumption that there exist two transport modes at the wall: the dominant one was transport by ejections, but sweeps can be involved in the removal of particles whose friction coecient with the wall is high

Agitation and Microstructure in non-dilute homogeneous bubbly flows, CD proc

International audienc

couplage hydrodynamique-biomasse dans les procédés de dépollution : approche locale des mécanismes de croissance et d'adhésion/détachement de micro-organismes sur substrat solide

présentation lors de la réunion du Cluster Environnement "Gestion des effluents industriels et urbains ex-situ

Induced agitation in homogeneous bubbly flows at moderate particle Reynolds number

International audienceWe investigated the agitation induced in a liquid by quasimonodispersed spherical bubbles. For Rep about 10 and for void fractions ranging from 0.002 to 0.1, the axial velocity fluctuation probability density functions happen to be uniquely determined by the velocity variance. In addition, the variance experiences a rupture from a nonlinear to a quasilinear increase with the volumic concentration. It is shown that the presence of a deficit of neighbors in the wake of any test inclusion forces the screening of velocity disturbances at a finite distance X--of order Rep / $alpha$ --and that the velocity variance does not depend on the system size when the later is large compared with X. Finally, the bubble-induced agitation happens to be much stronger for quasimonodispersed than for polydispersed size distributions because of the longer interaction duration between pairs in the former case