Spheres in the vicinity of a bifurcation: elucidating the Zweifach-Fung effect

International audienceThe problem of the splitting of a suspension in bifurcating channels dividing into two branches of non equal flow rates is addressed. As observed for long, in particular in blood flow studies, the volume fraction of particles generally increases in the high flow rate branch and decreases in the other one. In the literature, this phenomenon is sometimes interpreted as the result of some attraction of the particles towards this high flow rate branch. In this paper, we focus on the existence of such an attraction through microfluidic experiments and two-dimensional simulations and show clearly that such an attraction does not occur but is, on the contrary, directed towards the low flow rate branch. Arguments for this attraction are given and a discussion on the sometimes misleading arguments found in the literature is proposed. Finally, the enrichment in particles in the high flow rate branch is shown to be mainly a consequence of the initial distribution in the inlet branch, which shows necessarily some depletion near the walls

Domain decomposition methods in Feel++

International audienceThis paper presents our ongoing work on building a versatile domain decomposition methods framework in Feel++ that provides expressivity (e.g. closeness to the mathematical language) and efficiency. We display in particular the capabilities of Feel++ regarding Schwarz (non)overlapping and mortar methods. Some numerical tests and code snippets, taken from Feel++, support the explanations

Simulation of two-fluid flows using a Finite Element/level set method. Application to bubbles and vesicle dynamics

International audienceA new framework for two-fluids flow using a Finite Element/Level Set method is presented and verified through the simulation of the rising of a bubble in a viscous fluid. This model is then enriched to deal with vesicles (which mimic red blood cells mechanical behavior) by introducing a Lagrange multiplier to constrain the inextensibility of the membrane. Moreover, high order polynomial approximation is used to increase the accuracy of the simulations. A validation of this model is finally presented on known behaviors of vesicles under flow such as "tank treading" and tumbling motions

Microbubble moving in blood flow in microchannels: effect on the cell-free layer and cell local concentration

Gas embolisms can hinder blood flow and lead to occlusion of the vessels and ischemia. Bubbles in microvessels circulate as tubular bubbles (Taylor bubbles) and can be trapped, blocking the normal flow of blood. To understand how Taylor bubbles flow in microcirculation, in particular, how bubbles disturb the blood flow at the scale of blood cells, experiments were performed in microchannels at a low Capillary number. Bubbles moving with a stream of in vitro blood were filmed with the help of a high-speed camera. Cell-free layers (CFLs) were observed downstream of the bubble, near the microchannel walls and along the centerline, and their thicknesses were quantified. Upstream to the bubble, the cell concentration is higher and CFLs are less clear. While just upstream of the bubble the maximum RBC concentration happens at positions closest to the wall, downstream the maximum is in an intermediate region between the centerline and the wall. Bubbles within microchannels promote complex spatio-temporal variations of the CFL thickness along the microchannel with significant relevance for local rheology and transport processes. The phenomenon is explained by the flow pattern characteristic of low Capillary number flows. Spatio-temporal variations of blood rheology may have an important role in bubble trapping and dislodging.The authors acknowledge the financial support provided by PTDC/SAU-BEB/105650/2008, PTDC/SAU-ENB/ 116929/2010, EXPL/EMS-SIS/2215/2013 and PTDC/QEQ-FTT/4287/ 2014 from FCT (Science and Technology Foundation), COMPETE, QREN and European Union (FEDER).info:eu-repo/semantics/publishedVersio