Viscoelastic liquid curtains

International audienc

Ecoulements de fluides viscoélastiques en géométries confinées (application à la récupération assistée des hydrocarbures)

L'écoulement des fluides complexes à l'échelle micrométrique est une problématique qui intéresse notamment la récupération assistée du pétrole. Ici, les fluides sont des solutions de polymères et de tensioactifs capables de s'auto-assembler en micelles géantes. Nous étudions ces écoulements au sein d'outils microfluidiques fabriqués en résine SU-8 selon un protocole développé pendant cette thèse. Nous avons réalisé des expériences de drainage d'huile en milieux poreux et montré que la rhéofluidification et le glissement promeuvent le phénomène de digitation pendant l'invasion. Nos expériences montrent que ces solutions peuvent être élastiquement turbulentes à de faibles nombre de Reynolds. Ces instabilités de vitesse ont des conséquences sur la rhéologie locale du fluide dans un simple canal droit et sont une source de dissipation additionnelle dans des géométries plus complexes.The flow of complex fluids in confined geometries is an issue of interest notably in the field of oil recovery. In this work, the fluids are polymer solutions of high molecular weight and surfactant solutions enable to form wormlike micelles. We study the flow in microfluidic devices made-up with Su-8 resin following a protocol that has been set during this PhD. We carried out experiences of oil drainage in porous media and show that shear-thinning and slippage are propoting the fingering during the invasion. our experiences also show that these solutions can be turbulent at low reynods numbers. These instabilities have consequences on the local rheology in a simple straight channel and are source of additional dissipation in more complex geometries.BORDEAUX1-Bib.electronique (335229901) / SudocSudocFranceF

Sequential biogenesis of host cell membrane rearrangements induced by hepatitis C virus infection.: HCV-induced membrane rearrangements

International audienceLike most positive-strand RNA viruses, hepatitis C virus (HCV) forms a membrane-associated replication complex consisting of replicating RNA, viral and host proteins anchored to altered cell membranes. We used a combination of qualitative and quantitative electron microscopy (EM), immuno-EM, and the 3D reconstruction of serial EM sections to analyze the host cell membrane alterations induced by HCV. Three different types of membrane alteration were observed: vesicles in clusters (ViCs), contiguous vesicles (CVs), and double-membrane vesicles (DMVs). The main ultrastructural change observed early in infection was the formation of a network of CVs surrounding the lipid droplets. Later stages in the infectious cycle were characterized by a large increase in the number of DMVs, which may be derived from the CVs. These DMVs are thought to constitute the membranous structures harboring the viral replication complexes in which viral replication is firmly and permanently established and to protect the virus against double-stranded RNA-triggered host antiviral responses

Optimal Memory-aware Backpropagation of Deep Join Networks

International audienceDeep Learning training memory needs can preventthe user to consider large models and large batchsizes. In this work, we propose to use techniquesfrom memory-aware scheduling and AutomaticDifferentiation (AD) to execute a backpropagationgraph with a bounded memory requirement at thecost of extra recomputations. The case of a singlehomogeneous chain, i.e. the case of a networkwhose all stages are identical and form a chain,is well understood and optimal solutions havebeen proposed in the AD literature. The networksencountered in practice in the context of DeepLearning are much more diverse, both in terms ofshape and heterogeneity.In this work, we define the class of backpropagationgraphs, and extend those on which one can computein polynomial time a solution that minimizes the totalnumber of recomputations. In particular we considerjoin graphs which correspond to models such asSiamese or Cross Modal Networks

A Makespan Lower Bound for the Scheduling of the Tiled Cholesky Factorization based on ALAP Schedule

International audienceDue to the advent of multicore architectures and massive parallelism, the tiled Cholesky factorization algorithm has recently received plenty of attention and is often referenced by practitioners as a case study. It is also implemented in mainstream dense linear algebra libraries and is used as a testbed for runtime systems. However, we note that theoretical study of the parallelism of this algorithm is currently lacking. In this paper, we present new theoretical results about the tiled Cholesky factorization in the context of a parallel homogeneous model without communication costs. Based on the relative costs of involved kernels, we prove that only two different situations must be considered, typically corresponding to CPUs and GPUs. By a careful analysis on the number of tasks of each type that run simultaneously in the ALAP (As Late As Possible) schedule without resource limitation, we are able to determine precisely the number of busy processors at any time (as degree 2 polynomials). We then use this information to find a closed form formula for the minimum time to schedule a tiled Cholesky factorization of size n on P processors. We show that this bound outperforms classical bounds from the literature. We also prove that ALAP(P), an ALAP-based schedule where the number of resources is limited to P , has a makespan extremely close to the lower bound, thus proving both the effectiveness of ALAP(P) schedule and of the lower bound on the makespan

I/O-Optimal Algorithms for Symmetric Linear Algebra Kernels

International audienceIn this paper, we consider two fundamental symmetric kernels in linear algebra: the Cholesky factorization and the symmetric rank-$k$ update (SYRK), with the classical three nested loops algorithms for these kernels. In addition, we consider a machine model with a fast memory of size $S$ and an unbounded slow memory. In this model, all computations must be performed on operands in fast memory, and the goal is to minimize the amount of communication between slow and fast memories. As the set of computations is fixed by the choice of the algorithm, only the ordering of the computations (the schedule) directly influences the volume of communications.We prove lower bounds of $\frac{1}{3\sqrt{2}}\frac{N^3}{\sqrt{S}}$ for the communication volume of the Cholesky factorization of an $N\times N$ symmetric positive definite matrix, and of $\frac{1}{\sqrt{2}}\frac{N^2M}{\sqrt{S}}$ for the SYRK computation of $\mathbf{A}\cdot\mathbf{A}^{T}$, where $\mathbf{A}$ is an $N\times M$ matrix. Both bounds improve the best known lower bounds from the literature by a factor $\sqrt{2}$.In addition, we present two out-of-core, sequential algorithms with matching communication volume: \TBS for SYRK, with a volume of $\frac{1}{\sqrt{2}}\frac{N^2M}{\sqrt{S}} + \mathcal{O}(NM\log N)$, and \LBC for Cholesky, with a volume of $\frac{1}{3\sqrt{2}}\frac{N^3}{\sqrt{S}} + \mathcal{O}(N^{5/2})$. Both algorithms improve over the best known algorithms from the literature by a factor $\sqrt{2}$, and prove that the leading terms in our lower bounds cannot be improved further. This work shows that the operational intensity of symmetric kernels like SYRK or Cholesky is intrinsically higher (by a factor $\sqrt{2}$) than that of corresponding non-symmetric kernels (GEMM and LU factorization)

Turbulence élastique dans des solutions de polymères en canal droit à bas Re

Les solutions de polymères s’écoulant dans des géométries curvilinéaires à faibles nombres de Reynolds (Re >1), cette turbulence, dite élastique, est attribuée à l’anisotropie des contraintes normales dans la solution. Il a été montré seulement très récemment, à la fois théoriquement [2] et expérimentalement [3], que ce comportement se développait aussi en géométrie rectiligne si une perturbation d’amplitude suffisamment grande était apportée à l’écoulement. Dans ce travail, nous caractérisons expérimentalement les conséquences de cette turbulence élastique sur les profils de vitesse et sur la loi de comportement d’une solution de Polyaccrylamides de forte masse molaire en écoulement dans une géométrie rectiligne. Il a déjà été théoriquement montré dans la littérature que les spectres des vitesses turbulentes suivent une décroissance en loi de puissance des basses fréquences vers les hautes fréquences [4]. La mesure de ces spectres par une technique de LDV nous permet de mettre en évidence sur notre solution cette décroissance caractéristique de la turbulence élastique. A l’échelle microfluidique, un simple canal droit nous permet d’explorer une large gamme de Wi en variant le gradient de pression aux extrémités du canal. Une technique de vélocimétrie par suivi de particules, couplées à des mesures de rhéométrie rotationnelle classique, mettent en évidence une instabilité du front d’écoulement. La loi de comportement de la solution, déduite de la courbe d’écoulement contrainte / taux de déformation, dépend fortement du gradient de pression appliqué au canal. Pour de faibles Wi, les courbes se superposent. Par contre, au-delà d’un Wi critique, cette prédiction n’est plus valide et de fortes fluctuations sont observées. Ce comportement est attribué au degré de turbulence dans la solution en écoulement. [1] A. Groisman and V. Steinberg, Nature. 405 (2000). [2] A. Morozov and W. van Saarloos, Physics Report 447 (2007). [3] D. Bonn, F. Ingremeau, Y. Amarouchene and H. Kellay, Phys. Rev. E 84, 045301(2011). [4] A. Fouxon and V. Lebedev, Physics of Fluid 15(7) (2003)

Symmetric Block-Cyclic Distribution: Fewer Communications Leads to Faster Dense Cholesky Factorization

International audienceWe consider the distributed Cholesky factorization on homogeneous nodes. Inspired by recent progress on asymptotic lower bounds on the total communication volume required to perform Cholesky factorization, we present an original data distribution, Symmetric Block Cyclic (SBC), designed to take advantage of the symmetry of the matrix. We prove that SBC reduces the overall communication volume between nodes by a factor of square root of 2 compared to the standard 2D blockcyclic distribution. SBC can easily be implemented within the paradigm of task-based runtime systems. Experiments using the Chameleon library over the StarPU runtime system demonstrate that the SBC distribution reduces the communication volume as expected, and also achieves better performance and scalability than the classical 2D block-cyclic allocation scheme in all configurations. We also propose a 2.5D variant of SBC and prove that it further improves the communication and performance benefits

Dispersal strategies of juvenile pike (Esox lucius L.): influences and consequences for body size, somatic growth and trophic position

Individual variability in dispersal strategies, where some individuals disperse and others remain resident, is a common phenomenon across many species. Despite its important ecological consequences, however, the mechanisms and individual advantages of dispersal remain poorly understood. Here, riverine Northern pike (Esox lucius) juveniles (young-of-the-year and age 1+ year) were used to investigate the influence of body size and trophic position (at capture) on the dispersal from off-channel natal habitats, and the subsequent consequences for body sizes, specific growth rate and trophic position (at recapture). Individuals that dispersed into the river (‘dispersers’) were not significantly different in body size or trophic position than those remaining on nursery grounds (‘stayers’). Once in the river, however, the dispersers grew significantly faster than stayers and, on recapture, were significantly larger, but with no significant differences in their trophic positions. Early dispersal into the river was therefore not facilitated by dietary shifts to piscivory and the attainment of larger body sizes of individuals whilst in their natal habitats. These results suggest that there are long-term benefits for individuals dispersing early from natal areas via elevated growth rates and, potentially, higher fitness, with the underlying mechanisms potentially relating to competitive displacement