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Reduced-order representation of near-wall structures in the late transitional boundary layer
No full textInternational audienceDirect numerical simulations (DNS) of controlled H- and K-type transitions to turbulence in an M=0.2 (where M is the Mach number) nominally zero-pressure-gradient and spatially developing flat-plate boundary layer are considered. Sayadi, Hamman & Moin (J. Fluid Mech., vol. 724, 2013, pp. 480-509) showed that with the start of the transition process, the skin-friction profiles of these controlled transitions diverge abruptly from the laminar value and overshoot the turbulent estimation. The objective of this work is to identify the structures of dynamical importance throughout the transitional region. Dynamic mode decomposition (DMD) (Schmid, J. Fluid Mech., vol. 656, 2010, pp. 5-28) as an optimal phase-averaging process, together with triple decomposition (Reynolds & Hussain, J. Fluid Mech., vol. 54 (02), 1972, pp. 263-288), is employed to assess the contribution of each coherent structure to the total Reynolds shear stress. This analysis shows that low-frequency modes, corresponding to the legs of hairpin vortices, contribute most to the total Reynolds shear stress. The use of composite DMD of the vortical structures together with the skin-friction coefficient allows the assessment of the coupling between near-wall structures captured by the low-frequency modes and their contribution to the total skin-friction coefficient. We are able to show that the low-frequency modes provide an accurate estimate of the skin-friction coefficient through the transition process. This is of interest since large-eddy simulation (LES) of the same configuration fails to provide a good prediction of the rise to this overshoot. The reduced-order representation of the flow is used to compare the LES and the DNS results within this region. Application of this methodology to the LES of the H-type transition illustrates the effect of the grid resolution and the subgrid-scale model on the estimated shear stress of these low-frequency modes. The analysis shows that although the shapes and frequencies of the low-frequency modes are independent of the resolution, the amplitudes are underpredicted in the LES, resulting in underprediction of the Reynolds shear stress
A Practical Source of Chlorodifluoromethyl Radicals. Convergent Routes to gem-Difluoroalkenes and -dienes and (2,2-Difluoroethyl)-indoles, -azaindoles, and -naphthols.
No full textInternational audienceThe preparation of O-octadecyl-S-chlorodifluoromethyl xanthate from chlorodifluoroacetic acid and its use as a convenient source of chlordifluoromethyl radicals is described. This reagent may be used to access gem-difluoroalkenes and -dienes, as well as (2,2-difluoroethyl)indolines, -indoles, and -naphthols
Regulation of the ROS Response Dynamics and Organization to PDGF Motile Stimuli Revealed by Single Nanoparticle Imaging.
No full textInternational audienceAlthough reactive oxygen species (ROS) are better known for their harmful effects, more recently, H2O2, one of the ROS, was also found to act as a secondary messenger. However, details of spatiotemporal organization of specific signaling pathways that H2O2 is involved in are currently missing. Here, we use single nanoparticle imaging to measure the local H2O2 concentration and reveal regulation of the ROS response dynamics and organization to platelet-derived growth factor (PDGF) signaling. We demonstrate that H2O2 production is controlled by PDGFR kinase activity and EGFR transactivation, requires a persistent stimulation, and is regulated by membrane receptor diffusion. This temporal filtering is impaired in cancer cells, which may determine their pathological migration. H2O2 subcellular mapping reveals that an external PDGF gradient induces an amplification-free asymmetric H2O2 concentration profile. These results support a general model for the control of signal transduction based only on membrane receptor diffusion and second messenger degradation
A Concurrent Pattern Calculus
No full textInternational audienceConcurrent pattern calculus (CPC) drives interaction between processes by comparing data structures, just as sequential pattern calculus drives computation. By generalising from pattern matching to pattern unification, interaction becomes symmetrical, with information flowing in both directions. CPC provides a natural language to express trade where information exchange is pivotal to interaction. The unification allows some patterns to be more discriminating than others; hence, the behavioural theory must take this aspect into account, so that bisimulation becomes subject to compatibility of patterns. Many popular process calculi can be encoded in CPC; this allows for a gain in expressiveness, formalised through encodings
Study of mechanotransduction in blood vessel endothelial cells using optical tweezers and fluorescence microscopy (Poster)
No full textposter présenté par Karen Perrone
Splitting of a turbulent puff in pipe flow
No full textInternational audienceThe transition to turbulence of the flow in a pipe of constant radius is numerically studied over a range of Reynolds numbers where turbulence begins to expand by puff splitting. We first focus on the case Re = 2300 where splitting occurs as discrete events. Around this value only long-lived pseudo-equilibrium puffs can be observed in practice, as typical splitting times become very long. When Re is further increased, the flow enters a more continuous puff splitting regime where turbulence spreads faster. Puff splitting presents itself as a two-step stochastic process. A splitting puff first emits a chaotic pseudopod made of azimuthally localized streaky structures at the downstream (leading) laminar-turbulent interface. This structure can later expand azimuthally as it detaches from the parent puff. Detachment results from a collapse of turbulence over the whole cross-section of the pipe. Once the process is achieved a new puff is born ahead. Large-deviation consequences of elementary stochastic processes at the scale of the streak are invoked to explain the statistical nature of splitting and the Poisson-like distributions of splitting times reported by Avila, Moxey, de Lozar, Avila, Barkley and Hof (2011 Science 333 192–196)
Mixture of Gaussian regressions model with logistic weights, a penalized maximum likelihood approach
No full textInternational audienceIn the framework of conditional density estimation, we use candidates taking the form of mixtures of Gaussian regressions with logistic weights and means depending on the covariate. We aim at estimating the number of components of this mixture, as well as the other parameters, by a penalized maximum likelihood approach. We provide a lower bound on the penalty that ensures an oracle inequality for our estimator. We perform some numerical experiments that support our theoretical analysis
S’engager dans l’Open Innovation - Fondations, démarches et grandes pratiques
No full textDans ce rapport qui s’appuie sur une revue de travaux récents portant sur l’Open Innovation (articles scientifiques, livres publiés et articles de conférences), nous cherchons à expliquer les grands principes de l’Open Innovation tels qu’ils sont analysés et décrits par le monde de la recherche académique. Aussi, nous commençons par présenter les trois grands piliers de cette littérature et ce qu’ils apportent de nouveau : le livre fondateur d’Henri Chesbrough et sa représentation de ce qu’est l’Open Innovation, la notion de capacité d’absorption d’une entreprise et la notion de lead-user. Puis, nous présentons les résultats de l’étude des entreprises qui se sont engagées dans des démarches d’ouverture de leur innovation et la majeure difficulté rencontrée par celles-ci : la barrière culturelle à franchir. Enfin, nous terminons par balayer les grandes familles de pratiques rencontrées dans l’Open Innovation en action : la recherche de savoirs externes, leur intégration en interne puis leur commercialisation.Dans ce rapport qui s’appuie sur une revue de travaux récents portant sur l’Open Innovation (articles scientifiques, livres publiés et articles de conférences), nous cherchons à expliquer les grands principes de l’Open Innovation tels qu’ils sont analysés et décrits par le monde de la recherche académique. Aussi, nous commençons par présenter les trois grands piliers de cette littérature et ce qu’ils apportent de nouveau : le livre fondateur d’Henri Chesbrough et sa représentation de ce qu’est l’Open Innovation, la notion de capacité d’absorption d’une entreprise et la notion de lead-user. Puis, nous présentons les résultats de l’étude des entreprises qui se sont engagées dans des démarches d’ouverture de leur innovation et la majeure difficulté rencontrée par celles-ci : la barrière culturelle à franchir. Enfin, nous terminons par balayer les grandes familles de pratiques rencontrées dans l’Open Innovation en action : la recherche de savoirs externes, leur intégration en interne puis leur commercialisation
Bundle-based pruning in the max-plus curse of dimensionality free method
No full textSee also arXiv:1402.1436International audienceRecently a new class of techniques termed themax-plus curse of dimensionality-free methods have been de-veloped to solve nonlinear optimal control problems. In thesemethods the discretization in state space is avoided by using amax-plus basis expansion of the value function. This requiresstoring only the coefficients of the basis functions used forrepresentation. However, the number of basis functions growsexponentially with respect to the number of time steps ofpropagation to the time horizon of the control problem. Thisso called “curse of complexity” can be managed by applying apruning procedure which selects the subset of basis functionsthat contribute most to the approximation of the value function.The pruning procedures described thus far in the literature relyon the solution of a sequence of high dimensional optimizationproblems which can become computationally expensive.In this paper we show that if the max-plus basis functionsare linear and the region of interest in state space is convex, thepruning problem can be efficiently solved by the bundle method.This approach combining the bundle method and semidefiniteformulations is applied to the quantum gate synthesis problem,in which the state space is the special unitary group (whichis non-convex). This is based on the observation that theconvexification of the unitary group leads to an exact relaxation.The results are studied and validated via examples