515 research outputs found
Interface instability in shear banding flow
We report on the spatio-temporal dynamics of the interface in shear-banding
flow of a wormlike micellar system (cetyltrimethylammonium bromide and sodium
nitrate in water) during a start-up experiment. Using the scattering properties
of the induced structures, we demonstrate the existence of an instability of
the interface between bands along the vorticity direction. Different regimes of
spatio-temporal dynamics of the interface are indentified along the stress
plateau. We build a model based on the flow symetry which qualitatively
describes the observed patterns
Bifurcations in nonlinear models of fluid-conveying pipes supported at both ends
Stationary bifurcations in several nonlinear models of fluid conveying pipes
fixed at both ends are analyzed with the use of Lyapunov-Schmidt reduction and
singularity theory. Influence of gravitational force, curvature and vertical
elastic support on various properties of bifurcating solutions are
investigated. In particular the conditions for occurrence of supercritical and
subcritical bifurcations are presented for the models of Holmes, Thurman and
Mote, and Paidoussis.Comment: to appear in Journal of Fluids and Structures; 6 figure
The increasing trend in preterm birth in public hospitals in northern Argentina
Objective: To identify factors associated with the increasing incidence of preterm birth in northern Argentina. Methods: In an observational study, data were reviewed from a prospective, population-based registry of pregnancy outcomes in six cities in 2009-2012. The primary outcome was preterm birth (at 20-37 weeks). Bivariate tests and generalized estimating equations were used within a conceptual hierarchical framework to estimate the cluster-corrected annual trend in odds of preterm birth. Results: The study reviewed data from 11 433 live births. There were 484 (4.2%) preterm births. The incidence of preterm births increased by 38% between 2009 and 2012, from 37.5 to 51.7 per 1000 live births. Unadjusted risk factors for preterm birth included young or advanced maternal age, normal body mass index, nulliparity, no prenatal care, no vitamins or supplements during pregnancy, multiple gestation, and maternal hypertension or prepartum hemorrhage. The prevalence of many risk factors increased over the study period, but variations in these factors explained less than 1% of the increasing trend in preterm birth. Conclusion: The incidence of preterm births insix small cities in northern Argentina increased greatly between 2009 and 2012. This trend was unexplained by the risk factors measured. Other factors should be assessed in future studies.Fil: Weaver, Emily H.. University of North Carolina School at Chapel Hill; Estados UnidosFil: Gibbons, Luz. Instituto de Efectividad ClĂnica y Sanitaria; ArgentinaFil: Belizan, Jose. Instituto de Efectividad ClĂnica y Sanitaria; Argentina. Consejo Nacional de Investigaciones CientĂficas y TĂ©cnicas; ArgentinaFil: Althabe, Fernando. Consejo Nacional de Investigaciones CientĂficas y TĂ©cnicas; Argentina. Instituto de Efectividad ClĂnica y Sanitaria; Argentin
Interface dynamics in shear-banding flow of giant micelles
We report on a non trivial dynamics of the interface between shear bands
following a start-up of flow in a semi-dilute wormlike micellar system
investigated using a combination of mechanical and optical measurements. During
the building of the banding structure, we observed the stages of formation,
migration of the interface between bands and finally the destabilization of
this interface along the vorticity axis. The mechanical signature of these
processes has been indentified in the time series of the shear stress. The
interface instability occurs all along the stress plateau, the asymptotic
wavelength of the patterns increasing with the control parameter typically from
a fraction of the gap width to about four times the gap width. Three main
regimes of dynamics are highlighted : a spatially stable oscillating mode
approximately at the middle of the coexistence region flanked by two ranges
where the dynamics appears more exotic with propagative and chaotic events
respectively at low and high shear rates. The distribution of small particles
seeded in the solution strongly suggests that the flow is three-dimensional.
Finally, we demonstrate that the shear-banding scenario described in this paper
is not specific to our system
Noise induced transition from an absorbing phase to a regime of stochastic spatiotemporal intermittency
We introduce a stochastic partial differential equation capable of
reproducing the main features of spatiotemporal intermittency (STI).
Additionally the model displays a noise induced transition from laminarity to
the STI regime. We show by numerical simulations and a mean-field analysis that
for high noise intensities the system globally evolves to a uniform absorbing
phase, while for noise intensities below a critical value spatiotemporal
intermittence dominates. A quantitative computation of the loci of this
transition in the relevant parameter space is presented.Comment: 4 pages, 6 eps figures. Submitted to Phys. Rev. Lett. See for
additional information http://imedea.uib.es
Optimum control strategies for maximum thrust production in underwater undulatory swimming
Fish, cetaceans and many other aquatic vertebrates undulate their bodies to
propel themselves through water. Numerous studies on natural, artificial or
analogous swimmers are dedicated to revealing the links between the kinematics
of body oscillation and the production of thrust for swimming. One of the most
open and difficult questions concerns the best kinematics to maximize this
later quantity for given constraints and how a system strategizes and adjusts
its internal parameters to reach this maximum. To address this challenge, we
exploit a biomimetic robotic swimmer to determine the control signal that
produces the highest thrust. Using machine learning techniques and intuitive
models, we find that this optimal control consists of a square wave function,
whose frequency is fixed by the interplay between the internal dynamics of the
swimmer and the fluid-structure interaction with the surrounding fluid. We then
propose a simple implementation for autonomous robotic swimmers that requires
no prior knowledge of systems or equations. This application to aquatic
locomotion is validated by 2D numerical simulations
Pattern forming instability induced by light in pure and dye-doped nematic liquid crystals
We study theoretically the instabilities induced by a linearly polarized
ordinary light wave incident at a small oblique angle on a thin layer of
homeotropically oriented nematic liquid crystal with special emphasis on the
dye-doped case. The spatially periodic Hopf bifurcation that occurs as the
secondary instability after the stationary Freedericksz transition is analyzed.Comment: 8 pages, 7 figures, LaTeX, accepted to Phys. Rev.
A Realidade a Cerca da Subida de Preços: A Dinâmica dos Preços de Alimentos em Moçambique e as Implicações PolĂticas.
Resultados das Investigações do SIMA- Departamento de EstatĂstica e Departamento de Análise de PolĂticas MINAG - Direcção de Economia – Dpto. EstatĂsticafood security, food policy Mozambique, Demand and Price Analysis, Q18,
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