100 research outputs found
Oscillations and translation of a free cylinder in a confined flow
An oscillatory instability has been observed experimentally on an horizontal
cylinder free to move and rotate between two parallel vertical walls of
distance H; its characteristics differ both from vortex shedding driven
oscillations and from those of tethered cylinders in the same geometry. The
vertical motion of the cylinder, its rotation about its axis and its transverse
motion across the gap have been investigated as a function of its diameter D,
its density s, of the mean vertical velocity U of the fluid and of its
viscosity. For a blockage ratio D/H above 0.5 and a Reynolds number Re larger
then 14, oscillations of the rolling angle of the cylinder about its axis and
of its transverse coordinate in the gap are observed together with periodic
variations of the vertical velocity. Their frequency f is the same for the
sedimentation of the cylinder in a static fluid (U = 0) and for a non-zero mean
flow (U 6= 0). The Strouhal number St associated to the oscillation varies as
1/Re with : St.Re = 3 0.15. The corresponding period 1/f is then
independent of U and corresponds to a characteristic viscous diffusion time
over a distance ~ D, implying a strong influence of the viscosity. These
characteristics differ from those of vortex shedding and tethered cylinders for
which St is instead roughly constant with Re and higher than here
Hydrodynamic interactions of spherical particles in Poiseuille flow between two parallel walls
We study hydrodynamic interactions of spherical particles in incident
Poiseuille flow in a channel with infinite planar walls. The particles are
suspended in a Newtonian fluid, and creeping-flow conditions are assumed.
Numerical results, obtained using our highly accurate Cartesian-representation
algorithm [Physica A xxx, {\bf xx}, 2005], are presented for a single sphere,
two spheres, and arrays of many spheres. We consider the motion of freely
suspended particles as well as the forces and torques acting on particles
adsorbed at a wall. We find that the pair hydrodynamic interactions in this
wall-bounded system have a complex dependence on the lateral interparticle
distance due to the combined effects of the dissipation in the gap between the
particle surfaces and the backflow associated with the presence of the walls.
For immobile particle pairs we have examined the crossover between several
far-field asymptotic regimes corresponding to different relations between the
particle separation and the distances of the particles from the walls. We have
also shown that the cumulative effect of the far-field flow substantially
influences the force distribution in arrays of immobile spheres. Therefore, the
far-field contributions must be included in any reliable algorithm for
evaluating many-particle hydrodynamic interactions in the parallel-wall
geometry.Comment: submitted to Physics of Fluid
Échange international et distorsions internes:Comment gouverner la globalisation ?
La concurrence des pays émergents de la taille de la Chine ou de l’Inde fait resurgir avec force le débat qui oppose partisans et adversaires du libre-échange au sein même des pays développés. Pour les premiers, la croissance du commerce international est forcément bénéficiaire pour tous. Pour les seconds, elle est la cause de tous les maux et en particulier du chômage. Les choses sont pourtant bien différentes et beaucoup moins simples. Les changements dans le commerce international, comme d’ailleurs le progrès technique, créent inévitablement des distorsions qui perturbent le fonctionnement de l’économie. Ces distorsions ne sont pas réductibles à des dysfonctionnements des marchés des biens ou du travail. Elles ne peuvent donc pas être éliminées ab initio en choisissant des institutions optimales. Pour autant, des restrictions au commerce international ne sont pas une solution car elles risquent de créer d’autres distorsions qui viennent s’ajouter aux premières. Des distorsions avant tout intérieures appellent des solutions intérieures. Pour explorer ces solutions, il faut retenir une analyse conçue pour étudier, non les propriétés des positions d’équilibre avant et après l’ouverture à l’échange, mais les caractéristiques d’un processus de transition dont le succès n’est pas assuré. Ce type d’analyse permet de mettre en évidence la nécessité d’introduire une forme d’inertie dans les mécanismes d’ajustement. Si une relative flexibilité des salaires est encore possible quand l’intensité du changement est limitée, une certaine viscosité est requise dans le cas contraire, et un accès facilité aux ressources financières externes est nécessaire. Ce résultat pourrait aider à formuler les choix institutionnels et organisationnels à mettre en œuvre dans les économies qui veulent tirer avantage du commerce international et de la globalisation.Changes in the pattern of international trade inevitably create distortions that perturb the functioning of the economy. These distortions may not be reduced to malfunctioning goods or labour markets, and hence cannot be eliminated by simply choosing the optimal institutions. Domestic distortions call for domestic solutions. To explore these solutions it is useless to analyze the properties of equilibria before and after the opening to trade. Rather, we need to build an analytical framework suited to investigate the characteristics of a transition process whose success is not guaranteed ex ante. it appears that wage rigidity and an easy access to external financial resources are necessary in presence of fast pace of change, while if change is sufficiently slow the standard recipe of wage flexibility may be appropriate. These results may help in the institutional and organizational choices to be implemented in economies willing to profit from international trade and globalization
Inverse lift: a signature of the elasticity of complex fluids?
To understand the mechanics of a complex fluid such as a foam we propose a
model experiment (a bidimensional flow around an obstacle) for which an
external sollicitation is applied, and a local response is measured,
simultaneously. We observe that an asymmetric obstacle (cambered airfoil
profile) experiences a downards lift, opposite to the lift usually known (in a
different context) in aerodynamics. Correlations of velocity, deformations and
pressure fields yield a clear explanation of this inverse lift, involving the
elasticity of the foam. We argue that such an inverse lift is likely common to
complex fluids with elasticity.Comment: 4 pages, 4 figures, revised version, submitted to PR
Two-dimensional flow of foam around an obstacle: force measurements
A Stokes experiment for foams is proposed. It consists in a two-dimensional
flow of a foam, confined between a water subphase and a top plate, around a
fixed circular obstacle. We present systematic measurements of the drag exerted
by the flowing foam on the obstacle, \emph{versus} various separately
controlled parameters: flow rate, bubble volume, bulk viscosity, obstacle size,
shape and boundary conditions. We separate the drag into two contributions, an
elastic one (yield drag) at vanishing flow rate, and a fluid one (viscous
coefficient) increasing with flow rate. We quantify the influence of each
control parameter on the drag. The results exhibit in particular a power-law
dependence of the drag as a function of the bulk viscosity and the flow rate
with two different exponents. Moreover, we show that the drag decreases with
bubble size, and increases proportionally to the obstacle size. We quantify the
effect of shape through a dimensioned drag coefficient, and we show that the
effect of boundary conditions is small.Comment: 26 pages, 13 figures, resubmitted version to Phys. Rev.
Effects of Peroral Omega-3 Fatty Acid Supplementation on Cerebrospinal Fluid Biomarkers in Patients with Alzheimer’s Disease: A Randomized Controlled Trial—The OmegAD Study
Background:
Studies have suggested a connection between a decrease in the levels of polyunsaturated fatty acids (PUFAs) and Alzheimer’s disease (AD). We aimed to assess the effect of supplementation with omega-3 fatty acids (n-3 FAs) on biomarkers analyzed in the cerebrospinal fluid (CSF) of patients diagnosed with AD. /
Objective:
To investigate the effects of daily supplementation with 2.3 g of PUFAs in AD patients on the biomarkers in CSF described below. We also explored the possible correlation between these biomarkers and the performance in the cognitive test Mini-Mental State Examination (MMSE). /
Methods:
Thirty-three patients diagnosed with AD were randomized to either treatment with a daily intake of 2.3 g of n-3 FAs (n = 18) or placebo (n = 15). CSF samples were collected at baseline and after six months of treatment, and the following biomarkers were analyzed: Aβ 38, Aβ 40, Aβ 42, t-tau, p-tau, neurofilament light (NfL), chitinase-3-like protein 1 (YKL-40), acetylcholinesterase (AChE), butyrylcholinesterase (BuChE), soluble IL-1 receptor type II (sIL-1RII), and IL-6. /
Results:
There were no significant differences between the groups concerning the level of the different biomarkers in the CSF at baseline. Within the treatment group, there was a small but significant increase in both YKL-40 (p = 0.04) and NfL (p = 0.03), while the other CSF biomarkers remained stable. /
Conclusion:
Supplementation with n-3 FAs had a statistically significant effect on NfL and YKL-40, resulting in an increase of both biomarkers, indicating a possible increase of inflammatory response and axonal damage. This increase in biomarkers did not correlate with MMSE score. /
Trial registration: clinicaltrial.gov Identifier: NCT00211159
A user-friendly risk-score for predicting in-hospital cardiac arrest among patients admitted with suspected non ST-elevation acute coronary syndrome – the SAFER-score
Aim: To develop a simple risk-score model for predicting in-hospital cardiac arrest (CA) among patients hospitalized with suspected non-ST elevation acute coronary syndrome (NSTE-ACS). Methods: Using the Swedish Web-system for Enhancement and Development of Evidence-based care in Heart disease Evaluated According to Recommended Therapies (SWEDEHEART), we identified patients (n = 242 303) admitted with suspected NSTE-ACS between 2008 and 2014. Logistic regression was used to assess the association between 26 candidate variables and in-hospital CA. A risk-score model was developed and validated using a temporal cohort (n = 126 073) comprising patients from SWEDEHEART between 2005 and 2007 and an external cohort (n = 276 109) comprising patients from the Myocardial Ischaemia National Audit Project (MINAP) between 2008 and 2013. Results: The incidence of in-hospital CA for NSTE-ACS and non-ACS was lower in the SWEDEHEART-derivation cohort than in MINAP (1.3% and 0.5% vs. 2.3% and 2.3%). A seven point, five variable risk score (age ≥60 years (1 point), ST-T abnormalities (2 points), Killip Class >1 (1 point), heart rate <50 or ≥100 bpm (1 point), and systolic blood pressure <100 mmHg (2 points) was developed. Model discrimination was good in the derivation cohort (c-statistic 0.72) and temporal validation cohort (c-statistic 0.74), and calibration was reasonable with a tendency towards overestimation of risk with a higher sum of score points. External validation showed moderate discrimination (c-statistic 0.65) and calibration showed a general underestimation of predicted risk. Conclusions: A simple points score containing five variables readily available on admission predicts in-hospital CA for patients with suspected NSTE-ACS
Dynamics of a small neutrally buoyant sphere in a fluid and targeting in Hamiltonian systems
We show that, even in the most favorable case, the motion of a small
spherical tracer suspended in a fluid of the same density may differ from the
corresponding motion of an ideal passive particle. We demonstrate furthermore
how its dynamics may be applied to target trajectories in Hamiltonian systems.Comment: See home page http://lec.ugr.es/~julya
Мониторинг резистентности грамотрицательных возбудителей, выделенных у пациентов с хирургическими инфекциями кожи и мягких тканей
МЯГКИЕ ТКАНИХИРУРГИЧЕСКИЕ ИНФЕКЦИИГРАМОТРИЦАТЕЛЬНЫЕ БАКТЕРИИГРАМОТРИЦАТЕЛЬНЫЕ БАКТЕРИАЛЬНЫЕ ИНФЕКЦИИРЕЗИСТЕНТНОСТЬ БАКТЕРИЙ К АНТИБИОТИКАМНОЗОКОМИАЛЬНАЯ ИНФЕКЦИЯСИНЕГНОЙНАЯ ПАЛОЧКААЦИНЕТОБАКТЕ
Simple Viscous Flows: from Boundary Layers to the Renormalization Group
The seemingly simple problem of determining the drag on a body moving through
a very viscous fluid has, for over 150 years, been a source of theoretical
confusion, mathematical paradoxes, and experimental artifacts, primarily
arising from the complex boundary layer structure of the flow near the body and
at infinity. We review the extensive experimental and theoretical literature on
this problem, with special emphasis on the logical relationship between
different approaches. The survey begins with the developments of matched
asymptotic expansions, and concludes with a discussion of perturbative
renormalization group techniques, adapted from quantum field theory to
differential equations. The renormalization group calculations lead to a new
prediction for the drag coefficient, one which can both reproduce and surpass
the results of matched asymptotics
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