725 research outputs found
Essential considerations in the investigation of associations between insulin and cancer risk using prescription databases
Boyle, P Ford, I Robertson, Jfr La Vecchia, C Boffetta, P Autier, P eng England 2009/01/01 00:00 Ecancermedicalscience. 2009;3:174. doi: 10.3332/ecancer.2009.174. Epub 2009 Dec 11.International audienceno abstrac
Relative dispersion in fully developed turbulence: The Richardson's Law and Intermittency Corrections
Relative dispersion in fully developed turbulence is investigated by means of
direct numerical simulations. Lagrangian statistics is found to be compatible
with Richardson description although small systematic deviations are found. The
value of the Richardson constant is estimated as , in a close
agreement with recent experimental findings [S. Ott and J. Mann J. Fluid Mech.
{\bf 422}, 207 (2000)]. By means of exit-time statistics it is shown that the
deviations from Richardson's law are a consequence of Eulerian intermittency.
The measured Lagrangian scaling exponents require a set of Eulerian structure
function exponents which are remarkably close to standard ones
known for fully developed turbulence
Chaos or Noise - Difficulties of a Distinction
In experiments, the dynamical behavior of systems is reflected in time
series. Due to the finiteness of the observational data set it is not possible
to reconstruct the invariant measure up to arbitrary fine resolution and
arbitrary high embedding dimension. These restrictions limit our ability to
distinguish between signals generated by different systems, such as regular,
chaotic or stochastic ones, when analyzed from a time series point of view. We
propose to classify the signal behavior, without referring to any specific
model, as stochastic or deterministic on a certain scale of the resolution
, according to the dependence of the -entropy,
, and of the finite size Lyapunov exponent,
, on .Comment: 24 pages RevTeX, 9 eps figures included, two references added, minor
corrections, one section has been split in two (submitted to PRE
Conformal invariance in two-dimensional turbulence
Simplicity of fundamental physical laws manifests itself in fundamental
symmetries. While systems with an infinity of strongly interacting degrees of
freedom (in particle physics and critical phenomena) are hard to describe, they
often demonstrate symmetries, in particular scale invariance. In two dimensions
(2d) locality often promotes scale invariance to a wider class of conformal
transformations which allow for nonuniform re-scaling. Conformal invariance
allows a thorough classification of universality classes of critical phenomena
in 2d. Is there conformal invariance in 2d turbulence, a paradigmatic example
of strongly-interacting non-equilibrium system? Here, using numerical
experiment, we show that some features of 2d inverse turbulent cascade display
conformal invariance. We observe that the statistics of vorticity clusters is
remarkably close to that of critical percolation, one of the simplest
universality classes of critical phenomena. These results represent a new step
in the unification of 2d physics within the framework of conformal symmetry.Comment: 10 pages, 5 figures, 1 tabl
Universality and saturation of intermittency in passive scalar turbulence
The statistical properties of a scalar field advected by the non-intermittent
Navier-Stokes flow arising from a two-dimensional inverse energy cascade are
investigated. The universality properties of the scalar field are directly
probed by comparing the results obtained with two different types of injection
mechanisms. Scaling properties are shown to be universal, even though
anisotropies injected at large scales persist down to the smallest scales and
local isotropy is not fully restored. Scalar statistics is strongly
intermittent and scaling exponents saturate to a constant for sufficiently high
orders. This is observed also for the advection by a velocity field rapidly
changing in time, pointing to the genericity of the phenomenon. The persistence
of anisotropies and the saturation are both statistical signatures of the
ramp-and-cliff structures observed in the scalar field.Comment: 4 pages, 8 figure
Self-Similar Bootstrap of Divergent Series
A method is developed for calculating effective sums of divergent series.
This approach is a variant of the self-similar approximation theory. The
novelty here is in using an algebraic transformation with a power providing the
maximal stability of the self-similar renormalization procedure. The latter is
to be repeated as many times as it is necessary in order to convert into closed
self-similar expressions all sums from the series considered. This multiple and
complete renormalization is called self-similar bootstrap. The method is
illustrated by several examples from statistical physics.Comment: 1 file, 22 pages, RevTe
Transport properties of heavy particles in high Reynolds number turbulence
The statistical properties of heavy particle trajectories in high Reynolds
numbers turbulent flows are analyzed. Dimensional analysis assuming Kolmogorov
scaling is compared with the result of numerical simulation using a synthetic
turbulence advecting field. The non-Markovian nature of the fluid velocity
statistics along the solid particle trajectories is put into evidence, and its
relevance in the derivation of Lagrangian transport models is discussed.Comment: 30 pages, 11 eps figures included. To appear in Physics of Fluid
Predictability in the large: an extension of the concept of Lyapunov exponent
We investigate the predictability problem in dynamical systems with many
degrees of freedom and a wide spectrum of temporal scales. In particular, we
study the case of turbulence at high Reynolds numbers by introducing a
finite-size Lyapunov exponent which measures the growth rate of finite-size
perturbations. For sufficiently small perturbations this quantity coincides
with the usual Lyapunov exponent. When the perturbation is still small compared
to large-scale fluctuations, but large compared to fluctuations at the smallest
dynamically active scales, the finite-size Lyapunov exponent is inversely
proportional to the square of the perturbation size. Our results are supported
by numerical experiments on shell models. We find that intermittency
corrections do not change the scaling law of predictability. We also discuss
the relation between finite-size Lyapunov exponent and information entropy.Comment: 4 pages, 2 Postscript figures (included), RevTeX 3.0, files packed
with uufile
Second-Hand Tobacco Smoke in Never Smokers Is a Significant Risk Factor for Coronary Artery Calcification
ObjectivesThe aim of this study was to assess the relationship of the extent of subclinical atherosclerosis measured by coronary artery calcification (CAC) to the extent of second-hand tobacco smoke (SHTS) exposure in asymptomatic people who never smoked.BackgroundAn association between SHTS and CAC was recently reported in a single study, but the quantitative aspects of the relationship are not known.MethodsA cohort of 3,098 never smokers 40 to 80 years of age, enrolled in the FAMRI-IELCAP (Flight Attendant Medical Research Institute International Early Lung Cancer Action Program) screening program, completed a SHTS questionnaire, and had a low-dose nongated computed tomography scan. The questionnaire provided a quantitative score for total SHTS exposure, as well as separately as a child and as an adult at home and at work; 4 categories of exposure to SHTS were identified (minimal, low, moderate, and high exposure). CAC was graded using a previously validated ordinal scale score that ranged from 0 to 12. Logistic regression analysis of the prevalence and ordered logistic regression analysis of the extent of CAC were performed to assess the independent contribution of SHTS adjusted for age, sex, diabetes, hypercholesterolemia, hypertension, and renal disease. Linear and quadratic regression analyses of CAC and SHTS were performed.ResultsThe prevalence of CAC was 24.3% (n = 754) and was significantly higher in those with more than minimal SHTS exposure compared with those with minimal SHTS exposure (26.4% vs. 18.5%, p < 0.0001). The adjusted odds ratios for CAC prevalence were 1.54 (95% confidence interval: 1.17 to 2.20) for low SHTS exposure, 1.60 (95% confidence interval: 1.21 to 2.10) for moderate exposure, and 1.93 (95% confidence interval: 1.49 to 2.51) for high exposure. The association of the extent of SHTS with the extent of CAC was confirmed by the adjusted odds ratio (p < 0.0001).ConclusionsThe presence and extent of CAC were associated with extent of SHTS exposure even when adjusted for other risk factors for CAC, suggesting that SHTS exposure causes CAC
Mean- Field Approximation and a Small Parameter in Turbulence Theory
Numerical and physical experiments on two-dimensional (2d) turbulence show
that the differences of transverse components of velocity field are well
described by a gaussian statistics and Kolmogorov scaling exponents. In this
case the dissipation fluctuations are irrelevant in the limit of small
viscosity. In general, one can assume existence of critical
space-dimensionality , at which the energy flux and all odd-order
moments of velocity difference change sign and the dissipation fluctuations
become dynamically unimportant. At the flow can be described by the
``mean-field theory'', leading to the observed gaussian statistics and
Kolmogorov scaling of transverse velocity differences. It is shown that in the
vicinity of the ratio of the relaxation and translation
characteristic times decreases to zero, thus giving rise to a small parameter
of the theory. The expressions for pressure and dissipation contributions to
the exact equation for the generating function of transverse velocity
differences are derived in the vicinity of . The resulting equation
describes experimental data on two-dimensional turbulence and demonstrate onset
of intermittency as and in three-dimensional flows in
close agreement with experimental data. In addition, some new exact relations
between correlation functions of velocity differences are derived. It is also
predicted that the single-point pdf of transverse velocity difference in
developing as well as in the large-scale stabilized two-dimensional turbulence
is a gaussian.Comment: 25 pages, 1 figur
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