2,534 research outputs found
Statistical Mechanics of Shell Models for 2D-Turbulence
We study shell models that conserve the analogues of energy and enstrophy,
hence designed to mimic fluid turbulence in 2D. The main result is that the
observed state is well described as a formal statistical equilibrium, closely
analogous to the approach to two-dimensional ideal hydrodynamics of Onsager,
Hopf and Lee. In the presence of forcing and dissipation we observe a forward
flux of enstrophy and a backward flux of energy. These fluxes can be understood
as mean diffusive drifts from a source to two sinks in a system which is close
to local equilibrium with Lagrange multipliers (``shell temperatures'')
changing slowly with scale. The dimensional predictions on the power spectra
from a supposed forward cascade of enstrophy, and from one branch of the formal
statistical equilibrium, coincide in these shell models at difference to the
corresponding predictions for the Navier-Stokes and Euler equations in 2D. This
coincidence have previously led to the mistaken conclusion that shell models
exhibit a forward cascade of enstrophy.Comment: 25 pages + 9 figures, TeX dialect: RevTeX 3.
Disease proportions attributable to environment
Population disease proportions attributable to various causal agents are popular as they present a simplified view of the contribution of each agent to the disease load. However they are only summary figures that may be easily misinterpreted or over-interpreted even when the causal link between an exposure and an effect is well established. This commentary discusses several issues surrounding the estimation of attributable proportions, particularly with reference to environmental causes of cancers, and critically examines two recently published papers. These issues encompass potential biases as well as the very definition of environment and of environmental agent. The latter aspect is not just a semantic question but carries implications for the focus of preventive actions, whether centred on the material and social environment or on single individuals
Long-time behavior of MHD shell models
The long time behavior of velocity-magnetic field alignment is numerically
investigated in the framework of MHD shell model. In the stationary forced
case, the correlation parameter C displays a nontrivial behavior with long
periods of high variability which alternates with periods of almost constant C.
The temporal statistics of correlation is shown to be non Poissonian, and the
pdf of constant sign periods displays clear power law tails. The possible
relevance of the model for geomagnetic dynamo problem is discussed.Comment: 6 pages with 5 figures. In press on Europhysics Letter
Causation in the Presence of Weak Associations
none1siDespite their observational nature, epidemiologic studies have been used to make inductive inferences about the causes of
human diseases. In this context, I mainly consider the term “cause” in its cognitive (explanatory) meaning, that is, by detecting
causal factors and identifying mechanisms of diseases...openBoffetta, P.Boffetta, P
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A Case-Control Study of Multiple Myeloma Nested in the American Cancer Society Prospective Study
Among the subjects enrolled in the American Cancer Society Cancer Prevention Study II, a large nation-wide prospective study, 282 died from multiple myeloma (MM) during the first 4 years of follow-up. These were divided into incident cases who were initially free from disease and prevalent cases who reported MM or related symptoms at the time of enrollment. For each case, 4 controls matched for age, sex, ethnic group and residence were randomly selected. Previous history of diabetes [odds ratio (OR) = 2.0] and employment as farmer (OR = 2.7) were the risk factors consistently suggested by the results of the analysis. The risk associated with farming showed a linear trend with duration of exposure. Pesticide or herbicide exposure was not a risk factor per se but, when combined with farming, it increased the OR to 4.3. Low education, occupational exposure to dyes, and employment in a bank, on the railroad or as a maid were also suggested risk factors, but either they were not statistically significant or they were based on small numbers of exposed subjects. No association between MM and asbestos exposure was observed
Closure of two dimensional turbulence: the role of pressure gradients
Inverse energy cascade regime of two dimensional turbulence is investigated
by means of high resolution numerical simulations. Numerical computations of
conditional averages of transverse pressure gradient increments are found to be
compatible with a recently proposed self-consistent Gaussian model. An
analogous low order closure model for the longitudinal pressure gradient is
proposed and its validity is numerically examined. In this case numerical
evidence for the presence of higher order terms in the closure is found. The
fundamental role of conditional statistics between longitudinal and transverse
components is highlighted.Comment: 4 pages, 2 figures, in press on PR
Quantitative Assessment of Asbestos Fibers in Normal and Pathological Pleural Tissue—A Scoping Review
Background: pleural mesothelioma is a rare cancer in the general population, but it is more common in subjects occupationally exposed to asbestos. Studies with asbestos fiber quantification in pleural tissue are scarce: for this reason, we aimed at undertaking a scoping review to summarize the evidence provided by studies in which asbestos fibers were determined by electron microscopy (SEM or TEM) in human pleural tissues, whether normal or pathologic. Materials and methods: A scoping review of articles that quantified asbestos fibers in human pleural tissue (normal or pathologic) by electron microscopy (SEM or TEM), in subjects with asbestos exposure (if any) was performed. Results: The 12 studies selected comprised 137 cases, out of which 142 samples were analyzed. Asbestos fibers were detected in 111 samples (78%) and were below the detectable limit in 31 samples (22%). The concentration of asbestos fibers detected in the positive samples was distributed from as low as 0.01 mfgdt (millions of fibers per gram of dry tissue) up to 240 mfgdt. However, the minimum concentration of fibers overlaps in the three types of tissues (normal pleura, pleural plaque, mesothelioma) in terms of magnitude; therefore, it is not possible to distinguish a definite pattern which differentiates one tissue from the other. Conclusions: The studies included were heterogeneous as to the representativeness of the samples and analytical techniques; the possibility of false negatives must be considered. It would be desirable to systematically search for asbestos fibers to fill the knowledge gap about the presence of asbestos fibers in normal or pathological pleural tissue in order to better understand the development of the different pleural diseases induced by this mineral
Acceleration and vortex filaments in turbulence
We report recent results from a high resolution numerical study of fluid
particles transported by a fully developed turbulent flow. Single particle
trajectories were followed for a time range spanning more than three decades,
from less than a tenth of the Kolmogorov time-scale up to one large-eddy
turnover time. We present some results concerning acceleration statistics and
the statistics of trapping by vortex filaments.Comment: 10 pages, 5 figure
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