6,557 research outputs found
On the Clark-alpha model of turbulence: global regularity and long--time dynamics
In this paper we study a well-known three--dimensional turbulence model, the
filtered Clark model, or Clark-alpha model. This is Large Eddy Simulation (LES)
tensor-diffusivity model of turbulent flows with an additional spatial filter
of width alpha (). We show the global well-posedness of this model with
constant Navier-Stokes (eddy) viscosity. Moreover, we establish the existence
of a finite dimensional global attractor for this dissipative evolution system,
and we provide an anaytical estimate for its fractal and Hausdorff dimensions.
Our estimate is proportional to , where is the integral spatial
scale and is the viscous dissipation length scale. This explicit bound is
consistent with the physical estimate for the number of degrees of freedom
based on heuristic arguments. Using semi-rigorous physical arguments we show
that the inertial range of the energy spectrum for the Clark- model has
the usual Kolmogorov power law for wave numbers and
decay power law for This is evidence that the
Clark model parameterizes efficiently the large wave numbers within
the inertial range, , so that they contain much less translational
kinetic energy than their counterparts in the Navier-Stokes equations.Comment: 11 pages, no figures, submitted to J of Turbulenc
The Multiple Origin of Blue Straggler Stars: Theory vs. Observations
In this chapter we review the various suggested channels for the formation
and evolution of blue straggler stars (BSSs) in different environments and
their observational predictions. These include mass transfer during binary
stellar evolution - case A/B/C and D (wind Roche-lobe overflow) mass transfer,
stellar collisions during single and binary encounters in dense stellar
cluster, and coupled dynamical and stellar evolution of triple systems. We also
explore the importance of the BSS and binary dynamics in stellar clusters. We
review the various observed properties of BSSs in different environments (halo
and bulge BSSs, BSSs in globular clusters and BSSs in old open clusters), and
compare the current observations with the theoretical predictions for BSS
formation. We try to constrain the likely progenitors and processes that play a
role in the formation of BSSs and their evolution. We find that multiple
channels of BSS formation are likely to take part in producing the observed
BSSs, and we point out the strengths and weaknesses of each the formation
channel in respect to the observational constraints. Finally we point out
directions to further explore the origin of BSS, and highlight eclipsing binary
BSSs as important observational tool.Comment: Chapter 11, in Ecology of Blue Straggler Stars, H.M.J. Boffin, G.
Carraro & G. Beccari (Eds), Astrophysics and Space Science Library, Springe
Enhanced local-type inflationary trispectrum from a non-vacuum initial state
We compute the primordial trispectrum for curvature perturbations produced
during cosmic inflation in models with standard kinetic terms, when the initial
quantum state is not necessarily the vacuum state. The presence of initial
perturbations enhances the trispectrum amplitude for configuration in which one
of the momenta, say , is much smaller than the others, . For those squeezed configurations the trispectrum acquires the
so-called local form, with a scale dependent amplitude that can get values of
order . This amplitude can be larger than the
prediction of the so-called Maldacena consistency relation by a factor ,
and can reach the sensitivity of forthcoming observations, even for
single-field inflationary models.Comment: 11 pages, 1 figure. References added, typos corrected, minor change
Discovery of novel variants in genotyping arrays improves genotype retention and reduces ascertainment bias
<p>Abstract</p> <p>Background</p> <p>High-density genotyping arrays that measure hybridization of genomic DNA fragments to allele-specific oligonucleotide probes are widely used to genotype single nucleotide polymorphisms (SNPs) in genetic studies, including human genome-wide association studies. Hybridization intensities are converted to genotype calls by clustering algorithms that assign each sample to a genotype class at each SNP. Data for SNP probes that do not conform to the expected pattern of clustering are often discarded, contributing to ascertainment bias and resulting in lost information - as much as 50% in a recent genome-wide association study in dogs.</p> <p>Results</p> <p>We identified atypical patterns of hybridization intensities that were highly reproducible and demonstrated that these patterns represent genetic variants that were not accounted for in the design of the array platform. We characterized variable intensity oligonucleotide (VINO) probes that display such patterns and are found in all hybridization-based genotyping platforms, including those developed for human, dog, cattle, and mouse. When recognized and properly interpreted, VINOs recovered a substantial fraction of discarded probes and counteracted SNP ascertainment bias. We developed software (MouseDivGeno) that identifies VINOs and improves the accuracy of genotype calling. MouseDivGeno produced highly concordant genotype calls when compared with other methods but it uniquely identified more than 786000 VINOs in 351 mouse samples. We used whole-genome sequence from 14 mouse strains to confirm the presence of novel variants explaining 28000 VINOs in those strains. We also identified VINOs in human HapMap 3 samples, many of which were specific to an African population. Incorporating VINOs in phylogenetic analyses substantially improved the accuracy of a <it>Mus </it>species tree and local haplotype assignment in laboratory mouse strains.</p> <p>Conclusion</p> <p>The problems of ascertainment bias and missing information due to genotyping errors are widely recognized as limiting factors in genetic studies. We have conducted the first formal analysis of the effect of novel variants on genotyping arrays, and we have shown that these variants account for a large portion of miscalled and uncalled genotypes. Genetic studies will benefit from substantial improvements in the accuracy of their results by incorporating VINOs in their analyses.</p
BMQ
BMQ: Boston Medical Quarterly was published from 1950-1966 by the Boston University School of Medicine and the Massachusetts Memorial Hospitals
Formation and stability of self-assembled coherent islands in highly mismatched heteroepitaxy
We study the energetics of island formation in Stranski-Krastanow growth
within a parameter-free approach. It is shown that an optimum island size
exists for a given coverage and island density if changes in the wetting layer
morphology after the 3D transition are properly taken into account. Our
approach reproduces well the experimental island size dependence on coverage,
and indicates that the critical layer thickness depends on growth conditions.
The present study provides a new explanation for the (frequently found) rather
narrow size distribution of self-assembled coherent islands.Comment: 4 pages, 5 figures, In print, Phys. Rev. Lett. Other related
publications can be found at http://www.fhi-berlin.mpg.de/th/paper.htm
Investigating knowledge management factors affecting Chinese ICT firms performance: An integrated KM framework
This is an Author's Accepted Manuscript of an article published in the Journal of Information Systems Management, 28(1), 19 - 29, 2011, copyright Taylor & Francis, available online at: http://www.tandfonline.com/10.1080/10580530.2011.536107.This article sets out to investigate the critical factors of Knowledge Management (KM) which are considered to have an impact on the performance of Chinese information and communication technology (ICT) firms. This study confirms that the cultural environment of an enterprise is central to its success in the context of China. It shows that a collaborated, trusted, and learning environment within ICT firms will have a positive impact on their KM performance
Models of Individual Blue Stragglers
This chapter describes the current state of models of individual blue
stragglers. Stellar collisions, binary mergers (or coalescence), and partial or
ongoing mass transfer have all been studied in some detail. The products of
stellar collisions retain memory of their parent stars and are not fully mixed.
Very high initial rotation rates must be reduced by an unknown process to allow
the stars to collapse to the main sequence. The more massive collision products
have shorter lifetimes than normal stars of the same mass, while products
between low mass stars are long-lived and look very much like normal stars of
their mass. Mass transfer can result in a merger, or can produce another binary
system with a blue straggler and the remnant of the original primary. The
products of binary mass transfer cover a larger portion of the colour-magnitude
diagram than collision products for two reasons: there are more possible
configurations which produce blue stragglers, and there are differing
contributions to the blended light of the system. The effects of rotation may
be substantial in both collision and merger products, and could result in
significant mixing unless angular momentum is lost shortly after the formation
event. Surface abundances may provide ways to distinguish between the formation
mechanisms, but care must be taking to model the various mixing mechanisms
properly before drawing strong conclusions. Avenues for future work are
outlined.Comment: Chapter 12, in Ecology of Blue Straggler Stars, H.M.J. Boffin, G.
Carraro & G. Beccari (Eds), Astrophysics and Space Science Library, Springe
A Fully Tunable Single-Walled Carbon Nanotube Diode
We demonstrate a fully tunable diode structure utilizing a fully suspended
single-walled carbon nanotube (SWNT). The diode's turn-on voltage under forward
bias can be continuously tuned up to 4.3 V by controlling gate voltages, which
is ~6 times the nanotube bandgap energy. Furthermore, the same device design
can be configured into a backward diode by tuning the band-to-band tunneling
current with gate voltages. A nanotube backward diode is demonstrated for the
first time with nonlinearity exceeding the ideal diode. These results suggest
that a tunable nanotube diode can be a unique building block for developing
next generation programmable nanoelectronic logic and integrated circuits.Comment: 14 pages, 4 figure
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