6,987 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
Blue Straggler Stars: Early Observations that Failed to Solve the Problem
In this chapter, I describe early ideas on blue stragglers, and various
observations (some published, some not) that promised but failed to resolve the
question of their origin. I review the data and ideas that were circulating
from Allan Sandage's original discovery in 1953 of "anomalous blue stars" in
the globular cluster M3, up until about 1992, when what seems to have been the
only previous meeting devoted to Blue Straggler Stars (BSSs) was held at the
Space Telescope Science Institute.Comment: Chapter 2, in Ecology of Blue Straggler Stars, H.M.J. Boffin, G.
Carraro & G. Beccari (Eds), Astrophysics and Space Science Library, Springe
Stochastic Heterostructures in B/N-Doped Carbon Nanotubes
Carbon nanotubes are one-dimensional and very narrow. These obvious facts
imply that under doping with boron and nitrogen, microscopic doping
inhomogeneity is much more important than for bulk semiconductors. We consider
the possibility of exploiting such fluctuations to create interesting devices.
Using self-consistent tight-binding (SCTB), we study heavily doped highly
compensated nanotubes, revealing the spontaneous formation of structures
resembling chains of random quantum dots, or nano-scale diode-like elements in
series. We also consider truly isolated impurities, revealing simple scaling
properties of bound state sizes and energies.Comment: 4 pages RevTeX, 4 PostScript figure
Detection Rates for Kaluza-Klein Dark Matter
We consider the lightest Kaluza-Klein particle at N=1 mode (LKP) of universal
extra dimension to be the candidate for Dark Matter and predict the detection
rates for such particles for Germenium and NaI detectors. We have also
calculated the nature of annual modulation for the signals in these two types
of detectors for LKP Dark Matter. The rates with different values of speed of
solar system in the Galactic rest frame are also evaluated.Comment: Submitted to Phys. Rev.
Genetic diversity of the rain tree (Albizia saman) in Colombian seasonally dry tropical forest for informing conservation and restoration interventions
Albizia saman is a multipurpose tree species of seasonally dry tropical forests (SDTFs) of Mesoamerica and northern South America typically cultivated in silvopastoral and other agroforestry systems around the world, a trend that is bound to increase in light of multimillion hectare commitments for forest and landscape restoration. The effective conservation and sustainable use of A. saman requires detailed knowledge of its genetic diversity across its native distribution range of which surprisingly little is known to date. We assessed the genetic diversity and structure of A.saman across twelve representative locations of SDTF in Colombia, and how they may have been shaped by past climatic changes and human influence. We found four different genetic groups which may be the result of differentiation due to isolation of populations in preglacial times. The current distribution and mixture of genetic groups across STDF fragments we observed might be the result of range expansion of SDTFs during the last glacial period followed by range contraction during the Holocene and humanâinfluenced movement of germplasm associated with cattle ranching. Despite the fragmented state of the presumed natural A. saman stands we sampled, we did not find any signs of inbreeding, suggesting that gene flow is not jeopardized in humanized landscapes. However, further research is needed to assess potential deleterious effects of fragmentation on progeny. Climate change is not expected to seriously threaten the in situ persistence of A. saman populations and might present opportunities for future range expansion. However, the sourcing of germplasm for tree planting activities needs to be aligned with the genetic affinity of reference populations across the distribution of Colombian SDTFs. We identify priority source populations for in situ conservation based on their high genetic diversity, lack or limited signs of admixture, and/or genetic uniqueness
Large Fluctuations in the Horizon Area and what they can tell us about Entropy and Quantum Gravity
We evoke situations where large fluctuations in the entropy are induced, our
main example being a spacetime containing a potential black hole whose
formation depends on the outcome of a quantum mechanical event. We argue that
the teleological character of the event horizon implies that the consequent
entropy fluctuations must be taken seriously in any interpretation of the
quantal formalism. We then indicate how the entropy can be well defined despite
the teleological character of the horizon, and we argue that this is possible
only in the context of a spacetime or ``histories'' formulation of quantum
gravity, as opposed to a canonical one, concluding that only a spacetime
formulation has the potential to compute --- from first principles and in the
general case --- the entropy of a black hole. From the entropy fluctuations in
a related example, we also derive a condition governing the form taken by the
entropy, when it is expressed as a function of the quantal density-operator.Comment: 35 pages, plain Tex, needs mathmacros.tex and msmacros.te
The Impact of Selected Socioeconomic Factors on Residentsâ Perceptions of Benefits of Community Gardens
Participating in community gardens is believed to have several benefits; yet, there has been limited research on socioeconomic factors and their impact on community gardens. Therefore, this study assessed the impact of selected socioeconomic factors on residentsâ perceptions of benefits of community gardens. Using a questionnaire, data were obtained from a convenience sample of 217 participants from Macon County, Alabama, and analyzed using descriptive statistics and ordinal logit analysis. The results showed that a majority (at least 73%) agreed or strongly agreed with statements regarding nutritional benefits, health benefits, and community benefits of community gardens. In addition, several socioeconomic factors; age, education, and annual household income, had statistically significant effects on perceptions of benefits of participating in community gardens. Consequently, it was recommended that policies and programs that encourage participation in community gardens be put in place for residents in the study area, with cooperation among, county officials, University scientists, and community residents. This will lead to benefits of eating more fruits and vegetables, eating fresher foods, and possibly, in the long-term, reducing chronic diseases. Keywords: Community gardens, Socioeconomic factors, Macon County, Rural communitie
Enhanced sequential carrier capture into individual quantum dots and quantum posts controlled by surface acoustic waves
Individual self-assembled Quantum Dots and Quantum Posts are studied under
the influence of a surface acoustic wave. In optical experiments we observe an
acoustically induced switching of the occupancy of the nanostructures along
with an overall increase of the emission intensity. For Quantum Posts,
switching occurs continuously from predominantely charged excitons (dissimilar
number of electrons and holes) to neutral excitons (same number of electrons
and holes) and is independent on whether the surface acoustic wave amplitude is
increased or decreased. For quantum dots, switching is non-monotonic and shows
a pronounced hysteresis on the amplitude sweep direction. Moreover, emission of
positively charged and neutral excitons is observed at high surface acoustic
wave amplitudes. These findings are explained by carrier trapping and
localization in the thin and disordered two-dimensional wetting layer on top of
which Quantum Dots nucleate. This limitation can be overcome for Quantum Posts
where acoustically induced charge transport is highly efficient in a wide
lateral Matrix-Quantum Well.Comment: 11 pages, 5 figure
A new liver perfusion and preservation system for transplantation Research in large animals
A kidney perfusion machine, model MOX-100 (Waters Instruments, Ltd, Rochester, MN) was modified to allow continuous perfusion of the portal vein and pulsatile perfusion of the hepatic artery of the liver. Additional apparatus consists of a cooling system, a membrane oxygenator, a filter for foreign bodies, and bubble traps. This system not only allows hypothermic perfusion preservation of the liver graft, but furthermore enables investigation of ex vivo simulation of various circulatory circumstances in which physiological perfusion of the liver is studied. We have used this system to evaluate the viability of liver allografts preserved by cold storage. The liver was placed on the perfusion system and perfused with blood with a hematocrit of approximately 20% and maintained at 37°C for 3 h. The flows of the hepatic artery and portal vein were adjusted to 0.33 mL and 0.67 mL/g of liver tissue, respectively. Parameters of viability consisted of hourly bile output, oxygen consumption, liver enzymes, electrolytes, vascular resistance, and liver histology. This method of liver assessment in large animals will allow the objective evaluation of organ viability for transplantation and thereby improve the outcome of organ transplantation. Furthermore, this pump enables investigation into the pathophysiology of liver ischemia and preservation. © 1990 Informa UK Ltd All rights reserved: reproduction in whole or part not permitted
Search for Neutrinoless Double-Beta Decay in Xe with EXO-200
We report on a search for neutrinoless double-beta decay of Xe with
EXO-200. No signal is observed for an exposure of 32.5 kg-yr, with a background
of ~1.5 x 10^{-3} /(kg yr keV) in the region of interest. This
sets a lower limit on the half-life of the neutrinoless double-beta decay
(Xe) > 1.6 x 10 yr (90% CL),
corresponding to effective Majorana masses of less than 140-380 meV, depending
on the matrix element calculation
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