2,729 research outputs found
Fractal tracer distributions in turbulent field theories
We study the motion of passive tracers in a two-dimensional turbulent
velocity field generated by the Kuramoto-Sivashinsky equation. By varying the
direction of the velocity-vector with respect to the field-gradient we can
continuously vary the two Lyapunov exponents for the particle motion and
thereby find a regime in which the particle distribution is a strange
attractor. We compare the Lyapunov dimension to the information dimension of
actual particle distributions and show that there is good agreement with the
Kaplan-Yorke conjecture. Similar phenomena have been observed experimentally.Comment: 17 pages, 7 figures, elsart.sty, psfig.sty, LaTe
Is Rising Returns to Scale a Figment of Poor Data?
While using detailed firm-level data from the private business sector, this study identifies two empirical puzzles: (i) returns-to-scale (RTS) parameter estimates rise at higher levels of data aggregation, and (ii) estimates from the firm level suggest decreasing returns to scale. The analysis shows that, although consistent with rising estimates, the Basu-Fernald (1997) aggregation-bias effect does not drive this result. Rather, rising and too low returns-to-scale estimates probably reflect a mixture of random errors in factor inputs. It turns out, in fact, that a 7.5-10 percent error in labor (hours worked) can explain both puzzles.Business cycles; Data aggregation; External economies; Factor hoarding; Firm-level data; Monte Carlo simulation; Random errors; Returns to scale
Osmotically driven pipe flows and their relation to sugar transport in plants
In plants, osmotically driven flows are believed to be responsible for
translocation of sugar in the pipe-like phloem cell network, spanning the
entire length of the plant. In this paper, we present an experimental and
theoretical study of transient osmotically driven flows through pipes with
semipermeable walls. We extend the experimental work of Eschrich, Evert and
Young \cite[]{Eschrich:1972} by providing a more accurate version of their
experiment allowing for better comparison with theory. In the experiments we
measure the dynamics and structure of a "sugar front", i.e. the transport and
decay of a sudden loading of sugar in a pipe which is closed in both ends. We
include measurements of pressure inside the membrane tube allowing us to
compare the experiments directly with theory and, in particular, to confirm
quantitatively the exponential decay of the front in a closed tube.In a novel
setup we are able to measure the entire concentration profile as the sugar
front moves. In contrast to previous studies we find very good agreement
between experiment and theory.
In the limit of low axial resistance (valid in our experiments as well as in
many cases in plants) we show that the equations can be solved exactly by the
method of characteristics yielding, in general, an implicit solution. Further
we show that under more general conditions the equations of motion can be
rewritten as a single integro-differential equation, which can be readily
solved numerically. The applicability of our results to plants is discussed and
it is shown that it is probable that the pressure-flow hypothesis can account
for short distance transport of sugar in plants.Comment: 34 pages, Submitted to Journal of Fluid Mechanics on May 28, 200
Gut bacteria and necrotizing enterocolitis: cause or effect?
Development of necrotising enterocolitis (NEC) is considered to be dependent on the bacterial colonisation of the gut. With little concordance between published data and a recent study failing to detect a common strain in infants with NEC, more questions than answers are arising about our understanding of this complex disease
Spatio-temporal patterns in arctic fox (Vulpes alopex) diets revealed by molecular analysis of scats from Northeast Greenland
The arctic fox (Vulpes lagopus) is endemic to the Arctic where it holds a central position in the trophic interactions. The diet of the species has previously been described as being highly flexible, but whether this flexibility is a constant trait through time, or merely reflects fast temporal changes in abundance among prey taxa, has so far been poorly resolved. Using molecular analyses of arctic fox scats from Northeast Greenland, we first examined the temporal dynamics of arctic fox diets during the short snow-free season, and then examined whether local food availability at different sites affected arctic fox dependence on lemmings. Arctic fox diets included most terrestrial vertebrate species found in the region, and exhibited substantial temporal changes, generally reflecting the dynamic changes in prey availability from late winter through autumn. This dietary flexibility was also reflected geographically, with arctic foxes consuming a variety of local prey (mainly waterfowl and lemmings) in summer. Moreover, the dietary response of arctic foxes to changes in lemming abundance depended on access to non-lemming prey. Based on these findings, we discuss whether varying degrees of lemming-dependency, combined with geographical differences in winter food availability, may explain previously published differences in arctic fox breeding patterns in high arctic Greenland.Peer reviewe
An evaluation of the ability of Dichelobacter nodosus to survive in soil
<p>Abstract</p> <p>Background</p> <p><it>Dichelobacter nodosus</it> is the causative agent of footrot in sheep. The survival of the bacterium in soil is of importance for the epidemiology of the disease. The investigation evaluates the survival of <it>D. nodosus</it> in soil with and without added hoof powder stored under different temperatures.</p> <p>Results</p> <p>An experimental setup was used with bacteriological culture and real-time polymerase chain reaction (PCR), and the results indicate that the bacteria can survive in soil for longer time than previously expected. The survival time was found to be dependent on temperature and the addition of hoof powder to the soil, with the longest survival time estimated to be 24âdays in soil samples with hoof powder stored at 5°C.</p> <p>Conclusion</p> <p>Our findings indicate that the survival time of <it>D. nodosus</it> and its ability to infect susceptible sheep on pasture under different climatic conditions should be studied further.</p
Spatio-temporal patterns in arctic fox (Vulpes alopex) diets revealed by molecular analysis of scats from Northeast Greenland
The arctic fox (Vulpes lagopus) is endemic to the Arctic where it holds a central position in the trophic interactions. The diet of the species has previously been described as being highly flexible, but whether this flexibility is a constant trait through time, or merely reflects fast temporal changes in abundance among prey taxa, has so far been poorly resolved. Using molecular analyses of arctic fox scats from Northeast Greenland, we first examined the temporal dynamics of arctic fox diets during the short snow-free season, and then examined whether local food availability at different sites affected arctic fox dependence on lemmings. Arctic fox diets included most terrestrial vertebrate species found in the region, and exhibited substantial temporal changes, generally reflecting the dynamic changes in prey availability from late winter through autumn. This dietary flexibility was also reflected geographically, with arctic foxes consuming a variety of local prey (mainly waterfowl and lemmings) in summer. Moreover, the dietary response of arctic foxes to changes in lemming abundance depended on access to non-lemming prey. Based on these findings, we discuss whether varying degrees of lemming-dependency, combined with geographical differences in winter food availability, may explain previously published differences in arctic fox breeding patterns in high arctic Greenland.Peer reviewe
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