9,252 research outputs found
Environmental impact assessment of small scale resource exploitation: The case of gold panning in Zhulube Catchment, Limpopo Basin, Zimbabwe
Rim curvature anomaly in thin conical sheets revisited
This paper revisits one of the puzzling behaviors in a developable cone
(d-cone), the shape obtained by pushing a thin sheet into a circular container
of radius by a distance [E. Cerda, S. Chaieb, F. Melo, and L.
Mahadevan, {\sl Nature} {\bf 401}, 46 (1999)]. The mean curvature was reported
to vanish at the rim where the d-cone is supported [T. Liang and T. A. Witten,
{\sl Phys. Rev. E} {\bf 73}, 046604 (2006)]. We investigate the ratio of the
two principal curvatures versus sheet thickness over a wider dynamic range
than was used previously, holding and fixed. Instead of tending
towards 1 as suggested by previous work, the ratio scales as .
Thus the mean curvature does not vanish for very thin sheets as previously
claimed. Moreover, we find that the normalized rim profile of radial curvature
in a d-cone is identical to that in a "c-cone" which is made by pushing a
regular cone into a circular container. In both c-cones and d-cones, the ratio
of the principal curvatures at the rim scales as ,
where is the pushing force and is the Young's modulus. Scaling
arguments and analytical solutions confirm the numerical results.Comment: 25 pages, 12 figures. Added references. Corrected typos. Results
unchange
Dynamic buckling and fragmentation in brittle rods
We present experiments on the dynamic buckling and fragmentation of slender
rods axially impacted by a projectile. By combining the results of Saint-Venant
and elastic beam theory, we derive a preferred wavelength lambda for the
buckling instability, and experimentally verify the resulting scaling law for a
range of materials including teflon, dry pasta, glass, and steel. For brittle
materials, buckling leads to the fragmentation of the rod. Measured fragment
length distributions show two clear peaks near lambda/2 and lambda/4. The
non-monotonic nature of the distributions reflect the influence of the
deterministic buckling process on the more random fragmentation processes.Comment: 4 pages, 5 figures, submitted to Physical Review Letter
Morphology of meteoroid and space debris craters on LDEF metal targets
We measured the depths, average diameters, and circularity indices of over 600 micrometeoroid and space debris craters on various metal surfaces exposed to space on the Long Duration Exposure Facility (LDEF) satellite, as a test of some of the formalisms used to convert the diameters of craters on space-exposed surfaces into penetration depths for the purpose of calculating impactor sizes or masses. The topics covered include the following: targe materials orientation; crater measurements and sample populations; effects of oblique impacts; effects of projectile velocity; effects of crater size; effects of target hardness; effects of target density; and effects of projectile properties
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Conscientiousness predicts greater recovery from negative emotion
Greater levels of conscientiousness have been associated with lower levels of negative affect. We focus on one mechanism through which conscientiousness may decrease
negative affect: effective emotion regulation, as reflected by greater recovery from negative stimuli. In 273 adults who were 35 - 85 years old, we collected self-report measures of personality including conscientiousness and its self-control facet, followed
on average 2 years later by psychophysiological measures of emotional reactivity and recovery. Among middle-aged adults (35 - 65 years old), the measures of
conscientiousness and self-control predicted greater recovery from, but not reactivity to, negative emotional stimuli. The effect of conscientiousness and self-control on recovery was not driven by other personality variables or by greater task adherence on the part of high conscientiousness individuals. In addition, the effect was specific to negative emotional stimuli and did not hold for neutral or positive emotional stimuli
A semi-quantitative scattering theory of amorphous materials
It is argued that topological disorder in amorphous solids can be described
by local strains related to local reference crystals and local rotations. An
intuitive localization criterion is formulated from this point of view. The
Inverse Participation Ratio and the location of mobility edges in band tails is
directly related to the character of the disorder potential in amorphous solid,
the coordination number, the transition integral and the nodes of wave
functions of the corresponding reference crystal. The dependence of the decay
rate of band tails on temperature and static disorder are derived. \textit{Ab
initio} simulations on a-Si and experiments on a-Si:H are compared to these
predictions.Comment: 4 pages, 2 figures, will be submitted to Phys. Rev. Let
High-frequency homogenization for periodic media
This article is available open access through the publisher’s website at the link below. Copyright @ 2010 The Royal Society.An asymptotic procedure based upon a two-scale approach is developed for wave propagation in a doubly periodic inhomogeneous medium with a characteristic length scale of microstructure far less than that of the macrostructure. In periodic media, there are frequencies for which standing waves, periodic with the period or double period of the cell, on the microscale emerge. These frequencies do not belong to the low-frequency range of validity covered by the classical homogenization theory, which motivates our use of the term ‘high-frequency homogenization’ when perturbing about these standing waves. The resulting long-wave equations are deduced only explicitly dependent upon the macroscale, with the microscale represented by integral quantities. These equations accurately reproduce the behaviour of the Bloch mode spectrum near the edges of the Brillouin zone, hence yielding an explicit way for homogenizing periodic media in the vicinity of ‘cell resonances’. The similarity of such model equations to high-frequency long wavelength asymptotics, for homogeneous acoustic and elastic waveguides, valid in the vicinities of thickness resonances is emphasized. Several illustrative examples are considered and show the efficacy of the developed techniques.NSERC (Canada) and the EPSRC
Low-loss criterion and effective area considerations for photonic crystal fibers
We study the class of endlessly single-mode all-silica photonic crystal
fibers with a triangular air-hole cladding. We consider the sensibility to
longitudinal nonuniformities and the consequences and limitations for realizing
low-loss large-mode area photonic crystal fibers. We also discuss the
dominating scattering mechanism and experimentally we confirm that both macro
and micro-bending can be the limiting factor.Comment: Accepted for Journal of Optics A - Pure and Applied Optic
Improved large-mode area endlessly single-mode photonic crystal fibers
We numerically study the possibilities for improved large-mode area endlessly
single mode photonic crystal fibers for use in high-power delivery
applications. By carefully choosing the optimal hole diameter we find that a
triangular core formed by three missing neighboring air holes considerably
improves the mode area and loss properties compared to the case with a core
formed by one missing air hole. In a realized fiber we demonstrate an
enhancement of the mode area by ~30 % without a corresponding increase in the
attenuation.Comment: 3 pages including 3 eps-figures. Accepted for Optics Letter
Variation in plasma corticosterone in migratory songbirds: A test of the migration-modulation hypothesis
Physiological mechanisms underlying migration remain poorly understood, but recent attention has focused on the role of the glucocorticoid hormone corticosterone (CORT) as a key endocrine regulator of migration. The migrationmodulation hypothesis (MMH) proposes that baseline plasma CORT levels are elevated in migratory birds to facilitate hyperphagia and lipogenesis and that further elevation of CORT in response to acute stress is suppressed. Consequently, CORT may be a poor indicator of individual condition or environmental variation in migratory birds. We tested the MMH by measuring baseline and stress-induced CORT in common yellowthroats (Geothlypis trichas) during fall migration over 2 consecutive years in the Revelstoke Reach drawdown zone, a migratory stopover site affected by local hydroelectric operations. Birds had low baseline CORT at initial capture (!5 ng/mL) and then showed a robust stress response, with CORT increasing to ca. 50 ng/ mL within 10-20 min. Our data therefore do not support the MMH. Baseline CORT did not vary with body mass, time of capture, Julian day, or year, suggesting that variable flooding regimes did not affect baseline CORT. Individual variation in the rate of increase in CORT was correlated with Julian day, being higher later in the migration period. Our data suggest that plasma CORT can be a useful metric in migration studies
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