13,059 research outputs found
Kelvin Wave Cascade and Decay of Superfluid Turbulence
Kelvin waves (kelvons)--the distortion waves on vortex lines--play a key part
in the relaxation of superfluid turbulence at low temperatures. We present a
weak-turbulence theory of kelvons. We show that non-trivial kinetics arises
only beyond the local-induction approximation and is governed by three-kelvon
collisions; corresponding kinetic equation is derived. On the basis of the
kinetic equation, we prove the existence of Kolmogorov cascade and find its
spectrum. The qualitative analysis is corroborated by numeric study of the
kinetic equation. The application of the results to the theory of superfluid
turbulence is discussed.Comment: 4 pages, RevTe
Slack Dynamics on an Unfurling String
An arch will grow on a rapidly deployed thin string in contact with a rigid
plane. We present a qualitative model for the growing structure involving the
amplification, rectification, and advection of slack in the presence of a
steady stress field, validate our assumptions with numerical experiments, and
pose new questions about the spatially developing motions of thin objects.Comment: significant changes. removed one figur
Short wavelength spectrum and Hamiltonian stability of vortex rings
We compare dynamical and energetical stability criteria for vortex rings. It
is argued that vortex rings will be intrinsically unstable against
perturbations with short wavelengths below a critical wavelength, because the
canonical vortex Hamiltonian is unbounded from below for these modes. To
explicitly demonstrate this behaviour, we derive the oscillation spectrum of
vortex rings in incompressible, inviscid fluids, within a geometrical cutoff
procedure for the core. The spectrum develops an anomalous branch of negative
group velocity, and approaches the zero of energy for wavelengths which are
about six times the core diameter. We show the consequences of this dispersion
relation for the thermodynamics of vortex rings in superfluid He at low
temperatures.Comment: 7 pages, 4 figures, final version to appear in Phys. Rev.
Effects of crack tip geometry on dislocation emission and cleavage: A possible path to enhanced ductility
We present a systematic study of the effect of crack blunting on subsequent
crack propagation and dislocation emission. We show that the stress intensity
factor required to propagate the crack is increased as the crack is blunted by
up to thirteen atomic layers, but only by a relatively modest amount for a
crack with a sharp 60 corner. The effect of the blunting is far less
than would be expected from a smoothly blunted crack; the sharp corners
preserve the stress concentration, reducing the effect of the blunting.
However, for some material parameters blunting changes the preferred
deformation mode from brittle cleavage to dislocation emission. In such
materials, the absorption of preexisting dislocations by the crack tip can
cause the crack tip to be locally arrested, causing a significant increase in
the microscopic toughness of the crack tip. Continuum plasticity models have
shown that even a moderate increase in the microscopic toughness can lead to an
increase in the macroscopic fracture toughness of the material by several
orders of magnitude. We thus propose an atomic-scale mechanism at the crack
tip, that ultimately may lead to a high fracture toughness in some materials
where a sharp crack would seem to be able to propagate in a brittle manner.
Results for blunt cracks loaded in mode II are also presented.Comment: 12 pages, REVTeX using epsfig.sty. 13 PostScript figures. Final
version to appear in Phys. Rev. B. Main changes: Discussion slightly
shortened, one figure remove
Anomalous translational velocity of vortex ring with finite-amplitude Kelvin waves
We consider finite-amplitude Kelvin waves on an inviscid vortex assuming that
the vortex core has infinitesimal thickness. By numerically solving the
governing Biot-Savart equation of motion, we study how the frequency of the
Kelvin waves and the velocity of the perturbed ring depend on the Kelvin wave
amplitude. In particular, we show that, if the amplitude of the Kelvin waves is
sufficiently large, the perturbed vortex ring moves backwards.Comment: 6 pages, 5 figures, v2: minor changes, v3: typos correcte
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Vascular changes in diabetic retinopathy-a longitudinal study in the Nile rat.
Diabetic retinopathy is the most common microvascular complication of diabetes and is a major cause of blindness, but an understanding of the pathogenesis of the disease has been hampered by a lack of accurate animal models. Here, we explore the dynamics of retinal cellular changes in the Nile rat (Arvicanthis niloticus), a carbohydrate-sensitive model for type 2 diabetes. The early retinal changes in diabetic Nile rats included increased acellular capillaries and loss of pericytes that correlated linearly with the duration of diabetes. These vascular changes occurred in the presence of microglial infiltration but in the absence of retinal ganglion cell loss. After a prolonged duration of diabetes, the Nile rat also exhibits a spectrum of retinal lesions commonly seen in the human condition including vascular leakage, capillary non-perfusion, and neovascularization. Our longitudinal study documents a range and progression of retinal lesions in the diabetic Nile rat remarkably similar to those observed in human diabetic retinopathy, and suggests that this model will be valuable in identifying new therapeutic strategies
Callers’ attitudes and experiences of UK breastfeeding helpline support
Background: Breastfeeding peer support, is considered to be a key intervention for increasing breastfeeding duration rates. Whilst a number of national organisations provide telephone based breastfeeding peer support, to date there have been no published evaluations into callers’ experiences and attitudes of this support. In this study we report on the descriptive and qualitative insights provided by 908 callers as part of an evaluation of UK-based breastfeeding helpline(s).
Methods: A structured telephone interview, incorporating Likert scale responses and open-ended questions was undertaken with 908 callers over May to August, 2011 to explore callers’ experiences of the help and support received via the breastfeeding helpline(s).
Results: Overall satisfaction with the helpline was high, with the vast majority of callers’ recalling positive experiences of the help and support received. Thematic analysis was undertaken on all qualitative and descriptive data recorded during the evaluation, contextualised within the main areas addressed within the interview schedule in terms of ‘contact with the helplines’; ‘experiences of the helpline service’, ‘perceived effectiveness of support provision’ and ‘impact on caller wellbeing’.
Conclusion: Callers valued the opportunity for accessible, targeted, non-judgmental and convenient support. Whilst the telephone support did not necessarily influence women’s breastfeeding decisions, the support they received left them feeling reassured, confident and more determined to continue breastfeeding. We recommend extending the helpline service to ensure support can be accessed when needed, and ongoing training and support for volunteers. Further advertising and promotion of the service within wider demographic groups is warranted
Effective range function below threshold
We demonstrate that the kernel of the Lippmann-Schwinger equation, associated
with interactions consisting of a sum of the Coulomb plus a short range nuclear
potential, below threshold becomes degenerate. Taking advantage of this fact,
we present a simple method of calculating the effective range function for
negative energies. This may be useful in practice since the effective range
expansion extrapolated to threshold allows to extract low-energy scattering
parameters: the Coulomb-modified scattering length and the effective range.Comment: 14 pages, 1 figur
A wavefront orientation method for precise numerical determination of tsunami travel time
We present a highly accurate and computationally efficient method (herein,
the "wavefront orientation method") for determining the travel time of
oceanic tsunamis. Based on Huygens' Principle, the method uses an eight-point
grid-point pattern and the most recent information on the orientation of the
advancing wavefront to determine the time for a tsunami to travel to a
specific oceanic location. The method is shown to provide improved accuracy
and reduced anisotropy compared with the conventional multiple grid-point
method presently in widespread use
Sampling properties of directed networks
For many real-world networks only a small "sampled" version of the original
network may be investigated; those results are then used to draw conclusions
about the actual system. Variants of breadth-first search (BFS) sampling, which
are based on epidemic processes, are widely used. Although it is well
established that BFS sampling fails, in most cases, to capture the
IN-component(s) of directed networks, a description of the effects of BFS
sampling on other topological properties are all but absent from the
literature. To systematically study the effects of sampling biases on directed
networks, we compare BFS sampling to random sampling on complete large-scale
directed networks. We present new results and a thorough analysis of the
topological properties of seven different complete directed networks (prior to
sampling), including three versions of Wikipedia, three different sources of
sampled World Wide Web data, and an Internet-based social network. We detail
the differences that sampling method and coverage can make to the structural
properties of sampled versions of these seven networks. Most notably, we find
that sampling method and coverage affect both the bow-tie structure, as well as
the number and structure of strongly connected components in sampled networks.
In addition, at low sampling coverage (i.e. less than 40%), the values of
average degree, variance of out-degree, degree auto-correlation, and link
reciprocity are overestimated by 30% or more in BFS-sampled networks, and only
attain values within 10% of the corresponding values in the complete networks
when sampling coverage is in excess of 65%. These results may cause us to
rethink what we know about the structure, function, and evolution of real-world
directed networks.Comment: 21 pages, 11 figure
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