3,726 research outputs found
Dislocation microstructures and strain-gradient plasticity with one active slip plane
We study dislocation networks in the plane using the vectorial phase-field
model introduced by Ortiz and coworkers, in the limit of small lattice spacing.
We show that, in a scaling regime where the total length of the dislocations is
large, the phase field model reduces to a simpler model of the strain-gradient
type. The limiting model contains a term describing the three-dimensional
elastic energy and a strain-gradient term describing the energy of the
geometrically necessary dislocations, characterized by the tangential gradient
of the slip. The energy density appearing in the strain-gradient term is
determined by the solution of a cell problem, which depends on the line tension
energy of dislocations. In the case of cubic crystals with isotropic elasticity
our model shows that complex microstructures may form, in which dislocations
with different Burgers vector and orientation react with each other to reduce
the total self energy
The epidemiology of canine leishmaniasis: transmission rates estimated from a cohort study in Amazonian Brazil
We estimate the incidence rate, serological conversion rate and basic case reproduction number (R0) of Leishmania infantum from a cohort study of 126 domestic dogs exposed to natural infection rates over 2 years on Marajó Island, Pará State, Brazil. The analysis includes new methods for (1) determining the number of seropositives in cross-sectional serological data, (2) identifying seroconversions in longitudinal studies, based on both the number of antibody units and their rate of change through time, (3) estimating incidence and serological pre-patent periods and (4) calculating R0 for a potentially fatal, vector-borne disease under seasonal transmission. Longitudinal and cross-sectional serological (ELISA) analyses gave similar estimates of the proportion of dogs positive. However, longitudinal analysis allowed the calculation of pre-patent periods, and hence the more accurate estimation of incidence: an infection–conversion model fitted by maximum likelihood to serological data yielded seasonally varying per capita incidence rates with a mean of 8·66×10[minus sign]3/day (mean time to infection 115 days, 95% C.L. 107–126 days), and a median pre-patent period of 94 (95% C.L. 82–111) days. These results were used in conjunction with theory and dog demographic data to estimate the basic reproduction number, R0, as 5·9 (95% C.L. 4·4–7·4). R0 is a determinant of the scale of the leishmaniasis control problem, and we comment on the options for control
An Action Principle for Relativistic MHD
A covariant action principle for ideal relativistic magnetohydrodynamics
(MHD) in terms of natural Eulerian field variables is given. This is done by
generalizing the covariant Poisson bracket theory of Marsden et al., which uses
a noncanonical bracket to effect constrained variations of an action
functional. Various implications and extensions of this action principle are
also discussed. Two significant by-products of this formalism are the
introduction of a new divergence-free 4-vector variable for the magnetic field,
and a new Lie-dragged form for the theory
Joint Reconstruction of Multi-channel, Spectral CT Data via Constrained Total Nuclear Variation Minimization
We explore the use of the recently proposed "total nuclear variation" (TNV)
as a regularizer for reconstructing multi-channel, spectral CT images. This
convex penalty is a natural extension of the total variation (TV) to
vector-valued images and has the advantage of encouraging common edge locations
and a shared gradient direction among image channels. We show how it can be
incorporated into a general, data-constrained reconstruction framework and
derive update equations based on the first-order, primal-dual algorithm of
Chambolle and Pock. Early simulation studies based on the numerical XCAT
phantom indicate that the inter-channel coupling introduced by the TNV leads to
better preservation of image features at high levels of regularization,
compared to independent, channel-by-channel TV reconstructions.Comment: Submitted to Physics in Medicine and Biolog
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