852 research outputs found
A Bayesian spatio-temporal model of panel design data: airborne particle number concentration in Brisbane, Australia
This paper outlines a methodology for semi-parametric spatio-temporal
modelling of data which is dense in time but sparse in space, obtained from a
split panel design, the most feasible approach to covering space and time with
limited equipment. The data are hourly averaged particle number concentration
(PNC) and were collected, as part of the Ultrafine Particles from Transport
Emissions and Child Health (UPTECH) project. Two weeks of continuous
measurements were taken at each of a number of government primary schools in
the Brisbane Metropolitan Area. The monitoring equipment was taken to each
school sequentially. The school data are augmented by data from long term
monitoring stations at three locations in Brisbane, Australia.
Fitting the model helps describe the spatial and temporal variability at a
subset of the UPTECH schools and the long-term monitoring sites. The temporal
variation is modelled hierarchically with penalised random walk terms, one
common to all sites and a term accounting for the remaining temporal trend at
each site. Parameter estimates and their uncertainty are computed in a
computationally efficient approximate Bayesian inference environment, R-INLA.
The temporal part of the model explains daily and weekly cycles in PNC at the
schools, which can be used to estimate the exposure of school children to
ultrafine particles (UFPs) emitted by vehicles. At each school and long-term
monitoring site, peaks in PNC can be attributed to the morning and afternoon
rush hour traffic and new particle formation events. The spatial component of
the model describes the school to school variation in mean PNC at each school
and within each school ground. It is shown how the spatial model can be
expanded to identify spatial patterns at the city scale with the inclusion of
more spatial locations.Comment: Draft of this paper presented at ISBA 2012 as poster, part of UPTECH
projec
Life and Death at the Edge of a Windy Cliff
The survival probability of a particle diffusing in the two dimensional
domain near a ``windy cliff'' at is investigated. The particle dies
upon reaching the edge of the cliff. In addition to diffusion, the particle is
influenced by a steady ``wind shear'' with velocity , \ie, no average bias either toward or away from the cliff.
For this semi-infinite system, the particle survival probability decays with
time as , compared to in the absence of wind. Scaling
descriptions are developed to elucidate this behavior, as well as the survival
probability within a semi-infinite strip of finite width with particle
absorption at . The behavior in the strip geometry can be described in
terms of Taylor diffusion, an approach which accounts for the crossover to the
decay when the width of the strip diverges. Supporting numerical
simulations of our analytical results are presented.Comment: 13 pages, plain TeX, 5 figures available upon request to SR
(submitted to J. Stat. Phys.
Induced vertical disparity effects on local and global stereopsis
Purpose: Although significant amounts of vertical misalignment could have a noticeable effect on visual performance, there is no conclusive evidence about the effect of very small amount of vertical disparity on stereopsis and binocular vision. Hence, the aim of this study was to investigate the effects of induced vertical disparity on local and global stereopsis at near. Materials and Methods: Ninety participants wearing best-corrected refraction had local and global stereopsis tested with 0.5 and 1.0 prism diopter (Δ) vertical prism in front of their dominant and non-dominant eye in turn. This was compared to local and global stereopsis in the same subjects without vertical prism. Data were analyzed in SPSS.17 software using the independent samples T and the repeated measures ANOVA tests. Results: Induced vertical disparity decreases local and global stereopsis. This reduction is greater when vertical disparity is induced in front of the non-dominant eye and affects global more than local stereopsis. Repeated measures ANOVA showed differences in the mean stereopsis between the different measured states for local and global values. Local stereopsis thresholds were reduced by 10s of arc or less on average with 1.0Δ of induced vertical prism in front of either eye. However, global stereopsis thresholds were reduced by over 100s of arc by the same 1.0Δ of induced vertical prism. Conclusion: Induced vertical disparity affects global stereopsis thresholds by an order of magnitude (or a factor of 10) more than local stereopsis. Hence, using a test that measures global stereopsis such as the TNO is more sensitive to vertical misalignment than a test such as the Stereofly that measures local stereopsis. © 2014 Informa Healthcare USA, Inc. All rights reserved: reproduction in whole or part not permitted
Reconstruction of three-dimensional porous media using generative adversarial neural networks
To evaluate the variability of multi-phase flow properties of porous media at
the pore scale, it is necessary to acquire a number of representative samples
of the void-solid structure. While modern x-ray computer tomography has made it
possible to extract three-dimensional images of the pore space, assessment of
the variability in the inherent material properties is often experimentally not
feasible. We present a novel method to reconstruct the solid-void structure of
porous media by applying a generative neural network that allows an implicit
description of the probability distribution represented by three-dimensional
image datasets. We show, by using an adversarial learning approach for neural
networks, that this method of unsupervised learning is able to generate
representative samples of porous media that honor their statistics. We
successfully compare measures of pore morphology, such as the Euler
characteristic, two-point statistics and directional single-phase permeability
of synthetic realizations with the calculated properties of a bead pack, Berea
sandstone, and Ketton limestone. Results show that GANs can be used to
reconstruct high-resolution three-dimensional images of porous media at
different scales that are representative of the morphology of the images used
to train the neural network. The fully convolutional nature of the trained
neural network allows the generation of large samples while maintaining
computational efficiency. Compared to classical stochastic methods of image
reconstruction, the implicit representation of the learned data distribution
can be stored and reused to generate multiple realizations of the pore
structure very rapidly.Comment: 21 pages, 20 figure
Survival of a Diffusing Particle in a Transverse Shear Flow: A First-Passage Problem with Continuously Varying Persistence Exponent
We consider a particle diffusing in the y-direction, dy/dt=\eta(t), subject
to a transverse shear flow in the x-direction, dx/dt=f(y), where x \ge 0 and
x=0 is an absorbing boundary. We treat the class of models defined by f(y) =
\pm v_{\pm}(\pm y)^\alpha where the upper (lower) sign refers to y>0 (y<0). We
show that the particle survives with probability Q(t) \sim t^{-\theta} with
\theta = 1/4, independent of \alpha, if v_{+}=v_{-}. If v_{+} \ne v_{-},
however, we show that \theta depends on both \alpha and the ratio v_{+}/v_{-},
and we determine this dependence.Comment: 4 page
Mapping aerial metal deposition in metropolitan areas from tree bark : a case study in Sheffield, England
We investigated the use of metals accumulated on tree bark for mapping their deposition across metropolitan Sheffield by sampling 642 trees of three common species. Mean concentrations of metals were generally an order of
magnitude greater than in samples from a remote uncontaminated site. We found trivially small differences among tree species with respect to metal concentrations on
bark, and in subsequent statistical analyses did not discriminate between them. We mapped the concentrations of As, Cd and Ni by lognormal universal kriging using parameters estimated by residual maximum likelihood ({\sc reml}). The concentrations of Ni and Cd were greatest close to a large steel works, their probable source, and declined markedly within 500~metres of it and from there more gradually over several kilometres. Arsenic was much more
evenly distributed, probably as a result of locally mined coal burned in domestic fires for many years. Tree bark seems to integrate airborne pollution over time, and our findings show that sampling and analysing it are cost-effective means of mapping and identifying sources
Vertical Heterophoria and Postural Control in Nonspecific Chronic Low Back Pain
The purpose of this study was to test postural control during quiet standing in
nonspecific chronic low back pain (LBP) subjects with vertical heterophoria (VH)
before and after cancellation of VH; also to compare with healthy subjects with,
and without VH. Fourteen subjects with LBP took part in this study. The postural
performance was measured through the center of pressure displacements with a
force platform while the subjects fixated on a target placed at either 40 or 200
cm, before and after VH cancellation with an appropriate prism. Their postural
performance was compared to that of 14 healthy subjects with VH and 12 without
VH (i.e. vertical orthophoria) studied previously in similar conditions. For LBP
subjects, cancellation of VH with a prism improved postural performance. With
respect to control subjects (with or without VH), the variance of speed of the
center of pressure was higher, suggesting more energy was needed to stabilize
their posture in quiet upright stance. Similarly to controls, LBP subjects
showed higher postural sway when they were looking at a target at a far distance
than at a close distance. The most important finding is that LBP subjects with
VH can improve their performance after prism-cancellation of their VH. We
suggest that VH reflects mild conflict between sensory and motor inputs involved
in postural control i.e. a non optimal integration of the various signals. This
could affect the performance of postural control and perhaps lead to pain.
Nonspecific chronic back pain may results from such prolonged conflict
Perturbation theory for the effective diffusion constant in a medium of random scatterer
We develop perturbation theory and physically motivated resummations of the
perturbation theory for the problem of a tracer particle diffusing in a random
media. The random media contains point scatterers of density uniformly
distributed through out the material. The tracer is a Langevin particle
subjected to the quenched random force generated by the scatterers. Via our
perturbative analysis we determine when the random potential can be
approximated by a Gaussian random potential. We also develop a self-similar
renormalisation group approach based on thinning out the scatterers, this
scheme is similar to that used with success for diffusion in Gaussian random
potentials and agrees with known exact results. To assess the accuracy of this
approximation scheme its predictions are confronted with results obtained by
numerical simulation.Comment: 22 pages, 6 figures, IOP (J. Phys. A. style
Continuum Derrida Approach to Drift and Diffusivity in Random Media
By means of rather general arguments, based on an approach due to Derrida
that makes use of samples of finite size, we analyse the effective diffusivity
and drift tensors in certain types of random medium in which the motion of the
particles is controlled by molecular diffusion and a local flow field with
known statistical properties. The power of the Derrida method is that it uses
the equilibrium probability distribution, that exists for each {\em finite}
sample, to compute asymptotic behaviour at large times in the {\em infinite}
medium. In certain cases, where this equilibrium situation is associated with a
vanishing microcurrent, our results demonstrate the equality of the
renormalization processes for the effective drift and diffusivity tensors. This
establishes, for those cases, a Ward identity previously verified only to
two-loop order in perturbation theory in certain models. The technique can be
applied also to media in which the diffusivity exhibits spatial fluctuations.
We derive a simple relationship between the effective diffusivity in this case
and that for an associated gradient drift problem that provides an interesting
constraint on previously conjectured results.Comment: 18 pages, Latex, DAMTP-96-8
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