36,101 research outputs found
Neutron capture cross sections from surrogate reaction data and theory: connecting the pieces with a Markov-Chain Monte Carlo approach
The neutron capture cross section for has recently been
determined using surrogate data and nuclear reaction
theory. That work employed an approximate fitting method based on Bayesian
Monte Carlo sampling to determine parameters needed for calculating the
cross section. Here, we improve the approach by
introducing a more sophisticated Markov Chain Monte Carlo sampling method. We
present preliminary results.Comment: Accepted into the proceedings of the 6th International Workshop on
Compound-Nuclear Reactions and Related Topics, Berkeley, California,
September 24-28, 2018. 4 pages, 1 figur
Trapping and displacement of liquid collars and plugs in rough-walled tubes
A liquid film wetting the interior of a long circular cylinder redistributes
under the action of surface tension to form annular collars or occlusive plugs.
These equilibrium structures are invariant under axial translation within a
perfectly smooth uniform tube and therefore can be displaced axially by very
weak external forcing. We consider how this degeneracy is disrupted when the
tube wall is rough, and determine threshold conditions under which collars or
plugs resist displacement under forcing. Wall roughness is modelled as a
non-axisymmetric Gaussian random field of prescribed correlation length and
small variance, mimicking some of the geometric irregularities inherent in
applications such as lung airways. The thin film coating this surface is
modelled using lubrication theory. When the roughness is weak, we show how the
locations of equilibrium collars and plugs can be identified in terms of the
azimuthally averaged tube radius; we derive conditions specifying equilibrium
collar locations under an externally imposed shear flow, and plug locations
under an imposed pressure gradient. We use these results to determine the
probability of external forcing being sufficient to displace a collar or plug
from a rough-walled tube, when the tube roughness is defined only in
statistical terms
Drop spreading with random viscosity
We examine theoretically the spreading of a viscous liquid drop over a thin
film of uniform thickness, assuming the liquid's viscosity is regulated by the
concentration of a solute that is carried passively by the spreading flow. The
solute is assumed to be initially heterogeneous, having a spatial distribution
with prescribed statistical features. To examine how this variability
influences the drop's motion, we investigate spreading in a planar geometry
using lubrication theory, combining numerical simulations with asymptotic
analysis. We assume diffusion is sufficient to suppress solute concentration
gradients across but not along the film. The solute field beneath the bulk of
the drop is stretched by the spreading flow, such that the initial solute
concentration immediately behind the drop's effective contact lines has a
long-lived influence on the spreading rate. Over long periods, solute swept up
from the precursor film accumulates in a short region behind the contact line,
allowing patches of elevated viscosity within the precursor film to hinder
spreading. A low-order model provides explicit predictions of the variances in
spreading rate and drop location, which are validated against simulations
How closely do baryons follow dark matter on large scales?
We investigate the large-scale clustering and gravitational interaction of
baryons and dark matter (DM) over cosmic time using a set of collisionless
N-body simulations. Both components, baryons and DM, are evolved from distinct
primordial density and velocity power spectra as predicted by early-universe
physics. We first demonstrate that such two-component simulations require an
unconventional match between force and mass resolution (i.e. force softening on
at least the mean particle separation scale). Otherwise, the growth on any
scale is not correctly recovered because of a spurious coupling between the two
species at the smallest scales. With these simulations, we then demonstrate how
the primordial differences in the clustering of baryons and DM are
progressively diminished over time. In particular, we explicitly show how the
BAO signature is damped in the spatial distribution of baryons and imprinted in
that of DM. This is a rapid process, yet it is still not fully completed at low
redshifts. On large scales, the overall shape of the correlation function of
baryons and DM differs by 2% at z = 9 and by 0.2% at z = 0. The differences in
the amplitude of the BAO peak are approximately a factor of 5 larger: 10% at z
= 9 and 1% at z = 0. These discrepancies are, however, smaller than effects
expected to be introduced by galaxy formation physics in both the shape of the
power spectrum and in the BAO peak, and are thus unlikely to be detected given
the precision of the next generation of galaxy surveys. Hence, our results
validate the standard practice of modelling the observed galaxy distribution
using predictions for the total mass clustering in the Universe.Comment: 9 pages, 6 figures. Replaced with version published in MNRA
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