883 research outputs found
An Immersed Boundary method with divergence-free velocity interpolation and force spreading
The Immersed Boundary (IB) method is a mathematical framework for
constructing robust numerical methods to study fluid-structure interaction in
problems involving an elastic structure immersed in a viscous fluid. The IB
formulation uses an Eulerian representation of the fluid and a Lagrangian
representation of the structure. The Lagrangian and Eulerian frames are coupled
by integral transforms with delta function kernels. The discretized IB
equations use approximations to these transforms with regularized delta
function kernels to interpolate the fluid velocity to the structure, and to
spread structural forces to the fluid. It is well-known that the conventional
IB method can suffer from poor volume conservation since the interpolated
Lagrangian velocity field is not generally divergence-free, and so this can
cause spurious volume changes. In practice, the lack of volume conservation is
especially pronounced for cases where there are large pressure differences
across thin structural boundaries. The aim of this paper is to greatly reduce
the volume error of the IB method by introducing velocity-interpolation and
force-spreading schemes with the properties that the interpolated velocity
field in which the structure moves is at least C1 and satisfies a continuous
divergence-free condition, and that the force-spreading operator is the adjoint
of the velocity-interpolation operator. We confirm through numerical
experiments in two and three spatial dimensions that this new IB method is able
to achieve substantial improvement in volume conservation compared to other
existing IB methods, at the expense of a modest increase in the computational
cost. Further, the new method provides smoother Lagrangian forces (tractions)
than traditional IB methods. The method presented here is restricted to
periodic computational domains. Its generalization to non-periodic domains is
important future work.Comment: 49 pages, 13 figure
The geometry of thermodynamic control
A deeper understanding of nonequilibrium phenomena is needed to reveal the
principles governing natural and synthetic molecular machines. Recent work has
shown that when a thermodynamic system is driven from equilibrium then, in the
linear response regime, the space of controllable parameters has a Riemannian
geometry induced by a generalized friction tensor. We exploit this geometric
insight to construct closed-form expressions for minimal-dissipation protocols
for a particle diffusing in a one dimensional harmonic potential, where the
spring constant, inverse temperature, and trap location are adjusted
simultaneously. These optimal protocols are geodesics on the Riemannian
manifold, and reveal that this simple model has a surprisingly rich geometry.
We test these optimal protocols via a numerical implementation of the
Fokker-Planck equation and demonstrate that the friction tensor arises
naturally from a first order expansion in temporal derivatives of the control
parameters, without appealing directly to linear response theory
Verbal Learning and Memory After Cochlear Implantation in Postlingually Deaf Adults: Some New Findings with the CVLT-II
OBJECTIVES:
Despite the importance of verbal learning and memory in speech and language processing, this domain of cognitive functioning has been virtually ignored in clinical studies of hearing loss and cochlear implants in both adults and children. In this article, we report the results of two studies that used a newly developed visually based version of the California Verbal Learning Test-Second Edition (CVLT-II), a well-known normed neuropsychological measure of verbal learning and memory.
DESIGN:
The first study established the validity and feasibility of a computer-controlled visual version of the CVLT-II, which eliminates the effects of audibility of spoken stimuli, in groups of young normal-hearing and older normal-hearing (ONH) adults. A second study was then carried out using the visual CVLT-II format with a group of older postlingually deaf experienced cochlear implant (ECI) users (N = 25) and a group of ONH controls (N = 25) who were matched to ECI users for age, socioeconomic status, and nonverbal IQ. In addition to the visual CVLT-II, subjects provided data on demographics, hearing history, nonverbal IQ, reading fluency, vocabulary, and short-term memory span for visually presented digits. ECI participants were also tested for speech recognition in quiet.
RESULTS:
The ECI and ONH groups did not differ on most measures of verbal learning and memory obtained with the visual CVLT-II, but deficits were identified in ECI participants that were related to recency recall, the buildup of proactive interference, and retrieval-induced forgetting. Within the ECI group, nonverbal fluid IQ, reading fluency, and resistance to the buildup of proactive interference from the CVLT-II consistently predicted better speech recognition outcomes.
CONCLUSIONS:
Results from this study suggest that several underlying foundational neurocognitive abilities are related to core speech perception outcomes after implantation in older adults. Implications of these findings for explaining individual differences and variability and predicting speech recognition outcomes after implantation are discussed
The Gounkoto Au deposit, West Africa: Constraints on ore genesis and volatile sources from petrological, fluid inclusion and stable isotope data
The Loulo–Gounkoto complex in the Kédougou–Kéniéba Inlier hosts three multi-million ounce orogenic gold deposits, situated along the Senegal–Mali Shear Zone. This west Malian gold belt represents the largest West African orogenic gold district outside Ghana. The Gounkoto deposit is hosted to the south of the Gara and Yalea gold mines in the Kofi Series metasedimentary rocks. The ore body is structurally controlled and is characterised by sodic and phyllic alteration, As- and Fe-rich ore assemblages, with abundant magnetite, and overall enrichment in Fe–As–Cu–Au–Ag–W–Ni–Co–REE + minor Te–Pb–Se–Cd. Fluid inclusion analysis indicates that the deposit formed at P–T conditions of approximately 1.4 kbar and 340 °C and that two end member fluids were involved in mineralisation: (1) a moderate temperature (315–340 °C), low salinity (< 10 wt.% NaCl equiv.), low density (≤ 1 g·cm− 3), H2O–CO2–NaCl–H2S ± N2–CH4 fluid; (2) a high temperature (up to 445 °C), hypersaline (~ 40 wt.% NaCl equiv.), high density (~ 1.3 g·cm− 3), H2O–CO2–NaCl ± FeCl2 fluid. Partial mixing of these fluids within the Jog Zone at Gounkoto enhanced phase separation in the aqueo-carbonic fluid and acted as a precipitation mechanism for Au. These findings demonstrate the widespread, if heterogeneously distributed, nature of fluid mixing as an ore forming process in the Loulo–Gounkoto complex, operating over at least a 30 km strike length of the shear zone. Stable isotope analyses of ore components at Gounkoto indicate a dominant metamorphic source for H2O, H2S and CO2, and by extension Au. It thus can be reasoned that both the aqueo-carbonic and the hypersaline fluid at Gounkoto are of metamorphic origin and that the high levels of salinity in the brine are likely derived from evaporite dissolution
The Fundamental Plane of QSOs and the Relationship Between Host and Nucleus
We present results from an archival study of 70 medium-redshift QSOs observed
with the Wide Field Planetary Camera 2 on board the Hubble Space Telescope. The
QSOs have magnitudes M_V <= -23 (total nuclear plus host light) and redshifts
. The aim of the present study is to investigate the
connections between the nuclear and host properties of QSOs, using
high-resolution images and removing the central point source to reveal the host
structure. We confirm that more luminous QSO nuclei are found in more luminous
host galaxies. Using central black hole masses from the literature, we find
that nuclear luminosity also generally increases with black hole mass, but it
is not tightly correlated. Nuclear luminosities range from 2.3% to 200% of the
Eddington limit. Those in elliptical hosts cover the range fairly evenly, while
those in spirals are clustered near the Eddington limit. Using a principal
components analysis, we find a kind of fundamental plane relating the nuclear
luminosity to the size and effective surface magnitude of the bulge. Using
optical nuclear luminosity, this relationship explains 96.1% of the variance in
the overall sample, while another version of the relationship uses x-ray
nuclear luminosity and explains 95.2% of the variance. The form of this QSO
fundamental plane shows similarities to the well-studied fundamental plane of
elliptical galaxies, and we examine the possible relationship between them as
well as the difficulties involved in establishing this connection.Comment: 49 pages, 6 figures. Accepted by Ap
The host galaxies of luminous quasars
We present results of a deep HST/WFPC2 imaging study of 17 quasars at z~0.4,
designed to determine the properties of their host galaxies. The sample
consists of quasars with absolute magnitudes in the range -24>M_V>-28, allowing
us to investigate host galaxy properties across a decade in quasar luminosity,
but at a single redshift. We find that the hosts of all the RLQs, and all the
RQQs with nuclear luminosities M_V<-24, are massive bulge-dominated galaxies,
confirming and extending the trends deduced from our previous studies. From the
best-fitting model host galaxies we have estimated spheroid and black-hole
masses, and the efficiency (with respect to Eddington luminosity) with which
each quasar is radiating. The largest inferred black-hole mass in our sample is
\~3.10^9 M_sun, comparable to those at the centres of M87 and Cygnus A. We find
no evidence for super-Eddington accretion in even the most luminous objects. We
investigate the role of scatter in the black-hole:spheroid mass relation in
determining the ratio of quasar to host-galaxy luminosity, by generating
simulated populations of quasars lying in hosts with a Schechter mass function.
Within the subsample of the highest luminosity quasars, the observed variation
in nuclear-host luminosity ratio is consistent with being the result of the
scatter in the black-hole:spheroid relation. Quasars with high nuclear-host
ratios can be explained by sub-Eddington accretion onto black holes in the
high-mass tail of the black-hole:spheroid relation. Our results imply that,
owing to the Schechter cutoff, host mass should not continue to increase
linearly with quasar luminosity, at the very highest luminosities. Any quasars
more luminous than M_V=-27 should be found in massive elliptical hosts which at
the present day would have M_V ~ -24.5.Comment: Accepted for publication in MNRAS. 18 pages; 7 figures and 17
greyscale images are reproduced here at low quality due to space limitations.
High-resolution figures are available from
ftp://ftp.roe.ac.uk/pub/djef/preprints/floyd2004
American Black Bear Population Fragmentation Determined Through Pedigrees in the Trans-Border Canada-United States Region
Fragmentation of species with large numbers of individuals in adjacent areas can be challenging to detect using genetic tools as there often is no differentiation because genetic drift occurs very slowly. We used a genetic-based pedigree analysis to detect fragmentation in the American black bear (Ursus americanus) across 2 highways with large adjacent populations. We used 20 locus microsatellite genotypes to detect parent-offspring and full sibling pairs within a sample of 388 black bears. We used the spatial patterns of capture locations of these first order relatives relative to US Highway 2 in northwest Montana and Highway 3 in southeast British Columbia to estimate the number of close relatives sampled across the highways (migrants/km of highway length) as an index of fragmentation. We compared these values to an expected migrant/km rate derived from the mean values of simulated fractures in the Highway 2 and Highway 3 region. We found evidence that these highway corridors were fragmenting black bear populations, but not completely. The observed migrant/km rate for Highway 2 was 0.05, while the expected rate was 0.21 migrants/km. Highway 3 had an observed migrant/km rate of 0.09 compared to the expected rate of 0.26. None of the 16 bears carrying GPS radio collars for 1 year crossed Highway 2, yet 6 of 18 crossed Highway 3. Pedigree and telemetry results were more closely aligned in the Highway 2 system evidencing more intense fragmentation than we found along Highway 3. Our results demonstrate that pedigree analysis may be a useful tool for investigating population fragmentation in situations where genetic signals of differentiation are too weak to determine migration rates using individual-based methods, such as population assignment
Simulating Cardiac Fluid Dynamics in the Human Heart
Cardiac fluid dynamics fundamentally involves interactions between complex
blood flows and the structural deformations of the muscular heart walls and the
thin, flexible valve leaflets. There has been longstanding scientific,
engineering, and medical interest in creating mathematical models of the heart
that capture, explain, and predict these fluid-structure interactions. However,
existing computational models that account for interactions among the blood,
the actively contracting myocardium, and the cardiac valves are limited in
their abilities to predict valve performance, resolve fine-scale flow features,
or use realistic descriptions of tissue biomechanics. Here we introduce and
benchmark a comprehensive mathematical model of cardiac fluid dynamics in the
human heart. A unique feature of our model is that it incorporates
biomechanically detailed descriptions of all major cardiac structures that are
calibrated using tensile tests of human tissue specimens to reflect the heart's
microstructure. Further, it is the first fluid-structure interaction model of
the heart that provides anatomically and physiologically detailed
representations of all four cardiac valves. We demonstrate that this
integrative model generates physiologic dynamics, including realistic
pressure-volume loops that automatically capture isovolumetric contraction and
relaxation, and predicts fine-scale flow features. None of these outputs are
prescribed; instead, they emerge from interactions within our comprehensive
description of cardiac physiology. Such models can serve as tools for
predicting the impacts of medical devices or clinical interventions. They also
can serve as platforms for mechanistic studies of cardiac pathophysiology and
dysfunction, including congenital defects, cardiomyopathies, and heart failure,
that are difficult or impossible to perform in patients
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