16,125 research outputs found
Modelling crystal aggregation and deposition\ud in the catheterised lower urinary tract
Urethral catheters often become encrusted with crystals of magnesium struvite and calcium phosphate. The encrustation can block the catheter, which can cause urine retention in the bladder and reflux into the kidneys. We develop a mathematical model to investigate crystal deposition on the catheter surface, modelling the bladder as a reservoir of fluid and the urethral catheter as a rigid channel. At a constant rate, fluid containing crystal particles of unit size enters the reservoir, and flows from the reservoir through the channel and out of the system. The crystal particles aggregate, which we model using Becker–Döring coagulation theory, and are advected through the channel, where they continue to aggregate and are deposited on the channel’s walls. Inhibitor particles also enter the reservoir, and can bind to the crystals, preventing further aggregation and deposition. The crystal concentrations are spatially homogeneous in the reservoir, whereas the channel concentrations vary spatially as a result of advection, diffusion and deposition. We investigate the effect of inhibitor particles on the amount of deposition. For all parameter values, we find that crystals deposit along the full length of the channel, with maximum deposition close to the channel’s entrance
White Dwarf Cosmochronology in the Solar Neighborhood
The study of the stellar formation history in the solar neighborhood is a
powerful technique to recover information about the early stages and evolution
of the Milky Way. We present a new method which consists of directly probing
the formation history from the nearby stellar remnants. We rely on the volume
complete sample of white dwarfs within 20 pc, where accurate cooling ages and
masses have been determined. The well characterized initial-final mass relation
is employed in order to recover the initial masses (1 < M/Msun < 8) and total
ages for the local degenerate sample. We correct for moderate biases that are
necessary to transform our results to a global stellar formation rate, which
can be compared to similar studies based on the properties of main-sequence
stars in the solar neighborhood. Our method provides precise formation rates
for all ages except in very recent times, and the results suggest an enhanced
formation rate for the solar neighborhood in the last 5 Gyr compared to the
range 5 < Age (Gyr) < 10. Furthermore, the observed total age of ~10 Gyr for
the oldest white dwarfs in the local sample is consistent with the early
seminal studies that have determined the age of the Galactic disk from stellar
remnants. The main shortcoming of our study is the small size of the local
white dwarf sample. However, the presented technique can be applied to larger
samples in the future.Comment: 25 pages, 10 figures, accepted for publication in the Astrophysical
Journa
A study of adaptive multimode systems Final technical report, Mar. 1967 - Apr. 1968
Laboratory model of experiment for demonstrating applicability of adaptive multimode systems concept to scientific spacecraft instrumentatio
The Feeling of Color: A Haptic Feedback Device for the Visually Disabled
Tapson J, Gurari N, Diaz J, et al. The Feeling of Color: A Haptic Feedback Device for the Visually Disabled. Presented at the Biomedical Circuits and Systems Conference (BIOCAS), Baltimore, MD.We describe a sensory augmentation system designed to provide the visually disabled with a sense of color. Our system consists of a glove with short-range optical color sensors mounted on its fingertips, and a torso-worn belt on which tactors (haptic feedback actuators) are mounted. Each fingertip sensor detects the observed objectpsilas color. This information is encoded to the tactor through vibrations in respective locations and varying modulations. Early results suggest that detection of primary colors is possible with near 100% accuracy and moderate latency, with a minimum amount of training
Dispersal Dynamics in a Wind-Driven Benthic System
Bedload and water column traps were used with simultaneous wind and water velocity measurements to study postlarval macrofaunal dispersal dynamics in Manukau Harbour, New Zealand. A 12-fold range in mean wind condition resulted in large differences in water flow (12-fold), sediment flux (285-fold), and trap collection of total number of individuals (95-fold), number of the dominant infaunal organism (84-fold for the bivalve Macomona liliana), and number of species (4-fold). There were very strong, positive relationships among wind condition, water velocity, sediment flux, and postlarval dispersal, especially in the bedload. Local density in the ambient sediment was not a good predictor of dispersal. Results indicate that postlarval dispersal may influence benthic abundance pat- terns over a range of spatial scales
The Field White Dwarf Mass Distribution
We revisit the properties and astrophysical implications of the field white
dwarf mass distribution in preparation of Gaia applications. Our study is based
on the two samples with the best established completeness and most precise
atmospheric parameters, the volume-complete survey within 20 pc and the Sloan
Digital Sky Survey (SDSS) magnitude-limited sample. We explore the modelling of
the observed mass distributions with Monte Carlo simulations, but find that it
is difficult to constrain independently the initial mass function (IMF), the
initial-to-final-mass relation (IFMR), the stellar formation history (SFH), the
variation of the Galactic disk vertical scale height as a function of stellar
age, and binary evolution. Each of these input ingredients has a moderate
effect on the predicted mass distributions, and we must also take into account
biases owing to unidentified faint objects (20 pc sample), as well as unknown
masses for magnetic white dwarfs and spectroscopic calibration issues (SDSS
sample). Nevertheless, we find that fixed standard assumptions for the above
parameters result in predicted mean masses that are in good qualitative
agreement with the observed values. It suggests that derived masses for both
studied samples are consistent with our current knowledge of stellar and
Galactic evolution. Our simulations overpredict by 40-50% the number of massive
white dwarfs (M > 0.75 Msun) for both surveys, although we can not exclude a
Salpeter IMF when we account for all biases. Furthermore, we find no evidence
of a population of double white dwarf mergers in the observed mass
distributions.Comment: 15 pages, 16 figures, accepted for publication in MNRA
Population structure, long-term connectivity, and effective size of mutton snapper (Lutjanus analis) in the Caribbean Sea and Florida Keys
Genetic structure and average long-term connectivity and
effective size of mutton snapper (Lutjanus analis) sampled from offshore localities in the U.S. Caribbean and the Florida Keys were assessed by using nuclear-encoded microsatellites and a fragment of mitochondrial DNA. No significant differences in allele, genotype (microsatellites), or haplotype (mtDNA) distributions were detected; tests of selective neutrality (mtDNA) were nonsignificant after Bonferroni correction. Heuristic
estimates of average long-term rate of migration (proportion of migrant individuals/generation) between geographically adjacent localities varied from 0.0033 to 0.0054, indicating that local subpopulations could respond
independently of environmental perturbations. Estimates of average longterm effective population sizes varied from 341 to 1066 and differed significantly among several of the localities. These results indicate that over time larval drift and interregional adult movement may not be sufficient to maintain population sustainability across the region and that there may be different demographic stocks at
some of the localities studied. The estimate of long-term effective population size at the locality offshore of St. Croix was below the minimum threshold size considered necessary to maintain the equilibrium between the loss of adaptive genetic variance from genetic drift and its replacement by mutation. Genetic variability in mutton snapper likely is maintained at the intraregional level by aggregate spawning and random mating of local populations. This feature is perhaps ironic in that aggregate spawning also renders mutton snapper especially vulnerable to overexploitation
Anomalous Cosmic Rays: The Principal Source of High Energy Heavy Ions in the Radiation Belts
Recent observations from SAMPEX have shown that "anomalous cosmic rays" are
the principal source of high energy (> 10MeV/nuc) heavy ions trapped in the
radiation belts. This component of interplanetary particles is known to originate
from interstellar atoms that has been accelerated to high energies in the outer
heliosphere. The mechanism by which anomalous cosmic rays with ~1 to ~50 Me V/nuc are trapped in a radiation belt at L ≈ 2 has now been verified. We
discuss models for accelerating and trapping anomalous cosmic rays and review
observations of their composition, energy spectra, pitch angle distribution, and time
variations. Extrapolation of the fluxes observed at ~600 km to higher altitude and
other time periods is also discussed
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