7,537 research outputs found
Measurements of the Yield Stress in Frictionless Granular Systems
We perform extensive molecular dynamics simulations of 2D frictionless
granular materials to determine whether these systems can be characterized by a
single static yield shear stress. We consider boundary-driven planar shear at
constant volume and either constant shear force or constant shear velocity.
Under steady flow conditions, these two ensembles give similar results for the
average shear stress versus shear velocity. However, near jamming it is
possible that the shear stress required to initiate shear flow can differ
substantially from the shear stress required to maintain flow. We perform
several measurements of the shear stress near the initiation and cessation of
flow. At fixed shear velocity, we measure the average shear stress
in the limit of zero shear velocity. At fixed shear force, we
measure the minimum shear stress required to maintain steady flow
at long times. We find that in finite-size systems ,
which implies that there is a jump discontinuity in the shear velocity from
zero to a finite value when these systems begin flowing at constant shear
force. However, our simulations show that the difference , and thus the discontinuity in the shear velocity, tend to zero in
the infinite system size limit. Thus, our results indicate that in the large
system limit, frictionless granular systems are characterized by a single
static yield shear stress. We also monitor the short-time response of these
systems to applied shear and show that the packing fraction of the system and
shape of the velocity profile can strongly influence whether or not the shear
stress at short times overshoots the long-time average value.Comment: 7 pages and 6 figure
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Development and application of steelhead (Oncorhynchus mykiss) kelt identification techniques
The migrations upstream of pre-spawned adult steelhead (Oncorhynchus mykiss) and
downstream of post-spawned steelhead (referred to as "kelts") can geographically and
temporally overlap in the Snake River, leading to management challenges for populations
listed under the Endangered Species Act (ESA). This thesis describes the development and
application of a method to accurately identify kelts using ultrasound, which is viewed as the
first step towards development of effective management practices. In order to develop
classification criteria for pre- and post-spawn steelhead, ultrasound images of gonads were
taken and plasma levels of testosterone (T), I 1-ketotestosterone (1I-KT), and 17a,20β-
dihydroxyprogesterone (DHP) were determined in adult steelhead before and after spawning.
Results indicated that ultrasound images provide quantifiable selection criteria - based on the
size, number, location, and/or echogeneity of gonads - for the identification of pre-spawned
versus post-spawned adults. Pre-spawn females were easily identifiable by the presence of
numerous, well-developed eggs. Conversely, only a few mature eggs remained in the body
cavity of female kelts. Males were readily identified by the presence of testicular tissue, but
determining their maturation status (i.e., pre- or post-spawned) was more difficult compared to
females because some males retained gonad mass after spawning. The average cross-sectional
testis area in mature males was 2.86 cm² (SD = 0.74) relative to 0.62 cm² (SD = 0.24) in postspawned
males, demonstrating a statistically significant difference in testis size before and
after spawning. Distributions of testis measurements between the two maturational types did
not overlap and results of a discriminant function analysis suggest a classification criteria of
1.25 cm² could accurately (Ca. 99%, based on percentiles) distinguish males as pre- or post-spawned. Concentrations of T, 1 1-KT, and DHP were significantly higher in pre-spawn males
relative to post-spawned males, confirming an association between gonad size and
reproductive hormones. Using classification criteria developed from steelhead of known sex
and maturation type, ultrasound imaging of adult steelhead at Lower Granite Dam (LGR)
juvenile bypass facility revealed that 94.6% of 3,968 adult steelhead sampled from 1 April to
10 June 2000 were kelts. This included approximately 2,050 wild kelts, or about 17% of the
entire protected steelhead run above the dam. The majority of kelts examined were female
(77.0%) and most were in good overall morphological condition (69.5%), as indicated by the
lack of scars, infections, or other morphological damage. Based on the abundance of kelts at
LGR, management initiatives aimed at improving the repeat spawning rate of steelhead may
be considered an important part of a comprehensive plan to rebuild ESA-listed steelhead
populations throughout the Snake River basin
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Sources of error in clinical measurement of the amplitude of accommodation
Measurement of the amplitude of accommodation is established as a procedure in a routine optometric eye examination. However, clinical methods of measurement of this basic optical function have several sources of error. They are numerous and diverse, and include depth of focus, reaction time, instrument design, specification of the measurement end-point, specification of the reference point of measurement, measurement conditions, consideration of refractive error, and psychological factors. Several of these sources of inaccuracy are composed of multiple sub-sources, and many of the sub-sources influence the common methods of measurement of amplitude of accommodation. Consideration of these sources of measurement error casts doubt on the reliability of the results of measurement, on the validity of established normative values that have been produced using these methods, and on the value of reports of the results of surgery designed to restore accommodation. Clinicians can reduce the effects of some of the sources of error by modifying techniques of measurement with existing methods, but a new method may further improve accuracy
The Spatial Distribution of Star Formation in the Solar Neighbourhood: Do all stars form in clusters?
We present a global study of low mass, young stellar object (YSO) surface
densities in nearby (< 500 pc) star forming regions based on a comprehensive
collection of Spitzer Space Telescope surveys. We show that the distribution of
YSO surface densities in the solar neighbourhood is a smooth distribution,
being adequately described by a lognormal function from a few to 10^3 YSOs per
pc^2, with a peak at 22 stars/pc^2 and a dispersion of 0.85. We do not find
evidence for multiple discrete modes of star-formation (e.g. clustered and
distributed). Comparing the observed surface density distribution to previously
reported surface density threshold definitions of clusters, we find that the
fraction of stars in clusters is crucially dependent on the adopted
definitions, ranging from 40 to 90%. However, we find that only a low fraction
(< 26%) of stars are formed in dense environments where their
formation/evolution (along with their circumstellar disks and/or planets) may
be affected by the close proximity of their low-mass neighbours.Comment: 6 pages, 2 figures, MNRAS letters, accepte
Interaction between random heterogeneously charged surfaces in an electrolyte solution
We study, using Monte Carlo simulations, the interaction between infinite
heterogeneously charged surfaces inside an electrolyte solution. The surfaces
are overall neutral with quenched charged domains. An average over the quenched
disorder is performed to obtain the net force. We find that the interaction
between the surfaces is repulsive at short distances and is attractive for
larger separations
Cluster Mass Estimate and a Cusp of the Mass Density Distribution in Clusters of Galaxies
We study density cusps in the center of clusters of galaxies to reconcile
X-ray mass estimates with gravitational lensing masses. For various mass
density models with cusps we compute X-ray surface brightness distribution, and
fit them to observations to measure the range of parameters in the density
models. The Einstein radii estimated from these density models are compared
with Einstein radii derived from the observed arcs for Abell 2163, Abell 2218,
and RX J1347.5-1145. The X-ray masses and lensing masses corresponding to these
Einstein radii are also compared. While steeper cusps give smaller ratios of
lensing mass to X-ray mass, the X-ray surface brightnesses estimated from
flatter cusps are better fits to the observations. For Abell 2163 and Abell
2218, although the isothermal sphere with a finite core cannot produce giant
arc images, a density model with a central cusp can produce a finite Einstein
radius, which is smaller than the observed radii. We find that a total mass
density profile which declines as produces the largest radius
in models which are consistent with the X-ray surface brightness profile. As
the result, the extremely large ratio of the lensing mass to the X-ray mass is
improved from 2.2 to 1.4 for Abell 2163, and from 3 to 2.4 for Abell 2218. For
RX J1347.5-1145, which is a cooling flow cluster, we cannot reduce the mass
discrepancy.Comment: 23 pages, 10 figures, Latex, uses aasms4.sty, accepted for
publication in ApJ, Part
Spectroscopic properties of Young Stellar Objects in the Lupus Molecular Clouds
The results of an optical spectroscopic survey of a sample of young stellar
objects (YSOs) and pre-main sequence (PMS) stars in the Lupus Clouds are
presented. 92 objects were observed with VLT/FLAMES. All of those objects show
IR excess as discovered by the Spitzer Legacy Program "From Molecular Cores to
Planet-Forming Disks" (c2d). After reduction, 54 spectra with good
signal-to-noise ratio are spectrally classified. Effective temperatures and
luminosities are derived for these objects, and used to construct H-R diagrams
for the population. The sample consists mostly of M-type stars, with 10% K-type
stars. Individual ages and masses are inferred for the objects according to
theoretical evolutionary models. The mean population age is found to be between
3.6 and 4.4 Myr, depending on the model, while the mean mass is found to be
~0.3 M for either model. Together with literature data, the distribution of
spectral types is found to be similar to that in Chamaeleon I and IC348. The
H{\alpha} line in emission, found in 49% of the sample, is used to distinguish
between classical and weak-line T Tauri stars. 56% of the objects show
H{\alpha} in emission and are accreting T Tauri stars. Mass accretion rates
between 10-8 and 10-11 M yr-1 are determined from the full width at 10% of the
H{\alpha} peak intensity. These mass accretion rates are, within a large
scatter, consistent with the Mac \propto M 2 relation found in the literature.Comment: 13 figures, 4 tables, Accepted for publication in MNRA
Accounting for skew when post-processing MOGREPS-UK temperature forecast fields
This is the final version. Available from the American Meteorological Society via the DOI in this recordWhen statistically post-processing temperature forecasts, it is almost always assumed that
the future temperature follows a Gaussian distribution conditional on the output of an ensemble
prediction system. Recent studies, however, have demonstrated that it can at times be beneficial to employ alternative parametric families when post-processing temperature forecasts, that
are either asymmetric or heavier-tailed than the normal distribution. In this article, we compare choices of the parametric distribution used within the Ensemble Model Output Statistics
(EMOS) framework to statistically post-process 2m temperature forecast fields generated by
the Met Office’s regional, convection-permitting ensemble prediction system, MOGREPS-UK.
Specifically, we study the normal, logistic and skew-logistic distributions. A flexible alternative
is also introduced that first applies a Yeo-Johnson transformation to the temperature forecasts
prior to post-processing, so that they more readily conform to the assumptions made by established post-processing methods. It is found that accounting for the skewness of temperature
when post-processing can enhance the performance of the resulting forecast field, particularly
during summer and winter and in mountainous regions
Performance of Several Low‐Cost Accelerometers
Several groups are implementing low-cost host-operated systems of strong-motion accelerographs to support the somewhat divergent needs of seismologists and earthquake engineers. The Advanced National Seismic System Technical Implementation Committee (ANSS TIC, 2002), managed by the U.S. Geological Survey (USGS) in cooperation with other network operators, is exploring the efficacy of such systems if used in ANSS networks. To this end, ANSS convened a working group to explore available Class C strong-motion accelerometers (defined later), and to consider operational and quality control issues, and the means of annotating, storing, and using such data in ANSS networks. The working group members are largely coincident with our author list, and this report informs instrument-performance matters in the working group’s report to ANSS. Present examples of operational networks of such devices are the Community Seismic Network (CSN; csn.caltech.edu), operated by the California Institute of Technology, and Quake-Catcher Network (QCN; Cochran et al., 2009; qcn.stanford.edu; November 2013), jointly operated by Stanford University and the USGS. Several similar efforts are in development at other institutions. The overarching goals of such efforts are to add spatial density to existing Class-A and Class-B (see next paragraph) networks at low cost, and to include many additional people so they become invested in the issues of earthquakes, their measurement, and the damage they cause
1,1′-Fc(4-C6H4CO2Et)2and its unusual salt derivative withZ′ = 5,catena-[Na+]2[1,1′-Fc(4-C6H4CO2−)2]·0.6H2O [1,1′-Fc = (η5-(C5H4)2Fe]
The neutral diethyl 4,4'-(ferrocene-1,1'-diyl)dibenzoate, Fe[[eta]5-(C5H4)(4-C6H4CO2Et)]2 (I), yields (II) (following base hydrolysis) as the unusual complex salt poly[disodium bis[diethyl 4,4'-(ferrocene-1,1'-diyl)dibenzoate] 0.6-hydrate] or [Na+]2[Fe{[eta]5-(C5H4)-4-C6H4CO_2^-}2]·0.6H2O with Z' = 5. Compound (I) crystallizes in the triclinic system, space group P\bar 1, with two molecules having similar geometry in the asymmetric unit (Z' = 2). The salt complex (II) crystallizes in the orthorhombic system, space group Pbca, with the asymmetric unit comprising poly[decasodium pentakis[diethyl 4,4'-(ferrocene-1,1'-diyl)dibenzoate] trihydrate] or [Na+]10[Fe{[eta]5-(C5H4)-4-C6H4CO_2^-}2]5·3H2O. The five independent 1,1'-Fc[(4-C6H4CO2)-]2 dianions stack in an offset ladder (stepped) arrangement with the ten benzoates mutually oriented cisoid towards and bonded to a central layer comprising the ten Na+ ions and three water molecules [1,1'-Fc = [eta]5-(C5H4)2Fe]. The five dianions differ in the cisoid orientations of their pendant benzoate groups, with four having their -C6H4- groups mutually oriented at interplanar angles from 0.6 (3) to 3.2 (3)° (as [pi]...[pi] stacked C6 rings) and interacting principally with Na+ ions. The fifth dianion is distorted and opens up to an unprecedented -C6H4- interplanar angle of 18.6 (3)° through bending of the two 4-C6H4CO2 groups and with several ionic interactions involving the three water molecules (arranged as one-dimensional zigzag chains in the lattice). Overall packing comprises two-dimensional layers of Na+ cations coordinated mainly by the carboxylate O atoms, and one-dimensional water chains. The non-polar Fc(C6H4)2 groups are arranged perpendicular to the layers and mutually interlock through a series of efficient C-H...[pi] stacking contacts in a herringbone fashion to produce an overall segregation of polar and non-polar entities
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