294 research outputs found
Collective resonances in plasmonic crystals: Size matters
Periodic arrays of metallic nanoparticles may sustain Surface Lattice
Resonances (SLRs), which are collective resonances associated with the
diffractive coupling of Localized Surface Plasmon Resonances (LSPRs). By
investigating a series of arrays with varying number of particles, we traced
the evolution of SLRs to its origins. Polarization resolved extinction spectra
of arrays formed by a few nanoparticles were measured, and found to be in very
good agreement with calculations based on a coupled dipole model. Finite size
effects on the optical properties of the arrays are observed, and our results
provide insight into the characteristic length scales for collective plasmonic
effects: for arrays smaller than 5 x 5 particles, the Q-factors of SLRs are
lower than those of LSPRs; for arrays larger than 20 x 20 particles, the
Q-factors of SLRs saturate at a much larger value than those of LSPRs; in
between, the Q-factors of SLRs are an increasing function of the number of
particles in the array.Comment: 4 figure
Assessing how disruption of methanogenic communities and their syntrophic relationships in tidal freshwater marshes via saltwater intrusion may affect CH4 emissions
Tidal freshwater wetlands (TFW), which lie at the interface of saltwater and freshwater ecosystems, are predicted to experience moderate salinity increases due to sea level rise. Increases in salinity generally suppress CH4 production, but it is uncertain to what extent elevated salinity will affect CH4 cycling in TFW. It is also unknown whether CH4 production will resume when freshwater conditions return. The ability to produce CH4 is limited to a monophyletic group of the Euryarchaeota phylum called methanogens (MG), who are limited to a small number of substrates (e.g., acetate, H2, and formate) produced from the breakdown of fermentation products. In freshwater anaerobic soils, the degradation of certain fermentation products (e.g., butyrate, propionate) is only energetically favorable when their catabolic byproduct, H2 or formate, is consumed to low concentrations by MGs. This is considered a form of obligate syntrophy. Sulfate reducing bacteria (SRB) are capable of utilizing a larger variety of substrates than MG, including substrates degraded by methanogenic syntrophy (e.g., butyrate, propionate). The introduction of sulfate (SO4 -2) into TFW via saltwater intrusion events may allow SRB to disrupt syntrophic relationships between hydrogenotrophic MG and syntrophic fermenters. This may select for MG taxa that differ in their rate of CH4 production. The objectives of this study were to determine the effect of oligohaline SO4 -2 concentrations on MG community functions (i.e., CH4 production and syntrophic butyrate degradation); and, to assess whether these functions recover after competition with SRB has been removed
Computations for the 16-foot transonic tunnel, NASA, Langley Research Center, revision 1
The equations used by the 16 foot transonic tunnel in the data reduction programs are presented in eight modules. Each module consists of equations necessary to achieve a specific purpose. These modules are categorized in the following groups: tunnel parameters; jet exhaust measurements; skin friction drag; balance loads and model attitudes calculations; internal drag (or exit-flow distributions); pressure coefficients and integrated forces; thrust removal options; and turboprop options. This document is a companion document to NASA TM-83186, A User's Guide to the Langley 16 Foot Transonic Tunnel, August 1981
Counts-in-Cylinders in the Sloan Digital Sky Survey with Comparisons to N-body Simulations
Environmental statistics provide a necessary means of comparing the
properties of galaxies in different environments and a vital test of models of
galaxy formation within the prevailing, hierarchical cosmological model. We
explore counts-in-cylinders, a common statistic defined as the number of
companions of a particular galaxy found within a given projected radius and
redshift interval. Galaxy distributions with the same two-point correlation
functions do not necessarily have the same companion count distributions. We
use this statistic to examine the environments of galaxies in the Sloan Digital
Sky Survey, Data Release 4. We also make preliminary comparisons to four models
for the spatial distributions of galaxies, based on N-body simulations, and
data from SDSS DR4 to study the utility of the counts-in-cylinders statistic.
There is a very large scatter between the number of companions a galaxy has and
the mass of its parent dark matter halo and the halo occupation, limiting the
utility of this statistic for certain kinds of environmental studies. We also
show that prevalent, empirical models of galaxy clustering that match observed
two- and three-point clustering statistics well fail to reproduce some aspects
of the observed distribution of counts-in-cylinders on 1, 3 and 6-Mpc/h scales.
All models that we explore underpredict the fraction of galaxies with few or no
companions in 3 and 6-Mpc/h cylinders. Roughly 7% of galaxies in the real
universe are significantly more isolated within a 6 Mpc/h cylinder than the
galaxies in any of the models we use. Simple, phenomenological models that map
galaxies to dark matter halos fail to reproduce high-order clustering
statistics in low-density environments.Comment: 17 pages, 10 figures. Accepted, Ap
Evaluating the Need for Preoperative MRI Before Primary Hip Arthroscopy in Patients 40 Years and Younger With Femoroacetabular Impingement Syndrome: A Multicenter Comparative Analysis
BACKGROUND: Routine hip magnetic resonance imaging (MRI) before arthroscopy for patients with femoroacetabular impingement syndrome (FAIS) offers questionable clinical benefit, delays surgery, and wastes resources.
PURPOSE: To assess the clinical utility of preoperative hip MRI for patients aged ≤40 years who were undergoing primary hip arthroscopy and who had a history, physical examination findings, and radiographs concordant with FAIS.
STUDY DESIGN: Cohort study; Level of evidence, 3.
METHODS: Included were 1391 patients (mean age, 25.8 years; 63% female; mean body mass index, 25.6) who underwent hip arthroscopy between August 2015 and December 2021 by 1 of 4 fellowship-trained hip surgeons from 4 referral centers. Inclusion criteria were FAIS, primary surgery, and age ≤40 years. Exclusion criteria were MRI contraindication, reattempt of nonoperative management, and concomitant periacetabular osteotomy. Patients were stratified into those who were evaluated with preoperative MRI versus those without MRI. Those without MRI received an MRI before surgery without deviation from the established surgical plan. All preoperative MRI scans were compared with the office evaluation and intraoperative findings to assess agreement. Time from office to arthroscopy and/or MRI was recorded. MRI costs were calculated.
RESULTS: Of the study patients, 322 were not evaluated with MRI and 1069 were. MRI did not alter surgical or interoperative plans. Both groups had MRI findings demonstrating anterosuperior labral tears treated intraoperatively (99.8% repair, 0.2% debridement, and 0% reconstruction). Compared with patients who were evaluated with MRI and waited 63.0 ± 34.6 days, patients who were not evaluated with MRI underwent surgery 6.5 ± 18.7 days after preoperative MRI. MRI delayed surgery by 24.0 ± 5.3 days and cost a mean $2262 per patient.
CONCLUSION: Preoperative MRI did not alter indications for primary hip arthroscopy in patients aged ≤40 years with a history, physical examination findings, and radiographs concordant with FAIS. Rather, MRI delayed surgery and wasted resources. Routine hip MRI acquisition for the younger population with primary FAIS with a typical presentation should be challenged
Close Galaxy Counts as a Probe of Hierarchical Structure Formation
Standard LCDM predicts that the major merger rate of galaxy-size dark matter
halos rises rapidly with redshift. The average number of close companions per
galaxy, Nc, is often used to infer the galaxy merger rate, however, recent
observational studies suggest that Nc evolves very little with redshift. Here
we use a "hybrid" N- body simulation plus analytic substructure model to
predict Nc directly. We identify dark matter subhalos with galaxies and show
that the observed lack of close pair count evolution arises because the high
merger rate per halo at early times is counteracted by a decrease in the number
of halos massive enough to host a galaxy pair. We compare our results to data
compiled from the DEEP2, SSRS2, and the UZC redshift surveys. Observed pair
counts match our predictions if we assume a monotonic mapping between galaxy
luminosity and the maximum circular velocity that each subhalo had when it was
first accreted onto its host halo. This suggests that satellite galaxies are
significantly more resilient to destruction than are dissipationless dark
matter subhalos. We argue that while Nc does not provide a direct measure of
the halo merger rate, it offers a powerful means to constrain the Halo
Occupation Distribution and the spatial distribution of galaxies within halos.
Interpreted in this way, close pair counts provide a useful test of galaxy
formation processes on < 100 kpc scales.Comment: 16 pages, 16 figures, minor change to figure 10, figure captions
updated, typos corrected, Figure 4 corrected, version accepted for
publication by Ap
Tradeoffs in jet inlet design: a historical perspective
The design of the inlet(s) is one of the most demanding tasks of the development process of any gas turbine-powered aircraft. This is mainly due to the multi-objective and multidisciplinary nature of the exercise. The solution is generally a compromise between a number of conflicting goals and these conflicts are the subject of the present paper. We look into how these design tradeoffs have been reflected in the actual inlet designs over the years and how the emphasis has shifted from one driver to another. We also review some of the relevant developments of the jet age in aerodynamics and design and manufacturing technology and we examine how they have influenced and informed inlet design decision
Galactic populations of radio and gamma-ray pulsars in the polar cap model
We simulate the characteristics of the Galactic population of radio and
-ray pulsars using Monte Carlo techniques. At birth, neutron stars are
spatially distributed in the Galactic disk, with supernova-kick velocities, and
randomly dispersed in age back to years. They are evolved in the
Galactic gravitational potential to the present time. From a radio luminosity
model, the radio flux is filtered through a selected set of radio-survey
parameters. -ray luminosities are assigned using the features of recent
polar cap acceleration models invoking space-charge-limited flow, and a pulsar
death valley further attenuates the population of radio-loud pulsars. Assuming
a simple emission geometry with aligned radio and -ray beams of 1
steradian solid angle, our model predicts that EGRET should have seen 7
radio-loud and 1 radio-quiet, -ray pulsars. With much improved
sensitivity, GLAST, on the other hand, is expected to observe 76 radio-loud and
74 radio-quiet, -ray pulsars of which 7 would be identified as pulsed
sources. We also explore the effect of magnetic field decay on the
characteristics of the radio and -ray pulsar populations. Including
magnetic field decay on a timescale of 5 Myr improves agreement with the radio
pulsar population and increases the predicted number of GLAST detected pulsars
to 90 radio-loud and 101 radio-quiet (9 pulsed) -ray pulsars. The lower
flux threshold allows GLAST to detect -ray pulsars at larger distances
than those observed by the radio surveys used in this study.Comment: 38 pages, 11 figures, accepted for publication v565 n1 Ap
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