14,604 research outputs found
Shock transmission in coupled beams and rib stiffened structures
Shock transmission in a simple coupled beam structure and in a ring-stringer stiffened cylinder is investigated experimentally and analytically using wave transmission and statistical energy analysis concepts. The use of the response spectrum to characterize the excitation provided to a simple beam by a force pulse is studied. Analysis of the transmission of a dilatation wave in a periodically stiffened plate indicates that the stiffeners are fairly transparent to the wave, but some of the dilatational energy is scattered into bending at each support
Rate dependent shear bands in a shear transformation zone model of amorphous solids
We use Shear Transformation Zone (STZ) theory to develop a deformation map
for amorphous solids as a function of the imposed shear rate and initial
material preparation. The STZ formulation incorporates recent simulation
results [Haxton and Liu, PRL 99 195701 (2007)] showing that the steady state
effective temperature is rate dependent. The resulting model predicts a wide
range of deformation behavior as a function of the initial conditions,
including homogeneous deformation, broad shear bands, extremely thin shear
bands, and the onset of material failure. In particular, the STZ model predicts
homogeneous deformation for shorter quench times and lower strain rates, and
inhomogeneous deformation for longer quench times and higher strain rates. The
location of the transition between homogeneous and inhomogeneous flow on the
deformation map is determined in part by the steady state effective
temperature, which is likely material dependent. This model also suggests that
material failure occurs due to a runaway feedback between shear heating and the
local disorder, and provides an explanation for the thickness of shear bands
near the onset of material failure. We find that this model, which resolves
dynamics within a sheared material interface, predicts that the stress weakens
with strain much more rapidly than a similar model which uses a single state
variable to specify internal dynamics on the interface.Comment: 10 pages, 13 figures, corrected typos, added section on rate
strengthening vs. rate weakening material
A Serendipitous Search for High-Redshift Lyman alpha Emission: Two Primeval Galaxy Candidates at z~3
In the course of our ongoing search for serendipitous high-redshift Lyman
alpha (LyA) Emissionin deep archival Keck spectra, we discovered two very high
equivalent width (W_{obs} ~ 450A, 2-sigma) LyA emission line candidates at z ~3
in a moderate dispersion (R~1200) spectrogram. Both lines have low velocity
dispersions (sigma_v ~ 60 km/s) and deconvolved radii r ~ 1 kpc (h = 0.5). We
argue that the lines are LyA, and are powered by stellar ionization. The
surface density of robust, high equivalent width LyA candidates is estimated to
be ~3 \pm 2 per arcmin^2 per unit redshift at z ~ 3, consistent with the
estimate of Cowie etal (1998). The LyA emission line source characteristics are
consistent with the galaxies undergoing their first burst of star formation,
ie, with being primeval. Source sizes and velocity dispersions are comparable
to the theoretical primeval galaxy model of Lin and Murray (1992) based on the
inside-out, self-similar collapse of an isothermal sphere. In this model, star
formation among field galaxies is a protracted process. Galaxies are thought to
be able to display high equivalent widths for only the first few x 10 Myr. This
time is short in relation to the difference in look back times between z=3 and
z=4, and implies that a substantial fraction of strong line-emitting galaxies
at z=3 were formed at redshifts z < 4. We discuss the significance of
high-equivalent width LyA-emitting galaxies in terms of the emerging picture of
the environment, and the specific characteristics of primeval galaxy formation
at high redshift.Comment: 17 pages, 3 figures, one table. To appear in the Astrophysical
Journa
A five year record of high-frequency in situ measurements of non-methane hydrocarbons at Mace Head, Ireland
Continuous high-frequency in situ measurements of a range of non-methane hydrocarbons have been made at Mace Head since January 2005. Mace Head is a background Northern Hemispheric site situated on the eastern edge of the Atlantic. Five year measurements (2005–2009) of six C<sub>2</sub>–C<sub>5</sub> non-methane hydrocarbons have been separated into baseline Northern Hemispheric and European polluted air masses, among other sectors. Seasonal cycles in baseline Northern Hemispheric air masses and European polluted air masses arriving at Mace Head have been studied. Baseline air masses show a broad summer minima between June and September for shorter lived species, longer lived species show summer minima in July/August. All species displayed a winter maxima in February. European air masses showed baseline elevated mole fractions for all non-methane hydrocarbons. Largest elevations (of up to 360 ppt for ethane maxima) from baseline data were observed in winter maxima, with smaller elevations observed during the summer. Analysis of temporal trends using the Mann-Kendall test showed small (<6 % yr<sup>&minus;1</sup>) but statistically significant decreases in the butanes and <i>i</i>-pentane between 2005 and 2009 in European air. No significant trends were found for any species in baseline air
Strain localization in a shear transformation zone model for amorphous solids
We model a sheared disordered solid using the theory of Shear Transformation
Zones (STZs). In this mean-field continuum model the density of zones is
governed by an effective temperature that approaches a steady state value as
energy is dissipated. We compare the STZ model to simulations by Shi, et
al.(Phys. Rev. Lett. 98 185505 2007), finding that the model generates
solutions that fit the data,exhibit strain localization, and capture important
features of the localization process. We show that perturbations to the
effective temperature grow due to an instability in the transient dynamics, but
unstable systems do not always develop shear bands. Nonlinear energy
dissipation processes interact with perturbation growth to determine whether a
material exhibits strain localization. By estimating the effects of these
interactions, we derive a criterion that determines which materials exhibit
shear bands based on the initial conditions alone. We also show that the shear
band width is not set by an inherent diffusion length scale but instead by a
dynamical scale that depends on the imposed strain rate.Comment: 8 figures, references added, typos correcte
Photon collection from a trapped ion--cavity system
We present the design and implementation of a trapped ion cavity QED system.
A single ytterbium ion is confined by a micron-scale ion trap inside a 2 mm
optical cavity. The ion is coherently pumped by near resonant laser light while
the cavity output is monitored as a function of pump intensity and cavity
detuning. We observe a Purcell enhancement of scattered light into the solid
angle subtended by the optical cavity, as well as a three-peak structure
arising from strongly driving the atom. This system can be integrated into
existing atom{photon quantum network protocols and is a pathway towards an
efficient atom{photon quantum interface
A variant in LIN28B is associated with 2D:4D finger-length ratio, a putative retrospective biomarker of prenatal testosterone exposure
The ratio of the lengths of an individual's second to fourth digit (2D:4D) is commonly used as a noninvasive retrospective biomarker for prenatal androgen exposure. In order to identify the genetic determinants of 2D:4D, we applied a genome-wide association approach to 1507 11-year-old children from the Avon Longitudinal Study of Parents and Children (ALSPAC) in whom 2D:4D ratio had been measured, as well as a sample of 1382 12- to 16-year-olds from the Brisbane Adolescent Twin Study. A meta-analysis of the two scans identified a single variant in the LIN28B gene that was strongly associated with 2D:4D (rs314277: p = 4.1 108) and was subsequently independently replicated in an additional 3659 children from the ALSPAC cohort (p = 1.53 106). The minor allele of the rs314277 variant has previously been linked to increased height and delayed age at menarche, but in our study it was associated with increased 2D:4D in the direction opposite to that of previous reports on the correlation between 2D:4D and age at menarche. Our findings call into question the validity of 2D:4D as a simplistic retrospective biomarker for prenatal testosterone exposure
Temperature Effects on Threshold Counterion Concentration to Induce Aggregation of fd Virus
We seek to determine the mechanism of like-charge attraction by measuring the
temperature dependence of critical divalent counterion concentration
() for the aggregation of fd viruses. We find that an increase in
temperature causes to decrease, primarily due to a decrease in the
dielectric constant () of the solvent. At a constant ,
is found to increase as the temperature increases. The effects of
and on can be combined to that of one parameter:
Bjerrum length (). decreases exponentially as
increases, suggesting that entropic effect of counterions plays an important
role at the onset of bundle formation.Comment: 12 pages, 3 figure
Self-starting optical–electrical–optical homodyne clock recovery for phase-modulated signals
We propose a novel self-homodyne optical–electrical–optical clock recovery technique for binary phase-shift keying (BPSK) signals using commercial optical and electrical components. We present the principle of operation as well as a proof-of-concept experiment for a 10.7 Gb/s BPSK signal clock recovery transmitted over a dispersion-compensated link of 20 km of single-mode fiber. Suppression of pattern-related frequency noise at the output of the recovered clock is shown. The timing jitter of the recovered clock at 10.7 GHz was measured to be ∼450 fs (integration range: 100 Hz–10 MHz)
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Using Paired Depth Interviews to Collect Qualitative Data
In this manuscript, the authors discuss using paired depth interviews as a method of collecting qualitative data.Paired depth interviewing—also known as paired interviewing—is defined as one researcher interviewing two people together (Houssart & Evens, 2011) for the purposes of collecting information about how the pair perceives the same event or phenomenon (Arksey, 1996). Although this form of interviewing has much potential as a data collection tool, it has received scant attention in the qualitative research literature. Thus, the purpose of this article is to provide a framework for using paired depth interviews as a method of collecting qualitative data. In this manuscript, we define and describe paired depth interviews, discuss conceptualizations of paired depth interviews using Roulston’s (2010) framework, delineate the strengths and limitations of paired depth interviews, and provide examples of paired depth interviews utilized in helping professions. Furthermore, we present a case study of original work that illustrates the utility of paired depth interviews and provide suggestions for future directions for paired depth interviews
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