7,150 research outputs found
Turbulence measurements in a swirling confined jet flowfield using a triple hot-wire probe
An axisymmetric swirling confined jet flowfield, similar to that encountered in gas turbine combustors was investigated using a triple hot-wire probe. The raw data from the three sensors were digitized using ADC's and stored on a Tektronix 4051 computer. The data were further reduced on the computer to obtain time-series for the three instantaneous velocity components in the flowfield. The time-mean velocities and the turbulence quantities were deduced. Qualification experiments were performed and where possible results compared with independent measurements. The major qualification experiments involved measurements performed in a non-swirling flow compared with conventional X-wire measurements. In the swirling flowfield, advantages of the triple wire technique over the previously used multi-position single hot-wire method are noted. The measurements obtained provide a data base with which the predictions of turbulence models in a recirculating swirling flowfield can be evaluated
Molecular dynamics simulations of an anomalous response of diamond to shock compression
We performed molecular dynamics simulations of shock wave propagation in diamond in the [110] crystallographic direction and observed an anomalous response of the material. This regime is characterized by absence of plastic deformation in the intermediate interval of shock wave intensities between shear-deformation and overdriven rehybridization shock wave regimes
PSR J1453+1902 and the radio luminosities of solitary versus binary millisecond pulsars
We present 3 yr of timing observations for PSR J1453+1902, a 5.79-ms pulsar
discovered during a 430-MHz drift-scan survey with the Arecibo telescope. Our
observations show that PSR J1453+1902 is solitary and has a proper motion of
8(2) mas/yr. At the nominal distance of 1.2 kpc estimated from the pulsar's
dispersion measure, this corresponds to a transverse speed of 46(11) km/s,
typical of the millisecond pulsar population. We analyse the current sample of
55 millisecond pulsars in the Galactic disk and revisit the question of whether
the luminosities of isolated millisecond pulsars are different from their
binary counterparts. We demonstrate that the apparent differences in the
luminosity distributions seen in samples selected from 430-MHz surveys can be
explained by small-number statistics and observational selection biases. An
examination of the sample from 1400-MHz surveys shows no differences in the
distributions. The simplest conclusion from the current data is that the spin,
kinematic, spatial and luminosity distributions of isolated and binary
millisecond pulsars are consistent with a single homogeneous population.Comment: 8 pages, 5 figures and 3 tables, accepted for publication by MNRA
Induced antiferromagnetism and large magnetoresistances in RuSr2(Nd,Y,Ce)2Cu2O10-d ruthenocuprates
RuSr2(Nd,Y,Ce)2Cu2O10-d ruthenocuprates have been studied by neutron
diffraction, magnetotransport and magnetisation measurements and the electronic
phase diagram is reported. Separate Ru and Cu spin ordering transitions are
observed, with spontaneous Cu antiferromagnetic order for low hole doping
levels p, and a distinct, induced-antiferromagnetic Cu spin phase in the 0.02 <
p < 0.06 pseudogap region. This ordering gives rise to large negative
magnetoresistances which vary systematically with p in the
RuSr2Nd1.8-xY0.2CexCu2O10-d series. A collapse of the magnetoresistance (MR)
and magnetisation in the pre-superconducting region may signify the onset of
superconducting fluctuations.Comment: 22 pages, 11 figure
Sensitivity studies for r-process nucleosynthesis in three astrophysical scenarios
In rapid neutron capture, or r-process, nucleosynthesis, heavy elements are
built up via a sequence of neutron captures and beta decays that involves
thousands of nuclei far from stability. Though we understand the basics of how
the r-process proceeds, its astrophysical site is still not conclusively known.
The nuclear network simulations we use to test potential astrophysical
scenarios require nuclear physics data (masses, beta decay lifetimes, neutron
capture rates, fission probabilities) for all of the nuclei on the neutron-rich
side of the nuclear chart, from the valley of stability to the neutron drip
line. Here we discuss recent sensitivity studies that aim to determine which
individual pieces of nuclear data are the most crucial for r-process
calculations. We consider three types of astrophysical scenarios: a traditional
hot r-process, a cold r-process in which the temperature and density drop
rapidly, and a neutron star merger trajectory.Comment: 8 pages, 4 figures, submitted to the Proceedings of the International
Nuclear Physics Conference (INPC) 201
Ultracold, radiative charge transfer in hybrid Yb ion - Rb atom traps
Ultracold hybrid ion-atom traps offer the possibility of microscopic
manipulation of quantum coherences in the gas using the ion as a probe.
However, inelastic processes, particularly charge transfer can be a significant
process of ion loss and has been measured experimentally for the Yb ion
immersed in a Rb vapour. We use first-principles quantum chemistry codes to
obtain the potential energy curves and dipole moments for the lowest-lying
energy states of this complex. Calculations for the radiative decay processes
cross sections and rate coefficients are presented for the total decay
processes. Comparing the semi-classical Langevin approximation with the quantum
approach, we find it provides a very good estimate of the background at higher
energies. The results demonstrate that radiative decay mechanisms are important
over the energy and temperature region considered. In fact, the Langevin
process of ion-atom collisions dominates cold ion-atom collisions. For spin
dependent processes \cite{kohl13} the anisotropic magnetic dipole-dipole
interaction and the second-order spin-orbit coupling can play important roles,
inducing couplingbetween the spin and the orbital motion. They measured the
spin-relaxing collision rate to be approximately 5 orders of magnitude higher
than the charge-exchange collision rate \cite{kohl13}. Regarding the measured
radiative charge transfer collision rate, we find that our calculation is in
very good agreement with experiment and with previous calculations.
Nonetheless, we find no broad resonances features that might underly a strong
isotope effect. In conclusion, we find, in agreement with previous theory that
the isotope anomaly observed in experiment remains an open question.Comment: 7 figures, 1 table accepted for publication in J. Phys. B: At. Mol.
Opt. Phys. arXiv admin note: text overlap with arXiv:1107.114
Predicting the minimum liquid surface tension activity of pseudomonads expressing biosurfactants
Bacteria produce a variety of biosurfactants capable of significantly reducing liquid (aqueous) surface tension (γ) with a range of biological roles and biotechnological uses. In order to determine the lowest achievable surface tension (γMin), we tested a diverse collection of Pseudomonas-like isolates from contaminated soil and activated sludge, and identified those expressing biosurfactants by drop-collapse assay. Liquid surface tension reducing ability was quantitatively determined by tensiometry, with 57 isolates found to significantly lower culture supernatant surface tensions to 24.5 – 49.1 mN m−1. Differences in biosurfactant behaviour determined by foaming, emulsion and oil-displacement assays, was also observed amongst isolates producing surface tensions of 25 – 27 mN m−1, suggesting that a range of structurally-diverse biosurfactants were being expressed. Individual distribution identification (IDI) analysis was used to identify the theoretical probability distribution that best fitted the surface tension data, which predicted a γMin of 24.24 mN m−1. This was in agreement with predictions based on earlier work of published mixed–bacterial spp. data, suggesting a fundamental limit to the ability of bacterial biosurfactants to reduce surface tensions in aqueous systems. This implies a biological restriction on the synthesis and export of these agents or a physical-chemical restriction on their functioning once produced
The Double Pulsar Eclipses I: Phenomenology and Multi-frequency Analysis
The double pulsar PSR J0737-3039A/B displays short, 30 s eclipses that arise
around conjunction when the radio waves emitted by pulsar A are absorbed as
they propagate through the magnetosphere of its companion pulsar B. These
eclipses offer a unique opportunity to probe directly the magnetospheric
structure and the plasma properties of pulsar B. We have performed a
comprehensive analysis of the eclipse phenomenology using multi-frequency radio
observations obtained with the Green Bank Telescope. We have characterized the
periodic flux modulations previously discovered at 820 MHz by McLaughlin et
al., and investigated the radio frequency dependence of the duration and depth
of the eclipses. Based on their weak radio frequency evolution, we conclude
that the plasma in pulsar B's magnetosphere requires a large multiplicity
factor (~ 10^5). We also found that, as expected, flux modulations are present
at all radio frequencies in which eclipses can be detected. Their complex
behavior is consistent with the confinement of the absorbing plasma in the
dipolar magnetic field of pulsar B as suggested by Lyutikov & Thompson and such
a geometric connection explains that the observed periodicity is harmonically
related to pulsar B's spin frequency. We observe that the eclipses require a
sharp transition region beyond which the plasma density drops off abruptly.
Such a region defines a plasmasphere which would be well inside the
magnetospheric boundary of an undisturbed pulsar. It is also two times smaller
than the expected standoff radius calculated using the balance of the wind
pressure from pulsar A and the nominally estimated magnetic pressure of pulsar
B.Comment: 9 pages, 7 figures, 3 tables, ApJ in pres
Low temperature magnetic transition in RuSr2EuCeCu2O10 ruthenocuprate
A new magnetic transition in the ruthenocuprate parent compound
RuSr2EuCeCu2O10 has been observed below 10 K. It shows up only as a kink in the
imaginary part of the ac susceptibility and exhibits a pronounced frequency
dependence. At the same time, the real part of the ac susceptibility and the dc
magnetization study show very little change in the same temperature window
suggesting only a minor fraction of the material to be involved in the
transition. Frequency dependence shows excellent agreement with the predictions
of the Arrhenius law known to describe well the dynamics of the
superparamagnetic particles. The same type of the investigation on the
RuSr2Eu1.1Ce0.9Cu2O10 composition showed no evidence of the similar transition,
which points to a possible intrinsic behavior.Comment: to be published in Physica
Pulsar Science with the Green Bank 43m Telescope
The 43m telescope at the NRAO site in Green Bank, WV has recently been
outfitted with a clone of the Green Bank Ultimate Pulsar Processing Instrument
(GUPPI \cite{Ransom:2009}) backend, making it very useful for a number of
pulsar related studies in frequency ranges 800-1600 MHz and 220-440 MHz. Some
of the recent science being done with it include: monitoring of the Crab
pulsar, a blind search for transient sources, pulsar searches of targets of
opportunity, and an all-sky mapping project. For the Crab monitoring project,
regular observations are searched for giant pulses (GPs), which are then
correlated with -ray photons from the \emph{Fermi} spacecraft. Data
from the all-sky mapping project are first run through a pipeline that does a
blind transient search, looking for single pulses over a DM range of 0-500
pc~cm. These projects are made possible by MIT Lincoln Labs.Comment: 2 pages, 1 figure, to appear in AIP Conference Proceedings of Pulsar
Conference 2010 "Radio Pulsars: a key to unlock the secrets of the Universe",
Sardinia, October 201
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