711 research outputs found
Inherent-Structure Dynamics and Diffusion in Liquids
The self-diffusion constant D is expressed in terms of transitions among the
local minima of the potential (inherent structure, IS) and their correlations.
The formulae are evaluated and tested against simulation in the supercooled,
unit-density Lennard-Jones liquid. The approximation of uncorrelated
IS-transition (IST) vectors, D_{0}, greatly exceeds D in the upper temperature
range, but merges with simulation at reduced T ~ 0.50. Since uncorrelated IST
are associated with a hopping mechanism, the condition D ~ D_{0} provides a new
way to identify the crossover to hopping. The results suggest that theories of
diffusion in deeply supercooled liquids may be based on weakly correlated IST.Comment: submitted to PR
Progressive internal gravity waves with bounded upper surface climbing a triangular obstacle
In this paper we discuss a theoretical model for the interfacial profiles of
progressive non-linear waves which result from introducing a triangular
obstacle, of finite height, attached to the bottom below the flow of a
stratified, ideal, two layer fluid, bounded from above by a rigid boundary. The
derived equations are solved by using a nonlinear perturbation method. The
dependence of the interfacial profile on the triangular obstacle size, as well
as its dependence on some flow parameters, such as the ratios of depths and
densities of the two fluids, have been studied
Assessment of the Performance Potential of Advanced Subsonic Transport Concepts for NASA's Environmentally Responsible Aviation Project
NASA's Environmentally Responsible Aviation (ERA) project has matured technologies to enable simultaneous reductions in fuel burn, noise, and nitrogen oxide (NOx) emissions for future subsonic commercial transport aircraft. The fuel burn reduction target was a 50% reduction in block fuel burn (relative to a 2005 best-in-class baseline aircraft), utilizing technologies with an estimated Technology Readiness Level (TRL) of 4-6 by 2020. Progress towards this fuel burn reduction target was measured through the conceptual design and analysis of advanced subsonic commercial transport concepts spanning vehicle size classes from regional jet (98 passengers) to very large twin aisle size (400 passengers). Both conventional tube-and-wing (T+W) concepts and unconventional (over-wing-nacelle (OWN), hybrid wing body (HWB), mid-fuselage nacelle (MFN)) concepts were developed. A set of propulsion and airframe technologies were defined and integrated onto these advanced concepts which were then sized to meet the baseline mission requirements. Block fuel burn performance was then estimated, resulting in reductions relative to the 2005 best-in-class baseline performance ranging from 39% to 49%. The advanced single-aisle and large twin aisle T+W concepts had reductions of 43% and 41%, respectively, relative to the 737-800 and 777-200LR aircraft. The single-aisle OWN concept and the large twin aisle class HWB concept had reductions of 45% and 47%, respectively. In addition to their estimated fuel burn reduction performance, these unconventional concepts have the potential to provide significant noise reductions due, in part, to engine shielding provided by the airframe. Finally, all of the advanced concepts also have the potential for significant NOx emissions reductions due to the use of advanced combustor technology. Noise and NOx emissions reduction estimates were also generated for these concepts as part of the ERA project
Single ultrafast diffusive conduction based optoelectronic switch for multi-channel operation
For multi-channel optical switching, we report single ultrafast diffusive conduction based optoelectronic switches that accommodate >100 optical channels (with 2,000mm-2 channel density and <10% crosstalk), on 300μm×300μm devices with switching bandwidths of >50GHz. © 2005 IEEE
Influence of intensive melt shearing on the microstructure and mechanical properties of an Al-Mg alloy with high added impurity content
The official published version can be accessed from the link below - Copyright @ The Minerals, Metals & Materials Society and ASM International 2011We have investigated the influence of melt conditioning by intensive shearing on the mechanical behavior and microstructure of Al-Mg-Mn-Fe-Cu-Si alloy sheet produced from a small book mold ingot with high added impurity content. The melt conditioned ingot has fine grains throughout its cross section, whereas a conventionally cast ingot, without melt shearing, has coarser grains and shows a wider variation of grain size. Both needle-shaped and coarse Chinese script iron bearing intermetallic particles are found in the microstructure at the center of the conventionally processed ingot, but for the melt conditioned ingot, only fine Chinese script intermetallic particles are observed. In addition to the iron bearing intermetallics, Mg2Si particles are also observed. The ingots were rolled to thin sheet and solution heat treated (SHT). During rolling, the iron-based intermetallics and Mg2Si particles are broken and aligned along the rolling direction. Yield strength (YS), ultimate tensile strength (UTS), and elongation of the intensively melt sheared and processed sheet are all improved compared to the conventionally cast and processed sheet. Fractographic analysis of the tensile fracture surfaces shows that the clustered and coarse iron bearing intermetallic particles are responsible for the observed reduction in mechanical properties of the conventionally cast sheet. We have shown that by refining the initial microstructure of the ingot by intensive shear melt conditioning, it is possible to achieve improved mechanical properties at the final sheet gage of an AlMgMn alloy with a high content of impurities.This study is under the Technology
Strategy Board funded REALCAR projec
Vegetation history and climatic fluctuations on a transect along the Dead Sea west shore and impact on past societies over the last 3500 years.
This study represents the vegetation history of the last 3500 years and conducts an analysis of the climatic fluctuations on a 75 km long transect on the western Dead Sea shore. Palynological and sedimentological data are available from six cores near Mount Sedom, Ein Boqueq, and Ein Gedi and from outcrops near Ze'elim and Ein Feshkha. The comparison of the pollen data with the lake levels shows synchronous trends. During the Middle Bronze Age, Iron Age and Hellenistic to Byzantine Period the high lake level of the Dead Sea signals an increase in precipitation. Contemporaneously, values of cultivated plants indicate an increase in agriculture. Lake level is low during the Late Bronze Age, within the Iron Age and at the end of the Byzantine period, indicating dry periods when all pds show a decrease of cultivated plants. Forest regeneration led by drought-resistant pines is observed in all pollen diagrams (pds) following the agricultural decline in the Byzantine period and, in the pds near Ein Boqeq, Ze'elim and Ein Feshkha, during the late Iron Age. The modern vegetation gradient is reflected in the palaeo-records: a stronger expansion of Mediterranean vegetation and cultivated plants in the northern sites is recognisable
Prospects for the CERN Axion Solar Telescope Sensitivity to 14.4 keV Axions
The CERN Axion Solar Telescope (CAST) is searching for solar axions using the
9.0 T strong and 9.26 m long transverse magnetic field of a twin aperture LHC
test magnet, where axions could be converted into X-rays via reverse Primakoff
process. Here we explore the potential of CAST to search for 14.4 keV axions
that could be emitted from the Sun in M1 nuclear transition between the first,
thermally excited state, and the ground state of 57Fe nuclide. Calculations of
the expected signals, with respect to the axion-photon coupling, axion-nucleon
coupling and axion mass, are presented in comparison with the experimental
sensitivity.Comment: 4 pages, 1 figure. Submitted to Nucl. Instr. and Meth.
Search for solar axion emission from 7Li and D(p,gamma)3He nuclear decays with the CAST gamma-ray calorimeter
We present the results of a search for a high-energy axion emission signal
from 7Li (0.478 MeV) and D(p,gamma)3He (5.5 MeV) nuclear transitions using a
low-background gamma-ray calorimeter during Phase I of the CAST experiment.
These so-called "hadronic axions" could provide a solution to the long-standing
strong-CP problem and can be emitted from the solar core from nuclear M1
transitions. This is the first such search for high-energy pseudoscalar bosons
with couplings to nucleons conducted using a helioscope approach. No excess
signal above background was found.Comment: 20 pages, 8 figures, final version to be published in JCA
Atomic X-ray Spectroscopy of Accreting Black Holes
Current astrophysical research suggests that the most persistently luminous
objects in the Universe are powered by the flow of matter through accretion
disks onto black holes. Accretion disk systems are observed to emit copious
radiation across the electromagnetic spectrum, each energy band providing
access to rather distinct regimes of physical conditions and geometric scale.
X-ray emission probes the innermost regions of the accretion disk, where
relativistic effects prevail. While this has been known for decades, it also
has been acknowledged that inferring physical conditions in the relativistic
regime from the behavior of the X-ray continuum is problematic and not
satisfactorily constraining. With the discovery in the 1990s of iron X-ray
lines bearing signatures of relativistic distortion came the hope that such
emission would more firmly constrain models of disk accretion near black holes,
as well as provide observational criteria by which to test general relativity
in the strong field limit. Here we provide an introduction to this phenomenon.
While the presentation is intended to be primarily tutorial in nature, we aim
also to acquaint the reader with trends in current research. To achieve these
ends, we present the basic applications of general relativity that pertain to
X-ray spectroscopic observations of black hole accretion disk systems, focusing
on the Schwarzschild and Kerr solutions to the Einstein field equations. To
this we add treatments of the fundamental concepts associated with the
theoretical and modeling aspects of accretion disks, as well as relevant topics
from observational and theoretical X-ray spectroscopy.Comment: 63 pages, 21 figures, Einstein Centennial Review Article, Canadian
Journal of Physics, in pres
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