1,394 research outputs found
Flame zone of a composite propellant expanded by a laser source
Technique scales flame structure linearly with gas kinetic mean free path, which increases two to three orders of magnitude as pressure decreases like amount. Kinetic and transport time scales expand in proportion so that regression rates for laser-induced flames are two to three orders of magnitude slower
Composite solid propellant flame microstructure determination Annual report, 23 Jun. 1967 - 22 Jun. 1968
Composite solid propellant flame microstructure determination
Axion Cosmology and the Energy Scale of Inflation
We survey observational constraints on the parameter space of inflation and
axions and map out two allowed windows: the classic window and the inflationary
anthropic window. The cosmology of the latter is particularly interesting;
inflationary axion cosmology predicts the existence of isocurvature
fluctuations in the CMB, with an amplitude that grows with both the energy
scale of inflation and the fraction of dark matter in axions. Statistical
arguments favor a substantial value for the latter, and so current bounds on
isocurvature fluctuations imply tight constraints on inflation. For example, an
axion Peccei-Quinn scale of 10^16 GeV excludes any inflation model with energy
scale > 3.8*10^14 GeV (r > 2*10^(-9)) at 95% confidence, and so implies
negligible gravitational waves from inflation, but suggests appreciable
isocurvature fluctuations.Comment: 10 PRD pages, 4 figs, V3: updated to match published versio
Deep Equatorial Pacific Ocean Oxygenation and Atmospheric CO\u3csub\u3e2\u3c/sub\u3e Over the Last Ice Age
Ventilation of carbon stored in the deep ocean is thought to play an important role in atmospheric CO2 increases associated with Pleistocene deglaciations. The presence of this respired carbon has been recorded by an array of paleoceanographic proxies from various locations across the global ocean. Here we present a new sediment core from the Eastern Equatorial Pacific (EEP) Ocean spanning the last 180,000 years and reconstruct high-resolution 230Th-derived fluxes of 232Th and excess barium, along with redox-sensitive uranium concentrations to examine past variations in dust delivery, export productivity, and bottom-water oxygenation, respectively. Our bottom-water oxygenation record is compared to other similar high-resolution records from across the Pacific and in the Southern Ocean. We suggest that the deep Pacific is a site of respired carbon storage associated with periods of decreased global atmospheric CO2 concentration during the LGM, confirming the conclusions from a wealth of previous studies. However, our study is the first to show a similar relationship beyond the last glacial, extending to at least 70,000 years
Pistons modeled by potentials
In this article we consider a piston modelled by a potential in the presence
of extra dimensions. We analyze the functional determinant and the Casimir
effect for this configuration. In order to compute the determinant and Casimir
force we employ the zeta function scheme. Essentially, the computation reduces
to the analysis of the zeta function associated with a scalar field living on
an interval in a background potential. Although, as a model for a
piston, it seems reasonable to assume a potential having compact support within
, we provide a formalism that can be applied to any sufficiently smooth
potential.Comment: 10 pages, LaTeX. A typo in eq. (3.5) has been corrected. In
"Cosmology, Quantum Vacuum and Zeta Functions: In Honour of Emilio Elizalde",
Eds. S.D. Odintsov, D. Saez-Gomez, and S. Xambo-Descamps. (Springer 2011) pp
31
Warm tachyonic inflation in warped background
We analyze warm tachyonic inflation, proposed in the literature, but from the
viewpoint of four dimensional effective action for tachyon field on a non-BPS
D3-brane. We find that consistency with observational data on density
perturbation and validity of effective action requires warped compactification.
The number of background branes which source the flux is found to be of the
order of 10 in contrast to the order of in the standard cold
inflationary scenario.Comment: 9 pages, RevTe
Probing Dark Matter Substructure with Pulsar Timing
We demonstrate that pulsar timing measurements may potentially be able to
detect the presence of dark matter substructure within our own galaxy. As dark
matter substructure transits near the line-of-sight between a pulsar and an
observer, the change in the gravitational field will result in a delay of the
light-travel-time of photons. We calculate the effect of this delay due to
transiting dark matter substructure and find that the effect on pulsar timing
ought to be observable over decadal timescales for a wide range of substructure
masses and density profiles. We find that transiting dark matter substructure
with masses above 0.01 solar masses ought to be detectable at present by these
means. With improved measurements, this method may be able to distinguish
between baryonic, thermal non-baryonic, and non-thermal non-baryonic types of
dark matter. Additionally, information about structure formation on small
scales and the density profiles of galactic dark matter substructure can be
extracted via this method.Comment: 8 pages, 4 figures, replaced to match published versio
Nanostructures in Ti processed by severe plastic deformation
Metals and alloys processed by severe plastic deformation (SPD) can demonstrate superior mechanical properties, which are rendered by their unique defect structures. In this investigation, transmission electron microscopy and x-ray analysis were used to systematically study the defect structures, including grain and subgrain structures, dislocation cells, dislocation distributions, grain boundaries, and the hierarchy of these structural features, in nanostructured Ti produced by a two-step SPD procedure-warm equal channel angular pressing followed by cold rolling. The effects of these defect structures on the mechanical behaviors of nanostructured Ti are discussed
- …