268,485 research outputs found
Effect of velocity overshoot on the performance of magnetohydrodynamic subsonic diffusers
The evolution of an overshoot velocity distribution was studied in a plane two dimensional diffuser as a function of diffuser divergence angle. The diffuser performance for velocity overshoot was compared to that for a fully developed inlet velocity profile. Results indicate that the ratio of peak-to-center line velocity increases along the diffuser for a diffuser half angle greater than some critical value. It was also found that irrespective of the accompanying inlet temperature distribution, the wall shear stress and the wall heat flux is substantially larger when the inlet velocity profile has an overshoot than that for a fully developed inlet velocity profile
Embeddings of rearrangement invariant spaces that are not strictly singular
We give partial answers to the following conjecture: the natural embedding of
a rearrangement invariant space E into L_1([0,1]) is strictly singular if and
only if G does not embed into E continuously, where G is the closure of the
simple functions in the Orlicz space L_Phi with Phi(x) = exp(x^2)-1.Comment: Also available at http://www.math.missouri.edu/~stephen/preprint
Big Bang Nucleosynthesis with Independent Neutrino Distribution Functions
We have performed new Big Bang Nucleosynthesis calculations which employ
arbitrarily-specified, time-dependent neutrino and antineutrino distribution
functions for each of up to four neutrino flavors. We self-consistently couple
these distributions to the thermodynamics, the expansion rate and scale
factor-time/temperature relationship, as well as to all relevant weak,
electromagnetic, and strong nuclear reaction processes in the early universe.
With this approach, we can treat any scenario in which neutrino or antineutrino
spectral distortion might arise. These scenarios might include, for example,
decaying particles, active-sterile neutrino oscillations, and active-active
neutrino oscillations in the presence of significant lepton numbers. Our
calculations allow lepton numbers and sterile neutrinos to be constrained with
observationally-determined primordial helium and deuterium abundances. We have
modified a standard BBN code to perform these calculations and have made it
available to the community.Comment: 9 pages, 5 figure
End region and current consolidation effects upon the performance of an MHD channel for the ETF conceptual design
The effects of MHD channel end regions on the overall power generation were considered. The peak plant thermodynamic efficiency was found to be slightly lower than for the active region (41%). The channel operating point for the peak efficiency was shifted to the supersonic mode (Mach No., M sub c approx. 1.1) rather than the previous subsonic operation (M sub c approx. 0.9). The sensitivity of the channel performance to the B-field, diffuser recovery coefficient, channel load parameter, Mach number, and combustor pressure is also discussed. In addition, methods for operating the channel in a constant-current mode are investigated. This mode is highly desirable from the standpoint of simplifying the current and voltage consolidation for the inverter system. This simplification could result in significant savings in the cost of the equipment. The initial results indicate that this simplification is possible, even under a strict Hall field constraint, with resonable plant thermodynamic efficiency (40.5%)
Temperature distributions of a cesium-seeded hydrogen-oxygen supersonic free jet
The hydrogen-oxygen plasma was generated at combustion chamber pressures ranging from 0.5 to 2.0 megapascals and for various seed ratios (1 to 10 percent). The plasma was observed as the atmospheric exhaust from a Mach 2 rocket test facility. Transverse profiles of the absolute integrated intensity were measured with the optically thin CsI lines (0.5664 and 0.5636 microns) at a range of axial positions downstream of the 5-cm-diameter combustor nozzle exit. Radial profiles of the emission coefficient were obtained from the measured transverse profiles of intensity by Abel inversion. Temperatures were then determined from the emission coefficients for conditions of local thermodynamic equilibrium using particle densities generated by a two-dimensional free jet computer program. Temperature results show the inherent effects of compression and expansion pressure waves characteristic of a free jet exiting from a supersonic nozzle
Strong absorption and selective thermal emission from a mid-infrared metamaterial
We demonstrate thin-film metamaterials with resonances in the mid-infrared
wavelength range. Our structures are numerically modeled and experimentally
characterized by reflection and angularly-resolved thermal emission
spectroscopy. We demonstrate strong and controllable absorption resonances
across the mid-infrared wavelength range. In addition, the polarized thermal
emission from these samples is shown to be highly selective and largely
independent of emission angles from normal to 45 degrees. Experimental results
are compared to numerical models with excellent agreement. Such structures hold
promise for large-area, low-cost metamaterial coatings for control of gray- or
black-body thermal signatures, as well as for possible mid-IR sensing
applications.Comment: The following article has been submitted to Appl. Phys. Lett. After
it is published, it will be found at http://apl.aip.org/. 14 pages including
4 figure page
Arresting woodland bird decline in Australian agricultural landscapes: potential application of the European agri-environment model
This paper considered the applicability of the European model of land stewardship payments, in particular its support for biodiversity conservation in agricultural landscapes, to an Australian context. More broadly, the research approach described in the paper may also be applied to assessing the suitability of overseas stewardship schemes to the provision of any ecoservice in
Australia, such as carbon sequestration and floodwater regulation
Star Formation and the Interstellar Medium In Nearby Tidal Streams (SAINTS): Spitzer Mid-infrared Spectroscopy and Imaging of Intergalactic Star-forming Objects
A spectroscopic analysis of 10 intergalactic star forming objects (ISFOs) and
a photometric analysis of 67 ISFOs in a sample of 14 interacting systems is
presented. The majority of the ISFOs have relative polycyclic aromatic
hydrocarbon (PAH) band strengths similar to those of nearby spiral and
starburst galaxies. In contrast to what is observed in blue compact dwarfs
(BCDs) and local giant HII regions in the Milky Way (NGC 3603) and the
Magellanic Clouds (30 Doradus and N 66), the relative PAH band strengths in
ISFOs correspond to models with a significant PAH ion fraction (<50%) and
bright emission from large PAHs (~100 carbon atoms). The [NeIII]/[NeII] and
[SIV]/[SIII] line flux ratios indicate moderate levels of excitation with an
interstellar radiation field that is harder than the majority of the Spitzer
Infrared Nearby Galaxies Survey and starburst galaxies, but softer than BCDs
and local giant HII regions. The ISFO neon line flux ratios are consistent with
a burst of star formation < 6 million years ago. Most of the ISFOs have
~million solar masses of warm molecular hydrogen with a likely origin in
photo-dissociation regions (PDRs). Infrared Array Camera photometry shows the
ISFOs to be bright at 8 um, with one third having [4.5] - [8.0] > 3.7, i.e.,
enhanced non-stellar emission, most likely due to PAHs, relative to normal
spirals, dwarf irregulars and BCD galaxies. The relative strength of the 8 um
emission compared to that at 3.6 um or 24 um separates ISFOs from dwarf
galaxies in Spitzer two color diagrams. The infrared power in two thirds of the
ISFOs is dominated by emission from grains in a diffuse interstellar medium.
One in six ISFOs have significant emission from PDRs, contributing ~30 % - 60 %
of the total power. ISFOs are young knots of intense star formation.Comment: Accepted in ApJ. 49 pages 9 figure
Xenon in Mercury-Manganese Stars
Previous studies of elemental abundances in Mercury-Manganese (HgMn) stars
have occasionally reported the presence of lines of the ionized rare noble gas
Xe II, especially in a few of the hottest stars with Teff ~ 13000--15000 K. A
new study of this element has been undertaken using observations from Lick
Observatory's Hamilton Echelle Spectrograph. In this work, the spectrum
synthesis program UCLSYN has been used to undertake abundance analysis assuming
LTE. We find that in the Smith & Dworetsky sample of HgMn stars, Xe is vastly
over-abundant in 21 of 22 HgMn stars studied, by factors of 3.1--4.8 dex. There
does not appear to be a significant correlation of Xe abundance with Teff. A
comparison sample of normal late B stars shows no sign of Xe II lines that
could be detected, consistent with the expected weakness of lines at normal
abundance. The main reason for the previous lack of widespread detection in
HgMn stars is probably due to the strongest lines being at longer wavelengths
than the photographic blue. The lines used in this work were 4603.03A, 4844.33A
and 5292.22A.Comment: 8 pages, 4 figures. Accepted by Monthly Notices of the Royal
Astronomical Society, 8 January 200
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