5,926 research outputs found
Investigation of continuously traversing microphone system for mode measurement
The continuously Traversing Microphone System consists of a data acquisition and processing method for obtaining the modal coefficients of the discrete, coherent acoustic field in a fan inlet duct. The system would be used in fan rigs or full scale engine installations where present measurement methods, because of the excessive number of microphones and long test times required, are not feasible. The purpose of the investigation reported here was to develop a method for defining modal structure by means of a continuously traversing microphone system and to perform an evaluation of the method, based upon analytical studies and computer simulated tests. A variety of system parameters were examined, and the effects of deviations from ideal were explored. Effects of traverse speed, digitizing rate, run time, roundoff error, calibration errors, and random noise background level were determined. For constant fan operating speed, the sensitivity of the method to normal errors and deviations was determined to be acceptable. Good recovery of mode coefficients was attainable. Fluctuating fan speed conditions received special attention, and it was concluded that by employing suitable time delay procedures, satisfactory information on mode coefficients can be obtained under realistic conditions. A plan for further development involving fan rig tests was prepared
Relative Sizes of X-ray and Optical Images of Elliptical Galaxies; Correlation with X-ray Luminosity
Optical parameters of elliptical galaxies are tightly correlated, but their
x-ray parameters vary widely. The x-ray luminosity L_x ranges over more than an
order of magnitude for ellipticals having similar optical luminosity L_B. The
source of this scatter has been elusive. We show here that the dispersion in
L_x for fixed optical luminosity L_B correlates strongly with the dimensionless
ratio of the sizes of the x-ray and optical images, r_ex/r_e. Specifically, we
find that (L_x/L_B) is proportional to (r_{ex}/r_e)^{0.60 \pm 0.30}, a version
of the correlation that is independent of distance. This correlation may be a
natural result of mergings and tidal truncations that are expected during the
formation and early evolution of ellipticals in groups of galaxies. The radial
structure of x-ray images also varies: some are compact (e.g. NGC 4649, 7626,
5044), others diffuse (e.g. NGC 4636, 1399).Comment: 5 pages, 3 figures. Accepted for publication in Astrophysical Journal
Letter
Quantum oscillator and Kepler-Coulomb problems in curved spaces: deformed shape invariance, point canonical transformations, and rational extensions
The quantum oscillator and Kepler-Coulomb problems in -dimensional spaces
with constant curvature are analyzed from several viewpoints. In a deformed
supersymmetric framework, the corresponding nonlinear potentials are shown to
exhibit a deformed shape invariance property. By using the point canonical
transformation method, the two deformed Schr\"odinger equations are mapped onto
conventional ones corresponding to some shape-invariant potentials, whose
rational extensions are well known. The inverse point canonical transformations
then provide some rational extensions of the oscillator and Kepler-Coulomb
potentials in curved space. The oscillator on the sphere and the Kepler-Coulomb
potential in a hyperbolic space are studied in detail and their extensions are
proved to be consistent with already known ones in Euclidean space. The
partnership between nonextended and extended potentials is interpreted in a
deformed supersymmetric framework. Those extended potentials that are
isospectral to some nonextended ones are shown to display deformed shape
invariance, which in the Kepler-Coulomb case is enlarged by also translating
the degree of the polynomial arising in the rational part denominator.Comment: 32 pages, no figure; published versio
Eigen modes for the problem of anomalous light transmission through subwavelength holes
We show that the wide-spread concept of optical eigen modes in lossless
waveguide structures, which assumes the separation on propagating and
evanescent modes, fails in the case of metal-dielectric structures, including
photonic crystals. In addition to these modes, there is a sequence of new
eigen-states with complex values of the propagation constant and non-vanishing
circulating energy flow. The whole eigen-problem ceases to be hermitian because
of changing sign of the optical dielectric constant. The new anomalous modes
are shown to be of prime importance for the description of the anomalous light
transmission through subwavelength holes.Comment: 5 pages, 4 figure
Evolution of Hot Gas and Dark Halos in Group-Dominant Elliptical Galaxies: Influence of Cosmic Inflow
We study the complete dynamical evolution of hot interstellar gas in massive
elliptical galaxies born into a simple flat universe beginning with an
overdense perturbation. Within the turn-around radius dark matter flows in a
self-similar fashion into a stationary Navarro-Frenk-White halo and the
baryonic gas shocks. After a few gigayears, when enough gas accumulates within
the accretion shock, the de Vaucouleurs stellar system is constructed and the
energy from Type II supernovae is released. The stars and dark halo are matched
to NGC 4472. Gas continues to enter the galaxy by secondary infall and by
stellar mass loss based on a Salpeter IMF. After about 13 Gyrs the temperature
and density distribution in the hot gas agree quite well with the hot
interstellar gas observed in NGC 4472. As a result of supernova-driven outflow,
the present day baryonic fraction has a deep minimum in the outer galactic
halo. When relatively gas-rich, X-ray luminous models are spatially truncated
at early times, simulating tidal events that may have occurred during galaxy
group dynamics, the current locus of truncated models lies just along the
, X-ray size correlation among well-observed ellipticals, providing
another striking confirmation of our simple model of elliptical evolution.Comment: 16 pages in AASTEX LaTeX with 14 figures; accepted by Astrophysical
Journa
On-demand or Spot? Selling the cloud to risk-averse customers
In Amazon EC2, cloud resources are sold through a combination of an on-demand
market, in which customers buy resources at a fixed price, and a spot market,
in which customers bid for an uncertain supply of excess resources. Standard
market environments suggest that an optimal design uses just one type of
market. We show the prevalence of a dual market system can be explained by
heterogeneous risk attitudes of customers. In our stylized model, we consider
unit demand risk-averse bidders. We show the model admits a unique equilibrium,
with higher revenue and higher welfare than using only spot markets.
Furthermore, as risk aversion increases, the usage of the on-demand market
increases. We conclude that risk attitudes are an important factor in cloud
resource allocation and should be incorporated into models of cloud markets.Comment: Appeared at WINE 201
Quasar Proper Motions and Low-Frequency Gravitational Waves
We report observational upper limits on the mass-energy of the cosmological
gravitational-wave background, from limits on proper motions of quasars.
Gravitational waves with periods longer than the time span of observations
produce a simple pattern of apparent proper motions over the sky, composed
primarily of second-order transverse vector spherical harmonics. A fit of such
harmonics to measured motions yields a 95%-confidence limit on the mass-energy
of gravitational waves with frequencies <2e-9 Hz, of <0.11/h*h times the
closure density of the universe.Comment: 15 pages, 1 figure. Also available at
http://charm.physics.ucsb.edu:80/people/cgwinn/cgwinn_group/index.htm
Environmental determinants of acute respiratory symptoms and diarrhoea in young coloured children living in urban and peri-urban areas of South Africa
The impact of environmental risk factors associated with housing was examined in relation to diarrhoeal disease and acute respiratory symptoms in South African coloured children. A multistage cluster sample representative of all coloured people living in the major urban and peri-urban areas of South Africa was used for the study. Interviews were conducted with respondents from 1 227 households. Overall, 8,5% children under 5 years were reported to have had diarrhoea, while 29% had experienced coughing and breathing problems in a 2-week recall period. Individual risk factors identified using the odds ratios (ORs) for diarrhoea included not having an inside tap or a flush toilet in the homes (both yielded an OR of 3,3), not owning a refuse receptacle (OR = 2,5), not being connected to an electricity supply (OR = 2,5), low household income (OR = 1,8), more than 2 people per room (OR = 2,0) and less than Standard 5 maternal education (OR = 1,6). Absence of an inside toilet, not having a refuse receptacle and overcrowding all remained as independent risk factors after logistic regression analyses. Multiple logistic regression analyses revealed that not having a refuse receptacle and the absence of electricity for heating purposes were independently associated with respiratory symptoms. The overall preventive potentials for respiratory symptoms were significantly less than those for diarrhoea. Improving physical access to essential environmental health services in urban areas and improvements in the educational status of women are urgently needed if childhood infections are to be prevented
Revised Relativistic Hydrodynamical Model for Neutron-Star Binaries
We report on numerical results from a revised hydrodynamic simulation of
binary neutron-star orbits near merger. We find that the correction recently
identified by Flanagan significantly reduces but does not eliminate the
neutron-star compression effect. Although results of the revised simulations
show that the compression is reduced for a given total orbital angular
momentum, the inner most stable circular orbit moves to closer separation
distances. At these closer orbits significant compression and even collapse is
still possible prior to merger for a sufficiently soft EOS. The reduced
compression in the corrected simulation is consistent with other recent studies
of rigid irrotational binaries in quasiequilibrium in which the compression
effect is observed to be small. Another significant effect of this correction
is that the derived binary orbital frequencies are now in closer agreement with
post-Newtonian expectations.Comment: Submitted to Phys. Rev.
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