7,608 research outputs found
The NASA low thrust propulsion program
The NASA OAST Propulsion, Power, and Energy Division supports a low thrust propulsion program aimed at providing high performance options for a broad range of near-term and far-term mission and vehicles. Low thrust propulsion has a major impact on the mission performance of essentially all spacecraft and vehicles. On-orbit lifetimes, payloads, and trip times are significantly impacted by low thrust propulsion performance and integration features for Earth-to-orbit (ETO) vehicles, Earth-orbit and planetary spacecraft, and large platforms in Earth orbit. Major emphases are on low thrust chemical propulsion, both storables and hydrogen/oxygen; low-power (auxiliary) electric arcjects and resistojets; and high-power (primary) electric propulsion, including ion, magnetoplasmadynamic (MPD), and electrodeless concepts. The major recent accomplishments of the program are presented and their impacts discussed
Magnetic collimation of protostellar winds into bipolar outflows
Researchers describe self-consistent 2-D magnetohydrodynamic (MHD) simulations of the collimation of an isotropic protostellar wind into bipolar outflows by magnetic stresses in the ambient medium. A variety of ambient field strengths, wind luminosities, and density profiles were studied. Collimation occurs when the energy of the magnetic field swept up by the expanding bubble approaches the bubble thermal energy. Measured axial and radial expansion rates are in good agreement with the analytical predictions of Konigl (1982)
Including All the Lines
I present a progress report on including all the lines in the linelists,
including all the lines in the opacities, including all the lines in the model
atmosphere and spectrum synthesis calculations, producing high-resolution,
high-signal-to-noise atlases that show (not quite) all the lines, so that
finally we can determine the properties of stars from a few of the lines.Comment: 9 pages, no figures. Presented at "Dimitrifest" conference in
Boulder, Colorado, March 30 - April 3, 200
Foundation investigation -- Kaimuki High School auditorium, Honolulu, Hawaii
D.A.G.S. Job no. 02-16-6051.2Sections: investigation, site and subsurface conditions, recommendations, services during construction, field and laboratory data, specifications for installation of driven piles, site plan, subsurface profiles, settlement chart, strip footings settlement chart, boring logs, soil classification chart, plasticity chart, and consolidation test reports.Miyachi, HarryGroup Architect
Recurrent cerebellar architecture solves the motor-error problem
Current views of cerebellar function have been heavily influenced by the models of Marr and Albus, who suggested that the climbing fibre input to the cerebellum acts as a teaching signal for motor learning. It is commonly assumed that this teaching signal must be motor error (the difference between actual and correct motor command), but this approach requires complex neural structures to estimate unobservable motor error from its observed sensory consequences.
We have proposed elsewhere a recurrent decorrelation control architecture in which Marr-Albus models learn without requiring motor error. Here, we prove convergence for this architecture and demonstrate important advantages for the modular control of systems with multiple degrees of freedom. These results are illustrated by modelling adaptive plant compensation for the three-dimensional vestibular ocular reflex. This provides a functional role for recurrent cerebellar connectivity, which may be a generic anatomical feature of projections between regions of cerebral and cerebellar cortex
The Effect of the Hall Term on the Nonlinear Evolution of the Magnetorotational Instability: I. Local Axisymmetric Simulations
The effect of the Hall term on the evolution of the magnetorotational
instability (MRI) in weakly ionized accretion disks is investigated using local
axisymmetric simulations. First, we show that the Hall term has important
effects on the MRI when the temperature and density in the disk is below a few
thousand K and between 10^13 and 10^18 cm^{-3} respectively. Such conditions
can occur in the quiescent phase of dwarf nova disks, or in the inner part
(inside 10 - 100 AU) of protoplanetary disks. When the Hall term is important,
the properties of the MRI are dependent on the direction of the magnetic field
with respect to the angular velocity vector \Omega. If the disk is threaded by
a uniform vertical field oriented in the same sense as \Omega, the axisymmetric
evolution of the MRI is an exponentially growing two-channel flow without
saturation. When the field is oppositely directed to \Omega, however, small
scale fluctuations prevent the nonlinear growth of the channel flow and the MRI
evolves into MHD turbulence. These results are anticipated from the
characteristics of the linear dispersion relation. In axisymmetry on a field
with zero-net flux, the evolution of the MRI is independent of the size of the
Hall term relative to the inductive term. The evolution in this case is
determined mostly by the effect of ohmic dissipation.Comment: 31 pages, 3 tables, 12 figures, accepted for publication in ApJ,
postscript version also available from
http://www.astro.umd.edu/~sano/publications
Use of Post-Consumer Corrugated Fiberboard as Fine Aggregate Replacement in Controlled Low-Strength Materials
This study was conducted to investigate the use of post-consumer corrugated board in controlled low-strength material (CLSM) applications. Corrugated fiberboard (termed corrugate), which constitutes a significant fraction of the municipal solid waste stream in the United States (approximately one third by weight), was used as a partial replacement for fine aggregate in CLSM at aggregate replacement ratios ranging from 0 % (i.e., control) to 6 %. The corrugate was fiberized (i.e., repulped) in a blender prior to being mixed with other constituents in the CLSM. The density, air content, and flow consistency of the fresh CLSM were determined, and bleeding was qualitatively assessed. Also, the unconfined compressive strength was determined for the resulting mixtures at different test ages. As the corrugate content increased, air content and water demand increased, density and compressive strength decreased, and some mixtures exhibited excessive bleeding. Corrugated fiberboard was determined to be effective as a fine aggregate replacement to produce mixtures with 28-day compressive strengths within the range for excavatable CLSM
Cosmological Radiation Hydrodynamics with ENZO
We describe an extension of the cosmological hydrodynamics code ENZO to
include the self-consistent transport of ionizing radiation modeled in the
flux-limited diffusion approximation. A novel feature of our algorithm is a
coupled implicit solution of radiation transport, ionization kinetics, and gas
photoheating, making the timestepping for this portion of the calculation
resolution independent. The implicit system is coupled to the explicit
cosmological hydrodynamics through operator splitting and solved with scalable
multigrid methods. We summarize the numerical method, present a verification
test on cosmological Stromgren spheres, and then apply it to the problem of
cosmological hydrogen reionization.Comment: 14 pages, 3 figures, to appear in Recent Directions in Astrophysical
Quantitative Spectroscopy and Radiation Hydrodynamics, Ed. I. Hubeny,
American Institute of Physics (2009
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