2,344 research outputs found
Modeling of convection phenomena in Bridgman-Stockbarger crystal growth
Thermal convection phenomena in a vertically oriented Bridgman-Stockbarger apparatus were modeled by computer simulations for different gravity conditions, ranging from earth conditions to extremely low gravity, approximate space conditions. The modeling results were obtained by the application of a state-of-the art, transient, multi-dimensional, completely densimetrically coupled, discrete-element computational model which was specifically developed for the simulation of flow, temperature, and species concentration conditions in two-phase (solid-liquid) systems. The computational model was applied to the simulation of the flow and the thermal conditions associated with the convection phenomena in a modified Germanium-Silicon charge enclosed in a stationary fused-silica ampoule. The results clearly indicated that the gravitational field strength influences the characteristics of the coherent vortical flow patterns, interface shape and position, maximum melt velocity, and interfacial normal temperature gradient
Prehypertensive blood pressures and regional cerebral blood flow independently relate to cognitive performance in midlife
Background
High blood pressure is thought to contribute to dementia in late life, but our understanding of the relationship between individual differences in blood pressure (
BP
) and cognitive functioning is incomplete. In this study, cognitive performance in nonhypertensive midlife adults was examined as a function of resting
BP
and regional cerebral blood flow (
rCBF
) responses during cognitive testing. We hypothesized that
BP
would be negatively related to cognitive performance and that cognitive performance would also be related to
rCBF
responses within areas related to
BP
control. We explored whether deficits related to systolic
BP
might be explained by
rCBF
responses to mental challenge.
Methods and Results
Healthy midlife participants (n=227) received neuropsychological testing and performed cognitive tasks in a magnetic resonance imaging scanner. A pseudocontinuous arterial spin labeling sequence assessed
rCBF
in brain areas related to
BP
in prior studies. Systolic
BP
was negatively related to 4 of 5 neuropsychological factors (standardized β>0.13): memory, working memory, executive function, and mental efficiency. The
rCBF
in 2 brain regions of interest was similarly related to memory, executive function, and working memory (standardized β>0.17); however,
rCBF
responses did not explain the relationship between resting systolic
BP
and cognitive performance.
Conclusions
Relationships at midlife between prehypertensive levels of systolic
BP
and both cognitive and brain function were modest but suggested the possible value of midlife intervention.
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The first products made in space: Monodisperse latex particles
The preparation of large particle size 3 to 30 micrometer monodisperse latexes in space confirmed that original rationale unequivocally. The flight polymerizations formed negligible amounts of coagulum as compared to increasing amounts for the ground-based polymerizations. The number of offsize large particles in the flight latexes was smaller than in the ground-based latexes. The particle size distribution broadened and more larger offsize particles were formed when the polymerizations of the partially converted STS-4 latexes were completed on Earth. Polymerization in space also showed other unanticipated advantages. The flight latexes had narrower particle size distributions than the ground-based latexes. The particles of the flight latexes were more perfect spheres than those of the ground-based latexes. The superior uniformity of the flight latexes was confirmed by the National Bureau of Standards acceptance of the 10 micrometer STS-6 latex and the 30 micrometer STS-11 latexes as Standard Reference Materials, the first products made in space for sale on Earth. The polymerization rates in space were the same as those on Earth within experimental error. Further development of the ground-based polymerization recipes gave monodisperse particles as large as 100 micrometer with tolerable levels of coagulum, but their uniformity was significantly poorer than the flight latexes. Careful control of the polymerization parameters gave uniform nonspherical particles: symmetrical and asymmetrical doublets, ellipsoids, egg-shaped, ice cream cone-shaped, and popcorn-shaped particles
Quantum planes and quantum cylinders from Poisson homogeneous spaces
Quantum planes and a new quantum cylinder are obtained as quantization of
Poisson homogeneous spaces of two different Poisson structures on classical
Euclidean group E(2).Comment: 13 pages, plain Tex, no figure
Flux through a hole from a shaken granular medium
We have measured the flux of grains from a hole in the bottom of a shaken
container of grains. We find that the peak velocity of the vibration, vmax,
controls the flux, i.e., the flux is nearly independent of the frequency and
acceleration amplitude for a given value of vmax. The flux decreases with
increasing peak velocity and then becomes almost constant for the largest
values of vmax. The data at low peak velocity can be quantitatively described
by a simple model, but the crossover to nearly constant flux at larger peak
velocity suggests a regime in which the granular density near the container
bottom is independent of the energy input to the system.Comment: 14 pages, 4 figures. to appear in Physical Review
Sodium vacancy ordering and the co-existence of localized spins and itinerant charges in NaxCoO2
The sodium cobaltate family (NaxCoO2) is unique among transition metal oxides
because the Co sits on a triangular lattice and its valence can be tuned over a
wide range by varying the Na concentration x. Up to now detailed modeling of
the rich phenomenology (which ranges from unconventional superconductivity to
enhanced thermopower) has been hampered by the difficulty of controlling pure
phases. We discovered that certain Na concentrations are specially stable and
are associated with superlattice ordering of the Na clusters. This leads
naturally to a picture of co-existence of localized spins and itinerant charge
carriers. For x = 0.84 we found a remarkably small Fermi energy of 87 K. Our
picture brings coherence to a variety of measurements ranging from NMR to
optical to thermal transport. Our results also allow us to take the first step
towards modeling the mysterious ``Curie-Weiss'' metal state at x = 0.71. We
suggest the local moments may form a quantum spin liquid state and we propose
experimental test of our hypothesis.Comment: 16 pages, 5 figure
The PARSE Programming Paradigm. Part I: Software Development Methodology. Part II: Software Development Support Tools
The programming methodology of PARSE (parallel software environment), a software environment being developed for reconfigurable non-shared memory parallel computers, is described. This environment will consist of an integrated collection of language interfaces, automatic and semi-automatic debugging and analysis tools, and operating system —all of which are made more flexible by the use of a knowledge-based implementation for the tools that make up PARSE. The programming paradigm supports the user freely choosing among three basic approaches /abstractions for programming a parallel machine: logic-based descriptive, sequential-control procedural, and parallel-control procedural programming. All of these result in efficient parallel execution. The current work discusses the methodology underlying PARSE, whereas the companion paper, “The PARSE Programming Paradigm — II: Software Development Support Tools,” details each of the component tools
Ballistic Annihilation Kinetics: The Case of Discrete Velocity Distributions
The kinetics of the annihilation process, , with ballistic particle
motion is investigated when the distribution of particle velocities is {\it
discrete}. This discreteness is the source of many intriguing phenomena. In the
mean field limit, the densities of different velocity species decay in time
with different power law rates for many initial conditions. For a
one-dimensional symmetric system containing particles with velocity 0 and , there is a particular initial state for which the concentrations of all
three species as decay as . For the case of a fast ``impurity'' in a
symmetric background of and particles, the impurity survival
probability decays as . In a symmetric
4-velocity system in which there are particles with velocities and
, there again is a special initial condition where the two species
decay at the same rate, t^{-\a}, with \a\cong 0.72. Efficient algorithms
are introduced to perform the large-scale simulations necessary to observe
these unusual phenomena clearly.Comment: 18 text pages, macro file included, hardcopy of 9 figures available
by email request to S
Quantized algebras of functions on homogeneous spaces with Poisson stabilizers
Let G be a simply connected semisimple compact Lie group with standard
Poisson structure, K a closed Poisson-Lie subgroup, 0<q<1. We study a
quantization C(G_q/K_q) of the algebra of continuous functions on G/K. Using
results of Soibelman and Dijkhuizen-Stokman we classify the irreducible
representations of C(G_q/K_q) and obtain a composition series for C(G_q/K_q).
We describe closures of the symplectic leaves of G/K refining the well-known
description in the case of flag manifolds in terms of the Bruhat order. We then
show that the same rules describe the topology on the spectrum of C(G_q/K_q).
Next we show that the family of C*-algebras C(G_q/K_q), 0<q\le1, has a
canonical structure of a continuous field of C*-algebras and provides a strict
deformation quantization of the Poisson algebra \C[G/K]. Finally, extending a
result of Nagy, we show that C(G_q/K_q) is canonically KK-equivalent to C(G/K).Comment: 23 pages; minor changes, typos correcte
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