12,522 research outputs found
Earthquake-Resistant Design of Earth Dams
Lessons gained from observations of the field performance of earth darns during earthquakes are reviewed and used to illustrate the primary problems of concern. Defensive design measures which may be taken to mitigate the various hazards are reviewed and illustrated. Analytical approaches for evaluating seismic stability and the deformations of earth darns during earthquakes are discussed, together with recent developments which facilitate their implementation in special cases; situations which require careful consideration of special effects such as the three-dimensionality of the dam-valley system and pore pressure re-distribution following an earthquake are discussed and illustrated
Neural processes of proactive and reactive controls modulated by motor-skill experiences
This study investigated the experience of open and closed motor skills on modulating proactive and reactive control processes in task switching. Fifty-four participants who were open-skilled
Phase Transitions in Quantum Dots
We perform Hartree-Fock calculations to show that quantum dots (i.e. two
dimensional systems of up to twenty interacting electrons in an external
parabolic potential) undergo a gradual transition to a spin-polarized Wigner
crystal with increasing magnetic field strength. The phase diagram and ground
state energies have been determined. We tried to improve the ground state of
the Wigner crystal by introducing a Jastrow ansatz for the wavefunction and
performing a variational Monte Carlo calculation. The existence of so called
magic numbers was also investigated. Finally, we also calculated the heat
capacity associated with the rotational degree of freedom of deformed many-body
states.Comment: 14 pages, 7 postscript figure
Seaweed species diversity in South Africa
A dataset is presented on the diversity of South African seaweed species and their distribution in contiguous 50-km coastal sections, to demonstrate current knowledge of the flora in various coastal regions. The coastline has a rich flora, consisting of some 800 species. The South Coast has the highest species diversity (between 250 and 300 species in each section) relative to the West Coast (about 140 species per section) or the East Coast (about 200 species per section). There is a considerable increase in the number of species (>240 species per section), particularly in the green algae, in the easternmost regions that border the tropics. An analysis of seaweeds present in six small stretches of coastline shows that these sites contain 80–90% of the species in the West and South coastal sections, but only 60% of species over the entire coastline. Therefore, successful conservation of relatively few sites could theoretically preserve the majority of the seaweed flora. Many of the species missing from these detailed collections are subtidal, a habitat that is generally undersampled, particularly on the South and East coasts. Although species diversity is becoming relatively well known, systematic studies on many groups, using modern techniques, are needed. Detailed core distribution data for many species are lacking, and there is no national plan for seaweed systematics or herbarium collections. The distributions of green algae are particularly poorly known. Detailed systematic and biogeographic investigations on the overlap between the seaweeds of the temperate South Coast and those of the tropical Indian Ocean are necessary, and these should include subtidal collections and vegetation descriptions.Keywords: algae, conservation, seaweeds, South Africa, species diversityAfrican Journal of Marine Science 2002, 24: 9–1
Multilevel blocking approach to the fermion sign problem in path-integral Monte Carlo simulations
A general algorithm toward the solution of the fermion sign problem in
finite-temperature quantum Monte Carlo simulations has been formulated for
discretized fermion path integrals with nearest-neighbor interactions in the
Trotter direction. This multilevel approach systematically implements a simple
blocking strategy in a recursive manner to synthesize the sign cancellations
among different fermionic paths throughout the whole configuration space. The
practical usefulness of the method is demonstrated for interacting electrons in
a quantum dot.Comment: 4 pages RevTeX, incl. two figure
Ultraviolet observations of the X-ray photoionized wind of Cygnus X-1 during X-ray soft/high state
(Shortened) Ultraviolet observations of the black hole X-ray binary Cygnus
X-1 were obtained using the STIS on HSTubble. We detect P Cygni line features
show strong, broad absorption components when the X-ray source is behind the
companion star and noticeably weaker absorption when the X-ray source is
between us and the companion star. We fit the P Cygni profiles using the SEI
method applied to a spherically symmetric stellar wind subject to X-ray
photoionization from the black hole. The Si IV doublet provides the most
reliable estimates of the parameters of the wind and X-ray illumination. The
velocity increases with radius according to
, with and
km s.The microturbulent velocity was
km s. Our fit implies a ratio of X-ray luminosity to wind mass-loss rate
of L, measured at = 4.8. Our
models determine parameters that may be used to estimate the accretion rate
onto the black hole and independently predict the X-ray luminosity. Our
predicted L matches that determined by contemporaneous RXTE ASM remarkably
well, but is a factor of 3 lower than the rate according to
Bondi-Hoyle-Littleton spherical wind accretion. We suggest that some of the
energy of accretion may go into powering a jet.Comment: 34 pages, 21 figures, 4 tables, accepted for publication in Ap
Investigation of the Vibrational Modes of Edge-Constrained Fibrous Samples Placed in a Standing Wave Tube
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