9,303 research outputs found
Multimode circular integrated optical microresonators: Coupled mode theory modeling
A frequency domain model of multimode circular microresonators for filter
applications in integrated optics is investigated. Analytical basis modes of 2D
bent waveguides or curved interfaces are combined with modes of straight
channels in a spatial coupled mode theory framework. Free of fitting
parameters, the model allows to predict quite efficiently the spectral response
of the microresonators. It turns out to be sufficient to take only a few
dominant cavity modes into account. Comparisons of these simulations with
computationally more expensive rigorous numerical calculations show a
satisfactory agreement
Surface pressure measurements at two tips of a model helicopter rotor in hover
Surface pressures were measured near the tip of a hovering single-bladed model helicopter rotor with two tip shapes. The rotor had a constant-chord, untwisted blade with a square, flat tip which could be modified to a body-of-revolution tip. Pressure measurements were made on the blade surface along the chordwise direction at six radial stations outboard of the 94 percent blade radius. Data for each blade tip configuration were taken at blade collective pitch angles of 0, 6.18 and 11.4 degrees at a Reynolds number of 736,000 and a Mach number of 0.25 both based on tip speed. Chordwise pressure distributions and constant surface pressure contours are presented and discussed
Ascent control studies of the 049 and ATP parallel burn solid rocket motor shuttle configurations
The control authority approach is discussed as a major problem of the parallel burn soil shuttle configuration due to the many resulting system impacts regardless of the approach. The major trade studies and their results, which led to the recommendation of an SRB TVC control authority approach are presented
Density Functional Theory: Methods and Problems
The application of density functional theory to nuclear structure is
discussed, highlighting the current status of the effective action approach
using effective field theory, and outlining future challenges.Comment: 10 pages, 14 figures, invited talk at INT workshop on Nuclear Forces
and the Quantum Many-Body Problem, Seattle, October 200
Potential Energy Surface for H_2 Dissociation over Pd(100)
The potential energy surface (PES) of dissociative adsorption of H_2 on
Pd(100) is investigated using density functional theory and the full-potential
linear augmented plane wave (FP-LAPW) method. Several dissociation pathways are
identified which have a vanishing energy barrier. A pronounced dependence of
the potential energy on ``cartwheel'' rotations of the molecular axis is found.
The calculated PES shows no indication of the presence of a precursor state in
front of the surface. Both results indicate that steering effects determine the
observed decrease of the sticking coefficient at low energies of the H_2
molecules. We show that the topology of the PES is related to the dependence of
the covalent H(s)-Pd(d) interactions on the orientation of the H_2 molecule.Comment: RevTeX, 8 pages, 5 figures in uufiles forma
Hubble Space Telescope imaging of the CFRS and LDSS redshift surveys - IV. Influence of mergers in the evolution of faint field galaxies from z~1
HST images of a sample of 285 galaxies with measured z from the CFRS and
Autofib-LDSS redshift surveys are analysed to derive the evolution of the
merger fraction out to z~1. We have performed visual and machine-based merger
identifications, as well as counts of bright pairs of galaxies with magnitude
differences less than 1.5 mag. We find that the pair fraction increases with z,
with up to ~20% of the galaxies being in physical pairs at z~0.75-1. We derive
a merger fraction varying with z as (1+z)^{3.2 +/- 0.6}, after correction for
line-of-sight contamination, in excellent agreement with the merger fraction
derived from the visual classification of mergers for which m = 3.4 +/- 0.6.
After correcting for seeing effects on the ground-based selection of survey
galaxies, we conclude that the pair fraction evolves as (1+z)^{2.7 +/- 0.6}.
This implies that an average L* galaxy will have undergone 0.8 to 1.8 merger
events from z=1 to 0, with 0.5 to 1.2 merger events occuring in a 2 Gyr time
span at z~0.9. This result is consistent with predictions from semi-analytical
models of galaxy formation. From the simple co-addition of the observed
luminosities of the galaxies in pairs, physical mergers are computed to lead to
a brightening of 0.5 mag for each pair on average, and a boost in star
formation rate of a factor of 2, as derived from the average [O II] equivalent
widths. Mergers of galaxies are therefore contributing significantly to the
evolution of both the luminosity function and luminosity density of the
Universe out to z~1.Comment: 14 pages, 6 PS figures included. Accepted for publication in MNRA
Recursive self-organizing map as a contractive iterative function system
Recently, there has been a considerable research activity in extending topographic maps of vectorial data to more general data structures, such as sequences or trees. However, the representational capabilities and internal representations of the models are not well understood. We rigorously analyze a generalization of the Self-Organizing Map (SOM) for processing sequential data, Recursive SOM (RecSOM [1]), as a non-autonomous dynamical system consisting off a set of fixed input maps. We show that contractive fixed input maps are likely to produce Markovian organizations of receptive fields o the RecSOM map. We derive bounds on parameter (weighting the importance of importing past information when processing sequences) under which contractiveness of the fixed input maps is guaranteed
Cadaveric Renal transplantation with Cyclosporine: Experiences in 148 patients at a single institution.
Reinventing College Physics for Biologists: Explicating an epistemological curriculum
The University of Maryland Physics Education Research Group (UMd-PERG)
carried out a five-year research project to rethink, observe, and reform
introductory algebra-based (college) physics. This class is one of the Maryland
Physics Department's large service courses, serving primarily life-science
majors. After consultation with biologists, we re-focused the class on helping
the students learn to think scientifically -- to build coherence, think in
terms of mechanism, and to follow the implications of assumptions. We designed
the course to tap into students' productive conceptual and epistemological
resources, based on a theoretical framework from research on learning. The
reformed class retains its traditional structure in terms of time and
instructional personnel, but we modified existing best-practices curricular
materials, including Peer Instruction, Interactive Lecture Demonstrations, and
Tutorials. We provided class-controlled spaces for student collaboration, which
allowed us to observe and record students learning directly. We also scanned
all written homework and examinations, and we administered pre-post conceptual
and epistemological surveys. The reformed class enhanced the strong gains on
pre-post conceptual tests produced by the best-practices materials while
obtaining unprecedented pre-post gains on epistemological surveys instead of
the traditional losses.Comment: 35 pages including a 15 page appendix of supplementary material
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