25,293 research outputs found
Equivalent beam modeling using numerical reduction techniques
Numerical procedures that can accomplish model reductions for space trusses were developed. Three techniques are presented that can be implemented using current capabilities within NASTRAN. The proposed techniques accomplish their model reductions numerically through use of NASTRAN structural analyses and as such are termed numerical in contrast to the previously developed analytical techniques. Numerical procedures are developed that permit reductions of large truss models containing full modeling detail of the truss and its joints. Three techniques are presented that accomplish these model reductions with various levels of structural accuracy. These numerical techniques are designated as equivalent beam, truss element reduction, and post-assembly reduction methods. These techniques are discussed in detail
Lifetimes and Sizes from Two-Particle Correlation Functions
We discuss the Yano-Koonin-Podgoretskii (YKP) parametrization of the
two-particle correlation function for azimuthally symmetric expanding sources.
We derive model-independent expressions for the YKP fit parameters and discuss
their physical interpretation. We use them to evaluate the YKP fit parameters
and their momentum dependence for a simple model for the emission function and
propose new strategies for extracting the source lifetime. Longitudinal
expansion of the source can be seen directly in the rapidity dependence of the
Yano-Koonin velocity.Comment: 15 pages REVTEX, 2 figures included, submitted to Phys. Lett. B,
Expanded discussion of disadvantages of standard HBT fit and of Fig.
Stability of optically-active charged excitons in quasi-two dimensional systems
A negatively charged quasi-two dimensional exciton () is solved exactly
numerically in the presence of a uniform perpendicular B-field. Various
quasi-two dimensional geometries are studied. The charge distribution of the
parallel to the B-field is found to be crucial in determining the
stability of the optically-active and hence its photoluminescence (PL)
signature. The theory provides a quantitative explanation of recent
experimental results obtained for a GaAs quantum well. Effects are found which
cannot be described within a lowest Landau level approximation.
PACS: 78.20.Ls 78.66.-w 73.20.DxComment: Minor revisions, mainly removal of the term trion in favour of the
term charged-exciton to comply with Physical Review B. To be published as a
Rapid Communication in Physical Review
The rheology of dense, polydisperse granular fluids under shear
The solution of the Enskog equation for the one-body velocity distribution of
a moderately dense, arbitrary mixture of inelastic hard spheres undergoing
planar shear flow is described. A generalization of the Grad moment method,
implemented by means of a novel generating function technique, is used so as to
avoid any assumptions concerning the size of the shear rate. The result is
illustrated by using it to calculate the pressure, normal stresses and shear
viscosity of a model polydisperse granular fluid in which grain size, mass and
coefficient of restitution varies amoungst the grains. The results are compared
to a numerical solution of the Enskog equation as well as molecular dynamics
simulations. Most bulk properties are well described by the Enskog theory and
it is shown that the generalized moment method is more accurate than the simple
(Grad) moment method. However, the description of the distribution of
temperatures in the mixture predicted by Enskog theory does not compare well to
simulation, even at relatively modest densities.Comment: 8 postscript figures Replaced with new version correcting an error in
the SME calculations and misc. small corrections. Second replacement with
final correction of SME calculation
Hybrid Search: Effectively Combining Keywords and Semantic Searches
This paper describes hybrid search, a search method supporting both document and knowledge retrieval via the flexible combination of ontologybased search and keyword-based matching. Hybrid search smoothly copes with
lack of semantic coverage of document content, which is one of the main limitations of current semantic search methods. In this paper we define hybrid search formally, discuss its compatibility with the current semantic trends and present a reference implementation: K-Search. We then show how the method outperforms both keyword-based search and pure semantic search in terms of precision and recall in a set of experiments performed on a collection of about 18.000 technical documents. Experiments carried out with professional users show that users understand the paradigm and consider it very powerful and reliable. K-Search has been ported to two applications released at Rolls-Royce
plc for searching technical documentation about jet engines
On the scaling behaviour of cross-tie domain wall structures in patterned NiFe elements
The cross-tie domain wall structure in micrometre and sub-micrometre wide
patterned elements of NiFe, and a thickness range of 30 to 70nm, has been
studied by Lorentz microscopy. Whilst the basic geometry of the cross-tie
repeat units remains unchanged, their density increases when the cross-tie
length is constrained to be smaller than the value associated with a continuous
film. This occurs when element widths are sufficiently narrow or when the wall
is forced to move close to an edge under the action of an applied field. To a
very good approximation the cross-tie density scales with the inverse of the
distance between the main wall and the element edge. The experiments show that
in confined structures, the wall constantly modifies its form and that the need
to generate, and subsequently annihilate, extra vortex/anti-vortex pairs
constitutes an additional source of hysteresis.Comment: 4 pages, 5 figures, accepted for publication in Europhysics Letters
(EPL
Report of the Terrestrial Bodies Science Working Group. Volume 8: The comets
The determination of the nuclear and atmospheric properties of comets, and the interaction of the solar wind with the comet tail are scientific objectives for a mission to one or more comets in the next decade. Recommended priorities for direct cometary exploration are listed
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