9,188 research outputs found
Temporal Analysis of Activity Patterns of Editors in Collaborative Mapping Project of OpenStreetMap
In the recent years Wikis have become an attractive platform for social
studies of the human behaviour. Containing millions records of edits across the
globe, collaborative systems such as Wikipedia have allowed researchers to gain
a better understanding of editors participation and their activity patterns.
However, contributions made to Geo-wikis_wiki-based collaborative mapping
projects_ differ from systems such as Wikipedia in a fundamental way due to
spatial dimension of the content that limits the contributors to a set of those
who posses local knowledge about a specific area and therefore cross-platform
studies and comparisons are required to build a comprehensive image of online
open collaboration phenomena. In this work, we study the temporal behavioural
pattern of OpenStreetMap editors, a successful example of geo-wiki, for two
European capital cities. We categorise different type of temporal patterns and
report on the historical trend within a period of 7 years of the project age.
We also draw a comparison with the previously observed editing activity
patterns of Wikipedia.Comment: Submitte
Shock Diffraction by Convex Cornered Wedges for the Nonlinear Wave System
We are concerned with rigorous mathematical analysis of shock diffraction by
two-dimensional convex cornered wedges in compressible fluid flow governed by
the nonlinear wave system. This shock diffraction problem can be formulated as
a boundary value problem for second-order nonlinear partial differential
equations of mixed elliptic-hyperbolic type in an unbounded domain. It can be
further reformulated as a free boundary problem for nonlinear degenerate
elliptic equations of second order. We establish a first global theory of
existence and regularity for this shock diffraction problem. In particular, we
establish that the optimal regularity for the solution is across the
degenerate sonic boundary. To achieve this, we develop several mathematical
ideas and techniques, which are also useful for other related problems
involving similar analytical difficulties.Comment: 50 pages;7 figure
Characterization of gravity current formation for the use in detonation refraction experiments
Detonation propagation through an interface is being studied at Caltech. In these
experiments, the interface shape is determined by the gravity currents. This report
presents an experimental study of the formation and the development of these
gravity currents by an analog system in a water channel using water and salt water to
simulate the density differences in detonation experiments. The major parameters
such as the Reynolds number and the density difference were matched in both
experiments to be able to compare the gravity current in the water channel and the
gravity current in the Galcit detonation tube. In the present study, the gravity current
was generated by the removal of a plate, and was visualized by adding food dye. The
results confirm previous studies; Keulegan demonstrated in 1957 that the velocity of
the gravity current is a function of the square root of the density difference. The
interface is affect by the retracting of the plate which creates the wake effects. The
Kelvin-Helmholtz instabilities on the upstream side of the gravity current create the
visualized mixing zones. Finally, this study revealed how the gravity current interface
into the Galcit detonation tube should develop and where the mixing zones should
occur
The climate emergency across business, community and campaign groups: motivations and barriers to driving change for the common good
Initiatives to help tackle the climate emergency have tended to focus on large-scale actions that governments can take and smaller-scale actions for individuals, with relatively limited attention given to the mass of groups that form society between these two ends of the spectrum. The aim here is to help close that gap and the chosen areas of study are business, community, and campaign groups. The approach was to assess the existing literature and to conduct three focus groups to identify what motivates these groups to take actions on climate change, while also uncovering the barriers that may drive choices of inaction. The findings indicate that multiple factors motivate environmental engagements across business, community, and campaign groups, but personal ethics and concern for the future play the most vital roles. The most common barriers cited were difficulties in accessing support schemes, a lack of clear accessible information, and financial implications. Better networking and knowledge exchange are considered essential for meaningful progress. This research provides a new framework, upon which many organisations can be better motivated to take actions in helping deal with the global climate emergency facing humanity. Practical action guides may be developed from existing materials, and small taskforces could be trained to provide direct hands-on support to groups across society, especially those in SMEs and local communities. The energies of younger people and campaigners, combined with the experiences of other generations, would create a powerful force for good
Auger de-excitation of metastable molecules at metallic surfaces
We study secondary electron emission from metallic surfaces due to Auger
de-excitation of diatomic metastable molecules. Our approach is based on an
effective model for the two active electrons involved in the process -- a
molecular electron described by a linear combination of atomic orbitals when it
is bound and a two-center Coulomb wave when it is not and a metal electron
described by the eigenfunctions of a step potential -- and employs Keldysh
Green's functions. Solving the Dyson equation for the retarded Green's function
by exponential resummation we are able to treat time-nonlocal self-energies and
to avoid the wide-band approximation.Results are presented for the
de-excitation of \NitrogenDominantMetastableState\ on aluminum and tungsten and
discussed in view of previous experimental and theoretical investigations. We
find quantitative agreement with experimental data for tungsten indicating that
the effective model captures the physics of the process quite well. For
aluminum we predict secondary electron emission due to Auger de-excitation to
be one to two orders of magnitude smaller than the one found for resonant
charge-transfer and subsequent auto-detachment.Comment: 15 pages, 9 figures, revised version using an improved
single-electron basi
Collaborative semantic web browsing with Magpie
Web browsing is often a collaborative activity. Users involved in a joint information gathering exercise will wish to share knowledge about the web pages visited and the contents found. Magpie is a suite of tools supporting the interpretation of web pages and semantically enriched web browsing. By automatically associating an ontology-based semantic layer to web resources, Magpie allows relevant services to be invoked as well as remotely triggered within a standard web browser. In this paper we describe how Magpie trigger services can provide semantic support to collaborative browsing activities
Phase Space Transport in Noisy Hamiltonian Systems
This paper analyses the effect of low amplitude friction and noise in
accelerating phase space transport in time-independent Hamiltonian systems that
exhibit global stochasticity. Numerical experiments reveal that even very weak
non-Hamiltonian perturbations can dramatically increase the rate at which an
ensemble of orbits penetrates obstructions like cantori or Arnold webs, thus
accelerating the approach towards an invariant measure, i.e., a
near-microcanonical population of the accessible phase space region. An
investigation of first passage times through cantori leads to three
conclusions, namely: (i) that, at least for white noise, the detailed form of
the perturbation is unimportant, (ii) that the presence or absence of friction
is largely irrelevant, and (iii) that, overall, the amplitude of the response
to weak noise scales logarithmically in the amplitude of the noise.Comment: 13 pages, 3 Postscript figures, latex, no macors. Annals of the New
York Academy of Sciences, in pres
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