9,830 research outputs found
Flows on Graphs with Random Capacities
We investigate flows on graphs whose links have random capacities. For binary
trees we derive the probability distribution for the maximal flow from the root
to a leaf, and show that for infinite trees it vanishes beyond a certain
threshold that depends on the distribution of capacities. We then examine the
maximal total flux from the root to the leaves. Our methods generalize to
simple graphs with loops, e.g., to hierarchical lattices and to complete
graphs.Comment: 8 pages, 6 figure
Satellite downlink scheduling problem: A case study
The synthetic aperture radar (SAR) technology enables satellites to
efficiently acquire high quality images of the Earth surface. This generates
significant communication traffic from the satellite to the ground stations,
and, thus, image downlinking often becomes the bottleneck in the efficiency of
the whole system. In this paper we address the downlink scheduling problem for
Canada's Earth observing SAR satellite, RADARSAT-2. Being an applied problem,
downlink scheduling is characterised with a number of constraints that make it
difficult not only to optimise the schedule but even to produce a feasible
solution. We propose a fast schedule generation procedure that abstracts the
problem specific constraints and provides a simple interface to optimisation
algorithms. By comparing empirically several standard meta-heuristics applied
to the problem, we select the most suitable one and show that it is clearly
superior to the approach currently in use.Comment: 23 page
Noise Measurements of a Low-Noise Top-Mounted Propulsion Installation for a Supersonic Airliner
A model-scale exhaust system was tested to validate low-noise concepts and noise prediction methods. The tests involved far-field acoustics, translating phased array, and particle image velocimetry; this report covers the far-field acoustic measurements. Data were acquired for a series of nozzles with different chevron designs, both uninstalled and installed on a representative aircraft planform. The impact of the various chevron treatments on the far-field noise was documented, along with the impact of the pylon and planform. For the baseline nozzle, installation produced a 2EPNdB reduction, as assumed in system studies. Chevrons were used to shift noise sources upstream to maximize the installation benefits and to reduce unshielded sources downstream. These resulted in reductions of 4-5EPNdB relative to the uninstalled baseline nozzle. Detailed analysis of spectral directivities behind the integrated EPNL metric gave insight into how well these concepts actually work. When correlated with particle image velocimetry measurements and phased array measurements, reported in companion papers, the explanation of acoustic benefits from top-mounted propulsion is clear as is the path toward optimization of the concept
The Price of Anarchy in Transportation Networks: Efficiency and Optimality Control
Uncoordinated individuals in human society pursuing their personally optimal
strategies do not always achieve the social optimum, the most beneficial state
to the society as a whole. Instead, strategies form Nash equilibria which are
often socially suboptimal. Society, therefore, has to pay a price of anarchy
for the lack of coordination among its members. Here we assess this price of
anarchy by analyzing the travel times in road networks of several major cities.
Our simulation shows that uncoordinated drivers possibly waste a considerable
amount of their travel time. Counterintuitively,simply blocking certain streets
can partially improve the traffic conditions. We analyze various complex
networks and discuss the possibility of similar paradoxes in physics.Comment: major revisions with multicommodity; Phys. Rev. Lett., accepte
Hybrid density functional study of electronic and optical properties of phase change memory material:
In this article, we use hybrid density functional (HSE06) to study the
crystal and electronic structures and optical properties of well known phase
change memory material . We calculate the
structural parameters, band gaps and dielectric functions of three stable
structures of this material. We also analyze the electron charge distribution
using the Bader's theory of charge analysis. We find that hybrid density
functional slightly overestimate the value of 'C' parameter. However, overall,
our results calculated with the use of hybrid density functional (HSE06) are
very close to available experimental values than calculated with the use of PBE
functional. Specifically, the electronic band gap values of this material
calculated with HSE06 are in good agreement with the available experimental
data in the literature. Furthermore, we perform the charge analysis and find
that naive ionic model fails to explain the charge distribution between the
constituent atoms, showing the complex nature of this compound.Comment: 10 pages, 3 tables, 3 figure
Electronic structure and chemical bonding in Ti2AlC investigated by soft x-ray emission spectroscopy
The electronic structure of the nanolaminated transition metal carbide Ti2AlC
has been investigated by bulk-sensitive soft x-ray emission spectroscopy. The
measured Ti L, C K and Al L emission spectra are compared with calculated
spectra using ab initio density-functional theory including dipole matrix
elements. The detailed investigation of the electronic structure and chemical
bonding provides increased understanding of the physical properties of this
type of nanolaminates. Three different types of bond regions are identified;
the relatively weak Ti 3d - Al 3p hybridization 1 eV below the Fermi level, and
the Ti 3d - C 2p and Ti 3d - C 2s hybridizations which are stronger and deeper
in energy are observed around 2.5 eV and 10 eV below the Fermi level,
respectively. A strongly modified spectral shape of the 3s final states in
comparison to pure Al is detected for the buried Al monolayers indirectly
reflecting the Ti 3d - Al 3p hybridization. The differences between the
electronic and crystal structures of Ti2AlC, Ti3AlC2 and TiC are discussed in
relation to the number of Al layers per Ti layer in the two former systems and
the corresponding change of the unusual materials properties.Comment: 14 pages, 7 figures; PACS:78.70.En, 71.15.Mb, 71.20.-
Key influences in the design and implementation of mental health information systems in Ghana and South Africa
Introduction Strengthening of mental health information systems (MHIS) is essential to monitor and evaluate mental health services in low and middle-income countries. While research exists assessing wider health management information systems, there is limited published evidence exploring the design and implementation of MHIS in these settings. This paper aims to identify and assess the key factors affecting the design and implementation of MHIS, as perceived by the key stakeholders in Ghana and South Africa. Methods We report findings from the Mental Health and Poverty Project, a 5-year research programme implemented within four African countries. The MHIS strengthening in South Africa and Ghana included two related components: intervention and research. The intervention component aimed to strengthen MHIS in the two countries, and the research component aimed to document interventions in each country, including the key influences. Data were collected using semi structured interviews with key stakeholders and reviews of key documents and secondary data from the improved MHIS. We analyzed the qualitative data using a framework approach. Results Key components of the MHIS intervention involved the introduction of a redesigned patient registration form, entry into computers for analysis every 2 months by clinical managerial staff, and utilization of data in hospital management meetings in three psychiatric hospitals in Ghana; and the introduction of a new set of mental health indicators and related forms and tally sheets at primary care clinics and district hospitals in five districts in the KwaZulu-Natal and Northern Cape provinces in South Africa. Overall, the key stakeholders perceived the MHIS strengthening as an effective intervention in both countries with an enhanced set of indicators in South Africa and introduction of a computerized system in Ghana. Discussion Influences on the design and implementation of MHIS interventions in Ghana and South Africa relate to resources, working approaches (including degree of consultations during the design stage and communication during implementation stage) and the low priority of mental health. Although the influencing factors represent similar categories, more influences were identified on MHIS implementation, compared with the design stage. Different influences appear to be related within, and across, the MHIS design and implementation and may reinforce or negate each other thus leading to the multiplier or minimization effects. The wider context, similar to other studies, is important in ensuring the success of such interventions. Conclusion Future MHIS strengthening interventions can consider three policy implications which emerged from our analysis and experience: enhancing consultations during the intervention design, better consideration of implementation challenges during design, and better recognition of relations between different influence
Random manifolds in non-linear resistor networks: Applications to varistors and superconductors
We show that current localization in polycrystalline varistors occurs on
paths which are, usually, in the universality class of the directed polymer in
a random medium. We also show that in ceramic superconductors, voltage
localizes on a surface which maps to an Ising domain wall. The emergence of
these manifolds is explained and their structure is illustrated using direct
solution of non-linear resistor networks
Optimal Traffic Networks
Inspired by studies on the airports' network and the physical Internet, we
propose a general model of weighted networks via an optimization principle. The
topology of the optimal network turns out to be a spanning tree that minimizes
a combination of topological and metric quantities. It is characterized by a
strongly heterogeneous traffic, non-trivial correlations between distance and
traffic and a broadly distributed centrality. A clear spatial hierarchical
organization, with local hubs distributing traffic in smaller regions, emerges
as a result of the optimization. Varying the parameters of the cost function,
different classes of trees are recovered, including in particular the minimum
spanning tree and the shortest path tree. These results suggest that a
variational approach represents an alternative and possibly very meaningful
path to the study of the structure of complex weighted networks.Comment: 4 pages, 4 figures, final revised versio
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