268 research outputs found
Graph Annotations in Modeling Complex Network Topologies
The coarsest approximation of the structure of a complex network, such as the
Internet, is a simple undirected unweighted graph. This approximation, however,
loses too much detail. In reality, objects represented by vertices and edges in
such a graph possess some non-trivial internal structure that varies across and
differentiates among distinct types of links or nodes. In this work, we
abstract such additional information as network annotations. We introduce a
network topology modeling framework that treats annotations as an extended
correlation profile of a network. Assuming we have this profile measured for a
given network, we present an algorithm to rescale it in order to construct
networks of varying size that still reproduce the original measured annotation
profile.
Using this methodology, we accurately capture the network properties
essential for realistic simulations of network applications and protocols, or
any other simulations involving complex network topologies, including modeling
and simulation of network evolution. We apply our approach to the Autonomous
System (AS) topology of the Internet annotated with business relationships
between ASs. This topology captures the large-scale structure of the Internet.
In depth understanding of this structure and tools to model it are cornerstones
of research on future Internet architectures and designs. We find that our
techniques are able to accurately capture the structure of annotation
correlations within this topology, thus reproducing a number of its important
properties in synthetically-generated random graphs
Noise figure and photon probability distribution in Coherent Anti-Stokes Raman Scattering (CARS)
The noise figure and photon probability distribution are calculated for
coherent anti-Stokes Raman scattering (CARS) where an anti-Stokes signal is
converted to Stokes. We find that the minimum noise figure is ~ 3dB.Comment: 2 page
Hyperbolic Geometry of Complex Networks
We develop a geometric framework to study the structure and function of
complex networks. We assume that hyperbolic geometry underlies these networks,
and we show that with this assumption, heterogeneous degree distributions and
strong clustering in complex networks emerge naturally as simple reflections of
the negative curvature and metric property of the underlying hyperbolic
geometry. Conversely, we show that if a network has some metric structure, and
if the network degree distribution is heterogeneous, then the network has an
effective hyperbolic geometry underneath. We then establish a mapping between
our geometric framework and statistical mechanics of complex networks. This
mapping interprets edges in a network as non-interacting fermions whose
energies are hyperbolic distances between nodes, while the auxiliary fields
coupled to edges are linear functions of these energies or distances. The
geometric network ensemble subsumes the standard configuration model and
classical random graphs as two limiting cases with degenerate geometric
structures. Finally, we show that targeted transport processes without global
topology knowledge, made possible by our geometric framework, are maximally
efficient, according to all efficiency measures, in networks with strongest
heterogeneity and clustering, and that this efficiency is remarkably robust
with respect to even catastrophic disturbances and damages to the network
structure
Stabilization of Hydrodynamic Flows by Small Viscosity Variations
Motivated by the large effect of turbulent drag reduction by minute
concentrations of polymers we study the effects of a weakly space-dependent
viscosity on the stability of hydrodynamic flows. In a recent Letter [Phys.
Rev. Lett. {\bf 87}, 174501, (2001)] we exposed the crucial role played by a
localized region where the energy of fluctuations is produced by interactions
with the mean flow (the "critical layer"). We showed that a layer of weakly
space-dependent viscosity placed near the critical layer can have a very large
stabilizing effect on hydrodynamic fluctuations, retarding significantly the
onset of turbulence. In this paper we extend these observation in two
directions: first we show that the strong stabilization of the primary
instability is also obtained when the viscosity profile is realistic (inferred
from simulations of turbulent flows with a small concentration of polymers).
Second, we analyze the secondary instability (around the time-dependent primary
instability) and find similar strong stabilization. Since the secondary
instability develops around a time-dependent solution and is three-dimensional,
this brings us closer to the turbulent case. We reiterate that the large effect
is {\em not} due to a modified dissipation (as is assumed in some theories of
drag reduction), but due to reduced energy intake from the mean flow to the
fluctuations. We propose that similar physics act in turbulent drag reduction.Comment: 10 pages, 17 figs., REVTeX4, PRE, submitte
Drag Reduction by Polymers in Turbulent Channel Flows: Energy Redistribution Between Invariant Empirical Modes
We address the phenomenon of drag reduction by dilute polymeric additive to
turbulent flows, using Direct Numerical Simulations (DNS) of the FENE-P model
of viscoelastic flows. It had been amply demonstrated that these model
equations reproduce the phenomenon, but the results of DNS were not analyzed so
far with the goal of interpreting the phenomenon. In order to construct a
useful framework for the understanding of drag reduction we initiate in this
paper an investigation of the most important modes that are sustained in the
viscoelastic and Newtonian turbulent flows respectively. The modes are obtained
empirically using the Karhunen-Loeve decomposition, allowing us to compare the
most energetic modes in the viscoelastic and Newtonian flows. The main finding
of the present study is that the spatial profile of the most energetic modes is
hardly changed between the two flows. What changes is the energy associated
with these modes, and their relative ordering in the decreasing order from the
most energetic to the least. Modes that are highly excited in one flow can be
strongly suppressed in the other, and vice versa. This dramatic energy
redistribution is an important clue to the mechanism of drag reduction as is
proposed in this paper. In particular there is an enhancement of the energy
containing modes in the viscoelastic flow compared to the Newtonian one; drag
reduction is seen in the energy containing modes rather than the dissipative
modes as proposed in some previous theories.Comment: 11 pages, 13 figures, included, PRE, submitted, REVTeX
A simple model for drag reduction
Direct Numerical Simulations established that the FENE-P model of
viscoelastic flows exhibits the phenomenon of turbulent drag reduction which is
caused in experiments by dilute polymeric additives. To gain analytic
understanding of the phenomenon we introduce in this Letter a simple
1-dimensional model of the FENE-P equations. We demonstrate drag reduction in
the simple model, and explain analytically the main observations which include
(i) reduction of velocity gradients for fixed throughput and (ii) increase of
throughput for fixed dissipation.Comment: submitted to PR
877-3 Effect of percutaneous transluminal renal artery angioplasty with stenting on renal function in patients with atherosclerotic renal artery stenosis
INdigenous Systems and Policies Improved and Reimagined for Ear and hearing care (INSPIRE): A multi-method study protocol
Introduction Otitis media (middle ear disease) severity and chronicity among Aboriginal and Torres Strait Islander people, as well as gaps in socioeconomic outcomes related to hearing loss, indicates a breakdown in the current ear and hearing care system. The ear and hearing care system spans multiple sectors due to long-term impacts of otitis media and hearing loss in health, education and employment, necessitating a multi-disciplinary cross-sectorial approach to ear and hearing care. Public policies shape the current ear and hearing care system and here it is argued that a critical public policy analysis across different sectors is needed, with strong Aboriginal and Torres Strait Islander leadership and guidance. The current study aims to establish consensus-based ear and hearing care policy solutions for Aboriginal and Torres Strait Islander people in Australia. Methods and analysis This multi-method study will be guided by a Brains Trust with strong Aboriginal and Torres Strait Islander leadership. Public policies in hearing health, social services, and education will be scoped to identify policy gaps, using the World Health Organization framework. Qualitative data will be collected through a culturally specific process of yarning circles to identify policy challenges and/or limitations in enabling accessible ear and hearing care programs/services for Aboriginal and Torres Strait Islander people, using dimensions of Morestin's public policy appraisal tool as an interview guide for stakeholders. Themes from the yarning circles will be used to inform an expert Delphi process to establish consensus-based policy solutions for optimising the ear and hearing care system for Aboriginal and Torres Strait Islander people. Ethics and dissemination This study has approval from the Australian Institute of Aboriginal and Torres Strait Islander Studies Ethics Committee. Study findings will be disseminated to community through Brains Trust members and study participants, as well as through publications in peer-reviewed journals and research forum presentations
Structural efficiency of percolation landscapes in flow networks
Complex networks characterized by global transport processes rely on the
presence of directed paths from input to output nodes and edges, which organize
in characteristic linked components. The analysis of such network-spanning
structures in the framework of percolation theory, and in particular the key
role of edge interfaces bridging the communication between core and periphery,
allow us to shed light on the structural properties of real and theoretical
flow networks, and to define criteria and quantities to characterize their
efficiency at the interplay between structure and functionality. In particular,
it is possible to assess that an optimal flow network should look like a "hairy
ball", so to minimize bottleneck effects and the sensitivity to failures.
Moreover, the thorough analysis of two real networks, the Internet
customer-provider set of relationships at the autonomous system level and the
nervous system of the worm Caenorhabditis elegans --that have been shaped by
very different dynamics and in very different time-scales--, reveals that
whereas biological evolution has selected a structure close to the optimal
layout, market competition does not necessarily tend toward the most customer
efficient architecture.Comment: 8 pages, 5 figure
On the observational determination of squeezing in relic gravitational waves and primordial density perturbations
We develop a theory in which relic gravitational waves and primordial density
perturbations are generated by strong variable gravitational field of the early
Universe. The generating mechanism is the superadiabatic (parametric)
amplification of the zero-point quantum oscillations. The generated fields have
specific statistical properties of squeezed vacuum quantum states.
Macroscopically, squeezing manifests itself in a non-stationary character of
variances and correlation functions of the fields, the periodic structures of
the metric power spectra, and, as a consequence, in oscillatory behavior of the
higher order multipoles C_l of the cosmic microwave background anisotropy. We
start with the gravitational wave background and then apply the theory to
primordial density perturbations. We derive an analytical formula for the
positions of peaks and dips in the angular power spectrum l(l+1)C_l as a
function of l. This formula shows that the values of l at the peak positions
are ordered in the proportion 1:3:5:..., whereas at the dips they are ordered
as 1:2:3:.... We compare the derived positions with the actually observed
features, and find them to be in reasonably good agreement. It appears that the
observed structure is better described by our analytical formula based on the
(squeezed) metric perturbations associated with the primordial density
perturbations, rather than by the acoustic peaks reflecting the existence of
plasma sound waves at the last scattering surface. We formulate a forecast for
other features in the angular power spectrum, that may be detected by the
advanced observational missions, such as MAP and PLANCK. We tentatively
conclude that the observed structure is a macroscopic manifestation of
squeezing in the primordial metric perturbations.Comment: 34 pages, 3 figures; to appear in Phys. Rev. D66, 0435XX (2002);
includes Note Added in Proofs: "The latest CBI observations (T.J.Pearson et
al., astro-ph/0205388) have detected four peaks, at l ~ 550, 800, 1150, 1500,
and four dips, at l ~ 400, 700, 1050, 1400. These positions are in a very
good agreement with the theoretical formula (6.35) of the present paper. We
interpret this data as confirmation of our conclusion that it is gravity, and
not acoustics, that is responsible for the observed structure.
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