7,653 research outputs found
Vortex motion around a circular cylinder above a plane
The study of vortex flows around solid obstacles is of considerable interest
from both a theoretical and practical perspective. One geometry that has
attracted renewed attention recently is that of vortex flows past a circular
cylinder placed above a plane wall, where a stationary recirculating eddy can
form in front of the cylinder, in contradistinction to the usual case (without
the plane boundary) for which a vortex pair appears behind the cylinder. Here
we analyze the problem of vortex flows past a cylinder near a wall through the
lenses of the point-vortex model. By conformally mapping the fluid domain onto
an annular region in an auxiliary complex plane, we compute the vortex
Hamiltonian analytically in terms of certain special functions related to
elliptic theta functions. A detailed analysis of the equilibria of the model is
then presented. The location of the equilibrium in front of the cylinder is
shown to be in qualitative agreement with the experimental findings. We also
show that a topological transition occurs in phase space as the parameters of
the systems are variedComment: 17 pages, 8 figure
Gossip Algorithms for Distributed Signal Processing
Gossip algorithms are attractive for in-network processing in sensor networks
because they do not require any specialized routing, there is no bottleneck or
single point of failure, and they are robust to unreliable wireless network
conditions. Recently, there has been a surge of activity in the computer
science, control, signal processing, and information theory communities,
developing faster and more robust gossip algorithms and deriving theoretical
performance guarantees. This article presents an overview of recent work in the
area. We describe convergence rate results, which are related to the number of
transmitted messages and thus the amount of energy consumed in the network for
gossiping. We discuss issues related to gossiping over wireless links,
including the effects of quantization and noise, and we illustrate the use of
gossip algorithms for canonical signal processing tasks including distributed
estimation, source localization, and compression.Comment: Submitted to Proceedings of the IEEE, 29 page
Araçá: cultivar de mandioca para a microrregião do Alto Purus, Acre.
bitstream/CPAF-AC-2010/1262/1/araca-cultivar-de-mandioca-para-a-microrregiao-do-alto-purus.pd
Beyond Counting: New Perspectives on the Active IPv4 Address Space
In this study, we report on techniques and analyses that enable us to capture
Internet-wide activity at individual IP address-level granularity by relying on
server logs of a large commercial content delivery network (CDN) that serves
close to 3 trillion HTTP requests on a daily basis. Across the whole of 2015,
these logs recorded client activity involving 1.2 billion unique IPv4
addresses, the highest ever measured, in agreement with recent estimates.
Monthly client IPv4 address counts showed constant growth for years prior, but
since 2014, the IPv4 count has stagnated while IPv6 counts have grown. Thus, it
seems we have entered an era marked by increased complexity, one in which the
sole enumeration of active IPv4 addresses is of little use to characterize
recent growth of the Internet as a whole.
With this observation in mind, we consider new points of view in the study of
global IPv4 address activity. Our analysis shows significant churn in active
IPv4 addresses: the set of active IPv4 addresses varies by as much as 25% over
the course of a year. Second, by looking across the active addresses in a
prefix, we are able to identify and attribute activity patterns to network
restructurings, user behaviors, and, in particular, various address assignment
practices. Third, by combining spatio-temporal measures of address utilization
with measures of traffic volume, and sampling-based estimates of relative host
counts, we present novel perspectives on worldwide IPv4 address activity,
including empirical observation of under-utilization in some areas, and
complete utilization, or exhaustion, in others.Comment: in Proceedings of ACM IMC 201
Two-dimensional dispersion analytical approach: Eddy diffusivities depending on source distance
An analytical air quality dispersion approach based on the steadystate two-dimensional advection-diffusion equation is presented. The solution employs a spectral method and is analytical in the sense that no approximation is made along its derivation. The approach is valid for homogeneous turbulence and for situations of uniform mean wind speed, and for practical purposes, to elevated releases that occur in neutral stability conditions without strong buoyancy. To
simulate and compare the results of this approach against observed ground-level crosswind-integrated concentration two eddy diffusivities are considered. The first eddy diffusivity depends on the distance from the source while the second one assumes a constant value independent of the source distance. It is found that the memory effect contained in the eddy diffusivity, which is a function of downwind distance from the source, allows a better description of the turbulent dispersion of atmospheric contaminants released by an elevated continuous point source
Four-vortex motion around a circular cylinder
The motion of two pairs of counter-rotating point vortices placed in a
uniform flow past a circular cylinder is studied analytically and numerically.
When the dynamics is restricted to the symmetric subspace---a case that can be
realized experimentally by placing a splitter plate in the center plane---, it
is found that there is a family of linearly stable equilibria for same-signed
vortex pairs. The nonlinear dynamics in the symmetric subspace is investigated
and several types of orbits are presented. The analysis reported here provides
new insights and reveals novel features of this four-vortex system, such as the
fact that there is no equilibrium for two pairs of vortices of opposite signs
on the opposite sides of the cylinder. (It is argued that such equilibria might
exist for vortex flows past a cylinder confined in a channel.) In addition, a
new family of opposite-signed equilibria on the normal line is reported. The
stability analysis for antisymmetric perturbations is also carried out and it
shows that all equilibria are unstable in this case.Comment: 27 pages, 13 figures, to be published in Physics of Fluid
Respiratory versatility in Desulfovibrio desulfuricans ATCC 27774 – a proteomic approach
Poster presented at the Bacterial Electron Transfer Processes and their Regulation Meeting, European Federation of Biotechnology Microbial Physiology Section, 15-18 March 2015, Vimeiro, Portugal
- …