10,033 research outputs found
Max-Weight Revisited: Sequences of Non-Convex Optimisations Solving Convex Optimisations
We investigate the connections between max-weight approaches and dual
subgradient methods for convex optimisation. We find that strong connections
exist and we establish a clean, unifying theoretical framework that includes
both max-weight and dual subgradient approaches as special cases. Our analysis
uses only elementary methods, and is not asymptotic in nature. It also allows
us to establish an explicit and direct connection between discrete queue
occupancies and Lagrange multipliers.Comment: convex optimisation, max-weight scheduling, backpressure, subgradient
method
Proportional Fair MU-MIMO in 802.11 WLANs
We consider the proportional fair rate allocation in an 802.11 WLAN that
supports multi-user MIMO (MU-MIMO) transmission by one or more stations. We
characterise, for the first time, the proportional fair allocation of MU-MIMO
spatial streams and station transmission opportunities. While a number of
features carry over from the case without MU-MIMO, in general neither flows nor
stations need to be allocated equal airtime when MU-MIMO is available
Phase diagram of the vortex system in layered superconductors with strong columnar pinning
We present the results of a detailed investigation of the low-temperature
properties of the vortex system in strongly anisotropic layered superconductors
with a random array of columnar pinning centers. Our method involves numerical
minimization of a free energy functional in terms of the time-averaged local
vortex density. It yields the detailed vortex density distribution for all
local free-energy minima, and therefore allows the computation of any desired
correlation function of the time-averaged local vortex density. Results for the
phase diagram in the temperature vs. pin concentration plane at constant
magnetic induction are presented. We confirm that for very low pin
concentrations, the low-temperature phase is a Bragg glass, which melts into an
interstitial liquid phase via two first-order steps, separated by a Bose glass
phase. At higher concentrations, however, the low-temperature phase is a Bose
glass, and the melting transition becomes continuous. The transition is then
characterized by the onset of percolation of liquid-like regions across the
sample. Inhomogeneous local melting of the Bose glass is found to occur. There
is also a depinning crossover between the interstitial liquid and a completely
unpinned liquid at higher temperatures. At sufficiently large pin
concentrations, the depinning line merges with the Bose glass to interstitial
liquid transition. Many of the features we find have been observed
experimentally and in simulations. We discuss the implications of our results
for future experimental and theoretical work.Comment: 15 pages including Figure
The phase diagram of vortex matter in layered superconductors with tilted columnar pinning centers
We study the vortex matter phase diagram of a layered superconductor in the
presence of columnar pinning defects, {\it tilted} with respect to the normal
to the layers. We use numerical minimization of the free energy written as a
functional of the time averaged vortex density of the Ramakrishnan-Yussouff
form, supplemented by the appropriate pinning potential. We study the case
where the pin density is smaller than the areal vortex density. At lower pin
concentrations, we find, for temperatures of the order of the melting
temperature of the unpinned lattice, a Bose glass type phase which at lower
temperatures converts, via a first order transition, to a Bragg glass, while,
at higher temperatures, it crosses over to an interstitial liquid. At somewhat
higher concentrations, no transition to a Bragg glass is found even at the
lowest temperatures studied. While qualitatively the behavior we find is
similar to that obtained using the same procedures for columnar pins normal to
the layers, there are important and observable quantitative differences, which
we discuss.Comment: 12 pages, including figure
High rate photon irradiation test of an 8-plane ATLAS transition radiation tracker sector prototype
Strain injection techniques in dynamic fracture modeling
A computationally affordable modeling of dynamic fracture phenomena is performed in this study by using strain injection techniques and Finite Elements with Embedded strong discontinuities (E-FEM). In the present research, classical strain localization and strong discontinuity approaches are considered by injecting discontinuous strain and displacement modes in the finite element formulation without an increase of the total number of degrees of freedom. Following the Continuum Strong Discontinuity Approach (CSDA), stress–strain constitutive laws can be employed in the context of fracture phenomena and, therefore, the methodology remains applicable to a wide number of continuum mechanics models. The position and orientation of the displacement discontinuity is obtained through the solution of a crack propagation problem, i.e. the crack path field, based on the distribution of localized strains. The combination of the above mentioned approaches is envisaged to avoid stress-locking and directional mesh bias phenomena. Dynamic simulations are performed increasing the loading rate up to the appearance of crack branching, and the variation in terms of failure modes is investigated as well as the influence of the strain injection together with the crack path field algorithm. Objectivity of the presented methodology with respect to the spatial and temporal discretization is analyzed in terms of the dissipated energy during the fracture process. The dissipation at the onset of branching is studied for different loading rate conditions and is linked to the experimental maximum velocity observed before branching takes place.Fil: Lloberas Valls, Oriol. Universidad Politecnica de Catalunya; España. Centre Internacional de Metodes Numerics en Enginyeria; EspañaFil: Huespe, Alfredo Edmundo. Centre Internacional de Metodes Numerics en Enginyeria; España. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Santa Fe. Centro de Investigaciones en Métodos Computacionales. Universidad Nacional del Litoral. Centro de Investigaciones en Métodos Computacionales; ArgentinaFil: Oliver, J.. Centre Internacional de Metodes Numerics en Enginyeria; España. Universidad Politecnica de Catalunya; EspañaFil: Dias, I.F.. Laboratório Nacional de Engenharia Civil; Portuga
Phase diagram of vortex matter in layered superconductors with random point pinning
We study the phase diagram of the superconducting vortex system in layered
high-temperature superconductors in the presence of a magnetic field
perpendicular to the layers and of random atomic scale point pinning centers.
We consider the highly anisotropic limit where the pancake vortices on
different layer are coupled only by their electromagnetic interaction. The free
energy of the vortex system is then represented as a Ramakrishnan-Yussouff free
energy functional of the time averaged vortex density. We numerically minimize
this functional and examine the properties of the resulting phases. We find
that, in the temperature () -- pinning strength () plane at constant
magnetic induction, the equilibrium phase at low and is a Bragg glass.
As one increases or a first order phase transition occurs to another
phase that we characterize as a pinned vortex liquid. The weakly pinned vortex
liquid obtained for high and small smoothly crosses over to the
strongly pinned vortex liquid as is decreased or increased -- we do not
find evidence for the existence, in thermodynamic equilibrium, of a distinct
vortex glass phase in the range of pinning parameters considered here. %cdr We
present results for the density correlation functions, the density and defect
distributions, and the local field distribution accessible via SR
experiments. These results are compared with those of existing theoretical,
numerical and experimental studies.Comment: 15 pages, including figures. Higher resolution files for Figs 3a and
11 available from author
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