12,598 research outputs found
A note on Stokes' problem in dense granular media using the --rheology
The classical Stokes' problem describing the fluid motion due to a steadily
moving infinite wall is revisited in the context of dense granular flows of
mono-dispersed beads using the recently proposed --rheology. In
Newtonian fluids, molecular diffusion brings about a self-similar velocity
profile and the boundary layer in which the fluid motion takes place increases
indefinitely with time as , where is the kinematic
viscosity. For a dense granular visco-plastic liquid, it is shown that the
local shear stress, when properly rescaled, exhibits self-similar behaviour at
short-time scales and it then rapidly evolves towards a steady-state solution.
The resulting shear layer increases in thickness as analogous
to a Newtonian fluid where is an equivalent granular kinematic
viscosity depending not only on the intrinsic properties of the granular media
such as grain diameter , density and friction coefficients but also
on the applied pressure at the moving wall and the solid fraction
(constant). In addition, the --rheology indicates that this growth
continues until reaching the steady-state boundary layer thickness , independent of the grain size, at about a finite
time proportional to , where is
the acceleration due to gravity and is the
relative surplus of the steady-state wall shear-stress over the
critical wall shear stress (yield stress) that is needed to bring the
granular media into motion... (see article for a complete abstract).Comment: in press (Journal of Fluid Mechanics
Weak helix submanifolds of euclidean spaces
It is shown that there exist nonstrong weak 2-helix surfaces of R
Entanglement negativity in quantum field theory
We develop a systematic method to extract the negativity in the ground state
of a 1+1 dimensional relativistic quantum field theory, using a path integral
formalism to construct the partial transpose rho_A^{T_2} of the reduced density
matrix of a subsystem A=A1 U A2, and introducing a replica approach to obtain
its trace norm which gives the logarithmic negativity E=ln||\rho_A^{T_2}||.
This is shown to reproduce standard results for a pure state. We then apply
this method to conformal field theories, deriving the result E\sim(c/4) ln(L1
L2/(L1+L2)) for the case of two adjacent intervals of lengths L1, L2 in an
infinite system, where c is the central charge. For two disjoint intervals it
depends only on the harmonic ratio of the four end points and so is manifestly
scale invariant. We check our findings against exact numerical results in the
harmonic chain.Comment: 4 pages, 5 figure
Quantum Measurement and the Aharonov-Bohm Effect with Superposed Magnetic Fluxes
We consider the magnetic flux in a quantum mechanical superposition of two
values and find that the Aharonov-Bohm effect interference pattern contains
information about the nature of the superposition, allowing information about
the state of the flux to be extracted without disturbance. The information is
obtained without transfer of energy or momentum and by accumulated nonlocal
interactions of the vector potential with many charged particles
forming the interference pattern, rather than with a single particle. We
suggest an experimental test using already experimentally realized superposed
currents in a superconducting ring and discuss broader implications.Comment: 6 pages, 4 figures; Changes from version 3: corrected typo (not
present in versions 1 and 2) in Eq. 8; Changes from version 2: shortened
abstract; added refs and material in Section IV. The final publication is
available at: http://link.springer.com/article/10.1007/s11128-013-0652-
Entanglement entropy and quantum field theory: a non-technical introduction
In these proceedings we give a pedagogical and non-technical introduction to
the Quantum Field Theory approach to entanglement entropy. Particular attention
is devoted to the one space dimensional case, with a linear dispersion
relation, that, at a quantum critical point, can be effectively described by a
two-dimensional Conformal Field Theory.Comment: 10 Pages, 2 figures. Talk given at the conference "Entanglement in
Physical and information sciences", Centro Ennio de Giorgi, Pisa, December
200
An integrable modification of the critical Chalker-Coddington network model
We consider the Chalker-Coddington network model for the Integer Quantum Hall
Effect, and examine the possibility of solving it exactly. In the
supersymmetric path integral framework, we introduce a truncation procedure,
leading to a series of well-defined two-dimensional loop models, with two loop
flavours. In the phase diagram of the first-order truncated model, we identify
four integrable branches related to the dilute Birman-Wenzl-Murakami
braid-monoid algebra, and parameterised by the loop fugacity . In the
continuum limit, two of these branches (1,2) are described by a pair of
decoupled copies of a Coulomb-Gas theory, whereas the other two branches (3,4)
couple the two loop flavours, and relate to an Wess-Zumino-Witten (WZW) coset model for the particular values where is a positive integer. The truncated
Chalker-Coddington model is the point of branch 4. By numerical
diagonalisation, we find that its universality class is neither an analytic
continuation of the WZW coset, nor the universality class of the original
Chalker-Coddington model. It constitutes rather an integrable, critical
approximation to the latter.Comment: 34 pages, 18 figures, 3 appendice
Field trials and test results of portable DVB-T systems with transmit delay diversity
This paper describes work carried out by Brunel University and Broadreach Systems (UK) to quantify the advantages that can be achieved if Transmit Diversity is applied to systems employing the DVB standard. The techniques investigated can be applied to standard receiver equipment without modification. An extensive and carefully planned field trial was performed during the winter of 2007/2008 in Uxbridge (UK) to validate predictions from theoretical modeling and laboratory simulations. The transmissions were performed in the 730 MHz frequency band with a DVB-T transmitter and a mean power of 18.4dBW. Transmit delay diversity has been observed to deliver significant reception improvement in automotive and indoor- non line of sight situations
Curvature Matrix Models for Dynamical Triangulations and the Itzykson-DiFrancesco Formula
We study the large-N limit of a class of matrix models for dually weighted
triangulated random surfaces using character expansion techniques. We show that
for various choices of the weights of vertices of the dynamical triangulation
the model can be solved by resumming the Itzykson-Di Francesco formula over
congruence classes of Young tableau weights modulo three. From this we show
that the large-N limit implies a non-trivial correspondence with models of
random surfaces weighted with only even coordination number vertices. We
examine the critical behaviour and evaluation of observables and discuss their
interrelationships in all models. We obtain explicit solutions of the model for
simple choices of vertex weightings and use them to show how the matrix model
reproduces features of the random surface sum. We also discuss some general
properties of the large-N character expansion approach as well as potential
physical applications of our results.Comment: 37 pages LaTeX; Some clarifying comments added, last Section
rewritte
A novel decentralised system architecture for multi-camera target tracking
Target tracking in a multi-camera system is an active and challenging research that in many systems requires video synchronisation and knowledge of the camera set-up and layout. In this paper a highly flexible, modular and decentralised system architecture is presented for multi-camera target tracking with relaxed synchronisation constraints among camera views. Moreover, the system does not rely on positional information to handle camera hand-off events. As a practical application, the system itself can, at any time, automatically select the best target view available, to implicitly solve occlusion. Further, to validate the proposed architecture, an extension to a multi-camera environment of the colour-based IMS-SWAD tracker is used. The experimental results show that the tracker can successfully track a chosen target in multiple views, in both indoor and outdoor environments, with non-overlapping and overlapping camera views
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