102,118 research outputs found
Micro-Macro relations for flow through random arrays of cylinders
The transverse permeability for creeping flow through unidirectional random arrays of fibers with various structures is revisited theoretically and numerically using the finite element method (FEM). The microstructure at various porosities has a strong effect on the transport properties, like permeability, of fibrous materials. We compare different microstructures (due to four random generator algorithms) as well as the effect of boundary conditions, finite size, homogeneity and isotropy of the structure on the macroscopic permeability of the fibrous medium. Permeability data for different minimal distances collapse when their minimal value is subtracted, which yields an empirical macroscopic permeability master function of porosity. Furthermore, as main result, a microstructural model is developed based on the lubrication effect in the narrow channels between neighboring fibers. The numerical experiments suggest a unique, scaling power law relationship between the permeability obtained from fluid flow simulations and the mean value of the shortest Delaunay triangulation edges (constructed using the centers of the fibers), which is identical to the averaged second nearest neighbor fiber distances. This universal lubrication relation, as valid in a wide range of porosities, accounts for the microstructure, e.g. hexagonally ordered or disordered fibrous media. It is complemented by a closure relation that relates the effective microscopic length to the packing fraction
Effects of Disorder on Electron Transport in Arrays of Quantum Dots
We investigate the zero-temperature transport of electrons in a model of
quantum dot arrays with a disordered background potential. One effect of the
disorder is that conduction through the array is possible only for voltages
across the array that exceed a critical voltage . We investigate the
behavior of arrays in three voltage regimes: below, at and above the critical
voltage. For voltages less than , we find that the features of the
invasion of charge onto the array depend on whether the dots have uniform or
varying capacitances. We compute the first conduction path at voltages just
above using a transfer-matrix style algorithm. It can be used to
elucidate the important energy and length scales. We find that the geometrical
structure of the first conducting path is essentially unaffected by the
addition of capacitive or tunneling resistance disorder. We also investigate
the effects of this added disorder to transport further above the threshold. We
use finite size scaling analysis to explore the nonlinear current-voltage
relationship near . The scaling of the current near ,
, gives similar values for the effective exponent
for all varieties of tunneling and capacitive disorder, when the current is
computed for voltages within a few percent of threshold. We do note that the
value of near the transition is not converged at this distance from
threshold and difficulties in obtaining its value in the limit
Architectural support for task dependence management with flexible software scheduling
The growing complexity of multi-core architectures has motivated a wide range of software mechanisms to improve the orchestration of parallel executions. Task parallelism has become a very attractive approach thanks to its programmability, portability and potential for optimizations. However, with the expected increase in core counts, finer-grained tasking will be required to exploit the available parallelism, which will increase the overheads introduced by the runtime system. This work presents Task Dependence Manager (TDM), a hardware/software co-designed mechanism to mitigate runtime system overheads. TDM introduces a hardware unit, denoted Dependence Management Unit (DMU), and minimal ISA extensions that allow the runtime system to offload costly dependence tracking operations to the DMU and to still perform task scheduling in software. With lower hardware cost, TDM outperforms hardware-based solutions and enhances the flexibility, adaptability and composability of the system. Results show that TDM improves performance by 12.3% and reduces EDP by 20.4% on average with respect to a software runtime system. Compared to a runtime system fully implemented in hardware, TDM achieves an average speedup of 4.2% with 7.3x less area requirements and significant EDP reductions. In addition, five different software schedulers are evaluated with TDM, illustrating its flexibility and performance gains.This work has been supported by the RoMoL ERC Advanced Grant (GA 321253), by the European HiPEAC Network of Excellence, by the Spanish Ministry of Science and
Innovation (contracts TIN2015-65316-P, TIN2016-76635-C2-2-R and TIN2016-81840-REDT), by the Generalitat de Catalunya (contracts 2014-SGR-1051 and 2014-SGR-1272), and by the European Union’s Horizon 2020 research and innovation programme under grant agreement No 671697 and No. 671610. M. Moretó has been partially supported by the Ministry of Economy and Competitiveness under Juan de la Cierva postdoctoral fellowship number JCI-2012-15047.Peer ReviewedPostprint (author's final draft
Quantum Phase Transitions and Vortex Dynamics in Superconducting Networks
Josephson junction arrays are ideal model systems where a variety of
phenomena, phase transitions, frustration effects, vortex dynamics, chaos, to
mention a few of them, can be studied in a controlled way. In this review we
focus on the quantum dynamical properties of low capacitance Josephson junction
arrays. The two characteristic energy scales in these systems are the Josephson
energy, associated to the tunneling of Cooper pairs between neighboring
islands, and the charging energy, which is the energy cost to add an extra
electron charge to a neutral island. The phenomena described in this review
stem from the competition between single electron effects with the Josephson
effect. One example is the (quantum) Superconductor-Insulator phase transition
which occurs by varying the ratio between the coupling constants and/or by
means of external magnetic/electric fields. We will describe how the phase
diagram depends on the various control paramters and the transport properties
close to the quantum critical point. The relevant topological excitations on
the superconducting side of the phase diagram are vortices. In low capacitance
junction arrays vortices behave as massive underdamped particles that can
exhibit quantum behaviour. We will report on the various experiments and
theoretical treatments on quantum vortex dynamics.Comment: To be published in Physics Reports. Better quality figures can be
obtained upon reques
Stability and Vortex Shedding of Bluff Body Arrays
The primary purpose of this study was to develop an understanding of the stability of laminar
flow through bluff body arrays, and investigate the nature of the unsteady vortex shedding regime
that follows. The flow was numerically investigated using a specially developed multi-domain
spectral element solver. Important criteria in the solver development were flexibility, efficiency, and
accuracy. Flexibility was critical to the functionality of the code, as arrays of varying geometry
were investigated. Efficiency with a high degree of accuracy was also of primary importance, with
the code implemented to run efficiently on today's massively parallel architectures.
Numerical two-dimensional stability analysis of the flow in several configurations of inline and
staggered array geometries was performed. The growth rate, eigenfunction, and frequency of the
disturbances were determined. The critical Reynolds number for flow transition in each case was
identified and compared to that of flow over a single body. Based on the solutions of the laminar
flow, a one-dimensional analytical analysis was performed on selected velocity profiles in the wake
region. The results of this analysis were used to guide the interpretation of the two dimensional
results and formulate a general theory of stability of inline and staggered bluff body arrays. The
nature of the flow in the unsteady regime following the onset of instability was examined for an
inline and a staggered arrangement. Particular attention was focused on the vortex shedding which
was visualized and quantified through computation of the flow swirl, a quantity which identifies
regions of rotary motion. The conditions required for the generation of leading edge vortex shedding
were identified and discussed. Finally, a third geometry related to the inline and staggered arrays
was considered. Flow solution data for this geometry is presented and its suitability as a model for
louvered arrays was discussed.Air Conditioning and Refrigeration Project 11
Depinning and dynamics of vortices confined in mesoscopic flow channels
We study the behavior of vortex matter in artificial flow channels confined
by pinned vortices in the channel edges (CE's). The critical current is
governed by the interaction with static vortices in the CE's. We study
structural changes associated with (in)commensurability between the channel
width and the natural row spacing , and their effect on . The
behavior depends crucially on the presence of disorder in the CE arrays. For
ordered CE's, maxima in occur at matching ( integer), while
for defects along the CE's cause a vanishing . For weak CE
disorder, the sharp peaks in at become smeared via nucleation
and pinning of defects. The corresponding quasi-1D row configurations can
be described by a (disordered)sine-Gordon model. For larger disorder and
, levels at of the ideal lattice strength
. Around 'half filling' (), disorder causes new
features, namely {\it misaligned} defects and coexistence of and
rows in the channel. This causes a {\it maximum} in around mismatch,
while smoothly decreases towards matching due to annealing of the
misaligned regions. We study the evolution of static and dynamic structures on
changing , the relation between modulations of and transverse
fluctuations and dynamic ordering of the arrays. The numerical results at
strong disorder show good qualitative agreement with recent mode-locking
experiments.Comment: 29 pages, 32 figure
Water wave transmission by an array of floating disks
An experimental validation of theoretical models of transmission of regular
water waves by large arrays of floating disks is presented. The experiments are
conducted in a wave basin. The models are based on combined potential-flow and
thin-plate theories, and the assumption of linear motions. A low-concentration
array, in which disks are separated by approximately a disk diameter in
equilibrium, and a high-concentration array, in which adjacent disks are almost
touching in equilibrium, are used for the experiments. The proportion of
incident wave energy transmitted by the disks is presented as a function of
wave period, and for different wave amplitudes. Results indicate that the
models predict wave energy transmission accurately for small-amplitude waves
and low-concentration arrays. Discrepancies for large-amplitude waves and
high-concentration arrays are attributed to wave overwash of the disks and
collisions between disks. Validation of model predictions of rigid-body motions
of a solitary disk are also presented
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