972 research outputs found
Managing community membership information in a small-world grid
As the Grid matures the problem of resource discovery across communities,
where resources now include computational services, is becoming more
critical. The number of resources available on a world-wide grid is set to grow
exponentially in much the same way as the number of static web pages on
the WWW. We observe that the world-wide resource discovery problem can
be modelled as a slowly evolving very-large sparse-matrix where individual
matrix elements represent nodes’ knowledge of one another. Blocks in the
matrix arise where nodes offer more than one service. Blocking effects also
arise in the identification of sub-communities in the Grid. The linear algebra
community has long been aware of suitable representations of large, sparse
matrices. However, matrices the size of the world-wide grid potentially number
in the billions, making dense solutions completely intractable. Distributed
nodes will not necessarily have the storage capacity to store the addresses of
any significant percentage of the available resources. We discuss ways of modelling
this problem in the regime of a slowly changing service base including
phenomena such as percolating networks and small-world network effects
Sparse cross-products of metadata in scientific simulation management
Managing scientific data is by no means a trivial task even in a single site environment
with a small number of researchers involved. We discuss some issues concerned with posing
well-specified experiments in terms of parameters or instrument settings and the metadata
framework that arises from doing so. We are particularly interested in parallel computer
simulation experiments, where very large quantities of warehouse-able data are involved. We
consider SQL databases and other framework technologies for manipulating experimental data.
Our framework manages the the outputs from parallel runs that arise from large cross-products
of parameter combinations. Considerable useful experiment planning and analysis can be done
with the sparse metadata without fully expanding the parameter cross-products. Extra value
can be obtained from simulation output that can subsequently be data-mined. We have
particular interests in running large scale Monte-Carlo physics model simulations. Finding
ourselves overwhelmed by the problems of managing data and compute ¿resources, we have
built a prototype tool using Java and MySQL that addresses these issues. We use this example
to discuss type-space management and other fundamental ideas for implementing a laboratory
information management system
A framework and simulation engine for studying artificial life
The area of computer-generated artificial life-forms is a relatively recent
field of inter-disciplinary study that involves mathematical modelling, physical
intuition and ideas from chemistry and biology and computational science.
Although the attribution of “life” to non biological systems is still controversial,
several groups agree that certain emergent properties can be ascribed to
computer simulated systems that can be constructed to “live” in a simulated
environment. In this paper we discuss some of the issues and infrastructure
necessary to construct a simulation laboratory for the study of computer generated
artificial life-forms. We review possible technologies and present some
preliminary studies based around simple models
64-bit architechtures and compute clusters for high performance simulations
Simulation of large complex systems remains one of the most demanding
of high performance computer systems both in terms of raw compute performance
and efficient memory management. Recent availability of 64-bit
architectures has opened up the possibilities of commodity computers accessing
more than the 4 Gigabyte memory limit previously enforced by 32-bit
addressing. We report on some performance measurements we have made on
two 64-bit architectures and their consequences for some high performance
simulations. We discuss performance of our codes for simulations of artificial
life models; computational physics models of point particles on lattices; and
with interacting clusters of particles. We have summarised pertinent features
of these codes into benchmark kernels which we discuss in the context of wellknown
benchmark kernels of the 32-bit era. We report on how these these
findings were useful in the context of designing 64-bit compute clusters for
high-performance simulations
Investigation of taste tainting in salmon flesh in the Ribble catchment
This report presents the findings of the first phase of an investigation into the cause(s) of taints in salmonid fish in the River Ribble, commissioned by the North West Region of the Environment Agency. There have been reports of tainting in fish taken from both the estuary and the freshwater river for many years, but the contaminants involved and their source and transport pathway are unknown. Tainting by phenols has been of specific concern in the past.
The work programme comprised: examination of tainting reports; collection of salmonids; their submission for taste testing; literature review; analysis of fish flesh using gas chromatography-mass spectrometry (GCMS) and analysis of river bed sediments. From enquiries, three common descriptors of the 'taint' were identified: disinfectanty; diesely; and muddy. The incidence of taints appears transient/irregular and may therefore relate to
the incidence of discharges and specific threshold concentrations of pollutants. The literature review showed that a wide range of organic compounds including many industrial chemicals, and others which are naturally occurring, can taint fish flesh. Taste testing confirmed the presence of tainted salmon and trout in the Ribbie Catchment. It identified a low incidence of 'untainted' fish but demonstrated the 'taint' to be not specific to
one tainting substance. Differences were found both between the species and fish from different parts of the catchment. Overall, most fish exhibited an unpleasant flavour, though
this may have been influenced to some extent by the fact that most were sexually mature.
The worst tainting was found in trout from the river Calder: a soapy/chemical aftertaste. An unpleasant earthy/musty flavour distinguished the salmon from the trout. Phenol was shown to have been a minor issue during the present study, whilst no hydrocarbon taints were
identified.
Examination of tissue from the eight salmon exhibiting the worst taints revealed the presence of aromatic hydrocarbons, but no phenolic compounds. Other notable substances present in the fish were siioxanes and benzophenone. Data from sediment analysis is presented which shows the main compounds present to be aromatic and polyaromatic hydrocarbons, that concentrations at two locations R. Darwen and R. Calder were significantly higher than at other sites, and that some phenolic compounds were detected at low levels.
A paucity of fish flesh taste descriptors linked to specific compounds prevented an obvious correlation to be made between the tastes observed and the organic compounds detected.
Descriptors frequently used by the taste testing panel (e.g. earthy, musty, astringency, chemical) cannot be linked to any of the compounds identified in the tissue analyses. No taste information was available from the literature on siioxanes. Aromatic hydrocarbons though present in tissue and sediments were not identified as tainting
Confined granular packings: structure, stress, and forces
The structure and stresses of static granular packs in cylindrical containers
are studied using large-scale discrete element molecular dynamics simulations
in three dimensions. We generate packings by both pouring and sedimentation and
examine how the final state depends on the method of construction. The vertical
stress becomes depth-independent for deep piles and we compare these stress
depth-profiles to the classical Janssen theory. The majority of the tangential
forces for particle-wall contacts are found to be close to the Coulomb failure
criterion, in agreement with the theory of Janssen, while particle-particle
contacts in the bulk are far from the Coulomb criterion. In addition, we show
that a linear hydrostatic-like region at the top of the packings unexplained by
the Janssen theory arises because most of the particle-wall tangential forces
in this region are far from the Coulomb yield criterion. The distributions of
particle-particle and particle-wall contact forces exhibit
exponential-like decay at large forces in agreement with previous studies.Comment: 11 pages, 11 figures, submitted to PRE (v2) added new references,
fixed typo
Granular packings with moving side walls
The effects of movement of the side walls of a confined granular packing are
studied by discrete element, molecular dynamics simulations. The dynamical
evolution of the stress is studied as a function of wall movement both in the
direction of gravity as well as opposite to it. For all wall velocities
explored, the stress in the final state of the system after wall movement is
fundamentally different from the original state obtained by pouring particles
into the container and letting them settle under the influence of gravity. The
original packing possesses a hydrostatic-like region at the top of the
container which crosses over to a depth-independent stress. As the walls are
moved in the direction opposite to gravity, the saturation stress first reaches
a minimum value independent of the wall velocity, then increases to a
steady-state value dependent on the wall-velocity. After wall movement ceases
and the packing reaches equilibrium, the stress profile fits the classic
Janssen form for high wall velocities, while it has some deviations for low
wall velocities. The wall movement greatly increases the number of
particle-wall and particle-particle forces at the Coulomb criterion. Varying
the wall velocity has only small effects on the particle structure of the final
packing so long as the walls travel a similar distance.Comment: 11 pages, 10 figures, some figures in colo
Flux transitions in a superconducting ring
We perform a numeric study of the flux transitions in a superconducting ring
at fixed temperature, while the applied field is swept at an ideally slow rate.
The current around the ring and its free energy are evaluated. We partially
explain some of the known experimental features, and predict a considerably
large new feature: in the vicinity of a critical field, giant jumps are
expected
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