15,381 research outputs found
Fracture toughness of a zirconia engineering ceramic and the effects thereon of surface processing with fibre laser radiation
Vickers hardness indentation tests were employed to investigate the near-surface changes in the hardness of a fibre laser-treated and an as-received ZrO2 engineering ceramic. Indents were created using 5, 20, and 30 kg loads to obtain the hardness. Optical microscopy, white-light interferometry, and a coordinate measuring machine were then used to observe the crack lengths and crack geometry. Palmqvist and half-penny median crack profiles were found, which dictated the selection of the group of equations used herein. Computational and analytical approaches were then adapted to determine the K1c of ZrO2. It was found that the best applicable equation was: K1c = 0.016 (E/H)1/2 (P/c3/2), which was confirmed to be 42 per cent accurate in producing K1c values within the range of 8 to 12 MPa m1/2 for ZrO2. Fibre laser surface treatment reduced the surface hardness and produced smaller crack lengths in comparison with the as-received surface. The surface crack lengths, hardness, and indentation loads were found to be important, particularly the crack length, which significantly influenced the end K1c value when K1c = 0.016 (E/H)1/2 (P/c3/2) was used. This is because, the longer the crack lengths, the lower the ceramic's resistance to indentation. This, in turn, increased the end K1c value. Also, the hardness influences the K1c, and a softer surface was produced by the fibre laser treatment; this resulted in higher resistance to crack propagation and enhanced the ceramic's K1c. Increasing the indentation load also varied the end K1c value, as higher indentation loads resulted in a bigger diamond footprint, and the ceramic exhibited longer crack lengths
Risk factors for acute exacerbations of COPD in a primary care population: A retrospective observational cohort study
Objectives: To evaluate risk factors associated with exacerbation frequency in primary care. Information on exacerbations of chronic obstructive pulmonary disease (COPD) has mainly been generated by secondary care-based clinical cohorts. Design: Retrospective observational cohort study. Setting: Electronic medical records database (England and Wales). Participants: 58 589 patients with COPD aged ≥40 years with COPD diagnosis recorded between 1 April 2009 and 30 September 2012, and with at least 365 days of follow-up before and after the COPD diagnosis, were identified in the Clinical Practice Research Datalink. Mean age: 69 years; 47% female; mean forced expiratory volume in 1s 60% predicted. Outcome measures: Data on moderate or severe exacerbation episodes defined by diagnosis and/or medication codes 12 months following cohort entry were retrieved, together with demographic and clinical characteristics. Associations between patient characteristics and odds of having none versus one, none versus frequent (≥2) and one versus frequent exacerbations over 12 months follow-up were evaluated using multivariate logistic regression models. Results: During follow-up, 23% of patients had evidence of frequent moderate-to-severe COPD exacerbations (24% one; 53% none). Independent predictors of increased odds of having exacerbations during the follow-up, either frequent episodes or one episode, included prior exacerbations, increasing dyspnoea score, increasing grade of airflow limitation, females and prior or current history of several comorbidities (eg, asthma, depression, anxiety, heart failure and cancer). Conclusions: Primary care-managed patients with COPD at the highest risk of exacerbations can be identified by exploring medical history for the presence of prior exacerbations, greater COPD disease severity and co-occurrence of other medical conditions
Analysis of the 3DVAR Filter for the Partially Observed Lorenz '63 Model
The problem of effectively combining data with a mathematical model
constitutes a major challenge in applied mathematics. It is particular
challenging for high-dimensional dynamical systems where data is received
sequentially in time and the objective is to estimate the system state in an
on-line fashion; this situation arises, for example, in weather forecasting.
The sequential particle filter is then impractical and ad hoc filters, which
employ some form of Gaussian approximation, are widely used. Prototypical of
these ad hoc filters is the 3DVAR method. The goal of this paper is to analyze
the 3DVAR method, using the Lorenz '63 model to exemplify the key ideas. The
situation where the data is partial and noisy is studied, and both discrete
time and continuous time data streams are considered. The theory demonstrates
how the widely used technique of variance inflation acts to stabilize the
filter, and hence leads to asymptotic accuracy
Ion collection by oblique surfaces of an object in a transversely-flowing strongly-magnetized plasma
The equations governing a collisionless obliquely-flowing plasma around an
ion-absorbing object in a strong magnetic field are shown to have an exact
analytic solution even for arbitrary (two-dimensional) object-shape, when
temperature is uniform, and diffusive transport can be ignored. The solution
has an extremely simple geometric embodiment. It shows that the ion collection
flux density to a convex body's surface depends only upon the orientation of
the surface, and provides the theoretical justification and calibration of
oblique `Mach-probes'. The exponential form of this exact solution helps
explain the approximate fit of this function to previous numerical solutions.Comment: Four pages, 2 figures. Submitted to Phys. Rev. Letter
VIoLET: A Large-scale Virtual Environment for Internet of Things
IoT deployments have been growing manifold, encompassing sensors, networks,
edge, fog and cloud resources. Despite the intense interest from researchers
and practitioners, most do not have access to large-scale IoT testbeds for
validation. Simulation environments that allow analytical modeling are a poor
substitute for evaluating software platforms or application workloads in
realistic computing environments. Here, we propose VIoLET, a virtual
environment for defining and launching large-scale IoT deployments within cloud
VMs. It offers a declarative model to specify container-based compute resources
that match the performance of the native edge, fog and cloud devices using
Docker. These can be inter-connected by complex topologies on which
private/public networks, and bandwidth and latency rules are enforced. Users
can configure synthetic sensors for data generation on these devices as well.
We validate VIoLET for deployments with > 400 devices and > 1500 device-cores,
and show that the virtual IoT environment closely matches the expected compute
and network performance at modest costs. This fills an important gap between
IoT simulators and real deployments.Comment: To appear in the Proceedings of the 24TH International European
Conference On Parallel and Distributed Computing (EURO-PAR), August 27-31,
2018, Turin, Italy, europar2018.org. Selected as a Distinguished Paper for
presentation at the Plenary Session of the conferenc
Quantum Trivelpiece-Gould waves in a magnetized dense plasma
The dispersion relation for the electrostatic waves below the electron plasma
frequency in a dense quantum plasma is derived by using the magnetohydrodynamic
model. It is shown that in the classical case the dispersion relation reduces
to the expression obtained for the well-known Trivelpiece-Gould (TG) modes.
Attention is also devoted to the case of solitary waves associated with the
nonlinear TG modes.Comment: 8 pages, 0 figure
Exact Solution of Return Hysteresis Loops in One Dimensional Random Field Ising Model at Zero Temperature
Minor hysteresis loops within the main loop are obtained analytically and
exactly in the one-dimensional ferromagnetic random field Ising-model at zero
temperature. Numerical simulations of the model show excellent agreement with
the analytical results
Theory of nonlinear optical properties of phenyl-substituted polyacetylenes
In this paper we present a theoretical study of the third-order nonlinear
optical properties of poly(diphenyl)polyacetylene (PDPA) pertaining to the
third-harmonic-generation (THG) process. We study the aforesaid process in
PDPA's using both the independent electron Hueckel model, as well as
correlated-electron Pariser-Parr-Pople (P-P-P) model. The P-P-P model based
calculations were performed using various configuration interaction (CI)
methods such as the the multi-reference-singles-doubles CI (MRSDCI), and the
quadruples-CI (QCI) methods, and the both longitudinal and the transverse
components of third-order susceptibilities were computed. The Hueckel model
calculations were performed on oligo-PDPA's containing up to fifty repeat
units, while correlated calculations were performed for oligomers containing up
to ten unit cells. At all levels of theory, the material exhibits highly
anisotropic nonlinear optical response, in keeping with its structural
anisotropy. We argue that the aforesaid anisotropy can be divided over two
natural energy scales: (a) the low-energy response is predominantly
longitudinal and is qualitatively similar to that of polyenes, while (b) the
high-energy response is mainly transverse, and is qualitatively similar to that
of trans-stilbene.Comment: 13 pages, 7 figures (included), to appear in Physical Review B (April
15, 2004
Clustering-Based Materialized View Selection in Data Warehouses
Materialized view selection is a non-trivial task. Hence, its complexity must
be reduced. A judicious choice of views must be cost-driven and influenced by
the workload experienced by the system. In this paper, we propose a framework
for materialized view selection that exploits a data mining technique
(clustering), in order to determine clusters of similar queries. We also
propose a view merging algorithm that builds a set of candidate views, as well
as a greedy process for selecting a set of views to materialize. This selection
is based on cost models that evaluate the cost of accessing data using views
and the cost of storing these views. To validate our strategy, we executed a
workload of decision-support queries on a test data warehouse, with and without
using our strategy. Our experimental results demonstrate its efficiency, even
when storage space is limited
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