769 research outputs found
Streamer discharge inception in a sub-breakdown electric field from a dielectric body with a frequency dependent dielectric permittivity
We study positive streamer inception from the tip of an elongated ice particle. The dielectric permittivity of ice drops from 93 to 3 for electric fields changing on the millisecond timescale [1]. We demonstrate that this effect can be important on the nanosecond time scale of streamer discharge development. The effect makes the streamer propagate with only half the
velocity it would have for the dielectric permittivity fixed to 9
Adaptive selection of sampling points for uncertainty quantification
We present a simple and robust strategy for the selection of sampling points
in Uncertainty Quantification. The goal is to achieve the fastest possible
convergence in the cumulative distribution function of a stochastic output of
interest. We assume that the output of interest is the outcome of a
computationally expensive nonlinear mapping of an input random variable, whose
probability density function is known. We use a radial function basis to
construct an accurate interpolant of the mapping. This strategy enables adding
new sampling points one at a time, adaptively. This takes into full account the
previous evaluations of the target nonlinear function. We present comparisons
with a stochastic collocation method based on the Clenshaw-Curtis quadrature
rule, and with an adaptive method based on hierarchical surplus, showing that
the new method often results in a large computational saving.Comment: 22 pages, 15 figures; to appear in Int. J. Uncertainty Quantificatio
Can falls risk prediction tools correctly identify fall-prone elderly rehabilitation inpatients? A systematic review and meta-analysis
BACKGROUND: Falls of elderly people may cause permanent disability or death.
Particularly susceptible are elderly patients in rehabilitation hospitals. We
systematically reviewed the literature to identify falls prediction tools
available for assessing elderly inpatients in rehabilitation hospitals.
METHODS AND FINDINGS: We searched six electronic databases using comprehensive
search strategies developed for each database. Estimates of sensitivity and
specificity were plotted in ROC space graphs and pooled across studies. Our
search identified three studies which assessed the prediction properties of falls
prediction tools in a total of 754 elderly inpatients in rehabilitation
hospitals. Only the STRATIFY tool was assessed in all three studies; the other
identified tools (PJC-FRAT and DOWNTON) were assessed by a single study. For a
STRATIFY cut-score of two, pooled sensitivity was 73% (95%CI 63 to 81%) and
pooled specificity was 42% (95%CI 34 to 51%). An indirect comparison of the tools
across studies indicated that the DOWNTON tool has the highest sensitivity (92%),
while the PJC-FRAT offers the best balance between sensitivity and specificity
(73% and 75%, respectively). All studies presented major methodological
limitations.
CONCLUSIONS: We did not identify any tool which had an optimal balance between
sensitivity and specificity, or which were clearly better than a simple clinical
judgment of risk of falling. The limited number of identified studies with major
methodological limitations impairs sound conclusions on the usefulness of falls
risk prediction tools in geriatric rehabilitation hospitals
Hyperledger Fabric:Evaluating Endorsement Policy Strategies in Supply Chains
Hyperledger Fabric is a permissioned blockchain solution, in which network participation is controlled by predefined rules. This makes it an attractive platform for enterprise settings. In Fabric, endorsement policies are used to specify the peers that must confirm a transaction before it can be considered as valid and appended to the ledger. This work examines various implications of Fabric's endorsement policy component, for which different endorsement policy strategies (and subsequent trade-offs) are evaluated by modeling two real-world supply chain case studies. This work discusses how vulnerable endorsement policies can lead to admitting inauthentic data on the ledger. To address this issue, several approaches are proposed to balance integrity with limited disclosure of confidential information, with or without hosting a network peer directly. Furthermore, the concept of multi-tenancy in blockchain networks is introduced as a way of reducing the technological barrier in technology adoption
Dietary interventions for maintaining cognitive function in cognitively healthy people in late life
This is the protocol for a review and there is no abstract. The objectives are as follows: In this review we will set out to evaluate the effects of dietary interventions for maintaining cognitive function in cognitively healthy people in late-life
Prediction of lightning inception by large ice particles and extensive air showers
We derive that lightning can start if the electric field is 15% of the breakdown field, and if elongated ice particles of 6 cm length and 100 free electrons per cm3 are present. This is one particular example set from a parameter range that we discuss as well. Our simulations include the permittivity ε(ω) of ice. 100 free electrons per cm3 exist at 5.5 km altitude in air showers created by cosmic particles of at least 5×1015 eV. If the electric field zone is 3 m high and 0.2 km2 in the horizontal direction, at least one discharge per minute can be triggered. The size distribution of the ice particles is crucial for our argument; more detailed measurements would be desirable
Lightning inception from ice particles and extensive air showers
The electric fields in thunderstorms can exceed the breakdown value locally near ice particles present in thunderclouds. But are fields high enough and the regions large enough to initiate a streamer discharge? And where would a sufficient density of free electrons come from to start the discharge in the humid air that rapidly binds electrons in water-clusters
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