9,781 research outputs found
The effect of injector design on thrust- chamber erosion
Relation between injector design and erosion of ablative and pyrolytic graphite thrust chamber throa
Fast algorithm for detecting community structure in networks
It has been found that many networks display community structure -- groups of
vertices within which connections are dense but between which they are sparser
-- and highly sensitive computer algorithms have in recent years been developed
for detecting such structure. These algorithms however are computationally
demanding, which limits their application to small networks. Here we describe a
new algorithm which gives excellent results when tested on both
computer-generated and real-world networks and is much faster, typically
thousands of times faster than previous algorithms. We give several example
applications, including one to a collaboration network of more than 50000
physicists.Comment: 5 pages, 4 figure
Anomaly Detection in Paleoclimate Records using Permutation Entropy
Permutation entropy techniques can be useful in identifying anomalies in
paleoclimate data records, including noise, outliers, and post-processing
issues. We demonstrate this using weighted and unweighted permutation entropy
of water-isotope records in a deep polar ice core. In one region of these
isotope records, our previous calculations revealed an abrupt change in the
complexity of the traces: specifically, in the amount of new information that
appeared at every time step. We conjectured that this effect was due to noise
introduced by an older laboratory instrument. In this paper, we validate that
conjecture by re-analyzing a section of the ice core using a more-advanced
version of the laboratory instrument. The anomalous noise levels are absent
from the permutation entropy traces of the new data. In other sections of the
core, we show that permutation entropy techniques can be used to identify
anomalies in the raw data that are not associated with climatic or
glaciological processes, but rather effects occurring during field work,
laboratory analysis, or data post-processing. These examples make it clear that
permutation entropy is a useful forensic tool for identifying sections of data
that require targeted re-analysis---and can even be useful in guiding that
analysis.Comment: 15 pages, 7 figure
(Mis)perceptions of ethnic group size and consequences for community expectations and cooperation with law enforcement
The changing composition of race and ethnic group size has been noted for Western nations over the last 15 years. Analysis of this change has linked fear of crime and attitudes toward immigrants and prejudice. Changes in ethnic composition are associated with movement of White residents out of traditionally White communities, rising ethnic tension as the ethnic mix shifts, and a heightened sense of injustice regarding the justice system. (Mis)perceptions of ethnic groups size shape attitudes toward minority groups, as well as policy, practice, and individual behavior in the context of the community. This study seeks to understand the extent of such misperceptions in the Australian context and whether misperceptions of race and ethnic composition are associated with beliefs and attitudes toward formal and informal social control. Utilizing Blalock’s racial threat hypothesis, this study analyzes whether perceived relative ethnic group size is associated with self-reported willingness to cooperate with police as a way to minimize perceived threat. Findings suggest that respondents overestimate the size of minority populations while underestimating the majority White composition and that these misperceived distortions in ethnic group size have consequences for informal and formal social control
Numerical Linked-Cluster Algorithms. I. Spin systems on square, triangular, and kagome lattices
We discuss recently introduced numerical linked-cluster (NLC) algorithms that
allow one to obtain temperature-dependent properties of quantum lattice models,
in the thermodynamic limit, from exact diagonalization of finite clusters. We
present studies of thermodynamic observables for spin models on square,
triangular, and kagome lattices. Results for several choices of clusters and
extrapolations methods, that accelerate the convergence of NLC, are presented.
We also include a comparison of NLC results with those obtained from exact
analytical expressions (where available), high-temperature expansions (HTE),
exact diagonalization (ED) of finite periodic systems, and quantum Monte Carlo
simulations.For many models and properties NLC results are substantially more
accurate than HTE and ED.Comment: 14 pages, 16 figures, as publishe
Numerical Linked-Cluster Approach to Quantum Lattice Models
We present a novel algorithm that allows one to obtain temperature dependent
properties of quantum lattice models in the thermodynamic limit from exact
diagonalization of small clusters. Our Numerical Linked Cluster (NLC) approach
provides a systematic framework to assess finite-size effects and is valid for
any quantum lattice model. Unlike high temperature expansions (HTE), which have
a finite radius of convergence in inverse temperature, these calculations are
accurate at all temperatures provided the range of correlations is finite. We
illustrate the power of our approach studying spin models on {\it kagom\'e},
triangular, and square lattices.Comment: 4 pages, 5 figures, published versio
An experimental evaluation of 100-lb-thrust ablatively cooled rocket engines
Test firing of 100-lb thrust ablatively cooled rocket chamber
Vortex Formation by Interference of Multiple Trapped Bose-Einstein Condensates
We report observations of vortex formation as a result of merging together
multiple Rb Bose-Einstein condensates (BECs) in a confining potential.
In this experiment, a trapping potential is partitioned into three sections by
a barrier, enabling the simultaneous formation of three independent,
uncorrelated condensates. The three condensates then merge together into one
BEC, either by removal of the barrier, or during the final stages of
evaporative cooling if the barrier energy is low enough; both processes can
naturally produce vortices within the trapped BEC. We interpret the vortex
formation mechanism as originating in interference between the initially
independent condensates, with indeterminate relative phases between the three
initial condensates and the condensate merging rate playing critical roles in
the probability of observing vortices in the final, single BEC.Comment: 5 pages, 3 figure
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