12,315 research outputs found
Snakes and ladders: localized solutions of plane Couette flow
We demonstrate the existence of a large number of exact solutions of plane
Couette flow, which share the topology of known periodic solutions but are
localized in space. Solutions of different size are organized in a
snakes-and-ladders structure strikingly similar to that observed for simpler
pattern-forming PDE systems. These new solutions are a step towards extending
the dynamical systems view of transitional turbulence to spatially extended
flows.Comment: submitted to Physics Review Letter
A New Method for Protecting Interrelated Time Series with Bayesian Prior Distributions and Synthetic Data
Organizations disseminate statistical summaries of administrative data via the Web for unrestricted public use. They balance the trade-off between confidentiality protection and inference quality. Recent developments in disclosure avoidance techniques include the incorporation of synthetic data, which capture the essential features of underlying data by releasing altered data generated from a posterior predictive distribution. The United States Census Bureau collects millions of interrelated time series micro-data that are hierarchical and contain many zeros and suppressions. Rule-based disclosure avoidance techniques often require the suppression of count data for small magnitudes and the modification of data based on a small number of entities. Motivated by this problem, we use zero-inflated extensions of Bayesian Generalized Linear Mixed Models (BGLMM) with privacy-preserving prior distributions to develop methods for protecting and releasing synthetic data from time series about thousands of small groups of entities without suppression based on the of magnitudes or number of entities. We find that as the prior distributions of the variance components in the BGLMM become more precise toward zero, confidentiality protection increases and inference quality deteriorates. We evaluate our methodology using a strict privacy measure, empirical differential privacy, and a newly defined risk measure, Probability of Range Identification (PoRI), which directly measures attribute disclosure risk. We illustrate our results with the U.S. Census Bureau’s Quarterly Workforce Indicators
Numerical Measurements of Scaling Relations in Two-Dimensional Conformal Fluid Turbulence
We present measurements of relativistic scaling relations in
-dimensional conformal fluid turbulence from direct numerical
simulations, in the weakly compressible regime. These relations were
analytically derived previously in Westernacher-Schneider, Lehner, Oz (2015)
for a relativistic fluid; this work is a continuation of that study, providing
further analytical insights together with numerical experiments to test the
scaling relations and extract other important features characterizing the
turbulent behavior. We first explicitly demonstrate that the non-relativistic
limit of these scaling relations reduce to known results from the statistical
theory of incompressible Navier-Stokes turbulence. In simulations of the
inverse-cascade range, we find the relevant relativistic scaling relation is
satisfied to a high degree of accuracy. We observe that the non-relativistic
versions of this scaling relation underperform the relativistic one in both an
absolute and relative sense, with a progressive degradation as the rms Mach
number increases from to . In the direct-cascade range, the two
relevant relativistic scaling relations are satisfied with a lower degree of
accuracy in a simulation with rms Mach number . We elucidate the poorer
agreement with further simulations of an incompressible Navier-Stokes fluid.
Finally, as has been observed in the incompressible Navier-Stokes case, we show
that the energy spectrum in the inverse-cascade of the conformal fluid exhibits
scaling rather than the Kolmogorov/Kraichnan expectation of
, and that it is not necessarily associated with compressive effects.
We comment on the implications for a recent calculation of the fractal
dimension of a turbulent -dimensional AdS black brane.Comment: 18 pages, 10 figures, 1 table. To be submitted to JHEP - comments
welcome. V2 contains a technical edit for the JHEP compiler, as well as minor
grammatical correction
A Deep Survey of HI-Selected Galaxies: The Sample and the Data
In a 21 cm neutral hydrogen survey of approximately 55 sq deg out to a
redshift of cz=8340 km/s, we have identified 75 extragalactic HI sources. These
objects comprise a well-defined sample of extragalactic sources chosen by means
that are independent of optical surface brightness selection effects. In this
paper we describe the Arecibo survey procedures and HI data, follow-up VLA HI
observations made of several unusual sources, and Kitt Peak B-, R-, and I-band
photometry for nearly all of the galaxies. We have also gathered information
for some of the optically detected galaxies within the same search volume. We
examine how samples generated by different types of search techniques overlap
with selection by HI flux. Only the least massive HI object, which is among the
lowest mass HI sources previously found, does not have a clear optical
counterpart, but a nearby bright star may hide low surface brightness emission.
However the newly-detected systems do have unusual optical properties. Most of
the 40 galaxies that were not previously identified in magnitude-limited
catalogs appear to be gas-dominated systems, and several of these systems have
HI mass-to-light ratios among the largest values ever previously found. These
gas-dominated objects also tend to have very blue colors, low surface
brightnesses, and no central bulges, which correlate strongly with their
relative star-to-gas content.Comment: 48 pages, 10 figures, Figure 3 included as 3 separate JPG images. To
appear in Ap J Supplement
Extracting Formal Models from Normative Texts
We are concerned with the analysis of normative texts - documents based on
the deontic notions of obligation, permission, and prohibition. Our goal is to
make queries about these notions and verify that a text satisfies certain
properties concerning causality of actions and timing constraints. This
requires taking the original text and building a representation (model) of it
in a formal language, in our case the C-O Diagram formalism. We present an
experimental, semi-automatic aid that helps to bridge the gap between a
normative text in natural language and its C-O Diagram representation. Our
approach consists of using dependency structures obtained from the
state-of-the-art Stanford Parser, and applying our own rules and heuristics in
order to extract the relevant components. The result is a tabular data
structure where each sentence is split into suitable fields, which can then be
converted into a C-O Diagram. The process is not fully automatic however, and
some post-editing is generally required of the user. We apply our tool and
perform experiments on documents from different domains, and report an initial
evaluation of the accuracy and feasibility of our approach.Comment: Extended version of conference paper at the 21st International
Conference on Applications of Natural Language to Information Systems (NLDB
2016). arXiv admin note: substantial text overlap with arXiv:1607.0148
The Case for Community Colleges: Aligning Higher Education and Workforce Needs in Massachusetts
Reviews research on the need for middle-skilled workers with at least an associate's degree, Massachusetts' community college system, promising models for aligning community college curricula with workforce needs, and challenges. Makes recommendations
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