8,088 research outputs found
Similarity Measure Development for Case-Based Reasoning- A Data-driven Approach
In this paper, we demonstrate a data-driven methodology for modelling the
local similarity measures of various attributes in a dataset. We analyse the
spread in the numerical attributes and estimate their distribution using
polynomial function to showcase an approach for deriving strong initial value
ranges of numerical attributes and use a non-overlapping distribution for
categorical attributes such that the entire similarity range [0,1] is utilized.
We use an open source dataset for demonstrating modelling and development of
the similarity measures and will present a case-based reasoning (CBR) system
that can be used to search for the most relevant similar cases
Comparing Numerical Methods for Isothermal Magnetized Supersonic Turbulence
We employ simulations of supersonic super-Alfvenic turbulence decay as a
benchmark test problem to assess and compare the performance of nine
astrophysical MHD methods actively used to model star formation. The set of
nine codes includes: ENZO, FLASH, KT-MHD, LL-MHD, PLUTO, PPML, RAMSES, STAGGER,
and ZEUS. We present a comprehensive set of statistical measures designed to
quantify the effects of numerical dissipation in these MHD solvers. We compare
power spectra for basic fields to determine the effective spectral bandwidth of
the methods and rank them based on their relative effective Reynolds numbers.
We also compare numerical dissipation for solenoidal and dilatational velocity
components to check for possible impacts of the numerics on small-scale density
statistics. Finally, we discuss convergence of various characteristics for the
turbulence decay test and impacts of various components of numerical schemes on
the accuracy of solutions. We show that the best performing codes employ a
consistently high order of accuracy for spatial reconstruction of the evolved
fields, transverse gradient interpolation, conservation law update step, and
Lorentz force computation. The best results are achieved with divergence-free
evolution of the magnetic field using the constrained transport method, and
using little to no explicit artificial viscosity. Codes which fall short in one
or more of these areas are still useful, but they must compensate higher
numerical dissipation with higher numerical resolution. This paper is the
largest, most comprehensive MHD code comparison on an application-like test
problem to date. We hope this work will help developers improve their numerical
algorithms while helping users to make informed choices in picking optimal
applications for their specific astrophysical problems.Comment: 17 pages, 5 color figures, revised version to appear in ApJ, 735,
July 201
The Effects of Large-Scale Magnetic Fields on Disk Formation and Evolution
This is a draft chapter for a book, entitled Physical Processes in
Circumstellar Disks around Young Stars, which is scheduled for publication by
the University of Chicago Press as one of its Theoretical Astrophysics Series
volumes. Sect. 1 presents the motivation for considering the effects of a
large-scale, ordered magnetic field on the formation and evolution of
protostellar disks. Sect. 2 outlines the physical principles that underlie the
magnetohydrodynamics of disks that are threaded by such a field. Sect. 3
discusses the formation and early evolution of disks that result from the
collapse of a rotating molecular cloud core that is coupled to the
insterstellar magnetic field. Sect. 4 reviews the observational evidence for
the disk--wind connection and describes the structure of magnetically
accelerated disk outflows, focusing on centrifugally driven winds; it then goes
on to discuss the equilibrium and stability properties of weakly ionized
protostellar accretion disks in which the transport of angular momentum is
dominated by a wind of this type. Sect. 5 considers the coupling between the
central protostar and the surrounding disk through the protostellar magnetic
field, covering, in turn, the phenomenology, basic concepts, and results of
numerical simulations. The chapter is summarized in Sect. 6, which also
contains a discussion of future research directions.Comment: 68 pages, 8 figures, to appear in Physical Processes in Circumstellar
Disks around Young Stars, ed. P. J. V. Garcia (Chicago: University of Chicago
Press), uses svmult.cl
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