18,021 research outputs found
The Influence of Network Topology on Sound Propagation in Granular Materials
Granular materials, whose features range from the particle scale to the
force-chain scale to the bulk scale, are usually modeled as either particulate
or continuum materials. In contrast with either of these approaches, network
representations are natural for the simultaneous examination of microscopic,
mesoscopic, and macroscopic features. In this paper, we treat granular
materials as spatially-embedded networks in which the nodes (particles) are
connected by weighted edges obtained from contact forces. We test a variety of
network measures for their utility in helping to describe sound propagation in
granular networks and find that network diagnostics can be used to probe
particle-, curve-, domain-, and system-scale structures in granular media. In
particular, diagnostics of meso-scale network structure are reproducible across
experiments, are correlated with sound propagation in this medium, and can be
used to identify potentially interesting size scales. We also demonstrate that
the sensitivity of network diagnostics depends on the phase of sound
propagation. In the injection phase, the signal propagates systemically, as
indicated by correlations with the network diagnostic of global efficiency. In
the scattering phase, however, the signal is better predicted by meso-scale
community structure, suggesting that the acoustic signal scatters over local
geographic neighborhoods. Collectively, our results demonstrate how the force
network of a granular system is imprinted on transmitted waves.Comment: 19 pages, 9 figures, and 3 table
Automated Detection and Tracking of Solar Magnetic Bright Points
Magnetic Bright Points (MBPs) in the internetwork are among the smallest
objects in the solar photosphere and appear bright against the ambient
environment. An algorithm is presented that can be used for the automated
detection of the MBPs in the spatial and temporal domains. The algorithm works
by mapping the lanes through intensity thresholding. A compass search, combined
with a study of the intensity gradient across the detected objects, allows the
disentanglement of MBPs from bright pixels within the granules. Object growing
is implemented to account for any pixels that might have been removed when
mapping the lanes. The images are stabilized by locating long-lived objects
that may have been missed due to variable light levels and seeing quality.
Tests of the algorithm employing data taken with the Swedish Solar Telescope
(SST), reveal that ~90% of MBPs within a 75"x 75" field of view are detected
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