17 research outputs found
Sound propagation and force chains in granular materials
Granular materials are inherently heterogeneous, leading to challenges in
formulating accurate models of sound propagation. In order to quantify acoustic
responses in space and time, we perform experiments in a photoelastic granular
material in which the internal stress pattern (in the form of force chains) is
visible. We utilize two complementary methods, high-speed imaging and
piezoelectric transduction, to provide particle-scale measurements of both the
amplitude and speed of an acoustic wave in the near-field regime. We observe
that the wave amplitude is on average largest within particles experiencing the
largest forces, particularly in those chains radiating away from the source,
with the force-dependence of this amplitude in qualitative agreement with a
simple Hertzian-like model of particle contact area. In addition, we are able
to directly observe rare transient force chains formed by the opening and
closing of contacts during propagation. The speed of the leading edge of the
pulse is in quantitative agreement with predictions for one-dimensional chains,
while the slower speed of the peak response suggests that it contains waves
which have travelled over multiple paths even within just this near-field
region. These effects highlight the importance of particle-scale behaviors in
determining the acoustical properties of granular materials
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
The James Webb Space Telescope Mission
Twenty-six years ago a small committee report, building on earlier studies,
expounded a compelling and poetic vision for the future of astronomy, calling
for an infrared-optimized space telescope with an aperture of at least .
With the support of their governments in the US, Europe, and Canada, 20,000
people realized that vision as the James Webb Space Telescope. A
generation of astronomers will celebrate their accomplishments for the life of
the mission, potentially as long as 20 years, and beyond. This report and the
scientific discoveries that follow are extended thank-you notes to the 20,000
team members. The telescope is working perfectly, with much better image
quality than expected. In this and accompanying papers, we give a brief
history, describe the observatory, outline its objectives and current observing
program, and discuss the inventions and people who made it possible. We cite
detailed reports on the design and the measured performance on orbit.Comment: Accepted by PASP for the special issue on The James Webb Space
Telescope Overview, 29 pages, 4 figure