15,354 research outputs found
Topology of Force Networks in Granular Media under Impact
We investigate the evolution of the force network in experimental systems of
two-dimensional granular materials under impact. We use the first Betti number,
, and persistence diagrams, as measures of the topological properties
of the force network. We show that the structure of the network has a complex,
hysteretic dependence on both the intruder acceleration and the total force
response of the granular material. can also distinguish between the
nonlinear formation and relaxation of the force network. In addition, using the
persistence diagram of the force network, we show that the size of the loops in
the force network has a Poisson-like distribution, the characteristic size of
which changes over the course of the impact
Force and Mass Dynamics in Non-Newtonian Suspensions
Above a certain solid fraction, dense granular suspensions in water exhibit
non-Newtonian behavior, including impact-activated solidification. Although it
has been suggested that solidification depends on boundary interactions,
quantitative experiments on the boundary forces have not been reported. Using
high-speed video, tracer particles, and photoelastic boundaries, we determine
the impactor kinematics and the magnitude and timings of impactor-driven events
in the body and at the boundaries of cornstarch suspensions. We observe mass
shocks in the suspension during impact. The shockfront dynamics are strongly
correlated to those of the intruder. However, the total momentum associated
with this shock never approaches the initial impactor momentum. We also observe
a faster second front, associated with the propagation of pressure to the
boundaries of the suspension. The two fronts depend differently on the initial
impactor speed, , and the suspension packing fraction. The speed of the
pressure wave is at least an order of magnitude smaller than (linear)
ultrasound speeds obtained for much higher frequencies, pointing to complex
amplitude and frequency response of cornstarch suspensions to compressive
strains
Neutrinos from beta processes in a presupernova: probing the isotopic evolution of a massive star
We present a new calculation of the neutrino flux received at Earth from a
massive star in the hours of evolution prior to its explosion as a
supernova (presupernova). Using the stellar evolution code MESA, the neutrino
emissivity in each flavor is calculated at many radial zones and time steps. In
addition to thermal processes, neutrino production via beta processes is
modeled in detail, using a network of 204 isotopes. We find that the total
produced flux has a high energy spectrum tail, at
MeV, which is mostly due to decay and electron capture on isotopes with . In a tentative window of observability of MeV and hours pre-collapse, the contribution of beta processes to the flux
is at the level of . For a star at kpc distance, a 17 kt
liquid scintillator detector would typically observe several tens of events
from a presupernova, of which up to due to beta processes. These
processes dominate the signal at a liquid argon detector, thus greatly
enhancing its sensitivity to a presupernova.Comment: 14 pages, 5 figure
The Information Content of the NCREIF Index
This paper examines the dynamic behavior of the NCREIF index. NCREIF total return and appreciation indexes are smooth and exhibit strong autocorrelation and autoregressive heteroskedasticity. We test the information transmission from the NAREIT index to the NCREIF index. In our VAR analysis, the NAREIT index returns Granger cause the returns of the NCREIF indexes. In our ARCH information transmission analysis, the NCREIF indexes are observed to incorporate information spillover from the NAREIT indexes in both the mean and variance of the index returns. The ARCH dynamics between the NCREIF and NAREIT indexes suggest a nonlinear relation between the two indexes.
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