12,671 research outputs found
Towards Landslide Predictions: Two Case Studies
In a previous work [Helmstetter, 2003], we have proposed a simple physical
model to explain the accelerating displacements preceding some catastrophic
landslides, based on a slider-block model with a state and velocity dependent
friction law. This model predicts two regimes of sliding, stable and unstable
leading to a critical finite-time singularity. This model was calibrated
quantitatively to the displacement and velocity data preceding two landslides,
Vaiont (Italian Alps) and La Clapi\`ere (French Alps), showing that the former
(resp. later) landslide is in the unstable (resp. stable) sliding regime. Here,
we test the predictive skills of the state-and-velocity-dependent model on
these two landslides, using a variety of techniques. For the Vaiont landslide,
our model provides good predictions of the critical time of failure up to 20
days before the collapse. Tests are also presented on the predictability of the
time of the change of regime for la Clapi\`ere landslide.Comment: 30 pages with 12 eps figure
Transient vibration analysis of a completely free plate using modes obtained by Gorman's superposition method
This paper shows that the transient response of a plate undergoing flexural vibration can be calculated accurately and efficiently using the natural frequencies and modes obtained from the superposition method. The response of a completely free plate is used to demonstrate this. The case considered is one where all supports of a simply supported thin rectangular plate under self weight are suddenly removed. The resulting motion consists of a combination of the natural modes of a completely free plate. The modal superposition method is used for determining the transient response, and the natural frequencies and mode shapes of the plates used are obtained by Gorman's superposition method. These are compared with corresponding results based on the modes using the RayleighâRitz method using the ordinary and degenerated freeâfree beam functions. There is an excellent agreement between the results from both approaches but the superposition method has shown faster convergence and the results may serve as benchmarks for the transient response of completely free plates
Computation of atomic astrophysical opacities
The revision of the standard Los Alamos opacities in the 1980-1990s by a
group from the Lawrence Livermore National Laboratory (OPAL) and the Opacity
Project (OP) consortium was an early example of collaborative big-data science,
leading to reliable data deliverables (atomic databases, monochromatic
opacities, mean opacities, and radiative accelerations) widely used since then
to solve a variety of important astrophysical problems. Nowadays the precision
of the OPAL and OP opacities, and even of new tables (OPLIB) by Los Alamos, is
a recurrent topic in a hot debate involving stringent comparisons between
theory, laboratory experiments, and solar and stellar observations in
sophisticated research fields: the standard solar model (SSM), helio and
asteroseismology, non-LTE 3D hydrodynamic photospheric modeling, nuclear
reaction rates, solar neutrino observations, computational atomic physics, and
plasma experiments. In this context, an unexpected downward revision of the
solar photospheric metal abundances in 2005 spoiled a very precise agreement
between the helioseismic indicators (the radius of the convection zone
boundary, the sound-speed profile, and helium surface abundance) and SSM
benchmarks, which could be somehow reestablished with a substantial opacity
increase. Recent laboratory measurements of the iron opacity in physical
conditions similar to the boundary of the solar convection zone have indeed
predicted significant increases (30-400%), although new systematic improvements
and comparisons of the computed tables have not yet been able to reproduce
them. We give an overview of this controversy, and within the OP approach,
discuss some of the theoretical shortcomings that could be impairing a more
complete and accurate opacity accountingComment: 31 pages, 10 figures. This review is originally based on a talk given
at the 12th International Colloquium on Atomic Spectra and Oscillator
Strengths for Astrophysical and Laboratory Plasmas, Sao Paulo, Brazil, July
2016. It has been published in the Atoms online journa
Implementation of standard testbeds for numerical relativity
We discuss results that have been obtained from the implementation of the
initial round of testbeds for numerical relativity which was proposed in the
first paper of the Apples with Apples Alliance. We present benchmark results
for various codes which provide templates for analyzing the testbeds and to
draw conclusions about various features of the codes. This allows us to sharpen
the initial test specifications, design a new test and add theoretical insight.Comment: Corrected versio
Pseudogap and high-temperature superconductivity from weak to strong coupling. Towards quantitative theory
This is a short review of the theoretical work on the two-dimensional Hubbard
model performed in Sherbrooke in the last few years. It is written on the
occasion of the twentieth anniversary of the discovery of high-temperature
superconductivity. We discuss several approaches, how they were benchmarked and
how they agree sufficiently with each other that we can trust that the results
are accurate solutions of the Hubbard model. Then comparisons are made with
experiment. We show that the Hubbard model does exhibit d-wave
superconductivity and antiferromagnetism essentially where they are observed
for both hole and electron-doped cuprates. We also show that the pseudogap
phenomenon comes out of these calculations. In the case of electron-doped high
temperature superconductors, comparisons with angle-resolved photoemission
experiments are nearly quantitative. The value of the pseudogap temperature
observed for these compounds in recent photoemission experiments has been
predicted by theory before it was observed experimentally. Additional
experimental confirmation would be useful. The theoretical methods that are
surveyed include mostly the Two-Particle Self-Consistent Approach, Variational
Cluster Perturbation Theory (or variational cluster approximation), and
Cellular Dynamical Mean-Field Theory.Comment: 32 pages, 51 figures. Slight modifications to text, figures and
references. A PDF file with higher-resolution figures is available at
http://www.physique.usherbrooke.ca/senechal/LTP-toc.pd
A Parallel Algorithm for Solving the 3d Schrodinger Equation
We describe a parallel algorithm for solving the time-independent 3d
Schrodinger equation using the finite difference time domain (FDTD) method. We
introduce an optimized parallelization scheme that reduces communication
overhead between computational nodes. We demonstrate that the compute time, t,
scales inversely with the number of computational nodes as t ~ N_nodes^(-0.95
+/- 0.04). This makes it possible to solve the 3d Schrodinger equation on
extremely large spatial lattices using a small computing cluster. In addition,
we present a new method for precisely determining the energy eigenvalues and
wavefunctions of quantum states based on a symmetry constraint on the FDTD
initial condition. Finally, we discuss the usage of multi-resolution techniques
in order to speed up convergence on extremely large lattices.Comment: 18 pages, 7 figures; published versio
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