1,967 research outputs found
Adaptive asynchronous time-stepping, stopping criteria, and a posteriori error estimates for fixed-stress iterative schemes for coupled poromechanics problems
In this paper we develop adaptive iterative coupling schemes for the Biot
system modeling coupled poromechanics problems. We particularly consider the
space-time formulation of the fixed-stress iterative scheme, in which we first
solve the problem of flow over the whole space-time interval, then exploiting
the space-time information for solving the mechanics. Two common
discretizations of this algorithm are then introduced based on two coupled
mixed finite element methods in-space and the backward Euler scheme in-time.
Therefrom, adaptive fixed-stress algorithms are build on conforming
reconstructions of the pressure and displacement together with equilibrated
flux and stresses reconstructions. These ingredients are used to derive a
posteriori error estimates for the fixed-stress algorithms, distinguishing the
different error components, namely the spatial discretization, the temporal
discretization, and the fixed-stress iteration components. Precisely, at the
iteration of the adaptive algorithm, we prove that our estimate gives
a guaranteed and fully computable upper bound on the energy-type error
measuring the difference between the exact and approximate pressure and
displacement. These error components are efficiently used to design adaptive
asynchronous time-stepping and adaptive stopping criteria for the fixed-stress
algorithms. Numerical experiments illustrate the efficiency of our estimates
and the performance of the adaptive iterative coupling algorithms
Performance Of A Liquid Argon Time Projection Chamber Exposed To The WANF Neutrino Beam
We present the results of the first exposure of a Liquid Argon TPC to a
multi-GeV neutrino beam. The data have been collected with a 50 liters
ICARUS-like chamber located between the CHORUS and NOMAD experiments at the
CERN West Area Neutrino Facility (WANF). We discuss both the instrumental
performance of the detector and its capability to identify and reconstruct low
multiplicity neutrino interactions.Comment: 14 pages, 12 figures. Submitted for publication to Physical Review
Quasar Tomography: Unification of Echo Mapping and Photoionisation Models
Reverberation mapping uses time-delayed variations in photoionised emission
lines to map the geometry and kinematics of emission-line gas in active
galactic nuclei. In previous work, the light travel time delay
tau=R(1+cos(theta))/c and Doppler shift v give a 2-d map Psi(tau,v) for each
emission line. Here we combine the velocity-delay information with
photoionisation physics in a maximum entropy fit to the full reverberating
spectrum F_lam(lam,t) to recover a 5-d map of the differential covering
fraction f(R,theta,n,N,v), with n and N the density and column density of the
gas clouds. We test the method for a variety of geometries (shells, rings,
disks, clouds, jets) by recovering a 3-d map f(R,theta,n) from reverberations
in 7 uv emission lines. The best test recovers a hollow shell geometry,
defining R to 0.15 dex, n to 0.3 dex, and ionisation parameter U ~ 1/(n R^2) to
0.1 dex. The results are sensitive to the adopted distance and luminosity,
suggesting that these parameters may be measurable as well.Comment: Accepted 4 Sep 2002 for publication in MNRA
Deep shower interpretation of the cosmic ray events observed in excess of the Greisen-Zatsepin-Kuzmin energy
We consider the possibility that the ultra-high-energy cosmic ray flux has a
small component of exotic particles which create showers much deeper in the
atmosphere than ordinary hadronic primaries. It is shown that applying the
conventional AGASA/HiRes/Auger data analysis procedures to such exotic events
results in large systematic biases in the energy spectrum measurement. SubGZK
exotic showers may be mis-reconstructed with much higher energies and mimick
superGZK events. Alternatively, superGZK exotic showers may elude detection by
conventional fluorescence analysis techniques.Comment: 22 pages, 5 figure
Guaranteed and robust a posteriori error estimates and balancing discretization and linearization errors for monotone nonlinear problems
International audienceWe derive a posteriori error estimates for a class of second-order monotone quasi-linear diffusion-type problems approximated by piecewise affine, continuous finite elements. Our estimates yield a guaranteed and fully computable upper bound on the error measured by the dual norm of the residual, as well as a global error lower bound, up to a generic constant independent of the nonlinear operator. They are thus fully robust with respect to the nonlinearity, thanks to the choice of the error measure. They are also locally efficient, albeit in a different norm, and hence suitable for adaptive mesh refinement. Moreover, they allow to distinguish, estimate separately, and compare the discretization and linearization errors. Hence, the iterative (Newton--Raphson, quasi-Newton) linearization can be stopped whenever the linearization error drops to the level at which it does not affect significantly the overall error. This can lead to important computational savings, as performing an excessive number of unnecessary linearization iterations can be avoided. Numerical experiments for the -Laplacian illustrate the theoretical developments
Well-balanced finite volume schemes for hydrodynamic equations with general free energy
Well balanced and free energy dissipative first- and second-order accurate
finite volume schemes are proposed for a general class of hydrodynamic systems
with linear and nonlinear damping. The natural Liapunov functional of the
system, given by its free energy, allows for a characterization of the
stationary states by its variation. An analog property at the discrete level
enables us to preserve stationary states at machine precision while keeping the
dissipation of the discrete free energy. These schemes allow for analysing
accurately the stability properties of stationary states in challeging problems
such as: phase transitions in collective behavior, generalized Euler-Poisson
systems in chemotaxis and astrophysics, and models in dynamic density
functional theories; having done a careful validation in a battery of relevant
test cases.Comment: Videos from the simulations of this work are available at
https://sergioperezresearch.wordpress.com/well-balance
Limits to the muon flux from WIMP annihilation in the center of the Earth with the AMANDA detector
A search for nearly vertical up-going muon-neutrinos from neutralino
annihilations in the center of the Earth has been performed with the AMANDA-B10
neutrino detector. The data sample collected in 130.1 days of live-time in
1997, ~10^9 events, has been analyzed for this search. No excess over the
expected atmospheric neutrino background is oberved. An upper limit at 90%
confidence level on the annihilation rate of neutralinos in the center of the
Earth is obtained as a function of the neutralino mass in the range 100
GeV-5000 GeV, as well as the corresponding muon flux limit.Comment: 14 pages, 11 figures. Version accepted for publication in Physical
Review
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