1,283 research outputs found
Genealogical particle analysis of rare events
In this paper an original interacting particle system approach is developed
for studying Markov chains in rare event regimes. The proposed particle system
is theoretically studied through a genealogical tree interpretation of
Feynman--Kac path measures. The algorithmic implementation of the particle
system is presented. An estimator for the probability of occurrence of a rare
event is proposed and its variance is computed, which allows to compare and to
optimize different versions of the algorithm. Applications and numerical
implementations are discussed. First, we apply the particle system technique to
a toy model (a Gaussian random walk), which permits to illustrate the
theoretical predictions. Second, we address a physically relevant problem
consisting in the estimation of the outage probability due to polarization-mode
dispersion in optical fibers.Comment: Published at http://dx.doi.org/10.1214/105051605000000566 in the
Annals of Applied Probability (http://www.imstat.org/aap/) by the Institute
of Mathematical Statistics (http://www.imstat.org
Backpropagation Imaging in Nonlinear Harmonic Holography in the Presence of Measurement and Medium Noises
In this paper, the detection of a small reflector in a randomly heterogenous
medium using second-harmonic generation is investigated. The medium is
illuminated by a time-harmonic plane wave at frequency omega. It is assumed
that the reflector has a non-zero second-order nonlinear susceptibility, and
thus emits a wave at frequency two omega in addition to the fundamental
frequency linear scattering. It is shown how the fundamental frequency signal
and the second-harmonic signal propagate in the medium. A statistical study of
the images obtained by migrating the boundary data is performed. It is proved
that the second-harmonic image is more stable with respect to medium noise than
the one obtained with the fundamental signal. Moreover, the signal-to-noise
ratio for the second-harmonic image does not depend neither on the second-order
susceptibility tensor nor on the volume of the particle.Comment: 36 pages, 18 figure
Entropy production and time asymmetry in nonequilibrium fluctuations
The time-reversal symmetry of nonequilibrium fluctuations is experimentally
investigated in two out-of-equilibrium systems namely, a Brownian particle in a
trap moving at constant speed and an electric circuit with an imposed mean
current. The dynamical randomness of their nonequilibrium fluctuations is
characterized in terms of the standard and time-reversed entropies per unit
time of dynamical systems theory. We present experimental results showing that
their difference equals the thermodynamic entropy production in units of
Boltzmann's constant
Finite Element Simulation of Low Velocity Impact Damage on an Aeronautical Carbon Composite Structure
Low velocity barely visible impact damage (BVID) in laminated carbon composite structures has a major importance for aeronautical industries. This contribution leads with the development of finite element models to simulate the initiation and the propagation of internal damage inside a carbon composite structure due by a low velocity impact. Composite plates made from liquid resin infusion process (LRI) have been subjected to low energy impacts (around 25 J) using a drop weight machine. In the experimental procedure, the internal damage is evaluated using an infrared thermographic camera while the indentation depth of the face is measured by optical measurement technique. In a first time we developed a robust model using homogenised shells based on degenerated tri-dimensional brick elements and in a second time we decided to modelize the whole stacking sequence of homogeneous layers and cohesive interlaminar interfaces in order to compare and validate the obtained results. Both layer and interface damage initiation and propagation models based on the Hashin and the Benzeggagh-Kenane criteria have been used for the numerical simulations. Comparison of numerical results and experiments has shown the accuracy of
the proposed models
Poésie spatiale = Raumpoesie. - Zweisprachig: Französisch / Deutsch
Ilse und Pierre Garnier haben ein umfangreiches lyrisches Werk geschaffen. Seit Beginn der sechziger Jahre sind sie Initiatoren und Hauptvertreter der poésie spatiale. Die von Pierre Garnier redigierte Zeitschrift ´Les Lettres´ bildet zwischen 1963 und 1967 eine internationale Plattform für konkrete, visuelle und spatiale Poesie. Das Ehepaar Garnier hat darüber hinaus in Übersetzungen und Aufsätzen deutsche Literatur in Frankreich bekannt gemacht. Die maßgebliche französische Benn-Übersetzung stammt von Pierre Garnier. Die poésie spatiale integriert, reflektiert und erneuert die Konstellationen der konkreten und visuellen Poesie. Sie betont die Materialität und räumliche Gestalt von Sprach- und Textzeichen. Nach und nach wurden die spatialen Werke von Pierre und Ilse Garnier zunehmend asketischer und nachdenklicher. Ihre fragilen Wort- und Bildkompositionen verflechten vielfältige literarische und philosophische Assoziationen und versuchen, die Räume jenseits des Sagbaren zu ergründen
Scattered wavefield in the stochastic homogenization regime
In the context of providing a mathematical framework for the propagation of
ultrasound waves in a random multiscale medium, we consider the scattering of
classical waves (modeled by a divergence form scalar Helmholtz equation) by a
bounded object with a random composite micro-structure embedded in an unbounded
homogeneous background medium. Using quantitative stochastic homogenization
techniques, we provide asymptotic expansions of the scattered field in the
background medium with respect to a scaling parameter describing the spatial
random oscillations of the micro-structure. Introducing a boundary layer
corrector to compensate the breakdown of stationarity assumptions at the
boundary of the scattering medium, we prove quantitative - and -
error estimates for the asymptotic first-order expansion. The theoretical
results are supported by numerical experiments
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