56,754 research outputs found
Transport in a Levy ratchet: Group velocity and distribution spread
We consider the motion of an overdamped particle in a periodic potential
lacking spatial symmetry under the influence of symmetric L\'evy noise, being a
minimal setup for a ``L\'evy ratchet.'' Due to the non-thermal character of the
L\'evy noise, the particle exhibits a motion with a preferred direction even in
the absence of whatever additional time-dependent forces. The examination of
the L\'evy ratchet has to be based on the characteristics of directionality
which are different from typically used measures like mean current and the
dispersion of particles' positions, since these get inappropriate when the
moments of the noise diverge. To overcome this problem, we discuss robust
measures of directionality of transport like the position of the median of the
particles displacements' distribution characterizing the group velocity, and
the interquantile distance giving the measure of the distributions' width.
Moreover, we analyze the behavior of splitting probabilities for leaving an
interval of a given length unveiling qualitative differences between the noises
with L\'evy indices below and above unity. Finally, we inspect the problem of
the first escape from an interval of given length revealing independence of
exit times on the structure of the potential.Comment: 9 pages, 12 figure
Idle Period Propagation in Message-Passing Applications
Idle periods on different processes of Message Passing applications are
unavoidable. While the origin of idle periods on a single process is well
understood as the effect of system and architectural random delays, yet it is
unclear how these idle periods propagate from one process to another. It is
important to understand idle period propagation in Message Passing applications
as it allows application developers to design communication patterns avoiding
idle period propagation and the consequent performance degradation in their
applications. To understand idle period propagation, we introduce a methodology
to trace idle periods when a process is waiting for data from a remote delayed
process in MPI applications. We apply this technique in an MPI application that
solves the heat equation to study idle period propagation on three different
systems. We confirm that idle periods move between processes in the form of
waves and that there are different stages in idle period propagation. Our
methodology enables us to identify a self-synchronization phenomenon that
occurs on two systems where some processes run slower than the other processes.Comment: 18th International Conference on High Performance Computing and
Communications, IEEE, 201
Levy stable noise induced transitions: stochastic resonance, resonant activation and dynamic hysteresis
A standard approach to analysis of noise-induced effects in stochastic
dynamics assumes a Gaussian character of the noise term describing interaction
of the analyzed system with its complex surroundings. An additional assumption
about the existence of timescale separation between the dynamics of the
measured observable and the typical timescale of the noise allows external
fluctuations to be modeled as temporally uncorrelated and therefore white.
However, in many natural phenomena the assumptions concerning the
abovementioned properties of "Gaussianity" and "whiteness" of the noise can be
violated. In this context, in contrast to the spatiotemporal coupling
characterizing general forms of non-Markovian or semi-Markovian L\'evy walks,
so called L\'evy flights correspond to the class of Markov processes which
still can be interpreted as white, but distributed according to a more general,
infinitely divisible, stable and non-Gaussian law. L\'evy noise-driven
non-equilibrium systems are known to manifest interesting physical properties
and have been addressed in various scenarios of physical transport exhibiting a
superdiffusive behavior. Here we present a brief overview of our recent
investigations aimed to understand features of stochastic dynamics under the
influence of L\'evy white noise perturbations. We find that the archetypal
phenomena of noise-induced ordering are robust and can be detected also in
systems driven by non-Gaussian, heavy-tailed fluctuations with infinite
variance.Comment: 7 pages, 8 figure
Interstellar Sonic and Alfv\'enic Mach Numbers and the Tsallis Distribution
In an effort to characterize the Mach numbers of ISM magnetohydrodynamic
(MHD) turbulence, we study the probability distribution functions (PDFs) of
patial increments of density, velocity, and magnetic field for fourteen ideal
isothermal MHD simulations at resolution 512^3. In particular, we fit the PDFs
using the Tsallis function and study the dependency of fit parameters on the
compressibility and magnetization of the gas. We find that the Tsallis function
fits PDFs of MHD turbulence well, with fit parameters showing sensitivities to
the sonic and Alfven Mach numbers. For 3D density, column density, and
position-position-velocity (PPV) data we find that the amplitude and width of
the PDFs shows a dependency on the sonic Mach number. We also find the width of
the PDF is sensitive to global Alfvenic Mach number especially in cases where
the sonic number is high. These dependencies are also found for mock
observational cases, where cloud-like boundary conditions, smoothing, and noise
are introduced. The ability of Tsallis statistics to characterize sonic and
Alfvenic Mach numbers of simulated ISM turbulence point to it being a useful
tool in the analysis of the observed ISM, especially when used simultaneously
with other statistical techniques.Comment: 20 pages, 16 figures, ApJ submitte
An integrated source of broadband quadrature squeezed light
An integrated silicon nitride resonator is proposed as an ultra-compact
source of bright single-mode quadrature squeezed light at 850 nm. Optical
properties of the device are investigated and tailored through numerical
simulations, with particular attention paid to loss associated with interfacing
the device. An asymmetric double layer stack waveguide geometry with inverse
vertical tapers is proposed for efficient and robust fibre-chip coupling,
yielding a simulated total loss of -0.75 dB/facet. We assess the feasibility of
the device through a full quantum noise analysis and derive the output
squeezing spectrum for intra-cavity pump self-phase modulation. Subject to
standard material loss and detection efficiencies, we find that the device
holds promises for generating substantial quantum noise squeezing over a
bandwidth exceeding 1 GHz. In the low-propagation loss regime, approximately -7
dB squeezing is predicted for a pump power of only 50 mW.Comment: 23 pages, 12 figure
A new method for aspherical surface fitting with large-volume datasets
In the framework of form characterization of aspherical surfaces, European National Metrology Institutes (NMIs) have been developing ultra-high precision machines having the ability to measure aspherical lenses with an uncertainty of few tens of nanometers. The fitting of the acquired aspherical datasets onto their corresponding theoretical model should be achieved at the same level of precision. In this article, three fitting algorithms are investigated: the Limited memory-Broyden-Fletcher-Goldfarb-Shanno (L-BFGS), the Levenberg–Marquardt (LM) and one variant of the Iterative Closest Point (ICP). They are assessed based on their capacities to converge relatively fast to achieve a nanometric level of accuracy, to manage a large volume of data and to be robust to the position of the data with respect to the model. Nev-ertheless, the algorithms are first evaluated on simulated datasets and their performances are studied. The comparison of these algorithms is extended on measured datasets of an aspherical lens. The results validate the newly used method for the fitting of aspherical surfaces and reveal that it is well adapted, faster and less complex than the LM or ICP methods.EMR
Viscous fingering of miscible slices
Viscous fingering of a miscible high viscosity slice of fluid displaced by a
lower viscosity fluid is studied in porous media by direct numerical
simulations of Darcy's law coupled to the evolution equation for the
concentration of a solute controlling the viscosity of miscible solutions. In
contrast with fingering between two semi-infinite regions, fingering of finite
slices is a transient phenomenon due to the decrease in time of the viscosity
ratio across the interface induced by fingering and dispersion processes. We
show that fingering contributes transiently to the broadening of the peak in
time by increasing its variance. A quantitative analysis of the asymptotic
contribution of fingering to this variance is conducted as a function of the
four relevant parameters of the problem i.e. the log-mobility ratio R, the
length of the slice l, the Peclet number Pe and the ratio between transverse
and axial dispersion coefficients . Relevance of the results is
discussed in relation with transport of viscous samples in chromatographic
columns and propagation of contaminants in porous media.Comment: 10 pages, 13 figure
Numerical studies towards practical large-eddy simulation
Large-eddy simulation developments and validations are presented for an
improved simulation of turbulent internal flows. Numerical methods are proposed
according to two competing criteria: numerical qualities (precision and
spectral characteristics), and adaptability to complex configurations. First,
methods are tested on academic test-cases, in order to abridge with fundamental
studies. Consistent results are obtained using adaptable finite volume method,
with higher order advection fluxes, implicit grid filtering and "low-cost"
shear-improved Smagorinsky model. This analysis particularly focuses on mean
flow, fluctuations, two-point correlations and spectra. Moreover, it is shown
that exponential averaging is a promising tool for LES implementation in
complex geometry with deterministic unsteadiness. Finally, adaptability of the
method is demonstrated by application to a configuration representative of
blade-tip clearance flow in a turbomachine
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