5,920 research outputs found
Geometry of the ergodic quotient reveals coherent structures in flows
Dynamical systems that exhibit diverse behaviors can rarely be completely
understood using a single approach. However, by identifying coherent structures
in their state spaces, i.e., regions of uniform and simpler behavior, we could
hope to study each of the structures separately and then form the understanding
of the system as a whole. The method we present in this paper uses trajectory
averages of scalar functions on the state space to: (a) identify invariant sets
in the state space, (b) form coherent structures by aggregating invariant sets
that are similar across multiple spatial scales. First, we construct the
ergodic quotient, the object obtained by mapping trajectories to the space of
trajectory averages of a function basis on the state space. Second, we endow
the ergodic quotient with a metric structure that successfully captures how
similar the invariant sets are in the state space. Finally, we parametrize the
ergodic quotient using intrinsic diffusion modes on it. By segmenting the
ergodic quotient based on the diffusion modes, we extract coherent features in
the state space of the dynamical system. The algorithm is validated by
analyzing the Arnold-Beltrami-Childress flow, which was the test-bed for
alternative approaches: the Ulam's approximation of the transfer operator and
the computation of Lagrangian Coherent Structures. Furthermore, we explain how
the method extends the Poincar\'e map analysis for periodic flows. As a
demonstration, we apply the method to a periodically-driven three-dimensional
Hill's vortex flow, discovering unknown coherent structures in its state space.
In the end, we discuss differences between the ergodic quotient and
alternatives, propose a generalization to analysis of (quasi-)periodic
structures, and lay out future research directions.Comment: Submitted to Elsevier Physica D: Nonlinear Phenomen
Precision Measurements of Stretching and Compression in Fluid Mixing
The mixing of an impurity into a flowing fluid is an important process in
many areas of science, including geophysical processes, chemical reactors, and
microfluidic devices. In some cases, for example periodic flows, the concepts
of nonlinear dynamics provide a deep theoretical basis for understanding
mixing. Unfortunately, the building blocks of this theory, i.e. the fixed
points and invariant manifolds of the associated Poincare map, have remained
inaccessible to direct experimental study, thus limiting the insight that could
be obtained. Using precision measurements of tracer particle trajectories in a
two-dimensional fluid flow producing chaotic mixing, we directly measure the
time-dependent stretching and compression fields. These quantities, previously
available only numerically, attain local maxima along lines coinciding with the
stable and unstable manifolds, thus revealing the dynamical structures that
control mixing. Contours or level sets of a passive impurity field are found to
be aligned parallel to the lines of large compression (unstable manifolds) at
each instant. This connection appears to persist as the onset of turbulence is
approached.Comment: 5 pages, 5 figure
Severe Respiratory Insufficiency Complicating Epstein-Barr Virus Infection: Case Report and Review
We report a case involving a young adult who had life-threatening bilateral pneumonitis in the course of an acute Epstein-Barr virus (EBV) infection. Because of severe hypoxemia, the patient required mechanical ventilation and additional oxygenation by an intravascular oxygenator. The patient was treated with corticosteroids and survived without sequelae. Severe pulmonary involvement associated with EBV infection is a rare but potentially fatal complication of infectious mononucleosis. Similar cases reported in the literature are reviewed, and the therapeutic options for this particular complication are discusse
A configuration system for the ATLAS trigger
The ATLAS detector at CERN's Large Hadron Collider will be exposed to
proton-proton collisions from beams crossing at 40 MHz that have to be reduced
to the few 100 Hz allowed by the storage systems. A three-level trigger system
has been designed to achieve this goal. We describe the configuration system
under construction for the ATLAS trigger chain. It provides the trigger system
with all the parameters required for decision taking and to record its history.
The same system configures the event reconstruction, Monte Carlo simulation and
data analysis, and provides tools for accessing and manipulating the
configuration data in all contexts.Comment: 4 pages, 2 figures, contribution to the Conference on Computing in
High Energy and Nuclear Physics (CHEP06), 13.-17. Feb 2006, Mumbai, Indi
Detecting barriers to transport: A review of different techniques
We review and discuss some different techniques for describing local
dispersion properties in fluids. A recent Lagrangian diagnostics, based on the
Finite Scale Lyapunov Exponent (FSLE), is presented and compared to the Finite
Time Lyapunov Exponent (FTLE), and to the Okubo-Weiss (OW) and Hua-Klein (HK)
criteria. We show that the OW and HK are a limiting case of the FTLE, and that
the FSLE is the most efficient method for detecting the presence of
cross-stream barriers. We illustrate our findings by considering two examples
of geophysical interest: a kinematic meandering jet model, and Lagrangian
tracers advected by stratospheric circulation.Comment: 15 pages, 9 figures, submitted to Physica
Elementa physiologiae corporis humani auctore Alberto v. Haller... : tomus tertius, respiratio vox
Port. a dos tintas con grab. xil.Texto con notas a pie de pág.Enc. PielSign.: A-Z4, Aa-Oo4, Aaa-Ooo4, Ppp2, Qqq
Was ist Rechtens, wenn die oberste Staatsgewalt dem Zwecke des Staatsverbandes entgegenhandelt? : Nebst einer Darstellung der Theorie des Herrn von Haller, hinsichtlich dieser Frage / Von Friedrich Karl von Strombeck ...
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