163 research outputs found
Estimating long term behavior of flows without trajectory integration: the infinitesimal generator approach
The long-term distributions of trajectories of a flow are described by
invariant densities, i.e. fixed points of an associated transfer operator. In
addition, global slowly mixing structures, such as almost-invariant sets, which
partition phase space into regions that are almost dynamically disconnected,
can also be identified by certain eigenfunctions of this operator. Indeed,
these structures are often hard to obtain by brute-force trajectory-based
analyses. In a wide variety of applications, transfer operators have proven to
be very efficient tools for an analysis of the global behavior of a dynamical
system.
The computationally most expensive step in the construction of an approximate
transfer operator is the numerical integration of many short term trajectories.
In this paper, we propose to directly work with the infinitesimal generator
instead of the operator, completely avoiding trajectory integration. We propose
two different discretization schemes; a cell based discretization and a
spectral collocation approach. Convergence can be shown in certain
circumstances. We demonstrate numerically that our approach is much more
efficient than the operator approach, sometimes by several orders of magnitude
Transport in time-dependent dynamical systems: Finite-time coherent sets
We study the transport properties of nonautonomous chaotic dynamical systems
over a finite time duration. We are particularly interested in those regions
that remain coherent and relatively non-dispersive over finite periods of time,
despite the chaotic nature of the system. We develop a novel probabilistic
methodology based upon transfer operators that automatically detects maximally
coherent sets. The approach is very simple to implement, requiring only
singular vector computations of a matrix of transitions induced by the
dynamics. We illustrate our new methodology on an idealized stratospheric flow
and in two and three dimensional analyses of European Centre for Medium Range
Weather Forecasting (ECMWF) reanalysis data
How well-connected is the surface of the global ocean?
The Ekman dynamics of the ocean surface circulation is known to contain attracting regions such as the great oceanic gyres and the associated garbage patches. Less well-known are the extents of the basins of attractions of these regions and how strongly attracting they are. Understanding the shape and extent of the basins of attraction sheds light on the question of the strength of connectivity of different regions of the ocean, which helps in understanding the flow of buoyant material like plastic litter. Using short flow time trajectory data from a global ocean model, we create a Markov chain model of the surface ocean dynamics. The surface ocean is not a conservative dynamical system as water in the ocean follows three-dimensional pathways, with upwelling and downwelling in certain regions. Using our Markov chain model, we easily compute net surface upwelling and downwelling, and verify that it matches observed patterns of upwelling and downwelling in the real ocean. We analyze the Markov chain to determine multiple attracting regions. Finally, using an eigenvector approach, we (i) identify the five major ocean garbage patches, (ii) partition the ocean into basins of attraction for each of the garbage patches, and (iii) partition the ocean into regions that demonstrate transient dynamics modulo the attracting garbage patches
Almost sure invariance principle for random piecewise expanding maps
We prove a fiberwise almost sure invariance principle for random piecewise
expanding transformations in one and higher dimensions using recent
developments on martingale techniques
Optimally coherent sets in geophysical flows: A new approach to delimiting the stratospheric polar vortex
The "edge" of the Antarctic polar vortex is known to behave as a barrier to
the meridional (poleward) transport of ozone during the austral winter. This
chemical isolation of the polar vortex from the middle and low latitudes
produces an ozone minimum in the vortex region, intensifying the ozone hole
relative to that which would be produced by photochemical processes alone.
Observational determination of the vortex edge remains an active field of
research. In this letter, we obtain objective estimates of the structure of the
polar vortex by introducing a new technique based on transfer operators that
aims to find regions with minimal external transport. Applying this new
technique to European Centre for Medium-Range Weather Forecasts (ECMWF) ERA-40
three-dimensional velocity data we produce an improved three-dimensional
estimate of the vortex location in the upper stratosphere where the vortex is
most pronounced. This novel computational approach has wide potential
application in detecting and analysing mixing structures in a variety of
atmospheric, oceanographic, and general fluid dynamical settings
The Recoverable Robust Tail Assignment Problem
This is the author accepted manuscript. The final version is available from Institute for Operations Research and the Management Sciences (INFORMS) via the DOI in this record Schedule disruptions are commonplace in the airline industry with many flight-delaying events
occurring each day. Recently there has been a focus on introducing robustness into airline planning
stages to reduce the effect of these disruptions. We propose a recoverable robustness technique as
an alternative to robust optimisation to reduce the effect of disruptions and the cost of recovery. We
formulate the recoverable robust tail assignment problem (RRTAP) as a stochastic program, solved
using column generation in the master and subproblems of the Benders decomposition. We implement a two-phase algorithm for the Benders decomposition incorporating the Magnanti-Wong [21]
enhancement techniques. The RRTAP includes costs due to flight delays, cancellation, and passenger
rerouting, and the recovery stage includes cancellation, delay, and swapping options. To highlight
the benefits of simultaneously solving planning and recovery problems in the RRTAP we compare
our tail assignment solution with the tail assignment generated using a connection cost function
presented in Gr¨onkvist [15]. Using airline data we demonstrate that by developing a better tail assignment plan via the RRTAP framework, one can reduce recovery costs in the event of a disruption.Australian Research Council Centre of Excellence for MathematicsMASCOS
Ulam method for the Chirikov standard map
We introduce a generalized Ulam method and apply it to symplectic dynamical
maps with a divided phase space. Our extensive numerical studies based on the
Arnoldi method show that the Ulam approximant of the Perron-Frobenius operator
on a chaotic component converges to a continuous limit. Typically, in this
regime the spectrum of relaxation modes is characterized by a power law decay
for small relaxation rates. Our numerical data show that the exponent of this
decay is approximately equal to the exponent of Poincar\'e recurrences in such
systems. The eigenmodes show links with trajectories sticking around stability
islands.Comment: 13 pages, 13 figures, high resolution figures available at:
http://www.quantware.ups-tlse.fr/QWLIB/ulammethod/ minor corrections in text
and fig. 12 and revised discussio
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