11,652 research outputs found
The Kardar-Parisi-Zhang equation in the weak noise limit: Pattern formation and upper critical dimension
We extend the previously developed weak noise scheme, applied to the noisy
Burgers equation in 1D, to the Kardar-Parisi-Zhang equation for a growing
interface in arbitrary dimensions. By means of the Cole-Hopf transformation we
show that the growth morphology can be interpreted in terms of dynamically
evolving textures of localized growth modes with superimposed diffusive modes.
In the Cole-Hopf representation the growth modes are static solutions to the
diffusion equation and the nonlinear Schroedinger equation, subsequently
boosted to finite velocity by a Galilei transformation. We discuss the dynamics
of the pattern formation and, briefly, the superimposed linear modes.
Implementing the stochastic interpretation we discuss kinetic transitions and
in particular the properties in the pair mode or dipole sector. We find the
Hurst exponent H=(3-d)/(4-d) for the random walk of growth modes in the dipole
sector. Finally, applying Derrick's theorem based on constrained minimization
we show that the upper critical dimension is d=4 in the sense that growth modes
cease to exist above this dimension.Comment: 27 pages, 19 eps figs, revte
A primer on noise-induced transitions in applied dynamical systems
Noise plays a fundamental role in a wide variety of physical and biological
dynamical systems. It can arise from an external forcing or due to random
dynamics internal to the system. It is well established that even weak noise
can result in large behavioral changes such as transitions between or escapes
from quasi-stable states. These transitions can correspond to critical events
such as failures or extinctions that make them essential phenomena to
understand and quantify, despite the fact that their occurrence is rare. This
article will provide an overview of the theory underlying the dynamics of rare
events for stochastic models along with some example applications
High-fidelity rendering on shared computational resources
The generation of high-fidelity imagery is a computationally expensive process
and parallel computing has been traditionally employed to alleviate this cost.
However, traditional parallel rendering has been restricted to expensive shared
memory or dedicated distributed processors. In contrast, parallel computing on
shared resources such as a computational or a desktop grid, offers a low cost alternative. But, the prevalent rendering systems are currently incapable of seamlessly handling such shared resources as they suffer from high latencies, restricted
bandwidth and volatility. A conventional approach of rescheduling failed jobs in
a volatile environment inhibits performance by using redundant computations.
Instead, clever task subdivision along with image reconstruction techniques provides an unrestrictive fault-tolerance mechanism, which is highly suitable for
high-fidelity rendering. This thesis presents novel fault-tolerant parallel rendering algorithms for effectively tapping the enormous inexpensive computational
power provided by shared resources.
A first of its kind system for fully dynamic high-fidelity interactive rendering
on idle resources is presented which is key for providing an immediate feedback
to the changes made by a user. The system achieves interactivity by monitoring
and adapting computations according to run-time variations in the computational
power and employs a spatio-temporal image reconstruction technique for enhancing the visual fidelity. Furthermore, algorithms described for time-constrained offline rendering of still images and animation sequences, make it possible to deliver
the results in a user-defined limit. These novel methods enable the employment
of variable resources in deadline-driven environments
Role of particle conservation in self-propelled particle systems
Actively propelled particles undergoing dissipative collisions are
known to develop a state of spatially distributed coherently moving clusters.
For densities larger than a characteristic value, clusters grow in time and form
a stationary well-ordered state of coherent macroscopic motion. In this work
we address two questions. (i) What is the role of the particlesā aspect ratio in
the context of cluster formation, and does the particle shape affect the systemās
behavior on hydrodynamic scales? (ii) To what extent does particle conservation
influence pattern formation? To answer these questions we suggest a simple
kinetic model permitting us to depict some of the interaction properties between
freely moving particles and particles integrated in clusters. To this end, we
introduce two particle species: single and cluster particles. Specifically, we
account for coalescence of clusters from single particles, assembly of single
particles on existing clusters, collisions between clusters and cluster disassembly.
Coarse graining our kinetic model, (i) we demonstrate that particle shape (i.e.
aspect ratio) shifts the scale of the transition density, but does not impact the
instabilities at the ordering threshold and (ii) we show that the validity of particle
conservation determines the existence of a longitudinal instability, which tends to amplify density heterogeneities locally, and in turn triggers a wave pattern
with wave vectors parallel to the axis of macroscopic order. If the system is in
contact with a particle reservoir, this instability vanishes due to a compensation
of density heterogeneities
Conditioning the logistic branching process on non-extinction
We consider a birth and death process in which death is due to both `natural
death' and to competition between individuals, modelled as a quadratic function
of population size. The resulting `logistic branching process' has been
proposed as a model for numbers of individuals in populations competing for
some resource, or for numbers of species. However, because of the quadratic
death rate, even if the intrinsic growth rate is positive, the population will,
with probability one, die out in finite time. There is considerable interest in
understanding the process conditioned on non-extinction.
In this paper, we exploit a connection with the ancestral selection graph of
population genetics to find expressions for the transition rates in the
logistic branching process conditioned on survival until some fixed time ,
in terms of the distribution of a certain one-dimensional diffusion process at
time . We also find the probability generating function of the Yaglom
distribution of the process and rather explicit expressions for the transition
rates for the so-called Q-process, that is the logistic branching process
conditioned to stay alive into the indefinite future. For this process, one can
write down the joint generator of the (time-reversed) total population size and
what in population genetics would be called the `genealogy' and in
phylogenetics would be called the `reconstructed tree' of a sample from the
population.
We explore some ramifications of these calculations numerically
Out of equilibrium dynamics of classical and quantum complex systems
Equilibrium is a rather ideal situation, the exception rather than the rule
in Nature. Whenever the external or internal parameters of a physical system
are varied its subsequent relaxation to equilibrium may be either impossible or
take very long times. From the point of view of fundamental physics no generic
principle such as the ones of thermodynamics allows us to fully understand
their behaviour. The alternative is to treat each case separately. It is
illusionary to attempt to give, at least at this stage, a complete description
of all non-equilibrium situations. Still, one can try to identify and
characterise some concrete but still general features of a class of out of
equilibrium problems - yet to be identified - and search for a unified
description of these. In this report I briefly describe the behaviour and
theory of a set of non-equilibrium systems and I try to highlight common
features and some general laws that have emerged in recent years.Comment: 36 pages, to be published in Compte Rendus de l'Academie de Sciences,
T. Giamarchi e
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