134 research outputs found
Emergence of patterns in driven and in autonomous spatiotemporal systems
The relationship between a driven extended system and an autonomous
spatiotemporal system is investigated in the context of coupled map lattice
models. Specifically, a locally coupled map lattice subjected to an external
drive is compared to a coupled map system with similar local couplings plus a
global interaction. It is shown that, under some conditions, the emergent
patterns in both systems are analogous. Based on the knowledge of the dynamical
responses of the driven lattice, we present a method that allows the prediction
of parameter values for the emergence of ordered spatiotemporal patterns in a
class of coupled map systems having local coupling and general forms of global
interactions.Comment: 7 pages, 3 figs, submitted to PRE (2002
Kink-induced transport and segregation in oscillated granular layers
We use experiments and molecular dynamics simulations of vertically
oscillated granular layers to study horizontal particle segregation induced by
a kink (a boundary between domains oscillating out of phase). Counter-rotating
convection rolls carry the larger particles in a bidisperse layer along the
granular surface to a kink, where they become trapped. The convection
originates from avalanches that occur inside the layer, along the interface
between solidified and fluidized grains. The position of a kink can be
controlled by modulation of the container frequency, making possible systematic
harvesting of the larger particles.Comment: 4 pages, 5 figures. to appear in Phys. Rev. Let
Solitary vortex couples in viscoelastic Couette flow
We report experimental observation of a localized structure, which is of a
new type for dissipative systems. It appears as a solitary vortex couple
("diwhirl") in Couette flow with highly elastic polymer solutions. A unique
property of the diwhirls is that they are stationary, in contrast to the usual
localized wave structures in both Hamiltonian and dissipative systems which are
stabilized by wave dispersion. It is also a new object in fluid dynamics - a
couple of vortices that build a single entity somewhat similar to a magnetic
dipole. The diwhirls arise as a result of a purely elastic instability through
a hysteretic transition at negligible Reynolds numbers. It is suggested that
the vortex flow is driven by the same forces that cause the Weissenberg effect.
The diwhirls have a striking asymmetry between the inflow and outflow, which is
also an essential feature of the suggested elastic instability mechanism.Comment: 9 pages (LaTeX), 5 Postscript figures, submitte
Stationary and Oscillatory Spatial Patterns Induced by Global Periodic Switching
We propose a new mechanism for pattern formation based on the global
alternation of two dynamics neither of which exhibits patterns. When driven by
either one of the separate dynamics, the system goes to a spatially homogeneous
state associated with that dynamics. However, when the two dynamics are
globally alternated sufficiently rapidly, the system exhibits stationary
spatial patterns. Somewhat slower switching leads to oscillatory patterns. We
support our findings by numerical simulations and discuss the results in terms
of the symmetries of the system and the ratio of two relevant characteristic
times, the switching period and the relaxation time to a homogeneous state in
each separate dynamics.Comment: REVTEX preprint: 12 pages including 1 (B&W) + 3 (COLOR) figures (to
appear in Physical Review Letters
Continuum-type stability balloon in oscillated granular layers
The stability of convection rolls in a fluid heated from below is limited by
secondary instabilities, including the skew-varicose and crossroll
instabilities. We observe a stability boundary defined by the same
instabilities in stripe patterns in a vertically oscillated granular layer.
Molecular dynamics simulations show that the mechanism of the skew-varicose
instability in granular patterns is similar to that in convection. These
results suggest that pattern formation in granular media can be described by
continuum models analogous to those used in fluid systems.Comment: 4 pages, 6 ps figs, submitted to PR
Controlled Dynamics of Interfaces in a Vibrated Granular Layer
We present experimental study of a topological excitation, {\it interface},
in a vertically vibrated layer of granular material. We show that these
interfaces, separating regions of granular material oscillation with opposite
phases, can be shifted and controlled by a very small amount of an additional
subharmonic signal, mixed with the harmonic driving signal. The speed and the
direction of interface motion depends sensitively on the phase and the
amplitude of the subharmonic driving.Comment: 4 pages, 6 figures, RevTe
Hexagons, Kinks and Disorder in Oscillated Granular Layers
Experiments on vertically oscillated granular layers in an evacuated
container reveal a sequence of well-defined pattern bifurcations as the
container acceleration is increased. Period doublings of the layer center of
mass motion and a parametric wave instability interact to produce hexagons and
more complicated patterns composed of distinct spatial domains of different
relative phase separated by kinks (phase discontinuities). Above a critical
acceleration, the layer becomes disordered in both space and time.Comment: 4 pages. The RevTeX file has a macro allowing various styles. The
appropriate style is "myprint" which is the defaul
Resonant nucleation of spatio-temporal order via parametric modal amplification
We investigate, analytically and numerically, the emergence of
spatio-temporal order in nonequilibrium scalar field theories. The onset of
order is triggered by destabilizing interactions (DIs), which instantaneously
change the interacting potential from a single to a double-well, tunable to be
either degenerate (SDW) or nondegenerate (ADW). For the SDW case, we observe
the emergence of spatio-temporal coherent structures known as oscillons. We
show that this emergence is initially synchronized, the result of parametric
amplification of the relevant oscillon modes. We also discuss how these ordered
structures act as bottlenecks for equipartition. For ADW potentials, we show
how the same parametric amplification mechanism may trigger the rapid decay of
a metastable state. For a range of temperatures, the decay rates associated
with this resonant nucleation can be orders of magnitude larger than those
computed by homogeneous nucleation, with time-scales given by a simple power
law, , where depends weakly on the
temperature and is the free-energy barrier of a critical
fluctuation.Comment: 38 pages, 20 figures now included within the tex
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