141 research outputs found
Coexisting Pulses in a Model for Binary-Mixture Convection
We address the striking coexistence of localized waves (`pulses') of
different lengths which was observed in recent experiments and full numerical
simulations of binary-mixture convection. Using a set of extended
Ginzburg-Landau equations, we show that this multiplicity finds a natural
explanation in terms of the competition of two distinct, physical localization
mechanisms; one arises from dispersion and the other from a concentration mode.
This competition is absent in the standard Ginzburg-Landau equation. It may
also be relevant in other waves coupled to a large-scale field.Comment: 5 pages revtex with 4 postscript figures (everything uuencoded
Attractive Interaction Between Pulses in a Model for Binary-Mixture Convection
Recent experiments on convection in binary mixtures have shown that the
interaction between localized waves (pulses) can be repulsive as well as {\it
attractive} and depends strongly on the relative {\it orientation} of the
pulses. It is demonstrated that the concentration mode, which is characteristic
of the extended Ginzburg-Landau equations introduced recently, allows a natural
understanding of that result. Within the standard complex Ginzburg-Landau
equation this would not be possible.Comment: 7 pages revtex with 3 postscript figures (uuencoded
Parametric Forcing of Waves with Non-Monotonic Dispersion Relation: Domain Structures in Ferrofluids?
Surface waves on ferrofluids exposed to a dc-magnetic field exhibit a
non-monotonic dispersion relation. The effect of a parametric driving on such
waves is studied within suitable coupled Ginzburg-Landau equations. Due to the
non-monotonicity the neutral curve for the excitation of standing waves can
have up to three minima. The stability of the waves with respect to long-wave
perturbations is determined a phase-diffusion equation. It shows that the
band of stable wave numbers can split up into two or three sub-bands. The
resulting competition between the wave numbers corresponding to the respective
sub-bands leads quite naturally to patterns consisting of multiple domains of
standing waves which differ in their wave number. The coarsening dynamics of
such domain structures is addressed.Comment: 23 pages, 6 postscript figures, composed using RevTeX. Submitted to
PR
A Non-Equilibrium Defect-Unbinding Transition: Defect Trajectories and Loop Statistics
In a Ginzburg-Landau model for parametrically driven waves a transition
between a state of ordered and one of disordered spatio-temporal defect chaos
is found. To characterize the two different chaotic states and to get insight
into the break-down of the order, the trajectories of the defects are tracked
in detail. Since the defects are always created and annihilated in pairs the
trajectories form loops in space time. The probability distribution functions
for the size of the loops and the number of defects involved in them undergo a
transition from exponential decay in the ordered regime to a power-law decay in
the disordered regime. These power laws are also found in a simple lattice
model of randomly created defect pairs that diffuse and annihilate upon
collision.Comment: 4 pages 5 figure
Sources and sinks separating domains of left- and right-traveling waves: Experiment versus amplitude equations
In many pattern forming systems that exhibit traveling waves, sources and
sinks occur which separate patches of oppositely traveling waves. We show that
simple qualitative features of their dynamics can be compared to predictions
from coupled amplitude equations. In heated wire convection experiments, we
find a discrepancy between the observed multiplicity of sources and theoretical
predictions. The expression for the observed motion of sinks is incompatible
with any amplitude equation description.Comment: 4 pages, RevTeX, 3 figur
Temporal Modulation of Traveling Waves in the Flow Between Rotating Cylinders With Broken Azimuthal Symmetry
The effect of temporal modulation on traveling waves in the flows in two
distinct systems of rotating cylinders, both with broken azimuthal symmetry,
has been investigated. It is shown that by modulating the control parameter at
twice the critical frequency one can excite phase-locked standing waves and
standing-wave-like states which are not allowed when the system is rotationally
symmetric. We also show how previous theoretical results can be extended to
handle patterns such as these, that are periodic in two spatial direction.Comment: 17 pages in LaTeX, 22 figures available as postscript files from
http://www.esam.nwu.edu/riecke/lit/lit.htm
Whirling Hexagons and Defect Chaos in Hexagonal Non-Boussinesq Convection
We study hexagon patterns in non-Boussinesq convection of a thin rotating
layer of water. For realistic parameters and boundary conditions we identify
various linear instabilities of the pattern. We focus on the dynamics arising
from an oscillatory side-band instability that leads to a spatially disordered
chaotic state characterized by oscillating (whirling) hexagons. Using
triangulation we obtain the distribution functions for the number of pentagonal
and heptagonal convection cells. In contrast to the results found for defect
chaos in the complex Ginzburg-Landau equation and in inclined-layer convection,
the distribution functions can show deviations from a squared Poisson
distribution that suggest non-trivial correlations between the defects.Comment: 4 mpg-movies are available at
http://www.esam.northwestern.edu/~riecke/lit/lit.html submitted to New J.
Physic
Direct Hopf Bifurcation in Parametric Resonance of Hybridized Waves
We study parametric resonance of interacting waves having the same wave
vector and frequency. In addition to the well-known period-doubling instability
we show that under certain conditions the instability is caused by a Hopf
bifurcation leading to quasiperiodic traveling waves. It occurs, for example,
if the group velocities of both waves have different signs and the damping is
weak. The dynamics above the threshold is briefly discussed. Examples
concerning ferromagnetic spin waves and surface waves of ferro fluids are
discussed.Comment: Appears in Phys. Rev. Lett., RevTeX file and three postscript
figures. Packaged using the 'uufiles' utility, 33 k
Pattern selection as a nonlinear eigenvalue problem
A unique pattern selection in the absolutely unstable regime of driven,
nonlinear, open-flow systems is reviewed. It has recently been found in
numerical simulations of propagating vortex structures occuring in
Taylor-Couette and Rayleigh-Benard systems subject to an externally imposed
through-flow. Unlike the stationary patterns in systems without through-flow
the spatiotemporal structures of propagating vortices are independent of
parameter history, initial conditions, and system length. They do, however,
depend on the boundary conditions in addition to the driving rate and the
through-flow rate. Our analysis of the Ginzburg-Landau amplitude equation
elucidates how the pattern selection can be described by a nonlinear eigenvalue
problem with the frequency being the eigenvalue. Approaching the border between
absolute and convective instability the eigenvalue problem becomes effectively
linear and the selection mechanism approaches that of linear front propagation.
PACS: 47.54.+r,47.20.Ky,47.32.-y,47.20.FtComment: 18 pages in Postsript format including 5 figures, to appear in:
Lecture Notes in Physics, "Nonlinear Physics of Complex Sytems -- Current
Status and Future Trends", Eds. J. Parisi, S. C. Mueller, and W. Zimmermann
(Springer, Berlin, 1996
Boundary Limitation of Wavenumbers in Taylor-Vortex Flow
We report experimental results for a boundary-mediated wavenumber-adjustment
mechanism and for a boundary-limited wavenumber-band of Taylor-vortex flow
(TVF). The system consists of fluid contained between two concentric cylinders
with the inner one rotating at an angular frequency . As observed
previously, the Eckhaus instability (a bulk instability) is observed and limits
the stable wavenumber band when the system is terminated axially by two rigid,
non-rotating plates. The band width is then of order at small
() and agrees well with
calculations based on the equations of motion over a wide -range.
When the cylinder axis is vertical and the upper liquid surface is free (i.e.
an air-liquid interface), vortices can be generated or expelled at the free
surface because there the phase of the structure is only weakly pinned. The
band of wavenumbers over which Taylor-vortex flow exists is then more narrow
than the stable band limited by the Eckhaus instability. At small
the boundary-mediated band-width is linear in . These results are
qualitatively consistent with theoretical predictions, but to our knowledge a
quantitative calculation for TVF with a free surface does not exist.Comment: 8 pages incl. 9 eps figures bitmap version of Fig
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