108 research outputs found
Advectional enhancement of eddy diffusivity under parametric disorder
Frozen parametric disorder can lead to appearance of sets of localized
convective currents in an otherwise stable (quiescent) fluid layer heated from
below. These currents significantly influence the transport of an admixture (or
any other passive scalar) along the layer. When the molecular diffusivity of
the admixture is small in comparison to the thermal one, which is quite typical
in nature, disorder can enhance the effective (eddy) diffusivity by several
orders of magnitude in comparison to the molecular diffusivity. In this paper
we study the effect of an imposed longitudinal advection on delocalization of
convective currents, both numerically and analytically; and report subsequent
drastic boost of the effective diffusivity for weak advection.Comment: 14 pages, 6 figures, for Topical Issue of Physica Scripta "2nd Intl.
Conf. on Turbulent Mixing and Beyond
Uncertainty Principle for Control of Ensembles of Oscillators Driven by Common Noise
We discuss control techniques for noisy self-sustained oscillators with a
focus on reliability, stability of the response to noisy driving, and
oscillation coherence understood in the sense of constancy of oscillation
frequency. For any kind of linear feedback control--single and multiple delay
feedback, linear frequency filter, etc.--the phase diffusion constant,
quantifying coherence, and the Lyapunov exponent, quantifying reliability, can
be efficiently controlled but their ratio remains constant. Thus, an
"uncertainty principle" can be formulated: the loss of reliability occurs when
coherence is enhanced and, vice versa, coherence is weakened when reliability
is enhanced. Treatment of this principle for ensembles of oscillators
synchronized by common noise or global coupling reveals a substantial
difference between the cases of slightly non-identical oscillators and
identical ones with intrinsic noise.Comment: 10 pages, 5 figure
Noise Can Reduce Disorder in Chaotic Dynamics
We evoke the idea of representation of the chaotic attractor by the set of
unstable periodic orbits and disclose a novel noise-induced ordering
phenomenon. For long unstable periodic orbits forming the strange attractor the
weights (or natural measure) is generally highly inhomogeneous over the set,
either diminishing or enhancing the contribution of these orbits into system
dynamics. We show analytically and numerically a weak noise to reduce this
inhomogeneity and, additionally to obvious perturbing impact, make a
regularizing influence on the chaotic dynamics. This universal effect is rooted
into the nature of deterministic chaos.Comment: 11 pages, 5 figure
Diffusion of a passive scalar by convective flows under parametric disorder
We study transport of a weakly diffusive pollutant (a passive scalar) by
thermoconvective flow in a fluid-saturated horizontal porous layer heated from
below under frozen parametric disorder. In the presence of disorder (random
frozen inhomogeneities of the heating or of macroscopic properties of the
porous matrix), spatially localized flow patterns appear below the convective
instability threshold of the system without disorder. Thermoconvective flows
crucially effect the transport of a pollutant along the layer, especially when
its molecular diffusion is weak. The effective (or eddy) diffusivity also
allows to observe the transition from a set of localized currents to an almost
everywhere intense "global" flow. We present results of numerical calculation
of the effective diffusivity and discuss them in the context of localization of
fluid currents and the transition to a "global" flow. Our numerical findings
are in a good agreement with the analytical theory we develop for the limit of
a small molecular diffusivity and sparse domains of localized currents. Though
the results are obtained for a specific physical system, they are relevant for
a broad variety of fluid dynamical systems.Comment: 12 pages, 4 figures, the revised version of the paper for J. Stat.
Mech. (Special issue for proceedings of 5th Intl. Conf. on Unsolved Problems
on Noise and Fluctuations in Physics, Biology & High Technology, Lyon
(France), June 2-6, 2008
Towards quantitative prediction of proteasomal digestion patterns of proteins
We discuss the problem of proteasomal degradation of proteins. Though
proteasomes are important for all aspects of the cellular metabolism, some
details of the physical mechanism of the process remain unknown. We introduce a
stochastic model of the proteasomal degradation of proteins, which accounts for
the protein translocation and the topology of the positioning of cleavage
centers of a proteasome from first principles. For this model we develop the
mathematical description based on a master-equation and techniques for
reconstruction of the cleavage specificity inherent to proteins and the
proteasomal translocation rates, which are a property of the proteasome specie,
from mass spectroscopy data on digestion patterns. With these properties
determined, one can quantitatively predict digestion patterns for new
experimental set-ups. Additionally we design an experimental set-up for a
synthetic polypeptide with a periodic sequence of amino acids, which enables
especially reliable determination of translocation rates.Comment: 14 pages, 4 figures, submitted to J. Stat. Mech. (Special issue for
proceedings of 5th Intl. Conf. on Unsolved Problems on Noise and Fluctuations
in Physics, Biology & High Technology, Lyon (France), June 2-6, 2008
Interference patterns of multifacet 20x(0-pi-) Josephson junctions with ferromagnetic barrier
We have realized multifacet Josephson junctions with periodically alternating
critical current density (MJJs) using
superconductor-insulator-ferromagnet-superconductor heterostructures. We show
that anomalous features of critical current vs. applied magnetic field,
observed also for other types of MJJs, are caused by a non-uniform flux density
(parallel to the barrier) resulting from screening currents in the electrodes
in the presence of a (parasitic) off-plane field component.Comment: submitted to PR
Visualizing supercurrents in ferromagnetic Josephson junctions with various arrangements of 0 and \pi segments
Josephson junctions with ferromagnetic barrier can have positive or negative
critical current depending on the thickness of the ferromagnetic layer.
Accordingly, the Josephson phase in the ground state is equal to 0 (a
conventional or 0 junction) or to ( junction). When 0 and
segments are joined to form a "0- junction", spontaneous supercurrents
around the 0- boundary can appear. Here we report on the visualization of
supercurrents in superconductor-insulator-ferromagnet-superconductor (SIFS)
junctions by low-temperature scanning electron microscopy (LTSEM). We discuss
data for rectangular 0, , 0-, 0--0 and 20 \times 0-
junctions, disk-shaped junctions where the 0- boundary forms a ring, and
an annular junction with two 0- boundaries. Within each 0 or segment
the critical current density is fairly homogeneous, as indicated both by
measurements of the magnetic field dependence of the critical current and by
LTSEM. The parts have critical current densities up to
35\units{A/cm^2} at T = 4.2\units{K}, which is a record value for SIFS
junctions with a NiCu F-layer so far. We also demonstrate that SIFS technology
is capable to produce Josephson devices with a unique topology of the 0-
boundary.Comment: 29 pages, 8 figure
Simulation of non-stationary processes in industrial centrifugal cascades of uranium enrichment
The mathematical model of non-stationary dividing processes of uranium enrichment in industrial centrifugal cascades which can be used in a computer simulator to prepare experts in dividing production and application as an expert system in the automated control system of technological circuit has been developed and realized
Scaling of Transport Coefficients of Porous Media under Compaction
Porous sediments in geological systems are exposed to stress by the
above-laying mass and consequent compaction, which may be significantly
nonuniform across the massif. We derive scaling laws for the compaction of
sediments of similar geological origin. With these laws, we evaluate the
dependence of the transport properties of a fluid-saturated porous medium
(permeability, effective molecular diffusivity, hydrodynamic dispersion,
electrical and thermal conductivities) on its porosity. In particular, we
demonstrate that the assumption of a uniform geothermal gradient is not
adequate for systems with nonuniform compaction and show the importance of the
derived scaling laws for mathematical modelling of methane hydrate deposits;
these deposits are believed to have potential for impact on global climate
change and Glacial-Interglacial cycles.Comment: 6 pages, 2 figure
A tunable macroscopic quantum system based on two fractional vortices
We propose a tunable macroscopic quantum system based on two fractional
vortices. Our analysis shows that two coupled fractional vortices pinned at two
artificially created \kappa\ discontinuities of the Josephson phase in a long
Josephson junction can reach the quantum regime where coherent quantum
oscillations arise. For this purpose we map the dynamics of this system to that
of a single particle in a double-well potential. By tuning the \kappa\
discontinuities with injector currents we are able to control the parameters of
the effective double-well potential as well as to prepare a desired state of
the fractional vortex molecule. The values of the parameters derived from this
model suggest that an experimental realisation of this tunable macroscopic
quantum system is possible with today's technology.Comment: We updated our manuscript due to a change of the focus from qubit to
macroscopic quantum effect
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