1,307 research outputs found
Different transport regimes in a spatially-extended recirculating background
Passive scalar transport in a spatially-extended background of roll
convection is considered in the time-periodic regime. The latter arises due to
the even oscillatory instability of the cell lateral boundary, here accounted
for by sinusoidal oscillations of frequency . By varying the latter
parameter, the strength of anticorrelated regions of the velocity field can be
controled and the conditions under which either an enhancement or a reduction
of transport takes place can be created. Such two ubiquitous regimes are
triggered by a small-scale(random) velocity field superimposed to the
recirculating background. The crucial point is played by the dependence of
Lagrangian trajectories on the statistical properties of the small-scale
velocity field, e.g. its correlation time or its energy.Comment: 9 pages Latex; 5 figure
Interference phenomena in scalar transport induced by a noise finite correlation time
The role played on the scalar transport by a finite, not small, correlation
time, , for the noise velocity is investigated, both analytically and
numerically. For small 's a mechanism leading to enhancement of
transport has recently been identified and shown to be dominating for any type
of flow. For finite non-vanishing 's we recognize the existence of a
further mechanism associated with regions of anticorrelation of the Lagrangian
advecting velocity. Depending on the extension of the anticorrelated regions,
either an enhancement (corresponding to constructive interference) or a
depletion (corresponding to destructive interference) in the turbulent
transport now takes place.Comment: 8 pages, 3 figure
Transport in finite size systems: an exit time approach
In the framework of chaotic scattering we analyze passive tracer transport in
finite systems. In particular, we study models with open streamlines and a
finite number of recirculation zones. In the non trivial case with a small
number of recirculation zones a description by mean of asymptotic quantities
(such as the eddy diffusivity) is not appropriate. The non asymptotic
properties of dispersion are characterized by means of the exit time
statistics, which shows strong sensitivity on initial conditions. This yields a
probability distribution function with long tails, making impossible a
characterization in terms of a unique typical exit time.Comment: 16 RevTeX pages + 6 eps-figures include
Simple stochastic models showing strong anomalous diffusion
We show that {\it strong} anomalous diffusion, i.e. \mean{|x(t)|^q} \sim
t^{q \nu(q)} where is a nonlinear function of , is a generic
phenomenon within a class of generalized continuous-time random walks. For such
class of systems it is possible to compute analytically nu(2n) where n is an
integer number. The presence of strong anomalous diffusion implies that the
data collapse of the probability density function P(x,t)=t^{-nu}F(x/t^nu)
cannot hold, a part (sometimes) in the limit of very small x/t^\nu, now
nu=lim_{q to 0} nu(q). Moreover the comparison with previous numerical results
shows that the shape of F(x/t^nu) is not universal, i.e., one can have systems
with the same nu but different F.Comment: Final versio
Partner selection in indoor-to-outdoor cooperative networks: an experimental study
In this paper, we develop a partner selection protocol for enhancing the
network lifetime in cooperative wireless networks. The case-study is the
cooperative relayed transmission from fixed indoor nodes to a common outdoor
access point. A stochastic bivariate model for the spatial distribution of the
fading parameters that govern the link performance, namely the Rician K-factor
and the path-loss, is proposed and validated by means of real channel
measurements. The partner selection protocol is based on the real-time
estimation of a function of these fading parameters, i.e., the coding gain. To
reduce the complexity of the link quality assessment, a Bayesian approach is
proposed that uses the site-specific bivariate model as a-priori information
for the coding gain estimation. This link quality estimator allows network
lifetime gains almost as if all K-factor values were known. Furthermore, it
suits IEEE 802.15.4 compliant networks as it efficiently exploits the
information acquired from the receiver signal strength indicator. Extensive
numerical results highlight the trade-off between complexity, robustness to
model mismatches and network lifetime performance. We show for instance that
infrequent updates of the site-specific model through K-factor estimation over
a subset of links are sufficient to at least double the network lifetime with
respect to existing algorithms based on path loss information only.Comment: This work has been submitted to IEEE Journal on Selected Areas in
Communications in August 201
Flow networks: A characterization of geophysical fluid transport
We represent transport between different regions of a fluid domain by flow
networks, constructed from the discrete representation of the Perron-Frobenius
or transfer operator associated to the fluid advection dynamics. The procedure
is useful to analyze fluid dynamics in geophysical contexts, as illustrated by
the construction of a flow network associated to the surface circulation in the
Mediterranean sea. We use network-theory tools to analyze the flow network and
gain insights into transport processes. In particular we quantitatively relate
dispersion and mixing characteristics, classically quantified by Lyapunov
exponents, to the degree of the network nodes. A family of network entropies is
defined from the network adjacency matrix, and related to the statistics of
stretching in the fluid, in particular to the Lyapunov exponent field. Finally
we use a network community detection algorithm, Infomap, to partition the
Mediterranean network into coherent regions, i.e. areas internally well mixed,
but with little fluid interchange between them.Comment: 16 pages, 15 figures. v2: published versio
Spectral properties of quantum -body systems versus chaotic properties of their mean field approximations
We present numerical evidence that in a system of interacting bosons there
exists a correspondence between the spectral properties of the exact quantum
Hamiltonian and the dynamical chaos of the associated mean field evolution.
This correspondence, analogous to the usual quantum-classical correspondence,
is related to the formal parallel between the second quantization of the mean
field, which generates the exact dynamics of the quantum -body system, and
the first quantization of classical canonical coordinates. The limit of
infinite density and the thermodynamic limit are then briefly discussed.Comment: 15 pages RevTeX, 11 postscript figures included with psfig, uuencoded
gz-compressed .tar fil
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