7,121 research outputs found
Condensate in quasi two-dimensional turbulence
We investigate the process of formation of large-scale structures in a
turbulent flow confined in a thin layer. By means of direct numerical
simulations of the Navier-Stokes equations, forced at an intermediate scale, we
obtain a split of the energy cascade in which one fraction of the input goes to
small scales generating the three-dimensional direct cascade. The remaining
energy flows to large scales producing the inverse cascade which eventually
causes the formation of a quasi two-dimensional condensed state at the largest
horizontal scale. Our results shows that the connection between the two actors
of the split energy cascade in thin layers is tighter than what was established
before: the small scale three-dimensional turbulence acts as an effective
viscosity and dissipates the large-scale energy thus providing a
viscosity-independent mechanism for arresting the growth of the condensate.
This scenario is supported by quantitative predictions of the saturation energy
in the condensate
Split energy cascade in turbulent thin fluid layers
We discuss the phenomenology of the split energy cascade in a
three-dimensional thin fluid layer by mean of high resolution numerical
simulations of the Navier-Stokes equations. We observe the presence of both an
inverse energy cascade at large scales, as predicted for two-dimensional turbu-
lence, and of a direct energy cascade at small scales, as in three-dimensional
turbulence. The inverse energy cascade is associated with a direct cascade of
enstrophy in the intermediate range of scales. Notably, we find that the
inverse cascade of energy in this system is not a pure 2D phenomenon, as the
coupling with the 3D velocity field is necessary to guarantee the constancy of
fluxes
AGN Obscuration and the Unified Model
Unification Models of Active Galactic Nuclei postulate that all the observed
differences between Type 1 and Type 2 objects are due to orientation effects
with respect to the line-of-sight to the observer. The key ingredient of these
models is the obscuring medium, historically envisaged as a toroidal structure
on a parsec scale. However, many results obtained in the last few years are
clearly showing the need for a more complex geometrical distribution of the
absorbing media. In this paper we review the various pieces of evidence for
obscuring media on different scales, from the vicinity of the black hole to the
host galaxy, in order to picture an updated unification scenario explaining the
complex observed phenomenology. We conclude by mentioning some of the open
issues.Comment: 14 pages, 8 figures, review article accepted for publication on the
special issue of Advances in Astronomy "Seeking for the Leading Actor on the
Cosmic Stage: Galaxies versus Supermassive Black Holes
On positivity of parton distributions
We discuss the bounds on polarized parton distributions which follow from
their definition in terms of cross section asymmetries. We spell out how the
bounds obtained in the naive parton model can be derived within perturbative
QCD at leading order when all quark and gluon distributions are defined in
terms of suitable physical processes. We specify a convenient physical
definition for the polarized and unpolarized gluon distributions in terms of
Higgs production from gluon fusion. We show that these bounds are modified by
subleading corrections, and we determine them up to NLO. We examine the ensuing
phenomenological implications, in particular in view of the determination of
the polarized gluon distribution.Comment: 20 pages, 8 figures included by epsf, plain tex with harvma
An Improved Splitting Function for Small x Evolution
We summarize our recent result for a splitting function for small x evolution
which includes resummed small x logarithms deduced from the leading order BFKL
equation with the inclusion of running coupling effects. We compare this
improved splitting function with alternative approaches.Comment: 5 pages, 2 figures, presented by G.A.at DIS200
Concurrent enhancement of percolation and synchronization in adaptive networks
Co-evolutionary adaptive mechanisms are not only ubiquitous in nature, but
also beneficial for the functioning of a variety of systems. We here consider
an adaptive network of oscillators with a stochastic, fitness-based, rule of
connectivity, and show that it self-organizes from fragmented and incoherent
states to connected and synchronized ones. The synchronization and percolation
are associated to abrupt transitions, and they are concurrently (and
significantly) enhanced as compared to the non-adaptive case. Finally we
provide evidence that only partial adaptation is sufficient to determine these
enhancements. Our study, therefore, indicates that inclusion of simple adaptive
mechanisms can efficiently describe some emergent features of networked
systems' collective behaviors, and suggests also self-organized ways to control
synchronization and percolation in natural and social systems.Comment: Published in Scientific Report
Singlet parton evolution at small x: a theoretical update
This is an extended and pedagogically oriented version of our recent work, in
which we proposed an improvement of the splitting functions at small x which
overcomes the apparent problems encountered by the BFKL approach.Comment: 30 pages, 8 figures, latex with sprocl.sty and epsfi
DebtRank: A microscopic foundation for shock propagation
The DebtRank algorithm has been increasingly investigated as a method to
estimate the impact of shocks in financial networks, as it overcomes the
limitations of the traditional default-cascade approaches. Here we formulate a
dynamical "microscopic" theory of instability for financial networks by
iterating balance sheet identities of individual banks and by assuming a simple
rule for the transfer of shocks from borrowers to lenders. By doing so, we
generalise the DebtRank formulation, both providing an interpretation of the
effective dynamics in terms of basic accounting principles and preventing the
underestimation of losses on certain network topologies. Depending on the
structure of the interbank leverage matrix the dynamics is either stable, in
which case the asymptotic state can be computed analytically, or unstable,
meaning that at least one bank will default. We apply this framework to a
dataset of the top listed European banks in the period 2008 - 2013. We find
that network effects can generate an amplification of exogenous shocks of a
factor ranging between three (in normal periods) and six (during the crisis)
when we stress the system with a 0.5% shock on external (i.e. non-interbank)
assets for all banks.Comment: 10 pages, 2 figure
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