1,111 research outputs found
On the Heat Transfer in Rayleigh-Benard systems
In this paper we discuss some theoretical aspects concerning the scaling laws
of the Nusselt number versus the Rayleigh number in a Rayleigh-Benard cell. We
present a new set of numerical simulations and compare our findings against the
predictions of existing models. We then propose a new theory which relies on
the hypothesis of Bolgiano scaling. Our approach generalizes the one proposed
by Kadanoff, Libchaber and coworkers and solves some of the inconsistencies
raised in the recent literature.Comment: 10 pages, 5 figure
Extreme events in the dispersions of two neighboring particles under the influence of fluid turbulence
We present a numerical study of two-particle dispersion from point-sources in
3D incompressible Homogeneous and Isotropic turbulence, at Reynolds number Re
\simeq 300. Tracer particles are emitted in bunches from localized sources
smaller than the Kolmogorov scale. We report the first quantitative evidence,
supported by an unprecedented statistics, of the deviations of relative
dispersion from Richardson's picture. Deviations are due to extreme events of
pairs separating much faster than average, and of pairs remaining close for
long times. The two classes of events are the fingerprint of complete different
physics, the former being dominated by inertial subrange and large-scale
fluctuations, while the latter by the dissipation subrange. A comparison of
relative separation in surrogate white-in-time velocity field, with correct
viscous-, inertial- and integral-scale properties allows us to assess the
importance of temporal correlations along tracer trajectories.Comment: 5 pages, 6 figure
How to detect illegal waste shipments? The case of the international trade in polyethylene waste
The purpose of this research is to provide a methodological framework that is able to enhance our capability to detect
illegal waste shipment with particular reference to waste plastics. Based on a very large cross-sectional dataset
covering 187 countries over the period 2002-2012, our study aims to do this by using both the mirror statistics method
and the network analysis. Specifically, by using mirror statistics, we identify the existence of a set of “suspicious”
trade relations between pairs of countries. Then, we employ social network analysis in order to define the position of
each country in this illegal trade structure, and to have a clear exposition of the connections between them. Our main
findings reveal the central positions of the USA, Germany and the UK as sources and China and Malaysia as outlets of
illegal shipments of waste plastics. Moreover, our methodology allows us to highlight the presence of other countries,
which carry out an intermediary role within the global trade network, and to detect the changes in traditional illegal
shipment routes. Therefore, this paper shows how social network analysis provides a useful instrument by means of
which crime analysts and police detectives can develop effective strategies to interdict criminal activities
Universality in passively advected hydrodynamic fields: the case of a passive vector with pressure
Universality of statistical properties of passive quantities advected by
turbulent velocity fields at changing the passive forcing mechanism is
discussed. In particular, we concentrate on the statistical properties of an
hydrodynamic system with pressure. We present theoretical arguments and
preliminary numerical results which show that the fluxes of passive vector
field and of the velocity field have the same scaling behavior. By exploiting
such a property, we propose a way to compute the anomalous exponents of three
dimensional turbulent velocity fields. Our findings are in agreement within 5%
with experimental values of the anomalous exponents.Comment: 15 pages, 6 figure
Universality of anisotropic fluctuations from numerical simulations of turbulent flows
We present new results from a direct numerical simulation of a three
dimensional homogeneous Rayleigh-Benard system (HRB), i.e. a convective cell
with an imposed linear mean temperature profile along the vertical direction.
We measure the SO(3)-decomposition of both velocity structure functions and
buoyancy terms. We give a dimensional prediction for the values of the
anisotropic scaling exponents in this Rayleigh-Benard systems. Measured scaling
does not follow dimensional estimate, while a better agreement can be found
with the anisotropic scaling of a different system, the random-Kolmogorov-flow
(RKF). Our findings support the conclusion that scaling properties of
anisotropic fluctuations are universal, i.e. independent of the forcing
mechanism sustaining the turbulent flow.Comment: 4 pages, 3 figure
Intermittency and scaling laws for wall bounded turbulence
Well defined scaling laws clearly appear in wall bounded turbulence, even
very close to the wall, where a distinct violation of the refined Kolmogorov
similarity hypothesis (RKSH) occurs together with the simultaneous persistence
of scaling laws. A new form of RKSH for the wall region is here proposed in
terms of the structure functions of order two which, in physical terms,
confirms the prevailing role of the momentum transfer towards the wall in the
near wall dynamics.Comment: 10 pages, 5 figure
Mesoscopic modeling of heterogeneous boundary conditions for microchannel flows
We present a mesoscopic model of the fluid-wall interactions for flows in
microchannel geometries. We define a suitable implementation of the boundary
conditions for a discrete version of the Boltzmann equations describing a
wall-bounded single phase fluid. We distinguish different slippage properties
on the surface by introducing a slip function, defining the local degree of
slip for mesoscopic molecules at the boundaries. The slip function plays the
role of a renormalizing factor which incorporates, with some degree of
arbitrariness, the microscopic effects on the mesoscopic description. We
discuss the mesoscopic slip properties in terms of slip length, slip velocity,
pressure drop reduction (drag reduction), and mass flow rate in microchannels
as a function of the degree of slippage and of its spatial distribution and
localization, the latter parameter mimicking the degree of roughness of the
ultra-hydrophobic material in real experiments. We also discuss the increment
of the slip length in the transition regime, i.e. at O(1) Knudsen numbers.
Finally, we compare our results with Molecular Dynamics investigations of the
dependency of the slip length on the mean channel pressure and local slip
properties (Cottin-Bizonne et al. 2004) and with the experimental dependency of
the pressure drop reduction on the percentage of hydrophobic material deposited
on the surface -- Ou et al. (2004).Comment: 21 pages, 10 figure
Surface Roughness-Hydrophobicity Coupling in Microchannel and Nanochannel Flows
An approach based on a lattice version of the Boltzmann kinetic equation for describing multiphase flows in nano- and microcorrugated devices is proposed. We specialize it to describe the wetting-dewetting transition of fluids in the presence of nanoscopic grooves etched on the boundaries. This approach permits us to retain the essential supramolecular details of fluid-solid interactions without surrenderingÂżactually boostingÂżthe computational efficiency of continuum methods. The method is used to analyze the importance of conspiring effects between hydrophobicity and roughness on the global mass flow rate of the microchannel. In particular we show that smart surfaces can be tailored to yield very different mass throughput by changing the bulk pressure. The mesoscopic method is also validated quantitatively against the molecular dynamics results of [Cottin-Bizonne et al., Nat. Mater. 2, 237 (2003)]
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