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 surrendering¿actually boosting¿the 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)]