1,721,476 research outputs found
Joint statistics of acceleration and vorticity in fully developed turbulence
No full textWe report results from a high-resolution numerical study of fluid particles transported by a fully developed turbulent flow at R¿ = 280. Single-particle trajectories were followed for a time range spanning more than three decades, from less than a tenth of the Kolmogorov timescale up to one large-eddy turnover time. We present results concerning acceleration statistics and the statistics of trapping by vortex filaments conditioned to the local values of vorticity and enstrophy. We found that a suitable observable, a¿ = a v, connected to the centripetal acceleration, is strongly sensitive to the trapping of particles into vortex filaments. In particular, we found that its sign may persist up to 10-15 t¿, while the typical oscillation of the longitudinal acceleration happens on a time lag of a few t¿
Settling of particles in homogeneous shear turbulence
No full textThe settling of (inertial) particles is studied in homogeneous shear turbulence. A drift velocity perpendicular to gravity is measured due to the interplay between the homogeneous shear turbulence and gravity acting on the particles. We introduce a model to predict and understand this phenomenon
A Lagrangian sub-grid model for the dispersion of clouds of tracers
No full textTurbulence models are expected to satisfy the conflicting requirements of accuracy and computational efficiency. Here we discuss a new model that was recently developed in order to accurately and efficiently describe the dynamic of a clouds of tracers particles in Large Eddy Simulations of homogeneous and isotropic turbulent flows. The models incorporates the multi-scale nature of time and space turbulent velocity correlations that are essential in order to correctly reproduce the relative dispersion of multi-particle clouds. The model can be seen as an off-grid solver for the Eulerian velocity field at the positions of a given number of Lagrangian tracers that self-consistently move with it. Extensions to non homogeneous and isotropic turbulence as well as to the dynamics of particles will be discussed
Preparazione di materiali nanocompositi a base carbonio per applicazioni tecnologiche
Full text linkLa sintesi e le applicazioni dei materiali nanocompositi sono di importanza strategica nel campo della scienza dei materiali. In questo lavoro di tesi sono stati sviluppati diverse tipologie di materiali nanocampositi: Resina epossidica/nanotubi di carbonio (CNT), Poli(3,4-etilendiossitiofene) polistirensulfonato (PEDOT:PSS)/CNT, metallo/CNT. Questi materiali sono stati caratterizzati con le seguenti tecniche: Microscopia elettronica a scansione (SEM), Microscopia a forza atomica (AFM), microscopia acustica a forza atomica (AFAM), spettroscopia Raman e tecniche elettroanalitiche. La caratterizzazione funzionale è stata condotta testando i materiali in differenti applicazioni: Sensori resistivi, sensori di tipo nano-bilancia al quarzo e gestione del calore. I risultati sperimentali sono riportati nel lavoro di tesi.The synthesis and applications of nanocomposites are an important and strategical field of nanomaterial science. In this work has been developed several kind of nanocomposit materials, in particular: Epoxy resins and carbon nanotubes (CNT), Poly(3,4-ethylenedioxythiophene) polystyrenesulfonate (PEDOT:PSS) and CNT, metal and CNT. This material has been characterized by means of: scanning electron microscopy (SEM), Atomic force miscroscpy (AFM), acoustic atomic force microscopy (AFAM), Raman spectroscopy and Electroanalitical techniques. The developed nanocomposite material has been tested by means different kind of application: Resistive sensors, quartz nano-balance and thermal management. The results are reported in the thesis work
On the heat transfer in Rayleigh-Benard systems
No full textIn this paper we discuss some theoretical aspects concerning the scaling laws of the Nusselt number versus the Rayleigh number in a Rayleigh–Bénard 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
Intermittency in turbulence: Multiplicative random process in space and time
No full textWe present a simple stochastic algorithm for generating multiplicative processes with multiscaling both in space and in time. With this algorithm we are able to reproduce a synthetic signal with the same space and time correlation as the one coming from shell models for turbulence and the one coming from a turbulent velocity field in a quasi-Lagrangian reference frame
Hydrodynamical turbulence by fractal fourier decimation
No full textWe present a systematic numerical investigation of high-resolution 3D isotropic and homogeneous turbulence resolved on a decimated set of Fourier modes. Fractal decimation acts to decrease the effective dimensionality of the flow by allowing triadic interactions only in a set of Fourier modes N(k) proportional to k^DF for large k. While keeping the symmetries of the original 3D Navier-Stokes equations unchanged, a dramatic change in small-scale statistics is detected at decreasing the fractal dimension DF . Already at fractal dimension DF = 2.8, a global self-similar behaviour is observed in the inertial range of scales, the consequence of such transition are the restoration of the scaling symmetry and vorticity distribution that becomes close to Gaussian. We relate the results to the different roles of local vs non-local interactions in the energy transfer range
Effects of thermal fluctuations in the fragmentation of a nanoligament
Full text linkWe study the effects of thermally induced capillary waves in the fragmentation of a liquid ligament into multiple nanodroplets. Our numerical implementation is based on a fluctuating lattice Boltzmann (LB) model for nonideal multicomponent fluids, including nonequilibrium stochastic fluxes mimicking the effects of molecular forces at the nanoscales. We quantitatively analyze the statistical distribution of the breakup times and the droplet volumes after the fragmentation process at changing the two relevant length scales of the problem, i.e., the thermal length scale and the ligament size. The robustness of the observed findings is also corroborated by quantitative comparisons with the predictions of sharp interface hydrodynamics. Beyond the practical importance of our findings for nanofluidic engineering devices, our study also explores a novel application of LB in the realm of nanofluidic phenomena
Extreme Events in the Dispersions of Two Neighboring Particles Under the Influence of Fluid Turbulence
Full text linkWe present a numerical study of two-particle dispersion from point sources in three-dimensional incompressible homogeneous and isotropic turbulence at Reynolds number Re¿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 time. The two classes of events are the fingerprints of complete different physics, the former dominated by inertial subrange and large-scale fluctuations, and the latter by dissipation subrange. A comparison of the 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
A Lattice Boltzmann method for turbulent emulsions
No full textThe breakup of droplets in a turbulent flow is key to many natural and industrial applications. Here we present and validate a computationally efficient numerical method that allows to study turbulent emulsion for very long times. The numerical method is based on a multi-component Lattice Boltzmann method based on the Shan-Chen model and supplemented with a large scale force to stir turbulence. A special treatment to limit mobility between different fluid components is introduced and validated. We demonstrate the potential of our approach in sustaining a turbulent emulsion over extremely long integration times (necessary to collect firm turbulence statistics) and we present first results on the probability distribution function of droplets' accelerations
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