553 research outputs found
Rayleigh--Taylor turbulence in two dimensions
The first consistent phenomenological theory for two and three dimensional
Rayleigh--Taylor (RT) turbulence has recently been presented by Chertkov [Phys.
Rev. Lett. {\bf 91} 115001 (2003)]. By means of direct numerical simulations we
confirm the spatio/temporal prediction of the theory in two dimensions and
explore the breakdown of the phenomenological description due to intermittency
effects. We show that small-scale statistics of velocity and temperature follow
Bolgiano-Obukhov scaling. At the level of global observables we show that the
time-dependent Nusselt and Reynolds numbers scale as the square root of the
Rayleigh number. These results point to the conclusion that Rayleigh-Taylor
turbulence in two and three dimensions, thanks to the absence of boundaries,
provides a natural physical realization of the Kraichnan scaling regime
hitherto associated with the elusive ``ultimate state of thermal convection''.Comment: 4 pages, 5 figure
Point-source scalar turbulence
The statistics of a passive scalar randomly emitted from a point source is
investigated analytically. Our attention has been focused on the two-point
equal-time scalar correlation function. The latter is indeed easily related to
the spectrum, a statistical indicator widely used both in experiments and in
numerical simulations. The only source of inhomogeneity/anisotropy is in the
injection mechanism, the advecting velocity here being statistically
homogeneous and isotropic. Our main results can be summarized as follows. 1)
For a very large velocity integral scale, a pure scaling behaviour in the
distance between the two points emerges only if their separation is much
smaller than their distance from the point source. 2) The value we have found
for the scaling exponent suggests the existence of a direct cascade, in spite
of the fact that here the forcing integral scale is formally set to zero. 3)
The combined effect of a finite inertial-range extension and of inhomogeneities
causes the emergence of subleading anisotropic corrections to the leading
isotropic term, that we have quantified and discussed.Comment: 10 pages, 1 figure, submitted to Journal of Fluid Mechanic
Scaling and universality in turbulent convection
Anomalous correlation functions of the temperature field in two-dimensional
turbulent convection are shown to be universal with respect to the choice of
external sources. Moreover, they are equal to the anomalous correlations of the
concentration field of a passive tracer advected by the convective flow itself.
The statistics of velocity differences is found to be universal, self-similar
and close to Gaussian. These results point to the conclusion that temperature
intermittency in two-dimensional turbulent convection may be traced back to the
existence of statistically preserved structures, as it is in passive scalar
turbulence.Comment: 4 pages, 6 figure
Industry 4.0 and manufacturing in the city: a possible vertical development
Deindustrialization has moved factories and jobs elsewhere, creating voids, not just space, in Western cities. The definition of the fourth industrial revolution incorporates the tendency of modern manufacturing to produce with innovative methodologies and systems, exploiting the ever-increasing development of ICT technologies and adapting it for factory applications. The production plant changes, both for the conformation of several systems that interact with each other and for a consequent occupation of the spaces. The article analyzes the evolutionary scenario of industrial production and describes the ways in which some activities can develop vertically, creating the conditions for a location in the city
Bioturbation beyond Earth: potential, methods and models of astroichnology
Traces – burrows, borings, footprints – are important evidences of biological behaviour on Earth, yet they received relatively little attention in the field of astrobiology. This study aims to discuss the application of ichnology (i.e. the study of life activity traces) to the search for past and modern life beyond Earth (i.e. herein called Astroichnology)
Coarse-grained description of a passive scalar
The issue of the parameterization of small-scale dynamics is addressed in the
context of passive-scalar turbulence. The basic idea of our strategy is to
identify dynamical equations for the coarse-grained scalar dynamics starting
from closed equations for two-point statistical indicators. With the aim of
performing a fully-analytical study, the Kraichnan advection model is
considered. The white-in-time character of the latter model indeed leads to
closed equations for the equal-time scalar correlation functions. The classical
closure problem however still arises if a standard filtering procedure is
applied to those equations in the spirit of the large-eddy-simulation strategy.
We show both how to perform exact closures and how to identify the
corresponding coarse-grained scalar evolution.Comment: 22 pages; submitted to Journal of Turbulenc
Passive scalar turbulence in high dimensions
Exploiting a Lagrangian strategy we present a numerical study for both
perturbative and nonperturbative regions of the Kraichnan advection model. The
major result is the numerical assessment of the first-order -expansion by
M. Chertkov, G. Falkovich, I. Kolokolov and V. Lebedev ({\it Phys. Rev. E},
{\bf 52}, 4924 (1995)) for the fourth-order scalar structure function in the
limit of high dimensions 's. %Two values of the velocity scaling exponent
have been considered: % and . In the first case, the
perturbative regime %takes place at , while in the second at , %in agreement with the fact that the relevant small parameter %of the
theory is . In addition to the perturbative results, the
behavior of the anomaly for the sixth-order structure functions {\it vs} the
velocity scaling exponent, , is investigated and the resulting behavior
discussed.Comment: 4 pages, Latex, 4 figure
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