2,554 research outputs found
Inverse cascade in Charney-Hasegawa-Mima turbulence
The inverse energy cascade in Charney-Hasegawa-Mima turbulence is
investigated. Kolmogorov law for the third order velocity structure function is
shown to be independent on the Rossby number, at variance with the energy
spectrum, as shown by high resolution direct numerical simulations. In the
asymptotic limit of strong rotation, coherent vortices are observed to form at
a dynamical scale which slowly grows with time. These vortices form an almost
quenched pattern and induce strong deviation form Gaussianity in the velocity
field.Comment: 4 pages, 5 figure
Inertial floaters in stratified turbulence
We investigate numerically the dynamics and statistics of inertial particles
transported by stratified turbulence, in the case of particle density
intermediate in the average density profile of the fluid. In these conditions,
particles tend to form a thin layer around the corresponding fluid isopycnal.
The thickness of the resulting layer is determined by a balance between
buoyancy (which attracts the particle to the isopycnal) and inertia (which
prevents them from following it exactly). By means of extensive numerical
simulations, we explore the parameter space of the system and we find that in a
range of parameters particles form fractal cluster within the layer.Comment: 6 pages, 6 figure
Weak versus strong wave turbulence in the MMT model
Within the spirit of fluid turbulence, we consider the one-dimensional
Majda-McLaughlin-Tabak (MMT) model that describes the interactions of nonlinear
dispersive waves. We perform a detailed numerical study of the direct energy
cascade in the defocusing regime. In particular, we consider a configuration
with large-scale forcing and small scale dissipation, and we introduce three
non- dimensional parameters: the ratio between nonlinearity and dispersion,
{\epsilon}, and the analogues of the Reynolds number, Re, i.e. the ratio
between the nonlinear and dissipative time-scales, both at large and small
scales. Our numerical experiments show that (i) in the limit of small
{\epsilon} the spectral slope observed in the statistical steady regime
corresponds to the one predicted by the Weak Wave Turbulence (WWT) theory. (ii)
As the nonlinearity is increased, the WWT theory breaks down and deviations
from its predictions are observed. (iii) It is shown that such departures from
the WWT theoretical predictions are accompanied by the phenomenon of
intermittency, typical of three dimensional fluid turbulence. We calculate the
structure-function as well as the probability density function of the wave
field at each scale and show that the degree of intermittency depends on
{\epsilon}.Comment: 7 pages, 6 figure
Peripheral mixing of passive scalar at small Reynolds number
Mixing of a passive scalar in the peripheral region close to a wall is
investigated by means of accurate direct numerical simulations of both a
three-dimensional Couette channel flow at low Reynolds numbers and a
two-dimensional synthetic flow. In both cases, the resulting phenomenology can
be understood in terms of the theory recently developed by Lebedev and Turitsyn
[Phys. Rev. E 69, 036301, 2004]. Our results prove the robustness of the
identified mechanisms responsible for the persistency of scalar concentration
close to the wall with important consequences in completely different fields
ranging from microfluidic applications to environmental dispersion modeling.Comment: 4 pages, 5 figure
Eriophyoid mites (Acari: Prostigmata: Eriophyoidea) from the Maltese Islands (Central Mediterranean): new reports and a preliminary check-list
In the 20th Century, about 20 species of eriophyoids were listed for the Maltese Islands but these reports
were entirely based on plant gall surveys. In these last four years, plant collections were made in order to
investigate the species of eriophyoids present in Malta and Gozo. Most of the earlier reports have been
confirmed and eight species were found to be new for the Maltese islands: Aceria caulobia (Nalepa) gall-making
on Suaeda vera Gmelin; Aceria onychia (Nalepa) on Phlomis fruticosa L.; Aceria sheldoni (Ewing) on lemon; Aculus
tetanothrix (Nalepa) gall-making on Salix sp.; Cecidophyopsis hendersoni (Keifer) on yucca; and three species
associated with olive, Ditrymacus athiasella Keifer, Oxycenus maxwelli (Keifer) and Tegolophus hassani ( Keifer).
Additional remarks were included for Acalitus phloeocoptes (Nalepa), collected on cherry plum, whose earlier
Maltese record was doubtful. A complete morphometric description of Aceria carlinae (Nalepa) is here
provided.peer-reviewe
Irreversibility-inversions in 2 dimensional turbulence
In this paper we consider a recent theoretical prediction (Bragg \emph{et
al.}, Phys. Fluids \textbf{28}, 013305 (2016)) that for inertial particles in
2D turbulence, the nature of the irreversibility of the particle-pair
dispersion inverts when the particle inertia exceeds a certain value. In
particular, when the particle Stokes number, , is below a certain
value, the forward-in-time (FIT) dispersion should be faster than the
backward-in-time (BIT) dispersion, but for above this value, this
should invert so that BIT becomes faster than FIT dispersion. This non-trivial
behavior arises because of the competition between two physically distinct
irreversibility mechanisms that operate in different regimes of . In
3D turbulence, both mechanisms act to produce faster BIT than FIT dispersion,
but in 2D turbulence, the two mechanisms have opposite effects because of the
flux of energy from the small to the large scales. We supplement the
qualitative argument given by Bragg \emph{et al.} (Phys. Fluids \textbf{28},
013305 (2016)) by deriving quantitative predictions of this effect in the short
time limit. We confirm the theoretical predictions using results of inertial
particle dispersion in a direct numerical simulation of 2D turbulence. A more
general finding of this analysis is that in turbulent flows with an inverse
energy flux, inertial particles may yet exhibit a net downscale flux of kinetic
energy because of their non-local in-time dynamics
Geotropic tracers in turbulent flows: a proxy for fluid acceleration
We investigate the statistics of orientation of small, neutrally buoyant,
spherical tracers whose center of mass is displaced from the geometrical
center. If appropriate-sized particles are considered, a linear relation can be
derived between the horizontal components of the orientation vector and the
same components of acceleration. Direct numerical simulations are carried out,
showing that such relation can be used to reconstruct the statistics of
acceleration fluctuations up to the order of the gravitational acceleration.
Based on such results, we suggest a novel method for the local experimental
measurement of accelerations in turbulent flows.Comment: 14 pages, 6 figure
A statistical conservation law in two and three dimensional turbulent flows
Particles in turbulence live complicated lives. It is nonetheless sometimes
possible to find order in this complexity. It was proposed in [Falkovich et
al., Phys. Rev. Lett. 110, 214502 (2013)] that pairs of Lagrangian tracers at
small scales, in an incompressible isotropic turbulent flow, have a statistical
conservation law. More specifically, in a d-dimensional flow the distance
between two neutrally buoyant particles, raised to the power and
averaged over velocity realizations, remains at all times equal to the initial,
fixed, separation raised to the same power. In this work we present evidence
from direct numerical simulations of two and three dimensional turbulence for
this conservation. In both cases the conservation is lost when particles exit
the linear flow regime. In 2D we show that, as an extension of the conservation
law, a Evans-Cohen-Morriss/Gallavotti-Cohen type fluctuation relation exists.
We also analyse data from a 3D laboratory experiment [Liberzon et al., Physica
D 241, 208 (2012)], finding that although it probes small scales they are not
in the smooth regime. Thus instead of \left, we look for a
similar, power-law-in-separation conservation law. We show that the existence
of an initially slowly varying function of this form can be predicted but that
it does not turn into a conservation law. We suggest that the conservation of
\left, demonstrated here, can be used as a check of isotropy,
incompressibility and flow dimensionality in numerical and laboratory
experiments that focus on small scales
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