442 research outputs found
Joint statistics of acceleration and vorticity in fully developed turbulence
We report results from a high resolution numerical study of fluid particles
transported by a fully developed turbulent flow. Single particle trajectories
were followed for a time range spanning more than three decades, from less than
a tenth of the Kolmogorov time-scale 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 distinguish two different behaviors between the joint statistics
of vorticity and centripetal acceleration or vorticity and longitudinal
acceleration.Comment: 8 pages, 6 figure
Homogeneous and Isotropic Turbulence: a short survey on recent developments
We present a detailed review of some of the most recent developments on
Eulerian and Lagrangian turbulence in homogeneous and isotropic statistics. In
particular, we review phenomenological and numerical results concerning the
issue of universality with respect to the large scale forcing and the viscous
dissipative physics. We discuss the state-of-the-art of numerical versus
experimental comparisons and we discuss the dicotomy between phenomenology
based on coherent structures or on statistical approaches. A detailed
discussion of finite Reynolds effects is also presented.Comment: based on the talk presented by R. Benzi at DSFD 2-14. postprint
version, published online on 6 July 2015 J. Stat. Phy
Cascades and transitions in turbulent flows
Turbulence is characterized by the non-linear cascades of energy and other
inviscid invariants across a huge range of scales, from where they are injected
to where they are dissipated. Recently, new experimental, numerical and
theoretical works have revealed that many turbulent configurations deviate from
the ideal 3D/2D isotropic cases characterized by the presence of a strictly
direct/inverse energy cascade, respectively. We review recent works from a
unified point of view and we present a classification of all known transfer
mechanisms. Beside the classical cases of direct and inverse cascades, the
different scenarios include: split cascades to small and large scales
simultaneously, multiple/dual cascades of different quantities, bi-directional
cascades where direct and inverse transfers of the same invariant coexist in
the same scale-range and finally equilibrium states where no cascades are
present, including the case when a condensate is formed. We classify all
transitions as the control parameters are changed and we analyse when and why
different configurations are observed. Our discussion is based on a set of
paradigmatic applications: helical turbulence, rotating and/or stratified
flows, MHD and passive/active scalars where the transfer properties are altered
as one changes the embedding dimensions, the thickness of the domain or other
relevant control parameters, as the Reynolds, Rossby, Froude, Peclet, or Alfven
numbers. We discuss the presence of anomalous scaling laws in connection with
the intermittent nature of the energy dissipation in configuration space. An
overview is also provided concerning cascades in other applications such as
bounded flows, quantum, relativistic and compressible turbulence, and active
matter, together with implications for turbulent modelling. Finally, we present
a series of open problems and challenges that future work needs to address.Comment: accepted for publication on Physics Reports 201
Effects of forcing in three dimensional turbulent flows
We present the results of a numerical investigation of three-dimensional
homogeneous and isotropic turbulence, stirred by a random forcing with a power
law spectrum, . Numerical simulations are performed at
different resolutions up to . We show that at varying the spectrum slope
, small-scale turbulent fluctuations change from a {\it forcing independent}
to a {\it forcing dominated} statistics. We argue that the critical value
separating the two behaviours, in three dimensions, is . When the
statistics is forcing dominated, for , we find dimensional scaling, i.e.
intermittency is vanishingly small. On the other hand, for , we find the
same anomalous scaling measured in flows forced only at large scales. We
connect these results with the issue of {\it universality} in turbulent flows.Comment: 4 pages, 4 figure
Helicity Transfer in Turbulent Models
Helicity transfer in a shell model of turbulence is investigated. We show
that a Reynolds-independent helicity flux is present in the model when the
large scale forcing breaks inversion symmetry. The equivalent in Shell Models
of the ``2/15 law'', obtained from helicity conservation in Navier-Stokes eqs.,
is derived and tested. The odd part of helicity flux statistic is found to be
dominated by a few very intense events. In a particular model, we calculate
analytically leading and sub-leading contribution to the scaling of triple
velocity correlation.Comment: 4 pages, LaTex, 2 figure
Analytic calculation of anomalous scaling in random shell models for a passive scalar
An exact non-perturbative calculation of the fourth-order anomalous
correction to the scaling behaviour of a random shell-model for passive scalars
is presented. Importance of ultraviolet (UV) and infrared (IR) boundary
conditions on the inertial scaling properties are determined. We find that
anomalous behaviour is given by the null-space of the inertial operator and we
prove strong UV and IR independence of the anomalous exponent. A limiting case
where diffusive behaviour can influence inertial properties is also presented.Comment: 3 pages, 1 figure, revised versio
Helicity advection in Turbulent Models
Helicity transfer in a shell model of turbulence is investigated. In
particular, we study the scaling behavior of helicity transfer in a dynamical
model of turbulence lacking inversion symmetry. We present some
phenomenological and numerical support to the idea that Helicity becomes -at
scale small enough- a passively-advected quantity.Comment: 6 pages, 2 figures, contribution to the proceedings of the
conference: Disorder and Chaos, in honour of Giovanni Paladin, September
22-24, 1997, Rom
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
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