1,138 research outputs found
On the intermittent energy transfer at viscous scales in turbulent flows
In this letter we present numerical and experimental results on the scaling
properties of velocity turbulent fields in the range of scales where viscous
effects are acting. A generalized version of Extended Self Similarity capable
of describing scaling laws of the velocity structure functions down to the
smallest resolvable scales is introduced. Our findings suggest the absence of
any sharp viscous cutoff in the intermittent transfer of energy.Comment: 10 pages, plain Latex, 6 figures available upon request to
[email protected]
Stochastic Resonance in Two Dimensional Landau Ginzburg Equation
We study the mechanism of stochastic resonance in a two dimensional Landau
Ginzburg equation perturbed by a white noise. We shortly review how to
renormalize the equation in order to avoid ultraviolet divergences. Next we
show that the renormalization amplifies the effect of the small periodic
perturbation in the system. We finally argue that stochastic resonance can be
used to highlight the effect of renormalization in spatially extended system
with a bistable equilibria
Intermittency in Turbulence: computing the scaling exponents in shell models
We discuss a stochastic closure for the equation of motion satisfied by
multi-scale correlation functions in the framework of shell models of
turbulence. We give a systematic procedure to calculate the anomalous scaling
exponents of structure functions by using the exact constraints imposed by the
equation of motion. We present an explicit calculation for fifth order scaling
exponent at varying the free parameter entering in the non-linear term of the
model. The same method applied to the case of shell models for Kraichnan
passive scalar provides a connection between the concept of zero-modes and
time-dependent cascade processes.Comment: 12 pages, 5 eps 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
Intermittency and the Slow Approach to Kolmogorov Scaling
From a simple path integral involving a variable volatility in the velocity
differences, we obtain velocity probability density functions with exponential
tails, resembling those observed in fully developed turbulence. The model
yields realistic scaling exponents and structure functions satisfying extended
self-similarity. But there is an additional small scale dependence for
quantities in the inertial range, which is linked to a slow approach to
Kolmogorov (1941) scaling occurring in the large distance limit.Comment: 10 pages, 5 figures, minor changes to mirror version to appear in PR
Double scaling and intermittency in shear dominated flows
The Refined Kolmogorov Similarity Hypothesis is a valuable tool for the
description of intermittency in isotropic conditions. For flows in presence of
a substantial mean shear, the nature of intermittency changes since the process
of energy transfer is affected by the turbulent kinetic energy production
associated with the Reynolds stresses. In these conditions a new form of
refined similarity law has been found able to describe the increased level of
intermittency which characterizes shear dominated flows. Ideally a length scale
associated with the mean shear separates the two ranges, i.e. the classical
Kolmogorov-like inertial range, below, and the shear dominated range, above.
However, the data analyzed in previous papers correspond to conditions where
the two scaling regimes can only be observed individually.
In the present letter we give evidence of the coexistence of the two regimes
and support the conjecture that the statistical properties of the dissipation
field are practically insensible to the mean shear. This allows for a
theoretical prediction of the scaling exponents of structure functions in the
shear dominated range based on the known intermittency corrections for
isotropic flows. The prediction is found to closely match the available
numerical and experimental data.Comment: 7 pages, 3 figures, submitted to PR
Universal statistics of non-linear energy transfer in turbulent models
A class of shell models for turbulent energy transfer at varying the
inter-shell separation, , is investigated. Intermittent corrections in
the continuous limit of infinitely close shells () have
been measured. Although the model becomes, in this limit, non-intermittent, we
found universal aspects of the velocity statistics which can be interpreted in
the framework of log-poisson distributions, as proposed by She and Waymire
(1995, Phys. Rev. Lett. 74, 262). We suggest that non-universal aspects of
intermittency can be adsorbed in the parameters describing statistics and
properties of the most singular structure. On the other hand, universal aspects
can be found by looking at corrections to the monofractal scaling of the most
singular structure. Connections with similar results reported in other shell
models investigations and in real turbulent flows are discussed.Comment: 4 pages, 2 figures available upon request to [email protected]
A new scaling property of turbulent flows
We discuss a possible theoretical interpretation of the self scaling property
of turbulent flows (Extended Self Similarity). Our interpretation predicts
that, even in cases when ESS is not observed, a generalized self scaling, must
be observed. This prediction is checked on a number of laboratory experiments
and direct numerical simulations.Comment: Plain Latex, 1 figure available upon request to
[email protected]
Mean- Field Approximation and Extended Self-Similarity in Turbulence
Recent experimental discovery of extended self-similarity (ESS) was one of
the most interesting developments, enabling precise determination of the
scaling exponents of fully developed turbulence. Here we show that the ESS is
consistent with the Navier-Stokes equations, provided the pressure -gradient
contributions are expressed in terms of velocity differences in the mean field
approximation (Yakhot, Phys.Rev. E{\bf 63}, 026307, (2001)). A sufficient
condition for extended self-similarity in a general dynamical systemComment: 8 pages, no figure
Lagrangian Statistics of Navier-Stokes- and MHD-Turbulence
We report on a comparison of high-resolution numerical simulations of
Lagrangian particles advected by incompressible turbulent hydro- and
magnetohydrodynamic (MHD) flows. Numerical simulations were performed with up
to collocation points and 10 million particles in the Navier-Stokes
case and collocation points and 1 million particles in the MHD case. In
the hydrodynamics case our findings compare with recent experiments from
Mordant et al. [1] and Xu et al. [2]. They differ from the simulations of
Biferale et al. [3] due to differences of the ranges choosen for evaluating the
structure functions. In Navier-Stokes turbulence intermittency is stronger than
predicted by a multifractal approach of [3] whereas in MHD turbulence the
predictions from the multifractal approach are more intermittent than observed
in our simulations. In addition, our simulations reveal that Lagrangian
Navier-Stokes turbulence is more intermittent than MHD turbulence, whereas the
situation is reversed in the Eulerian case. Those findings can not consistently
be described by the multifractal modeling. The crucial point is that the
geometry of the dissipative structures have different implications for
Lagrangian and Eulerian intermittency. Application of the multifractal approach
for the modeling of the acceleration PDFs works well for the Navier-Stokes case
but in the MHD case just the tails are well described.Comment: to appear in J. Plasma Phy
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