1,731 research outputs found
Passive scalar intermittency in compressible flow
A compressible generalization of the Kraichnan model (Phys. Rev. Lett. 72,
1016 (1994)) of passive scalar advection is considered. The dynamical role of
compressibility on the intermittency of the scalar statistics is investigated
for the direct cascade regime. Simple physical arguments suggest that an
enhanced intermittency should appear for increasing compressibility, due to the
slowing down of Lagrangian trajectory separations. This is confirmed by a
numerical study of the dependence of intermittency exponents on the degree of
compressibility, by a Lagrangian method for calculating simultaneous N-point
tracer correlations.Comment: 4 pages, 3 figures Revised version, accepted for publication in PRE -
Rapid communication
Relative dispersion in fully developed turbulence: from Eulerian to Lagrangian statistics in synthetic flows
The effect of Eulerian intermittency on the Lagrangian statistics of relative
dispersion in fully developed turbulence is investigated. A scaling range
spanning many decades is achieved by generating a multi-affine synthetic
velocity field with prescribed intermittency features. The scaling laws for the
Lagrangian statistics are found to depend on Eulerian intermittency in
agreement with a multifractal description. As a consequence of the Kolmogorov's
law, the Richardson's law for the variance of pair separation is not affected
by intermittency corrections.Comment: 4 pages RevTeX, 4 PostScript figure
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
Pair dispersion in synthetic fully developed turbulence
The Lagrangian statistics of relative dispersion in fully developed
turbulence is numerically investigated. A scaling range spanning many decades
is achieved by generating a synthetic velocity field with prescribed Eulerian
statistical features. When the velocity field obeys Kolmogorov similarity, the
Lagrangian statistics is self similar too, and in agreement with Richardson's
predictions. For an intermittent velocity field the scaling laws for the
Lagrangian statistics are found to depend on Eulerian intermittency in
agreement with a multifractal description. As a consequence of the Kolmogorov
law the Richardson law for the variance of pair separation is not affected by
intermittency corrections. A new analysis method, based on fixed scale averages
instead of usual fixed time statistics, is shown to give much wider scaling
range and should be preferred for the analysis of experimental data.Comment: 9 pages, 9 ps figures, submitted to Physics of Fluid
Active vs passive scalar turbulence
Active and passive scalars transported by an incompressible two-dimensional
conductive fluid are investigated. It is shown that a passive scalar displays a
direct cascade towards the small scales while the active magnetic potential
builds up large-scale structures in an inverse cascade process. Correlations
between scalar input and particle trajectories are found to be responsible for
those dramatic differences as well as for the behavior of dissipative
anomalies.Comment: Revised version, Phys. Rev. Lett., in pres
Front propagation in laminar flows
The problem of front propagation in flowing media is addressed for laminar
velocity fields in two dimensions. Three representative cases are discussed:
stationary cellular flow, stationary shear flow, and percolating flow.
Production terms of Fisher-Kolmogorov-Petrovskii-Piskunov type and of Arrhenius
type are considered under the assumption of no feedback of the concentration on
the velocity. Numerical simulations of advection-reaction-diffusion equations
have been performed by an algorithm based on discrete-time maps. The results
show a generic enhancement of the speed of front propagation by the underlying
flow. For small molecular diffusivity, the front speed depends on the
typical flow velocity as a power law with an exponent depending on the
topological properties of the flow, and on the ratio of reactive and advective
time-scales. For open-streamline flows we find always , whereas for
cellular flows we observe for fast advection, and for slow advection.Comment: Enlarged, revised version, 37 pages, 14 figure
Eulerian Statistically Preserved Structures in Passive Scalar Advection
We analyze numerically the time-dependent linear operators that govern the
dynamics of Eulerian correlation functions of a decaying passive scalar
advected by a stationary, forced 2-dimensional Navier-Stokes turbulence. We
show how to naturally discuss the dynamics in terms of effective compact
operators that display Eulerian Statistically Preserved Structures which
determine the anomalous scaling of the correlation functions. In passing we
point out a bonus of the present approach, in providing analytic predictions
for the time-dependent correlation functions in decaying turbulent transport.Comment: 10 pages, 10 figures. Submitted to Phys. Rev.
Mimicking a turbulent signal: sequential multiaffine processes
An efficient method for the construction of a multiaffine process, with
prescribed scaling exponents, is presented. At variance with the previous
proposals, this method is sequential and therefore it is the natural candidate
in numerical computations involving synthetic turbulence. The application to
the realization of a realistic turbulent-like signal is discussed in detail.
The method represents a first step towards the realization of a realistic
spatio-temporal turbulent field.Comment: 4 pages, 3 figures (included), RevTeX 3.
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