281 research outputs found
Multi-scale model of gradient evolution in turbulent flows
A multi-scale model for the evolution of the velocity gradient tensor in
fully developed turbulence is proposed. The model is based on a coupling
between a ``Restricted Euler'' dynamics [{\it P. Vieillefosse, Physica A, {\bf
14}, 150 (1984)}] which describes gradient self-stretching, and a deterministic
cascade model which allows for energy exchange between different scales. We
show that inclusion of the cascade process is sufficient to regularize the
well-known finite time singularity of the Restricted Euler dynamics. At the
same time, the model retains topological and geometrical features of real
turbulent flows: these include the alignment between vorticity and the
intermediate eigenvector of the strain-rate tensor and the typical teardrop
shape of the joint probability density between the two invariants, , of
the gradient tensor. The model also possesses skewed, non-Gaussian longitudinal
gradient fluctuations and the correct scaling of energy dissipation as a
function of Reynolds number. Derivative flatness coefficients are in good
agreement with experimental data.Comment: 4 pages, 4 figure
An accurate and efficient Lagrangian sub-grid model
A computationally efficient model is introduced to account for the sub-grid
scale velocities of tracer particles dispersed in statistically homogeneous and
isotropic turbulent flows. The model embeds the multi-scale nature of turbulent
temporal and spatial correlations, that are essential to reproduce
multi-particle dispersion. It is capable to describe the Lagrangian diffusion
and dispersion of temporally and spatially correlated clouds of particles.
Although the model neglects intermittent corrections, we show that pair and
tetrad dispersion results nicely compare with Direct Numerical Simulations of
statistically isotropic and homogeneous turbulence. This is in agreement
with recent observations that deviations from self-similar pair dispersion
statistics are rare events
Droplet size distribution in homogeneous isotropic turbulence
We study the physics of droplet breakup in a statistically stationary
homogeneous and isotropic turbulent flow by means of high resolution numerical
investigations based on the multicomponent lattice Boltzmann method. We
verified the validity of the criterion proposed by Hinze (1955) for droplet
breakup and we measured the full probability distribution function (pdf) of
droplets radii at different Reynolds numbers and for different volume fraction.
By means of a Lagrangian tracking we could follow individual droplets along
their trajectories, define a local Weber number based on the velocity gradients
and study its cross-correlation with droplet deformation.Comment: 10 pages, 6 figure
On the Anomalous Scaling Exponents in Nonlinear Models of Turbulence
We propose a new approach to the old-standing problem of the anomaly of the
scaling exponents of nonlinear models of turbulence. We achieve this by
constructing, for any given nonlinear model, a linear model of passive
advection of an auxiliary field whose anomalous scaling exponents are the same
as the scaling exponents of the nonlinear problem. The statistics of the
auxiliary linear model are dominated by `Statistically Preserved Structures'
which are associated with exact conservation laws. The latter can be used for
example to determine the value of the anomalous scaling exponent of the second
order structure function. The approach is equally applicable to shell models
and to the Navier-Stokes equations.Comment: revised version with new data on Navier-Stokes eq
Velocity gradients statistics along particle trajectories in turbulent flows: the refined similarity hypothesis in the Lagrangian frame
We present an investigation of the statistics of velocity gradient related
quantities, in particluar energy dissipation rate and enstrophy, along the
trajectories of fluid tracers and of heavy/light particles advected by a
homogeneous and isotropic turbulent flow. The Refined Similarity Hypothesis
(RSH) proposed by Kolmogorov and Oboukhov in 1962 is rephrased in the
Lagrangian context and then tested along the particle trajectories. The study
is performed on state-of-the-art numerical data resulting from numerical
simulations up to Re~400 with 2048^3 collocation points. When particles have
small inertia, we show that the Lagrangian formulation of the RSH is well
verified for time lags larger than the typical response time of the particle.
In contrast, in the large inertia limit when the particle response time
approaches the integral-time-scale of the flow, particles behave nearly
ballistic, and the Eulerian formulation of RSH holds in the inertial-range.Comment: 7 pages, 7 figures; Physical Review E (accepted Dec 7, 2009
Intermittency and Universality in Fully-Developed Inviscid and Weakly-Compressible Turbulent Flows
We performed high resolution numerical simulations of homogenous and
isotropic compressible turbulence, with an average 3D Mach number close to 0.3.
We study the statistical properties of intermittency for velocity, density and
entropy. For the velocity field, which is the primary quantity that can be
compared to the isotropic incompressible case, we find no statistical
differences in its behavior in the inertial range due either to the slight
compressibility or to the different dissipative mechanism. For the density
field, we find evidence of ``front-like'' structures, although no shocks are
produced by the simulation.Comment: Submitted to Phys. Rev. Let
Inverse energy cascade in three-dimensional isotropic turbulence
In turbulent flows kinetic energy is spread by nonlinear interactions over a
broad range of scales. Energy transfer may proceed either toward small scales
or in the reverse direction. The latter case is peculiar of two-dimensional
(2D) flows. Interestingly, a reversal of the energy flux is observed also in
three-dimensional (3D) geophysical flows under rotation and/or confined in thin
layers. The question is whether this phenomenon is enforced solely by external
anisotropic mechanisms or it is intimately embedded in the Navier-Stokes (NS)
equations. Here we show that an inverse energy cascade occurs also in 3D
isotropic flow. The flow is obtained from a suitable surgery of the NS
equations, keeping only triadic interactions between sign-defined helical
modes, preserving homogeneity and isotropy and breaking reflection invariance.
Our findings highlight the role played by helicity in the energy transfer
process and show that both 2D and 3D properties naturally coexist in all flows
in nature.Comment: 4 pages, 4 figure
Population dynamics at high Reynolds number
We study the statistical properties of population dynamics evolving in a
realistic two-dimensional compressible turbulent velocity field. We show that
the interplay between turbulent dynamics and population growth and saturation
leads to quasi-localization and a remarkable reduction in the carrying
capacity. The statistical properties of the population density are investigated
and quantified via multifractal scaling analysis. We also investigate
numerically the singular limit of negligibly small growth rates and
delocalization of population ridges triggered by uniform advection.Comment: 5 pages, 5 figure
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
Numerical simulations of aggregate breakup in bounded and unbounded turbulent flows
Breakup of small aggregates in fully developed turbulence is studied by means
of direct numerical simulations in a series of typical bounded and unbounded
flow configurations, such as a turbulent channel flow, a developing boundary
layer and homogeneous isotropic turbulence. The simplest criterion for breakup
is adopted, whereas aggregate breakup occurs when the local hydrodynamic stress
, with being the energy dissipation
at the position of the aggregate, overcomes a given threshold
, which is characteristic for a given type of aggregates.
Results show that the breakup rate decreases with increasing threshold. For
small thresholds, it develops a universal scaling among the different flows.
For high thresholds, the breakup rates show strong differences between the
different flow configurations, highlighting the importance of non-universal
mean-flow properties. To further assess the effects of flow inhomogeneity and
turbulent fluctuations, theresults are compared with those obtained in a smooth
stochastic flow. Furthermore, we discuss the limitations and applicability of a
set of independent proxies.Comment: 15 pages, 12 figures, Refinded discussion in Section 2.1, results
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