496 research outputs found
Intermittency in the relative separations of tracers and of heavy particles in turbulent flows
Results from Direct Numerical Simulations of particle relative dispersion in
three dimensional homogeneous and isotropic turbulence at Reynolds number
are presented. We study point-like passive tracers and
heavy particles, at Stokes number St = 0, 0.6, 1 and 5. Particles are emitted
from localised sources, in bunches of thousands, periodically in time, allowing
to reach an unprecedented statistical accuracy, with a total number of events
for two-point observables of the order of . The right tail of the
probability density function for tracers develops a clear deviation from
Richardson's self-similar prediction, pointing to the intermittent nature of
the dispersion process. In our numerical experiment, such deviations are
manifest once the probability to measure an event becomes of the order of -or
rarer than- one part over one million, hence the crucial importance of a large
dataset. The role of finite-Reynolds effects and the related fluctuations when
pair separations cross the boundary between viscous and inertial range scales
are discussed. An asymptotic prediction based on the multifractal theory for
inertial range intermittency and valid for large Reynolds numbers is found to
agree with the data better than the Richardson theory. The agreement is
improved when considering heavy particles, whose inertia filters out viscous
scale fluctuations. By using the exit-time statistics we also show that events
associated to pairs experiencing unusually slow inertial range separations have
a non self-similar probability distribution function.Comment: 22 pages, 14 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
Effects of vertical shear in modelling horizontal oceanic dispersion
Abstract. The effect of vertical shear on the horizontal dispersion properties of passive tracer particles on the continental shelf of the South Mediterranean is investigated by means of observation and model data. In situ current measurements reveal that vertical gradients of horizontal velocities in the upper mixing layer decorrelate quite fast ( ∼  1 day), whereas an eddy-permitting ocean model, such as the Mediterranean Forecasting System, tends to overestimate such decorrelation time because of finite resolution effects. Horizontal dispersion, simulated by the Mediterranean sea Forecasting System, is mostly affected by: (1) unresolved scale motions, and mesoscale motions that are largely smoothed out at scales close to the grid spacing; (2) poorly resolved time variability in the profiles of the horizontal velocities in the upper layer. For the case study we have analysed, we show that a suitable use of deterministic kinematic parametrizations is helpful to implement realistic statistical features of tracer dispersion in two and three dimensions. The approach here suggested provides a functional tool to control the horizontal spreading of small organisms or substance concentrations, and is thus relevant for marine biology, pollutant dispersion as well as oil spill applications
Statistics of pressure and of pressure-velocity correlations in isotropic turbulence
Some pressure and pressure-velocity correlation in a direct numerical
simulations of a three-dimensional turbulent flow at moderate Reynolds numbers
have been analyzed. We have identified a set of pressure-velocity correlations
which posseses a good scaling behaviour. Such a class of pressure-velocity
correlations are determined by looking at the energy-balance across any
sub-volume of the flow. According to our analysis, pressure scaling is
determined by the dimensional assumption that pressure behaves as a ``velocity
squared'', unless finite-Reynolds effects are overwhelming. The SO(3)
decompositions of pressure structure functions has also been applied in order
to investigate anisotropic effects on the pressure scaling.Comment: 21 pages, 8 figur
cAMP inhibits translation by inducing Ca2+/calmodulin-independent elongation factor 2 kinase activity in IPC-81 cells
AbstractTreatment of IPC-81 cells led to inhibition of protein synthesis, which was accompanied by an increase in the average size of polysomes and a decreased rate of elongation, indicating that it involved inhibition of peptide chain elongation. This inhibition was also associated with increased phosphorylation of elongation factor eEF2 (which inhibits its activity) and enhanced Ca2+/calmodulin-independent activity of eEF2 kinase. Previous work has shown that phosphorylation of eEF2 kinase by cAMP-dependent protein kinase (cAPK) in vitro induces such activator-independent activity, and the present data show that such a mechanism can occur in intact cells to link physiological levels of cAPK activation with inhibition of protein synthesis
Universality and saturation of intermittency in passive scalar turbulence
The statistical properties of a scalar field advected by the non-intermittent
Navier-Stokes flow arising from a two-dimensional inverse energy cascade are
investigated. The universality properties of the scalar field are directly
probed by comparing the results obtained with two different types of injection
mechanisms. Scaling properties are shown to be universal, even though
anisotropies injected at large scales persist down to the smallest scales and
local isotropy is not fully restored. Scalar statistics is strongly
intermittent and scaling exponents saturate to a constant for sufficiently high
orders. This is observed also for the advection by a velocity field rapidly
changing in time, pointing to the genericity of the phenomenon. The persistence
of anisotropies and the saturation are both statistical signatures of the
ramp-and-cliff structures observed in the scalar field.Comment: 4 pages, 8 figure
VLBI-derived troposphere parameters during CONT08
Time-series of zenith wet and total troposphere delays as well as north and east gradients are compared, and zenith total delays (ZTD) are combined on the level of parameter estimates. Input data sets are provided by ten Analysis Centers (ACs) of the International VLBI Service for Geodesy and Astrometry (IVS) for the CONT08 campaign (12-26 August 2008). The inconsistent usage of meteorological data and models, such as mapping functions, causes systematics among the ACs, and differing parameterizations and constraints add noise to the troposphere parameter estimates. The empirical standard deviation of ZTD among the ACs with regard to an unweighted mean is 4.6 mm. The ratio of the analysis noise to the observation noise assessed by the operator/software impact (OSI) model is about 2.5. These and other effects have to be accounted for to improve the intra-technique combination of VLBI-derived troposphere parameters. While the largest systematics caused by inconsistent usage of meteorological data can be avoided and the application of different mapping functions can be considered by applying empirical corrections, the noise has to be modeled in the stochastic model of intra-technique combination. The application of different stochastic models shows no significant effects on the combined parameters but results in different mean formal errors: the mean formal errors of the combined ZTD are 2.3 mm (unweighted), 4.4 mm (diagonal), 8.6 mm [variance component (VC) estimation], and 8.6 mm (operator/software impact, OSI). On the one hand, the OSI model, i.e. the inclusion of off-diagonal elements in the cofactor-matrix, considers the reapplication of observations yielding a factor of about two for mean formal errors as compared to the diagonal approach. On the other hand, the combination based on VC estimation shows large differences among the VCs and exhibits a comparable scaling of formal errors. Thus, for the combination of troposphere parameters a combination of the two extensions of the stochastic model is recommended
Anisotropic Homogeneous Turbulence: hierarchy and intermittency of scaling exponents in the anisotropic sectors
We present the first measurements of anisotropic statistical fluctuations in
perfectly homogeneous turbulent flows. We address both problems of
intermittency in anisotropic sectors and hierarchical ordering of anisotropies
on a direct numerical simulation of a three dimensional random Kolmogorov flow.
We achieved an homogeneous and anisotropic statistical ensemble by randomly
shifting the forcing phases. We observe high intermittency as a function of the
order of the velocity correlation within each fixed anisotropic sector and a
hierarchical organization of scaling exponents at fixed order of the velocity
correlation at changing the anisotropic sector.Comment: 6 pages, 3 eps figure
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