1,722 research outputs found
Local 4/5-Law and Energy Dissipation Anomaly in Turbulence
A strong local form of the ``4/3-law'' in turbulent flow has been proved
recently by Duchon and Robert for a triple moment of velocity increments
averaged over both a bounded spacetime region and separation vector directions,
and for energy dissipation averaged over the same spacetime region. Under
precisely stated hypotheses, the two are proved to be proportional, by a
constant 4/3, and to appear as a nonnegative defect measure in the local energy
balance of singular (distributional) solutions of the incompressible Euler
equations. Here we prove that the energy defect measure can be represented also
by a triple moment of purely longitudinal velocity increments and by a mixed
moment with one longitudinal and two tranverse velocity increments. Thus, we
prove that the traditional 4/5- and 4/15-laws of Kolmogorov hold in the same
local sense as demonstrated for the 4/3-law by Duchon-Robert.Comment: 14 page
Scaling Analysis and Evolution Equation of the North Atlantic Oscillation Index Fluctuations
The North Atlantic Oscillation (NAO) monthly index is studied from 1825 till
2002 in order to identify the scaling ranges of its fluctuations upon different
delay times and to find out whether or not it can be regarded as a Markov
process. A Hurst rescaled range analysis and a detrended fluctuation analysis
both indicate the existence of weakly persistent long range time correlations
for the whole scaling range and time span hereby studied. Such correlations are
similar to Brownian fluctuations. The Fokker-Planck equation is derived and
Kramers-Moyal coefficients estimated from the data. They are interpreted in
terms of a drift and a diffusion coefficient as in fluid mechanics. All partial
distribution functions of the NAO monthly index fluctuations have a form close
to a Gaussian, for all time lags, in agreement with the findings of the scaling
analyses. This indicates the lack of predictive power of the present NAO
monthly index. Yet there are some deviations for large (and thus rare) events.
Whence suggestions for other measurements are made if some improved
predictability of the weather/climate in the North Atlantic is of interest. The
subsequent Langevin equation of the NAO signal fluctuations is explicitly
written in terms of the diffusion and drift parameters, and a characteristic
time scale for these is given in appendix.Comment: 6 figures, 54 refs., 16 pages; submitted to Int. J. Mod. Phys. C:
Comput. Phy
Constraining the neutrino magnetic moment with anti-neutrinos from the Sun
We discuss the impact of different solar neutrino data on the
spin-flavor-precession (SFP) mechanism of neutrino conversion. We find that,
although detailed solar rates and spectra allow the SFP solution as a
sub-leading effect, the recent KamLAND constraint on the solar antineutrino
flux places stronger constraints to this mechanism. Moreover, we show that for
the case of random magnetic fields inside the Sun, one obtains a more stringent
constraint on the neutrino magnetic moment down to the level of \mu_\nu \lsim
few \times 10^{-12}\mu_B, similar to bounds obtained from star cooling.Comment: 4 pages, 3 figures. Final version to appear in Phys. Rev. Let
On the approach to equilibrium of an Hamiltonian chain of anharmonic oscillators
In this note we study the approach to equilibrium of a chain of anharmonic
oscillators. We find indications that a sufficiently large system always
relaxes to the usual equilibrium distribution. There is no sign of an
ergodicity threshold. The time however to arrive to equilibrium diverges when
, being the anharmonicity.Comment: 8 pages, 5 figure
Log-Poisson Cascade Description of Turbulent Velocity Gradient Statistics
The Log-Poisson phenomenological description of the turbulent energy cascade
is evoked to discuss high-order statistics of velocity derivatives and the
mapping between their probability distribution functions at different Reynolds
numbers. The striking confirmation of theoretical predictions suggests that
numerical solutions of the flow, obtained at low/moderate Reynolds numbers can
play an important quantitative role in the analysis of experimental high
Reynolds number phenomena, where small scales fluctuations are in general
inaccessible from direct numerical simulations
Interactions of a Light Hypersonic Jet with a Non-Uniform Interstellar Medium
We present three dimensional simulations of the interaction of a light
hypersonic jet with an inhomogeneous thermal and turbulently supported disk in
an elliptical galaxy. We model the jet as a light, supersonic non-relativistic
flow with parameters selected to be consistent with a relativistic jet with
kinetic power just above the FR1/FR2 break.
We identify four generic phases in the evolution of such a jet with the
inhomogeneous interstellar medium: 1) an initial ``flood and channel'' phase,
where progress is characterized by high pressure gas finding changing weak
points in the ISM, flowing through channels that form and re-form over time, 2)
a spherical, energy-driven bubble phase, were the bubble is larger than the
disk scale, but the jet remains fully disrupted close to the nucleus, 3) a
rapid, jet break--out phase the where jet breaks free of the last dense clouds,
becomes collimated and pierces the spherical bubble, and 4) a classical phase,
the jet propagates in a momentum-dominated fashion leading to the classical jet
+ cocoon + bow-shock structure.
Mass transport in the simulations is investigated, and we propose a model for
the morphology and component proper motions in the well-studied Compact
Symmetric Object 4C31.04.Comment: 66 pages, 22 figures, PDFLaTeX, aastex macros, graphicx and amssymb
packages, Accepted, to be published 2007 ApJ
Considerations on bubble fragmentation models
n this paper we describe the restrictions that the probability density function (p.d.f.) of the size of particles resulting from the rupture of a drop or bubble must satisfy. Using conservation of volume, we show that when a particle of diameter, D0, breaks into exactly two fragments of sizes D and D2 = (D30−D3)1/3 respectively, the resulting p.d.f., f(D; D0), must satisfy a symmetry relation given by D22 f(D; D0) = D2 f(D2; D0), which does not depend on the nature of the underlying fragmentation process. In general, for an arbitrary number of resulting particles, m(D0), we determine that the daughter p.d.f. should satisfy the conservation of volume condition given by m(D0) ∫0D0 (D/D0)3 f(D; D0) dD = 1. A detailed analysis of some contemporary fragmentation models shows that they may not exhibit the required conservation of volume condition if they are not adequately formulated. Furthermore, we also analyse several models proposed in the literature for the breakup frequency of drops or bubbles based on different principles, g(ϵ, D0). Although, most of the models are formulated in terms of the particle size D0 and the dissipation rate of turbulent kinetic energy, ϵ, and apparently provide different results, we show here that they are nearly identical when expressed in dimensionless form in terms of the Weber number, g*(Wet) = g(ϵ, D0) D2/30 ϵ−1/3, with Wet ~ ρ ϵ2/3 D05/3/σ, where ρ is the density of the continuous phase and σ the surface tension
Turbulent luminance in impassioned van Gogh paintings
We show that the patterns of luminance in some impassioned van Gogh paintings display the mathematical structure of fluid turbulence. Specifically, we show that the probability distribution function (PDF) of luminance fluctuations of points (pixels) separated by a distance R compares notably well with the PDF of the velocity differences in a turbulent flow, as predicted by the statistical theory of A.N. Kolmogorov. We observe that turbulent paintings of van Gogh belong to his last period, during which episodes of prolonged psychotic agitation of this artist were frequent. Our approach suggests new tools that open the possibility of quantitative objective research for art representation
Degree of randomness: numerical experiments for astrophysical signals
Astrophysical and cosmological signals such as the cosmic microwave
background radiation, as observed, typically contain contributions of different
components, and their statistical properties can be used to distinguish one
from the other. A method developed originally by Kolmogorov is involved for the
study of astrophysical signals of randomness of various degrees. Numerical
performed experiments based on the universality of Kolmogorov distribution and
using a single scaling of the ratio of stochastic to regular components, reveal
basic features in the behavior of generated signals also in terms of a critical
value for that ratio, thus enable the application of this technique for various
observational datasetsComment: 6 pages, 9 figures; Europhys.Letters; to match the published versio
Intermittency in the Joint Cascade of Energy and Helicity
The statistics of the energy and helicity fluxes in isotropic turbulence are
studied using high resolution direct numerical simulation. The scaling
exponents of the energy flux agree with those of the transverse velocity
structure functions through refined similarity hypothesis, consistent with
Kraichnan's prediction \cite{Kr74}. The helicity flux is even more intermittent
than the energy flux and its scaling exponents are closer to those of the
passive scalar. Using Waleffe's helical decomposition, we demonstrate that the
existence of positive mean helicity flux inhibits the energy transfer in the
negative helical modes, a non-passive effect
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