8,446 research outputs found
Comment on Bramwell et al, "Universal Fluctuations in Correlated Systems"
This is a comment on "Universal Fluctuations in Correlated Systems", by
Bramwell et al, Phys. Rev. Lett., 84, 3744 (2000.Comment: To appear in Phys. Rev. Let
What can we infer about the underlying physics from burst distributions observed in an RMHD simulation ?
We determine that the sizes of bursts in mean-square current density in a
reduced magnetohydrodynamic (RMHD)simulation follow power-law probability
density function (PDF). The PDFs for burst durations and waiting time between
bursts are clearly not exponential and could also be power-law. This suffices
to distinguish their behaviour from the original Bak et al. sandpile model
which had exponential waiting time PDFs. However, it is not sufficient to
distinguish between turbulence, some other SOC-like models, and other red noise
sources.Comment: In press, Planetary and Space Science. Proceedings of a session at
European Geophysical Society General Assembly, Nice, 200
Power law burst and inter-burst interval distributions in the solar wind: turbulence or dissipative SOC ?
We calculate for the first time the probability density functions (PDFs) P of
burst energy e, duration T and inter-burst interval tau for a known turbulent
system in nature. Bursts in the earth-sun component of the Poynting flux at 1
AU in the solar wind were measured using the MFI and SWE experiments on the
NASA WIND spacecraft. We find P(e) and P(T) to be power laws, consistent with
self-organised criticality (SOC). We find also a power law form for P(tau) that
distinguishes this turbulent cascade from the exponential P(tau) of ideal SOC,
but not from some other SOC-like sandpile models. We discuss the implications
for the relation between SOC and turbulence.Comment: 3 pages, 1 figure. Submitted to PRL on 25th February 2000. Revised
version re-submitted on 9th May 2000. Second revised version submitted Phys.
Rev. E on 26th June, 200
Extreme-value statistics from Lagrangian convex hull analysis for homogeneous turbulent Boussinesq convection and MHD convection
We investigate the utility of the convex hull of many Lagrangian tracers to
analyze transport properties of turbulent flows with different anisotropy. In
direct numerical simulations of statistically homogeneous and stationary
Navier-Stokes turbulence, neutral fluid Boussinesq convection, and MHD
Boussinesq convection a comparison with Lagrangian pair dispersion shows that
convex hull statistics capture the asymptotic dispersive behavior of a large
group of passive tracer particles. Moreover, convex hull analysis provides
additional information on the sub-ensemble of tracers that on average disperse
most efficiently in the form of extreme value statistics and flow anisotropy
via the geometric properties of the convex hulls. We use the convex hull
surface geometry to examine the anisotropy that occurs in turbulent convection.
Applying extreme value theory, we show that the maximal square extensions of
convex hull vertices are well described by a classic extreme value
distribution, the Gumbel distribution. During turbulent convection,
intermittent convective plumes grow and accelerate the dispersion of Lagrangian
tracers. Convex hull analysis yields information that supplements standard
Lagrangian analysis of coherent turbulent structures and their influence on the
global statistics of the flow.Comment: 18 pages, 10 figures, preprin
Optimal Moments for the Analysis of Peculiar Velocity Surveys
We present a new method for the analysis of peculiar velocity surveys which
removes contributions to velocities from small scale, nonlinear velocity modes
while retaining information about large scale motions. Our method utilizes
Karhunen--Lo\`eve methods of data compression to construct a set of moments out
of the velocities which are minimally sensitive to small scale power. The set
of moments are then used in a likelihood analysis. We develop criteria for the
selection of moments, as well as a statistic to quantify the overall
sensitivity of a set of moments to small scale power. Although we discuss our
method in the context of peculiar velocity surveys, it may also prove useful in
other situations where data filtering is required.Comment: 25 Pages, 3 figures. Submitted to Ap
La unidad del pensamiento de Popper
Fil: Watkins, J. W. N. London School of Economics. Department of Philosophy, Logic and Scientific Method. Londres, Gran Bretañ
- …