126 research outputs found
Decorrelating a compressible turbulent flow: an experiment
Floating particles that are initially distributed uniformly on the surface of
a turbulent fluid, subsequently coagulate, until finally a steady state is
reached. This being so, they manifestly form a compressible system. In this
experiment, the information dimension D_1, and the Lyapunov exponents of the
coagulated floaters, are measured. The trajectories and the velocity fields of
the particles are captured in a sequence of rapidly acquired images. Then the
velocity sequence is randomly shuffled in time to generate new trajectories.
This analysis mimics the Kraichnan ensemble and yields properties of a velocity
correlation function that is delta-correlated in time (but not in space). The
measurements are compared with theoretical expectations and with simulations of
Boffetta et al., that closely mimic the laboratory experiment reported here.Comment: 6 pages, 5 figure
Dispersion of tracer particles in a compressible flow
The turbulent diffusion of Lagrangian tracer particles has been studied in a
flow on the surface of a large tank of water and in computer simulations. The
effect of flow compressibility is captured in images of particle fields. The
velocity field of floating particles has a divergence, whose probability
density function shows exponential tails. Also studied is the motion of pairs
and triplets of particles. The mean square separation is fitted to
the scaling form ~ t^alpha, and in contrast with the
Richardson-Kolmogorov prediction, an extended range with a reduced scaling
exponent of alpha=1.65 pm 0.1 is found. Clustering is also manifest in strongly
deformed triangles spanned within triplets of tracers.Comment: 6 pages, 4 figure
Measurement of entropy production rate in compressible turbulence
The rate of change of entropy is measured for a system of particles
floating on the surface of a fluid maintained in a turbulent steady state. The
resulting coagulation of the floaters allows one to relate to the
velocity divergence and to the Lyapunov exponents characterizing the behavior
of this system. The quantities measured from experiments and simulations are
found to agree well with the theoretical predictions.Comment: 7 Pages, 4 figures, 1 tabl
Introduction: Second Annual Gallery of Nonlinear Images (Los Angeles, California, 2005)
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/87886/2/041101_1.pd
Energy flux fluctuations in a finite volume of turbulent flow
The flux of turbulent kinetic energy from large to small spatial scales is
measured in a small domain B of varying size R. The probability distribution
function of the flux is obtained using a time-local version of Kolmogorov's
four-fifths law. The measurements, made at a moderate Reynolds number, show
frequent events where the flux is backscattered from small to large scales,
their frequency increasing as R is decreased. The observations are corroborated
by a numerical simulation based on the motion of many particles and on an
explicit form of the eddy damping.Comment: 10 Pages, 5 figures, 1 tabl
Where surface physics and fluid dynamics meet: rupture of an amphiphile layer by fluid flow
We investigate the fluctuating pattern created by a jet of fluid impingent
upon an amphiphile-covered surface. This microscopically thin layer is
initially covered with 50 m floating particles so that the layer can be
visualized. A vertical jet of water located below the surface and directed
upward drives a hole in this layer. The hole is particle-free and is surrounded
by the particle-laden amphiphile region. The jet ruptures the amphiphile layer
creating a particle-free region that is surrounded by the particle-covered
surface. The aim of the experiment is to understand the (fluctuating) shape of
the ramified interface between the particle-laden and particle-free regions.Comment: published in Journal of Chemical Physic
Null Cones in Schwarzschild Geometry
Light cones of Schwarzschild geometry are studied in connection to the Null
Surface Formulation and gravitational lensing. The paper studies the light cone
cut function's singularity structure, gives exact gravitational lensing
equations, and shows that the "pseudo-Minkowski" coordinates are well defined
within the model considered.Comment: 31 pages, 5 figure
Persistent global power fluctuations near a dynamic transition in electroconvection
This is a study of the global fluctuations in power dissipation and light
transmission through a liquid crystal just above the onset of
electroconvection.
The source of the fluctuations is found to be the creation and annihilation
of defects. They are spatially uncorrelated and yet temporally correlated. The
temporal correlation is seen to persist for extremely long times. There seems
to be an especially close relation between defect creation/annihilat ion in
electroconvection and thermal plumes in Rayleigh-B\'enard convection
Hysteresis at low Reynolds number: the onset of 2D vortex shedding
Hysteresis has been observed in a study of the transition between laminar
flow and vortex shedding in a quasi-two dimensional system. The system is a
vertical, rapidly flowing soap film which is penetrated by a rod oriented
perpendicular to the film plane. Our experiments show that the transition from
laminar flow to a periodic K\'arm\'an vortex street can be hysteretic, i.e.
vortices can survive at velocities lower than the velocity needed to generate
them.Comment: RevTeX file 4 pages + 5 (encapsulated postscript) figures. to appear
in Phys.Rev.E, Rapid Communicatio
Turbulence in a free surface
We report an experimental and numerical study of turbulent fluid motion in a
free surface. The flow is realized experimentally on the surface of a tank
filled with water stirred by a vertically oscillating grid positioned well
below the surface. Particles floating on the surface are used to visualize the
flow. The effect of surface waves appears to be negligible. The flow is
unconventional in that it is confined to two dimensions but does not have
squared vorticity as a conservation law, that it is not divergence free and
that it inherits scaling features of the mean square velocity differences
S_2(R) and the vorticity fluctuations Omega(R) from the bulk 3-d turbulence.Comment: 4 pages, 4 Postscript figure
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