120 research outputs found
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
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
Marangoni shocks in unobstructed soap-film flows
It is widely thought that in steady, gravity-driven, unobstructed soap-film
flows, the velocity increases monotonically downstream. Here we show
experimentally that the velocity increases, peaks, drops abruptly, then lessens
gradually downstream. We argue theoretically and verify experimentally that the
abrupt drop in velocity corresponds to a Marangoni shock, a type of shock
related to the elasticity of the film. Marangoni shocks induce locally intense
turbulent fluctuations and may help elucidate the mechanisms that produce
two-dimensional turbulence away from boundaries.Comment: 4 pages, 5 figures, published in PR
Evidence for Coupling of Velocity and Composition Fluctuations in a Binary Liquid Mixture
A critical mixture of isobutyric acid and water was quenched from the one-phase region into the two-phase region and, after the spinodal ring was well developed, a reverse quench returned the system to the one-phase region. Light-scattering measurements for this process exhibit a clearly nondiffusive relaxation which, at least for early times after the quench reversal, is in good agreement with Ruiz\u27s scheme for the coupling of velocity and composition fluctuations
The Characteristics of Catholic Schools: Comparative Perspectives from the USA and Queensland, Australia
The faith-based identity of Catholic schools is increasingly problematic in a secularised society where the numbers of teachers belonging to religious orders are diminishing rapidly. Teachers’ views regarding the characteristics of Catholic schools are an important aspect of the identity of such schools. The authors locate Catholic schools in the USA and Queensland, Australia, in their respective contexts and compare teachers’ ratings of the importance of eleven given characteristics of Catholic schools as seen by 3,389 teachers in USA Catholic schools and 2,287 teachers in Queensland Catholic schools. When the mean ratings for each jurisdiction were statistically correlated, USA teachers were much more likely to rate these given characteristics as essential and the resulting χ² and associated Odds Ratio values indicated very statistically significant jurisdictional differences. Some tentative explanations are suggested including the differing political contexts, the conditions of teachers’ employment and the support structures for the spiritual and faith formation of teachers in the respective jurisdictions
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
Fluctuation and Dissipation in Liquid Crystal Electroconvection
In this experiment a steady state current is maintained through a liquid
crystal thin film. When the applied voltage is increased through a threshold, a
phase transition is observed into a convective state characterized by the
chaotic motion of rolls. Above the threshold, an increase in power consumption
is observed that is manifested by an increase in the mean conductivity. A sharp
increase in the ratio of the power fluctuations to the mean power dissipated is
observed above the transition. This ratio is compared to the predictions of the
fluctuation theorem of Gallavotti and Cohen using an effective temperature
associated with the rolls' chaotic motion.Comment: 4 pages, 3 figures, revtex forma
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
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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