1,095 research outputs found
Velocity statistics in excited granular media
We present an experimental study of velocity statistics for a partial layer
of inelastic colliding beads driven by a vertically oscillating boundary. Over
a wide range of parameters (accelerations 3-8 times the gravitational
acceleration), the probability distribution P(v) deviates measurably from a
Gaussian for the two horizontal velocity components. It can be described by
P(v) ~ exp(-|v/v_c|^1.5), in agreement with a recent theory. The characteristic
velocity v_c is proportional to the peak velocity of the boundary. The granular
temperature, defined as the mean square particle velocity, varies with particle
density and exhibits a maximum at intermediate densities. On the other hand,
for free cooling in the absence of excitation, we find an exponential velocity
distribution. Finally, we examine the sharing of energy between particles of
different mass. The more massive particles are found to have greater kinetic
energy.Comment: 27 pages, 13 figures, to appear in Chaos, September 99, revised 3
figures and tex
Oscillatory Flows Induced by Microorganisms Swimming in Two-dimensions
We present the first time-resolved measurements of the oscillatory velocity
field induced by swimming unicellular microorganisms. Confinement of the green
alga C. reinhardtii in stabilized thin liquid films allows simultaneous
tracking of cells and tracer particles. The measured velocity field reveals
complex time-dependent flow structures, and scales inversely with distance. The
instantaneous mechanical power generated by the cells is measured from the
velocity fields and peaks at 15 fW. The dissipation per cycle is more than four
times what steady swimming would require.Comment: 4 pages, 4 figure
Temporal Modulation of the Control Parameter in Electroconvection in the Nematic Liquid Crystal I52
I report on the effects of a periodic modulation of the control parameter on
electroconvection in the nematic liquid crystal I52. Without modulation, the
primary bifurcation from the uniform state is a direct transition to a state of
spatiotemporal chaos. This state is the result of the interaction of four,
degenerate traveling modes: right and left zig and zag rolls. Periodic
modulations of the driving voltage at approximately twice the traveling
frequency are used. For a large enough modulation amplitude, standing waves
that consist of only zig or zag rolls are stabilized. The standing waves
exhibit regular behavior in space and time. Therefore, modulation of the
control parameter represents a method of eliminating spatiotemporal chaos. As
the modulation frequency is varied away from twice the traveling frequency,
standing waves that are a superposition of zig and zag rolls, i.e. standing
rectangles, are observed. These results are compared with existing predictions
based on coupled complex Ginzburg-Landau equations
Experimental Measurements of Stretching Fields in Fluid Mixing
The mixing of an impurity into a flowing fluid is an important process in many areas of science, including geophysical processes, chemical reactors, and microfluidic devices. In some cases, for example periodic flows, the concepts of nonlinear dynamics provide a deep theoretical basis for understanding mixing. Unfortunately, the building blocks of this theory, i.e. the fixed points and invariant manifolds of the associated Poincaré map, have remained inaccessible to direct experimental study, thus limiting the insight that could be obtained. Using precision measurements of tracer particle trajectories in a two-dimensional fluid flow producing chaotic mixing, we directly measure the time-dependent stretching fields. These quantities, previously available only numerically, attain local maxima along lines coinciding with the stable and unstable manifolds, thus revealing the dynamical structures that control mixing. Contours or level sets of a passive impurity field are found to be aligned parallel to the lines of large stretching at each instant, thus explaining what happens as one stirs milk into coffee. --author-supplied descriptio
Amplitude Modulation and Relaxation-Oscillation of Counterpropagating Rolls within a Broken-Symmetry Laser-Induced Electroconvection Strip
We report a liquid-crystal pattern-formation experiment in which we break the
lateral (translational) symmetry of a nematic medium with a laser-induced
thermal gradient. The work is motivated by an improved measurement (reported
here) of the temperature dependence of the electroconvection threshold voltage
in planar-nematic 4-methoxybenzylidene-4-butylaniline (MBBA). In contrast with
other broken-symmetry-pattern studies that report a uniform drift, we observe a
strip of counterpropagating rolls that collide at a sink point, and a strong
temporally periodic amplitude modulation within a width of 3-4 rolls about the
sink point. The time dependence of the amplitude at a fixed position is
periodic but displays a nonsinusoidal relaxation-oscillation profile. After
reporting experimental results based on spacetime contours and wavenumber
profiles, along with a measurement of the change in the drift frequency with
applied voltage at a fixed control parameter, we propose some potential
guidelines for a theoretical model based on saddle-point solutions for
Eckhaus-unstable states and coupled complex Ginzburg-Landau equations.
Published in PRE 73, 036317 (2006).Comment: Published in Physical Review E in March 200
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