454 research outputs found
Hydrodynamics of dense granular systems
The properties of dense granular systems are analyzed from a hydrodynamical
point of view, based on conservation laws for the particle number density and
linear momentum. We discuss averaging problems associated with the nature of
such systems and the peculiarities of the sources of noise. We perform a
quantitative study by combining analytical methods and numerical results
obtained by ensemble-averaging of data on creep during compaction and molecular
dynamics simulations of convective flow. We show that numerical integration of
the hydrodynamic equations gives the expected evolution for the time-dependent
fields.Comment: 10 pages, 7 figure
Propagation of an Acoustic Pulse of Finite Amplitude in a Granular Medium
A study of propagation of a wide-band acoustic signal in a granular medium is reported. Experimental data on the propagation of pulses with an amplitude up to 3 MPa and characteristic length about 1 µs through a sample of cobalt-manganese nodules are compared with a computer model of the process. An anomalous sig'rfal absorption in the high-frequency range observed with relatively weak sounding pulses is explained under the assumption of a fractal sample structure on a certain scale. When the signal amplitude increases, the ahsorption assumes a normal power form which is evidence of substance structural changes
Helping Russian Students See the Invisible Hand
Teaching/Communication/Extension/Profession,
Modeling the Role of the Cell Cycle in Regulating Proteus mirabilis Swarm-Colony Development
We present models and computational results which indicate that the spatial
and temporal regularity seen in Proteus mirabilis swarm-colony development is
largely an expression of a sharp age of dedifferentiation in the cell cycle
from motile swarmer cells to immotile dividing cells (also called swimmer or
vegetative cells.) This contrasts strongly with reaction-diffusion models of
Proteus behavior that ignore or average out the age structure of the cell
population and instead use only density-dependent mechanisms. We argue the
necessity of retaining the explicit age structure, and suggest experiments that
may help determine the underlying mechanisms empirically. Consequently, we
advocate Proteus as a model organism for a multiscale understanding of how and
to what extent the life cycle of individual cells affects the macroscopic
behavior of a biological system
Transient response of photoexcited electrons: negative and oscillating current
Time-dependent current of the electrons excited in the conduction band after
ultrafast interband photogeneration is studied theoretically. The transient
photocurrent is calculated for the nonlinear regime of response to a stationary
electric field. The response demonstrates transient absolute negative
conductivity when the electrons are excited slightly below the optical phonon
energy, while the periodic oscillations of the electric current appear after
formation of the streaming distribution. The quenching of these peculiarities
by the elastic scattering of electrons is also considered.Comment: 5 pages, 3 figure
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