7,973 research outputs found
Swimming in Granular Media
We study a simple model of periodic contraction and extension of large
intruders in a granular bed to understand the mechanism for swimming in an
otherwise solid media. Using an event-driven simulation, we find optimal
conditions that idealized swimmers must use to critically fluidize a sand bed
so that it is rigid enough to support a load when needed, but fluid enough to
permit motion with minimal resistance. Swimmers - or other intruders - that
agitate the bed too rapidly produce large voids that prevent traction from
being achieved, while swimmers that move too slowly cannot travel before the
bed re-solidifies around them i.e., the swimmers locally probe the fundamental
time-scale in a granular packing
Identifying States of a Financial Market
The understanding of complex systems has become a central issue because
complex systems exist in a wide range of scientific disciplines. Time series
are typical experimental results we have about complex systems. In the analysis
of such time series, stationary situations have been extensively studied and
correlations have been found to be a very powerful tool. Yet most natural
processes are non-stationary. In particular, in times of crisis, accident or
trouble, stationarity is lost. As examples we may think of financial markets,
biological systems, reactors or the weather. In non-stationary situations
analysis becomes very difficult and noise is a severe problem. Following a
natural urge to search for order in the system, we endeavor to define states
through which systems pass and in which they remain for short times. Success in
this respect would allow to get a better understanding of the system and might
even lead to methods for controlling the system in more efficient ways.
We here concentrate on financial markets because of the easy access we have
to good data and because of the strong non-stationary effects recently seen. We
analyze the S&P 500 stocks in the 19-year period 1992-2010. Here, we propose
such an above mentioned definition of state for a financial market and use it
to identify points of drastic change in the correlation structure. These points
are mapped to occurrences of financial crises. We find that a wide variety of
characteristic correlation structure patterns exist in the observation time
window, and that these characteristic correlation structure patterns can be
classified into several typical "market states". Using this classification we
recognize transitions between different market states. A similarity measure we
develop thus affords means of understanding changes in states and of
recognizing developments not previously seen.Comment: 9 pages, 8 figure
Death kinetics of Escherichia coli in goat milk and Bacillus licheniformis in cloudberry jam treated by ohmic heating
In recent years, the worldâs food industry has focused increasing attention on electrical techniques of
food processing. Ohmic heating is one of these techniques that can be considered as a high temperature
short time and a purely bulk heating method, having potential applications in processes such as
blanching, evaporation and pasteurization in the food industry. However such technology would have
to assure the microbiological safety obtained by the conventional cooking methods. Concerning this,
the influence of heat treatment by ohmic and conventional technology on death kinetic parameters (D
and z values) of Escherichia coli ATCCÂź 25922 was studied in goat milk. In ohmic treatment lower D
values were obtained (D60ÂșC = 4.2 min, D63ÂșC = 1.9 min, D65ÂșC = 0.86 min) as compared to conventional
treatment (D63ÂșC = 3.9 min, D65ÂșC = 3.5, D67ÂșC = 2.8 min, D75ÂșC = 1.5 min). The increase of temperature
required for a ten fold decrease in D value was also lower in the ohmic inactivation (z = 8.4 ÂșC)
comparing with the conventional inactivation (z = 23.1 ÂșC). The death kinetics for Bacillus
licheniformis ATCCÂź 14580 spores in cloudberry jam were also studied under both types of heat
inactivation (ohmic and conventional) and similar conclusions were drawn for the D values; lower D
values were also obtained for ohmic treatment (D70ÂșC = 57.1 min, D75ÂșC = 25.2 min, D80ÂșC = 7.2 min) as
compared to conventional treatment (D70ÂșC = 85.3 min, D75ÂșC = 51.0, D80ÂșC = 18.1 min, D85ÂșC = 6.0 min,
D90ÂșC = 1.6 min). However, between the z values obtained for those treatments (z ohmic = 11.1 ÂșC and z
conventional = 11.4 ÂșC) the differences were not significant. In general the results of present work indicate
that the ohmic heating provides quicker death kinetics. This opens the perspective for shorter, less
aggressive treatments
Interminiband Rabi oscillations in biased semiconductor superlattices
Carrier dynamics at energy level anticrossings in biased semiconductor
superlattices, was studied in the time domain by solving the time-dependent
Schroedinger equation. The resonant nature of interminiband Rabi oscillations
has been explicitly demonstrated to arise from interference of intrawell and
Bloch oscillations. We also report a simulation of direct Rabi oscillations
across three minibands, in the high field regime, due to interaction between
three strongly coupled minibands.Comment: 13 pages, 16 figure
The Escherichia coli RutR transcription factor binds at targets within genes as well as intergenic regions.
The Escherichia coli RutR protein is the master regulator of genes involved in pyrimidine catabolism. Here we have used chromatin immunoprecipitation in combination with DNA microarrays to measure the binding of RutR across the chromosome of exponentially growing E. coli cells. Twenty RutR-binding targets were identified and analysis of these targets generated a DNA consensus logo for RutR binding. Complementary in vitro binding assays showed high-affinity RutR binding to 16 of the 20 targets, with the four low-affinity RutR targets lacking predicted key binding determinants. Surprisingly, most of the DNA targets for RutR are located within coding segments of the genome and appear to have little or no effect on transcript levels in the conditions tested. This contrasts sharply with other E. coli transcription factors whose binding sites are primarily located in intergenic regions. We suggest that either RutR has yet undiscovered function or that evolution has been slow to eliminate non-functional DNA sites for RutR because they do not have an adverse effect on cell fitness
Nonequilibrium Microscopic Distribution of Thermal Current in Particle Systems
A nonequilibrium distribution function of microscopic thermal current is
studied by a direct numerical simulation in a thermal conducting steady state
of particle systems. Two characteristic temperatures of the thermal current are
investigated on the basis of the distribution. It is confirmed that the
temperature depends on the current direction; Parallel temperature to the
heat-flux is higher than antiparallel one. The difference between the parallel
temperature and the antiparallel one is proportional to a macroscopic
temperature gradient.Comment: 4 page
Effects of demographic stochasticity on biological community assembly on evolutionary time scales
We study the effects of demographic stochasticity on the long-term dynamics
of biological coevolution models of community assembly. The noise is induced in
order to check the validity of deterministic population dynamics. While
mutualistic communities show little dependence on the stochastic population
fluctuations, predator-prey models show strong dependence on the stochasticity,
indicating the relevance of the finiteness of the populations. For a
predator-prey model, the noise causes drastic decreases in diversity and total
population size. The communities that emerge under influence of the noise
consist of species strongly coupled with each other and have stronger linear
stability around the fixed-point populations than the corresponding noiseless
model. The dynamics on evolutionary time scales for the predator-prey model are
also altered by the noise. Approximate fluctuations are observed with
noise, while fluctuations are found for the model without demographic
noise
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