1,218 research outputs found
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
VISHNU hybrid model for viscous QCD matter at RHIC and LHC energies
In this proceeding, we briefly describe the viscous hydrodynamics + hadron
cascade hybrid model VISHNU for relativistic heavy ion collisions and report
the current status on extracting the QGP viscosity from elliptic flow data.Comment: 4 pages, 1 figure, the proceedings of 7th International Workshop on
Critical Point and Onset of Deconfinement, Wuhan, China, Nov. 7-11, 201
High-order harmonic generation with a strong laser field and an attosecond-pulse train: the Dirac Delta comb and monochromatic limits
In recent publications, it has been shown that high-order harmonic generation
can be manipulated by employing a time-delayed attosecond pulse train
superposed to a strong, near-infrared laser field. It is an open question,
however, which is the most adequate way to approximate the attosecond pulse
train in a semi-analytic framework. Employing the Strong-Field Approximation
and saddle-point methods, we make a detailed assessment of the spectra obtained
by modeling the attosecond pulse train by either a monochromatic wave or a
Dirac-Delta comb. These are the two extreme limits of a real train, which is
composed by a finite set of harmonics. Specifically, in the monochromatic
limit, we find the downhill and uphill sets of orbits reported in the
literature, and analyze their influence on the high-harmonic spectra. We show
that, in principle, the downhill trajectories lead to stronger harmonics, and
pronounced enhancements in the low-plateau region. These features are analyzed
in terms of quantum interference effects between pairs of quantum orbits, and
compared to those obtained in the Dirac-Delta limit.Comment: 10 pages, 7 figures (eps files). To appear in Laser Physic
New Measurements and Quantitative Analysis of Electron Backscattering in the Energy Range of Neutron Beta-Decay
We report on the first detailed measurements of electron backscattering from
plastic scintillator targets, extending our previous work on beryllium and
silicon targets. The scintillator experiment posed several additional
experimental challenges associated with charging of the scintillator target,
and those challenges are addressed in detail. In addition, we quantitatively
compare the energy and angular distributions of this data, and our previous
data, with electron transport simulations based on the Geant4 and Penelope
Monte Carlo simulation codes. The Penelope simulation is found globally to give
a superior description of the data. Such information is crucial for a broad
array of weak-interaction physics experiments, where electron backscattering
can give rise to the dominant detector-related systematic uncertainty.Comment: 7 pages, 3 figure
Force correlations and arches formation in granular assemblies
In the context of a simple microscopic schematic scalar model we study the
effects of spatial correlations in force transmission in granular assemblies.
We show that the parameters of the normalized weights distribution function,
, strongly depend on the spatial extensions,
, of such correlations. We show, then, the connections between
measurable macroscopic quantities and microscopic mechanisms enhancing
correlations. In particular we evaluate how the exponential cut-off,
, and the small forces power law exponent, , depend
on the correlation length, . If correlations go to infinity, weights are
power law distributed.Comment: 6 page
Enhanced Perturbative Continuous Unitary Transformations
Unitary transformations are an essential tool for the theoretical
understanding of many systems by mapping them to simpler effective models. A
systematically controlled variant to perform such a mapping is a perturbative
continuous unitary transformation (pCUT) among others. So far, this approach
required an equidistant unperturbed spectrum. Here, we pursue two goals: First,
we extend its applicability to non-equidistant spectra with the particular
focus on an efficient derivation of the differential flow equations, which
define the enhanced perturbative continuous unitary transformation (epCUT).
Second, we show that the numerical integration of the flow equations yields a
robust scheme to extract data from the epCUT. The method is illustrated by the
perturbation of the harmonic oscillator with a quartic term and of the two-leg
spin ladders in the strong-rung-coupling limit for uniform and alternating rung
couplings. The latter case provides an example of perturbation around a
non-equidistant spectrum.Comment: 27 pages, 18 figures; separated methodological background from
introduction, added perturbed harmonic oscillator for additional
illustration, added explicit solution of deepCUT equation
Vector lattice model for stresses in granular materials
A vector lattice model for stresses in granular materials is proposed. A two
dimensional pile built by pouring from a point is constructed numerically
according to this model. Remarkably, the pile violates the Mohr Coulomb
stability criterion for granular matter, probably because of the inherent
anisotropy of such poured piles. The numerical results are also compared to the
earlier continuum FPA model and the (scalar) lattice -model
A Model for Force Fluctuations in Bead Packs
We study theoretically the complex network of forces that is responsible for
the static structure and properties of granular materials. We present detailed
calculations for a model in which the fluctuations in the force distribution
arise because of variations in the contact angles and the constraints imposed
by the force balance on each bead of the pile. We compare our results for force
distribution function for this model, including exact results for certain
contact angle probability distributions, with numerical simulations of force
distributions in random sphere packings. This model reproduces many aspects of
the force distribution observed both in experiment and in numerical simulations
of sphere packings
Mesophilic Digestion Kinetics of Manure Slurry
Anaerobic Digestion Kinetics Study of Cow Manure Was Performed at 35°C in Bench-Scale Gas-Lift Digesters (3.78 L Working Volume) at Eight Different Volatile Solids (VS) Loading Rates in the Range of 1.11-5.87 G L-1 Day-1. the Digesters Produced Methane at the Rates of 0.44-1.18 L L-1 Day-1, and the Methane Content of the Biogas Was Found to Increase with Longer Hydraulic Retention Time (HRT). based on the Experimental Observations, the Ultimate Methane Yield and the Specific Methane Productivity Were Estimated to Be 0.42 L CH4 (G vs. Loaded)-1 and 0.45 L CH4 (G vs. Consumed)-1, Respectively. Total and Dissolved Chemical Oxygen Demand (COD) Consumptions Were Calculated to Be 59-17% and 78-43% at 24.4-4.6 Days HRTs, respectively. Maximum Concentration of Volatile Fatty Acids in the Effluent Was Observed as 0.7 G L-1 at 4.6 Days HRT, While It Was Below Detection Limit at HRTs Longer Than 11 Days. the Observed Methane Production Rate Did Not Compare Well with the Predictions of Chen and Hashimoto\u27s [1] and Hill\u27s [2] Models using their Recommended Kinetic Parameters. However, under the Studied Experimental Conditions, the Predictions of Chen and Hashimoto\u27s [1] Model Compared Better to the Observed Data Than that of Hill\u27s [2] Model. the Nonlinear Regression Analysis of the Experimental Data Was Performed using a Derived Methane Production Rate Model, for a Completely Mixed Anaerobic Digester, Involving Contois Kinetics [3] with Endogenous Decay. the Best Fit Values for the Maximum Specific Growth Rate (Μm) and Dimensionless Kinetic Parameter (K) Were Estimated as 0.43 Day-1 and 0.89, Respectively. the Experimental Data Were Found to Be within 95% Confidence Interval of the Prediction of the Derived Methane Production Rate Model with the Sum of Residual Squared Error as 0.02. © Humana Press Inc. 2007
Perceptual Context in Cognitive Hierarchies
Cognition does not only depend on bottom-up sensor feature abstraction, but
also relies on contextual information being passed top-down. Context is higher
level information that helps to predict belief states at lower levels. The main
contribution of this paper is to provide a formalisation of perceptual context
and its integration into a new process model for cognitive hierarchies. Several
simple instantiations of a cognitive hierarchy are used to illustrate the role
of context. Notably, we demonstrate the use context in a novel approach to
visually track the pose of rigid objects with just a 2D camera
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