7,942 research outputs found
Homogeneous cooling of rough, dissipative particles: Theory and simulations
We investigate freely cooling systems of rough spheres in two and three
dimensions. Simulations using an event driven algorithm are compared with
results of an approximate kinetic theory, based on the assumption of a
generalized homogeneous cooling state. For short times , translational and
rotational energy are found to change linearly with . For large times both
energies decay like with a ratio independent of time, but not
corresponding to equipartition. Good agreement is found between theory and
simulations, as long as no clustering instability is observed. System
parameters, i.e. density, particle size, and particle mass can be absorbed in a
rescaled time, so that the decay of translational and rotational energy is
solely determined by normal restitution and surface roughness.Comment: 10 pages, 10 eps-figure
Aspects of management options for pasture-based dairy production stocked at two cows per hectare
End of project reportWhite clover in association with Rhizobium bacteria have the capacity to fix or convent atmospheric N into plant available N. This can make a considerable contribution to sward productivity. One of the objectives of this experiment was to determine the upper carrying capacity of grass-white clover swards receiving 90 kg fertilizer N/ha. A second objective was to examine the impact of grass-clover swards on mineral-N in the soil and losses of nitrate-N from soil to drainage water during the winter. This experiment was conducted at Solohead Research Farm. There were three treatments: (i) A grass-only treatment (FN) stocked at 2.0 cows per ha in 2003 and 2.2 cows per ha during 2004, 2005 and 2006. This treatment received an average of 226 kg per ha of fertilizer N per year during these years. (ii) A grass-clover treatment (WC) stocked at the same rates as FN and received an average of 90 kg per ha of fertilizer N per year during the experiment. (iii) A grass-only treatment (CC) that was gradually converted over to grass-clover during the experiment and stocked at 2.0 cows per ha throughout the experiment. Fertilizer N input was gradually lowered from 150 kg per ha in 2003 to a target of 90 kg per ha in 2005 and 2006
Second order statistics of NLOS indoor MIMO channels based on 5.2 GHz measurements
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Energy flows in vibrated granular media
We study vibrated granular media, investigating each of the three components
of the energy flow: particle-particle dissipation, energy input at the
vibrating wall, and particle-wall dissipation. Energy dissipated by
interparticle collisions is well estimated by existing theories when the
granular material is dilute, and these theories are extended to include
rotational kinetic energy. When the granular material is dense, the observed
particle-particle dissipation rate decreases to as little as 2/5 of the
theoretical prediction. We observe that the rate of energy input is the weight
of the granular material times an average vibration velocity times a function
of the ratio of particle to vibration velocity. `Particle-wall' dissipation has
been neglected in all theories up to now, but can play an important role when
the granular material is dilute. The ratio between gravitational potential
energy and kinetic energy can vary by as much as a factor of 3. Previous
simulations and experiments have shown that E ~ V^delta, with delta=2 for
dilute granular material, and delta ~ 1.5 for dense granular material. We
relate this change in exponent to the departure of particle-particle
dissipation from its theoretical value.Comment: 19 pages revtex, 10 embedded eps figures, accepted by PR
Inelastic Collapse of Three Particles
A system of three particles undergoing inelastic collisions in arbitrary
spatial dimensions is studied with the aim of establishing the domain of
``inelastic collapse''---an infinite number of collisions which take place in a
finite time. Analytic and simulation results show that for a sufficiently small
restitution coefficient, , collapse can
occur. In one dimension, such a collapse is stable against small perturbations
within this entire range. In higher dimensions, the collapse can be stable
against small variations of initial conditions, within a smaller range,
.Comment: 6 pages, figures on request, accepted by PR
Dynamics of inelastically colliding rough spheres: Relaxation of translational and rotational energy
We study the exchange of kinetic energy between translational and rotational
degrees of freedom for inelastic collisions of rough spheres. Even if
equipartition holds in the initial state it is immediately destroyed by
collisions. The simplest generalisation of the homogeneous cooling state allows
for two temperatures, characterizing translational and rotational degrees of
freedom separately. For times larger than a crossover frequency, which is
determined by the Enskog frequency and the initial temperature, both energies
decay algebraically like with a fixed ratio of amplitudes, different
from one.Comment: 5 pages, RevTeX, 2 eps figures, slightly expanded discussion, new
figures with dimensionless units, added references, accepted for publication
in PRE as a Rapid Com
Modeling Ellipsometry Measurements of Molecular Thin-Film Contamination on Genesis Array Samples
The discovery of a molecular thin-film contamination on Genesis flown array samples changed the course of preliminary assessment strategies. Analytical techniques developed to measure solar wind elemental abundances must now compensate for a thin-film contamination. Currently, this is done either by experimental cleaning before analyses or by depth-profiling techniques that bypass the surface contamination. Inside Johnson Space Center s Genesis dedicated ISO Class 4 (Class 10) cleanroom laboratory, the selection of collector array fragments allocated for solar wind analyses are based on the documentation of overall surface quality, visible surface particle contamination greater than 1 m, and the amount of thin film contamination measured by spectroscopic ellipsometry. Documenting the exact thickness, surface topography, and chemical composition of these contaminates is also critical for developing accurate cleaning methods. However, the first step in characterization of the molecular film is to develop accurate ellipsometry models that will determine an accurate thickness measurement of the contamination film
X-ray Polarization Signatures of Compton Scattering in Magnetic Cataclysmic Variables
Compton scattering within the accretion column of magnetic cataclysmic
variables (mCVs) can induce a net polarization in the X-ray emission. We
investigate this process using Monte Carlo simulations and find that
significant polarization can arise as a result of the stratified flow structure
in the shock-ionized column. We find that the degree of linear polarization can
reach levels up to ~8% for systems with high accretion rates and low
white-dwarf masses, when viewed at large inclination angles with respect to the
accretion column axis. These levels are substantially higher than previously
predicted estimates using an accretion column model with uniform density and
temperature. We also find that for systems with a relatively low-mass white
dwarf accreting at a high accretion rate, the polarization properties may be
insensitive to the magnetic field, since most of the scattering occurs at the
base of the accretion column where the density structure is determined mainly
by bremsstrahlung cooling instead of cyclotron cooling.Comment: 7 pages, 8 figures, accepted by MNRA
Two-dimensional Burgers Cellular Automaton
A two-dimensional cellular automaton(CA) associated with a two-dimensional
Burgers equation is presented. The 2D Burgers equation is an integrable
generalization of the well-known Burgers equation, and is transformed into a 2D
diffusion equation by the Cole-Hopf transformation. The CA is derived from the
2D Burgers equation by using the ultradiscrete method, which can transform
dependent variables into discrete ones. Some exact solutions of the CA, such as
shock wave solutions, are studied in detail.Comment: Latex2.09, 17 pages including 7 figure
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