5,346 research outputs found
Inertial particles distribute in turbulence as Poissonian points with random intensity inducing clustering and supervoiding
This work considers the distribution of inertial particles in turbulence
using the point-particle approximation. We demonstrate that the random point
process formed by the positions of particles in space is a Poisson point
process with log-normal random intensity ("log Gaussian Cox process" or LGCP).
The probability of having a finite number of particles in a small volume is
given in terms of the characteristic function of a log-normal distribution.
Corrections due to discreteness of the number of particles to the previously
derived statistics of particle concentration in the continuum limit are
provided. These are relevant for dealing with experimental or numerical data.
The probability of having regions without particles, i.e. voids, is larger for
inertial particles than for tracer particles where voids are distributed
according to Poisson processes. Further, the probability of having large voids
decays only log-normally with size. This shows that particles cluster, leaving
voids behind. At scales where there is no clustering there can still be an
increase of the void probability so that turbulent voiding is stronger than
clustering. The demonstrated double stochasticity of the distribution
originates in the two-step formation of fluctuations. First, turbulence brings
the particles randomly close together which happens with Poisson-type
probability. Then, turbulence compresses the particles' volume in the
observation volume. We confirm the theory of the statistics of the number of
particles in small volumes by numerical observations of inertial particle
motion in a chaotic ABC flow. The improved understanding of clustering
processes can be applied to predict the long-time survival probability of
reacting particles. Our work implies that the particle distribution in weakly
compressible flow with finite time correlations is a LGCP, independently of the
details of the flow statistics
Are standards and regulations of organic farming moving away from small farmers' knowledge?
Organic farming is a promising agricultural method with positive effects on the human ecological and social environment. Governments have taken over a major role in defining organic farming by creating legal standards. Many countries all over the world have established a certification and accreditation system in order to protect the justified expectations of consumers with regard to processing and controlling the product quality of organic goods and to protecting producers from fraudulent trade practices. As they are relevant to international trade, these standards do not only influence the organic farming movement on the national level but also have a converse impact across national borders. Organic farming was established in a bottom-up process as farmers aimed to design sustainable ways of using natural resources. Farmers’ traditional knowledge and their awareness of ecological, as well as, of social affairs was the main base for the development of organic farming. Since public interest in organic farming has grown rapidly, the ownership on the process of defining organic farming is no longer in the hands of farmers and the original principles and aims of themovement seem to be threatened by a bureaucratic view of “recipe”-organic farming. However, unsolved problems also exist between the necessities of global harmonization and the local adaptability of the standards on organic farming. This paper structures the current discussion and gives future prospects for further development
Statistical properties of supersonic turbulence in the Lagrangian and Eulerian frameworks
We present a systematic study of the influence of different forcing types on
the statistical properties of supersonic, isothermal turbulence in both the
Lagrangian and Eulerian frameworks. We analyse a series of high-resolution,
hydrodynamical grid simulations with Lagrangian tracer particles and examine
the effects of solenoidal (divergence-free) and compressive (curl-free) forcing
on structure functions, their scaling exponents, and the probability density
functions of the gas density and velocity increments. Compressively driven
simulations show a significantly larger density contrast, a more intermittent
behaviour, and larger fractal dimension of the most dissipative structures at
the same root mean square Mach number. We show that the absolute values of
Lagrangian and Eulerian structure functions of all orders in the integral range
are only a function of the root mean square Mach number, but independent of the
forcing. With the assumption of a Gaussian distribution for the probability
density function of the velocity increments on large scales, we derive a model
that describes this behaviour.Comment: 24 pages, 13 figures, Journal of Fluid Mechanics in pres
Origins of Extragalactic Cosmic Ray Nuclei by Contracting Alignment Patterns induced in the Galactic Magnetic Field
We present a novel approach to search for origins of ultra-high energy cosmic
rays. These particles are likely nuclei that initiate extensive air showers in
the Earth's atmosphere. In large-area observatories, the particle arrival
directions are measured together with their energies and the atmospheric depth
at which their showers maximize. The depths provide rough measures of the
nuclear charges. In a simultaneous fit to all observed cosmic rays we use the
galactic magnetic field as a mass spectrometer and adapt the nuclear charges
such that their extragalactic arrival directions are concentrated in as few
directions as possible. Using different simulated examples we show that, with
the measurements on Earth, reconstruction of extragalactic source directions is
possible. In particular, we show in an astrophysical scenario that source
directions can be reconstructed even within a substantial isotropic background.Comment: 14 pages, 15 figure
Heteroepitaxial growth of ZnO branches selectively on TiO2 nanorod tips with improved light harvesting performance
A seeded heteroepitaxial growth of ZnO nanorods selectively on TiO2 nanorod tips was achieved by restricting crystal growth on highly hydrophobic TiO2 nanorod film surfaces. Intriguing light harvesting performance and efficient charge transport efficiency has been found, which suggest potential applications in photovoltaics and optoelectronics
Yukawa Couplings for the Spinning Particle and the World Line Formalism
We construct the world-line action for a Dirac particle coupled to a
classical scalar or pseudo-scalar background field. This action can be used to
compute loop diagrams and the effective action in the Yukawa model using the
world-line path-integral formalism for spinning particles.Comment: 10 pages Latex, two uuencoded postscript figures. Note added at the
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