538 research outputs found
50 Years Later: Women, Work & the Work Ahead (Infographic)
How have things changed for women in the labor force over the last 50 years
Particle Image Thermometry for Natural Convection Flows
Particle Image Thermometry (PIT) is a technique by which temperature fields can be obtained non-invasively using thermochromic liquid crystals (TLCs) through image processing of experimental true-colour photographs. This is done using a calibration curve (hue versus temperature). With the calibration data, every pixel of the colour photograph is transformed to a temperature value, and thus accurate experimental temperature maps are obtained. Using this technique, examples of steady and unsteady natural convection are presented, which include steady magnetic convection of paramagnetic fluids in a cubic enclosure heated and cooled from opposite walls, and unsteady convective flows in a reservoir model cooled from above (night-time cooling). The instantaneous measurement of temperature fields is very useful for understanding flow characteristics in situations where conventional flow visualisation is not sufficient. This method also provides additional quantitative information for comparisons with numerical modelling
How expensive space-zero-gravity convection experiments can be carried out in terrestrial conditions – magnetic convection of a paramagnetic fluid
Over the last decade or so it has became possible to build high-temperature super-conducting magnets that operate in a laboratory environment. Many new phenomena connected with strong magnetic fields have been reported (e.g. promotion of combustion, magnetic levitation, separation methods for weakly magnetic materials etc.). There are many applications of the use of magnetic force on the Earth. For instance, knowing how to control such a force makes it possible to negate the influence of the gravitational force and study a particular phenomenon as it would occur in the Cosmos, but under terrestrial conditions, avoiding the need for expensive space travel. The use of a magnetic field may also help in many processes such as crystal growth, mixing and material processing. The present work is concerned with magnetic convection of a paramagnetic fluid in a cubical enclosure heated and cooled from the sidewalls. The influence of a 10-T magnetic field on the convection mode of the paramagnetic fluid and the heat transfer rate were investigated numerically and experimentally, and compared with gravitational natural convection. The present study clearly shows that natural convection can be enhanced, and the direction of the convection flow can be changed using a strong magnetic field in terrestrial conditions
Cosmic-ray Acceleration at Ultrarelativistic Shock Waves: Effects of a "Realistic" Magnetic Field Structure
First-order Fermi acceleration processes at ultrarelativistic shocks are
studied with Monte Carlo simulations. The accelerated particle spectra are
derived by integrating the exact particle trajectories in a turbulent magnetic
field near the shock. ''Realistic'' features of the field structure are
included. We show that the main acceleration process at superluminal shocks is
the particle compression at the shock. Formation of energetic spectral tails is
possible in a limited energy range only for highly perturbed magnetic fields,
with cutoffs occuring at low energies within the resonance energy range
considered. These spectral features result from the anisotropic character of
particle transport in the downstream magnetic field, where field compression
produces effectively 2D perturbations. Because of the downstream field
compression, the acceleration process is inefficient in parallel shocks for
larger turbulence amplitudes, and features observed in oblique shocks are
recovered. For small-amplitude turbulence, wide-energy range particle spectra
are formed and modifications of the process due to the existence of long-wave
perturbations are observed. In both sub- and superluminal shocks, an increase
of \gamma leads to steeper spectra with lower cut-off energies. The spectra
obtained for the ``realistic'' background conditions assumed here do not
converge to the ``universal'' spectral index claimed in the literature. Thus
the role of the first-order Fermi process in astrophysical sources hosting
relativistic shocks requires serious reanalysis.Comment: submitted to Ap
Cosmic Ray Acceleration at Relativistic Shock Waves with a "Realistic" Magnetic Field Structure
The process of cosmic ray first-order Fermi acceleration at relativistic
shock waves is studied with the method of Monte Carlo simulations. The
simulations are based on numerical integration of particle equations of motion
in a turbulent magnetic field near the shock. In comparison to earlier studies,
a few "realistic" features of the magnetic field structure are included. The
upstream field consists of a mean field component inclined at some angle to the
shock normal with finite-amplitude sinusoidal perturbations imposed upon it.
The perturbations are assumed to be static in the local plasma rest frame.
Their flat or Kolmogorov spectra are constructed with randomly drawn wave
vectors from a wide range . The downstream field structure
is derived from the upstream one as compressed at the shock. We present
particle spectra and angular distributions obtained at mildly relativistic sub-
and superluminal shocks and also parallel shocks. We show that particle spectra
diverge from a simple power-law, the exact shape of the spectrum depends on
both the amplitude of the magnetic field perturbations and the wave power
spectrum. Features such as spectrum hardening before the cut-off at oblique
subluminal shocks and formation of power-law tails at superluminal ones are
presented and discussed. At parallel shocks, the presence of finite-amplitude
magnetic field perturbations leads to the formation of locally oblique field
configurations at the shock and the respective magnetic field compressions.
This results in the modification of the particle acceleration process,
introducing some features present in oblique shocks, e.g., particle reflections
from the shock. We demonstrate for parallel shocks a (nonmonotonic) variation
of the particle spectral index with the turbulence amplitude.Comment: revised version (37 pages, 13 figures
A stochastic model of jaguar abundance in the Peruvian Amazon under climate variation scenarios
The jaguar (Panthera onca) is the dominant predator in Central and South America, but is now considered near-threatened. Estimating jaguar population size is difficult, due to uncertainty in the underlying dynamical processes as well as highly variable and sparse data. We develop a stochastic temporal model of jaguar abundance in the Peruvian Amazon, taking into account prey availability, under various climate change scenarios. The model is calibrated against existing data sets and an elicitation study in Pacaya Samiria. In order to account for uncertainty and variability, we construct a population of models over four key parameters, namely three scaling parameters for aquatic, small land, and large land animals and a hunting index. We then use this population of models to construct probabilistic evaluations of jaguar populations under various climate change scenarios characterized by increasingly severe flood and drought events and discuss the implications on jaguar numbers. Results imply that jaguar populations exhibit some robustness to extreme drought and flood, but that repeated exposure to these events over short periods can result in rapid decline. However, jaguar numbers could return to stability—albeit at lower numbers—if there are periods of benign climate patterns and other relevant factors are conducive
Triple coalescence singularity in a dynamical atomic process
We show that the high energy limit for the amplitude of the double electron
capture to the bound state of the Coulomb field of a nucleus with emission of a
single photon is determined by behavior of the wave function in the vicinity of
the singular triple coalescence point.Comment: 3 page
Particle Acceleration at Relativistic Shocks
I review the current status of Fermi acceleration theory at relativistic
shocks. I first discuss the relativistic shock jump conditions, then describe
the non-relativistic Fermi mechanism and the differences introduced by
relativistic flows. I present numerical calculations of the accelerated
particle spectrum, and examine the maximum energy attainable by this process. I
briefly consider the minimum energy for Fermi acceleration, and a possible
electron pre-acceleration mechanism.Comment: 17 pages, 4 figures. To appear in "Relativistic Flows in
Astrophysics", A.W. Guthmann, M. Georganopoulos, A. Marcowith and K.
Manolokou, eds., Lecture Notes in Pysics, Springer Verla
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