38,480 research outputs found
Poynting Flux Dominated Jets in Decreasing Density Atmospheres. I. The Non-relativistic Current-driven Kink Instability and the Formation of "Wiggled" Structures
Non-relativistic three-dimensional magnetohydrodynamical (MHD) simulations of
Poynting flux dominated (PFD) jets are presented. Our study focuses on the
propagation of strongly magnetized hypersonic, but sub-Alfv\'enic () flow and on the subsequent development
of a current-driven (CD) kink instability. This instability may be responsible
for the ``wiggled'' structures seen in sub-parsec scale (VLBI) jets. In the
present paper, we investigate the nonlinear behavior of PFD jets in a variety
of external ambient magnetized gas distributions, including those with density,
pressure, and temperature gradients. Our numerical results show that the jets
can develop CD distortions in the trans-Alfv\'enic flow case, even when the
flow itself is still strongly magnetically dominated. An internal
non-axisymmetric body mode grows on time scales of order of the Alfv\'en
crossing time and distorts the structure and magnetic configuration of the jet.
The kink () mode of the CD instability, driven by the radial component of
the Lorentz force, grows faster than other higher order modes (). In the
jet frame the mode grows locally and expands radially at each axial position
where the jet is unstable: the instability, therefore, does not propagate as a
wave along the jet length. A naturally-occurring, external helically magnetized
wind, which is (quasi-) axially current-free, surrounds the well-collimated
current-carrying jet and reduces velocity shear between the jet and external
medium. This stabilizes the growth of MHD Kelvin-Helmholtz surface modes in the
inner jet flow.Comment: 70 pages, 23 figures, 3 tables, Appendix, submitted to Ap
3-D Simulations of MHD Jets - The Stability Problem
Non-relativistic three-dimensional magnetohydrodynamic simulations of
Poynting-flux-dominated (PFD) jets are presented. Our study focuses on the
propagation of strongly magnetized hypersonic but sub-Alfv\'enic flow () and the development of a current-driven
(CD) kink instability. This instability may be responsible for the "wiggled"
structures seen in VLBI-scale AGN jets. In the present paper we investigate the
nonlinear behavior of PFD jets in a variety of external ambient magnetized gas
distributions, including those with density, pressure, and temperature
gradients. Our numerical results show that PFD jets can develop kink
distortions in the trans-Alfv\'enic flow case, even when the flow itself is
still strongly magnetically dominated. In the nonlinear development of the
instability, a non-axisymmetric mode grows on time scales of order the Alfv\'en
crossing time (in the jet frame) and proceeds to disrupt the kinematic and
magnetic structure of the jet. Because of a large scale poloidal magnetic field
in the ambient medium, the growth of surface modes ({\it i.e.}, MHD
Kelvin-Helmholtz instabilities) is suppressed. The CD kink mode () grows
faster than the other higher order modes (), driven in large part by the
radial component of the Lorentz force.Comment: 6 pages, 3 figures; to appear in Plasmas in the Laboratory and in the
Universe, Como, Italy, 16-19 Sep, 200
Synopsis of biological data on the cobia Rachycentron canadum (Pisces: Rachycentridae)
Information on the biology and fisheries of cobia, Rachycentron canadum, is compiled and reviewed in the FAD species synopsis style. Topics include taxonomy, morphology, distribution, reproduction, pre-adult and adult stages, food, growth, migration, population characteristics, and various aspects of exploitation.
Data and information were obtained from unpublished as
well as published sources.
Cobia, the only species in the family Rachycentridae, is a
migratory pelagic fish that occurs in tropical and subtropical seas of the world, except in the central and eastern Pacific Ocean. In the western Atlantic Ocean, spawning occurs during the warm months. Eggs and larvae are planktonic. Females grow faster than males: at 1 year, females are 36 cm FL and 0.4 kg; at 4 years, 99 cm and 11 kg; and at 8 years, 137 cm and 31 kg. Comparable data for males are: at 1 year, 31 cm and 0.3 kg; 4 years, 82 cm and 6 kg; and 8 years, 108 cm and 15 kg. Sexual maturity is attained by males at about 52 cm FL in their second
year and by females at about 70 cm in their third year. Fecundity for females 100-125 cm FL varies from 1.9 to 5.4 million eggs. Cobia favor crustaceans for food, but will feed on other invertebrates and fishes as well. They attain a maximum size of over 60 kg. Cobia are fished both commercially and recreationally. Commercially, they are usually caught incidentally in both hook-and-Iine and net fISheries. In the United States, which ranks behind Pakistan, Mexico, and the Philippines in commercial
production of cobia, recreational landings exceed
commercial landings by more than ten-fold. (PDF file contains 32 pages.
A new solution-adaptive grid generation method for transonic airfoil flow calculations
The clustering algorithm is controlled by a second-order, ordinary differential equation which uses the airfoil surface density gradient as a forcing function. The solution to this differential equation produces a surface grid distribution which is automatically clustered in regions with large gradients. The interior grid points are established from this surface distribution by using an interpolation scheme which is fast and retains the desirable properties of the original grid generated from the standard elliptic equation approach
Perturbations of Matter Fields in the Second-order Gauge-invariant Cosmological Perturbation Theory
Some formulae for the perturbations of the matter fields are summarized
within the framework of the second-order gauge-invariant cosmological
perturbation theory in a four dimensional homogeneous isotropic universe, which
is developed in the papers [K.Nakamura, Prog.Theor.Phys., 117 (2007), 17.]. We
derive the formulae for the perturbations of the energy momentum tensors and
equations of motion for a perfect fluid, an imperfect fluid, and a signle
scalar field, and show that all equations are derived in terms of
gauge-invariant variables without any gauge fixing.Comment: (v1) 76 pages, no figure; (v2) minor revision, typos are corrected,
references are added; (v3) Title is changed, Compactified into 55 pages,
Comment on the comparison with the other work is added; (v4)typos are
correcte
Symmetry and Codon Usage Correlations in the Genetic Code
The ratios of the codon usage in the quartets and sextets for the vertebrate
series exhibit a correlated behaviour which fits naturally in the framework of
the crystal basis model of the genetic code. Moreover the observed universal
behaviour of these suitably normalized ratios can be easily explained.Comment: 6 figures, documen
Lightweight refractory insulation and method of preparing the same Patent
Preparation and characteristics of lightweight refractory insulatio
Modeling the emergence of a new language: Naming Game with hybridization
In recent times, the research field of language dynamics has focused on the
investigation of language evolution, dividing the work in three evolutive
steps, according to the level of complexity: lexicon, categories and grammar.
The Naming Game is a simple model capable of accounting for the emergence of a
lexicon, intended as the set of words through which objects are named. We
introduce a stochastic modification of the Naming Game model with the aim of
characterizing the emergence of a new language as the result of the interaction
of agents. We fix the initial phase by splitting the population in two sets
speaking either language A or B. Whenever the result of the interaction of two
individuals results in an agent able to speak both A and B, we introduce a
finite probability that this state turns into a new idiom C, so to mimic a sort
of hybridization process. We study the system in the space of parameters
defining the interaction, and show that the proposed model displays a rich
variety of behaviours, despite the simple mean field topology of interactions.Comment: 12 pages, 10 figures, presented at IWSOS 2013 Palma de Mallorca, the
final publication will be available at LNCS http://www.springer.com/lnc
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