12,760 research outputs found
Shock-like solutions of the electrostatic Vlasov equation
Shock like solutions of electrostatic Vlasov equatio
Discrete spectra and damped waves in quasilinear theory
Discrete spectra and damped waves in quasilinear theor
Magnetic dynamo action in two-dimensional turbulent magneto-hydrodynamics
Two-dimensional magnetohydrodynamic turbulence is explored by means of numerical simulation. Previous analytical theory, based on non-dissipative constants of the motion in a truncated Fourier representation, is verified by following the evolution of highly non-equilibrium initial conditions numerically. Dynamo action (conversion of a significant fraction of turbulent kinetic energy into long-wavelength magnetic field energy) is observed. It is conjectured that in the presence of dissipation and external forcing, a dual cascade will be observed for zero-helicity situations. Energy will cascade to higher wave numbers simultaneously with a cascade of mean square vector potential to lower wave numbers, leading to an omni-directional magnetic energy spectrum which varies as 1/k 3 at lower wave numbers, simultaneously with a buildup of magnetic excitation at the lowest wave number of the system. Equipartition of kinetic and magnetic energies is expected at the highest wave numbers in the system
Applications of spectral methods to turbulent magnetofluids in space and fusion research
Recent and potential applications of spectral method computation to incompressible, dissipative magnetohydrodynamics are surveyed. Linear stability problems for one dimensional, quasi-equilibria are approachable through a close analogue of the Orr-Sommerfeld equation. It is likely that for Reynolds-like numbers above certain as-yet-undetermined thresholds, all magnetofluids are turbulent. Four recent effects in MHD turbulence are remarked upon, as they have displayed themselves in spectral method computations: (1) inverse cascades; (2) small-scale intermittent dissipative structures; (3) selective decays of ideal global invariants relative to each other; and (4) anisotropy induced by a mean dc magnetic field. Two more conjectured applications are suggested. All the turbulent processes discussed are sometimes involved in current carrying confined fusion magnetoplasmas and in space plasmas
Small scale structures in three-dimensional magnetohydrodynamic turbulence
We investigate using direct numerical simulations with grids up to 1536^3
points, the rate at which small scales develop in a decaying three-dimensional
MHD flow both for deterministic and random initial conditions. Parallel current
and vorticity sheets form at the same spatial locations, and further
destabilize and fold or roll-up after an initial exponential phase. At high
Reynolds numbers, a self-similar evolution of the current and vorticity maxima
is found, in which they grow as a cubic power of time; the flow then reaches a
finite dissipation rate independent of Reynolds number.Comment: 4 pages, 3 figure
Velocity field distributions due to ideal line vortices
We evaluate numerically the velocity field distributions produced by a
bounded, two-dimensional fluid model consisting of a collection of parallel
ideal line vortices. We sample at many spatial points inside a rigid circular
boundary. We focus on ``nearest neighbor'' contributions that result from
vortices that fall (randomly) very close to the spatial points where the
velocity is being sampled. We confirm that these events lead to a non-Gaussian
high-velocity ``tail'' on an otherwise Gaussian distribution function for the
Eulerian velocity field. We also investigate the behavior of distributions that
do not have equilibrium mean-field probability distributions that are uniform
inside the circle, but instead correspond to both higher and lower mean-field
energies than those associated with the uniform vorticity distribution. We find
substantial differences between these and the uniform case.Comment: 21 pages, 9 figures. To be published in Physical Review E
(http://pre.aps.org/) in May 200
A numerical study of the alpha model for two-dimensional magnetohydrodynamic turbulent flows
We explore some consequences of the ``alpha model,'' also called the
``Lagrangian-averaged'' model, for two-dimensional incompressible
magnetohydrodynamic (MHD) turbulence. This model is an extension of the
smoothing procedure in fluid dynamics which filters velocity fields locally
while leaving their associated vorticities unsmoothed, and has proved useful
for high Reynolds number turbulence computations. We consider several known
effects (selective decay, dynamic alignment, inverse cascades, and the
probability distribution functions of fluctuating turbulent quantities) in
magnetofluid turbulence and compare the results of numerical solutions of the
primitive MHD equations with their alpha-model counterparts' performance for
the same flows, in regimes where available resolution is adequate to explore
both. The hope is to justify the use of the alpha model in regimes that lie
outside currently available resolution, as will be the case in particular in
three-dimensional geometry or for magnetic Prandtl numbers differing
significantly from unity. We focus our investigation, using direct numerical
simulations with a standard and fully parallelized pseudo-spectral method and
periodic boundary conditions in two space dimensions, on the role that such a
modeling of the small scales using the Lagrangian-averaged framework plays in
the large-scale dynamics of MHD turbulence. Several flows are examined, and for
all of them one can conclude that the statistical properties of the large-scale
spectra are recovered, whereas small-scale detailed phase information (such as
e.g. the location of structures) is lost.Comment: 22 pages, 20 figure
MORPHOLOGY AND FUNCTION OF CELLS OF HUMAN EMBRYONIC LIVER IN MONOLAYER CULTURE
A system for culturing human fetal liver cells in monolayers is described and the effects of various conditions of growth on the morphology and function of the cultured cells are presented. The addition of 10% calf serum or 1% human serum to the growth medium accelerated the proliferation of the liver cells, with subsequent loss of characteristic morphology and specific functional activity. In the absence of serum, the cultured liver cells retained their morphology and their function for at least 4 wk, as evidenced by secretion of serum albumin and storage of glycogen and iron
New types of bialgebras arising from the Hopf equation
New types of bialgebras arising from the Hopf equation (pentagonal equation)
are introduced and studied. They will play from the Hopf equation the same role
as the co-quasitriangular do from the quantum Yang Baxter equation.Comment: Latex2e, Comm Algebra, in pres
Seasat data utilization project
During the three months of orbital operations, the satellite returned data from the world's oceans. Dozens of tropical storms, hurricanes and typhoons were observed, and two planned major intensive surface truth experiments were conducted. The utility of the Seasat-A microwave sensors as oceanographic tools was determined. Sensor and geophysical evaluations are discussed, including surface observations, and evaluation summaries of an altimeter, a scatterometer, a scanning multichannel microwave radiometer, a synthetic aperture radar, and a visible and infrared radiometer
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