18,468 research outputs found
Description of two new exotic species of the genus Chordodes
In the course of certain studies upon Gordiacea, I have determined two new species of Chordodes (CREPLIN), a genus closely allied to Gordius, from Borneo and Africa, and these forms are here described; the publication of the systematic results of my studies upon the american species of this group being postponed until I have an opportunity to compare more material. 1. Chordodes furnessi n. sp. 2. Chordodes albibarbatus n. sp
Recent Results and Perspectives at CDF and D0
Over the course of the past years the experimental measurements performed by
the two large collaborations, CDF and D{\O}, at the
Fermilab Tevatron Collider have fueled advances in our understanding of
physics at the energy frontier. At the present time the accelerator complex and
the two detectors are undergoing substantial improvements. In this paper, we
provide a discussion of some recent results which in turn provides a framework
within which we can look to future prospects.Comment: 22 pages, including 14 figures. Latex, crckapb.st
Resistive Magnetohydrodynamic Equilibria in a Torus
It was recently demonstrated that static, resistive, magnetohydrodynamic
equilibria, in the presence of spatially-uniform electrical conductivity, do
not exist in a torus under a standard set of assumed symmetries and boundary
conditions. The difficulty, which goes away in the ``periodic straight cylinder
approximation,'' is associated with the necessarily non-vanishing character of
the curl of the Lorentz force, j x B. Here, we ask if there exists a spatial
profile of electrical conductivity that permits the existence of zero-flow,
axisymmetric r esistive equilibria in a torus, and answer the question in the
affirmative. However, the physical properties of the conductivity profile are
unusual (the conductivity cannot be constant on a magnetic surface, for
example) and whether such equilibria are to be considered physically possible
remains an open question.Comment: 17 pages, 4 figure
Evidence of resonant mode coupling and the relationship between low and high frequencies in a rapidly rotating A star
In the theory of resonant mode coupling, the parent and child modes are
directly related in frequency and phase. The oscillations present in the fast
rotating Delta Scuti star KIC 8054146 allow us to test the most general and
generic aspects of such a theory. The only direct way to separate the parent
and coupled (child) modes is to examine the correlations in amplitude
variability between the different frequencies. For the dominant family of
related frequencies, only a single mode and a triplet are the origins of nine
dominant frequency peaks ranging from 2.93 to 66.30 cycles per day (as well as
dozens of small-amplitude combination modes and a predicted and detected third
high-frequency triplet). The mode-coupling model correctly predicts the large
amplitude variations of the coupled modes as a product of the amplitudes of the
parent modes, while the phase changes are also correctly modeled. This differs
from the behavior of 'normal' combination frequencies in that the amplitudes
are three orders of magnitude larger and may exceed even the amplitudes of the
parent modes. We show that two dominant low frequencies at 5.86 and 2.93 cycles
per day in the gravity-mode region are not harmonics of each other, and their
properties follow those of the almost equidistant high-frequency triplet. We
note that the previously puzzling situation of finding two strong peaks in the
low-frequency region related by nearly a factor of two in frequency has been
seen in other Delta Scuti stars as well.Comment: To be published in the Astrophysical Journa
Symmetry Reduction of Optimal Control Systems and Principal Connections
This paper explores the role of symmetries and reduction in nonlinear control
and optimal control systems. The focus of the paper is to give a geometric
framework of symmetry reduction of optimal control systems as well as to show
how to obtain explicit expressions of the reduced system by exploiting the
geometry. In particular, we show how to obtain a principal connection to be
used in the reduction for various choices of symmetry groups, as opposed to
assuming such a principal connection is given or choosing a particular symmetry
group to simplify the setting. Our result synthesizes some previous works on
symmetry reduction of nonlinear control and optimal control systems. Affine and
kinematic optimal control systems are of particular interest: We explicitly
work out the details for such systems and also show a few examples of symmetry
reduction of kinematic optimal control problems.Comment: 23 pages, 2 figure
Dynamic alignment and selective decay in MHD
Under some circumstances, incompressible magnetohydrodynamic turbulence will evolve toward a state in which the velocity fields and magnetic fields are aligned or anti-aligned. We propose a mechanism for this effect and illustrate with numerical computations. Under some other circumstances, the energy appears to decay selectively toward a minimum energy state in which the kinetic energy has disappeared. It has not been possible so far to identify a boundary in the phase space which divides the two regimes
Alternative statistical-mechanical descriptions of decaying two-dimensional turbulence in terms of "patches" and "points"
Numerical and analytical studies of decaying, two-dimensional (2D)
Navier-Stokes (NS) turbulence at high Reynolds numbers are reported. The effort
is to determine computable distinctions between two different formulations of
maximum entropy predictions for the decayed, late-time state. Both formulations
define an entropy through a somewhat ad hoc discretization of vorticity to the
"particles" of which statistical mechanical methods are employed to define an
entropy, before passing to a mean-field limit. In one case, the particles are
delta-function parallel "line" vortices ("points" in two dimensions), and in
the other, they are finite-area, mutually-exclusive convected "patches" of
vorticity which in the limit of zero area become "points." We use
time-dependent, spectral-method direct numerical simulation of the
Navier-Stokes equations to see if initial conditions which should relax to
different late-time states under the two formulations actually do so.Comment: 21 pages, 24 figures: submitted to "Physics of Fluids
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