32,441 research outputs found
How strong are the Rossby vortices?
The Rossby wave instability, associated with density bumps in differentially
rotating discs, may arise in several different astrophysical contexts, such as
galactic or protoplanetary discs. While the linear phase of the instability has
been well studied, the nonlinear evolution and especially the saturation phase
remain poorly understood. In this paper, we test the non-linear saturation
mechanism analogous to that derived for wave-particle interaction in plasma
physics. To this end we perform global numerical simulations of the evolution
of the instability in a two-dimensional disc. We confirm the physical mechanism
for the instability saturation and show that the maximum amplitude of vorticity
can be estimated as twice the linear growth rate of the instability. We provide
an empirical fitting formula for this growth rate for various parameters of the
density bump. We also investigate the effects of the azimuthal mode number of
the instability and the energy leakage in the spiral density waves. Finally, we
show that our results can be extrapolated to 3D discs.Comment: Accepted for publication in MNRA
Single side damage simulations and detection in beam-like structures
Beam-like structures are the most common components in real engineering, while single side damage is often encountered. In this study, a numerical analysis of single side damage in a free-free beam is analysed with three different finite element models; namely solid, shell and beam models for demonstrating their performance in simulating real structures. Similar to experiment, damage is introduced into one side of the beam, and natural frequencies are extracted from the simulations and compared with experimental and analytical results. Mode shapes are also analysed with modal assurance criterion. The results from simulations reveal a good performance of the three models in extracting natural frequencies, and solid model performs better than shell while shell model performs better than beam model under intact state. For damaged states, the natural frequencies captured from solid model show more sensitivity to damage severity than shell model and shell model performs similar to the beam model in distinguishing damage. The main contribution of this paper is to perform a comparison between three finite element models and experimental data as well as analytical solutions. The finite element results show a relatively well performanc
Forebrain Origins of Glutamatergic Innervation to the Rat Paraventricular Nucleus of the Hypothalamus: Differential Inputs to the Anterior Versus Posterior Subregions
The hypothalamic paraventricular nucleus (PVN) regulates numerous homeostatic systems and functions largely under the influence of forebrain inputs. Glutamate is a major neurotransmitter in forebrain, and glutamate neurosignaling in the PVN is known to mediate many of its functions. Previous work showed that vesicular glutamate transporters (VGluTs; specific markers for glutamatergic neurons) are expressed in forebrain sites that project to the PVN; however, the extent of this presumed glutamatergic innervation to the PVN is not clear. In the present study retrograde FluoroGold (FG) labeling of PVN-projecting neurons was combined with in situ hybridization for VGluT1 and VGluT2 mRNAs to identify forebrain regions that provide glutamatergic innervation to the PVN and its immediate surround in rats, with special consideration for the sources to the anterior versus posterior PVN. VGluT1 mRNA colocalization with retrogradely labeled FG neurons was sparse. VGluT2 mRNA colocalization with FG neurons was most abundant in the ventromedial hypothalamus after anterior PVN FG injections, and in the lateral, posterior, dorsomedial, and ventromedial hypothalamic nuclei after posterior PVN injections. Anterograde tract tracing combined with VGluT2 immunolabeling showed that 1) ventromedial nucleus-derived glutamatergic inputs occur in both the anterior and posterior PVN; 2) posterior nucleus-derived glutamatergic inputs occur predominantly in the posterior PVN; and 3) medial preoptic nucleus-derived inputs to the PVN are not glutamatergic, thereby corroborating the innervation pattern seen with retrograde tracing. The results suggest that PVN subregions are influenced by varying amounts and sources of forebrain glutamatergic regulation, consistent with functional differentiation of glutamate projections. J. Comp. Neurol. 519:1301–1319, 2011. © 2010 Wiley-Liss, Inc
Tidal Interaction between a Fluid Star and a Kerr Black Hole in Circular Orbit
We present a semi-analytic study of the equilibrium models of close binary
systems containing a fluid star (mass and radius ) and a Kerr black
hole (mass ) in circular orbit. We consider the limit where
spacetime is described by the Kerr metric. The tidally deformed star is
approximated by an ellipsoid, and satisfies the polytropic equation of state.
The models also include fluid motion in the stellar interior, allowing binary
models with nonsynchronized stellar spin (as expected for coalescing neutron
star-black hole binaries) to be constructed. Tidal disruption occurs at orbital
radius , but the dimensionless ratio depends on the spin parameter of
the black hole as well as on the equation of state and the internal rotation of
the star. We find that the general relativistic tidal field disrupts the star
at a larger than the Newtonian tide; the difference is
particularly prominent if the disruption occurs in the vicinity of the black
hole's horizon. In general, is smaller for a (prograde
rotating) Kerr black hole than for a Schwarzschild black hole. We apply our
results to coalescing black hole-neutron star and black hole-white dwarf
binaries. The tidal disruption limit is important for characterizing the
expected gravitational wave signals and is relevant for determining the
energetics of gamma ray bursts which may result from such disruption.Comment: 29 pages including 8 figures. Minor changes and update. To appear in
ApJ, March 20, 2000 (Vol.532, #1
Coulomb Drag near the metal-insulator transition in two-dimensions
We studied the drag resistivity between dilute two-dimensional hole systems,
near the apparent metal-insulator transition. We find the deviations from the
dependence of the drag to be independent of layer spacing and
correlated with the metalliclike behavior in the single layer resistivity,
suggesting they both arise from the same origin. In addition, layer spacing
dependence measurements suggest that while the screening properties of the
system remain relatively independent of temperature, they weaken significantly
as the carrier density is reduced. Finally, we demonstrate that the drag itself
significantly enhances the metallic dependence in the single layer
resistivity.Comment: 6 pages, 5 figures; revisions to text, to appear in Phys. Rev.
Post-Newtonian Models of Binary Neutron Stars
Using an energy variational method, we calculate quasi-equilibrium
configurations of binary neutron stars modeled as compressible triaxial
ellipsoids obeying a polytropic equation of state. Our energy functional
includes terms both for the internal hydrodynamics of the stars and for the
external orbital motion. We add the leading post-Newtonian (PN) corrections to
the internal and gravitational energies of the stars, and adopt hybrid orbital
terms which are fully relativistic in the test-mass limit and always accurate
to PN order. The total energy functional is varied to find quasi-equilibrium
sequences for both corotating and irrotational binaries in circular orbits. We
examine how the orbital frequency at the innermost stable circular orbit
depends on the polytropic index n and the compactness parameter GM/Rc^2. We
find that, for a given GM/Rc^2, the innermost stable circular orbit along an
irrotational sequence is about 17% larger than the innermost secularly stable
circular orbit along the corotating sequence when n=0.5, and 20% larger when
n=1. We also examine the dependence of the maximum neutron star mass on the
orbital frequency and find that, if PN tidal effects can be neglected, the
maximum equilibrium mass increases as the orbital separation decreases.Comment: 53 pages, LaTex, 9 figures as 10 postscript files, accepted by Phys.
Rev. D, replaced version contains updated reference
Validity of numerical trajectories in the synchronization transition of complex systems
We investigate the relationship between the loss of synchronization and the
onset of shadowing breakdown {\it via} unstable dimension variability in
complex systems. In the neighborhood of the critical transition to strongly
non-hyperbolic behavior, the system undergoes on-off intermittency with respect
to the synchronization state. There are potentially severe consequences of
these facts on the validity of the computer-generated trajectories obtained
from dynamical systems whose synchronization manifolds share the same
non-hyperbolic properties.Comment: 4 pages, 4 figure
Controlling complex networks: How much energy is needed?
The outstanding problem of controlling complex networks is relevant to many
areas of science and engineering, and has the potential to generate
technological breakthroughs as well. We address the physically important issue
of the energy required for achieving control by deriving and validating scaling
laws for the lower and upper energy bounds. These bounds represent a reasonable
estimate of the energy cost associated with control, and provide a step forward
from the current research on controllability toward ultimate control of complex
networked dynamical systems.Comment: 4 pages paper + 5 pages supplement. accepted for publication in
Physical Review Letters;
http://link.aps.org/doi/10.1103/PhysRevLett.108.21870
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