8,746 research outputs found
Solution to the Equations of the Moment Expansions
We develop a formula for matching a Taylor series about the origin and an
asymptotic exponential expansion for large values of the coordinate. We test it
on the expansion of the generating functions for the moments and connected
moments of the Hamiltonian operator. In the former case the formula produces
the energies and overlaps for the Rayleigh-Ritz method in the Krylov space. We
choose the harmonic oscillator and a strongly anharmonic oscillator as
illustrative examples for numerical test. Our results reveal some features of
the connected-moments expansion that were overlooked in earlier studies and
applications of the approach
Three-dimensional Simulations of Disk Accretion to an Inclined Dipole: I. Magnetospheric Flow at Different Theta
We present results of fully three-dimensional MHD simulations of disk
accretion to a rotating magnetized star with its dipole moment inclined at an
angle Theta to the rotation axis of the disk. We observed that matter accretes
from the disk to a star in two or several streams depending on Theta. Streams
may precess around the star at small Theta. The inner regions of the disk are
warped. The warping is due to the tendency of matter to co-rotate with inclined
magnetosphere. The accreting matter brings positive angular momentum to the
(slowly rotating) star tending to spin it up. The corresponding torque N_z
depends only weakly on Theta. The angular momentum flux to the star is
transported predominantly by the magnetic field; the matter component
contributes < 1 % of the total flux. Results of simulations are important for
understanding the nature of classical T Tauri stars, cataclysmic variables, and
X-ray pulsars.Comment: 26 pages, 22 figures, LaTeX, macros: emulapj.sty, avi simulations are
available at http://www.astro.cornell.edu/us-rus/inclined.ht
q-Breathers and the Fermi-Pasta-Ulam Problem
The Fermi-Pasta-Ulam (FPU) paradox consists of the nonequipartition of energy
among normal modes of a weakly anharmonic atomic chain model. In the harmonic
limit each normal mode corresponds to a periodic orbit in phase space and is
characterized by its wave number . We continue normal modes from the
harmonic limit into the FPU parameter regime and obtain persistence of these
periodic orbits, termed here -Breathers (QB). They are characterized by time
periodicity, exponential localization in the -space of normal modes and
linear stability up to a size-dependent threshold amplitude. Trajectories
computed in the original FPU setting are perturbations around these exact QB
solutions. The QB concept is applicable to other nonlinear lattices as well.Comment: 4 pages, 4 figure
Properties and nature of Be stars: 29. Orbital and long-term spectral variations of \gamma\ Cassiopei\ae
A detailed analysis of more than 800 electronic high-resolution spectra of
gamma Cas, which were obtained during a time interval of over 6000 days (16.84
yrs) at several observatories, documents the smooth variations in the density
and/or extend of its circumstellar envelope. We found a clear anticorrelation
between the peak intensity and FWHM of the H alpha emission, which seems to
agree with recent models of such emission lines. The main result of this study
is a confirmation of the binary nature of the object, determination of a
reliable linear ephemeris T_{min.RV} = HJD (2452081.90.6) +
(203.520.08)*E, and a rather definitive set of orbital elements. We
clearly demonstrated that the orbit is circular within the limits of accuracy
of our measurements and has a semi-amplitude of radial-velocity curve of
4.300.09 (km/s). No trace of the low-mass secondary was found. The time
distribution of our spectra does not allow a reliable investigation of rapid
spectral variations, which are undoubtedly present in the spectra. We postpone
this investigation for a future study, based on series of dedicated whole-night
spectral observations
Multiple stressors: using the honeybee model BEEHAVE to explore how spatial and temporal forage stress affects colony resilience
The causes underlying the increased mortality of honeybee colonies (Apis mellifera) observed over the past decade remain unclear. Since so far the evidence for monocausal explanations is equivocal, involvement of multiple stressors is generally assumed. We here focus on various aspects of forage availability, which have received less attention than other stressors because it is virtually impossible to explore them empirically. We applied the colony model BEEHAVE, which links within-hive dynamics and foraging, to stylized landscape settings to explore how foraging distance, forage supply, and “forage gaps”, i.e. periods in which honeybees cannot find any nectar and pollen, affect colony resilience and the mechanisms behind. We found that colony extinction was mainly driven by foraging distance, but the timing of forage gaps had strongest effects on time to extinction. Sensitivity to forage gaps of 15 days was highest in June or July even if otherwise forage availability was sufficient to survive. Forage availability affected colonies via cascading effects on queen's egg-laying rate, reduction of new-emerging brood stages developing into adult workers, pollen debt, lack of workforce for nursing, and reduced foraging activity. Forage gaps in July led to reduction in egg-laying and increased mortality of brood stages at a time when the queen's seasonal egg-laying rate is at its maximum, leading to colony failure over time. Our results demonstrate that badly timed forage gaps interacting with poor overall forage supply reduce honeybee colony resilience. Existing regulation mechanisms which in principle enable colonies to cope with varying forage supply in a given landscape and year, such as a reduction in egg-laying, have only a certain capacity. Our results are hypothetical, as they are obtained from simplified landscape settings, but they are consistent with existing empirical knowledge. They offer ample opportunities for testing the predicted effects of forage stress in controlled experiments
Reentrant superconductivity in superconductor/ferromagnetic-alloy bilayers
We studied the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) like state
establishing due to the proximity effect in superconducting Nb/Cu41Ni59
bilayers. Using a special wedge-type deposition technique, series of 20-35
samples could be fabricated by magnetron sputtering during one run. The layer
thickness of only a few nanometers, the composition of the alloy, and the
quality of interfaces were controlled by Rutherford backscattering
spectrometry, high resolution transmission electron microscopy, and Auger
spectroscopy. The magnetic properties of the ferromagnetic alloy layer were
characterized with superconducting quantum interference device (SQUID)
magnetometry. These studies yield precise information about the thickness, and
demonstrate the homogeneity of the alloy composition and magnetic properties
along the sample series. The dependencies of the critical temperature on the Nb
and Cu41Ni59 layer thickness, Tc(dS) and Tc(dF), were investigated for constant
thickness dF of the magnetic alloy layer and dS of the superconducting layer,
respectively. All types of non-monotonic behaviors of Tc versus dF predicted by
the theory could be realized experimentally: from reentrant superconducting
behavior with a broad extinction region to a slight suppression of
superconductivity with a shallow minimum. Even a double extinction of
superconductivity was observed, giving evidence for the multiple reentrant
behavior predicted by theory. All critical temperature curves were fitted with
suitable sets of parameters. Then, Tc(dF) diagrams of a hypothetical F/S/F
spin-switch core structure were calculated using these parameters. Finally,
superconducting spin-switch fabrication issues are discussed in detail in view
of the achieved results.Comment: 34 pages, 9 figure
Interdisciplinary design methodology for systems of mechatronic systems focus on highly dynamic environmental applications
This paper discusses a series of research challenges in the design of systems of mechatronic systems. A focus is given to environmental mechatronic applications within the chain “Renewable energy production - Smart grids - Electric vehicles”. For the considered mechatronic systems, the main design targets are formulated, the relations to state and parameter estimation, disturbance observation and rejection as well as control algorithms are highlighted. Finally, the study introduces an interdisciplinary design approach based on the intersectoral transfer of knowledge and collaborative experimental activities
Neural network based automatic limit prediction and avoidance system and method
A method for performance envelope boundary cueing for a vehicle control system comprises the steps of formulating a prediction system for a neural network and training the neural network to predict values of limited parameters as a function of current control positions and current vehicle operating conditions. The method further comprises the steps of applying the neural network to the control system of the vehicle, where the vehicle has capability for measuring current control positions and current vehicle operating conditions. The neural network generates a map of current control positions and vehicle operating conditions versus the limited parameters in a pre-determined vehicle operating condition. The method estimates critical control deflections from the current control positions required to drive the vehicle to a performance envelope boundary. Finally, the method comprises the steps of communicating the critical control deflection to the vehicle control system; and driving the vehicle control system to provide a tactile cue to an operator of the vehicle as the control positions approach the critical control deflections
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