143,729 research outputs found
Oscillation of higher order difference equations via comparison
In this paper we shall present some new oscillation criteria for difference equations of the form
x(n) + q(n)f(x[n - ]) = 0
and
x(n) = q(t)f(x[n - ]) + p(n)F(x[n + ])
via comparison with some difference equations of lower order whose oscillatory behavior are known
Non-radial oscillations of the rapidly rotating Be star HD 163868
We study the pulsational stability of the rapidly rotating Be star HD 163868
using a newly developed 2D oscillation code which takes the Coriolis force
fully into account and compare our results with observations (MOST) and recent
other stability analyses of this ~ 6 Msun star. We find both prograde and
retrograde overstable modes (although more prograde than retrograde modes) and
confirm the existence of low degree odd r-modes destabilised by the
kappa-mechanism. The ultra-low frequency modes that could not be explained in a
previous analysis are interpreted as high degree, retrograde m=1 modes. A
reasonably good fit to the observed oscillation spectrum is possible if we
assume that only even modes are observed. This requires a nearly equator-on
view of the observed star, consistent with the measured high v sin i value of
250 km/s.Comment: 7 pages, 7 figures; accepted by A&
Augmenting Numerical Stability of the Galerkin Finite Element Formulation for Electromagnetic Flowmeter Analysis
The magnetic flow meter is one of the best possible choice for the
measurement of flow rate of liquid metals in fast breeder reactors. Due to the
associated complexities in the measuring environment, theoretical evaluation of
their sensitivity is always preferred. In order to consider the 3D nature of
the problem and the general flow patterns, numerical field computational
approach is inevitable. When classical Galerkin's finite element formulation is
employed for the solution, it is known to introduce numerical oscillations at
high flow rates. The magnetic field produced by the flow induced currents
circulate within the fluid and forms the source of this numerical problem. To
overcome this, modified methods like stream-line upwind Petrov-Galerkin schemes
are generally suggested in the allied areas like fluid dynamics, in which a
similar dominance of advective (curl or circulation) component occurs over
diffusion (divergence) component. After a careful analysis of the numerical
instability through a reduced one dimensional problem, an elegant stable
approach is devised. In this scheme, a pole-zero cancellation approach is
adopted. The proposed scheme is shown to be absolutely stable. However, at
lower flow rates numerical results exhibits small oscillation, which can be
controlled by reducing the element size. The source of stability at higher flow
rates, as well as, oscillations at lower flow rates are analysed using
analytical solution of the associated difference equation. Finally the proposed
approach is applied to the original flow meter problem and the solution is
shown to be stable.Comment: IET Science, Measurement & Technology, 201
The Frequency Dependence of Critical-velocity Behavior in Oscillatory Flow of Superfluid Helium-4 Through a 2-micrometer by 2-micrometer Aperture in a Thin Foil
The critical-velocity behavior of oscillatory superfluid Helium-4 flow
through a 2-micrometer by 2-micrometer aperture in a 0.1-micrometer-thick foil
has been studied from 0.36 K to 2.10 K at frequencies from less than 50 Hz up
to above 1880 Hz. The pressure remained less than 0.5 bar. In early runs during
which the frequency remained below 400 Hz, the critical velocity was a
nearly-linearly decreasing function of increasing temperature throughout the
region of temperature studied. In runs at the lowest frequencies, isolated 2 Pi
phase slips could be observed at the onset of dissipation. In runs with
frequencies higher than 400 Hz, downward curvature was observed in the decrease
of critical velocity with increasing temperature. In addition, above 500 Hz an
alteration in supercritical behavior was seen at the lower temperatures,
involving the appearance of large energy-loss events. These irregular events
typically lasted a few tens of half-cycles of oscillation and could involve
hundreds of times more energy loss than would have occurred in a single
complete 2 Pi phase slip at maximum flow. The temperatures at which this
altered behavior was observed rose with frequency, from ~ 0.6 K and below, at
500 Hz, to ~ 1.0 K and below, at 1880 Hz.Comment: 35 pages, 13 figures, prequel to cond-mat/050203
Oscillation-free method for semilinear diffusion equations under noisy initial conditions
Noise in initial conditions from measurement errors can create unwanted
oscillations which propagate in numerical solutions. We present a technique of
prohibiting such oscillation errors when solving initial-boundary-value
problems of semilinear diffusion equations. Symmetric Strang splitting is
applied to the equation for solving the linear diffusion and nonlinear
remainder separately. An oscillation-free scheme is developed for overcoming
any oscillatory behavior when numerically solving the linear diffusion portion.
To demonstrate the ills of stable oscillations, we compare our method using a
weighted implicit Euler scheme to the Crank-Nicolson method. The
oscillation-free feature and stability of our method are analyzed through a
local linearization. The accuracy of our oscillation-free method is proved and
its usefulness is further verified through solving a Fisher-type equation where
oscillation-free solutions are successfully produced in spite of random errors
in the initial conditions.Comment: 19 pages, 9 figure
Fundamental properties of solar-like oscillating stars from frequencies of minimum : II. Model computations for different chemical compositions and mass
The large separations between the oscillation frequencies of solar-like stars
are measures of stellar mean density. The separations have been thought to be
mostly constant in the observed range of frequencies. However, detailed
investigation shows that they are not constant, and their variations are not
random but have very strong diagnostic potential for our understanding of
stellar structure and evolution. In this regard, frequencies of the minimum
large separation are very useful tools. From these frequencies, in addition to
the large separation and frequency of maximum amplitude, Y\i ld\i z et al.
recently have developed new methods to find almost all the fundamental stellar
properties. In the present study, we aim to find metallicity and helium
abundances from the frequencies, and generalize the relations given by Y\i ld\i
z et al. for a wider stellar mass range and arbitrary metallicity () and
helium abundance (). We show that the effect of metallicity is {
significant} for most of the fundamental parameters. For stellar mass, for
example, the expression must be multiplied by (Z/Z_{\sun})^{0.12}. For
arbitrary helium abundance, M \propto (Y/Y_{\sun})^{0.25} . Methods for
determination of and from pure asteroseismic quantities are based on
amplitudes (differences between maximum and minimum values of \Dnu) in the
oscillatory component in the spacing of oscillation frequencies. Additionally,
we demonstrate that the difference between the first maximum and the second
minimum is very sensitive to . It also depends on and small separation between the frequencies. Such a dependence leads us
to develop a method to find (and ) from oscillation frequencies. The
maximum difference between the estimated and model values is about 14 per
cent. It is 10 per cent for .Comment: 8 pages, 13 figures; published in MNRAS (2015
Electron-nuclear correlations for photo-induced dynamics in molecular dimers
Ultrafast photoinduced dynamics of electronic excitation in molecular dimers
is drastically affected by the dynamic reorganization of inter- and intra-
molecular nuclear configuration modeled by a quantized nuclear degree of
freedom [Cina et. al, J. Chem Phys. {118}, 46 (2003)]. The dynamics of the
electronic population and nuclear coherence is analyzed by solving the chain of
coupled differential equations for %mean coordinate, population inversion,
electron-vibrational correlation, etc. [Prezhdo, Pereverzev, J. Chem. Phys.
{113} 6557 (2000)]. Intriguing results are obtained in the approximation of a
small change of the nuclear equilibrium upon photoexcitation. In the limiting
case of resonance between the electronic energy gap and the frequency of the
nuclear mode these results are justified by comparison to the exactly solvable
Jaynes-Cummings model. It is found that the photoinduced processes in the model
dimer are arranged according to their time scales: (i) fast scale of nuclear
motion, (ii) intermediate scale of dynamical redistribution of electronic
population between excited states as well as growth and dynamics of
electron-nuclear correlation, (iii) slow scale of electronic population
approach to the quasi-equilibrium distribution, decay of electron-nuclear
correlation, and decrease of the amplitude of mean coordinate oscillation. The
latter processes are accompanied by a noticeable growth of the nuclear
coordinate dispersion associated with the overall nuclear wavepacket width. The
demonstrated quantum relaxation features of the photoinduced vibronic dynamics
in molecular dimers are obtained by a simple method, applicable to systems with
many degrees of freedom
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