67,023 research outputs found
Second-Order Convergence of a Projection Scheme for the Incompressible Navier–Stokes Equations with Boundaries
A rigorous convergence result is given for a projection scheme for the Navies–Stokes equations in the presence of boundaries. The numerical scheme is based on a finite-difference approximation, and the pressure is chosen so that the computed velocity satisfies a discrete divergence-free condition. This choice for the pressure and the particular way that the discrete divergence is calculated near the boundary permit the error in the pressure to be controlled and the second-order convergence in the velocity and the pressure to the exact solution to be shown. Some simplifications in the calculation of the pressure in the case without boundaries are also discussed
Understanding the effects of geometry and rotation on pulsar intensity profiles
We have developed a method to compute the possible distribution of radio
emission regions in a typical pulsar magnetosphere, taking into account the
viewing geometry and rotational effects of the neutron star. Our method can
estimate the emission altitude and the radius of curvature of particle
trajectory as a function of rotation phase for a given inclination angle,
impact angle, spin-period, Lorentz factor, field line constant and the
observation frequency. Further, using curvature radiation as the basic emission
mechanism, we simulate the radio intensity profiles that would be observed from
a given distribution of emission regions, for different values of radio
frequency and Lorentz factor. We show clearly that rotation effects can
introduce significant asymmetries into the observed radio profiles. We
investigate the dependency of profile features on various pulsar parameters. We
find that the radiation from a given ring of field lines can be seen over a
large range of pulse longitudes, originating at different altitudes, with
varying spectral intensity. Preferred heights of emission along discrete sets
of field lines are required to reproduce realistic pulsar profiles, and we
illustrate this for a known pulsar. Finally, we show how our model provides
feasible explanations for the origin of core emission, and also for one-sided
cones which have been observed in some pulsars.Comment: 21 pages, 11 figures, accepted for publication in MNRA
The Magnetic Rayleigh-Taylor Instability in Three Dimensions
We study the magnetic Rayleigh-Taylor instability in three dimensions, with
focus on the nonlinear structure and evolution that results from different
initial field configurations. We study strong fields in the sense that the
critical wavelength l_c at which perturbations along the field are stable is a
large fraction of the size of the computational domain. We consider magnetic
fields which are initially parallel to the interface, but have a variety of
configurations, including uniform everywhere, uniform in the light fluid only,
and fields which change direction at the interface. Strong magnetic fields do
not suppress instability, in fact by inhibiting secondary shear instabilities,
they reduce mixing between the heavy and light fluid, and cause the rate of
growth of bubbles and fingers to increase in comparison to hydrodynamics.
Fields parallel to, but whose direction changes at, the interface produce long,
isolated fingers separated by the critical wavelength l_c, which may be
relevant to the morphology of the optical filaments in the Crab nebula.Comment: 14 pages, 9 pages, accepted by Ap
Advanced aeroservoelastic stabilization techniques for hypersonic flight vehicles
Advanced high performance vehicles, including Single-Stage-To-Orbit (SSTO) hypersonic flight vehicles, that are statically unstable, require higher bandwidth flight control systems to compensate for the instability resulting in interactions between the flight control system, the engine/propulsion dynamics, and the low frequency structural modes. Military specifications, such as MIL-F-9490D and MIL-F-87242, tend to limit treatment of structural modes to conventional gain stabilization techniques. The conventional gain stabilization techniques, however, introduce low frequency effective time delays which can be troublesome from a flying qualities standpoint. These time delays can be alleviated by appropriate blending of gain and phase stabilization techniques (referred to as Hybrid Phase Stabilization or HPS) for the low frequency structural modes. The potential of using HPS for compensating structural mode interaction was previously explored. It was shown that effective time delay was significantly reduced with the use of HPS; however, the HPS design was seen to have greater residual response than a conventional gain stablized design. Additional work performed to advance and refine the HPS design procedure, to further develop residual response metrics as a basis for alternative structural stability specifications, and to develop strategies for validating HPS design and specification concepts in manned simulation is presented. Stabilization design sensitivity to structural uncertainties and aircraft-centered requirements are also assessed
Hubungan Tingkat Pendidikan dengan Preferensi Pemilihan Model Bisnis Toko Online
Online store as part of E-commerce from day to day growing rapidly, and increasingly organized according to the grouping\u27s business model E-commerce. Visitors E-commerce demographically divided into several groups one of which is the level of education. This study aims to determine the effect of education on elections E-commerce business model as measured by the number of visits to the site. Correlation analysis was used to measure the effect of the general level of education tersebut.Secara have a strong influence on the selection of E-commerce business model
Technicolor Models with Color-Singlet Technifermions and their Ultraviolet Extensions
We study technicolor models in which all of the technifermions are
color-singlets, focusing on the case in these fermions transform according to
the fundamental representation of the technicolor gauge group. Our analysis
includes a derivation of restrictions on the weak hypercharge assignments for
the technifermions and additional color-singlet, technisinglet fermions arising
from the necessity of avoiding stable bound states with exotic electric
charges. Precision electroweak constraints on these models are also discussed.
We determine some general properties of extended technicolor theories
containing these technicolor sectors.Comment: 17 pages, latex, 2 figure
Temperature dependent d-d excitations in manganites probed by resonant inelastic x-ray scattering
We report the observation of temperature dependent electronic excitations in
various manganites utilizing resonant inelastic x-ray scattering (RIXS) at the
Mn K-edge. Excitations were observed between 1.5 and 16 eV with temperature
dependence found as high as 10 eV. The change in spectral weight between 1.5
and 5 eV was found to be related to the magnetic order and independent of the
conductivity. On the basis of LDA+U and Wannier function calculations, this
dependence is associated with intersite d-d excitations. Finally, the
connection between the RIXS cross-section and the loss function is addressed.Comment: 5 pages, 5 figure
Coefficient of Restitution for Viscoelastic Spheres: The Effect of Delayed Recovery
The coefficient of normal restitution of colliding viscoelastic spheres is
computed as a function of the material properties and the impact velocity. From
simple arguments it becomes clear that in a collision of purely repulsively
interacting particles, the particles loose contact slightly before the distance
of the centers of the spheres reaches the sum of the radii, that is, the
particles recover their shape only after they lose contact with their collision
partner. This effect was neglected in earlier calculations which leads
erroneously to attractive forces and, thus, to an underestimation of the
coefficient of restitution. As a result we find a novel dependence of the
coefficient of restitution on the impact rate.Comment: 11 pages, 2 figure
Collective Dynamics of Random Polyampholytes
We consider the Langevin dynamics of a semi-dilute system of chains which are
random polyampholytes of average monomer charge and with a fluctuations in
this charge of the size and with freely floating counter-ions in the
surrounding. We cast the dynamics into the functional integral formalism and
average over the quenched charge distribution in order to compute the dynamic
structure factor and the effective collective potential matrix. The results are
given for small charge fluctuations. In the limit of finite we then find
that the scattering approaches the limit of polyelectrolyte solutions.Comment: 13 pages including 6 figures, submitted J. Chem. Phy
- …