968 research outputs found
T-spheres as a limit of Lemaitre-Tolman-Bondi solutions
In the Tolman model there exist two quite different branches of solutions -
generic Lemaitre-Tolman-Bondi (LTB) ones and T-spheres as a special case. We
show that, nonetheless, T-spheres can be obtained as a limit of the class of
LTB solutions having no origin and extending to infinity with the areal radius
approaching constant. It is shown that all singularities of T-models are
inherited from those of corresponding LBT solutions. In doing so, the disc type
singularity of a T-sphere is the analog of shell-crossing.Comment: 6 pages. 1 Reference added. To appear in Phys. Rev.
Redshift drift in axially symmetric quasi-spherical Szekeres models
Models of inhomogeneous universes constructed with exact solutions of
Einstein's General Relativity have been proposed in the literature with the aim
of reproducing the cosmological data without any need for a dark energy
component. Besides large scale inhomogeneity models spherically symmetric
around the observer, Swiss-cheese models have also been studied. Among them,
Swiss-cheeses where the inhomogeneous patches are modeled by different
particular Szekeres solutions have been used for reproducing the apparent
dimming of the type Ia supernovae (SNIa). However, the problem of fitting such
models to the SNIa data is completely degenerate and we need other constraints
to fully characterize them. One of the tests which is known to be able to
discriminate between different cosmological models is the redshift-drift. This
drift has already been calculated by different authors for
Lema\^itre-Tolman-Bondi (LTB) models. We compute it here for one particular
axially symmetric quasi-spherical Szekeres (QSS) Swiss-cheese which has
previously been shown to reproduce to a good accuracy the SNIa data, and we
compare the results to the drift in the CDM model and in some LTB
models that can be found in the literature. We show that it is a good
discriminator between them. Then, we discuss our model's remaining degrees of
freedom and propose a recipe to fully constrain them.Comment: 15 pages, 7 figures, minor changes in title, text, figures and
references; conclusions unchanged, this version matches the published versio
The Fermi-Pasta-Ulam recurrence and related phenomena for 1D shallow-water waves in a finite basin
In this work, different regimes of the Fermi-Pasta-Ulam (FPU) recurrence are
simulated numerically for fully nonlinear "one-dimensional" potential water
waves in a finite-depth flume between two vertical walls. In such systems, the
FPU recurrence is closely related to the dynamics of coherent structures
approximately corresponding to solitons of the integrable Boussinesq system. A
simplest periodic solution of the Boussinesq model, describing a single soliton
between the walls, is presented in an analytical form in terms of the elliptic
Jacobi functions. In the numerical experiments, it is observed that depending
on a number of solitons in the flume and their parameters, the FPU recurrence
can occur in a simple or complicated manner, or be practically absent. For
comparison, the nonlinear dynamics of potential water waves over nonuniform
beds is simulated, with initial states taken in the form of several pairs of
colliding solitons. With a mild-slope bed profile, a typical phenomenon in the
course of evolution is appearance of relatively high (rogue) waves, while for
random, relatively short-correlated bed profiles it is either appearance of
tall waves, or formation of sharp crests at moderate-height waves.Comment: revtex4, 10 pages, 33 figure
The hodograph method applicability in the problem of long-scale nonlinear dynamics of a thin vortex filament near a flat boundary
Hamiltonian dynamics of a thin vortex filament in ideal incompressible fluid
near a flat fixed boundary is considered at the conditions that at any point of
the curve determining shape of the filament the angle between tangent vector
and the boundary plane is small, also the distance from a point on the curve to
the plane is small in comparison with the curvature radius. The dynamics is
shown to be effectively described by a nonlinear system of two
(1+1)-dimensional partial differential equations. The hodograph transformation
reduces that system to a single linear differential equation of the second
order with separable variables. Simple solutions of the linear equation are
investigated at real values of spectral parameter when the filament
projection on the boundary plane has shape of a two-branch spiral or a smoothed
angle, depending on the sign of .Comment: 9 pages, revtex4, 6 eps-figure
Nonlinear interfacial waves in a constant-vorticity planar flow over variable depth
Exact Lagrangian in compact form is derived for planar internal waves in a
two-fluid system with a relatively small density jump (the Boussinesq limit
taking place in real oceanic conditions), in the presence of a background shear
current of constant vorticity, and over arbitrary bottom profile. Long-wave
asymptotic approximations of higher orders are derived from the exact
Hamiltonian functional in a remarkably simple way, for two different
parametrizations of the interface shape.Comment: revtex, 4.5 pages, minor corrections, summary added, accepted to JETP
Letter
ПОРІВНЯЛЬНИЙ АНАЛІЗ МЕТОДІВ ВІДНОВЛЕННЯ ПРОПУЩЕНИХ ДАНИХ ЧАСУ ДОСТУПНОСТІ СЕРВЕРА В ІНФОРМАЦІЙНО- EКОНОМІЧНИХ ПРОСТОРАХ
In the article presents results of research of primary statistical characteristics of time of processing and data transmission are given in systems of information economic area. On the basis of the received results the analysis of methods of recovery of the passed data is made. In work methods of data processing with admissions are analyzed and the most rational are chosen. Change of the law of distribution of selection depending on a way of elimination of the passed data is investigated. Forecasting of time of availability of the server of information economic area is executed. The goal of this paper is primary statistical characteristics research of data processing and transmission time in IES. Particularly, the server availability time (SAT) was studied within this work. This time consists of Client-to-Server connection time, Server data processing time, and Server answer transfer time. The values of service time were taken for research, that were received while studying of cloud computing performance. It was established that if data loss is not over 6% distribution law of initial model selection saves. If data loss is between 9% and 15% methods primary statistical characteristics of studied selection were determined without consideration of data missing. Missed data was filled with data distributed by the same law as for initial selection with admissions.В статье приведены результаты исследования первичных статистических характеристик времени обработки и передачи данных в информационно-экономических пространствах. На основании полученных результатов выполнен анализ методов восстановления пропущенных данных. В работе проанализированы приемы обработки данных с пропускам и выбраны наиболее рациональные. Исследовано изменение закона распределения выборки в зависимости от способа устранения пропущенных данных. Выполнено прогнозирование времени доступности сервера в информационно-экономических пространствах. Цель работы состояла в исследование первичных статистических характеристик времени обработки и передачи данных в информационно-экономических пространствах. В частности, в рамках данной работы изучалось время доступности сервера.У статті наведено результати дослідження первинних статистичних характеристик часу обробки і передачі даних в інформаційно-економічних просторах. На підставі отриманих результатів виконано аналіз методів відновлення пропущених даних. Проаналізовано прийоми обробки даних з пропускам та обрано найбільш раціональні. Досліджена зміна закону розподілу вибірки залежно від способу усунення пропущених даних. Виконано прогнозування часу доступності сервера економічних просторів. Мета роботи полягала у дослідження первинних статистичних характеристик часу обробки і передачі даних в інформаційно-економічних просторах. Зокрема, в рамках даної роботи вивчалося час доступності сервера
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