A Multiscale Diffuse-Interface Model for Two-Phase Flow in Porous Media

In this paper we consider a multiscale phase-field model for capillarity-driven flows in porous media. The presented model constitutes a reduction of the conventional Navier-Stokes-Cahn-Hilliard phase-field model, valid in situations where interest is restricted to dynamical and equilibrium behavior in an aggregated sense, rather than a precise description of microscale flow phenomena. The model is based on averaging of the equation of motion, thereby yielding a significant reduction in the complexity of the underlying Navier-Stokes-Cahn-Hilliard equations, while retaining its macroscopic dynamical and equilibrium properties. Numerical results are presented for the representative 2-dimensional capillary-rise problem pertaining to two closely spaced vertical plates with both identical and disparate wetting properties. Comparison with analytical solutions for these test cases corroborates the accuracy of the presented multiscale model. In addition, we present results for a capillary-rise problem with a non-trivial geometry corresponding to a porous medium

Noise enhanced performance of ratchet cellular automata

We present the first experimental realization of a ratchet cellular automaton (RCA) which has been recently suggested as an alternative approach for performing logical operations with interacting (quasi) particles. Our study was performed with interacting colloidal particles which serve as a model system for other dissipative systems i.e. magnetic vortices on a superconductor or ions in dissipative optical arrays. We demonstrate that noise can enhance the efficiency of information transport in RCA and consequently enables their optimal operation at finite temperatures.Comment: accepted for publication at Phys. Rew. Let

INVESTIGATIONS OF THE DYNAMICS OF A BISTABLE ELECTROMAGNET WITH IMPROVED CHARACTERISTICS FOR MEDIUM VOLTAGE VACUUM CIRCUIT BREAKERS

Introduction. Currently, for switching medium voltage circuits, vacuum circuit breakers are widely used, which have good arcing properties and high breaking capacity. One of the problems of creating the drive mechanism of such apparatus is the need to ensure the absence of contact welding when a through current of a short circuit of a given duration flows through them, which is achieved due to a certain amount of contact pressure. One of the problems arising in the design of circuit breakers is the need to fix the mechanism with a mechanical lock, which should hold the mechanism securely. This leads to significant specific mechanical loads, which in turn reduces the reliability of the circuit breaker. One way to solve these problems is to create a drive based on monostable or bistable electromagnetic actuators with highly coercive permanent magnets, which provide reliable fixation of the position of the contacts. Purpose. Investigation of the improved design of a bistable electromagnetic actuator based on permanent magnets of a medium voltage vacuum circuit breaker. Methods. Theoretical and experimental research and comparative analysis of existing and developed electromagnetic actuators. Conclusions. A new design of an electromagnetic bistable actuator with reduced overall dimensions is developed and tested. The electromechanical characteristics of the actuator correspond to the technical specifications, which is confirmed by both theoretical and experimental studies. The proposed actuator can be used as a drive mechanism for medium voltage vacuum circuit breakers.В статье исследован новый бистабильный электромагнит с высококоэрцитивными постоянными магнитами, который предполагается использовать в качестве актуатора вакуумных выключателей средних напряжений. Приводится теоретическое и экспериментальное исследование усовершенствованной конструкции электромагнита с целью сравнительного анализа его параметров с параметрами имеющейся конструкции. Теоретическое исследование базируется на мультифизической модели, которая включает расчет статического и динамического электромагнитных полей в нелинейной проводящей неоднородной среде с учетом постоянных магнитов, нелинейных уравнений разрядной цепи накопительного конденсатора, нелинейных уравнений движения. Экспериментальные исследования, которые проводились на реальном вакуумном выключателе, показали соответствие параметров нового бистабильного электромагнита расчетным показателям. Направление дальнейших исследований представляются в виде оптимизации геометрии электромагнита и схемы управления

On the HI-Hole and AGB Stellar Population of the Sagittarius Dwarf Irregular Galaxy

Using two HST/ACS data-sets that are separated by ~2 years has allowed us to derive the relative proper-motion for the Sagittarius dwarf irregular (SagDIG) and reduce the heavy foreground Galactic contamination. The proper-motion decontaminated SagDIG catalog provides a much clearer view of the young red-supergiant and intermediate-age asymptotic giant branch populations. We report the identification of 3 Milky Way carbon-rich dwarf stars, probably belonging to the thin disk, and pointing to the high incidence of this class at low Galactic latitudes. A sub-group of 4 oxygen-rich candidate stars depicts a faint, red extension of the well-defined SagDIG carbon-rich sequence. The origin of these oxygen-rich candidate stars remains unclear, reflecting the uncertainty in the ratio of carbon/oxygen rich stars. SagDIG is also a gas-rich galaxy characterized by a single large cavity in the gas disk (HI-hole), which is offset by ~360 pc from the optical centre of the galaxy. We nonetheless investigate the stellar feedback hypothesis by comparing the proper-motion cleaned stellar populations within the HI-hole with appropriately selected comparison regions, having higher HI densities external to the hole. The comparison shows no significant differences. In particular, the centre of the HI-hole (and the comparison regions) lack stellar populations younger than ~400 Myr, which are otherwise abundant in the inner body of the galaxy. We conclude that there is no convincing evidence that the SagDIG HI-hole is the result of stellar feedback, and that gravitational and thermal instabilities in the gas are the most likely mechanism for its formation.Comment: Accepted for publication in A&A, 11 pages, 6 jpeg figure

VLT Spectroscopy of Globular Clusters in Low Surface Brightness Dwarf Galaxies

We present VLT/FORS2 spectroscopic observations of globular clusters (GCs) in five low surface brightness (LSB) dwarf galaxies: KK211 and KK221, which are both dwarf spheroidal satellites (dSph) of NGC 5128, dSph KK84 located close to the isolated S0 galaxy NGC 3115, and two isolated dwarf irregular (dIrr) galaxies UGC 3755 and ESO 490-17. Our sample is selected from the Sharina et al. (2005) database of Hubble Space Telescope WFPC2 photometry of GC candidates in dwarf galaxies. For objects with accurate radial velocity measurements we confirm 26 as genuine GCs out of the 27 selected candidates from our WFPC2 survey. Lick absorption line indices in the spectra of confirmed GCs and the subsequent comparison with SSP model predictions show that all confirmed GCs in dSphs are old, except GC KK211-3-149 (6 +/- 2 Gyr), which we consider to be the nucleus of KK211. GCs in UGC 3755 and ESO 490-17 show a large spread in ages ranging from old objects (t > 10 Gyr) to clusters with ages around 1 Gyr. Most of our sample GCs have low metallicities [Z/H] <= -1. Two relatively metal-rich clusters with [Z/H] ~ -0.3 are likely to be associated with NGC 3115. Our sample GCs show in general a complex distribution of alpha-element enhancement with a mean [alpha/Fe]=0.19 +/-0.04 derived with the chi2 minimization technique and 0.18+/-0.12 dex computed with the iterative approach. These values are slightly lower than the mean [alpha/Fe]=0.29+/-0.01 for typical Milky Way GCs. We compare other abundance ratios with those of Local Group GCs and find indications for systematic differences in N and Ca abundance. The specific frequencies, S_N, of our sample galaxies are in line with the predictions of a simple mass-loss model for dwarf galaxies and compare well with S_N values found for dwarf galaxies in nearby galaxy clusters.Comment: accepted for publication in Ap

A Pulsational Model for the Orthogonal Polarization Modes in Radio Pulsars

In an earlier paper, we introduced a model for pulsars in which non-radial oscillations of high spherical degree (\el) aligned to the magnetic axis of a spinning neutron star were able to reproduce subpulses like those observed in single-pulse measurements of pulsar intensity. The model did not address polarization, which is an integral part of pulsar emission. Observations show that many pulsars emit radio waves that appear to be the superposition of two linearly polarized emission modes with orthogonal polarization angles. In this paper, we extend our model to incorporate linear polarization. As before, we propose that pulsational displacements of stellar material modulate the pulsar emission, but now we apply this modulation to a linearly-polarized mode of emission, as might be produced by curvature radiation. We further introduce a second polarization mode, orthogonal to the first, that is modulated by pulsational velocities. We combine these modes in superposition to model the observed Stokes parameters in radio pulsars.Comment: 19 pages, 4 figures accepted Ap

Triploid Plover Female Provides Support for a Role of the W Chromosome in Avian Sex Determination

Two models, Z Dosage and Dominant W, have been proposed to explain sex determination in birds, in which males are characterized by the presence of two Z chromosomes, and females are hemizygous with a Z and a W chromosome. According to the Z Dosage model, high dosage of a Z-linked gene triggers male development, whereas the Dominant W model postulates that a still unknown W-linked gene triggers female development. Using 33 polymorphic microsatellite markers, we describe a female triploid Kentish plover Charadrius alexandrinus identified by characteristic triallelic genotypes at 14 autosomal markers that produced viable diploid offspring. Chromatogram analysis showed that the sex chromosome composition of this female was ZZW. Together with two previously described ZZW female birds, our results suggest a prominent role for a female determining gene on the W chromosome. These results imply that avian sex determination is more dynamic and complex than currently envisioned.Organismic and Evolutionary Biolog

Skeleton-stabilized ImmersoGeometric Analysis for incompressible viscous flow problems

A Skeleton-stabilized ImmersoGeometric Analysis technique is proposed for incompressible viscous flow problems with moderate Reynolds number. The proposed formulation fits within the framework of the finite cell method, where essential boundary conditions are imposed weakly using a Nitsche-type method. The key idea of the proposed formulation is to stabilize the jumps of high-order derivatives of variables over the skeleton of the background mesh. The formulation allows the use of identical finite-dimensional spaces for the approximation of the pressure and velocity fields in immersed domains. The stability issues observed for inf-sup stable discretizations of immersed incompressible flow problems are avoided with this formulation. For B-spline basis functions of degree $k$ with highest regularity, only the derivative of order $k$ has to be controlled, which requires specification of only a single stabilization parameter for the pressure field. The Stokes and Navier-Stokes equations are studied numerically in two and three dimensions using various immersed test cases. Oscillation-free solutions and high-order optimal convergence rates can be obtained. The formulation is shown to be stable even in limit cases where almost every elements of the physical domain is cut, and hence it does not require the existence of interior cells. In terms of the sparsity pattern, the algebraic system has a considerably smaller stencil than counterpart approaches based on Lagrange basis functions. This important property makes the proposed skeleton-stabilized technique computationally practical. To demonstrate the stability and robustness of the method, we perform a simulation of fluid flow through a porous medium, of which the geometry is directly extracted from 3D $\mu{CT}$ scan data

PECULIARITIES OF CALCULATING STATIONARY HEATING OF WINDINGS OPERATING IN COMPLEX FORCED CONTROL SYSTEMS

General description of the research topic. A technique and an algorithm for calculating the thermal field of electromagnets operating in complex forced systems proposed by authors are considered. The widespread use of such devices in electromechanical switching devices allows not only to increase their speed but also significantly reduce the size, mass and energy losses, which indicates the relevance of this topic. The mathematical model of heating the windings of forced electromagnets proposed by the authors is a system of 1D differential equations of stationary heat transfer in a cylindrical coordinate system, supplemented by equations of electrical and magnetic circuits. This model allows to take into account the ripple of the currents in the windings and the losses in the magnetic core due to these ripples, contains certain signs of scientific novelty and represents the goal of the paper. The algorithm developed by the authors for calculating the thermal field of electromagnets operating in forced control systems is a complex iterative cycle. Its implementation is greatly simplified by using the Maple computing environment which allows to realize complicated and cumbersome mathematical transformations, automates the process of computations, and obtain results of numerical simulation in a convenient tabular and/or graphic form, which indicates the practical significance of this works. The results of comparison of computation results with experimental data presented in the paper indicate the adequacy of the model and algorithm proposed.Загальний опис теми дослідження. Розглядаються запропоновані авторами методика і алгоритм розрахунку теплового поля електромагнітів, що працюють в складних форсованих системах. Широке застосування в електромеханічних комутаційних апаратах подібних пристроїв дозволяє не тільки підвищити їх швидкодію, але також істотно зменшити розміри, масу і втрати енергії, що свідчить про актуальність даної теми. Запропонована авторами математична модель нагріву обмоток форсованих електромагнітів являє собою систему одновимірних диференціальних рівнянь стаціонарної теплопровідності в циліндричній системі координат, доповнену рівняннями електричного та магнітного кіл. Ця модель дозволяє врахувати пульсації струмів в обмотках, а також втрати в магнітопроводі, обумовлені цими пульсаціями, містить певні ознаки наукової новизни і є метою статті. Розроблений авторами алгоритм розрахунку теплового поля електромагнітів, що працюють в системах форсованого керування, являє собою складний ітераційний цикл, програмування якого істотно спрощується за рахунок застосування математичного пакету Maple, що дозволяє здійснювати складні і громіздкі математичні перетворення, автоматизувати процес розрахункових досліджень, отримувати результати комп'ютерного моделювання в зручній табличній та / або графічній формі, що свідчить про практичну значимість даної роботи. Наведені в статті результати зіставлення розрахунків з експериментальними даними, свідчать про адекватність запропонованих моделі та алгоритму

Evolutionary calculations of phase separation in crystallizing white dwarf stars

We present an exploration of the significance of Carbon/Oxygen phase separation in white dwarf stars in the context of self-consistent evolutionary calculations. Because phase separation can potentially increase the calculated ages of the oldest white dwarfs, it can affect the age of the Galactic disk as derived from the downturn in the white dwarf luminosity function. We find that the largest possible increase in ages due to phase separation is 1.5 Gyr, with a most likely value of approximately 0.6 Gyr, depending on the parameters of our white dwarf models. The most important factors influencing the size of this delay are the total stellar mass, the initial composition profile, and the phase diagram assumed for crystallization. We find a maximum age delay in models with masses of 0.6 solar masses, which is near the peak in the observed white dwarf mass distribution. We find that varying the opacities (via the metallicity) has little effect on the calculated age delays. In the context of Galactic evolution, age estimates for the oldest Galactic globular clusters range from 11.5 to 16 Gyr, and depend on a variety of parameters. In addition, a 4 to 6 Gyr delay is expected between the formation of the globular clusters and that of the Galactic thin disk, while the observed white dwarf luminosity function gives an age estimate for the thin disk of 9.5 +/-1.0 Gyr, without including the effect of phase separation. Using the above numbers, we see that phase separation could add between 0 to 3 Gyr to the white dwarf ages and still be consistent with the overall picture of Galaxy formation. Our calculated maximum value of 1.5 Gyr fits within these bounds, as does our best guess value of 0.6 Gyr.Comment: 13 total pages, 8 figures, 3 tables, accepted for publication in the Astrophysical Journal on May 25, 199