305 research outputs found
Dynamical Systems approach to Saffman-Taylor fingering. A Dynamical Solvability Scenario
A dynamical systems approach to competition of Saffman-Taylor fingers in a
channel is developed. This is based on the global study of the phase space
structure of the low-dimensional ODE's defined by the classes of exact
solutions of the problem without surface tension. Some simple examples are
studied in detail, and general proofs concerning properties of fixed points and
existence of finite-time singularities for broad classes of solutions are
given. The existence of a continuum of multifinger fixed points and its
dynamical implications are discussed. The main conclusion is that exact
zero-surface tension solutions taken in a global sense as families of
trajectories in phase space spanning a sufficiently large set of initial
conditions, are unphysical because the multifinger fixed points are
nonhyperbolic, and an unfolding of them does not exist within the same class of
solutions. Hyperbolicity (saddle-point structure) of the multifinger fixed
points is argued to be essential to the physically correct qualitative
description of finger competition. The restoring of hyperbolicity by surface
tension is discussed as the key point for a generic Dynamical Solvability
Scenario which is proposed for a general context of interfacial pattern
selection.Comment: 3 figures added, major rewriting of some sections, submitted to Phys.
Rev.
Interface dynamics in Hele-Shaw flows with centrifugal forces. Preventing cusp singularities with rotation
A class of exact solutions of Hele-Shaw flows without surface tension in a
rotating cell is reported. We show that the interplay between injection and
rotation modifies drastically the scenario of formation of finite-time cusp
singularities. For a subclass of solutions, we show that, for any given initial
condition, there exists a critical rotation rate above which cusp formation is
prevented. We also find an exact sufficient condition to avoid cusps
simultaneously for all initial conditions. This condition admits a simple
interpretation related to the linear stability problem.Comment: 4 pages, 2 figure
Effects of small surface tension in Hele-Shaw multifinger dynamics: an analytical and numerical study
We study the singular effects of vanishingly small surface tension on the
dynamics of finger competition in the Saffman-Taylor problem, using the
asymptotic techniques described in [S. Tanveer, Phil. Trans. R. Soc. Lond. A
343, 155 (1993)]and [M. Siegel, and S. Tanveer, Phys. Rev. Lett. 76, 419
(1996)] as well as direct numerical computation, following the numerical scheme
of [T. Hou, J. Lowengrub, and M. Shelley,J. Comp. Phys. 114, 312 (1994)]. We
demonstrate the dramatic effects of small surface tension on the late time
evolution of two-finger configurations with respect to exact (non-singular)
zero surface tension solutions. The effect is present even when the relevant
zero surface tension solution has asymptotic behavior consistent with selection
theory.Such singular effects therefore cannot be traced back to steady state
selection theory, and imply a drastic global change in the structure of
phase-space flow. They can be interpreted in the framework of a recently
introduced dynamical solvability scenario according to which surface tension
unfolds the structually unstable flow, restoring the hyperbolicity of
multifinger fixed points.Comment: 16 pages, 15 figures, submitted to Phys. Rev
Two-finger selection theory in the Saffman-Taylor problem
We find that solvability theory selects a set of stationary solutions of the
Saffman-Taylor problem with coexistence of two \it unequal \rm fingers
advancing with the same velocity but with different relative widths
and and different tip positions. For vanishingly small
dimensionless surface tension , an infinite discrete set of values of the
total filling fraction and of the relative
individual finger width are selected out of a
two-parameter continuous degeneracy. They scale as
and . The selected values of differ from
those of the single finger case. Explicit approximate expressions for both
spectra are given.Comment: 4 pages, 3 figure
Equilibration of Concentrated Hard Sphere Fluids
We report a systematic molecular dynamics study of the isochoric
equilibration of hard-sphere fluids in their metastable regime close to the
glass transition. The thermalization process starts with the system prepared in
a non-equilibrium state with the desired final volume fraction {\phi} but with
a prescribed non-equilibrium static structure factor S_0(k; {\phi}). The
evolution of the {\alpha}- relaxation time {\tau}{\alpha} (k) and long-time
self-diffusion coefficient DL as a function of the evolution time tw is then
monitored for an array of volume fractions. For a given waiting time the plot
of {\tau}{\alpha} (k; {\phi}, tw) as a function of {\phi} exhibits two regimes
corresponding to samples that have fully equilibrated within this waiting time
({\phi} \leq {\phi}(c) (tw)), and to samples for which equilibration is not yet
complete ({\phi} \geq {\phi}(c) (tw)). The crossover volume fraction {\phi}(c)
(tw) increases with tw but seems to saturate to a value {\phi}(a) \equiv
{\phi}(c) (tw \rightarrow \infty) \approx 0.582. We also find that the waiting
time t^(eq)_w({\phi}) required to equilibrate a system grows faster than the
corresponding equilibrium relaxation time, t^(eq)({\phi}) \approx 0.27 \times
[{\tau}{\alpha} (k; {\phi})]^1.43, and that both characteristic times increase
strongly as {\phi} approaches {\phi}^(a), thus suggesting that the measurement
of equilibrium properties at and above {\phi}(a) is experimentally impossible
General Non-equilibrium Theory of Colloid Dynamics
A non-equilibrium extension of Onsager's canonical theory of thermal
fluctuations is employed to derive a self-consistent theory for the description
of the statistical properties of the instantaneous local concentration profile
n(r,t) of a colloidal liquid in terms of the coupled time evolution equations
of its mean value n(r,t) and of the covariance {\sigma}(r,r';t) \equiv
of its fluctuations {\delta}n(r, t) = n(r, t) -
n(r, t). These two coarse-grained equations involve a local mobility function
b(r, t) which, in its turn, is written in terms of the memory function of the
two-time correlation function C(r, r' ; t, t') \equiv <{\delta}n(r,
t){\delta}n(r',t')>. For given effective interactions between colloidal
particles and applied external fields, the resulting self-consistent theory is
aimed at describing the evolution of a strongly correlated colloidal liquid
from an initial state with arbitrary mean and covariance n^0(r) and
{\sigma}^0(r,r') towards its equilibrium state characterized by the equilibrium
local concentration profile n^(eq)(r) and equilibrium covariance
{\sigma}^(eq)(r,r').
This theory also provides a general theoretical framework to describe
irreversible processes associated with dynamic arrest transitions, such as
aging, and the effects of spatial heterogeneities
Elogios honorificos y sublimes priuilegios de la cabeza del Serafico doctor de la Iglesia S. Buenauentura : propuestos al ... Concurso de Religiones Sagradas, Doctores, y Maestros de la Vniuersidad de Alcalà, que asistieron a la celebridad de su Translacion en el Conuento de S. Diego
Copia digital. Madrid : Ministerio de Cultura. Subdirección General de Coordinación Bibliotecaria, 2008Sign.: ¶\p6\s, A-B\p4\s, C\p2\sPort. con orla tip
Topological gravity on plumbed V-cobordisms
An ensemble of cosmological models based on generalized BF-theory is
constructed where the role of vacuum (zero-level) coupling constants is played
by topologically invariant rational intersection forms (cosmological-constant
matrices) of 4-dimensional plumbed V-cobordisms which are interpreted as
Euclidean spacetime regions. For these regions describing topology changes, the
rational and integer intersection matrices are calculated. A relation is found
between the hierarchy of certain elements of these matrices and the hierarchy
of coupling constants of the universal (low-energy) interactions.
PACS numbers: 0420G, 0240, 0460Comment: 29 page
Phase-field model for Hele-Shaw flows with arbitrary viscosity contrast. I. Theoretical approach
We present a phase-field model for the dynamics of the interface between two
inmiscible fluids with arbitrary viscosity contrast in a rectangular Hele-Shaw
cell. With asymptotic matching techniques we check the model to yield the right
Hele-Shaw equations in the sharp-interface limit and compute the corrections to
these equations to first order in the interface thickness. We also compute the
effect of such corrections on the linear dispersion relation of the planar
interface. We discuss in detail the conditions on the interface thickness to
control the accuracy and convergence of the phase-field model to the limiting
Hele-Shaw dynamics. In particular, the convergence appears to be slower for
high viscosity contrasts.Comment: 17 pages in revtex. changes: 1 reference adde
A bibliographic analysis of transformer literature 1990-2000
This paper presents an analysis of the bibliography on transformers covering the period from 1990 to 2000. It contains all the transformer subjects: a) Transformer design, b) Transformer protection, c) Transformer connections, d) Transformer diagnostics, e) Transformer failures, f) Transient analysis of transformers (overvoltages, overcurrents), g) Modeling and analysis of transformer using FEM (thermal modeling, losses modeling, insulation modeling, windings modeling). Several international journals were investigated including the following: Advances in Electrical and Computer Engineering, Canadian Journal of Electrical and Computer Engineering, COMPEL (The International Journal for Computation and Mathematics in Electrical and Electronic Engineering), Electrical Engineering, Electric Power Components and Systems, Electric Power Systems Research, European Transactions on Electrical Power, IEEE Transactions on Magnetics, IEEE Transactions on Power Delivery, International Journal of Electrical Power and Energy Systems, and IET Generation Transmission & Distribution. Due to the high number of publication in journals, we are not considering publications of conferences and symposia. A total of 700 publications are analyzed in this paper. The research presented in this paper is important because it contains and analyzes the best research papers on transformers coming from many countries all over the world and published in top rated scientific electrical engineering journals
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