159,789 research outputs found
Theory of electrical conductivities of ferrogels
Conductive organic polymers can be formulated with polymers that incorporate
fine dispersed metallic particles.
In this work, we present a general model for ferrogels which are chemically
cross-linked polymer networks swollen with a ferrofluid. Our aim is to study
the effect of the shape and/or material (conductivity) anisotropy on the
effective electrical conductivity of the ferrogel in the presence of an
external magnetic field. Our theory can reproduce the known results, and
provides a link between the particle property and orientation distribution and
the effective electrical conductivity. To this end, we find that material
(conductivity) anisotropies are more important to yield a high effective
electrical conductivity than shape anisotropies, while magnetic fields can
offer a correction.Comment: 15 pages, 2 figure
Dielectric behavior of oblate spheroidal particles: Application to erythrocytes suspensions
We have investigated the effect of particle shape on the eletrorotation (ER)
spectrum of living cells suspensions. In particular, we consider coated oblate
spheroidal particles and present a theoretical study of ER based on the
spectral representation theory. Analytic expressions for the characteristic
frequency as well as the dispersion strength can be obtained, thus simplifying
the fitting of experimental data on oblate spheroidal cells that abound in the
literature. From the theoretical analysis, we find that the cell shape, coating
as well as material parameters can change the ER spectrum. We demonstrate good
agreement between our theoretical predictions and experimental data on human
erthrocytes suspensions.Comment: RevTex; 5 eps figure
Many-body dipole-induced dipole model for electrorheological fluids
Theoretical investigations on electrorheological (ER) fluids usually rely on
computer simulations. An initial approach for these studies would be the
point-dipole (PD) approximation, which is known to err considerably when the
particles approach and finally touch due to many-body and multipolar
interactions. Thus various work attempted to go beyond the PD model. Being
beyond the PD model, previous attempts have been restricted to either
local-field effects only or multipolar effects only, but not both. For
instance, we recently proposed a dipole-induced-dipole (DID) model which is
shown to be both more accurate than the PD model and easy to use. This work is
necessary because the many-body (local-field) effect is included to put forth
the many-body DID model. The results show that the multipolar interactions can
indeed be dominant over the dipole interaction, while the local-field effect
may yield an important correction.Comment: RevTeX, 3 eps figure
Nonlinear ac responses of electro-magnetorheological fluids
We apply a Langevin model to investigate the nonlinear ac responses of
electro-magnetorheological (ERMR) fluids under the application of two crossed
dc magnetic (z axis) and electric (x axis) fields and a probing ac sinusoidal
magnetic field. We focus on the influence of the magnetic fields which can
yield nonlinear behaviors inside the system due to the particles with a
permanent magnetic dipole moment.
Based on a perturbation approach, we extract the harmonics of the magnetic
field and orientational magnetization analytically. To this end, we find that
the harmonics are sensitive to the degree of anisotropy of the structure as
well as the field frequency. Thus, it is possible to real-time monitor the
structure transformation of ERMR fluids by detecting the nonlinear ac
responses.Comment: 21 pages, 4 figure
CRLBs for Pilot-Aided Channel Estimation in OFDM System under Gaussian and Non-Gaussian Mixed Noise
The determination of Cramer-Rao lower bound (CRLB) as an optimality criterion for the problem of channel estimation in wireless communication is a very important issue. Several CRLBs on channel estimation have been derived for Gaussian noise. However, a practical channel is affected by not only Gaussian background noise but also non-Gaussian noise such as impulsive interference. This paper derives the deterministic and stochastic CRLBs for Gaussian and non-Gaussian mixed noise. Due to the use of the non-parametric kernel method to build the PDF of non-Gaussian noise, the proposed CRLBs are suitable for practical channel environments with various noise distributions
Assessment of closure coefficients for compressible-flow turbulence models
A critical assessment is made of the closure coefficients used for turbulence length scale in existing models of the transport equation, with reference to the extension of these models to compressible flow. It is shown that to satisfy the compressible 'law of the wall', the model coefficients must actually be functions of density gradients. The magnitude of the errors that result from neglecting this dependence on density varies with the variable used to specify the length scale. Among the models investigated, the k-omega model yields the best performance, although it is not completely free from errors associated with density terms. Models designed to reduce the density-gradient effect to an insignificant level are proposed
Open-closed field algebras
We introduce the notions of open-closed field algebra and open-closed field
algebra over a vertex operator algebra V. In the case that V satisfies certain
finiteness and reductivity conditions, we show that an open-closed field
algebra over V canonically gives an algebra over a \C-extension of the
Swiss-cheese partial operad. We also give a tensor categorical formulation and
categorical constructions of open-closed field algebras over V.Comment: 55 pages, largely revised, an old subsection is deleted, a few
references are adde
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