40,806 research outputs found
Gaussian approximation for finitely extensible bead-spring chains with hydrodynamic interaction
The Gaussian Approximation, proposed originally by Ottinger [J. Chem. Phys.,
90 (1) : 463-473, 1989] to account for the influence of fluctuations in
hydrodynamic interactions in Rouse chains, is adapted here to derive a new
mean-field approximation for the FENE spring force. This "FENE-PG" force law
approximately accounts for spring-force fluctuations, which are neglected in
the widely used FENE-P approximation. The Gaussian Approximation for
hydrodynamic interactions is combined with the FENE-P and FENE-PG spring force
approximations to obtain approximate models for finitely-extensible bead-spring
chains with hydrodynamic interactions. The closed set of ODE's governing the
evolution of the second-moments of the configurational probability distribution
in the approximate models are used to generate predictions of rheological
properties in steady and unsteady shear and uniaxial extensional flows, which
are found to be in good agreement with the exact results obtained with Brownian
dynamics simulations. In particular, predictions of coil-stretch hysteresis are
in quantitative agreement with simulations' results. Additional simplifying
diagonalization-of-normal-modes assumptions are found to lead to considerable
savings in computation time, without significant loss in accuracy.Comment: 26 pages, 17 figures, 2 tables, 75 numbered equations, 1 appendix
with 10 numbered equations Submitted to J. Chem. Phys. on 6 February 200
Search method for long-duration gravitational-wave transients from neutron stars
We introduce a search method for a new class of gravitational-wave signals,
namely long-duration O(hours - weeks) transients from spinning neutron stars.
We discuss the astrophysical motivation from glitch relaxation models and we
derive a rough estimate for the maximal expected signal strength based on the
superfluid excess rotational energy. The transient signal model considered here
extends the traditional class of infinite-duration continuous-wave signals by a
finite start-time and duration. We derive a multi-detector Bayes factor for
these signals in Gaussian noise using \F-statistic amplitude priors, which
simplifies the detection statistic and allows for an efficient implementation.
We consider both a fully coherent statistic, which is computationally limited
to directed searches for known pulsars, and a cheaper semi-coherent variant,
suitable for wide parameter-space searches for transients from unknown neutron
stars. We have tested our method by Monte-Carlo simulation, and we find that it
outperforms orthodox maximum-likelihood approaches both in sensitivity and in
parameter-estimation quality.Comment: 20 pages, 9 figures; submitted to PR
Simulations of Solid-on-Solid Models of Spreading of Viscous Droplets
We have studied the dynamics of spreading of viscous non-volatile fluids on
surfaces by MC simulations of SOS models. We have concentrated on the complete
wetting regime, with surface diffusion barriers neglected for simplicity.
First, we have performed simulations for the standard SOS model. Formation of a
single precursor layer, and a density profile with a spherical cap shaped
center surrounded by Gaussian tails can be reproduced with this model.
Dynamical layering (DL), however, only occurs with a very strongly attractive
van der Waals type of substrate potential. To more realistically describe the
spreading of viscous liquid droplets, we introduce a modified SOS model. In the
new model, tendency for DL and the effect of the surface potential are in part
embedded into the dynamics of the model. This allows a relatively simple
description of the spreading under different conditions, with a temperature
like parameter which strongly influences the droplet morphologies. Both rounded
droplet shapes and DL can easily be reproduced with the model. Furthermore, the
precursor width increases proportional to the square root of time, in
accordance with experimental observations. PACS: 68.10.Gw, 05.70.Ln, 61.20.Ja.Comment: to appear in Physica A (1994), standard LaTex, 20 page
Simulations of Solid-on-Solid Models of Spreading of Viscous Droplets
We have studied the dynamics of spreading of viscous non-volatile fluids on
surfaces by MC simulations of SOS models. We have concentrated on the complete
wetting regime, with surface diffusion barriers neglected for simplicity.
First, we have performed simulations for the standard SOS model. Formation of a
single precursor layer, and a density profile with a spherical cap shaped
center surrounded by Gaussian tails can be reproduced with this model.
Dynamical layering (DL), however, only occurs with a very strongly attractive
van der Waals type of substrate potential. To more realistically describe the
spreading of viscous liquid droplets, we introduce a modified SOS model. In the
new model, tendency for DL and the effect of the surface potential are in part
embedded into the dynamics of the model. This allows a relatively simple
description of the spreading under different conditions, with a temperature
like parameter which strongly influences the droplet morphologies. Both rounded
droplet shapes and DL can easily be reproduced with the model. Furthermore, the
precursor width increases proportional to the square root of time, in
accordance with experimental observations. PACS: 68.10.Gw, 05.70.Ln, 61.20.Ja.Comment: to appear in Physica A (1994), standard LaTex, 20 page
Phase structure of intrinsic curvature models on dynamically triangulated disk with fixed boundary length
A first-order phase transition is found in two types of intrinsic curvature
models defined on dynamically triangulated surfaces of disk topology. The
intrinsic curvature energy is included in the Hamiltonian. The smooth phase is
separated from a non-smooth phase by the transition. The crumpled phase, which
is different from the non-smooth phase, also appears at sufficiently small
curvature coefficient . The phase structure of the model on the disk is
identical to that of the spherical surface model, which was investigated by us
and reported previously. Thus, we found that the phase structure of the fluid
surface model with intrinsic curvature is independent of whether the surface is
closed or open.Comment: 9 pages with 10 figure
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