3,364 research outputs found
Image charges in spherical geometry: Application to colloidal systems
The effects of image charges (i.e., induced surface charges of polarization)
in spherical geometry and their implication for charged colloidal systems are
investigated. We study analytically and exactly a single microion interacting
with a dielectric sphere and discuss the similarities and discrepancies with
the case of a planar interface. By means of extensive Monte Carlo (MC)
simulations, we study within the framework of the primitive model the effects
of image charges on the structure of the electrical double layer. Salt-free
environment as well as salty solutions are considered. A remarkable finding of
this study is that the position of the maximum in the counterion density
(appearing at moderately surface charge density) remains quasi-identical,
regardless of the counterion valence and the salt content, to that obtained
within the \textit{single}-counterion system.Comment: 14 pages, 12 figures, RevTeX4, to appear in J. Chem. Phy
Hydrodynamic capabilities of an SPH code incorporating an artificial conductivity term with a gravity-based signal velocity
This paper investigates the hydrodynamic performances of an SPH code
incorporating an artificial heat conductivity term in which the adopted signal
velocity is applicable when gravity is present. In accordance with previous
findings it is shown that the performances of SPH to describe the development
of Kelvin-Helmholtz instabilities depend strongly on the consistency of the
initial condition set-up and on the leading error in the momentum equation due
to incomplete kernel sampling. An error and stability analysis shows that the
quartic B-spline kernel (M_5) possesses very good stability properties and we
propose its use with a large neighbor number, between ~50 (2D) to ~ 100 (3D),
to improve convergence in simulation results without being affected by the
so-called clumping instability. SPH simulations of the blob test show that in
the regime of strong supersonic flows an appropriate limiting condition, which
depends on the Prandtl number, must be imposed on the artificial conductivity
SPH coefficients in order to avoid an unphysical amount of heat diffusion.
Results from hydrodynamic simulations that include self-gravity show profiles
of hydrodynamic variables that are in much better agreement with those produced
using mesh-based codes. In particular, the final levels of core entropies in
cosmological simulations of galaxy clusters are consistent with those found
using AMR codes. Finally, results of the Rayleigh-Taylor instability test
demonstrate that in the regime of very subsonic flows the code has still
several difficulties in the treatment of hydrodynamic instabilities. These
problems being intrinsically due to the way in which in standard SPH gradients
are calculated and not to the implementation of the artificial conductivity
term.Comment: 26 pages, 15 figures, accepted for publication in A&
A Case Study of Small Scale Structure Formation in 3D Supernova Simulations
It is suggested in observations of supernova remnants that a number of large-
and small-scale structures form at various points in the explosion.
Multidimensional modeling of core-collapse supernovae has been undertaken since
SN1987A, and both simulations and observations suggest/show that
Rayleigh-Taylor instabilities during the explosion is a main driver for the
formation of structure in the remnants.
We present a case study of structure formation in 3D in a \msol{15} supernova
for different parameters. We investigate the effect of moderate asymmetries and
different resolutions of the formation and morphology of the RT unstable
region, and take first steps at determining typical physical quantities (size,
composition) of arising clumps. We find that in this progenitor the major RT
unstable region develops at the He/OC interface for all cases considered. The
RT instabilities result in clumps that are overdense by 1-2 orders of magnitude
with respect to the ambient gas, have size scales on the level of a few % of
the remnant diameter, and are not diffused after the first yrs of the
remnant evolution, in the absence of a surrounding medium.Comment: 59 pages, 34 figure
Smoothed Dissipative Particle Dynamics model for mesoscopic multiphase flows in the presence of thermal fluctuations
Thermal fluctuations cause perturbations of fluid-fluid interfaces and highly
nonlinear hydrodynamics in multiphase flows. In this work, we develop a novel
multiphase smoothed dissipative particle dynamics model. This model accounts
for both bulk hydrodynamics and interfacial fluctuations. Interfacial surface
tension is modeled by imposing a pairwise force between SDPD particles. We show
that the relationship between the model parameters and surface tension,
previously derived under the assumption of zero thermal fluctuation, is
accurate for fluid systems at low temperature but overestimates the surface
tension for intermediate and large thermal fluctuations. To analyze the effect
of thermal fluctuations on surface tension, we construct a coarse-grained Euler
lattice model based on the mean field theory and derive a semi-analytical
formula to directly relate the surface tension to model parameters for a wide
range of temperatures and model resolutions. We demonstrate that the present
method correctly models the dynamic processes, such as bubble coalescence and
capillary spectra across the interface
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