5,405 research outputs found
Atomic discreteness and the nature of structural equilibrium in conductance histograms of electromigrated Cu-nanocontacts
We investigate the histograms of conductance values obtained during
controlled electromigration thinning of Cu thin films. We focus on the question
whether the most frequently observed conductance values, apparent as peaks in
conductance histograms, can be attributed to the atomic structure of the wire.
To this end we calculate the Fourier transform of the conductance histograms.
We find all the frequencies matching the highly symmetric crystallographic
directions of fcc-Cu. In addition, there are other frequencies explainable by
oxidation and possibly formation of hcp-Cu. With these structures we can
explain all peaks occurring in the Fourier transform within the relevant range.
The results remain the same if only a third of the samples are included. By
comparing our results to the ones available in the literature on work-hardened
nanowires we find indications that even at low temperatures of the environment,
metallic nanocontacts could show enhanced electromigration at low current
densities due to defects enhancing electron scattering
Likelihood-Free Parallel Tempering
Approximate Bayesian Computational (ABC) methods (or likelihood-free methods)
have appeared in the past fifteen years as useful methods to perform Bayesian
analyses when the likelihood is analytically or computationally intractable.
Several ABC methods have been proposed: Monte Carlo Markov Chains (MCMC)
methods have been developped by Marjoramet al. (2003) and by Bortotet al.
(2007) for instance, and sequential methods have been proposed among others by
Sissonet al. (2007), Beaumont et al. (2009) and Del Moral et al. (2009). Until
now, while ABC-MCMC methods remain the reference, sequential ABC methods have
appeared to outperforms them (see for example McKinley et al. (2009) or Sisson
et al. (2007)). In this paper a new algorithm combining population-based MCMC
methods with ABC requirements is proposed, using an analogy with the Parallel
Tempering algorithm (Geyer, 1991). Performances are compared with existing ABC
algorithms on simulations and on a real example
The finite-volume method in computational rheology
The finite volume method (FVM) is widely used in traditional computational fluid dynamics (CFD), and many commercial CFD codes are based on this technique which is typically less demanding in computational resources than finite element methods (FEM). However, for historical reasons, a large number of Computational Rheology codes are based on FEM. There is no clear reason why the FVM should not be as successful as finite element based techniques in Computational Rheology and its applications, such as polymer processing or, more recently, microfluidic systems using complex fluids. This chapter describes the major advances on this topic since its inception in the early 1990’s, and is organized as follows. In the next section, a review of the major contributions to computational rheology using finite volume techniques is carried out, followed by a detailed explanation of the methodology developed by the authors. This section includes recent developments and methodologies related to the description of the viscoelastic constitutive equations used to alleviate the high-Weissenberg number problem, such as the log-conformation formulation and the recent kernel-conformation technique. At the end, results of numerical calculations are presented for the well-known benchmark flow in a 4:1 planar contraction to ascertain the quality of the predictions by this method
Channel Flow of a Tensorial Shear-Thinning Maxwell Model: Lattice Boltzmann Simulations
We introduce a nonlinear generalized tensorial Maxwell-type constitutive
equation to describe shear-thinning glass-forming fluids, motivated by a recent
microscopic approach to the nonlinear rheology of colloidal suspensions. The
model captures a nonvanishing dynamical yield stress at the glass transition
and incorporates normal-stress differences. A modified lattice-Boltzmann (LB)
simulation scheme is presented that includes non-Newtonian contributions to the
stress tensor and deals with flow-induced pressure differences. We test this
scheme in pressure-driven 2D Poiseuille flow of the nonlinear generalized
Maxwell fluid. In the steady state, comparison with an analytical solution
shows good agreement. The transient dynamics after startup and cessation of the
pressure gradient are studied; the simulation reproduces a finite stopping time
for the cessation flow of the yield-stress fluid in agreement with previous
analytical estimates
Lattice Boltzmann Simulations of Liquid Crystal Hydrodynamics
We describe a lattice Boltzmann algorithm to simulate liquid crystal
hydrodynamics. The equations of motion are written in terms of a tensor order
parameter. This allows both the isotropic and the nematic phases to be
considered. Backflow effects and the hydrodynamics of topological defects are
naturally included in the simulations, as are viscoelastic properties such as
shear-thinning and shear-banding.Comment: 14 pages, 5 figures, Revte
Transport coefficients of multi-particle collision algorithms with velocity-dependent collision rules
Detailed calculations of the transport coefficients of a recently introduced
particle-based model for fluid dynamics with a non-ideal equation of state are
presented. Excluded volume interactions are modeled by means of biased
stochastic multiparticle collisions which depend on the local velocities and
densities. Momentum and energy are exactly conserved locally. A general scheme
to derive transport coefficients for such biased, velocity dependent collision
rules is developed. Analytic expressions for the self-diffusion coefficient and
the shear viscosity are obtained, and very good agreement is found with
numerical results at small and large mean free paths. The viscosity turns out
to be proportional to the square root of temperature, as in a real gas. In
addition, the theoretical framework is applied to a two-component version of
the model, and expressions for the viscosity and the difference in diffusion of
the two species are given.Comment: 31 pages, 8 figures, accepted by J. Phys. Cond. Matte
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