172,268 research outputs found
Locally preferred structures and many-body static correlations in viscous liquids
We investigate the influence of static correlations beyond the pair level on
the dynamics of selected model glass-formers. We compare the pair structure,
angular distribution functions, and statistics of Voronoi polyhedra of two
well-known Lennard-Jones mixtures as well as of the corresponding
Weeks-Chandler-Andersen variants, in which the attractive part of the potential
is truncated. By means of the Voronoi construction we identify the atomic
arrangements corresponding to the locally preferred structures of the models.
We find that the growth of domains formed by interconnected locally preferred
structures signals the onset of the slow dynamics regime and allows to
rationalize the different dynamic behaviors of the models. At low temperature,
the spatial extension of the structurally correlated domains, evaluated at
fixed relaxation time, increases with the fragility of the models and is
systematically reduced by truncating the attractions. In view of these results,
proper inclusion of many-body static correlations in theories of the glass
transition appears crucial for the description of the dynamics of fragile
glass-formers.Comment: 9 pages, 8 figures, added two tables, minor revisions to the tex
Approximation of the critical buckling factor for composite panels
This article is concerned with the approximation of the critical buckling factor for thin composite plates. A new method to improve the approximation of this critical factor is applied based on its behavior with respect to lamination parameters and loading conditions. This method allows accurate approximation of the critical buckling factor for non-orthotropic laminates under complex combined loadings (including shear loading). The influence of the stacking sequence and loading conditions is extensively studied as well as properties of the critical buckling factor behavior (e.g concavity over tensor D or out-of-plane lamination parameters). Moreover, the critical buckling factor is numerically shown to be piecewise linear for orthotropic laminates under combined loading whenever shear remains low and it is also shown to be piecewise continuous in the general case. Based on the numerically observed behavior, a new scheme for the approximation is applied that separates each buckling mode and builds linear, polynomial or rational regressions for each mode. Results of this approach and applications to structural optimization are presented
Disentangling causal webs in the brain using functional Magnetic Resonance Imaging: A review of current approaches
In the past two decades, functional Magnetic Resonance Imaging has been used
to relate neuronal network activity to cognitive processing and behaviour.
Recently this approach has been augmented by algorithms that allow us to infer
causal links between component populations of neuronal networks. Multiple
inference procedures have been proposed to approach this research question but
so far, each method has limitations when it comes to establishing whole-brain
connectivity patterns. In this work, we discuss eight ways to infer causality
in fMRI research: Bayesian Nets, Dynamical Causal Modelling, Granger Causality,
Likelihood Ratios, LiNGAM, Patel's Tau, Structural Equation Modelling, and
Transfer Entropy. We finish with formulating some recommendations for the
future directions in this area
A study of the vibration of a horizontal U-bend subjected to an internal upwards flowing air-water mixture
U-bends are a common geometry in heat exchangers. In this paper, a U-bend in the vertical plane connected to horizontal straight pipes is considered. An initially stratified water/air flow moves upwards against gravity. The aim of this research is to investigate the internal flow profile and resulting force when the U-bend is subjected to a stratified air-water flow at the inlet. This is done numerically, i.e. by solving the unsteady Reynolds-averaged Navier-Stokes equations. For low mass flow rates, large gas bubbles are naturally formed at the entrance of the bend. The transient force on the tube allows to determine precisely the time instants of bubble initiation and thus to quantify the bubble frequency. Firstly, the tube is assumed to be rigid and the dependence of force oscillation on the inlet conditions is investigated. Secondly, the influence of the viscosity, wall wetting and the mass flow rate is analyzed. Finally, a fluid-structure interaction calculation is performed in order to quantify the vibration characteristics of the tube
Monte Carlo simulations of interfaces in polymer blends
We review recent simulation studies of interfaces between immiscible
homopolymer phases. Special emphasis is given to the presentation of efficient
simulation techniques and powerful methods of data analysis, such as the
analysis of capillary wave spectra. Possible reasons for polymer
incompatibility and ways to relate model dependent interaction parameters to an
effective Flory Huggins parameter are discussed. Various interfaces are then
considered and characterised with respect to their microscopic structure and
thermodynamic properties. In particular, interfaces between homopolymers of
equal or disparate stiffness are studied, interfaces containing diblock
copolymers, and interfaces confined in thin films. The results are related to
the phase behaviour of ternary homopolymer/copolymer systems, and to wetting
transitions in thin films.Comment: To appear in Annual Reviews of Computational Physics, edt. D.
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