Search results for " method"
showing 10 items of 10455 documents
Analytical design of nonlinear optical loop mirrors for fiber-optic communication systems
2006
International audience; We propose an easy and efficient method for analytically designing nonlinear optical loop mirrors (NOLMs) for fiber-optic communication systems. This analytical design is based on a Taylor series expansion of the transfer function of the NOLM, from which highly stable dynamical regimes can be readily obtained for any desired pulse parameters. We present numerical simulations showing dramatically improved performances in a 160 Gb/s transmission system that incorporates the NOLMs designed analytically.
New QM/MM implementation of the DFTB3 method in the gromacs package.
2015
The approximate density-functional tight-binding theory method DFTB3 has been implemented in the quantum mechanics/molecular mechanics (QM/MM) framework of the Gromacs molecular simulation package. We show that the efficient smooth particle–mesh Ewald implementation of Gromacs extends to the calculation of QM/MM electrostatic interactions. Further, we make use of the various free-energy functionalities provided by Gromacs and the PLUMED plugin. We exploit the versatility and performance of the current framework in three typical applications of QM/MM methods to solve biophysical problems: (i) ultrafast proton transfer in malonaldehyde, (ii) conformation of the alanine dipeptide, and (iii) el…
Multiple Crack Localization and Debonding Mechanisms for Thin Thermal Coating Films
2020
Experimental tests, carried out on small scale alloy specimens covered on one side with a thin thermal coating, have shown complex failure mechanisms. The failure mechanisms observed are due to the competition between two fracture mechanisms. The two mechanisms are: (i) Vertical tensile coating surface cracks and (ii) debonding shear decohesion mechanisms along the interface between the coating and the substrate. The present paper analyzes the mechanical problem of the nonlinear behavior thin film on a stiff substrate adopting a computational approach. Namely, incremental 2D nonlinear finite element simulations. The stiff superalloy substrate is modeled as a thermo-elastic material. The coa…
Thermoconvective instability and local thermal non-equilibrium in a porous layer with isoflux-isothermal boundary conditions
2014
The effects of lack of local thermal equilibrium between the solid phase and the fluid phase are taken into account for the convective stability analysis of a horizontal porous layer. The layer is bounded by a pair of plane parallel walls which are impermeable and such that the lower wall is subject to a uniform flux heating, while the upper wall is isothermal. The local thermal non-equilibrium is modelled through a two-temperature formulation of the energy exchange between the phases, resulting in a pair of local energy balance equations: one for each phase. Small-amplitude disturbances of the basic rest state are envisaged to test the stability. Then, the standard normal mode procedure is…
An Explicit Model for the Thermal-Mechanical Analysis of Hot Metal Forming Processes
1995
Abstract In the paper the authors propose a new finite element code for the coupled thermal-mechanical analysis of hot metal forming processes. As regards the mechanical problem, an explicit algorithm based on the solution of the dynamic equilibrium equation and an explicit time integration scheme is used, while the heat transfer analysis is based on the solution of the thermal equilibrium equations; in order to put the thermal problem in an explicit linear form a three level scheme has been employed for the discretization of the time variable. The model is based on a staggered procedure, in which the mechanical and the thermal analysis are carried out with respect to different time horizon…
Indium-Gallium Segregation inCuInxGa1−xSe2: AnAb Initio–Based Monte Carlo Study
2010
Thin-film solar cells with ${\mathrm{CuIn}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}{\mathrm{Se}}_{2}$ (CIGS) absorber are still far below their efficiency limit, although lab cells already reach 20.1%. One important aspect is the homogeneity of the alloy. Large-scale simulations combining Monte Carlo and density functional calculations show that two phases coexist in thermal equilibrium below room temperature. Only at higher temperatures, CIGS becomes more and more a homogeneous alloy. A larger degree of inhomogeneity for Ga-rich CIGS persists over a wide temperature range, which contributes to the observed low efficiency of Ga-rich CIGS solar cells.
Simulation of fluid-solid coexistence in finite volumes: A method to study the properties of wall-attached crystalline nuclei
2012
The Asakura-Oosawa model for colloid-polymer mixtures is studied by Monte Carlo simulations at densities inside the two-phase coexistence region of fluid and solid. Choosing a geometry where the system is confined between two flat walls, and a wall-colloid potential that leads to incomplete wetting of the crystal at the wall, conditions can be created where a single nanoscopic wall-attached crystalline cluster coexists with fluid in the remainder of the simulation box. Following related ideas that have been useful to study heterogeneous nucleation of liquid droplets at the vapor-liquid coexistence, we estimate the contact angles from observations of the crystalline clusters in thermal equil…
Simulation of binary fluids exposed to selectively adsorbing walls: a method to estimate contact angles and line tensions
2011
For an understanding of interfacial phenomena of fluids on the nanoscale a detailed knowledge of the excess free energies of fluids due to walls is required, as well as of the interfacial tension between coexisting fluid phases. A description of simulation approaches to solve this task is given for a suitable model binary (A + B) fluid. Sampling the order parameter distribution of the system without walls, the curvature dependent and flat interfacial tensions of coexisting ‘bulk’ phases is extracted. In a thin film geometry, the difference in wall free energies is found via a new thermodynamic integration method. Thus the contact angle θ of macroscopic droplets is estimated from Young's equ…
Microcanonical Determination of the Interface Tension of Flat and Curved Interfaces from Monte Carlo Simulations
2012
The investigation of phase coexistence in systems with multi-component order parameters in finite systems is discussed, and as a generic example, Monte Carlo simulations of the two-dimensional q-state Potts model (q=30) on LxL square lattices (40<=L<=100) are presented. It is shown that the microcanonical ensemble is well-suited both to find the precise location of the first order phase transition and to obtain an accurate estimate for the interfacial free energy between coexisting ordered and disordered phases. For this purpose, a microcanonical version of the heatbath algorithm is implemented. The finite size behaviour of the loop in the curve describing the inverse temperature vers…
Computer Simulation Studies of Chain Dynamics in Polymer Brushes
2012
Center-of-mass and single monomer motion in grafted chains comprising a strongly stretched polymer brush in thermal equilibrium are studied by large scale molecular dynamics and Monte Carlo simulations of a coarse-grained model. Good solvent conditions are assumed. Our findings seriously question earlier theoretical predictions about the relaxation described by Rouse dynamics of brush coatings. Thus, the correlation functions of parallel and perpendicular components of the mean distance of the center-of-mass from the grafting site, the squared gyration radius and end-to-end distance, are found to deviate strongly from a simple exponential decay. While the relaxation times extracted from the…