Gaussian beam conversion using an axicon

Abstract

The paper presents a theoretical investigation of laser beam tailoring using a transparent diffracting phase plate. It is shown that a linear axicon is able to transform a Gaussian beam either into a super-Gaussian profile, a ring-shaped profile or a doughnut profile. All these different intensity distributions are easily obtained by only changing the width of the incident Gaussian beam.     

Foundations and Practical Implementations of the Cluster Expansion

Different versions of the cluster expansion are explored using the Mo-Ta system as an example. One of the objectives of this work is to establish a clear distinction between phenomenological expansions that express the energy of an alloy in the form of a generalized Ising model, i.e. with constant pair and many body interactions, and cluster expansions that use a set of complete basis functions in configurational space and define the interactions as projections of the energy onto the basis functions. For the latter case, the interactions are functions of concentration and depend, furthermore, on the full state of order of the system. Such dependence is expected since the configurational energy is shown to be a homogeneous function of degree one in the complete set of configurational variables, or correlation functions, with the interactions being the Euler derivatives of the energy with respect to the correlation functions. For the Mo-Ta system we show that, by including the concentration dependence of the interactions either explicitly or through their dependence on volume, the cluster expansion converges significantly faster than the phenomenological Ising-like models commonly used to represent the energies of disordered alloys.     

Modelization of hybrid systems with hydrogen and renewable energy oriented to electric propulsion in sailboats

This paper presents a conceptual model of a hybrid electric sailboat in which energy from electric grid is stored in batteries and energy from renewable energies (eolic, solar and hydro) is stored as hydrogen. The main objective of this model is to study the viability of electrifying traditional sailboats with internal combustion engines into hybrid systems with batteries and fuel cell. The most important advantage of this design is the possibility to reduce up to zero emissions of traditional sailboat. Conversion of renewable energy to hydrogen is performed through an electrolyzer and post conversion to energy is carried out by a fuel cell. The fuel cell with the batteries forms the hybrid system (batteries-fuel cell) for propulsion electrical energy supply. In order to model the boat dynamic and energy systems, modular mathematical models were developed under Matlab^®-Simulink^®, using a fixed-step solver for the simulation of global model. A simulated logic controller manages the global model. In this paper, many models have been used: some of them are based in literature models and others were developed from experimental data. A control strategy has also been developed to manage energy flows and then it has been embedded to Matlab^® language. The global model permits test the performance of the sailboat.     

Ethylene production,respiration and gas exchange modelling in modified atmosphere packaging for banana fruits

A mathematical model was developed from experimental measurements to describe the evolution of the O₂, CO₂ and ethylene in a modified atmosphere packaging system for Cavendish bananas. The respiration and ethylene production in the fruits were experimentally obtained from a closed system method and then represented by Michaelis–Menten equations of enzyme kinetics. The gas transfer through the packaging was described by a Fick's diffusion equation, and the temperature dependence was represented based on the Arrhenius law. The model was validated by packaging the fruit in perforated bags of polypropylene and low density polyethylene at 12 °C for a period of 8 days. With the developed model it was possible to satisfactorily describe the experimental evolution of the gas content in the headspace of the packages, obtaining coefficients of determination (R²) of 0.93 for the O₂ levels, 0.90–0.91 for the CO₂ levels, and 0.89–0.93 for the ethylene levels.     

A Dynamic Harmonic Regression Approach to Power System Modal Identification and Prediction

Abstract—Time series models provide a powerful tool to extract nonstationary features from measured data. In this article, a statistical framework based upon a dynamic harmonic regression model for examining modal behavior is provided. In this model, temporal patterns in measured data are modeled within a stochastic state space setting. Estimates of the states or time-varying parameters are then obtained using an optimal estimation method based on the Kalman filter. Techniques to estimate future values of the unobserved signal are also analyzed. The widely applicable technique is illustrated on both simulated and measured data. Factors that affect the performance of the method are discussed, including the effects of non-linear trends, data quality, and sampling design. Connections with other modal identification methods are also investigated.     

Edible films and coatings to prevent the detrimental effect of oxygen on food quality: Possibilities and limitations

Oxygen has a deleterious effect on the quality of a wide variety of food products. The application of edible films and coatings to food products represents a new approach to solve this problem. Edible films and coatings can include antioxidant agents in their formulation and at the same time, they represent a barrier to oxygen, which results in a better preservation of quality. The water activity of the product, as well as the ambient relative humidity, determines the antioxidant effect of films and coatings. This paper reviews the latest studies dealing with the effectiveness and application of antioxidant films and coatings.     

Theoretical considerations on the modelling of transport in a three-phase electrode and application to a proton conducting solid oxide electrolysis cell

This paper presents a numerical model for fuel cells and electrolysis cells that use cermets as electrodes. The mass and charge continuity equations were demonstrated inside the electrodes and in the divergence term, surface ratios were used instead of more usual volumetric ratios. The Butler–Volmer equation for electrokinetics was used with concentration coefficients in order to predict correctly the concentration effects on the value of the transfer current density in the cermets. In addition, it was considered that the reaction takes place inside all of cermet's volume inste1 ad of a thin layer near the electrolyte. The model was tested for the proton-conducting SOEC technology on a generic cell design and the calculations were performed with COMSOL Multiphysics 4.1™. A parametric analysis was carried out on a proton-conducting SOEC in galvanostatic mode in order to evaluate the influence of parameters on oxygen production across the electrode in the anodic compartment. This analysis showed that the structural parameters of the cermets such as grain radius or volumetric ratios of the conductors play a major role in the distribution of reaction kinetics.