Control of the quality of laser surface texturing

Since the conception of the first lasers in the 60’s, the knowledge and consequently the applications of laser have been widely increased. Laser surface texturing is a specific field among the various applications of the laser. This technique is employed in the aim of improving tribological performances for instance. This paper presents the laser surface texturing of a heterogeneous material. This material is lamellar cast iron. It has been chosen for its good friction properties. The textured surface is composed of grooves or dimples. The dimensions are micrometric. For the grooves, different cross sections have been engraved: semicircular, rectangular, trapezoidal and triangular. The dimples are cylindrical. All these laser textured surfaces are engraved with the laser manufacturing machine: DML 40 SI of Gildemeister (Germany). This machine is equipped with a galvanometric scanner, which allows tilted surfaces to be engraved. Various machining strategies and the following results are discussed in function of the different desired laser surface texturing results. In addition, to increase the aspect of the engraved surfaces, laser polishing is employed. Thanks to a defocalisation of the laser beam on the surface, the material is not ablated but melted. To be able to compare the different laser machining process, several techniques of control have been used: roughness profilometry, scanning electron microscopy and non contacting optical measurement. The rules of use of these methods must be defined bearing in mind their inherent limitations.     

Polynomial chaos representation of a stochastic preconditioner

A method is developed in this paper to accelerate the convergence in computing the solution of stochastic algebraic systems of equations. The method is based on computing, via statistical sampling, a polynomial chaos decomposition of a stochastic preconditioner to the system of equations. This preconditioner can subsequently be used in conjunction with either chaos representations of the solution or with approaches based on Monte Carlo sampling. In addition to presenting the supporting theory, the paper also presents a convergence analysis and an example to demonstrate the significance of the proposed algorithm. Copyright © 2005 John Wiley & Sons, Ltd.     

Coagulation of Concentrated Suspensions of Ultrafine Alumina Powders by pH Shift

Highly concentrated suspensions of ultrafine alumina powders are prepared using 2-phosphonobutane-1,2,4-tricarboxylic acid as dispersant. The coagulation of these suspensions is carried out at around 278 K by adding a blend of acetic anhydride plus ethylene glycol. Coagulation takes place through a pH shift because of a time-delayed hydration of the acetic anhydride. The coagulation kinetics are studied by viscosity measurements with respect to the temperature and composition of the coagulant blend. Finally, the method is used to produce complex 3D parts of large dimensions for bio-applications by direct coagulation casting and the fired bodies are characterized.     

Comparing Numerical Methods for Solving the Competitive Storage Model

This paper compares numerical methods for solving the competitive storage model. Because storage implies a nonnegativity constraint on stocks, the solution methods must be considered carefully. The model is solved using value function iteration and several projection approaches, including parameterised expectations and decision rules approximation. In considering a storage model with convenience yield, in which the inequality constraint is smoothed, perturbation methods are also applied. Parameterised expectations approximation proves to be the most accurate method, whereas perturbation techniques are shown inadequate for solving this highly nonlinear model. The endogenous grid method allows rapid solution if supply is assumed to be inelastic.     

Autocalibration-based partioning relationship and parallax relation for head-mounted eye trackers

This paper presents new methods to calibrate a head-mounted Eye Tracker (ET) automatically, as well as a new way to obtain an estimated point of regard (POR), taking account of the parallax. Calibration is performed in real time; it is easy for the user who just needs to look at one calibration pattern for a few seconds before starting. This method provides a very important couple of points which helps to use a local relationship to compute the POR instead of a global one. This approach significantly improves the precision of the points of regard when the scene camera is mounted with a short focal lens. An estimation of POR when the user looks somewhere outside the calibration distance is also proposed. This estimation is based on an ET modelling such as a stereovision system, to take account of the parallax effect. The aim of this study is to simplify the use of ET techniques for “non-initiated” people, especially here learner drivers.     

Combined Experimental and Numerical Flow Field Study of Inclined Louvered Fins

In this study the flow behavior within an interrupted fin design, the inclined louvered fin, is investigated experimentally through visualization and numerically through computational fluid dynamics (CFD) simulation. The inclined louvered fin is a hybrid of the offset strip fin and standard louvered fin, aimed at improved performance at low Reynolds numbers for compact heat exchangers. The flow behavior is studied in six geometrically different configurations over a range of Reynolds numbers and quantified using the concept of “fin angle alignment factor,” which is related to the flow efficiency in louvered fins. The experimental data resulted in a discrete data set of local fin angle alignment factor values, which were used to validate the simulations. Using these validated cases it is shown that the graphical measurement method can be distorted by recirculation zones, resulting in erroneous values. Care should thus be taken when performing graphical measurement of the mean flow angle based on dye injection images. The transition from steady laminar to unsteady flow in inclined louvered fins is geometrically triggered and occurs at lower Reynolds numbers compared to slit fins and standard louvered fins. This property can potentially be used to further improve on the performance of interrupted fin surfaces.     

Arginine of Chlamydomonas reinhardtii [Fe–Fe] hydrogenase HydA1 plays a crucial role in electron transfer to its catalytic center

[Fe–Fe] hydrogenases, with hydrogen evolution activities outperforming [Ni–Fe] hydrogenases by 3–4 orders of magnitude, are still the most promising enzyme class for hydrogen production purposes. For Chlamydomonas reinhardtii [Fe–Fe] hydrogenase HydA1 the question of catalytic activity and electron transport is of main importance. Here we report the characterization of two mutant forms of C. reinhardtii HydA1. An aspartic acid in place of arginine¹⁷¹ leads to a six-fold increase of the catalytic activity in comparison to the wild type protein during methyl viologen-dependent hydrogen production. Tryptophan in position 171 does not result in any change in methyl viologen-induced activity. At the same time these mutations lead to a strong decrease in ferredoxin-dependent hydrogen production while the catalytic center of mutant forms stays intact. The localization of this amino acid (arginine¹⁷¹) in the environment of CrHydA1 H-cluster indicates that the limitation of the catalytic activity of this hydrogenase is due to the electron transfer step to the catalytic center where the reduction of protons takes place.     

Multi-scale coupling between two dynamical models for PEMFC aging prediction

In this paper, an approach allowing connecting two numerical models for the simulation of the PEMFC operation and durability at different physical scales is presented. After explaining the interest of coupling them to study the interactions between the fuel cell system and the fuel cell itself, along with lifetime concerns, the feasibility of the task is assessed. Then the numerical approach to achieve this coupling is presented. Finally the response of the coupled models is examined to check its validity, and first results are presented. Predicted fuel cell lifetime trends when changing the stack operation conditions are shown, which highlight the presence of optima concerning temperature, demanded current, pressure and O₂ stoichiometry. Two operation modes are then compared in terms of their impact on the fuel cell performance decay, showing that power cycles are more damaging the fuel cell than nominal operation.