Discrimination des modes pour l’étude des discontinuités dans les circuits planaires

The study of the dispersion phenomena of a planar transmission line in boxed structure is very intricate. In this paper we will try to determine the origin of the different modes by varying the geometrical parameters of the structure at a given frequency. We will also study the complex modes and their effects on thee characterisation of the planar discontinuities.     

Evidence for the activation-controlled galvanostatic growth of thin Anode Films on Tin in some neutral media

Growth of thin transparent anode films on Sn in neutral media has been studied by measuring galvanostatic anodic charging curves in phosphate buffer, 0.1M KCl and 0.1M Na₂SO₄ solutions (pH 6.6–6.7) at low current densities. The experimental technique is essentially the same as that used in previous investigations on valve metals. Although the rise in the anode potential of Sn does not exceed 2.0 V, the shape of the anodic charging curve is identical to that observed on valve metals: being thus composed of a linear and a non-linear region. Application of the kinetics of galvanostatic anodization to the results on Sn show that: (i) the oxide formation rate is linearly related to the ionic current density i by a double logarithmic plot, (ii) the reciprocal capacity is linearly related to log i, and (iii) the Tafel behaviour is exhibited at constant charge. These relations indicate that the anode film growth occurs by an activation-controlled ion conduction under the influence of the electric field across the film phase according to an exponential law. Treatment of the results allows the estimation of some kinetic parameters of film growth, e.g.: (i) the constants a and b of the empirical relation between oxide formation rate and i, (ii) the constants A and B of the exponential law, (iii) the electric field which is of the order of 10⁶ V/cm in phosphate, and 10⁷ V/cm in both chloride and sulphate solutions, and (iv) the effective activation distance for the ionic jump over the energy barrier associated with cation transport within the film, whereupon relative (and not absolute) values can only be obtained. Comparison between the present results and previous ones (also on Sn) taken by potentiodynamic technique shows that while diffusion kinetics play an important role in the formation of thick anode films by the potentiodynamic technique, activation-controlled kinetics explain the present results on the galvanostatic formation of thin transparent anode films.     

Response Time Performance of Videotex Systems

Videotex is an interactive information system which provides a variety of services to its users. Examples of such services are information retrieval, software distribution, transaction processing, and message handling. An important aspect of the quality of service experienced by a videotex user is the response time. We consider the use of mixed individual/broadcast delivery to enhance the response time performance. Broadcast delivery is attractive for information retrieval applications where several users may be requesting the same information page, and a single broadcast of this page will satisfy all requests simultaneously. Individual response, however, is required for transaction-oriented services and for the retrieval of confidential information. A queueing model is developed to study the performance of videotex systems under mixed delivery. Analytic results are derived for the mean response time. Numerical examples are presented to show the performance characteristics of mixed delivery, and how it can be used to enhance the response time performance without increasing the processing capacity of the system.     

A tool to minimize the time costs of parallel computations through optimal processing power allocation

In this paper, we present a software system, OPAS (Optimal Allocation System), that incorporates the optimal allocation policy in the analysis of the time-cost behaviour of parallel computations. OPAS assumes that the underlying system which supports the executions of parallel computations has a finite number of processors, that all the processors have the same speed and that the communication is achieved through a shared memory. OPAS defines the time cost as a function of the input, the algorithm, the data structure, the processor speed, the number of processors and the processing power allocation. In analysing the time cost of a computation, OPAS first uses the optimal allocation policy that we developed previously to determine the amount of processing power each node receives and then derives the computation's time cost. OPAS can evaluate different time-cost behaviours, such as the minimum time cost, the maximum time cost, the average time cost and the time-cost variance. It can also determine the speed-up and efficiency, and plot the time-cost curve and time-cost distribution.     

A comparative analysis of hardware and software fault tolerance: Impact on software reliability engineering

Today's digital systems are growing increasingly complex, and are being used in increasingly critical functions. The first premise makes them more prone to contain faults, and the second premise makes their failure less tolerable. This widening gap highlights the need for fault tolerant techniques, which make provisions for reliable operation of digital systems despite the presence and occasional manifestation of faults. In this paper we present a brief comparative survey of fault tolerance as it arises in hardware systems and software systems. We discuss logical models as well as statistical models of fault tolerance, and use these models to analyze design tradeoffs of fault tolerant systems. This revised version was published online in June 2006 with corrections to the Cover Date.     

A scenario-based reliability analysis approach for component-based software

This paper introduces a reliability model, and a reliability analysis technique for component-based software. The technique is named Scenario-Based Reliability Analysis (SBRA). Using scenarios of component interactions, we construct a probabilistic model named Component-Dependency Graph (CDG). Based on CDG, a reliability analysis algorithm is developed to analyze the reliability of the system as a function of reliabilities of its architectural constituents. An extension of the proposed model and algorithm is also developed for distributed software systems. The proposed approach has the following benefits: 1) It is used to analyze the impact of variations and uncertainties in the reliability of individual components, subsystems, and links between components on the overall reliability estimate of the software system. This is particularly useful when the system is built partially or fully from existing off-the-shelf components; 2) It is suitable for analyzing the reliability of distributed software systems because it incorporates link and delivery channel reliabilities; 3) The technique is used to identify critical components, interfaces, and subsystems; and to investigate the sensitivity of the application reliability to these elements; 4) The approach is applicable early in the development lifecycle, at the architecture level. Early detection of critical architecture elements, those that affect the overall reliability of the system the most, is useful in delegating resources in later development phases.     

Factorial design of experiment (DOE) for parametric exergetic investigation of a steam methane reforming process for hydrogen production

Hydrogen is expected to play a significant role in future energy systems. The efficient production of hydrogen at a minimum cost and in an environmentally acceptable manner is crucial for the development of a hydrogen-including economy. The exergy analysis is a powerful tool to quantify sustainable development potential. An important aspect of sustainable development is minimizing irreversibility. The purpose of this study is to perform the exergy analysis of a steam methane reforming (SMR) process for hydrogen production. As a first step, an exergy analysis of an existing process is shown to be an efficient tool to critically examine the process energy use and to test for possible savings in primary energy consumption. The results of this investigation prove that the exergetic efficiency of the SMR process is 65.47%, and the majority of destroyed exergy is localized in the reformer with a 65.81% contribution to the whole process destroyed exergy. Next, an exergetic parametric study of the SMR has been carried out with a factorial design of experiment (DOE) method. The influence of the reformer operating temperature and pressure and of the steam to carbon ratio (S/C) on the process exergetic efficiency has been studied. A second-order polynomial mathematical model has been obtained through correlating the exergetic efficiencies with the reformer operating parameters. The results of this study show that the rational choice of these parameters can improve the process exergetic performance.     

Characterization of graphite implanted with chlorine ions using combined Raman microspectrometry and transmission electron microscopy on thin sections prepared by focused ion beam

Graphite was modified by 250 keV ³⁷Cl⁺ ion implantation. Combined Raman microspectrometry/transmission electron microscopy (TEM) studies have been used to characterize the multiscale organization of the graphite structure. The penetration depth of ³⁷Cl⁺ into the graphite sample was limited to the surface (∼200 nm) because of the dissipation of the irradiating ion energy as expected by secondary ion mass spectrometry analysis. Raman microspectrometry appears to be an appropriate tool for studying such scales. Spectra showed a strong increase of defect bands after implantation at a fluence of 5 · 10¹³ ions/cm². In order to examine the structural degradation of the graphite versus the depth at the nanometer scale, the focused ion beam technique seems to be a well-suited method for a relevant coupling of Raman and TEM observations.     

Cylindrical projection improved formulation for sunshine assessment in buildings

A method which extends the use of the cylindrical projection tool for the assessment of sunshine was developed. This method is a computer-based mathematical analysis using trigonometric functions. It enables to project building components and sun’s position in the sky on a two-dimensional cylindrical projection chart for a wide range of simple and complex plane shapes, namely; inclined and non inclined vertical planes, segment of circular planes, semi-circular planes and circular planes. In this method, cylindrical projection will be more handled and manageable and will give more sensitive results. It uses Matlab programming language and its graphical potentialities. It will provide to designers and architects a quick and reasonable assessment of sunshine availability for complex building configurations at any time on any date of a given locality and consequently saving time and achieving a successful project. However, this projection method, may exhibit some limitations of its use in low latitudes (<20°) if the sun is high in the sky.     

An efficient reduced simulation of residual stresses in composite forming processes

An efficient simulation of thermal and mechanical models involved in thermosetting composites forming needs to overcome some numerical difficulties related to: (i) the multi-scale behaviour; (ii) the complex geometries involved needing too many degrees of freedom; (iii) the large time intervals where the solution has to be computed; (iv) the non-linearity of the involved evolution equations; (v) the numerous couplings… In this work, an efficient strategy based on a separated representation is proposed. This method enables to avoid the use of an incremental strategy and can lead to impressive computing time savings especially when the model involves fine meshes and very small time steps. The local non-linear chemical kinetics and its coupling with the global heat balance equation are naturally introduced in the separated representation algorithm. Moreover, the dependence of the thermal conductivity and the specific heat on the temperature and on the reaction advancement degree are also taken into account. Knowing the history of the temperature field, the separated representation is again used to solve the thermo-mechanical problem in order to determine the residual stresses.