Pseudospectral method of solution of the Fitzhugh–Nagumo equation

We present a study of the convergence of different numerical schemes in the solution of the Fitzhugh–Nagumo equations in the form of two coupled reaction diffusion equations for activator and inhibitor variables. The diffusion coefficient for the inhibitor is taken to be zero. The Fitzhugh–Nagumo equations, have spatial and temporal dynamics in two different scales and the solutions exhibit shock-like waves. The numerical schemes employed are a Chebyshev multidomain method, a finite difference method and the method developed by Barkley [D. Barkley, A model for fast computer simulation of excitable media, Physica D, 49 (1991) 61–70]. We consider two different models for the local dynamics. We present results for plane wave propagation in one dimension and spiral waves for two dimensions. We use an operator splitting method with the Chebyshev multidomain approach in order to reduce the computational time. Zero flux boundary conditions are imposed on the solutions.     

A new approach to automatic radiation spectrum analysis

The application of adaptive methods to the solution of the automatic radioisotope identification problem using the energy spectrum is described. The identification is carried out by means of neural networks, which allow the use of relatively reduced computational structures, while keeping high pattern recognition capability. In this context, it has been found that one of these simple structures, once adequately trained, is quite suitable to identify a given isotope present in a mixture of elements as well as the relative proportions of each identified substance. Preliminary results are presented, and are deemed good enough to consider these adaptive structures as powerful and simple tools in the automatic spectrum analysis     

Evaluation of Three Methods Proposed for the Computation of Inter-TSO Payments in the Internal Electricity Market of the European Union

Parties to the Internal Electricity Market of the European Union (IEM) decided in 2001 to abolish the method of pancaking of transmission tariffs for cross-border transactions that was originally in place. Instead, they have agreed to implement a system whereby national transmission tariffs provide access to the entire IEM. This system is supplemented by a scheme of inter-TSO payments. However, conflict may arise if the compensation that a country must pay another one is not in accordance with the electrical usage that the former is making of the grid of the latter. For instance, Inter TSO Compensation methods (ITC methods) implicitly allocate the cost of any existing or new transmission line. Therefore, the adoption of an inefficient method may be an obstacle for building some needed regional grid investments. Consequently, one should give careful consideration to the selection of the ITC method. This paper analyzes, both qualitatively and quantitatively, the implementation of the most relevant ITC methods that have been considered so far in the European debate. When assessing each method from a conceptual point of view, considerable attention is devoted to the critical examination of its main underlying assumptions.     

Pulse shaping for high data rate ultra‐wideband wireless transmission under the Russian spectral emission mask

This paper addresses impulse‐radio ultra‐wideband (IR‐UWB) transmission under the Russian spectral emission mask for unlicensed UWB radio communications. Four pulse shapes are proposed and their bit error rate (BER) performance is both estimated analytically and evaluated experimentally. Well‐known shapes such as the Gaussian, root‐raised cosine, hyperbolic secant, and the frequency B‐spline wavelet are used to form linear combinations of component pulses, shaped to make efficient use of the spectral emission mask. Analytical BER values are derived using a Nakagami‐m model, and good agreement is found with the experimentally obtained BER. The proposed pulse shapes allow IR‐UWB transmission with BERs below the limit for a 7% overhead forward error correction, achieving distances of up to 6.5 m at 1 Gbit/s, 4.5 m at 1.25 Gbit/s, and 1 m at  Gbit/s. These results confirm the viability of IR‐UWB transmission under the strict regulations of the Russian spectral emission mask.     

Synthesis and characterisation of Ti6Al4V/xTa alloy processed by solid state sintering

ABSTRACT

This work investigates the effect of Ta particle addition into a Ti6Al4V alloy processed by solid state sintering. The volume fraction of Ta ranged between 0 and 30?vol.-%. The sintering kinetics of powder mixes are evaluated by dilatometry. Sintered materials are characterised by SEM and XRD, and their mechanical properties are obtained from microhardness and compression tests. Sintering behaviour and final microstructure are affected by Ta particles, which slow down the densification, lower the temperature of α-to-β phase transition and stabilise the β phase. Mechanical properties, as microhardness, Young’s modulus and yield stress, depend on the microstructure reached after sintering and on the residual porosity. An equation expressing the Young’s modulus of Ti6Al4V/xTa alloy as function of x and porosity is proposed and validated. The materials with at least 20?vol.-% of Ta exhibited a high strength to modulus ratio, which is suitable for orthopaedic implants.     

Phase‐separation process in a poly(methyl methacrylate)‐modified epoxy system: A novel approach to understanding the effect of the curing temperature on the final morphology

In this study, the physical and chemical changes of a poly(methyl methacrylate) (PMMA)‐modified epoxy system were examined to understand the effect of the curing conditions on its final morphology. The curing process of the PMMA–epoxy reactive system was complementarily analyzed by Fourier transform infrared spectroscopy in the near range (FT‐NIR) and fluorescence spectroscopy. The relationships among (1) the chemical conversion of the curing reaction, (2) the first moment of the fluorescence emission band (〈ν〉) arising from a chromophore chemically bonded to the epoxy reactive system, (3) the phase‐separation process, and (4) the dynamics of the epoxy thermoset during its curing process are discussed. From a chemical point of view, FT‐NIR did not reveal any significant change in the curing reaction with the presence of 2 wt % PMMA. However, in terms of physical changes, the analysis of the fluorescence response clearly showed variations in the curing reaction due to the presence of the thermoplastic polymer. Also, fluorescence allowed the estimation of the glass‐transition temperature of the system with curing when the reaction was diffusion‐controlled, whereas Fourier transform infrared spectroscopy was not sensible enough. In the second part of this study, scanning electron microscopy images of the PMMA‐modified epoxy system were analyzed to understand the effect of the temperature on the final morphology when the amount of thermoplastic was below the critical volume fraction. A linear dependence between the inverse of the mean area of the thermoplastic‐rich domains and the inverse of the absolute temperature was obtained. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Hydrogen car fill-up process modeling and simulation

A novel mathematical model is proposed, based on thermodynamics and transport phenomena fundamentals, that aims to capture the hydrogen pressure, temperature and molar volume evolution during a hydrogen vehicle’s fill-up process. Hydrogen’s thermodynamic properties are calculated through the use of the generic cubic equation of state and residual properties. The obtained model gives rise to a set of differential-algebraic equations (DAE), which are then simulated using a hybrid Newton/Runge–Kutta method. The model’s pressure, temperature, volume, and mass flowrate predictions match, within 2%, corresponding experimental data obtained, during a fill-up process, from a fuel cell vehicle’s and the fueling station’s storage tanks. The model also elucidates the two mechanisms contributing to the temperature increase in the vehicle storage tank: heating by Joule–Thomson expansion and heating by compression. It is shown that Joule–Thomson heating dominates at the beginning of the fill-up process, while compression heating dominates towards the end of the fill-up process.     

Optimization of the Impeller Design for Mesenchymal Stem Cell Culture on Microcarriers in Bioreactors

When agitating mesenchymal stem cells adhered on microcarriers in bioreactors, a compromise has to be found between sufficient particle suspension and limitation of hydromechanical stresses. The present study proposes a strategy to improve the design of an ‘elephant ear' impeller at the just‐suspended state by varying its relative size, blade slope angle, and position in the reactor. To do that, computational fluid dynamics simulations were coupled with multi‐objective optimization to minimize the hydromechanical stress encountered by the microcarriers. Two minimization criteria were considered: (P/V)_@p and the energy dissipation function EDC. On the basis of 31 conditions, an optimal impeller geometry is proposed.