Generalized Nakagami-m phase crossing rate

This paper provides simple, exact, new closed-form expressions for the generalized phase crossing rate of Nakagami-m fading channels. Sample numerical results obtained by simulation are presented that validate the formulations developed here. A special case of this formulation is the Rayleigh case, whose result agrees with that obtained elsewhere in the literature. In passing, several new closed-form results concerning the statistics of the envelope, its in-phase and quadrature components, phase, and their time derivatives are obtained.     

Densification of transparent yttrium aluminum garnet (YAG) by SPS processing

Nano-size YAG powder co-mixed with 0.25 wt.% LiF was used to fabricate transparent polycrystalline YAG specimens by means of the Spark Plasma Sintering (SPS) technique. The presence of the LiF additive in the initial nano-powder allows obtaining fully dense disc shaped (up to 4 mm thick) transparent specimens at the outcome of a 2 h treatment at 1300 °C. The presence of LiF plays a key role in the mass transport related effects during the densification of YAG to full density and also in the elimination of the residual carbon contamination, allowing reaching a level of optical transmittance close to the theoretical value.     

Second-order statistics of maximal-ratio and equal-gain combining in Weibull fading

Exact expressions for the level crossing rate and average fade duration of M-branch equal-gain and maximal-ratio combining systems in a Weibull fading environment are presented. The expressions apply to unbalanced, nonidentical, independent diversity channels. In addition, new closed-form solutions for some special cases are obtained.     

Brazilian Solar Resource Atlas CD-ROM

The Brazilian Solar Resource Atlas CD-ROM is intended to be a tool for solar system design, delivering information that relies on a large interactive database, including, also, a modern device for calculating the solar radiation collected by planes inclined at different tilt angles or tracking the sun around one or two axes. The spatial distribution of the daily solar radiation, monthly and yearly averaged, that hits Brazil is presented in 13 colored maps, together with the methodological procedures used for harmonizing the information and designing the maps. A map containing the location of pyranometric and actinographic terrestrial stations, the information which was used for designing the maps, is also presented. The period of collected data runs from 1978 to 1990. Similarly, the atlas contains 13 colored maps, including daily insolation, monthly and yearly averaged, in addition to the map containing the location of the heliographic stations. The CD-ROM stores a database that includes over 500 stations located in Brazil and on the borders of its neighboring countries. The information is given either as daily, monthly average global solar radiation, or daily, monthly average insolation. A search device is included in the database, allowing to navigate over the maps or to select a desired location. The information may be printed as a report. The CD-ROM also includes a software to perform the following calculations: solar radiation estimates on a fixed plane at any inclination on North-South orientation; estimates of solar radiation collected by a plane tracking the sun about one or two axes; generation of synthetic series based on Markov’s Transition Matrixes.     

Physical coloring of multilayered polymeric structures

Multilayered polymer structures made of two transparent, hyperelastic thermoplastic polyurethanes (TPUs) have been studied. The sheets, made by co‐extrusion and layer multiplication techniques, exhibited “physical coloring”, where a preferred wavelength range was reflected and visualized, due to the refractive index difference and nanometric thicknesses of the alternating layers. The extrusion system comprises two extruders and a series of multiplying units, each doubling the number of layers, such that 12 multiplying units render 2^{12 + 1}+1 (= 8,193) layers. For a 0.8 mm‐thick die, the resulting layers after die swell are ～ 150 nm thick. The extrusion temperatures of both grades were adjusted to equilibrate the viscosities and thus stabilize the layers. TPUs are highly elastic, and large stretching ratios allow for significant reduction in thickness. Physical coloring developed in a controlled, reversible manner. The visual color correlates with elongation, and can be offset by choice of initial thickness. Both reflected and transmitted wavelengths change throughout the whole visible range. A model was developed to fit the reflectance data, by introducing a “stress–volume coefficient” due to volume changes during stretch. This model fits the data very well. Characterization by AFM and TEM showed few discontinuous layers and some defects in the structure, nevertheless the multilayer structure is demonstrated. POLYM. ENG. SCI., 59:112–119, 2019. © 2018 Society of Plastics Engineers     

Legacy code and parallel computing: updating and parallelizing a numerical model

In this paper, we present several important details in the process of legacy code parallelization, mostly related to the problem of maintaining numerical output of a legacy code while obtaining a balanced workload for parallel processing. Since we maintained the non-uniform mesh imposed by the original finite element code, we have to develop a specially designed data distribution among processors so that data restrictions are met in the finite element method. In particular, we introduce a data distribution method that is initially used in shared memory parallel processing and obtain better performance than the previous parallel program version. Besides, this method can be extended to other parallel platforms such as distributed memory parallel computers. We present results including several problems related to performance profiling on different (development and production) parallel platforms. The use of new and old parallel computing architectures leads to different behavior of the same code, which in all cases provides better performance in multiprocessor hardware.

     

Dynamic behavior of water wells coupled to PV pumping systems

Evaluation of a water supply system requires an overall analysis of its behavior, including the well performance and drawdown characteristics. The response of the water well is usually characterized by a single relationship between water level and water flow, representing its quasi‐equilibrium behavior. However, for days with large and rapid variations of irradiance this relationship does not correspond to the well‐equilibrium curve. In order to simulate the well behavior it has been developed a model that takes into account the variation of water level, pressure at the entrance of the well and static pressure of the aquifer, all of them related to the water flow. The model also considers the energy conservation equation of the system through the relationship among hydraulic load, water flow, and collected irradiance. Results of simulations show that, for days with large variations in solar irradiance, the static and dynamic analysis exhibit large differences in the instantaneous water head. The dynamic analysis shows that the water well tends to attenuate the fluctuations of the water head through a negative feedback mechanism. The model helps to understand some facts observed in the field. For example, the dispersion of points in the flow rate versus solar irradiance curves. The maximum drawdown obtained with the dynamic model is considerably smaller than the one predicted with the quasi‐equilibrium analysis. That difference can represent some savings in terms of water well hardware (drilling, wiring, and piping) for programs involving the installation of a large number of systems. The slowdown of water column variations, shown by the dynamic model, signifies that the motor‐pump system is not so intensively demanded as the quasi‐equilibrium analysis would suggest, resulting in longer lifetime prospects. Copyright © 2007 John Wiley & Sons, Ltd.     

Design procedure of V-trough cavities for photovoltaic systems

The combination of photovoltaic (PV) systems with V-trough cavities has been identified as an attractive option to reduce, in the short time scale, the prices of the PV electrical energy. In places of good radiation level, the output energy of these devices can be almost doubled, compared to PV flat-plate fixed systems. Additionally, V-trough cavities are simple to manufacture and can be used with conventional (1-sun) solar cells. In this work we present a design procedure for V-trough cavities used in combination with PV generators. The main design requirements are: uniform illumination on the plane of the PV module, within a finite interval of incidence angles; minimum cost of energy; and heat dissipation by natural, passive means. The V-trough cavities depend on two parameters. We obtain a first analytical relation between the concentration ratio (C) and the V-trough angle (ψ) for concentrators with uniform illumination at the absorber. The region of minimum cost of the V-trough PV ensemble yields a second relation. Then, a unique pair of cavity parameters, satisfying the above criteria, is found. A design example of a V-trough cavity for the city of Recife, PE, Brazil, is presented. © 1998 John Wiley & Sons, Ltd.     

A Simple Accurate Method for Generating Autocorrelated Nakagami-m Envelope Sequences

We propose a simple accurate method for generating autocorrelated Nakagami-m envelope sequences. The method allows for arbitrary values of fading parameter and nonisotropic fading scenarios. In essence, Nakagami-m samples are first drawn and then rearranged to match the Nakagami-m autocorrelation. The rearrangement is made in accordance with the rank statistics of an underlying Rayleigh reference sequence with the desired autocorrelation. Examples illustrate the excellent performance of the new method