Parallel Processing

Parallel processing is appearing as a promising approach to large-scale engineering computations. High performance can be achieved by using powerful microprocessors in parallel. The research challenge is to reformulate the problem, develop parallel algorithms, and devise new computational stratagems in order to utilize fully the capabilities of parallel machines. This article presents a review of parallel processing including various interconnection networks in multiprocessors, commercial parallel processors, and parallel programming languages and algorithms. Present applications of parallel processing in civil engineering are also described.     

T6热处理对共晶Al-Si合金力学性能、耐磨和抗蚀性能的影响(英文)

对共晶Al-Si合金的耐磨、抗蚀性能进行研究。通过控制T6热处理参数来改变共晶Al-Si合金的组织,研究组织变化对合金的硬度、强度和拉伸性能的影响,以及对抗冲蚀和腐蚀性能的影响。采用Al-Si合金制备了一典型的农机零部件。并将Al-Si合金与常用的农机用铝材的性能进行比较。结果表明,合金的化学成分、显微组织、载荷、滑动距离和试验环境对材料的耐磨、抗冲蚀和腐蚀性能有很大的影响。铸态Al-Si合金的性能要明显优于传统的铝材,而且T6热处理能够改善Al-Si合金的性能。因此,可采用Al-Si合金来替代传统的铝材制造农机。     

Residual strength evaluation of concrete structural components under fatigue loading

This paper presents methodologies for residual strength evaluation of concrete structural components using linear elastic and nonlinear fracture mechanics principles. The effect of cohesive forces due to aggregate bridging has been represented mathematically by employing tension softening models. Various tension softening models such as linear, bilinear, trilinear, exponential and power curve have been described with appropriate expressions. These models have been validated by predicting the remaining life of concrete structural components and comparing with the corresponding experimental values available in the literature. It is observed that the predicted remaining life by using power model and modified bi-linear model is in good agreement with the corresponding experimental values. Residual strength has also been predicted using these tension softening models and observed that the predicted residual strength is in good agreement with the corresponding analytical values in the literature. In general, it is observed that the variation of predicted residual moment with the chosen tension softening model follows the similar trend as in the case of remaining life. Linear model predicts large residual moments followed by trilinear, bilinear and power models.     

Apparatus and method for heat-run test on high-power PWM converters with low energy expenditure

Before installation, a voltage source converter is usually subjected to heat-run test to verify its thermal design and performance under load. For heat-run test, the converter needs to be operated at rated voltage and rated current for a substantial length of time. Hence, such tests consume huge amount of energy in case of high-power converters. Also, the capacities of the source and loads available in the research and development (R&D) centre or the production facility could be inadequate to conduct such tests. This paper proposes a method to conduct heat-run tests on high-power, pulse width modulated (PWM) converters with low energy consumption. The experimental set-up consists of the converter under test and another converter (of similar or higher rating), both connected in parallel on the ac side and open on the dc side. Vector-control or synchronous reference frame control is employed to control the converters such that one draws certain amount of reactive power and the other supplies the same; only the system losses are drawn from the mains. The performance of the controller is validated through simulation and experiments. Experimental results, pertaining to heat-run tests on a high-power PWM converter, are presented at power levels of 25 kVA to 150 kVA.     

Swift heavy ion irradiation induced modification of structure and surface morphology of BiFeO3 thin film

BiFeO₃ (BFO) thin films of thickness about 800 nm deposited on Si (100) substrates by sol–gel spin coating method were irradiated by 200 MeV Ag ions. Modification of structure and surface morphology of the films under irradiation was studied using glancing incidence X-ray diffraction (GIXRD) and atomic force microscope (AFM). Fluence dependence of GIXRD peak intensity indicated formation of 10 nm diameter cylindrical amorphous columns in crystalline BFO due to 200 MeV Ag ion irradiation. AFM analysis indicated that the pristine film consists of agglomerated grains with diffuse grain boundary. Irradiation led to reduced agglomeration of the grains with the formation of sharper grain boundaries. The rms roughness (σ _rms) estimated from AFM analysis increased from 6·2 in pristine film to 12·7 nm when the film irradiated at a fluence of 1 × 10¹¹ ions cm ^???2 . Further irradiation led to decrease of σ _rms which finally saturated at a value of 7–8 nm at high ion fluences. The power spectral density analysis indicated that the evolution of surface morphology of the pristine film is governed by the combined effect of evaporation condensation and volume diffusion processes. Swift heavy ion irradiation seems to increase the dominance of volume diffusion in controlling surface morphology of the film at high ion fluences.     

Impedance and a.c. conductivity studies of Ba(Pr1/2Nb1/2)O3 ceramic

Impedance and electrical conduction studies of Ba(Pr_1/2Nb_1/2)O₃ ceramic prepared through conventional ceramic fabrication technique are presented. The crystal symmetry, space group and unit cell dimensions were estimated using Rietveld analysis. X-ray diffraction analysis indicated the formation of a single-phase cubic structure with space group, ${Pm\overline 3 m}$ . EDAX and SEM studies were carried out to study the quality and purity of compound. To find a correlation between the response of the real system and idealized model circuit composed of discrete electrical components, the model fittings were presented using impedance data. Complex impedance as well as electric modulus analyses suggested dielectric relaxation to be of non-Debye type and negative temperature coefficient of resistance character. The correlated barrier hopping model was employed to successfully explain the mechanism of charge transport in Ba(Pr_1/2Nb_1/2)O₃. The a.c. conductivity data were used to evaluate density of states at Fermi level, minimum hopping length and apparent activation energy.     

Grain boundary space charge region potential barrier in polycrystalline semiconductors

The grain boundary space charge region potential barrier is derived in an analytical form under equilibrium as well as non-equilibrium conditions, using various energy distributions to represent the grain boundary surface states, and the Shockley- Read-Hall theory for the carrier generation-recombination processes. The energy value of the grain boundary states is taken as a single level in the energy band gap of the semiconductor and located at the intrinsic Fermi level or displaced from it by a given amount. The restriction of the single level is later removed by permitting a certain width to the energy distribution of these states. It is shown that the results derived for non-equilibrium case reduce to the equilibrium case results derived separately, under the appropriate limit of zero excitation potential. The numerical results obtained from this formulation for the case of polycrystalline silicon are presented graphically for different values of the bulk doping concentration, grain boundary surface state density, grain boundary state energy distributions and the excitation potential, under equilibrium and non-equilibrium conditions. It is found that the grain boundary energy states located higher than the intrinsic Fermi level give greater values of grain boundary potential barrier height for a p-type semiconductor and the reverse applies to an n-type semiconductor. Further a wider energy distribution of these states gives rise to a towering of the potential barrier height at the grain boundary. Under non-equilibrium conditions, it is seen that the grain boundary barrier height remains unaltered for values of the excitation potential below a certain threshold which in turn depends upon the bulk dopant concentration in the semiconductor and the grain boundary surface state density and its energy distribution. Above this threshold value the barrier height reduces nearly linearly with the increase of the excitation potential with a slope lying-between ?0.5 and ? 1.0. For comparison with the available experimental results of Seager and Castner for n-type polycrystalline silicon we have obtained the grain boundary surface density and energy distribution parameters that provide excellent matching with the experimentally determined variation of the grain boundary potential barrier height with the dopant concentration in the range of 10¹³ to 10¹⁷/cm³, and based upon these parameters we have calculated the non-equilibrium barrier heights at different excitation potentials for this experimental case. A comparison is also made with Seager's experimental observations of non-equilibrium grain boundary barrier height for n-type silicon bicrystals in the illumination range of 3 × 10^?6 to 3 × 10^?2 SUN. For particular values of the density of grain boundary surface states and their energy distribution parameters, we could match our theoretical results with his experimental observations of barrier height.     

DIGITAL SIMULATION OF A RECTIFIER LOADED SYNCHRONOUS MACHINE

ABSTRACT

The digital computer simulation of a rectifier loaded synchronous generator is described in this paper. The digital model of the synchronous generator with either a controlled or uncontrolled rectifier bridge deals efficiently with the complex commutation conditions when the source impedance is significant. From the equations developed in this paper, the performance of synchronous generators with or without damper windings, connected to controlled or uncontrolled bridge-rectifier loads, can be predicted from given loading data, field current, load current, generator speed and bridge delay angle. The approach described in this paper is valid for any bridge topology, and analysis of abnormal fault conditions presents no difficulties once the appropriate transformation matrix is available. The numerical analysis includes calculation of the performance of a synchronous generator with rectifier bridge loads for a variety of operating conditions.     

IMPROVEMENT OF THE PERFORMANCE OF A SUPERCONDUCTING TURBOGENERATOR THROUGH USE OF A CONTROLLABLE REACTIVE POWER COMPENSATOR

ABSTRACT

This paper presents the dynamic performance of a superconducting turbo-generator which employes a thyristor controlled static reactive power compensator at its terminals. The application of the fast acting compensator external to the machine, in a power system, is foreseen as one possible alternative to alleviate the excitation problems which arise on account of the extremely large field time constant of a superconducting generator. This application is compatible with the fast development in the rating of static compensators and imminent use of superconducting machines in the future.

The paper dwells on the transient aspects of performance to evaluate the feasibility of use of an external controller for voltage and also incorporating stabilizing signals. Simulation of the load rejection and transient stability dynamic performance of the superconducting alternator with and without thyristor controlled static compensator brings out its superiority over the field forcing techniques usually used with the conventional generators.