Sharif Digital Repository

The prediction of material microstructure is of great interest to the material designers since the property and performance of materials depend strongly on their microstructures. In this research the Monte Carlo method and grain growth phenomenon have been used to predict the limiting grain size in the presence of second phase particles. The results showed a good agreement with the previous published results predicted by the analytical models. It was found that the lattice orientation parameter, Q, strongly affects the kinetic of the grain growth. The experimental and theoretical results of limiting grain size predicted by Monte Carlo method showed that by decreasing the value of Q the... 

The stretched hypercube has recently been introduced as an attractive alternative to the well-known hypercube. Previous research on this network topology has mainly focused on topological properties, VLSI and algorithmic aspects of this network. Several analytical models have been proposed in the literature for different interconnection networks, as the most cost-effective tools to evaluate the performance merits of such systems. This paper proposes an analytical performance model to predict message latency in wormhole-switched stretched hypercube interconnection networks with fully adaptive routing. The analysis focuses on a fully adaptive routing algorithm which has been shown to be the... 

Most of the strategies yet implemented to optimize fuel loading pattern design in the nuclear power reactors, are based on maximizing the core effective multiplication factor (Keff) in order to extract maximum energy and to reduce the local power peaking factor (Pq) from a set value. However, a new optimization criterion would be of interest in order to maximize the burn-up of the plutonium content (Pu) in the fuel assemblies. In this research we have developed a new strategy based on multi objective optimization, to optimize fuel core loading pattern for a VVER-1000 reactor. In this approach we simultaneously have reduced the Pq, maximized Keff and also minimized the plutonium content in... 

This paper considers a cooperative space-frequency (SF) coded OFDM system and evaluates its performance over quasi-static frequency selective Rayleigh fading channels. An expression for exact outage error probability is derived and its tight closed form lower bound is presented. The tightness of the lower bound is demonstrated through the Monte Carlo simulations. The asymptotic analysis indicates that the proposed protocol achieves full spatial and frequency diversities available in the cooperative communication system. The theoretical analysis of the proposed space-frequency coded cooperation protocol is further confirmed by computer simulations. © 2007 IEEE  

In a production process, when the quality of a product depends on more than one correlated characteristic, multivariate quality control techniques are used. Although multivariate statistical process control is receiving increased attention in the literature, little work has been done to deal with multi-attribute processes. In monitoring the quality of a product or process in multi-attribute environments in which the attributes are correlated, several issues arise. For example, a high number of false alarms (type I error) occur and the probability of not detecting defects (type II error) increases when the process is monitored by a set of independent uni-attribute control charts. In this... 

By occurring failures in computer networks, routing protocols are triggered to update routing and forwarding tables. Because of invalid tables during update-time, transient loop may occur and packet-drop rate and end-to-end delay increase which means that the quality of service decreases. This paper studies the effect of routing-tables update-time on networks' performability, i.e. the ability of network to deliver services at predefined level. A sample network is studied and the simulation results show that faster updates of routing table, improve network's performability in the presence of failures. Since it may not worth or even be practical to accelerate all routers in the network, this... 

Based on the solution of the Wigner-Boltzmann (WB) equation, quantum correction in the drift-diffusion (DD) model is proposed. The new model, which is a form of the quantum DD model, can show quantum phenomena such as tunneling and quantum repulsion. To evaluate the proposed model, potential barrier and potential wells are simulated. Simulation results show the electron tunneling through and repulsion from the potential barrier. © 2007 The Japan Society of Applied Physics  

The application of artificial neural network to compressor performance map prediction is investigated. Different types of artificial neural network such as multilayer perceptron network, radial basis function network, general regression neural network, and a rotated general regression neural network proposed by the authors are considered. Two different models are utilized in simulating the performance map. The results indicate that while the rotated general regression neural network has the least mean error and best agreement to the experimental data, it is however limited to curve fitting application. On the other hand, if one considers a tool for curve fitting as well as for interpolation... 

In this paper, a new approach based on hybrid Particle Swarm-Based- Simulated Annealing Optimization (PSO-B-SA) for solving thermal unit commitment (UC) problems is proposed. The PSO-B-SA presented in this paper solves the two sub-problems simultaneously and independently; unit-scheduled problem that determines on/off status of units and the economic dispatch problem for production amount of generating units. Problem formulation of UC is defined as minimization of total objective function while satisfying all the associated constraints such as minimum up and down time, production limits and the required demand and spinning reserve. Simulation results show that the proposed approach can... 

In this paper, we develop a theory to study the nano defects of various geometries within thin films. The considered thin films have face centered cubic (fee) structure. The eigenstrain method is combined with the long-range Sutton-Chen (SC) inter-atomic potential function which is appropriate for fee crystals. The disturbance caused by a defect in a thin film is determined from the equilibrium equation using the discrete Fourier transformation. The disturbed field is also determined using three dimensional (3D) Molecular Dynamics (MD) simulation in which the constant NVT ensemble is applied to the atomic system. For illustration, the problem of nano disk shape defect in thin film is studied... 

The necklace hypercube network has recently been introduced as an attractive alternative to the well-known hypercube. Previous research on this network topology has mainly focused on topological properties and VLSI aspects of this network. In this paper, we propose some adaptive routing algorithm for the necklace hypercubes. The performance of necklace hypercubes using the proposed routing algorithm is then evaluated by means of simulation experiments. Experiments are realized under different working loads and for different network factors. Moreover, a comparison between the necklace hypercube and the well-known hypercube network is conducted. The comparison has been done considering... 

The mechanism and kinetics of the hydrogen evolution reaction (HER) on tin electrode in 0.1 M H2SO4 solution were studied using the methods of steady-state polarization and electrochemical impedance spectroscopy. The simulation of the data obtained from these two methods, using nonlinear fitting procedure allowed us to determine the rate constants of Volmer, Heyrovsky and Tafel steps associated with the mentioned reaction. The kinetic results indicate that HER mechanism at low negative potentials is a serial combination of Volmer step and parallel Tafel and Heyrovsky steps. At high negative potentials where the hydrogen coverage reaches its limiting value, a Tafel line with the slope of -... 

In this paper, impact of the stationary nodes on the link excess life as an important performance parameter of wireless mobile networks is investigated First, a model for the link excess life is introduced. Based on this model, the relation between the probability density function (PDF) of the link excess life and the PDF of relative velocity is explored. In fact, the PDF of relative velocity is divided into three PDFs according to the three categories of links. The effect of each group on the resultant PDF of global link excess life for different percentages of stationary nodes is explored. Moreover, an approximation method defined in our previous work is proposed as a tool to evaluate the... 

Based on a new derived radial distribution function (RDF) for potentials with a hard-core we have presented in this paper a method to apply the derived RDF for calculating thermodynamic properties of real fluids up to moderate densities. In order to use the derived RDF for real fluids, one of the potential parameters is chosen in such a way that the RDF behaves more like that for a real fluid. Hence we have been able to calculate all thermodynamic properties of a simple fluid analytically. We have then applied our procedure to a Lennard-Jones fluid and compared the results with simulation data. The agreement is good up to moderate densities, i.e. ρ* ≤ 0.6, which lies in the liquid range of... 

In this paper, a three-invariant cap plasticity with a kinematic hardening rule is presented for powder materials. A general form is developed for the cap plasticity which can be compared with some common double-surface plasticity models proposed for powders in literature. The constitutive elasto-plastic matrix and its components are derived based on the definition of yield surface, hardening parameter and non-linear elastic behavior, as function of relative density of powder. The procedure for determination of powder parameters is described. Finally, the applicability of the proposed model is demonstrated in numerical simulation of triaxial and confining pressure tests. © 2006 Elsevier B.V.... 

Feed-forward is one of the main methods for linearising microwave power amplifiers. The level of inter-modulation rejection (IMR) values can be predicted by appropriate modelling of the feed-forward microwave amplifier. Here, we have presented closed form expressions, which can be used for predicting the IMR for the first and second adjacent channels. Fifth- and third-order nonlinear models have been used for the main and error amplifiers, respectively. The predicted values have been verified using ADS simulator and a MATLAB program. © The Institution of Engineering and Technology 2007  

Artificial neural network was used to calculate the performance of a model supercapacitor as signified by the power density, energy density and utilization to the synthetic, intrinsic and operating characteristics. A four-layer neural net having two hidden layers having 6 and 15 nodes was found to be well capable of simulating the capacitor performance with the convergence achieved often a relatively small number of epochs. As for the input parameters, crystal size, surface lattice length, exchange current density of the capacitor active material and the cell current employed while utilization, energy density and power density were the outputs. © 2006 Elsevier B.V. All rights reserved  

In this work, a mathematical model has been developed to predict distributions of temperature, strain and strain rate during hot rolling as well as the subsequent microstructural changes after hot deformation. For doing so, a 2-D finite element analysis together with a stream function method have been coupled to calculate temperature distribution and velocity field within the rolling metal while the roll force has been estimated, through using an upper bound solution. Also the additivity rule and Avrami-type equation have been employed to predict the kinetics of static recrystallization after hot rolling. The model has been examined on AA5083. Hot rolling experiments have been conducted... 

The overall mechanical properties of composite materials are dependent on the mechanical response of individual constituents and their interactions while they may be relatively easy to determine. This paper represents a simulation process by which the cyclic stresses and fatigue loadings on its constituents could be predicted for an under fatigue loading lamina. Hence, the unidirectional composites fatigue would be studied through its constituents. The proposed model introduces a new coupled stiffness/strength technique by relating lamina stiffness to the stress field in its constituents. Therefore, the stress field and strength considerations in its constituents could be studied when the... 

In this paper, an approach is proposed for controlling the uncertain Lorenz system. Based on an identification technique, a controller is designed that guarantees the regulation of all states in the presence of system uncertainty. Since in some applications the challenging problem of output tracking is desired, we have proposed several effective set-point tracking control techniques. The control schemes that are based on the feedback linearization method, can stabilize the internal dynamics of the system. Simulation results have illustrated the effectiveness of the proposed schemes. © 2005 Elsevier Ltd. All rights reserved