Sharif Digital Repository

Short circuit current is one of the major threats for power system stability. Power system should have the ability to detect and clear these faults within a specified time. The protection system can clear the faults only if the interruption capability of the circuit breakers is higher than the fault level. Expansion of the grid and installation of new power on the scale that is envisaged will require practical solutions to a number of technical challenges. One of the technical challenges is fault level management as system expansion often results in increased fault levels beyond the design limits of the existing switchgears. Replacing the old equipments particularly circuit breakers with the... 

We use the weighted integral form of spherical Bessel functions and introduce a new analytical set of complete and biorthogonal potential-density basis functions. The potential and density functions of the new set have finite central values and they fall off, respectively, similar to r-(1+ l) and r-(4+ l) at large radii, where l is the latitudinal quantum number of spherical harmonics. The lowest order term associated with l = 0 is the perfect sphere of de Zeeuw. Our basis functions are intrinsically suitable for the modelling of three-dimensional, soft-centred stellar systems and they complement the basis sets of Clutton-Brock, Hernquist & Ostriker and Zhao. We test the performance of our... 

One of the shortcomings of photoconductive (PC) antennas in terahertz (THz) generation is low effective radiated power in the desirable direction. In this paper, we propose a defective photonic crystal (DPC) substrate consisting of a customized 2D array of air holes drilled into a solid substrate in order to improve the radiation characteristics of THz PC antennas. The effect of the proposed structure on the performance of a conventional THz PC antenna has been examined from several aspects including radiation efficiency, directivity, and field distribution. By comparing the radiation performance of the THz antenna on the proposed DPC substrate to that of the conventional solid substrate, it... 

Iran has been experiencing slow growth for the past ten years. Using plant-level information, we show that on average firm-specific productivity in manufacturing sectors declined at the rate of 2.6 percent annually, while large top decile firms experienced a modest growth in productivity between 2005 and 2011. We decompose this trend and find that within-plant variation is its main driving force while the between firms and industries component is insignificant. We test several alternative explanations that may contribute to these negative trends. We show that the subsidy reform had a negative effect, while privatization seems to have had no effect. Private management not affected on... 

In this paper, arrays of two different terahertz (THz) antennas based on defective photonic crystal (DPC) substrates are proposed and their radiation characteristics are investigated by using full-wave simulation. We show that employing DPC substrates in THz photoconductive antenna (PCA) arrays reduces the amount of power coupled to the substrate surface waves and, as a result, decreases the mutual coupling between the elements of the antenna array and increases the directivity in the boresight direction. By studying 1 × 2 arrays of bowtie and four-leaf-clover-shaped antennas, we show that either the improvement of directivity in a broad bandwidth or the enhancement of photomixer antenna... 

Using plant level panel data of manufacturing firms in Iran from 2003 to 2013, we test the Dutch disease predictions, including the impact of a rise in oil prices on intensive and extensive margins of trade. Our findings indicate that during periods of high oil prices domestic currency appreciates, but contrary to resource curse theories, domestic firms export more, and new firms enter foreign markets. We reconcile this paradox by proposing a new channel, which we call “subsidy disease.” Despite rising oil prices, the government of Iran kept domestic energy prices at low levels, which increased the implicit subsidy accruing to energy intensive firms. In effect, these firms gain a competitive... 

A set of bi-orthogonal potential-density basis functions is introduced to model the density and its associated gravitational field of three dimensional stellar systems. Radial components of our basis functions are weighted integral forms of spherical Bessel functions. We discuss on the properties of our basis functions and demonstrate their shapes for the latitudinal Fourier number l = 2. © 2008 Copyright International Astronomical Union 2008  

At least four source impedances are required to characterize the noise parameters of a two-port network. The accuracy of the noise parameters depends on the configuration of the employed source impedances (pattern). The configuration of a specified pattern, generated generally by an impedance tuner, changes with frequency, and as such, the accuracy is usually degraded in a wideband characterization. To realize a low-error wideband noise characterization, the present article thoroughly investigates the method of choosing a four-source impedance pattern. A Monte Carlo simulation-based approach is proposed to obtain the desired impedance patterns. The measurement and simulation results show... 

Low scalability and power efficiency of the shared bus in SoCs is a motivation to use on chip networks instead of traditional buses. In this paper we have modified the Orion power model to reach an analytical model to estimate the average message energy in K-Ary n-Cubes with focus on the number of virtual channels. Afterward by using the power model and also the performance model proposed in [11] the effect of number of virtual channels on Energy-Delay product have been analyzed. In addition a cycle accurate power and performance simulator have been implemented in VHDL to verify the results. ©2008 IEEE  

This paper presents a small open economy, dynamic general equilibrium model with cash-in advance constraint and a currency attack to explain the dynamic effects of changes in trade barriers in the short and long run. The model is calibrated using data on macroeconomic variables in Iran's economy from 1990 to 2013 to identify and analyze the effects of barriers imposed on trade. We analyze the effects of counterfactual fiscal, monetary and exchange rate policies in response to trade barriers and show how the economy would respond in different time horizons. We also evaluate the effects of eliminating each of the factors causing this stagflation. Our findings show that the short-term drop in... 

Thermodynamic and kinetic studies have been carried out on the adsorption of uranium(VI) by Amberlite IRA-910 resin. The adsorption process has been investigated as a function of adsorbate concentration, solution acidity, contact time, adsorbent dosage, and temperature. The experiments were preformed in batch mode, where uranium initial concentration on the solution samples were 185.5, 277.6 and 456.8 (mg/lit), sulfuric acid concentration range was 0.02-9 (mol/lit) and sorbent dosages were 0.2, 0.3 and 0.5 g. Equilibrium isotherm data were analyzed using Freundlich and Dubinin-Radushkevich isotherm models. The results showed that the adsorption process was well described by Freundlich... 

Heating, Ventilating and Air Conditioning (HVAC) plant is a multivariable, nonlinear and non minimum phase system, which control of this plant, is very difficult. This paper presents a new approach to control of HVAC system. The proposed method is a hybrid of fuzzy logic and PID controller. Simulation results show that this control strategy is very robust, flexible and alternative performance. To evaluate the usefulness of the proposed method, we compare the response of this method with PID controller. The simulation results show that our method has the better control performance than PID controller  

In this work, the goal is to find the optimum working point for intelligent wells that is equipped with down-hole chokes and producing from a multi-layer reservoir under pressure maintenance process. Two evolutionary optimization algorithms are used and results are compared from different points of view. Therefore, the differential evolution technique and genetic algorithm has been used in conjugation with a reservoir simulator. The overall optimization technique is applied to heterogeneous example problems involving smart and multi segment wells. Improvement of predicted performance, which is as high as 7% in one case is demonstrated for all of the examples studied  

A rapid wax injection tool of a gearbox shift fork was designed, simulated, and manufactured using rapid prototyping and rapid tooling technology to save time and cost of producing wax models used for the investment casting process. CAE simulation softwares, in particular, MoldFlow, are used to get wax injection moulding parameters such as filling parameters, temperature profiles, freeze time, speed, and pressure. The results of this research were compared with conventional wax model production methods. The criteria of such comparison were based upon parameters such as time, cost, and other related characteristics, which resulted in saving of 50% in time and 60% in cost. In this research,... 

We estimate the price elasticity of the demand for gasoline smuggling in Iran. For this purpose, we employ a detailed panel of monthly gasoline consumption data from 160 distribution hubs during the period 2005–2014. We apply two different approaches which are diff-in-diff and panel data estimations. The results suggest that the foreign-to-home gasoline price ratios have a significant impact on the time-varying elasticity of demand. This finding supports the hypothesis that an increase in domestic gasoline prices will significantly reduce the demand for smuggling. In addition, as the distance of a region from its closest higher-price neighboring country diminishes, the price elasticity of... 

According to International Technology Roadmap for Semiconductors (ITRS), before the end of this decade, we will be entering the era of a billion transistors on a single chip. The major threat toward the achievement of billion transistor on a chip is poor scalability of current interconnect infrastructure. With the advent of "Network on Chip (NoC)" various characters and methodologies of traditional networks were hardly considered on-chip. Failure, Power and Area are the major concepts that should be considered when migrating from traditional interconnection networks to NoCs. In this paper we study the effects of faulty links and nodes on power and performance of mesh based NoC, Also several... 

To what extent can a short-term decline in the output of a small open economy be explained by trade barriers? To answer this, we extend the Business Cycle Accounting method of Chari et al. (2007) to a small open economy model. We include an additional time-varying wedge to model financial trade frictions caused by barriers on imports. International sanctions on Iran provide an empirical opportunity to apply this method to data on Iran's recession in 2012-13. The results indicate that efficiency and investment wedges account for most of the fluctuations in aggregate variables during the sanctions, and trade barriers had little contemporaneous explanatory power. The effect of oil boycotts... 

To what extent can a short-term decline in the output of a small open economy be explained by trade barriers? To answer this, we extend the Business Cycle Accounting method of Chari et al. (2007) to a small open economy model. We include an additional time-varying wedge to model financial trade frictions caused by barriers on imports. International sanctions on Iran provide an empirical opportunity to apply this method to data on Iran's recession in 2012-13. The results indicate that efficiency and investment wedges account for most of the fluctuations in aggregate variables during the sanctions, and trade barriers had little contemporaneous explanatory power. The effect of oil boycotts... 

In the last decades, numerous attempts have been made to prevent microbial pollution spreading, using antibacterial agents. Zeolitic imidazolate framework-8 (ZIF-8) belongs to a subgroup of metal organic frameworks (MOFs) merits of attention due to the zinc ion clusters and its effective antibacterial activity. In this work, Ag-doped magnetic microporous γ-Fe2O3@SiO2@ZIF-8-Ag (FSZ-Ag) was successfully synthesized by a facile methodology in room temperature and used as an antibacterial agent against the growth of the Gram-negative Escherichia coli and Gram-positive Staphylococcus aureus bacteria. Several characterization methods were applied to analyze the properties of the materials, and the... 

The pool boiling is one of the heat transfer mechanisms used in cooling systems. Using an external force to improve the efficiency of thermo-fluid systems is among the most effective approaches, and also highly applicable in enhancing heat transfer. In this study, the pool boiling of water, as the working fluid, in the presence of a variable magnetic field, is studied. To this end, ferromagnetic beads were placed on the boiling surface, and the effect of their movements due to a variable magnetic field was observed and measured. The number of beads was selected such that they would cover 1/3, 1/2 and 2/3 of the surface in different cases and the magnetic field was controlled by altering...