Sharif Digital Repository

Modeling and simulation of acid transport in fractured porous medium and the formation of wormholes are the main goals of this dissertation. Injection of acid in rocks is one of the enhanced oil recovery methods which is extensively used in industry. The flow of acid in a porous medium produces a highly permeable path which is called wormholes. The formation of these wormholes is dependent on parameters of rock, acid and injection properties. As a result, simulation and prediction of this phenomenon are very important for optimization. Some researches developed methods to model this phenomenon in porous media but few of them investigated discontinuities in a medium. This research uses... 

Nowadays wireless technology is widespread all over the world and there is a competition among companies to provide the most secure and high range wireless networks for their customers. This thesis focuses on the security of wireless ad hoc networks. There are lots of different encryption algorithms for securing wireless ad hoc networks. Some of the most important are: WEP, TKIP, WPA, WPA2. All other encryption methods of wireless ad hoc networks are derived from these methods. This thesis focuses on WEP and WPA2. For both algorithms, a literature review is conducted, a new approach to cracking is suggested, and tested on real data, and the future works are mentioned. For WEP, which is an... 

In recent years, we have seen the proliferation of real-time and time-dependent systems. They have increased both in number and in complexity. Due to their increasing complexity, we need to model them and become assured of their correctness. Several formalisms have been devised for time-dependent systems. They include timed automaton and timed extensions of Petri nets. Our focus is on timed extension of activity networks and its model checking. Activity networksare powerful formalisms which allow designers to model the system in a natural, small and conceivable manner. This way, timed extension of activity networks can greatly help modelers. In this thesis, we propose a method to specify the... 

The purpose of this research is to investigate the seismic behavior of plane moment resisting frame consisting of steel-concrete columns and steel I beams utilizing fragility curves. Due to the great influence of the column type in seismic performance of structure. To reach an optimal system, different types of steel-concrete composite columns has been used in the models and the fragility curve of each one is compared. Steel-Concrete composite columns can be divided into three types: (1) concrete filled steel tube (CFT), (2) steel reinforced concrete (SRC) and (3) new combinations of (1) and (2). Due to the abundance of the use of concrete filled steel tube (CFT) and steel reinforced... 

Estimating the life-time of electronic devices has always been an important issue in their mass production. Nowadays, electronic chips are widely used due to technological advances and due to their smaller size. They may be more likely to break down because of their complexity. Therefore, estimating their life-time has received much attention. In the first chapters of this thesis, the concepts and standards that exist in the field of reliability and the life expectancy of electronic devices are discussed. There are many reasons for chip failure, which are beyond the scope of this thesis but what is known as the most important cause of failure which is named electromigration has been... 

A gas cyclone is a fixed mechanical device that uses centrifugal force to separate solid particles from gas. Gas containing particles is brought tangentially into the cyclone. This results in the development of a vortex, which applies strong centrifugal forces to the particles. The larger particles move toward the outer wall of the system as a result of centrifugal forces. gases spiral down from a tangential inlet towards the conical section, where the flow is reversed and the particles are collected in a hopper. The gas phase then proceeds upward in an inner vortex towards the gas exit via the vortex finder.
In this study swirling flow of two-phase gas-solid in cyclone separator is... 

Opinion formation is one of the major topics in social networks analysis and mining. A number of methods have been introduced for describing and simulating this phenomenon. In this thesis, we proposed a novel optimization framework based on an opinion formation model that used the Deffuant’s model with some changes in neighbors’ selection policy. Its efficiency was compared with a number of benchmark optimization methods including genetic algorithms, differential evolution and particle swarm optimization.The proposed model demonstrated better performance than the others.In the proposed, each person changes his opinion based on one of his neighbors who has the best performance in terms of the... 

Laufer and Hulstijn (2001) proposed the Task-Induced Involvement Load Hypothesis stating that in the retention of new words in an incidental context, tasks with higher involvement load will be more effective than tasks with lower involvement load. The present study aimed to examine whether the retention of vocabulary is enhanced with an increase in the amount of task-induced involvement load in an intentional learning environment. Moreover, it was investigated whether proficiency level of learners had a role in the effectiveness of the vocabulary tasks with different involvement loads. Four groups of university students, male and female, participated in the study. They were given a TOEFL... 

Regardless of the extensive research on different aspects of vocabulary and the great emphasis that leading scholars (Meara, 1996; Nation, 990; Read, 1988) placed on the significance of the number of words a learner knows as a substantial component in language learning and vocabulary learning, few investigations have been conducted regarding the size of the vocabulary each language learner knows (Nation, 2012). In addition, Nation (2012) debates the need to combine modern technology and language testing. Therefore, it is necessary to provide innovative methods of testing this knowledge. The present study developed the Computerized Adaptive Test (CAT) of Nation and Beglar’s (2007) Vocabulary... 

Self-regulated learning, though having swept educational psychology's literature like a tidal wave (Boekaerts, 1999), and also, compared to other constructs such as metacognition and learning strategies, has remained comparatively uncharted in L2 research. One preliminary step to chart this concept is to develop questionnaires for it. With these and other things in mind, the principal purpose of this study was to develop a questionnaire in English and Persian to measure self-regulated language learning capacity and behavior of Iranian EFL learners. The development of the questionnaire was guided by two pilots (412 participants), content validation through expert judgment, cognitive... 

A power transmission system includes transmission lines that are interconnected for economic and security reasons. Power systems are often prone to faults at the transmission level, since transmission lines are spread over vast geographical areas. Being the most serious events in power systems, blackouts are often initiated by a fault and a sequence of events afterward leads to partial or complete power outage. In this condition, fault diagnosis is the first step for power system restoration. In this research work, a new approach utilizing impedance-based fault location is introduced for fault diagnosis. By developing wide-area measurements in the context of smart grids, accurate... 

Internet of Things (IoT) is one of the most emerging fields in the computer science context. It is predicted that more than 9 billion devices will be connected to IoT networks by 2025. Considering the specifications of IoT devices such as low battery capacity and small memory, these networks face major challenges in energy consumption and reliability. One of the most important elements which have an indisputable effect on network efficiency is routing protocol. In this manner, IETF presented RPL in 2012. It is designed to create a robust network among IoT devices.Although RPL performs well in static networks, it is not designed originally for mobile networks and it has serious flaws in that... 

In conventional method of modeling and design of structures it is assumed that structures are fixedly supported, however, this assumption is generally far from reality; because, not only the majority of buildings’ bases are not stiff enough to suppose them rigid, but also if their foundations are shallow there exist no piles underneath them, they may separate from their bases – the event which is called foundation uplift. Although according to past studies permission of foundation uplift as well as incorporation of base flexibility may markedly affect any structure’s dynamic response, is spite of about half a century which has elapsed since the beginning of these studies, it seems that still... 

In this thesis, a modified method based on frequency discriminator technique is presented for measuring phase noise of microwave oscillators. In the proposed method, we use two oscillators with similar phase noise, but their free running frequencies are different in the range of 10MHz~50MHz. Mixing these two oscillators results in the IF frequency, for which the phase noise power at each offset has been doubled.The delay line is used in IF frequency and a phase shifter at microwave or millimeter wave range is used to provide quadrarure condition. Hence the cable loss at IF frequency is low and the sensitivity of system will be very good. Comparison of the measured data at X-band (the... 

In various fields of engineering sciences, SUT occupies the highest rank among other technical universities in Iran based on the ranking of Islamic World Science Citation Center (ISC) in 2013. It is, therefore, the host of the majority of exceptionally talented engineering students. In this respect, students of SUT are expected to perform considerably well in their future academic and vocational careers. The primary aim of this investigation is to shed light on a more appropriate course syllabus customized to the needs of these gifted students. A triangulation approach to data collection was employed to gain more insights into the study. A five-point Likert scale questionnaire was... 

A highly reliable tool for transient simulation is vital in the safety analysis of a nuclear reactor. Despite this fact most tools still use diffusion theory and point-kinetics that involve many approximations such as discretization in space, energy, angle and time. However, Monte Carlo method inherently overcomes these restrictions and provides an appropriate foundation to accurately calculate the parameters of a reactor. For this reason, in this thesis, a dynamic Monte Carlo code named MCSP (Monte Carlo dynamic Simulation of Particles tracking) is developed for both the steady state and time-dependent simulation of particle (neutrons, photons and electrons) tracking in an arbitrary 3D...