Sharif Digital Repository

One of the most important parts of vehicles and most of mechanical machines is power transmission system, especially the Gearbox used in them.In order to optimize the use of energy, especially fossil fuel, researchers are looking for new mechanisms to refine the operation of vehicles and other machines.One of the mechanisms which absorbed researcher’s attention during recent years is the Continuously Variable Transmission. By using these gearboxes which are replaced with older samples, we can have the continuous range of aspect ratio between two constant values. This important property causes the decrease of the use of fuel.In recent decades many manufacturers have used different kinds of... 

After discovery of superconductivity in 1911 people try to find superconductors with higher Tc. High Tc superconductivity in cuprates and iron based superconductors was discovered in 1986 and 2008 respectively. However, there is not any unit theory for high Tc superconductivity in cuprates or iron based superconductors. Study of electronic and phonon structures of high Tc superconductor compounds can lead to find theory of high Tc superconductivity.
In this dissertation electronic structure of Ba1-xKxFe2As2, which belongs to class of 122 of iron based superconductor, under potassium doping x=0,0.5,1 has been studied. To this end ab-initio method and Density functional theory have been... 

The discovery of Iron-based superconductors with maximum transition temperature of 56 Kelvin and High critical magnetic field of the order of 50T at temperature 20K has created evolution in different fields of superconductivity including practical issues. BaFe2As2 family is one of the most important compounds of Iron-based superconductors and with different doping elements for instanse elements of the first column like Potassium inters superconducting state with critical temperature of 36K.
The purpose of this thesis is to localize the technology of making this compound in Iran which no project has been done in this field in the country before. In this thesis manufacturing, Optimization... 

In this thesis, a low-power low-phase-noise 28 GHz voltage-controlled oscillator for 5G applications is presented. At first, to select the best structure in terms of FoM, various types of inductors and transformers have been designed and EM simulated. The class-C structure which leads to the highest FoM is chosen eventually. In order to increase oscillation amplitude and reduce phase noise while preserving FoM, a high-frequency noise filter is presented. This filter which is based on a λ/8 microstrip transmission line has solved the conventional filter problems e.g. narrow bandwidth, coupling with oscillator inductor and practical implementation issues.The oscillator is designed, EM... 

mismatch between skills and labor market demands reduce productivity and harm public welfare and economic growth. For this reason, governments implement active labor market policies to align the skills of the workforce with market needs and reduce unemployment. In this research, focusing on the issue of employment in deprived and rural areas of Iran, the effectiveness of one of the active labor market policies implemented by the Barkat Foundation is evaluated. From 2018 to the end of 2022, the Barkat Foundation granted nearly 290,000 micro-loans worth 14 trillion Tomans. The results show that, on average, if the loan amount exceeds 40 million Tomans, it can have a positive effect on the... 

Despite having major advantages such as expert management, economy of scale and lower analysis costs for investors, the mutual funds may still underperform (at suboptimal levels) due to factors like information asymmetry between fund managers and unitholders along with the difficulties involving assessment of manager's performance by investors. So the conflict of interest between the manager and unitholders propels them towards deciding to manipulate the risk measure of the fund for reasons other than the maximization of profits for unitholders by shifting risk level of the portfolio. For instance, the promise of non-linear growth in attracting money by obtaining a considerable ranking... 

Nowadays there are various means in the field of orbital mechanics used in different levels and their accuracy depends on the theory and complexity of the governing equations used in their development. Most of them just simulate the motion of an orbiting satellite while there exist some commercial products able to provide the user with the orbital performance specifications and almost none of them have a module to design an orbit, based on a required performance such as specific coverage / ground track, shadow/light duration, etc .... Although satisfying performance requirements and a set of orbital elements simultaneously only seems to be possible for some particular orbits, there is an... 

The topic of knowledge creation and knowledge management is one of the most leading issues in management studies in today’s world’s organizations.Knowledge creation is known as the first and the most basic step in knowledge management and it can provide the prefaces of developing a management work frame in an organization.In this thesis after reviewing the goals and purpose of the project under consideration, the literature of knowledge management is reviewed and the most important models of knowledge management are selected and analyzed.Also, the organization selected as a case study is fully studied and the necessary information is gathered on the basis of proposed indexes at two public... 

The most important task of a power system is generating and transmitting electricity for consumers, according to electricity quality standards. One of these quality criteria is preservation of grid frequency close to nominal value with continuity of energy delivery to consumers. Electricity is a specific commodity that its generation and consumption should be equal continually; otherwise the network frequency will change and may become unstable, causing severe damage to the network equipment. There for different control and protective actions are deployed to maintain balance between generation and consumption, and thus to keep grid frequency close to nominal value. Characteristics of... 

Poorly damped Low Frequency electromechanical Oscillations (LFOs) confine the capability of power system transmission lines. This phenomenon, which is the result of dynamic interactions among generator groups, may make the system prone to small- signal instability in some cases. Due to the use of some system controllers such as AVRs, these oscillations are inevitable.
Hence, electromechanical modes detection and assessing stability margin are important requirement in power system planning and operation, and can provide significant help to power system operators by preventing stability problems.
From the distant past, in order to monitor small-signal stability, power system operators... 

Global growth of electrical energy consumption and various limitations on the use of fossil fuels, such as environmental problems, rising prices and their limited available resources, have resulted in growing interest and investment in the usage of renewable energy generation, especially photovoltaic (PV) systems. Although the off-grid operation of PV systems was its dominant operation form in the past, proliferation of these systems has made the grid-connected operating mode widespread in recent years. However, their grid connection causes technical problems which need to be resolved. This thesis studies and analyses the steady state and dynamic operation of PV systems in grid-connected... 

This thesis aims at developing control system of a full converter wind turbine with modular and parallel back-to-back converters. Modular structures and paralleling of power electronic converters are the state-of-the-art methods for increasing their transferable power to the grid. Technological advances in high power wind turbines, especially for the offshore installations, which contain permanent magnet synchronous generators with back-to-back full converters, promote the use of modular structures.
In addition to developing the regular control systems of wind turbines for modular structures, selection of appropriate structures, power sharing between parallel converters and mitigation of... 

According to authorities and statistical figures, Iran is seriously facing insufficient housing crisis. Studies show that through an increase of 1.5 million units in annual housing construction capacity, this crisis will be overcome in 5 years. Although successful execution of such a big project is not unprecedented in the world, housing experts believe fulfillment of this plan through traditional method of construction in the country is impossible. The country's capacity of industrialized construction which is known as the remedy, is only limited to 2-3% of total construction. Thus development of this approach through financial, legal, ... supports of investment in prefabrication... 

Due to daily decrease of fossil energy resources, increase of environmental pollution which is included of toxic and greenhouse gases, and extensive growing rate of demands in transportation industry, especially vehicle industry, developing solutions in using fuel cells are vital for this industry in our country. Fuel cells are green, efficient and also powerful energy generators. According to potential advantages of fuel cells, structure, performance, benefits and also types of fuel cells are introduced as a desired energy resource. Indentifying existing choices of energy generators and ranking them using MCDM (Multi-Criteria Decision Making) technique was the first step to perform a... 

Although voltage control and stability issues are not new to the electricity industry, but nowadays, in most modern power systems, special attention has been paid to this issue. Once, only weak systems with long lines were involved in voltage instability, while today due to heavy loading, voltage stability is a serious concern even in developed networks. Over tim, with increasing demand for electricity and emergence of electricity market and economic constraints, power systems are more exposed to voltage instability and even voltage collapse. Therefore, reliable operation of the system in such circumstances requires evaluating voltage stability of the network at both stages of planning and... 

Global trends of electrical energy production indicate high penetration of renewable wind energy resources in power grids around the world.Although high penetration of these resources benefits the grid operators and end-users in various aspects, several new challenges should be addressed. These challenges can be focused on in various studies with different time intervals and different technical and economic issues. One of the most important issues relates to the system frequency regulation and control aspects in high wind and photovoltaic (PV) penetrated grids. This is due to the fact that wind turbine generators (WTGs) as well as PV systems do not provide inertial response for the system... 

Nowadays, concerns about the reduction of fossil fuels and environmental issues, have made renewable resources more important. Among these resources, photovoltaic (PV) systems are more popular than others, due to their environmental compatibility and unlimited availability of sunlight. Increasing the penetration level of PV systems in electricity networks presents various technical and control challenges such as voltage and frequency variations; and protection and harmonics issues. In this thesis, voltage rise problem due to reverse power flow in distribution feeders is discussed. This is one of the main obstacles to the expansion of these resources in distribution networks.Various solutions... 

Design and construction of an optical fiber sensor for liquid concentration detection are reported. This sensor operates based on light intensity modulation.Such modulation results from alteration of total internal reflection into partial reflection at the interface. It is well known that the extinction ratio is varied by changing the concentration.In this system,two optical fiber swere used, one transmitting the light source, another acting as the receiving fiber and a glass prism providing the total and partial reflections. The modulated intensity has been measured by using a receiving fiber. During the concentration measurements, when a liquid in a vessel touches the 45° faces of the... 

Periodic orbits are one of the basic objects of interest in Dynamical systems. And methods for detecting, counting and determining the periods of periodic orbits are of great interest. In spite of this interest, periodic orbits remain difficult to detect. The most celebrated existence result is the Poincare-Bendixson theorem. Unfortunately, this theorem is false in general for higher dimensional systems. The success of fixed point theory inspired the hope that similar topological techniques could be of use in proving the existence of periodic orbits. In this Thesis we have analyzed some topological and geometrical theories which could be of use in proving the existence of periodic orbits.... 

Various applications of speech processing systems are gaining more and more attention. Performances of these systems are dramatically affected in presence of background noise. Attempts toward improving quality of degraded speech have emerged into filed of speech enhancement which is one of the interesting topics in speech processing domain. In this thesis various single channel speech enhancement methods are studied briefly. Statistical methods as one of the most successful methods are studied more deeply. These methods are divided into two categories of short-term modeling of speech and long-term modeling. The aim of this thesis is to improve the performance of long-term methods of...