Sharif Digital Repository

According to the rapid growing in communication devices, users need higher rates for their usages. Fiber optics is one the major choices to support such rates. One of the simplest and cheapest networks is Local Area Network (LAN) as a Passive Optical Network (PON). In these networks, some computers are locally connected to a passive optical device and communicate through it. Multiple access to a shared channel is an important task in such a network. In this thesis, Code Division Multiple Access (CDMA) is used and a new algorithm for Multiple Access Control Sublayer (MAC) based on scheduling will be developed to improve Quality of Service (QoS) and stabilize the network.
In the scheduling... 

The amount of fresh water suitable for various uses is decreasing day by day. This may be due to evaporation or contamination of water or such. Heavy metal cations, dyes, oils and solvents are the most important substances that contaminate water. There are various ways to remove these contaminants, one of which is the absorption of these substances by suitable absorbents. Carbon compounds have attracted much attention among all the absorbents used so far. The aim of this study is to produce carbon aerogels from an economic and modify its structure and properties by adding graphene nanosheets. In this study, Novalak-graphene oxide (GO) gel was fabricated using solvent-saturated vapor... 

Future of the Karkheh River Basin and its people’s livelihoods clearly depends on natural resources like water, soil, vegetation and livestock. As water is the most limiting natural resource in this basin, any increase in water productivity will almost certainly benefit rural livelihoods. To ensure the sustainability of the improvements in water productivity, assessment of the possible impacts of climate change on hydrology and water resources in the basin is necessary. In this study the potential impacts of climate change on hydrology in the Karkheh River Basin were assessed using a macroscale hydrology model driven by 21st century simulations of temperature and precipitation downscaled... 

In recent years, two-layer sheets have been used in various industries. In this research, the formability of a two-layer sheet which is made of 1.35 mm steel and 0.45 mm copper sheets has been studied. The two-layer sheet used in this study was made by explosive welding method. In order to investigate the formability of the sheet, forming limit diagram was determined. First, Marciniak-Kuczynski (M-K) model was used. In order to determine forming limit diagram using this method, the two-layer sheet was equated with a single-layer sheet and MATLAB software was used. The two-layer sheet was also modeled in finite element software ABAQUS. In order to determine the forming limit diagram, Nakazima... 

Nanocrystallization kinetics of amorphous Finemet-type alloys was studied using Isokinetic and Isoconversional methods (Kissinger model-fitting method, Vyazovkin advance method, Kissinger–Akahira–Sunose(KSA),Flynn–Wall–Ozawa(FWO),Tang and Starink) under isothermal and non-isothermal conditions. Effects of chemical composition on the nanocrystallization kinetics, microstructure and magnetic properties of Fe73.5Si13.5B9Nb3Cu1 and Fe77Si11B9Nb2.4Cu0.6 alloys were also investigated. Kinetic triplet (activation energy, frequency factor and reaction model) were determined by these methods and compared for the alloys. According to isoconversional methods, the activation energy is variable as a... 

A great deal of information are continuously generated by users in different contexts such as social networks and online service providers in terms of temporal marked events. These events indicate that what happened to who by when and where.Modeling such events and predicting future ones has interesting applications in different domains such as item recommendation in online service providers and trending topic prediction in online social networks. However, complex longitudinal dependencies among such events makes the prediction task challenging. Moreover, nonstationarity of generative model of events and large size of events, makes the modeling and learning the models challenging.In this... 

A fully coupled numerical model is developed for the modeling of the cracks in porous media with flow of two fluids using the extended finite element method.The governing equations of two-phase fluid flow in deformable porous media, which account for the coupling between various physical phenomena, are derived within the framework of the generalized Biot theory. The solid phase displacement, the wetting phase pressure and the capillary pressure are taken as the primary variables of the formulation. The other variables are incorporated into the model via the experimentally determined functions that specify the relationship between the hydraulic properties of the porous medium, i.e.... 

The emergence of applications such as spam detection and online advertising coupled with the dramatic growth of user-generated content has attracted more and more attention to stream classification. The data stream in such applications is large or even unbounded; moreover, the system is often required to respond in an online manner. Furthermore, one of the main challenges of stream classification is that often the process that generates the data is non-stationary. This phenomenon, known as concept drift, poses different challenges to the classification problem.Therefore, an adaptive approach is required that can manage concept drift in an online fashion. This thesis presents a probabilistic... 

In this project, we have assessed the electrochemical treatment method. In this method, pH decrease of the place where a tumor is located engenders the necrosis of cancer cells and consequently they are destroyed. The process of pH decrease is caused by exerting voltage on electrods that are located near the tumor. The aims of optimizing this methods are to minimize the pH that results in the increase of cancer cell annihilation and to minimize the HClO and Cl2 products which are toxic and chage the healthy tissues to cancer cells. In this project, the model of electrochemical treatment method is explained, and then this model is simulated using the parametes which are provided in the... 

In the past, opinion polls have generally been conducted using statistical techniques and the comments of those samples, which, in addition to its various challenges, require much power and cost per poll. Today, due to the high penetration rate of social networks in society, a very large fraction of people discuss such issues in terms of their opinions and preferences in these networks, and hence, these networks can be a valuable source of information to get the opinions of people in a short time, and by lower cost.The purpose of this thesis is to enhancing the results of the surveys which have already performed using various methods of text processing in social networks by using the graph... 

Scattering theory on locally symmetric space first was studied by Peter Lax and Ralph Phillips. They studied the and develop scattering theory on it. Without the Fourier analysis tools this theory was completed with a lot of computations. In this thesis first we try to construct Fourier analysis by definition Fourier transform then by means of new tools we prove very short version of scattering theory on locally symmetric space . In general case for studying scattering theory we need firs spectral decomposition of Hilbert space of square integrable functions locally symmetric spaces of rational rank one is achieved by quotient of symmetric space where G is Lie group and K is maximal... 

Controlled release is a crucial factor in tissue engineering and cancer-therapy applications. The main purpose of current research is to synthesis smart magnetic nanocarriers for hydrophobic drug and embedding them in a fibrous platform for anti-cancer/ tissue engineering applications. In this regard, three different drug delivery systems of magnetic nanocolloid, magnetic fibers and hydrogels were studied. In the first phase, superparamagnetic iron oxide nanoparticles (SPIONs) were synthesized and then were modified using oleic acid and thermo-sensitive polymer of pluronic F127/F68. After characterization of this composite, Response Surface Methodology (RSM) was applied to model the lower... 

Reviewing trends of energy consumption in the world reveals that sooner or later energy resources will be depleted. This concerning rate of consumption has brought about more greenhouse gases (GHG) emission influencing deleteriously on environment and climate. Human has continued energy consumption to maintain and improve the quality of his life through industrial production. Industry as a significant energy user owns considerable contribution to this consumption. Several attempts have been carried out to develop energy management systems in order to improve the energy efficiency of industrial sectors. Whilst they are comprehensive and could be applied to any sector, there is lack of... 

During the most recent years, investigating novel nanomaterials based on porous strucures has become an interesting field of research. Among these kinds of materials, mesoporous silica has extensive applications in catalysis, absorbants and biomedical engineering. By combining specific surface characteristics of mesoporous silica with magnetic properties of iron oxide nanoparticles, new nanocomposites owing unique properties, can be obtained. In the present research, magnetic mesoporous nanocomposite has been synthesized. Iron oxide nanoparticles were produced through reduction of water solution containing Fe2+/Fe3+ ions, followed by hydrothermal treatment of precipitan. Then, iron oxide... 

In this study in-situ Al/Al3Ti was fabricated by mechanical alloying, hot extrusion and subsequent heat treatment. First, initial pure powders of Al and Ti were mechanically alloyed with 5 weight percentages of Ti for 40 hours. The as-hot extruded samples containing 5 wt% of initial Ti particles, were exposed to heating at 600 oC for various time intervals up to 10 hours. Microstructure analysis by OM and SEM proved that in-situ Al3Ti phase was formed as a layer around Ti particles by solid reactive diffusion between Al and Ti particles. After 10 hours of heating, all the Ti particles were consumed and transformed to Al3Ti. Mechanical properties of samples were investigated by Brinell... 

Wireless sensor networks (WSNs) are composed of many independent spatially-distributed sensor nodes where communicate their information acquired from environment to shape a uniform view of their managed scope. Using WSNs in places which human can hardly attend uninterruptedly, as well as constraints such as low power consumption, low price, small size, and limited radio range, are the biggest challenges for researchers and WSN designers. Supplying security is yet another common challenge of using these networks. The beginning chapters of this dissertation introduce the most important security mechanisms in WSNs, that is, key management and cryptography. Since each application has its own... 

The Tilor welded blank (TWB) two or more sheets with different thickness or strength which are welded together. The automobile designer are searching for new methods to reduce the cars’ weight and producing cost in order to diminish fuel consumption so they can survive in the competitive market. One way to achieve to this seemingly Contradictory demands, is using TWB in cars’ body. One of the most applicable methods to investigate the formability of metals sheets, is Forming Limit Diagrams (FLD). Three ways to determine the FLD are: Analytical Method, Experimental Methods, and Numerical Method. The experimental method to determine FLD, require metal sheets to undergo in different strain... 

Lipases (Triacyl Glycerol Acyl Hydrolase, EC.3.1.1.3) are multidimensional biocatalysts which have many applications in food industrials, dairy, fats and oils processing, detergents, synthesis of pure chemical and pharmaceutical materials, paper industrial and cosmetics. Generally, industrial enzymes are produced extracellularly, so their separation from a low-cost complex medium (usually used for production) is one of the cost stages in their production process. Filtration is the most economical method of separation in the production of enzymes, but due to the small size of colloidal particles and cell masses present in the fermentation environment, its use is subject to limitations. In... 

Development of new engineered materials for bone tissue engineering applications is rapidly growing. The most important characteristics of materials, which can be used for bone tissue engineering applications, are appropriate mechanical properties, degradation rate, swelling behavior, and bioactivity. The goal of the current investigation is to study the feasibility of incorporating a modified nano-biocomposite based on biodegradable thermoplastic starch for bone tissue engineering. Nano particles of ?-tricalcium phosphate and hydroxyapatite were incorporated for reinforcing the matrix as well as improving the bioactivity. A second biodegradable polymer, i.e. polycaprolactone, was used... 

A biocompatible ferrofluid have been prepared by co-precipitation of FeCl2.4H2O and FeCl3.6H2O under ultrasonic irradiation and with NaOH as alkaline agent. Also cystein was used as capping agent in the solution. Magnetic properties of the produced ferrofluid then determined by VSM test and magnetite nanoparticles were characterized by XRD and TEM techniques. Results indicate the presence of a biocompatible superparamagnetic ferrofluid including magnetite nanoparticles with particle size ranging under 15 nm. The effect of ultrasonic irradiation on presence of a sharp particle size distribution then denoted by comparing the condition with production without ultrasonic irradiation