Sharif Digital Repository

Currently, precision positioning plays a pivotal role in technology. The need for accurate positioning in applications such as robotics, optoelectronics and micro and nano technology increases day by day. To begin with, we wanted to make a miniature prototype of a micro actuator but in this regard, we are faced with several problems. Some major problems were the high cost of this prototype plus the fact that we did not have the necessary equipment with acceptable resolution for making such a system, so we decided to decrease the scale and build it in micro scale which is accessible for us. The main idea and design of this micro actuator is by Mrs. Akbari. During this project, the researcher... 

In the process of extraction and refining of crude oil, a considerable amount of waste water are produced and if they are not removed, they would become one of the most hazardous environmental factors. Furthermore, purification and separation of such wastewaters are extremely costly, which with an increase in separation efficiency can lead to a decrease in valuable product loss. On the other hand, in the extraction process from oil reservoirs, because of porous and permeable nature of them, solid particles may be added to the extracted crude oil, which can result in an obstruction in pipelines and transport equipment.The most conventional method for separating a mixture of water, oil, and... 

Let M be a closed manifold, oriented manifold of dimension -d. Let be the space of smooth loops in M. Chas and Sullivan defined a product on the homology H⋆(LM) of degree -d. They then investigated other structure that this product induces including a Batalin-Vilkovisky structure, and a lie algebra structure, and a lie algebra structure on the S1 equivariant homology HS1 ⋆ (LM). This algebraic structure as well as the others came under the general heading of the string topology of M  

In this research, a new step-by-step and comprehensive method for the design of back-to-back converters in the presence of nonlinear and constant power loads is proposed. Common methods for the design and control of back-to-back converters consider the design procedure independent of the linearity and dynamic of the load, also, the control system design and performance of the load-side and grid-side inverters of back-to-back converters are assumed to be independent of each other which is shown in this thesis that this assumption is not valid in the presence of nonlinear loads. These methods result in challenges and problems regarding stability and power quality indices of back-to-back... 

High-quality online services demand reliable and fast packet delivery at the network layer. However, clear evidence documents the existence of compromised routers in the ISP and enterprise networks, threatening network availability and reliability. A compromised router can stealthily drop, modify, inject, or delay packets in the forwarding path to launch Denial-of-Service, surveillance, man-in-the-middle attacks, etc. So researches tried to create intrusion detection methods to identify adversarial routers and switches. To this end, data-plane fault localization (FL) aims to identify faulty links and is an effective means of achieving high network availability. FL protocols use... 

Confining nematic liquid crystal between two curved boundary conditions while the nematic molecules have a degenerate planar anchoring leads to complex and beautiful textures of molecular disparagement(defect) in the bulk and on the surfaces. Such the liquid crystal shells are made with double-emulsion techniques in microfluidic devices have provided applications for micro-scale colloidal linkers.In this work, we have numerically minimized the elastic energy in one-constant approximation in present of Fournier’s degenerate surface potential with finite element method. The nematic shell is confined between two spherical surfaces that we have studied the final energies and their related... 

A liquid crystal is a state of matter composed of highly anisotropic building blocks. When liquid crystals are exposed to surfaces with given shapes and anchoring conditions, the orientation of their molecules gets spatially disordered and topological defects may form in the system. The interplay between liquid crystals and the geometry and topology of the colloidal particles has motivated extensive recent studies.In this thesis, we present the numerical modeling results for two faceted colloidal particles in a thermotropic nematic liquid crystal. To obtain the equilibrium nematic field with a finiteelement method, we numerically minimize the free energy of the system that based on the... 

Providing high bandwidth network infrastructures for ever increasing need of data transport is of great importance. The underling infrastructure for many communication services such as GSM/3G/4G mobile networks and Internet services is Synchronous Digital Hierarchy (SDH) optical transport systems. SDH are standardized protocols that multiplex multiple lower rate digital bit streams, such as E1 and Ethernet, and transfer them synchronously over optical fiber using lasers or LEDs. In addition to high data transfer rates, flexible network management and protection mechanisms have great importance, hence are part of SDH standards. In order to obtain flexible network architecture and protected... 

Development of Smart Grid and deployment of smart meters in large scale has raised a lot of concerns regarding customers’ privacy. Consequently, several schemes have been proposed to overcome the above mentioned issue. These schemes mainly rely on data aggregation as a method of protecting users’ privacy from the grid operators. However, the main problem with most of these schemes is the fact that they require a large amount of processing power at the meter side. This, together with the fact that smart meters don’t usually have a powerful processor, can cause the unavailability of smart meter data at the required time for operators of the grid, and at the same time disables smart meters from... 

Secure routing is a major key to service maintenance in wireless ad hoc networks. Ad hoc nature exposes the network to several types of node misbehavior or attacks. Wormhole attack is one of the most severe active attacks to mobile ad hoc networks. Noting the resource limitations in such networks, nodes may have a tendency to behave selfishly. Selfish behavior can have drastic impacts on network performance. We have proposed a Misbehavior-Tolerant Multipath Routing protocol (MTMR) which detects and punishes all types of misbehavior such as selfish behavior, wormhole, sinkhole and grey-hole attacks. The protocol utilizes a proactive approach to enforce cooperation. In addition, it uses a... 

Dynamic spectrum access with the aid of cognitive radio (CR) technology is a viable solution to spectrum scarcity. There are many military and emergency applications that require the use of cognitive radio ad-hoc networks (CRAHNs). In these net- works, nodes must first find each other in a wide spectrum and then exchange their desired data. The process of finding each other in a multichannel environment is a challenging task and is usually known as rendezvous. Since rendezvous is the first step in communication, disrupting will disable the nodes from establishing proper communication sessions. That results in a drastic degrade in the performance of the network. Therefore, it is important to... 

Spray-wall impingement is one of the phenomenon that associated in different subject such as spray cooling, combustion chamber and gas turbine. With the impingement of spray to the wall, liquid film begins to form. Wall film characteristics is still an important subject to study. Due to the various stages of liquid film formation from injection of spray till spray-wall impingement, the investigation of this phenomenon is far more complicated. This paper is meant to provide detailed data on spray-wall impingement and wall film formation physics with the help of computational analysis of OpenFOAM . A Lagrangian-Eulerian method based on discrete phase model (DPM) was employed to model... 

A structural system will normally fail as a result of a chain of failures of different components. In this thesis, fatigue reliability of fixed offshore platforms is investigated through analyzing different failure scenarios. In order to evaluate the occurrence probability of a special scenario, each failure scenario is divided into a finite number of sub-scenarios. To do this, all combinations of time sequences are generated for a given sequence of failures, using a specially developed program.
In order to calculate the occurrence probability of each scenario, a massive reliability analysis should be done for each of corresponding sub-scenarios. Knowing that there are a large number of... 

Light propagation through turbid media, such as biological tissues, experience scattering due to inhomogeneous distribution of refractive indices of their microscopic structure. Scattering and diffusion of light not only limit many optical imaging techniques, but are also big challenges for telecommunication, spectroscopy, and other optical techniques. Recently many new imaging methods are developed that work in strongly scattering media such as optical coherence tomography, diffusion tomography, and laser speckle velocimetry. In this thesis, we have studied how wavefront shaping techniques can tackle the problem of scattering. By spatially shaping the wavefront of the incident beam, using...