Sharif Digital Repository

The two central topics of information theory are the compression and the transmission of data. Shannon, in his seminal work, formalized both these problems and determined their fundamental limits. Since then the main goal of coding theory has been to find practical schemes that approach these limits.Polar codes, recently invented by Arıkan, are the first practical codes that are known to achieve the capacity for a large class of channels. Their code construction is based on a phenomenon called “channel polarization”.
Since the performance of polar code degrades when the block length is small, several kinds of Reed – Solomon concatenation is considered in order to improve the... 

In this thesis, a one-dimensional simulation, developed in previous studies, is utilized to study the stability phenomenon in axial compressors of gas turbine engines. The transient and dynamic operation of the axial compressor is determined by solving the conservation equations, expressed as one dimensional and time dependent Euler equations with related turbomachinery source terms. As known, stall and surge are one of the most destructive dynamic phenomenon in gas turbine engine compression systems starting with a flow separation. This occurrence is investigated in detail. In general, while choking phenomenon may occur during engine startup in the first stages, stall... 

Every day thousands of questions and answers are posted on CQA websites. Stack Overflow is known as one of the most famous CQA websites in the field of programming. Asking high-quality questions on this website is one of the significant challenges that users face, leading to negative experiences in some cases. In this research history of questions, answers, and post-edits are studied to get insights for predicting questions’ quality. There is a new approach proposed that takes both post scores, and their edits into account to predict their quality. The questions edits that are studied are the one that happened before getting an accepted answer. Furthermore, it is shown that predicting these... 

In this research, a new method is proposed for constructing concrete structures with complex geometry, including topology-optimized concrete dams on a small scale. According to global statistics, concrete is the most consumed material in the world after water, and its production process causes a lot of damage to the environment. One of the best ways to reduce concrete consumption is to use tools such as topology optimization to design concrete structures. Nevertheless, few research has been done in the field of topology optimization of concrete dams, one of the important reasons for which is the difficulty of building physical model to perform physical tests, validating numerical models, and... 

Little research has been conducted on the topic of water resources management in case of deep uncertainty. It is vital for the management method to be upgraded in order to accompany future changes. The purpose of this study is to inspect some viable scenarios and deriving proper reservoir management solutions in case of deep uncertainty. This is important due to the fact that future changes are often not considered when water resources structured are being designed. In the past, some researches in this field of study were done; For instance, the effects of climate change on reservoirs have been assessed. Also methods for coming up with scenarios have been proposed, most of which are based on... 

Ejtehadi, M. R. (Mohammad Reza)All cells store their hereditary information in the same linear chemical code, DNA molecule. These information organize all activities of the cell. Therefore, studying various properties of DNA is a big step toward understanding life. Physicists are interested in elastic properties of DNA molecule. DNA is somehow a chain of successive atoms that have been arranged in a helical ladder shape. DNA is a stiff polymer so one has to use a large amount of energy to bend it. Interestingly, in most of the natural cases, DNA is highly packed into spaces that are much smaller that its total length. Cell’s nucleus and viral capsids are some examples. Virus maybe the... 

Network design problem has been, and is, an important problem in transportation. Network design based on different objectives affects network's performance in different dimensions, so it is crucial to define a good objective for the network design problem that improves the network's performance in more than one aspect. The purpose of this study is to obtain the differences between the two network design problems, one based on resiliency, and the other based on consumer surplus.Network design problem based on consumer surplus minimizes the total travel time for the users in the network. Network design problem based on resiliency is defined as: (1) before the incident, (2) after the incident... 

The use of carbon nanotubes in Nano-switches is the new technology employed in NEMS. Nano-switches can also perform as a micro-switch where nano-switches have longer life time, consume lower power and have smaller size with respect to micro-switches. As the dimensions get smaller, continuous models lose their accuracy and molecular methods should be utilized. Non local theory, is attempting to change the responses of continuum models assuaging them to the molecular results. In the current project, non local theory is used to rewrite the governing equations of nano tubes motion and the results are compared with the classical results. In this modeling, the axial force and the mid-plane... 

Human action recognition is one of the computer vision branches. Video surveillance and human-computer interaction is among its modern applications. The main goal of this subject is to label RGB-D videos which are captured from acting human. Therefore, labeling the input videos among pre-learned action is called as action recognition.The action recognition problem consists of two primary parts. The first part is selecting a suitable descriptor to describe input videos, and the other part is the way that the system has been learned based on the learning action data. The main goal of this research title is to propose a method for recognizing each action video (that is acted by human and... 

The thesis attempts to investigate one of the factors influencing enterprise innovation, namely, innovation cooperation, its types and its diversity by using the data of about 2,500 companies in different fields of industry of national innovation survey that had been collected in 1395 hijri. In this research, firstly, by reviewing the literature on the field of innovative cooperation and the performance of enterprise innovation, the model of the association of colleagues with innovation performance has been extracted. In the following, it answers to this question is whether the cooperation with colleagues is essentially affecting the innovation performance of Iranian firms or not? The... 

Single nucleotide polymorphs, There are changes caused by a mutation in a nucleotide in the Dena sequence. Mononucleotide polymorphisms are the most common type of genetic variation. Some of these changes have little or no effect on cells, while others cause significant changes in the expression of cell genes that can lead to disease or resistance to certain diseases. Because of the importance of these changes and their effect on cell function, the relationships between these changes are also important. Over the past decade, thousands of single disease-related mononucleotide polymorphisms have been identified in genome-related studies. Studies in this field have shown that the expression of... 

Studying the interaction of active particles with walls and obstacles has a great importance for practical purposes. Using this knowledge, it is possible to control the motion of active particles in different environments.Experimental evidences show that after encountering with a wall, most of the active particles slide along the wall for some time and then detach in a certain direction, independent of their incoming angle. This special collision is considered to guide, sort or trap active particles.In this dissertation, using Brownian dynamics simulations and theories, we study how different selfpropelled particles and walls influence each other. We explore the behavior of two important... 

In this thesis, combustion of liquid fuel spray under the condition of hot and diluted oxidizer, has been investigated. To this end, a novel type laboratory-scale test rig was designed to study the physics governing the MILD-Spray combustion conditions with an applied approach. The underlined test rig eliminates the operational problems of its predecessor test stands and is used for a heavier fuel with much more complex chemical composition (kerosene). The test section is axially symmetrical, in which the fuel is injected by a pressure-swirl atomizer in the direction of hot and diluted co-flowing air. With the use of the aforesaid test rig, the effect of variables such as oxygen...