Sharif Digital Repository

Austenite in the 300 stainless steels is not a table phase in deformation processing during which martensite phase forms. This causes significant changes in the microstructures as well as the mechanical properties of deformed steel. In this work, the effects of rolling speed, reduction, temperature and strain pass on the strain-induced martensite during strip rolling operation of stainless steel 304L are studied. Cold rolling experiments under different speeds, reduction and temperature are conducted and then, the microstructural events and mechanical properties of the rolled steel are evaluated. For doing so, different testing techniques including tensile test, hardness measurement, optical... 

Entanglement plays a major role in quantum communication as the main resource of the network and makes it possible to perform protocols such as quantum teleportation and quantum cryptography which are not achievable in classical networks. Yet, being a very fragile resource against noise, which inevitably is more destructive as the distance increases and as time passes, makes it a very hard to share entangled pairs between two points at a long distance. This discouraging result for linear networks, has prompted investigation of entanglement percolation and entanglement distribution in regular one- and two-dimensional networks with various (i.e rectangular, triangular and hexagonal)... 

In this research, utilizing the concepts of thermohydraulics theory, optimization of cost due to the scale formation in heat exchangers 4131 and 4132 in Khorasan petrochemical Co. urea unit. The objective function involves the minimization of the energy consumption and production losses. First, via ASPEN we simulated shortcuts of exchangers in order to examine the effects of temperature and velocity on the scale formation. The project optimization was done by GAMS and due to the presence of nonlinear terms in the scale rate equations using CONOPT solver. Eventually, modern practical methods were used with the purpose of reduction and eradication of the heat exchanger's scale formation. The... 

In this thesis, we investigate the previously introduced Code-Shifted Reference (CSR) time hopping (TH) ultra-wideband (UWB) communication systems as a potential transmitted scheme reference technique. In this system, the orthogonality between the reference and data bearing signals is acquired in code domain. Also, we consider a simple scheme to clean to obtain a clean template to be used in the correlator receiver for the CSR –UWB, this method was used in UWB system before. In this method, which we call as averaged (Ave.) CSR-UWB, the template for each symbol is obtained by taking the average over all the frames of the symbol interval. It is shown that Ave-CSR system achieves higher... 

In recent years, coded cooperation has been considered as one of the best methods to cope with fading and at the same time to achieve channel coding gain. On the other hand, the idea of network coding has been introduced for increasing the efficiency of cooperative networks. In addition, it has been shown that only by jointly designing of network coding and channel coding, the capacity will be achievable. Also, to adjust the rate to the channel capacity without requiring the channel state information at the transmitter, Rateless coding has been proposed. In this thesis above techniques are combined to gain their advantages and to improve system performance. To this end, first we consider a... 

Powerful spectrum sensing and access strategies enable cognitive radios to find transmission opportunity in spectral resources dedicated exclusively to the incumbent/primary users (PUs). One of the key effecting factors on cognitive radio network (CRN) throughput is the spectrum sensing sequence used by each secondary user (SU). In this thesis, modeling, performance evaluation, and throughput maximization of a CRN are investigated. More specifically, firstly we evaluate the performance of sequential method for handovers regarding a SU’s average throughput and consumed energy, and an optimization problem is formulated to maximize the throughput through selecting optimal channel sensing time.... 

Opportunistic spectrum access in cognitive radio networks has introduced multiple challenges in the realization of this technology. One of these key challenges is spectrum mobility, in which a secondary user (SU) must be able to vacate channels upon the appearance of a primary user (PU) and reestablish a communication link to avoid service termination, known as spectrum handoff. For a connection-based modeling, multiple interruptions can occur, which lead to multiple handoffs; hence, in a proactive approach, a sequence of target channels, rather than a single target channel must be determined. The first parts of this thesisis dedicated to the analysis of a time-slotted cognitive radio, in... 

Cognitive radio faces specific challenges due to its special spectrum access scheme. Providing required QoS and preventing irrecoverable interference on the primary users is among the crucial requirements. Beam forming is one of the interference avoidance schemes utilized in MIMO networks. Most of the studies on this scheme assume that each symbol is transmitted via vectors in order to grant desired beam forming and specific combinations in the receiver. Since transmitting multiple symbols at every transmission can provide higher data rates, it seems necessary to exploit beam forming through matrixes instead of vectors. In this thesis, by neglecting the interference of the secondary user,... 

In Massive multiple input multiple output (MIMO) systems, due to utilizing a large antenna array in each base station, each user’s channel is orthogonal to one another. These systems have some valuable advantages such as the reduction of energy needed for transmission and significant improvement in spectral efficiency, which makes them be suitable for the next generation of wireless communication systems. In this thesis a massive MIMO working is considered on TDD mode, for wich the channel estimation is done in the base station using the pilot sequences recieved from users. At the beginning of each coherence block, users send their pilot sequences and the rest of the times is allocated to... 

Salicylaldehyde-amino acid Schiff base complexes are used as non-enzymatic models for the metal-pyridoxal (Vitamin B6) amino acid Schiff base systems which are key intermediates in many metabolic reactions of amino acids catalyzed by enzymes which require pyridoxal as a cofactor (transamination, decarboxylation, α and β elimination, racemization, etc.). In addition, complexes of amino acid Schiff bases are considered to constitute new kinds of potential anticancer, antitumor, antiradical and antibacterial reagents. On the other hand nuclear magnetic resonance imaging (MRI), has become a powerful tool for clinical diagnostics thanks to the development of a new class of pharmaceuticals... 

In this thesis CCM/CVM operation and modeling of the Sheppard-Taylor topology is reviewed and a predictive control strategy is applied for a Sheppard-Taylor-based power factor correction (PFC) rectifier. Compared to conventional boost or buck boost PFC’s, this topology allows a better current tracking at the AC side, with a relatively reduced voltage at the DC side. Consequently, the high frequency AC filters required by the buck PFCs are avoided, and the voltage stresses on the boost switches are significantly reduced. Furthermore In predictive control strategy the duty cycle required to achieve unity power factor in a half line period can be calculated in advance. The main advantage of... 

A steady laminar flow coupled with heat transfer and chemical reaction corresponding to metalorganic chemical vapor deposition (MOCVD) in an axis-symmetrical rotating disc reactor has been simulated in a detail manner based on Computational Fluid Dynamics approach. The simulation results have been validated with the benchmark solutions obtained from the literatures for the corresponding casestudy.Three decision variables have been used whose optimum values are sought inorder to minimize the nonuniformity of the film on the susceptor. Their optimum values have been obtained by the elder-Mead algorithm which is a geometric optimization method which does not require gradient of the objective... 

In the study of supersonic air intakes, two important issues including performance and stability are considered. In these types of intakes, when inlet mass flow rate is reduced to a specified amount, a self-sustained instability called Buzz phenomenon occurs. During this phenomenon, shocks and expansion waves oscillate ahead of the intake that will degrade the performance considerably due to the periodic pressure fluctuations inside the intake. In the current study, a supersonic axisymmetric air intake with mixed compression which has designed for a free stream Mach number of 2.0 has been tested were conducted for free stream Mach numbers of 1.8, 2.0, and 2.2, and the results were analyzed... 

Each of us at some point in our life has been astonished by the observation of the collective motion of certain animals such as birds, fishes or insects. These coherent and synchronized structures are apparently produced without the active role of a leader in the grouping and it has been reported even for some microorganisms such as bacteria or red blood cells. In all the mentioned examples agents are surrounding with the moving fluid which can affect the collective behavior of the system. The collaborative flock of birds in windy conditions and the swimming of fishes in rivers or along oceanic currents are some examples. Moreover, micro-organisms produce different collective behavior in the...