Sharif Digital Repository

Offshore structures are valuable structures used in the offshore petroleum and gas fields for the purpose of hydrocarbon production operation. These structures are influenced by wave loads. Sea waves are inherently alternative and for this reason fatigue failure is considered as one of the most important modes of failure in these structures. Joints in these structures are made by tubes and they are called tubular joints. The joining process in manufacturing of these joints is welding. Welding process induces residual stresses, mostly tensional, in the substructure of materials. This tensional residual stress has a destructive effect on fatigue life of the joint and increases probability of... 

Terahertz waves have three important features that make them suitable for security imaging. 1. THz waves can penetrate or can be absorbed from many of non-metallic and non-polar materials and are perfectly reflected from most of the metals, 2. Drugs and explosives have terahertz spectra that can be detected by THz imaging, 3. THz waves do not have an ionizing effect and are harmless to the human body. Lenses can be considered as a major component of the terahertz imaging systems. Because of the relatively large terahertz wavelengths compared with wavelengths, compression, and decreasing the dimensions of the lenses in these systems matters a lot. In addition, reducing the wave reflection... 

Beam shaping, the transformation of radiated Gaussian beam from the millimeter wave source (conical horn antenna) into a flat-top beam at a certain distance, in the Millimeter wave active stand-off imaging system can be done in different ways, among them, beam shaping with a concave-convex lens (double-sided lens) in transmission mode (previous research done) can be mentioned. This lens consists of a convex-plane and a plane-concave lenses. when the electromagnetic wave passes through the convex-plane part, the wave front of the incident electric field changes from spherical to plane so that the divergent incident rays become parallel, and then the phase profile of the electric field changes... 

Terahertz passive imaging has unique properties applicable to the field of surveillance and security systems due to terahertz radiation can penetrate fabrics as well as other non-metallic materials and it obtains good resolution for standoff imaging application. In this thesis, we investigate an approach to model and analyse a terahertz passive imaging system performance. We use a reported model for predicting target acquisition performance for a terahertz imaging system and also we present a ray-tracing method and a new metric to compare different quasi-optical structures and scanning systems. Eventually, to evaluate the system performance model, we investigate a commercial terahertz... 

In this thesis a waveguiding structure is introduced and analyzed for terahertz applications. The proposed structure is called Inhomogeneous Parallel-Plate Ladder Waveguide (IPPLWG). This waveguide is formed by locating a periodic structure inside parallel-plate waveguide.This periodic structure itself consists of a array of metallic posts surrounded by silicon. The combination of metallic posts and silicon have the capability of guiding and confining of EM waves in terahertz band. IPPLWG can be used in implementing the passive terahertz devices such as filters and couplers. The main objective in this thesis is to characterize the IPPLWG for potential applications. Because IPPLWG has open... 

The purpose of this thesis is the analysis, design and fabrication of polarization converters in the millimeter-wave and terahertz regions using metasurfaces in order to generate different polarizations in the transmitter of terahertz stand-off imaging systems. In this project, the design and fabrication of two linear cross-polarizers and a circular polarization converter have been done at the frequency of 100 GHz. These polarizers are easy to fabricate by printed circuit board (PCB) technology and are capable of converting the polarization of a linear incident wave to the cross-polarized transmission wave and circular polarization, respectively. Linear polarizers are double-layer and... 

Once the satellite is in space, it is very important to make sure that it is in its correct and prescribe orbit, along with the appropriate attitude, at any given point of time. Orbit determination and finding the exact position and velocity of a satellite is one of the most important factors in space missions. The purpose of this work is improvement in orbit determination of LEO satellites. In order to achieve this goal, combination of SGP4 and HPOP algorithms with Kalman filter is used. The HPOP algorithm uses from integration of equations of motion for finding the position and velocity of a satellite. So this algorithm is very accurate for the short time propagation, but since it is... 

The effect of dielectric cladding on the waveguiding characteristics of an array of metallic pillars on a metal plane in the terahertz band is explored. Firstly, the 2D structure made up of a metallic grating of infinite lateral width is studied analytically to get more insight into the problem. It is seen that by using dielectric cladding decaying length of the domino waveguide is less than one tenth of the wavelength in a frequency bandwidth.
Then the ideas inferred from the 2D structure are applied to the realistic 3D structure that has a finite lateral width. It is shown that by proper design of the dielectric medium surrounding the metallic pillars the modal field confinement can... 

Formation, precipitation and deposition of heavy organic compounds of petroleum fluids including, but not limited to, asphaltene in oil reservoirs and production systems have been by far the most important and challenging flow assurance problems. Therefore, preventative actions to control organic deposits in oil formations and production systems which can lead inevitably to reduction or cessation in production, have always been a main challenge for oil producers.In this thesis, a one dimensional simulator have been developed to study asphaltene precipitation, aggregation and deposition in an oil well column at isothermal and non-isothermal conditions. While the previously proposed PC-SAFT... 

The Nurse Scheduling Problem (NSP), is one of the most complicated problem in health care management systems. The problem is to determine a work schedule by considering some requirements such as governmental policies, staff and their managers satisfaction, etc. Nowadays, while the demand of treatment centers for nurses has increased especially in hospitals, the population of qualified nurses has decreased. In such a fuzzy situation, the managers of the treatment centers want a better nurse scheduling in order to increase the quality of giving services to their patients and consequently increase their income. In this thesis, a two-stage nurse scheduling problem with uncertainties in... 

In this dissertation, to improve the specifications of the available standoff imaging system in the THz and far-infrared Lab. of the electrical engineering department, we designed and analyzed some examples of reflective optical configurations as an optics of active millimeter-wave imaging systems. The superiority of reflective optics versus refractive optics due to the specifications such as loss reduction, broadband frequency response, proper aberration control, beam scanning capability, and easier transmitter and receiver positioning justifies the utilization of reflective configurations for designing millimeter-wave imaging optics. The defined design goals of this dissertation are as... 

In recent years, given the rapid advancements in various technologies, there has been a heightened demand for establishing swift communications. Terahertz telecommunication, with its distinctive spectral characteristics, has garnered significant attention among researchers. Consequently, for the operationalization of next-generation wireless telecommunications devices, special considerations must be taken into account in their design. The selection of an appropriate substrate and possessing comprehensive information about substrate behavior at terahertz frequencies are among the crucial factors in these designs. In this thesis, we have examined the interaction of terahertz waves with... 

In recent decades, the technology of utilizing millimeter wave frequency bands has made remarkable progress. This frequency band is widely employed in important areas such as security applications, medical imaging, quality control testing, and future telecommunications. The main characteristics of waves in this band are their low risk due to low-energy photons compared to X-rays, and higher image resolution in imaging due to their shorter wavelength compared to microwaves. This thesis aims to investigate the effect of irregular reflection of electromagnetic waves from the edges of a metallic body located adjacent to an dielectric background by analyzing the electromagnetic scattering. Then,...