Sharif Digital Repository

Analog-to-digital conversion and signal processing has been increasing due to its many advantages. So that mostly we prefer to convert signal from analog area to digital samples, then they are processed and finaly put the result signal at the system output. How ever because the restriction of the sampling rate, Prevent the spread of digital processing for the high-frequency signal (RF). In recent years, ADCs sampling rate rise up to several GHz (for example ADC with 4 GSPS and 12 bits for TI) that output of the these ADCs by powerful and fast FPGAs are processed but According to Shannon theorem band width of these ADCs is not desirable.the goal of this thesis uses of the compressed sensing... 

Sparse representation and compressed sensing have been widely used in various fields, one of the most popular of which is ISAR imaging. Inverse Synthetic Aperture Radar (ISAR) provides an electromagnetic image of the target, which is mainly used to identify and classify targets. In some applications, recognizing targets from a 2D image can be difficult and error-prone. One idea to deal with this problem is 3D ISAR imaging. The most widely used method of ISAR imaging is direct method based on Fourier transform. This method requires the measurement of radar data with high measurement density in 3 directions, which increases the data collection time and volume, which is the main problem of... 

Recovery of low-rank matrices from compressed linear measurements is an extension for the more well-known topic of recovery of sprse vectors from underdetermined measurements.Since the natural approach (i.e., rank minimization) for recovery of low-rank matrices is generally NP-hard, several alternatives have been proposed. However, there is a large gap between what can be achieved from these alternatives and the natural approach in terms of maximum rank of the unique solutions and the error of recovery. To narrow this gap, two novel algorithms are proposed. The main idea of both algorithms is to closely approximate the rank with a smooth function of singular values and then minimize the... 

Sparse signal processing (SSP), as a powerful tool and an efficient alternative to traditional complete transforms, has become a focus of attention during the last decade. In this ap-proach, we want to approximate a given signal as a linear combination of as few as possible basis signals. Each basis signal is called an atom and their collection is called a dictionary. This problem is generally difficult and belongs to the NP-hard problems; since it requires a combinatorial search. In recent years however, it has been shown both theoretically and experimentally that the sparset possible representation of a signal in an overcomplete dictio-nary is unique under some conditions and can be found in... 

Owing to the unprecedented increasing rate of introducing new means of editing and transferring videos, appropriate and efficient methods for authentication and tampering detection are already playing a pivotal role in today's world. Amongst various methods for authentication, tampering detection and property right protection, the focus of this thesis is on watermarking. In the first place, we will present watermarking methods based upon H.264 standard. Due to the fact that almost all of the video products are stored and distributed in compressed file formats, the ability to retrieve the watermark after video compression is of crucial importance. Hence watermarking whilst video compression... 

Orthogonal Frequency Division Multiplexing (OFDM) is a digital transmission method developed to meet the increasing demand for higher data rates in communications which can be used in both wired and wireless environments. This thesis describes the issue of the Peak to Average Power Ratio (PAPR) in OFDM system, based on compressed sensing at the receiver of a peak-reducing sparse clipper applied to an OFDM signal at the transmitter. By choosing proper clipping threshold, clipping signal will be sparse so locations, magnitudes, and phases of the clipping signal can be recovered by compressed sensing method. We demonstrate that in the absence of optimization algorithms at the transmitter,... 

Regarding to the emergence of quantum computers and advances in the large scale implementation of them, exploiting hard problems of coding theory is of great importance.This family of problems, like those of lattices, has an acceptable resistance against classic and quantum attacks. Thus, they can be replaced with hard problems in number theory and algebra. Increasing dominance of cyberspace made us highly dependent to it. Consequently, preparing tools and methods to mitigate future threats is necessary. Digital signatures constitute an important family of cryptographic primitives. Code-based digital signatures are mostly suffering from two main drawbacks: large public key and long time of... 

Understanding and defining mechanical behavior of engineering materials play an important role in the prediction of material response as well as analysis and design of engineering structures. In this investigation, polymer concrete (PC) with three different epoxy resin contents, ordinary cement concrete (OCC), lightweight concrete (LWC) and lime-mortar soil (LMS) have been studied under uniaxial and triaxial compression tests to determine their mechanical behavior by measuring axial stress-strain and volumetric strain versus axial strain curves. According to the results, PC showed higher strength, ductility and energy absorption than that of OCC and LWC. Nowadays, waste tire rubber is used... 

A large number of metals and alloys show an irregular plastic behavior called serrated flow when being deformed in a certain temperature and strain rate range. The occurrence of this phenomenon affects the mechanical properties of the material, such as work hardening rate and ductility; consequently, it exerts limitations on the service conditions in which the alloy can be used.
IN600, IN625 and MP35N are wrought Ni based and Ni-Co based alloys which are thermo-mechanically processed. When these alloys are deformed in temperatures between 300-700 ˚C, serrated plastic flow occurs. In order to investigate the mechanisms responsible for irregularities, hot compression tests were conducted... 

Due to restrictions on connecting beams to columns in steel structures when moment-resisting connections in two perpendicular directions are employed, the use of ordinary wide-flange sections or fabricated boxes becomes impractical. Application of Cruciform sections, especially when fabricated from two split-I sections (IPE sections) seems to be a reasonable solution to this problem. In this study, using Finite Elements Method and a commercial FEM software, the nonlinear behaviour of a number of cruciform columns, already tested by others, were modeled numerically. After reaching results in acceptable agreement with those of the tests, potentially suitable sections for fabrication of... 

Due to the wide application of lightweight concretes, there have been a lot of research activities in this field. The objectives of these activities have been mainly to increase the strength and endurance, and to reduce the weight and the cost of production of such concretes.
Lightweight concretes, in general, have the following advantages: relatively high strength to density ratio, high thermal and acoustic insulation, and fire resistance. However, foam concretes, as a particular type of lightweight concretes, compared with other types, has the advantage of ease of production. Therefore, in this work various aspects of foam concretes were studied. In this project, using a particular... 

Ultrasonically assisted machining process has been introduced as an effective solution to improve machining quality of mechanical parts. This study aims to reduce the drilling induced delamination to increase mechanical properties of CFRP laminates. Delamination decreases the strength of laminates under static and dynamic loads. Drilling parameters (feed and spindle speed) and ultrasonic vibrations amplitude are chosen as design experiment factors. Taguchi design of experiments method is used to study the effect of input factors on drilling thrust force, delamination factor and residual strength of drilled laminates. Input factors are in three levels and Taguchi L9 array is utilized. To... 

The benefits of fiber reinforced concrete (FRC) have been thoroughly investigated. Much of this work has focused on polypropylene FRC notched beams. Data on the bending behavior of beams with side notch is scarce. A pilot investigation on the compressive behavior of macro-synthetic FRC and on the splitting tensile strength of FRC was carried out. Twenty beam specimens with two different strength of 35 and 60 MPa was tested. The beams dimensions were 15*15*55 centimeter and they were notched at the bottom at mid and side span. Two different notch depth of 5 and 7.5 centimeter was implemented. At the end it was found out addition of fiber decreases compressive strength of 35 Mega Pascal... 

The seepage through foundation of earth dams causes the serious menaces and the reduction in their efficiency. Therefore essential contrivances must be provided against mentioned problem. One of the common and efficient ways to control the seepage through pervious layers under earth dams is plastic concrete cut off wall which is used as a filling material in dam foundation due to the low permeability, plastic behavior and low enough elastic modulus compatible with the adjusted soil. In addition to the mentioned properties, it is important that plastic concrete have an adequate compressive strength to sustain the lateral and vertical loads. Since the change in the ratio W/C (water to cement)... 

In present research forming process of nanopowders, which is a part of powder metallurgy was investigated by molecular dynamics method. Powder metallurgy is a relatively new method for production of industrial parts by pouring powder into die and compaction to desired density. One can reach parts with higher quality and strength by decreasing size of powder’s particles and entering the nano scale. Particle with smaller size have higher specific surface and due more intensity to react. Classic methods for investigation of this process don’t cover the atomic scale effects, so using newer procedures such as molecular dynamics is highly recommended. In present research, at first compaction of... 

In this thesis, gas-liquid two phase flow through channels of spatially varying cross-sections are investigated numerically. These flows have several industrial applications such as nozzles, diffusers, pipeline blowdown, valves and channels with sudden expansions or contractions. Finite volume method in conjunction with a second order Advection Upstream Splitting Method (AUSM+_up) is employed to discretize the governing equations. At low Mach numbers, a pressure-based algorithm with a pressure correction equation is derived. Using the pressure-based algorithm accelerates the convergence rate of the numerical solution of the flows through sudden expansions. The pressure adjacent to the... 

Compression systems are widely used in industry. Dynamic Modeling is a necessary step in design procedure of a safe compression system. A dynamic model should be adequate and simulate dominant elements of system. One important element of industrial compression systems is downstream pipelines. In this work, a dynamic model including pipeline acoustics is developed. It is accepted that changes of working gas molecular weight can cause inception of instabilities in compression systems. The dynamic model used to study mechanisms of these instabilities. A conventional control system is modeled and tuned for the compression system. The ability of this control system to prevent such instabilities... 

In the limit case of incompressibility, the displacement-based finite element methods are not capable of finding the solutions with adequate accuracy. Moreover, discontinuities in displacement field or strain field which exist in the interior of the elements should be dealt with appropriately. The u/p mixed formulation provides a suitable context for modeling the incompressible problems. It is capable of solving general problems in which there exist geometrical or material nonlinearities. In the case of employing the eXtended Finite Element Method (XFEM), uniform meshes can be used for problems with discontinuities and in fact the discontinuities can be decoupled from the mesh. In this... 

In general, the output power of the gas turbines decreases as the ambient temperature increases. One of the ways to deal with such a performance loss is to use a wet compression system, which reduces the air temperature rise throughout the compressor. In this system, the water droplets are injected into the compressor inlet and evaporated throughout the stages. As a result, the work of the compressor decreases, and the power and efficiency of the whole engine increase. Most of the researchers in this field have tried to develop mathematical models to investigate the effects of this system on compressor and gas turbine performance in steady-state conditions. However, it is necessary to... 

Concrete-filled FRP tubes have gained popularity among structures in areas with harsh environmental condition. Despite their unique material properties such as resistance to corrosion, noticeable axial stiffness, and durability, FRP materials have poor resistance to elevated temperatures. This paper presents numerical investigations on the behavior and capacity of concrete-filled pultruded GFRP tubes (CFGFTs) after exposure to elevated temperatures under concentric compression. Variables in this study were the tube’s thickness of 3, 5, and 7mm, the infill concrete’s compressive strengths of 30 and 60MPa, and the exposure temperature of 25, 100, 200, 300, and 400°C. The results in this study...