Sharif Digital Repository

In this thesis, we first briefly review the basic concepts of stochastic processes. After reviewing and studying the dynamic equation that can explain a stochastic process, we show how one can find on a data-driven basis, the first-, second- and higher-order interactions between different subunits of a complex system by disentangling the dynamics of multivariate time series into stochastic and deterministic parts. Our data-driven approach is to detect different degrees of interactions obtained using conditional moments of Kramers-Moyal coefficients from unconditioned correlation functions and statistical moments of multivariate N-dimensional multivariate time series. Finally, we study the... 

Control of complex networks is the main topic in this study. In this thesis, we consider the methods to steer the dynamics of a network from an initial state to the desired final state. To achieve this goal, one needs to inject appropriate control signals from some of the network nodes, known as driver nodes. We consider the quadratic sum of control signals as a cost function, to control the dynamics of the network. By minimising the cost function, we obtain the control signals as well as driver nodes for a linear time-invariant system. As an example, we provide the results of an energy minimization strategy to control the second order Kuramoto and Rössler dynamics in Erdos-Renyi and... 

In today’s microwave communication, oscillators play a very important role as signal sources, so, the design of an oscillator with suitable parameters can affect the final performance of a communication system. Among different parameters, phase noise is of utmost importance since this parameter can lead to interference in multi-frequency channel systems and thus its estimation and measurement is vital. In an oscillator, in addition to measurement, the most important problem is to reduce phase noise. In fact, for improving the performance of a transmitter-receiver ( e.g. receiver sensitivity ), one of the solutions is using a local oscillator with low phase noise. Our purpose in this project... 

Phase noise is one factor determining the quality of radar and telecommunication systems. Oscillators play an important role in system phase noise, as a signal generator for information modulation. Due to the greater impact of flicker noise at near carrier frequency than other types of phase noise sources, its reduction will greatly impact the phase noise of the telecommunication system. In this research, the simulation of phase noise and flicker noise of an oscillator with various types of bias circuit methods, such as bias with two independent sources, self-bias, improved self-bias, active bias, and low-frequency feedback, has been done. Then, a microwave dielectric resonator oscillator... 

In this thesis, we first introduce the required biological preparations and popular models for modeling single neuron, synapse and cable. Then by introduction of limit cycle oscillators and the necessary prerequisites, investigations are limited to systems involving weakly coupled oscillators. As two examples of such models, famous Kuramoto and Wilson- Cowan models are described. In the following, we introduce some methods for reduction dimension of weakly coupled oscillators and finally we apply one of the expressed methods on the dynamics of cortical network  

In this research, two of the most prominent known sources of interannual and interdecadal climate variability in the form of El Niño-Southern Oscillation (ENSO) and Pacific Decadal Oscillation (PDO) are analyzed to assess the influences of these large-scale climate phenomena on water supply in the Zayandeh-rood River Basin. For this purpose, any shifts in the mean and variance of the inflow volume of the Zayandeh-Rood Dam in different climate conditions (resulted by different combination of ENSO and PDO phases) has been analyzed and compared with similar statistics in neutral condition to find out if the differences are statistically significant. Correlation analysis indicates that the... 

Vibration of a stationary structure excited by another structure moving on the surface of the stationary Structure is very common in engineering, such as vibration of bridges or railway track under travelling vehicles. In the simplest case, the moving structure maybe modeled as a constant load or a mass or an oscillator, in this case the moving mass is attached to the structure either by a spring or by a mass-spring assembly.The model of a moving mass brings some improvements to the model of the moving load where there is no inertia effect. However, the relative displacement between the moving structures and the beam cannot be considered in the moving mass model. A more appropriate model in... 

This thesis revises the conventional design of oscillators and presents new methods for shaping and improving the phase noise curve of the oscillators. These methods are based on the position of the zeros and poles of transfer functions in the S-plane and choosing their orders also. This thesis shows that the position of the zeroes and poles of the transfer functions can affect the phase noise performance of the oscillators. The basic idea is maximizing the slope of the phase of the transfer functions. It is proved that the maximizing the slope of the phase can expectedly reduce the phase noise of the oscillators. Increasing the order of the transfer functions with the proper selection of... 

In this thesis, we investigate some transport and many body properties of the 2DEG at the interface of LaAlO3=SrTiO3. First, following Boltzmann approach,we calculate conductivity and mobility of the system. We find that the mobility varies inversely with the cubic power of the carrier density ( / n3 2D) in good agreement with the experimental results. Taking in to account the variations of the dielectric constant of SrTiO3 with carrier density together with the variable impurity density are remarkably important in order to explain the abrupt decrease of the mobility with increasing the density in this system. We find that although the multiband nature of the system and interband... 

The cosmic ray astronomy was born in 1937 when Victor Hess flied with his balloon to the height of 17,500 feet. He found that the cosmic rays come from out of atmosphere. There are three fundamental questions from that time. The goal of all cosmic ray's experimental activities and related theories is finding out the answer of these questions. The first question is about the energy and composition spectrum of cosmic rays. The second one is finding the sources which can product the very high energy particles such as the protons with the energy of 1021eV found in some observatory such as AGASA and Haverah Park. The last question is the process of diffusion of cosmic rays in the galaxy. One of... 

Energy production from fossil fuels such as oil and gas is one of the non-renewable energies, which causes many environmental problems such as global warming and environmental pollution. The existence of these problems has led to agreements such as the Glasgow Environmental Agreement, which obligates governments to apply methods to reduce the production of greenhouse gases, which include reducing the consumption of fossil fuels and increasing the use of renewable energy. Sea wave energy is one of the renewable energies that can be used to generate electricity. In the seas of Iran, including the Caspian Sea, Persian Gulf and Oman, there are sources of wave energy that can be used and are... 

Drop formation is an interesting field of research due to its wide-range of applications, such as drug delivery, biomedical studies, pharmaceutical industry, Lab-On-Chip (LOC) devices, etc. In recent years, researchers have used numerical studies to better understand how the process of forming and carefully examining the factors that affect quality improvement. One of the most effective methods in controlling the size and the process of drop formation is use of external force in microfluidic devices. Among the external force in this study, an electric field with oscillating current was used. Rapid methods for creating, deforming, and breaking up of drops in microfluidic applications are... 

In recent years, LBM has shown to be a suitable computational method for most flow simulations. Its simplicity, adaptability for complex geometries, and capability in parallel processing are among many reasons for broad implementation of LBM these days. In this research, we have tried to take advantage of these features in the simulation of a turbulent flow over a moving circular cylinder at low CPU cost. In order to use LBM in a high Reynolds number incompressible flow over a moving solid body, one needs to pay special attention to grid quality, curved solid wall boundary condition, turbulence model, moving boundary, etc. In order to obtain a suitable practical LBM computer code, the... 

The purpose of this research is to design and construct a test stand for the study of the kinematic and flow pattern of bluff bodies oscillating motion due to the induction of wake vortices. An experimental investigation was conducted to explore the effect of bluff bodies cross section to optimize the amount of electrical energy withdraw by electromagnetic harvester. The tests are carried out on cylinders with square section and rectangular sections with a length to width ratio of 1/2 and 2/1 and height of 50 cm, at speeds of 5 to 35 m/s in eiffel type wind tunnel. The effect of cylinder cross-section on the amount of energy harvested showed that the amplitude of oscillation in square-shaped... 

Coral reefs are one of the most important ecosystems in terms of animal diversity and affect the life of organisms in different ways. This ecosystem is under many stresses today. Among the most important events that threaten corals is the event of coral bleaching, the most important cause of which is the increase in Sea Surface Temperature (SST). Therefore, it is highly important to monitor changes in SST affecting corals. Generally, temperature anomalies are investigated using the Degree Heating Weeks index (DHW) and SST data. DHW index is variable in different coral areas and changes with time. Consequently, in the present study, this index has been investigated due to the lack of... 

Quantum mechanics, in the first place, investigates systems by introducing some principals about Hilbert space, Schrodinger equation etc. which are recognized as closed quantum systems. In principal, this is not possible to separate a system from its surrounding environment. Thus, the implications of the environment on the evolution of the system must be taken into consideration. As a matter of fact, all systems are considered open in a sense that, it is interacting with another system called the environment. In such circumstances, we tend to represent a description of open quantum systems in a way that following the evolution of the total system, i.e. system and the environment, is not... 

Obtaining polymers with desired properties is very important from industrial point of view. Weight average molecular weight and polydispersity are two important polymer properties. It is also important to obtain a high monomer conversion. In this thesis production of polyvinyl acetate is considered. Under special conditions, the vinyl acetate reactor dynamics become oscillatory which makes the reactor control difficult. For obtaining polymer with desired properties, an objective function which includes weight average molecular weight, polydispersity and monomer conversion has been defined. Then by minimizing this objective function the optimal temperature trajectory is obtained. In this... 

The subject of control and stabilization of dc-dc series resonant converter (SRC) is still a challenge in power electronics. The conventional controller design and stability analysis for this converter are based on the model which is derived using the sinusoidal approximation and averaging followed by linearization about an operating point. This model is not applicable to a SRC that operates below resonance especially in discontinuous conduction mode (DCM) because the sinusoidal approximation is no longer acceptable. However, a SRC may be purposely designed to operate in DCM. Therefore, it is essential to investigate the stability analysis and controller design using a more sophisticated... 

In this thesis, a laminated composite beam with single delamination under the action of moving force/oscillatory mass has been modeled and analyzed. The beam has been divided into four sub-beams and based on Bernoulli-Euler and also shear deformation theories and by applying the Hamilton’s principle, equations of motion along with boundary conditions and continuity and compatibility conditions are obtained. Primarily, based on the constrained mode model the delaminated layers have been analyzed then by implementing the Lagrange multipliers method, the nature of dynamic contact is incorporated. In the analysis of delaminated beam based on the constrained mode, by applying the mode summation... 

We present a model for noise in Ring Oscillator in order to improve their phase noise. Each Ring Oscillator consists of some nonlinear blocks. We study effect of these blocks on phase noise. In this study, noise behavior of each block is analyzed and a design guide for improvement of noise characteristic is presented.
It is essential for phase noise improvement to obtain insight about operation mechanism of oscillator. A comprehensive model is presented to get this insight. In this model, we show how a noise current produces phase noise also, a formula for phase noise is derived.
In second step, phase noise of Ring oscillator is improved by using the model. Active inductor is used in...