Sharif Digital Repository

With the rising penetration of wind power into electric power systems, more accurate and comprehensive studies are required to identify the interaction between wind farm(s) and power system. This requires accurate models of different types of wind turbine-generators. This paper describes modeling and small signal analysis of a grid connected fixed speed wind turbine generator (FSWTG). A complete model of FSWTG is derived and some reduced-order models are deduced. Eigenvalues and participation factors of the system for these models are calculated. Then, using the eigen analysis results, the models are compared and the simplifying assumptions which have been considered in the literatures are... 

In this paper, an effort has been made to model the 60 GHz wireless channel using a precise yet fast and simple method for short outdoor links that are a possible area for the use of 60 GHz wireless connectivity for high data rate services with dense frequency reuse. Considering the certain properties of this frequency band, it is possible to obtain a simple channel model to replace time consuming ray tracing methods with simpler models by disregarding low power multipath components. The street canyon model has been considered here as a replacement for ray tracing in an urban environment with below rooftop antennas to predict received power along streets and the results have been compared to... 

The aim of this study is to assess the functional connectivity from resting state functional magnetic resonance imaging (fMRI) data. Spectral clustering algorithm was applied to the realistic and real fMRI data acquired from a resting healthy subject to find functionally connected brain regions. In order to make computation of the spectral decompositions of the entire brain volume feasible, the similarity matrix has been sparsified with the t-nearestneighbor approach. Realistic data were created to investigate the performance of the proposed algorithm and comparing it to the recently proposed spectral clustering algorithm with the Nystrom approximation and also with some well-known... 

Challenging problems in the design and analysis of photovoltaic (PV) systems stem from the nonlinear current–voltage (I–V) characteristics of solar cells. This paper presents an analytical analysis based on a describing function method to investigate the transient and steady-state characteristics of a three-phase single-stage grid-connected PV system. In this study, the nonlinear I–V characteristic of the PV array is linearized around the operating point. The nonlinear dynamic of the maximum power point tracking controller is divided into two parts of continuous and discrete. For the continuous part, the common small-signal linearization is applied, while for the discontinuous part, a... 

In this paper, a learning algorithm is developed for Dynamic Plastic Continuous Neural Networks (DPCNNs) to improve their learning of highly nonlinear time dependent problems. A DPCNN is comprised of a base medium, which is nonlinear and plastic, and a number of neurons that are attached to the base by wire-like connections similar to perceptrons. The information is distributed within DPCNNs gradually and through wave propagation mechanism. While a DPCNN is adaptive due to its connection weights, the material properties of its base medium can also be adjusted to improve its learning. The material of the medium is plastic and can contribute to memorizing the history of input-response similar... 

In this study, the distributed six degree-of-freedom (6-DOF) coordinated control of spacecraft formation flying in low earth orbit (LEO) has been investigated. For this purpose, an accurate coupled translational and attitude relative dynamics model of the spacecraft with respect to the reference orbit (virtual leader) is presented by considering the most effective perturbation acceleration forces on LEO satellites, i.e. the second zonal harmonic and the atmospheric drag. Subsequently, the 6-DOF coordinated control of spacecraft in formation is studied. During the mission, the spacecraft communicate with each other through a switching network topology in which the weights of its graph... 

Background: Transcranial direct current stimulation (tDCS) is a noninvasive brain stimulation tool suited to alter cortical excitability and activity via the application of weak direct electrical currents. An increasing number of studies in the addiction literature suggests that tDCS modulates subjective self-reported craving through stimulation of dorsolateral prefrontal cortex (DLPFC). The major goal of this study was to explore effects of bilateral DLPFC stimulation on resting state networks (RSNs) in association with drug craving modulation. We targeted three large-scale RSNs; the default mode network (DMN), the executive control network (ECN), and the salience network (SN). Methods:... 

Alzheimer's Disease (AD) begins several years before the symptoms develop. It starts with Mild Cognitive Impairment (MCI) which can be separated into Early MCI and Late MCI (EMCI and LMCI). Functional connectivity analysis and classification are done among the different stages of illness with Functional Magnetic Resonance Imaging (fMRI). In this study, in addition to the four stages including healthy, EMCI, LMCI, and AD, the patients have been tracked for a year. Indeed, the classification has been done among 7 groups to analyze the functional connectivity changes in one year in different stages. After generating the functional connectivity graphs for eliminating the weak links, three... 

This paper presents a mathematical model for prediction of temperature history, final microstructures, and transformation kinetics in steel rods subjected to non-uniform cooling conditions. To achieve this goal, a mathematical model based on two-dimensional finite element method is developed to solve the governing heat conduction equation with non-uniform boundary conditions. The additivity rule is coupled with the finite element analysis to assess the kinetics of austenite decomposition during continuous cooling. The effect of decarburization during heating stage is also considered in the model employing Fick's second equation. To verify the predictions, time-temperature histories during... 

The main problem with airway segmentation methods which significantly influences their accuracy is leakage into the extra-luminal regions due to thinness of the airway wall during the process of segmentation. This phenomenon potentially makes large regions of lungparenchyma to be wrongly identified as airways. A solution to this problem in the previous methods was based on leak detection followed by reducing leakage during the segmentation process. This has been dealt with adjusting the segmentation parameters and performing the re-segmentation process on the pre-segmented area. This makes the algorithm very exhaustive and more dependent on the user interaction. The method presented here is... 

Functional magnetic resonance imaging (fMRI) is a non-invasive method that helps to analyze brain function based on BOLD signal fluctuations. Functional Connectivity (FC) catches the transient relationship between various brain regions usually measured by correlation analysis. The elements of the correlation matrix are between -1 to 1. Some of them are very small values usually related to weak and spurious correlations due to noises and artifacts. They can not be concluded as real strong correlations between brain regions and their existence could make a misconception and leads to fake results. It is crucial to make a conclusion based on reliable and informative correlations. In order to... 

Functional magnetic resonance imaging (fMRI) is a non-invasive method that helps to analyze brain function based on BOLD signal fluctuations. Functional Connectivity (FC) catches the transient relationship between various brain regions usually measured by correlation analysis. The elements of the correlation matrix are between -1 to 1. Some of them are very small values usually related to weak and spurious correlations due to noises and artifacts. They can not be concluded as real strong correlations between brain regions and their existence could make a misconception and leads to fake results. It is crucial to make a conclusion based on reliable and informative correlations. In order to... 

Purpose: Alzheimer's disease (AD) starts years before its signs and symptoms including the dementia become apparent. Diagnosis of the AD in the early stages is important to reduce the speed of brain decline. Aim of the study: Identifying the alterations in the functional connectivity of the brain during the disease stages is among the main important issues in this regard. Therefore, in this study, the changes in the functional connectivity during the AD stages were analyzed. Materials and methods: By employing the functional magnetic resonance imaging (fMRI) data and graph theory, weighted undirected graphs of the whole-brain and default mode network (DMN) network were investigated... 

Fluid flow behavior in a porous medium is a function of the geometry and topology of its pore space. The construction of a three dimensional pore space model of a porous medium is therefore an important first step in characterizing the medium and predicting its flow properties. A stochastic technique for reconstruction of the 3D pore structure of unstructured random porous media from a 2D thin section training image is presented. The proposed technique relies on successive 2D multiple point statistics simulations coupled to a multi-scale conditioning data extraction procedure. The Single Normal Equation Simulation Algorithm (SNESIM), originally developed as a tool for reproduction of... 

In this paper, a simpler implementation of the well-known droop method for the control of parallel Uninterruptible Power Supply (UPS) systems is presented. In this method, in the power-sharing control scheme, the output current is calculated by software without the need for a current sensor, resulting in a simpler and cheaper structure. By doing so, the number of feedback sensors is reduced from three to two. The paralleling strategy uses the droop method in which the control strategy is based on the drop in the inverter output frequency and amplitude. The application of Proportional-Resonant (PR) controllers is also extended to parallel inverter and its superior performance over the... 

This study investigated the behavior of ferrofluids containing superparamagnetic iron oxide nanoparticles (SPION) of various compositions for potential applications in drug delivery and imaging. To ensure biocompatibility, the interaction of these SPION with two cell lines (adhesive and suspended) was also investigated using an MTT (3-(4,5-dimethylthiazol-2-yl)-2,5- diphenyltetrazolium bromide) assay. The cell lines studied were primary mouse connective tissue cells (adhesive) and human leukemia cells (suspended). SPION were synthesized with a co-precipitation method under different stirring rates and NaOH molarities. The SPION demonstrated a range of magnetic saturations due to their... 

Asthma is a chronic inflammatory disease associated with a high prevalence of psychiatric disorders. There are specific brain networks responsible for emotional processes, including two important networks associated with psychiatric problems: the default mode network (DMN), which is more active in the resting state, and the salience network (SN), which is structurally connected to DMN. Although previous studies suggested that neuro-phenotypes of asthma may be recognizable by the neural activity of brain circuits, an association between the brain's functional alterations and psychiatric impairments induced by asthma remains unknown. We aimed to assess DMN and SN activity and its association... 

The performance of cellular solids in biomedical applications relies strongly on a detailed understanding of the effects of pore topology on mechanical properties. This study aims at characterizing the failure mechanism of scaffolds based on nodal connectivity (number of struts that meet in joints) and geometry of the pores. Plastic models of scaffolds having the same relative density but different cubic and trigonal unit cells were designed and then fabricated via three dimensional (3-D) printing. Unit cells were repeated in different arrangements in 3-D space. An in-situ imaging technique was utilized to study the progressive deformation of the scaffold models. Different nodal...