Sharif Digital Repository

Many factors cause degeneration of the measured signal in magnetic flux leakage (MFL). One of these factors is non-uniformity of the magnetic flux inside the sample under test. Uniformity of the magnetic flux results in smoother and more predictable signals from various discontinuity types and also a reduction in the eddy current distortions induced because of the movement of the magnetizing yoke. The goal of this research is to find a near-optimal length for the magnetizing yoke. Toward this end, yokes with different lengths are simulated with the finite element method and the quality of the distribution of the flux lines inside the sample is studied. In this way a criterion for... 

Several target position finding methods are proposed in various papers mainly regarding sensor networks. However, the problem of position finding in passive radar systems is somewhat different from the general case of sensor networks. Generally, in a passive radar system, there are few receivers located at short distances when compared with the it distance from the target. In this case, a problem known as geometric dilution of precision (GDP) occurs, which considerably increases the error of many proposed methods. This phenomenon can even make the position finding equations non-solvable. Regarding this fact, we have developed a least mean square error (LMSE) based method, which uses both the... 

Due to the importance of environmental protection and the need to use new energy and alternative to conventional fuels, renewable energy has received much attention. Due to this necessity, a discontinuous bioreactor producing methane gas from animal waste has been modeled and simulated in this research. Monod kinetics was used to express the relationship between the growth rate of microorganisms and substrate concentration. The fourth-order Rong Kuta numerical method solved the substrate consumption and production of microorganisms and methane gas. The effect of the initial concentration of microorganisms on methane production has also been investigated. The initial concentrations of... 

This paper is aimed at investigating some computational aspects of different isoperimetric problems on weighted trees. In this regard, we consider different connectivity parameters called minimum normalized cuts/isoperimetric numbers defined through taking the minimum of the maximum or the mean of the normalized outgoing flows from a set of subdomains of vertices, where these subdomains constitute a partition/subpartition. We show that the decision problem for the case of taking k-partitions and the maximum (called the max normalized cut problem NCP^M), and the other two decision problems for the mean version (referred to as IPP^m and NCP^m) are NP-complete problems for weighted trees. On... 

Small diameter vascular grafts (<6 mm) are highly demanded for patients suffering from severe occluded arteries to be used as a bypass or substituted conduit. Fabricating a graft with appropriate structural, mechanical and cell growth properties which has simultaneously anti-thrombogenic trait is a challenge nowadays. Here, we proposed a bilayer heparinized vascular graft that can mimic the structural and mechanical characteristics close to those of the native coronary artery by combining electrospinning and freeze drying methods. In this study, the inner layer was made by co-electrospinning of synthetic polymer, poly-caprolactone (PCL) and the natural polymer, gelatin (Gel). Also, heparin... 

Small diameter vascular grafts (<6 mm) are highly demanded for patients suffering from severe occluded arteries to be used as a bypass or substituted conduit. Fabricating a graft with appropriate structural, mechanical and cell growth properties which has simultaneously anti-thrombogenic trait is a challenge nowadays. Here, we proposed a bilayer heparinized vascular graft that can mimic the structural and mechanical characteristics close to those of the native coronary artery by combining electrospinning and freeze drying methods. In this study, the inner layer was made by co-electrospinning of synthetic polymer, poly-caprolactone (PCL) and the natural polymer, gelatin (Gel). Also, heparin... 

In many radar and EW applications, after some processing, the envelope of the received pulse is calculated and is used for target detection. Most of the time only the effect of thermal noise is considered in such detectors, but in most of the practical applications, before envelope detection, the signal should be down converted from radio frequency (RF) to an intermediate frequency (IF). In this case, the phase noise of the local oscillator will affect the signal, too. Surprisingly, in contrast to thermal noise, the effect of phase noise on the envelope of signal becomes more considerable whenever the IF bandwidth is decreased. So there exists an optimum IF bandwidth for the receiver. In... 

Improvement of energy efficiency in Software Defined Networking (SDN) is a challenging problem. In order to achieve higher energy efficiency, the routing and scheduling in software defined networks should be planned robustly based on the topological specifications of the SDN. The topological specifications of the SDN depends on the size of the network graph which changes rapidly as the virtualized network devices in the network turned on and off by the controller. Therefore, modelling the SDN graph is an important issue in designing a reliability aware energy efficient SDN. In this paper we propose a new algorithm for reliable energy efficient traffic engineering in SDN. In the proposed... 

In recent years, there has been a great tendency to optimize energy consumption in the oil and gas industry’s upstream and downstream equipment. One of the most energy-intensive processes in natural gas refineries is the condensate stabilization unit (gas condensate). The main bottlenecks of energy consumption in the old units are condensated stabilization (dehumidification with ethylene glycol), heater reboiler, and air coolers (air coolers). Therefore, much attention should be paid to these applications and electricity and steam consumption in this unit. In this study, a simulated model based on the Gachsaran gas refinery’s new layout has been developed. Optimization of this part of the... 

In this paper the problem of target detection using coherent pulse terrain in auto regressive (AR) clutter has been considered for MIMO radars. We have formulated this problem as a hypothesis test. Using the generalized likelihood ratio test (GLRT) an adaptive decision scheme has been developed for clutter with known order but unknown parameters. The performance of the proposed detectors have been evaluated using Monte-Carlo simulations. The results show the superiority of the MIMO radars with temporal coherent processing over the conventional phased arrays due to both angular spread and a newly presented phenomenon which is called doppler spread in this paper. © 2006 IEEE  

In this paper1 we consider higher isoperimetric numbers of a (finite directed) graph. In this regard we focus on the nth mean isoperimetric constant of a directed graph as the minimum of the mean outgoing normalized flows from a given set of n disjoint subsets of the vertex set of the graph. We show that the second mean isoperimetric constant in this general setting, coincides with (the mean version of) the classical Cheeger constant of the graph, while for the rest of the spectrum we show that there is a fundamental difference between the nth isoperimetric constant and the number obtained by taking the minimum over all n-partitions. In this direction, we show that our definition is the... 

Let LL be a reversible Markovian generator on a finite set View the MathML sourceV. Relations between the spectral decomposition of LL and subpartitions of the state space View the MathML sourceV into a given number of components which are optimal with respect to min–max or max–min Dirichlet connectivity criteria are investigated. Links are made with higher-order Cheeger inequalities and with a generic characterization of subpartitions given by the nodal domains of an eigenfunction. These considerations are applied to generators whose positive rates are supported by the edges of a discrete cycle ZNZN, to obtain a full description of their spectra and of the shapes of their eigenfunctions, as... 

Non-destructive identification of voids in ferromagnetic materials is of great importance for industrial applications. Magnetic flux leakage technique is used here to examine the defected structure. To this end, an inverse problem should be solved in order to infer the location and depth of internal flaws from the measured leaked magnetic signals. Currently generalized inverse method and singular value decomposition are used for solving such inverse problem. Considering the cracks separation has significant effect on the absolute value of magnetic flux leakage signals, we study different distributions of cracks. In this paper, the magnetic dipole model is proposed to reconstruct the extent...