Sharif Digital Repository

Performance of polymer electrolyte membrane fuel cells (PEMFC) at high current densities is limited to transport reactants and products. Furthermore, large amounts of water are generated and may be condensed due to the low temperature of the PEMFC. Development of a two-phase flow model is necessary in order to predict water flooding and its effects on the PEMFC performance. In this paper, a multiphase mixture model (M2) is used, accurately, to model two-phase transport in porous media of a PEMFC. The cathode side, which includes channel, gas diffusion layer (GDL), microporous layer (MPL), and catalyst layer (CL), is considered as the computational domain. A multidomain approach has been used... 

One of the well known block cipher is Gost which was accepted as a Russian Standard, and was proposed to be included in ISO/IEC 18033-3. But, recent attacks on this cipher result in extracting more weaknesses of it and so the standardization of Gost was rejected. In 2015, the designers proposed a modified version of Gost, namely Gost2, which has a new key schedule and explicit choice for Sboxes. In this paper, we present an improved algorithm for fixed point attack on Gost2 with computational complexity of 2233. This computational complexity has a factor of 24 improvement in comparison to previous attack which has 2237 computational complexity. © 2017 IEEE  

Meet in the middle (MITM) attack is one of the most important and applicable methods for cryptanalysis of block ciphers. In this paper, a more generalized method for MITM attack is considered. For this purpose, a notion, namely cut-set, is utilized by which several numbers of MITM attacks can be performed. However, manual investigation on these cases is time-consuming and sometimes not error-free. Therefore, a new search algorithm is also provided to obtain proper attacks in a timely manner. For examination, this new search algorithm, which could make an automated attack along with some certain ideas, is applied on HIGHT, Piccolo-128, CRAFT and AES-128 block ciphers. The least time... 

In this work, a new approach is described for the calculation of the relaxation time and magnetic anisotropy energy of magnetic nanoparticles. Ferrofluids containing monodispersed magnetite nanoparticles were synthesized via hydrothermal method and then heated using the 10 kA/m external AC magnetic fields in three different frequencies: 10, 50 and 100 kHz. By measuring the temperature variations during the application of the magnetic field, the total magnetic time constant including both Brownian and Neel relaxation times can be calculated. By measuring the magnetic core size and hydrodynamic size of particles, the magnetic anisotropy can be calculated too. Synthesized ferrofluids were... 

In this study, some stabilized magnetite based ferrofluids were synthesized using Dextran as a stabilizing agent. In order to achieve optimum experimental conditions for synthesizing ferrofluids as MRI contrast agents, the Taguchi method was used. This approach was employed to design and minimize the number of required experiments. By using the Taguchi orthogonal (L16) array, four parameters including solution temperature and alkalinity, reaction temperature and stirring rate were selected at four predetermined levels for 16 experiments. Synthesizing processes established based on this set of experimental conditions were carried out and the obtained ferrofluids were characterized using PCS,... 

Single floor facility layout problem (FLP) is related to finding the arrangement of a given number of departments within a facility; while in multi-floor FLP, the departments should be imbedded in some floors inside the facility. The significant influence of layout design on the effectiveness of any organization has turned FLP into an important issue. This paper presents a three- (two-) stage mathematical programming method to find competitive solutions for multi- (single-) floor problems. At the first stage, the departments are assigned to the floors through a mixed integer programming model (the single floor version does not require this stage). At the second stage, a nonlinear programming... 

In this article, the flow behavior around a NACA 0012 airfoil which is oscillating with different Reynolds numbers and in various amplitudes has been investigated numerically. Numerical simulations have been performed with ANSYS software. First, the 2-D geometry has been studied in different Reynolds numbers and angles of attack with various numerical methods in its static condition. This analysis was to choose the best turbulent model and comparing the grids to have the optimum one for dynamic simulations. Because the analysis was to study the blades of wind turbines, the Reynolds numbers were not arbitrary. They were in the range of 9.71e5 to 22.65e5. The angle of attack was in the range... 

The present study was aimed at performance and energy analysis of a hybrid wind-hydrogen power system. Such system consists of wind turbines, batteries for the short time energy storage, electrolyzer, fuel cell and hydrogen tank for long time energy storage. The proposed configuration is used to supply energy demand of a region with discrete seasonal wind speed regime. Temporary wind energy profiles restrict using batteries for electricity storage as they lose much electrical stored energy for the long time. Based on direct wind turbine usage, batteries and hydrogen storage, different energy supply strategies are introduced and analyzed to power the household electricity demand. The energy... 

Ni-Co/SiC nanocomposite coatings were electrodeposited in a modified watt type of Ni-Co bath containing 20 nm SiC particles to be codeposited. Potentiodynamic polarization tests were conducted to study the effect of the SiC particulates on the electrodeposition of Ni and Co. Scanning electron microscopy was used to assess the morphology of the Ni-Co alloy and Ni-Co/SiC nanocomposite coatings. The distribution of the particulates in the matrix was considered by means of transmission electron microscopy. Applying nanomechanical testing instruments coupled to atomic force microscopy, mechanical properties of the alloy and composite coatings were studied and compared. The presence of 11 vol.%... 

The relative velocity between ground surface and receiver platform leads to clutter expansion in the range-doppler surface. The expanded clutters can mask slow moving targets. The passive radar is a low cost and light weight radar with high detection ability, due to its multistatic geometry. In this paper, we employ the MISO passive radar with at least four transmitters of opportunity to constitute an over-determined system of equations. We propose an algorithm to localize moving targets in the presence of clutters. In the first step, the clutters are separated from moving targets and in the second one, the position and velocity of the target are computed by solving the system of equations.... 

Metal forming processes are widely used in industrial productions, automobile bodies, food industries, oil refineries, and liquid and gas transmission systems. Analyzing these processes is very important to reduce wastes and optimize the processes. Study of some main factors such as physical and mechanical properties of material and its formability, die geometry, die material, lubrication and pressing speed has been the topic of many research projects. In this paper, forming limit diagrams (FLDs) for LC and ULC steels and the effect of different parameters like the work-hardening exponent, n, and the plastic strain ratio, r, on these diagrams have been evaluated and simulated using... 

In the present paper, a comprehensive study on the prediction of forming limit diagrams (FLDs) for an AA3003-O aluminium alloy is developed theoretically and experimentally. For obtaining the experimental FLDs, an out-of-plane formability test was performed based on the technique proposed by Ozturk and Lee [F. Ozturk, D. Lee, J. Mater. Process. Technol. 170 (2005) 247-253]. The classical Marciniak-Kuczynski (M-K) model and some new yield criteria are utilized to simulate the necking phenomenon and calculate the limit strains theoretically. The employed yield functions are: the BBC2000, BBC2002, and BBC2003 yield criteria proposed by Banabic et al. [D. Banabic, S.D. Comsa, T. Balan, in:... 

Cyclic voltammetry, j-t transients and transient time measurements have been used to characterize the Au-Ni alloy deposition and to obtain the nucleation and growth mechanism. The electrodeposition of gold, nickel and gold-nickel alloys was studied in cyanide-citrate electrolytes. The analysis of the voltammograms and j-t transients clearly shows that deposition of the Au and hence, of Au-Ni alloys, is affected by adsorbed cyanide ligands mainly at low overpotentials and a direct reduction occurs at high overpotentials. In this case, the Au deposition is kinetically favored, while the Ni deposition is controlled by the cyanide-citrate adsorbed adlayer. In an alloy electrolyte, the Au and Ni... 

Various parameters affect thermal conductivity of nanofluid; however, some of them are more influential such as temperature, size and type of nano particles and volumetric concentration. In this study, artificial neural network as well as least square support vector machine (LSSVM) are applied in order to predict thermal conductivity ratio of alumina/water nanofluid as a function of particle size, temperature and volumetric concentration. LSSVM, Self-Organizing Map and Levenberg-Marquardt Back Propagation algorithms are applied to predict thermal conductivity ratio. Obtained results indicated that these algorithms are appropriate tool for thermal conductivity ratio prediction. The... 

Realistic description of fractured reservoirs demands primarily for a comprehensive understanding of fracture networks and their geometry including various individual fracture parameters as well as network connectivities. Newly developed multiple-point geostatistical simulation methods like SIMPAT and FILTERSIM are able to model connectivity and complexity of fracture networks more effectively than traditional variogrambased methods. This approach is therefore adopted to be used in this paper. Among the multiple-point statistics algorithms, FILTERSIM has the priority of less computational effort than does SIMPAT by applying filters and modern dimensionality reduction techniques to the... 

In this work, magnetite nanoparticles (mean particle size about 20 nm) were synthesized via coprecipitation method. In order to investigate the kinetics of nanoparticle formation, variation in the amount of reactants within the process was measured using pH-meter and atomic absorption spectroscopy (AAS) instruments. Results show that nanoparticle formation behavior can be described by Avrami equations. Transmission electron microscopy (TEM) and X-ray diffraction (XRD) were performed to study the chemical and morphological characterization of nanoparticles. Some simplifying assumptions were employed for estimating the nucleation and growth rate of magnetite nanoparticles  

Leak detection based on acoustic emission needs robust time delay estimation in the presence of various types of noise. Considering several aspects of leakage signal and noise, the present work introduces wavelet based optimized residual complexity (WORC), in order to minimize the probability of false alarms in the presence of correlated, uncorrelated, and impulsive noise. The proposed method utilizes two approaches simultaneously. First, it optimizes residual complexity “for robustness against both correlated and uncorrelated noise”, then it employs wavelet basis in order to reduce the complexity of mixture of correlated and impulsive noise. In order to compare the resolution and percentage... 

Urban air quality is a major concern throughout the world. The concentration of human activities in a relatively small area puts enormous pressure on urban systems and has led to numerous environmental problems which have created major problems for urban air quality management. The complex nature of air pollution, especially with respect to health impacts in urban areas, has prompted attempts to define the so-called indicators that condense and simplify the available monitoring data to make them suitable for public reporting and decision makers. Several concepts and indicators exist to measure and rank urban areas in terms of their infrastructural, socioeconomic and environment-related... 

In this paper, the topic model is adopted to learn traffic phases from video sequence. Phase detection is applied to determine where a video clip is in the traffic light sequence. Each video clip is labeled by a certain traffic phase, based on which, videos are segmented clip by clip. Using topic models, without any prior knowledge of the traffic rules, activities are detected as distributions over quantized optical flow vectors. Then, traffic phases are discovered as clusters over activities according to the traffic signals. We employ the Fully Sparse Topic Model (FSTM) as the topic model. The results show that our method can successfully discover both activities and traffic phases which... 

Liquefaction is one of the most serious geotechnical hazards that may cause irreparable and financial damages. Based on in-situ testing results, several methods have been developed to evaluate the liquefaction potential. Due to continuous and repeated records, the piezocone (CPTu) has gained wide acceptance among other in-situ tests in geotechnical practice. In this paper, a new approach is proposed for liquefaction potential evaluation based on CPTu results. The new approach is verified by ten CPTu soundings (case histories) derived from different sites. By analyzing the liquefied sites, an area on the soil classification charts was introduced in this paper, showing the soil types which are...