Sharif Digital Repository

In this investigation we have taken the viewpoint of the Landau theory for the normal state and have suggested a covariant equation for current density. This suggestion is based on the exact analysis of electrodynamic relations in the intermediate state of superconductors. Our analysis is restricted to the simplest electrodynamic relations. At first we have employed Maxwell's equations with the general Ohm-London equation and have found the general covariant current which takes a covariant form after Lorentz transformation. Finally we have checked it by considering two inertial frames in a simple state  

We use the Dissipative Particle Dynamics (DPD) method to simulate the variation in the configuration of a polymer chain, whose two ends are fixed in a nanochannel, subject to bulk motion of fluid particles. We aim to find suitable relations between the polymer chain curvature and the mean flow velocity magnitudes. In this regard, we introduce some important parameters, which can effectively affect the polymer chain curvature and we examine their magnitudes on the amount of polymer chain deflection. In fact, our main objective is to quantify the relation between some major flow parameters, possibly with different polymer chain properties. For example, we show that different flow velocity... 

The distributed nature and complexity of computer networks and various services provided via them, makes the networks vulnerable to numerous attacks. The TCP/IP presumptions which are based on using this protocol to provide a simple, open communication infrastructure in an academic and collaborative environment, causes this protocol lack of built-in mechanisms for authentication, integrity and privacy. Even though in the last few years a more systematic approach to TCP/IP network security problem has been followed, a formal approach to this problem is lacking. In this paper, we propose using Description Logics as a formal model which could be used to analyze TCP/IP networks against attacks.... 

Modeling and analysis of information system vulnerabilities helps us to predict possible attacks to networks using the network configuration and vulnerabilities information. As a fact, exploiting most of vulnerabilities result in access rights alteration. In this paper, we propose a new vulnerability analysis method based on the Take-Grant protection model. We extend the initial Take-Grant model to address the notion of vulnerabilities and introduce the vulnerabilities rewriting rules to specify how the protection state of the system can be changed by exploiting vulnerabilities. Our analysis is based on a bounded polynomial algorithm, which generates the closure of the Take-Grant graph... 

In this paper, we present our results about simulation of 2D-EOF in Nano/Micro scales porous media using lattice Boltzmann method (LBM) in micro-channel for EOF. The high efficient numerical code use strongly high nonlinear Poisson Boltzmann equation to predicate behavior of EOF in complex geometry. The results are developed with precisely investigation of several effective parameters on permeability of EOF, such as geometry (channel height and number and location of charge), external electric field, thickness of Debye length (ionic concentration), and zeta potential. Our results are in excellent agreement with available analytical results. Our results show that for certain external electric... 

In this investigation, we have studied the kinetics and mechanism of desorption of CO from the Cu(110) surface using a new Monte Carlo simulation and putting emphasis on high order lateral interaction. According to our simulated TPD spectra, for β 0 = 10 K/s the maximum desorption rate occurs at Tm = 218.6 K. Furthermore, analysis of simulated TPD spectra of CO desorption shows that it is strongly lateral-interactive and results an activation energy of CO desorption from Cu(110) that is Ed = 66.6 Kj/mol. These simulated results are compared with other reported results and show excellent agreement. After that we have investigated the kinetics and mechanism of desorption of CO2 from the... 

Mechanical behaviour of sandwich panels under different conditions have been exprimentally studied in this research to increase the knowledge of aircraft sandwich panel structures and facilitate design criteria for aircraft structures. Tests were concentrated on the honeycomb sandwich structures under different loads including flexural, insert shear, flat wise tension and compression loads. Furthermore, effect of core density and face material on mechanical behavior of different samples were investigated and compared with analytical and FEM method. Effects of skin thickness on strength of honycomb sandwhich panels under shear pull out and moments have also been considerd in this study.... 

The present study was undertaken to investigate a kinetic model for cell growth evaluation and biopolymer production by Xanthomonas campestris in a laboratory-scale batch fermentor. The optimum conditions selected for biopolymer production were 30 °C for media temperature, 500 rpm for agitation rate, 65 g/l for sugarcane concentration and 1.5 vvm for air flow rate. A detailed, unstructured, mathematical kinetic model is presented here for batch production of xanthan biopolymer from X. campestris. Set of ordinary differential equations were developed as logistic model for X. campestris growth and logistic incorporated Luedeking–Piret model for xanthan production. These models would offer more... 

The current study is an attempt to contribute for efficient and cost-effective substrates for xanthan gum production. In this context, the sugar cane molasses wastes can be used as a cheap substrate for xanthan gum production. Xanthan biopolymer production by a novel Xanthomonas campestris strain IBRC-M 10644 was optimized with statistical approaches. Based on the results of Response Surface Methodology (RSM) with Central Composite Design (CCD) technique, a second-order polynomial model was developed and evaluated the effects of variables on the maximum xanthan production. Agitation rate (X1: 200-500 rpm), sugar cane molasses concentration (X2: 30-90 g/L) and operation temperature (X3: 25-35... 

Effects of the amount and mode of input energy to an active multi-stage solar still were investigated in this work. To control the input energy, an electrical heater controlled by a PLC was utilized to simulate the energy absorbed by solar collectors. The study of the amount of input energy indicated that the freshwater production was a quadratic function of the collector over basin area (CBA) ratio. It was also found that stages 1 to 4 produced about 36%, 26%, 20% and 18% of the overall yield, respectively. Moreover, the effect of employing a thermal energy storage (TES) on the system performance was studied by comparing the mode of feeding energy according to the daily solar radiation... 

Our purpose is to model the multilayer laser solid freeform fabrication (LSFF) process for the material properties of low carbon steel 1015 for workpiece and cobalt alloy stellite 21 with 1.5 wt.% nano CeO2 as the powder particles. In this paper, transient heat transfer and mass transfer equations in laser solid freeform fabrication process are solved by Finite Element Method (FEM). In this approach, the geometry of the deposited material, temperature and thermal stress fields across the process area are predicted. For each layer the clad height is computed and computations are done by the use of MATLAB and COMSOL software  

A crude phenolic glycolipid extract from Mycobacterium bovis BCG was fractionated by column chromatography. A reversed-phase high performance liquid chromatography (HPLC) method with UV detection at 275nm was developed for simultaneous detection and separation of phenolic glycolipids (PGLs) in Mycobacterium bovis BCG. This analysis provides a good resolution. Different solvent systems and columns for HPLC were compared. A system composed of acetonitrile-water in the ratio of 0→80% at a flow rate of 0.8 mL/min and C8 analytical column were found to be optimum for HPLC of the phenolic glycolipids. This simple method is therefore appropriate to purify these compounds present in M. bovis extract... 

In this investigation heat treatment optimization of 17-4PH stainless steel has been carried out by a genetic algorithm. The optimum technique of heat treatment, adaptive to 17-4PH stainless steel, was obtained from the initial data set by the use of genetic algorithms based on modeling with artificial neural network. The results strongly indicate that the presented model has the great ability for heat treatment optimization of 17-4PH stainless steel to yield the highest strength levels in different working temperatures. © 2006 Elsevier Ltd. All rights reserved  

Different techniques including toluene-ethanol, guanidine hydrochloride, guanidine hydrochloride-EDTA, lysozyme, lysozyme-EDTA and sonication treatments were used to extract penicillin G acylase from Escherichia coli ATCC 11105 cells. The obtained results show that penicillin G acylase extraction by guanidine-EDTA treatment is more specific in comparison to the other applied methods. The maximum specific activity of the enzyme (5.61 U/mg), i.e. the maximum purification was found when 1 M guanidine + 100 mM EDTA solution was used for penicillin G acylase extraction at culture condition including 0.50% (w/v) yeast extract as carbon source. In such a condition more than 95% of the enzyme was... 

The kinetics of the enzymic reaction of penicillin G acylase from a mutant of Escherichia coli ATCC 11105 in forward and reverse directions were studied and the kinetic constants determined. Results show that the enzyme is inhibited by excess substrate, penicillin G (Pen G), and by both products. The non-competitive inhibition by 6-aminopenicillanic acid (6-APA) and competitive inhibition by phenylacetic acid were observed for the ordered uni bi deacylation reaction in the forward direction. The optimum pH value for the reverse acylation reaction was 5.7. The bi uni mechanism for the reverse reaction was investigated and the inhibitory effects of the substrates, 6-APA and phenyl acetic acid,...