Sharif Digital Repository

In this research, copper matrix composites reinforced with different amounts of titanium carbide particles were produced by mechanical milling and in-situ formation of reinforcements. Morphology and size of milled powders were inspected by scanning electron microscopy (SEM) several times during milling process. Changes in lattice parameter, crystallite size, lattice strain, dislocation density and Gibbs free energy changes (due to increasing in dislocation densities and grain boundaries) in different samples (with different TiC particles contents) were studied by X-Ray Diffraction technique with Cu-kα radiation and using Nelson-Riley method and Williamson-Hall equation. Microstructure of... 

Shell and tube heat exchanger with single twisted tube bundle in five different twist angles, are studied using computational fluid dynamics (CFD) and compared to the conventional shell and tube heat exchanger with single segmental baffles. Effect of shell-side nozzles configurations on heat exchanger performance is studied as well. Heat transfer rate and pressure drop are the main issues investigated in the paper. The results show that, for the same shell-side flow rate, the heat transfer coefficient of heat exchanger with twisted tube bundle is lower than that of the heat exchanger with segmental baffles while shell-side pressure drop of the former is even much lower than that of the... 

An experimental investigation is conducted here to study the effects of applying frictional sliding fuses (FSF) in concrete infilled steel frames. Firstly, the influences of some parameters on the behavior of the sliding fuse are studied: Methods of adjusting the FSF for a certain sliding strength are explained and influences of time duration, welding and corrosion are investigated as well. Based on the results, time duration does not significantly affect the FSF, however influences of welding and corrosion of the constitutive plates are substantial. Then, the results of testing two 1/3 scale single-storey single-bay concrete infilled steel frames having FSF are presented. The specimens were... 

This article presents a new method for determination of meshing stiffness of the gear teeth using three dimensional analysis of tooth stiffness. The teeth are treated as explicit capacitive fields and their compliance matrix is determined by considering virtual interconnected springs located at defined nodes on tooth profile. The stiffness of any arbitrary point on the profile can then be determined as a linear function of neighboring nodes stiffness. The results of this analysis help in determination of load distribution for a pair of meshing gears under real operation conditions where deflections and misalignments are inevitable. Copyright © 2005 by ASME  

Because of corrosion resistance and antibacterial effects, shape memory Ni-Ti-Ag alloy can be considered for different biomedical applications. Mechanical alloying is used to produce nanostructured Ni-Ti-Ag alloy from elemental powders. X-ray diffraction (XRD) and field emission scanning electron microscopy (FESEM) are used to characterize the product. Results show that after 1h milling, homogenous distribution of the elements occurs; while no intermetallic compounds is observed. After 3h milling, titanium dissolves in nickel to form amorphous and nanostructured solid phases. Peaks of B2 phase appear in the XRD pattern after the 3h milling of the powder mixture. Sintering of the 3h-milled... 

Let T be a partial Latin square and L a Latin square such that T ⊆ L.Then T is called a Latin trade, if there exists a partial Latin square T * such that T* ∩ T = ∅ and (LT) ∪ T* is a Latin square.We call T* a disjoint mate of T and the pair (T,T*) is called a Latin itrade.A Latin bitrade where empty rows and columns are ignored, is called a Κ-homogeneous Latin bitrade, if in each row and each column it contains exactly κ elements, and each element appears exactly k times.The number of filled cells in a Latin trade is referred to as its volume.Following the earlier work on κ-homogeneous Latin bitrades by Cavenagh, Donovan, and Drápal (2003 and 2004) Bean, Bidkhori, Khosravi, and... 

Monitoring embankment dams is of crucial importance. In earth dams, the pore pressures, earth pressures, and displacements occurring during construction and function are measured at the time of the first impounding and exploitation by installing essential instruments, and so the dam's performance is evaluated and analyzed. Scope of the present research is the evaluation of Siah-Sang Dam performance through the results of instruments and back analysis, conducted by FLAC software. The Mohr-Coulomb elasticplastic model has been considered as the behavioral model of the dam, and the effect of the upstream shell's materials deformation has been modeled at the time of the initial impounding.... 

A nonlinear robust adaptive sliding mode admittance controller is proposed for exoskeleton rehabilitation robots. The proposed controller has robustness against uncertainties of dynamic parameters using an adaptation law. Furthermore, an adaptive Sliding Mode Control (SMC) scheme is employed in the control law to provide robustness against disturbances (non-parametric uncertainties) with unknown bounds. For this purpose, another adaptation law is defined for the variation of the SMC gain. The proposed scheme is augmented with an admittance control method to provide the patient with compliance during interaction with the rehabilitation robot. The stability of the proposed controller and the... 

This paper investigates resonance phenomenon in active filters. The paper studies the effect of system/filter parameters on resonance conditions. It also proposes an active technique for damping such resonances. The study is based on modal analysis in which unstable modes or modes with slow damping are first extracted. Then, using a new technique, such modes are considerably damped. PSCAD/EMTDC software is also employed to verify the performance of the proposed technique. © 2008 ACECR  

High strength TRIP-aided steel sheets with high formability and better ductility are of industrial interest. Texture control and retained austenite characterization are considered as the main factors with respect to the formability and ductility. Processing parameter such as initial conditions of hot rolling, cold rolling to final size of sheet, intercritical annealing after cold rolling and subsequent isothermal heat treatment to achieve TRIP specification in product, affect bulk texture of sheets. Moreover, chemical composition of steel should be noted especially because it can change the intensity of orientation and final macro texture of sheet. In this work, the effect of cold rolling... 

Optimization of high strength and high formability of multiphase cold rolled sheet TRIP-aided steels is based on the composition and the austempering conditions. The effect of intercritical annealing temperature on the volume fraction and carbon concentration of the retained austenite was investigated in two different TRIP-aided steels. Experimental results show that the optimum annealing temperatures are 860 °C for Al-containing and 810 °C for Si-containing TRIP steels. It was demonstrated that the measurement of retained austenite can be successfully performed for textured TRIP steels by XRD. © 2006 Elsevier Inc. All rights reserved  

This paper presents the fuzzy logic computing for the design and implementation of fuel controller for gas turbine engines. For this purpose, a fuzz controller is designed in order to be implemented on an electronic control unit where it is used to drive a servo operated fuel control valve. The fuzzy logic computing approach for different parts of the controller including fuzzification, rule base, inference engine and defuzzification are then described. Finally, computer simulation of the fuzzy controllers integrated with the engine model is performed to investigate the effectiveness of the proposed fuzzy controller on the performance of a turbojet engine. The results are provided to show... 

A μ-way Latin trade of volume s is a collection of μ partial Latin squares T1, T2,⋯, Tμ, containing exactly the same s filled cells, such that, if cell (i,j) is filled, it contains a different entry in each of the μ partial Latin squares, and such that row i in each of the μ partial Latin squares contains, set-wise, the same symbols, and column j likewise. It is called a μ-wayk-homogeneous Latin trade if, in each row and each column, Tr, for 1≤r≤μ, contains exactly k elements, and each element appears in Tr exactly k times. It is also denoted as a (μ,k,m) Latin trade, where m is the size of the partial Latin squares. We introduce some general constructions for μ-way k-homogeneous Latin... 

Platinum content of two commercial spent reforming catalysts were extracted in aqua-regia solutions under atmospheric pressure and at temperatures up to 100 °C. Three factors, including presence of coke, catalyst particle size, and impeller agitation speed were first tested to study the relative importance of mass-transfer resistances during Pt extraction reaction. Catalyst particle sizes < 100 μm and agitation speeds > 700 rpm eliminated the internal and external mass-transfer resistances, respectively. The effect of other factors, including HNO3-to-HCl volume ratio, liquid-to-solid mass ratio, and the reaction temperature on the extraction rate of platinum were then examined. Pt extraction... 

In this paper, performance of power system stabilizer (PSS) of a large thermal power plant, as tuned by the manufacturer, is investigated. Then, the stabilizer is redesigned based on extended phase compensation of the exciter input - electrical torque transfer function and root locus analysis; and its effect on local, interplant and inter-area electromechanical oscillations is thoroughly analyzed. As Bisotoun PSS has a special structure with two inputs, frequency and power, its performance is compared with power-speed PSS. The effect of PSS with existing and new tunings on transient stability is also studied. Simulation results are verified by experimental test records. © 2007 JD  

Ab initio quantum chemical calculations at the density functional theory (DFT) level were performed on eight pyridine derivative molecules as corrosion inhibitors for iron in an acidic solution. In this regard, the geometry of the molecules were optimized using the B3LYP/6-31G method first, and then interactions of these optimized structures with the iron atom were explored using the B3LYP/LANL1MB method. Two modes of adsorption were considered, i.e., planar adsorption (P) via the pyridine ring and vertical adsorption (V) through a nitrogen atom. The interaction energy was minimized through the variation of the inhibitor molecule-iron atom distance. These minimum energy values, along with... 

The interaction energies of 3,5 di-methyl pyridine (A) and 2,4 di-methyl pyridine (B) on the (100) surface of body-centered cubic (bcc) iron were determined via a cluster model and ab initio quantum chemical calculations at density functional theory level. The obtained energies were used to compare the inhibition behavior of these molecules for iron corrosion in hydrochloric acid solution. The iron surface and its adsorption sites (on-top, bridge, hollow) were considered as some clusters taken from two-layered 〈100〉 planes, i.e. Fe1(1), Fe4(2,2), and Fe5(4,1). So, the process for which quantum chemical calculations was carried out consists of adsorption of molecule A (B) on these clusters.... 

Two-dimensional (2D) and three-dimensional (3D) numerical simulations of an external compression supersonic ramped inlet are presented for a free stream Mach number of 2. A comparison made between numerical results and experimental data showed that multi-block structured gird using standard k - " turbulence model gives acceptable results. The shape of present inlet diffuser was transformed gradually into a circular one to encompass the Aerodynamic Interface Plane (AIP). It was observed that the 3D simulation predicted a more accurate static pressure distribution during the length of supersonic inlet and total pressure distribution at the AIP in comparison with the 2D one. Further, a better...