Sharif Digital Repository

With the advent of various advanced materials, the idea of flying like birds has attracted considerable attention in recent years. In addition, aeroacoustics has become an important issue and is being widely studied. In this work, based on the shape of Owls’ wings, an attempt was made to improve the aeroacoustic and aerodynamic performances of conventional aircraft wings. For this purpose, wings with different elements, namely, square, triangular, and semicircular, on their top surface were examined. In addition, three different spatial distributions of the elements according to the Owl’s wings shape were considered. For incompressible airflow, aerodynamic and aeroacoustic parameters of wing... 

Holding yokes made of investment cast 17-4PH stainless steel were too soft (below 20. HRC). Optical microscopy of the parts showed that there is a high fraction of retained austenite after oil quench due to high amount of molybdenum found in parts as an impurity. Subzero treatment in liquid nitrogen after solution heat treating was used to convert retained austenite to martensite but parts became too brittle and fractured during post-heat treating machining. Double and triple aging at 538 °C (1000 °F) after subzero cooling was done on parts. Detailed fractography and tensile testing along with optical microscopy (OM) investigation proved that repeated aging at 538 °C (1000 °F) led to lower... 

Heavy-duty gas turbines are usually devised in power plants to generate electrical energy. Sudden failure in any of its parts or subdivisions will result in a decrement of the efficiency of the system or emergency shutdown of the system. The highest risk of failure in these turbines is subjected to the hot gas path (HGP) of the turbine. Due to the existence of uncertainty in diagnosing process or damage growth, in this research, a modified risk-based probabilistic failure analysis model using Bayesian networks (BN) was developed. First, a failure model was developed using the Fault Tree Analysis, and then it is transformed into a BN model. This model is capable of predicting and diagnosing... 

First stage nozzles are considered as one of the most critical components in the heavy-duty gas turbines because of their operational in-service condition in the hot gas path. In the present research, the failure analysis of a first stage nozzle in a 159 MW gas turbine is investigated in order to specify the modes and mechanisms of the failure and their root causes. Various examinations including macroscopic and visual inspection of the nozzle outer surface, the micro examination of the fracture surface, chemical analysis and metallographic analysis were carried out to complete the investigation. The micro and macro examinations revealed cracks, erosion and oxidation damages in the nozzle... 

The objective of this study was to investigate the synergistic effect of TBP on the system D2EHPA/kerosense/M2+ with respect to the extraction of some transitional metals. Although the presence of M2EHPA as an impurity in D2EHPA shifts the extraction curves to lower pH, empirical results show that the extraction curves shift to the right due to the addition of TBP and the counterbalance effect of M2EHPA. The results indicate that the equivalent concentration of D2EHPA decreases as a function of concentration of TBP because of the formation of hydrogen bonds between TBP and D2EHPA. Based on this finding, the D2EHPA equivalent concentration was determined by using a modifying factor for TBP... 

Four 5-, 10-, 20- and 30-story moment frames, representing low-, mid-, and two high-rise structures, were subjected to a great number of idealized directivity pulses. The amplitudes and periods of pulses vary from 0.02 g to 1.0 g and 0.5 to 12 sec, respectively. Over 1400 nonlinear dynamic analyses of low- to high-rise moment frames were performed which were feasible through using modified fish-bone model. The distribution of interstory drift along the height was studied and two applied contours were proposed: (i) the maximum interstory drift contour, and (ii) the critical story contour. These contours were demonstrated versus the ratio of natural period of the structure to the pulse period... 

In this article singular points of a parallel manipulator are obtained based on a novel geometrical method. Here we introduce the constrained plain method (CPM) and some of its application in parallel mechanism. Given the definition of constraint plane (CP) and infinite constraint plane (ICP) the dependency conditions of constraints is achieved with the use of a new theorem based on the Ceva geometrical theorem. The direction of angular velocity of a body is achieved by having three ICPs with the use of another theorem. Finally, with the use of the above two novel theorems singularities of the 3UPF_PU mechanism are obtained. It should be emphasized that this method is completely geometrical,... 

An analytical model is proposed to study the effect of particle size on melting enthalpy and entropy of metallic nanoparticles (NPs). The Mott's and Regel's equations for melting entropy in the combination of core average coordination number (CAC) and surface average coordination number (SAC) of freestanding NPs are considered. Clusters of icosahedral (IC), body centered cubic (BCC), and body centered tetragonal (BCT) structure without any vacancies and defects are modeled. Using the variable coordination number made this model to be in good agreement with experimental and molecular dynamic (MD) results of different crystal structures. The model predicts melting entropy and enthalpy of... 

Grinding is widely used for manufacturing of components that require fine surface finish and good dimensional accuracy. In this study a thermo-mechanical finite element analysis is conducted to find out how grinding parameters can affect temperature and residual stress distribution in the workpiece. Results of parametric study presented in this work indicate, by carefully selecting the grinding parameters, minimum thermal and mechanical damage can be achieved. Higher workpiece velocities produce higher surface residual stress. By increasing depths of cut, depth of tensile residual stresses increases. Convection heat coefficient does not have any considerable effect on surface residual stress... 

Adaptive Noise Cancellation (ANC) is a well-known technique for background noise reduction in automobile and vehicular environments. The noise fields in automobile and other vehicle interior obey the diffuse noise field model closely. On the other hand, the ANC does not provide sufficient noise reduction in the diffuse noise fields. In this paper, a new multistage post-filter is designed for ANC as a solution to diffuse noise conditions. The designed post-filter is a single channel Linear Prediction (LP) based speech enhancement system. The LP is performed by an adaptive lattice filter and attempts to extract speech components by using intermediate ANC signals. The post-filter has no... 

Shot peening is widely used to improve the fretting fatigue strength of critical surfaces. Fretting fatigue occurs in contacting parts that are subjected to fluctuating loads and sliding movements at the same time. This paper presents a sequential finite element simulation to investigate the shot peening effects on normal stress, shear stress, bulk stress and slip amplitude, which are considered to be the controlling parameters of fretting damage. The results demonstrated that among the modifications related to shot peening, compressive residual stress has a dominant effect on the fretting parameters  

One-dimensional (1D) MgO structures were successfully synthesized via carbothermic reduction of mechanically activated mixture of MgO and graphite. Mechanical activation of source materials before carbothermic reduction can substantially enhance the formation of MgO products at a temperature (1000 °C) relatively lower than that required in previous approaches (≥1200 °C). However, the morphology of MgO formed is dependent on the degree of mechanical activation and the condition of the subsequent carbothermic reduction. Two distinctive morphologies were found for MgO products synthesized using our method: single crystalline nanorods with rectangular cross-sections whose diameters range from 50... 

Attitude and vibration control of a general form of flexible satellites is addressed in this paper. Partial differential dynamic equations are derived considering new details such as multi sectioned solar panels and elastic connections between main hub and solar panels. Boundary control approach is adopted to eliminate simplification errors of discrete models, using just one actuator in the hub. Asymptotic stability of attitude dynamics is proved for a group of boundary controllers and necessary conditions for asymptotic stability of vibrations are discussed. Being independent of modeling accuracy and using easily measurable feedbacks are among advantages of the proposed class of... 

Characterization and calcination behavior of a low-grade manganese ore, as a part of Mn ferroalloys production, was studied by XRF, ex-situ XRD, in-situ XRD, and SEM-EDS techniques. Calcination experiments were carried out at and up to 900 °C (1173 K) in air and argon atmospheres. The samples were in particles and powder forms. The results indicated that both quartz and calcite phases in the ore exhibit a bimodal spatial distribution; as relatively large regions and finely distributed in the Mn- and Fe-containing phases. By Rietveld analysis of the in-situ XRD data, the reactions occurring upon heating during the calcination process were deduced. Thermal decomposition and reactive diffusion... 

Estimating the position of a mobile set is of great importance in new mobile services. However, in most cases, the accuracy should be less than 100 meters. This accuracy is hard to reach especially in urban areas. The main problem is that there are a lot of obstacles like buildings between the BTS and the mobile set. Thus the time measured between BTS and the mobile set is somehow greater than the time it takes the wave to travel directly between two points. This paper introduces an optimized solution for TDOA as one of the most efficient ways for finding the location of a mobile phone. Considering the standards and limitations of both GSM and UMTS, the Authors present a solution for... 

Weighting elements to achieve radiation patterns with desired characteristics is a classical work in phased array antennas. These characteristics can be low sidelobe level, narrow beamwidth, high directivity, pattern nulling in special angle and etc. For each of these characteristics, different methods have been introduced. Most of methods have been presented for uniform arrays, however there are lots of methods to obtain a desired pattern for antennas with non-uniform element distances. The problem with these methods is complexity or not very good results. In this paper, fast and easy methods based on Least Square Error that leads to good results are presented. In addition, weighting of... 

The elastic behavior of an edge dislocation, which is positioned outside of a nanoscale elliptical inhomogeneity, is studied within the interface elasticity approach incorporating the elastic moduli and surface tension of the interface. The complex potential function method is used. The dislocation stress field and the image force acting on the dislocation are found and analyzed in detail. The difference between the solutions obtained within the classical-elasticity and interface-elasticity approaches is discussed. It is shown that for the stress field, this difference can be significant in those points of the inhomogeneity-matrix interface, where the radius of curvature is smaller and which... 

In this paper, optimal control strategy is applied to parallel three-phase inverters using SVPWM technique. The three-phase system is transformed to a synchronous-stationary frame, suitable cost functions are defined, and the switching schemes are determined for the inverters. The control strategy minimizes the circulating current between the inverters, therefore, proper load sharing scheme is achieved. Simulations are performed for steady state and load change operation, and the results which show the suitability of the controller are provided. ©2009 IEEE  

A number of human diseases, including Alzheimer's and Parkinson's have been linked to amyloid formation. To search for an anti-amyloidogenic product, alkaloid enriched extract from borage leaves was examined for anti-amyloidogenic activity using Hen Egg White Lysozyme (HEWL) as a model protein. After isolation of the plant extract using rHPLC, only one fraction indicated a significant bioactivity. TEM analysis confirmed a remarkable reduction of amyloid fibrils in the presence of the bioactive fraction. To identify the effective substance in the fraction, mass spectrometry, FTIR, and NMR were performed. Our analyses determined that the bioactive compound as... 

In this work, after formulating the weakly nonlocal micromorphic equations of motion for non-Bravais crystals with general anisotropy, specialization to diamond structures is made. A critical dilemma is the determination of the elastic moduli tensor appearing in the equations of motion. From the equivalency of these equations with the pertinent equations obtained in the context of lattice dynamics, the expressions of the components of the elastic moduli tensors in terms of the atomic force constants are derived analytically. Subsequently, the atomic force constants are calculated via ab initio density functional perturbation theory (DFPT) with high precision. As a benchmark for the accuracy...