نشریه مهندسی مکانیک مدرس
سال سیزدهم شماره 12 (اسفند 1392)

The preliminary design of multistage axial compressor requires a vast number of design variables and limitations, the simultaneous investigation of which needs a complicated design program. This study provides a new algorithm by the use of combining multi-variable/multi-objective optimizations, meanline simulator and preliminary design program. On the one hand the two-layer optimization is used to consider all design variables and on the other the design limitations are put into the cost function together with surge margin and efficiency. The comparison of the results with the NASA transonic eight-stage compressor shows the ability of the new algorithm to achieve a compressor with better performance and lower weight.

Functionally graded materials have been taken into consideration by many researchers in the last two decades. Gradual changes of mechanical properties in FGMs decrease stress concentration, crack initiation and propagation and delamination. Many of the present and potential applications of FGM contain contact loading.This kind of loading causes surface crack initiation which is followed by subcritical crack propagation.Thus, propagation of surface cracks is one of the most important failure mechanisms in FG structures. In this article two dimensional sliding contact of a rigid flat punch on a homogeneous substrate with an FGM coating is studied. Plane strain condition is considered in this problem. The Properties of the substrate and the FGM layer are assumed to be elastic and the Poisson’s ratio is assumed to be constant. The modulus of elasticity in the graded layer is calculated based on TTO model approximation. This model defines a parameter q which considers the microstructural interactions. The governing equations are solved by Finite Difference method by means of MATLAB software. The influence of different parameters such nonhomogeneity,q, the dimensions of the punch, the thickness of the graded layer and the coefficient of friction on the mode I and II stress intensify factors are investigated.

The main purpose of the present research is analytical and numerical analyses of graphene/epoxy nanocomposites with a random distribution of nanoparticles. For this purpose, by combining the molecular dynamics and micromechanics methods, a new approach is presented. The molecular dynamics method is used to model the stiffness of the graphene/epoxy nanocomposites containing one layer of nano graphene embedded in epoxy resin. A multi-scale modeling strategy from macro to meso, then from meso to micro and finally from micro to nano scales is introduced. A representative volume element (RVE) is selected and for a nanocomposite having a single monolayer graphene embedded in epoxy resin, the longitudinal (E11), transverse (E22) and normal (E33) stiffnesses for three RVEs with arbitary graphene size are simulated. The best curve is fitted to each stiffness diagram and stiffnesses of the RVE in three directions with true graphene size are investigated. In order to consider the effect of randomly graphene sheets distribution in epoxy resin, micromechanical approach is used. Finally, the stiffness of the nanocomposites with randomly distributed graphene is calculated. For evaluation of the present approach in this research an experimental program is conducted. The result of the modeling is well agreed with the experimental data.

In the present study, the effect of intra-pore turbulence within porous burnershas been investigated on combustion of methane/air mixture in such burners. A model is adapted to the porous structure to models turbulence flow. The GRI 3.0 chemical reaction mechanism is utilized for the combustion of methane/air mixture and radiative part of the solid phase energy equation is obtained using the discrete ordinate method. The numerical results show that the gas temperature obtained from turbulence model stays below the corresponding laminar model temperature all over the combustion region, and the flame thickness becomes wider in turbulence model. Although the CO emission are insensitive to laminar or turbulence model, the burning speed and NO emission predictions are found to be significantly improved when the effects of turbulence are taken into account.

In this study the interaction of material and ultrasonic vibration of workpiece at different pulse on times (Ti) and discharge currents (I) in the electrical discharge machining (EDM) has been studied. The materials of machined samples were AISI H13 tool steel and FW4 weld steel. The results show that ultrasonic vibration of workpiece, independent of workpiece material increase material removal rate (MRR) and reduce tool wear ratio (TWR) and surface roughness. Also the results indicate that the effect of ultrasonic vibration on the material removal rate increase of FW4weld steel is higher than AISI H13 tool steel, and the reduction of tool wear ratio of FW4 weld steel is more than AISI H13 tool steel.

The aircraft aluminum alloys generally present low weldability by traditional fusion welding process. The development of the friction stir welding has provided an improved way of producing aluminum joints. In this present work the influence of process and tool parameters on strength properties of AA7075-T6 joints produced by friction stir welding was analyzed. Joints were fabricated by varying process parameters and tool parameters. Strength properties of the joints were evaluated and correlated with the microstructure and hardness of weld nugget. This investigation demonstrates, Joints fabricated at rotational speed of 1400 rpm and welding speed of 60 mm/min using the tool with 15mm shoulder diameter, 5mm pin diameter and 45 HRc tool hardness, yielded higher strength properties.

During modal testing, the test structure is connected to its surroundings by a suspension system. Cable suspension system is one of the most common suspension systems which is used to simulate the free-free condition, due to its low stiffness. In this condition, the measured modal properties are assumed to be free-free dynamic properties of the test structure. But in situations in which the test structure is big and flexible, the stiffness of the suspension system is comparable with the stiffness of the test structure and cannot be neglected. In this paper, the suspension effects are eliminated from measured modal frequencies of a flexible structure which is suspended by a cable suspension system. The eigenvalue sensitivity analysis is done and then, the effective stiffness of the cable suspension system is derived. Based on the eigenvalue sensitivity and the effective stiffness of the suspension system, an iterative procedure is proposed to eliminate the suspension effects from measured modal frequencies of the test structure. The proposed iterative procedure is verified using a finite element model. Finally, the proposed iterative procedure is used to eliminate the suspension effects from measured modal frequencies of a real flexible structure.

In this study, the finite element technique was used to analyze the thermo-mechanical behavior of the submerged arc butt-welded plates. In order to investigate the effects of the welding parameters on the magnitude of the submerged arc welding process efficiency, the distortions obtained from the finite element simulation were compared with experimental results. For studying the effects of welding voltage, current and speed on SAW process, finite element model with acceptable accuracy was developed. Welding efficiency of each sample were estimated by calibration of the finite element model. The angular distortions were studied in carbon steel plates of 12 millimeter thickness. Statistical analysis shows that the welding voltage and speed have no significant effect on the SAW efficiency. It is observed that by increasing the welding current, the magnitude of the welding efficiency increased significantly.

Application of carbon nanotube reinforced polymers in space industry is widely dispread due to the unique and multi-purpose properties of them Therefore, extraction of electrical and electromagnetic properties of nanocomposite materials in the frequency band of 12.4 to 18 GHz is an important issue in their development procedure. In this paper, experimental investigations on electrical and electromagnetic properties of carbon nanotube reinforced polymers are performed. The investigated properties include AC and DC electrical conductivities, permittivity, transmission and reflection coefficients, loss tangent and skin depth in Ku frequency band (12.4-18 GHz). The in situ polymerization method is selected to fabricate multi-walled carbon nanotube (MWCNT)/Vinylester nanocomposite. Ultrasonic device is used for dispersion of CNTs in resin and then Vector Network Analyzer (VNA) is employed for measurement of electrical properties of specimens. Weight fraction of MWCNT is chosen between 0.1 to 3 % in order to evaluate the influence of CNT content on investigated properties. Finally, equivalent circuit model is used to describe the observed behavior on the basis of semi-empirical study.

Reorganizations of cyclic variations, depending on fuel type, equivalence ratio, engine load and speed, and engine geometry, are the major purposes and may cause fluctuations of output power and unburned hydro-carbon. During this study, the effects of gasoline and natural gas (NG) as fuel on cyclic variations were investigated utilizing the recorded cylinder pressure of a research SI engine over more than 400 successive cycles. This work was performed at full load, 1800rpm and compression ratio of 8 with 0. 94 equivalence ratio using gasoline-air and NG-air mixtures. ُStatistical analysis of the obtained results showed that at the above conditions the coefficient of variations (COV) of indicated mean effective pressure (imep), peak pressure (Pmax) and the crank angle position of the peak pressure for gasoline-air mixture were 2. 4, 1. 29 and 1. 04 times of those for NG-air mixture, respectively; at the optimum ignition timing, imep of gasoline-air mixture is increasing with rising Pmax and decreasing with enhancing, however, imep of NG-air mixture seems to be independent to Pmax and.

In this study, nonlinear bending analysis of ring-stiffened annular laminated composite plates is studied. A discretely stiffened plate theory for elastic large deflection analysis of uniformly distributed loaded is introduced. The governing equations are derived based on a first-order shear deformation plate theory (FSDT) and large deflection von Karman equations. The numerical results are obtained using the dynamic relaxation (DR) method combined with the central finite difference discretization technique. For this purpose, a FORTRAN computer program is developed to generate the numerical results. In order to verify the accuracy of the present method the results are compared with those available in the literatures and ABAQUS finite element package as well. The computer code can handle symmetric, unsymmetrical and general theta-ply schemes. The effects of the plate thicknesses, different ratio of outer to inner radius, depth of stiffener, boundary condition and laminates lay-up are studied in detail.

In this study, factors affecting ballistic performance of fabrics used, including material properties, projectile geometry, boundary conditions, fabric dimensions, multiple plies of fabric armors and friction, has been studied. Ballistic limit was obtained as a criterion of ballistic performance of fabric to identify and compare the effect of the mentioned factor. To obtain the ballistic limit, ballistic tests were performed on the fabric. Also, a finite element model was created using LS-DYNA software and the results of the the simulation of this model show an acceptable agreement between the experimental and numerical analysis. Due to limitation in experimental tests,many of factors affecting performance of armors can be evaluated using this model.

Design of a suitable Control Strategy for operating a hybrid propulsion system in different types of roads and driving cycles is one of the most challenging subjects in hybrid vehicle research areas. Intelligent Control Strategies have been designed to meet the above requirement. The control signals in an intelligent control strategy of the hybrid vehicles are generated according to the driving cycle type. This is done by using a driving cycle identification unit. In this paper, design of a fuzzy based driving cycle identifier has been presented. The main idea in this unit is that any arbitrary driving cycle is similar to a group of standard driving cycles according to some degrees of similarity. As a result, the control strategy of the hybrid powertrain in the arbitrary driving cycle is affected by the optimized control strategy of the standard diving cycle based on the degree of similarity. Here, the subset of sufficient features is determined by using the floating search method as a useful feature selection algorithm. Also, the fuzzy clustering method is used to generate the values of similarity degrees to each standard driving cycle. Finally, the performance of the fuzzy driving cycle identification unit is assessed.

Abstract- The comfortability of car passengers, reduction of fuel consumption in order to reduce the emission of CO2, and increasing the efficiency of fuel usage are the main goals of car manufacturers. Therefore the combustions in internal combustion engines must be done at temperatures as high as possible. These high temperatures have negative effects such as impact, rattling status in gearbox, low-pitched sound in gearbox, and vibration of vehicles. These vibrations interfere with the comfortability of passengers. In order to reduce the unwanted vibrations as much as possible there is a need to uniform the radial velocity of the flywheel and the inlet torque to the gearbox. In this study the available methods are reviewed and a new approach is proposed which is the two-mass flywheel. The effects of this new flywheel are carefully investigated. The experimental results are compared with the numerical ones and a very good conformation is observed.

Magnetorheological (MR) materials indicate variations in their rheological properties when subjected to different magnetic fields. This study presents vibration analysis of laminated composite plates using MR fluid lumps. A structural dynamic modeling approach is presented to investigate the vibration characteristics of MR adaptive structures for different magnetic fields. The effects of laminate configurations and locations of MR fluid lamps on the controlled response are investigated. Vibration responses of the laminated plate have been simulated to demonstrate the accuracy and efficiency of the present approach. The results of this work may improve the dynamic performance of composite structures which are subjected to undesirable vibration during operation such as helicopters blades