Modares Mechanical Engineering
Year:2016 | Volume:16 | Issue:2|Papers Count:39

In this paper, acoustic emission monitoring of repaired aluminum 2024-T3 sheet with FML patch is studied. For the experimental investigation, 12 samples were made and classified into 4 categories according to the crack angle (zero and 45 degrees), and repaired or unrepaired state. To reduce manufacturing errors, composite prepreg is used for producing patches, aluminum surfaces are anodized and curing is done in an autoclave. In fatigue crack initiation process, using Acoustic Emission data acquisition crack initiation moment is detected. Onset of delamination, delamination advancement and critical delamination growth rate are identified using Acoustic Emission signal cumulative energy parameter. SEM image and investigation of failure surface are used for detecting failure mechanism. By introducing one frequency analysis method we tried to classify frequency range of failure mechanism signals. Because of frequency range intersection of matrix cracking, fiber/matrix separation and delamination of patch from aluminum sheet, force-displacement curve is divided to 3 zones and frequency analysis is done in each zone where probability of certain failure mechanism is higher than the others. Aluminum plastic deformation and crack growth occur in a same frequency range 440-480 kHz while, delamination happens in 100-150 kHz and 200-220 kHz frequency ranges.

Recent observations have shown that artery stenosis occurs as multiple-stenosis in 70% of patients with atherosclerosis plaques. Accordingly, the frequent occurrence of double-stenosis in blood arteries has inspired this paper to investigate and compare the plaque rupture risk in different arrangements of common plaque shapes in a double-stenosis. The plaque von- Mises stress in plaque fibrous cap is calculated by finite element modeling of the fluid-structure interaction (FSI) between the blood flow, artery and plaque components. Arbitrary Lagrangian- Eulerian approach is employed for FSI simulations and a benchmark problem dealing with wave propagation in a fluid-filled elastic tube is used for model verification. Transient velocity and pressure conditions of actual pulsatile blood flow through coronary artery are prescribed. The blood is assumed to be a Newtonian fluid and hyper-elastic material model is employed for describing nonlinear behavior of the human tissue composed of the arterial wall, lipid core and fibrous cap. It was observed that the arrangement, composed of two diffused plaques, is subjected to the maximum von- Mises stress, while the arrangement of ascendingdescending plaques experiences the minimum von- Mises stress. The effect of different parameters such as the stenosis degree, the space length between the plaques, and the plaque length is studied and discussed.

Boiling heat transfer is one of the most applicable heat transfer processes within the industry. In this paper, the pool boiling heat transfer of Fe3O4 /water nanofluid (ferrofluid) in atmospheric pressure has been analyzed experimentally. The nanofluid in this study has been synthesized in a single step and retains high stability. The replication and accuracy of the testing machine has been studied for deionized water three times, indicating an appropriate concordance with the literature. Considering different volume concentrations of the nanofluid has revealed that boiling heat transfer in high concentrations decreases with an increase of concentration, while it rises with the increase of concentration in low concentrations. Hence, boiling heat transfer coefficient in 0.1% volume concentration nanofluid has been measured to be the optimum value which increases up to 43%. The roughness of boiling surface was varied with the deposition of nanoparticles in various conditions of nanofluid concentration, and heat flux. It is noteworthy that in the present research, the effects of surface roughness change due to nano particles deposition and the impact of passing time on boiling process have been investigated for the first time. Therefore, several experiments have been designed in order to study the change of nanoparticles deposition due to the change of nanofluid concentration and boiling surface heat flux. The results indicate that boiling heat transfer of deposited surfaces at low heat fluxes decreases, while it rises at high heat fluxes.

A novel approach is presented for the reconfiguration of satellite constellations based on Lambert’s theorem. The reconfiguration problem in this article, is considered with the constraint of overall fuel cost minimization. Hence, orbital maneuvers required for the operation of reconfiguration are designed in such a way that, transferring globally the satellites to the desired configuration of constellation will be possible at minimal cost. Also, the introduced method of orbital transfer for implementing the reconfiguration phase of satellite constellation has no limitation on the shape and orientation of initial and target orbits such as: co-planarity, coaxiality, circularity and/or the existence of a common point.Moreover, a method is offered for modeling the cost function of reconfiguration problem in which the two important tasks of optimal orbital transfer of satellites to the target configuration of constellation and optimal assignment of each satellite to a specific terminal position or final orbit will be done in one single step. For this purpose and in order to achieve globally optimal solution of the reconfiguration problem of constellation the hybrid PSO/GA is used. Finally, two different scenarios of reconfiguration of satellite constellation will be modeled once by the presented approach and once by considering determined positions of flight and deployment for the satellites. The obtained results indicate the superiority of the idea presented in this article.

Titanium alloy Ti-6Al-4V is one of the most used industrial alloys, often employed in important and risky applications. One of the requirements for machining such parts is to achieve the appropriate surface integrity. Powder mixed electrical discharge machining is a process which has different mechanisms compared with traditional electrical discharge machining process; it is often used in order to obtain good surface finish. In this study two different kinds of Nano powders, SiO2 and Al2O3 were added in dielectric for machining of Ti-6Al-4V titanium alloy so that the effect of adding them on the output characteristics of the electric discharge process, including removal rate, tool wear ratio, surface roughness and integrity is investigated and compared. In order to investigate surface micro cracks and heat altered layer, surface and cross section of it were studied by scanning electron microscopy imaging.The results show addition of Nano powders into dielectric, especially SiO2, increases material removal rate, the effect of Nano powders on tool wear ratio depends on machining condition and setting. SiO2 Nano powder decreases surface roughness more than Al2O3 Nano powder. Surface integrity of machined sample in terms of micro-cracks and depth of the heat altered layer is improved with the addition of nanoparticles.

In this paper, a new approach has been presented for dynamic control of active suspension vehicle system subject to the road disturbances. The active suspension system (ASS) which has been considered in this paper is operated by a hydraulic actuator. The input of this hydraulic actuator is a servo valve. In other words, both mechanical equation of system (related to hydraulic actuator) and its electrical equation (related to servo valve) are considered. Therefore, the equations are complicated and only the input current of servo valve is accessible as the input control signal. The proposed approach is based on dynamic sliding mode control (DSMC).In DSMC chattering is removed due to the integrator which is placed before the input control signal of the plant. However, in DSMC the augmented system (the system plus the integrator) is one dimension bigger than the actual system and then, control of the plant is more complicated. But, its advantage is that the input control signal is obtained from a dynamic system or a low pass filter, while the robust performance (invariance property) of the system is reserved even in the presence of disturbance. Another advantage of proposed approach is that the desired output force of the hydraulic actuator is obtained by the controller.

In this paper a new method for motion control in humanoid robots has been introduced. The problem of movement learning, especially dance and repetitive actions of human beings to humanoid robots is a major challenge in the field of robotics. Imitation learning, which is a subset of supervised learning, is one of the main forms used to teach complex tasks to the humanoid robot, and is, accordingly based on the concept that an artificial system can imitate a great deal of information through learning from a human trainer. The main technique uses Central Pattern Generators structures which are able to produce required motion trajectories based on imitation learning. Systematic design of these neural networks is the main problem that is solved in this paper. The proposed model is a basic paradigm for imitation learning in the humanoid robots which do not require direct design of controller and programming. The proposed model has many benefits including smooth walking patterns and modulation during imitation.Simulation results of this learning system in the robot simulator (WEBOTS) have been linked with MATLAB software and its implementation on a NAO robot demonstrates that the robot has learned the desired motion with high accuracy. This model can be extended and used in the Nao soccer player both for the standard platform and the 3D soccer simulation leagues of Robocup SPL competitions to train different types of motions.

The study of the boundary layer flow is considered as one of the fundamental issues in fluid mechanics that attracts the attention of many of the researchers’ in this field. Most of the previous researches on boundary layer problem are limited to Newtonian fluids and only a few researches have considered the non-Newtonian fluids. The main objective of this research is to better recognize the viscoelastic properties effect on characteristics of the boundary layer. In this study, the hydrodynamic and thermal boundary layer of viscoelastic Falkner-Skan problem is investigated numerically. Here, the second order model has been used as the viscoelastic constitutive equation and MATLAB software is used for analysis. Both constant temperature and constant heat flux at walls are used as thermal boundary conditions. The effect of Reynolds number, the coefficient of the first normal stress difference, Prandtl number and wedge shape factor on the thickness of dynamic and heat transfer boundary layer, momentum thickness, displacement thickness, drag coefficient and Nusselt number are studied. The effects of both adverse pressure gradient and favorable pressure gradient in heat and hydrodynamics boundary layer characteristics are investigated. The magnitude of the non-Newtonian hydrodynamic and heat boundary layer are found to increase with increasing the coefficient of the first normal stress and finally at constant pressure gradient, average Nusselt number decreases along plate by increasing first normal stress coefficient.

Deep Drawing with rubber components is one of the conventional methods to reduce the cost of manufacturing, also this method is core cause for increase of LDR and has positive effect on improving the thinning defect. Moreover, the punch or matrix is made of rubber. Deep drawing of two-layer sheets is one of the new ways to achieve the desired properties in the produced parts in which two layer metal sheets are connected to each other by glue and are transformed together to the desired shape. Thinning control is different in the single layer when there are different materials or thicknesses. In this paper used the technique of initial gap distance between blank holder and fixed ring. In this study square sample using die along with rubber matrix by experimental and three-dimensional simulation has been formed. In this paper, using the finite element method and hyper-elastic model and the numerical model simulation for this process is three-dimensional.Then, to validate the obtained result from simulations, die with rubber components is made for square cups by considering permutation of layers for aluminum and steel and then, experimental and numerical results were compared. Finally, for evaluation effect of process parameters on thinning defect, force of punch and blank holder force used from Taguchi methodology.

This article presents a numerical investigation of fluid flow in one of the centrifugal pumps of pump-Iran Corporation. A computational fluid dynamics (CFD) analysis is performed by using the CFX software for a wide range of volumetric flow rates for two different rotor speeds of 1450 rpm and 2900 rpm and the numerical results of water are validated against measured values of head and total efficiency with an overall acceptable agreement. The results have been obtained for crude oil as diagrams of head, total efficiency and other variables with respect to volumetric flow rate and then compared with results of water. Numerical results show that the absolute pressure on blade surfaces for crude oil is 705 kPa less than when using water. The absolute pressure difference between inlet and outlet of impeller and spiral volute for crude oil is comparatively less than those amounts in comparison with water. Also, by increasing the angular velocity of rotor, it was observed that high levels of turbulence intensity are transmitted from outlet pipe bending to the impeller outlet at volumetric flow rate of 30 m3/h which causes reduction in efficiency. Also, high levels of turbulence intensity for crude oil are less than those amounts in comparison with water within impeller area. Finally, to present an impeller pump head curve for crude oil over the overall operation range of the pump, a second order polynomial equation was fit to numerical data.

Aerodynamic study of flows at low Reynolds for special applications such as micro unmanned underwater vehicles, underwater robots and explorers are investigated. In this paper, an improved progressive preconditioning method named power-law preconditioning method for analyzing unsteady laminar flows around hydrofoils is presented. In this method, the 2D Navier-Stokes equations are modified by altering the time derivative terms of the governing equations. The preconditioning matrix is adapted from the velocity flow-field by a power-law relation. The governing equation is integrated with a numerical resolution derived from the cell-centered Jameson’s finite volume algorithm and a dualtime implicit procedure is applied for solution of unsteady flows. The stabilization is achieved via the second- and fourth-order artificial dissipation scheme. Explicit four-step Runge–Kutta time integration is applied to achieve the steady-state condition. The computations are presented for unsteady laminar flows around NACA0012 hydrofoil at various angles of attack and Reynolds number. Results presented in the paper focus on the velocity profiles, lift and drag coefficient and effect of the power-law preconditioning method on convergence speed. The results show satisfactory agreement with numerical works of others and also indicate that using the power-law preconditioner improves the convergence rate and decreases the computational cost, significantly.

Stratospheric airships have introduced interesting solutions for many challenges in the aerospace industries. Buoyant and propulsion forces produced by airships make them capable of long-time flight and efficient operation. In spite of much progress, there are still many challenges in this exciting field of study. In this paper, first the dynamic model of fully-actuated stratospheric airship with 6-DOF is expressed by the generalized coordinates, then desired values of the airship attitude, linear and angular velocities are obtained according to desired path and using pseudo inversion of the kinematics and dynamics equations. In view of the unknown inertial parameters, first in adaptive inverse dynamic control, inertial parameters are estimated online using linearization parameters and gradient update law.Next, control law and nonlinear dynamic equation arededuced by designing passivity based control algorithm, and according to that, adaptive and robust control based on passivity is applied to control the airship. The stability of the closed loop control system is briefly proved using the Lyapunov stability theory. Finally, the simulation results for tracking of a desired path are shown and comparison between the results of all methods is presented.

In this paper, free vibration of rotating microbeams based on the strain gradient theory and Euler-Bernoulli beam assumptions is investigated. The Hamilton's Principle is applied on the attained strain and kinetic energy relations to obtain the equations of motion for the rotating microbeam. Then, by employing the adimensional parameters, the nondimensional form of the equations of motion is derived.After that, by applying the Galerkin approach on the dynamic equations of motion, the flapping and axial natural frequencies are calculated. Subsequently, the current results are validated by the existed papers results. After validation of the present results, the effects of the thickness to the material length scale parameter ratio, rotation speed and Poisson's coefficient on the flapping and axial frequencies are studied and the strain gradient theory results are compared with the modified couple stress and classical theories. The results show that the type of theory that is appointed has essential effects on the predicted natural frequencies. The effect of rotation speed on the possibility of the occurrence of internal resonances is also examined. In addition, for the first time, the effect of different mentioned theories on the axial natural frequencies are inspected. The presented results illustrate that, by considering the strain gradient theory, varying the Poisson's coefficient changes the axial frequencies, while the modified couple stress and classical theories are ineffectual in predicting any variations on the axial frequencies and the mentioned theories predict the same results for axial frequencies.

Gas launchers are an important part of impact testing apparatus which have many applications in material parameters identification. Some experiments call for very high velocity which is beyond the limit of one-stage gas launchers; thus, two-stage gas launchers are employed. Several parameters affect the operation of such launchers. For optimum adjustment of such parameters, modeling and simulation is necessary and inevitable. To this end, in this paper, a one dimensional model for a two-stage light gas launcher is proposed and utilized for performance optimization. To simulate combustion, experimental data for burning rate has been used. The proposed model is verified by comparing its predictions with the available experimental data. It is shown that the proposed model is accurate enough to predict the two-stage light gas launcher performance. The results of one dimensional model can be used in the basic design of the launcher to investigate the feasibility of manufacturing and estimate the costs.Moreover, the model is used to optimize the launcher performance as well as to determine optimum parameters. The statistical method of response surface is employed to find suitable second order polynomial models to predict the projectile velocity and maximum base pressure. The presented models are used to maximize the projectile velocity as well as to minimize the maximum projectile base pressure. To this end, Simplex method is employed to minimize the maximum base pressure in different conditions. Finally, the table of optimum conditions is presented to simplify the optimum use of the two-stage light gas launcher.

Ironing is a conventional metal forming process for producing thin walled cans with uniform thickness components manufactured from deep drawn cups. The most important drawback of the conventional ironing is that the lower thickness reduction ratio (TRR) requires annealing process and multi stage ironing. Recently a new ironing process named constrained ironing was presented by the current authors to achieve an extra TRR to solve the conventional ironing problems. This process which is based on the compressive stresses makes it possible to achieve high TRR without interruption for additional processing such as multi-stage ironing and annealing. In this paper FEM simulation was performed to investigate the effective parameters. The simulation results showed the process load increases with increasing the friction coefficient. Also the state of the stresses is fully compressive in constrained ironing process while it is tensile in the conventional ironing method. Thus compressive stress components minimize formability problems and higher thickness reduction ratio is achievable in the new ironing method. Moreover experimental results showed that the tensile strength and hardness increased after constrained ironed of the deep drawn cup.

A Stirling engine cycle is combined with a Spark Ignition (SI) engine cycle to recover the SI engine exhaust gas waste heat. One dimensional combustion simulation code is prepared for Spark Ignition type engine (M355G) simulation. The accuracy of numerical simulated results were validated with M355G experimentally. The experimental generated power and exhaust gas temperature vary in the range of 84.1- 176.7 kW and 610-710, respectively. The 1D code estimates the generated power with maximum 5.9% error and average exhaust gas temperature with 3.8% error in the operating range of the engine. Thermal analysis was done, and the results show that about 25% of input energy transfers by the exhaust gas are waste. The results indicate that, by installing a Stirling engine heater on the exhaust pipe of the SI engine about 8.4 kW of the waste heat can be recovered in the best condition. The simulation of Alpha-type Stirling engine was done by GT-Suit program and the Solo V161 experimental results were used for the validation. According to 9% error in generated power calculation for validation, the new Stirling engine is suggested for installation in exhaust pipe. The generated power and thermal efficiency were estimated for Stirling engine in various exhaust gas temperature which occurred in various SI engine working conditions. The coupled engines heat balance showed that the thermal efficiency is about 2-3% more than the ordinary one.

In this paper, FGMs are used as non-uniform materials in high temperature environments, especially in industries like aircraft, aerospace vehicles, nuclear plants and engineering structures. Different industries use them in thin and thick walled cylindrical pressure vessels. Based on the governing equations, differential equation of stresses is obtained in plastic state which can be widely used in the study of vessel and pipe behavior in elasto- plastic state. This research studies the temperature distribution and stress - strain relationships in the tube under internal pressure and temperature difference. Properties of these materials are considered as variable parameters function of radius. In this work, effects of these parameters have been investigated on yielding and the yield temperatures and stress changes under different loading in thickness of the tubes. Furthermore, it is shown that the structure of the tubes can be optimized by choosing appropriate parameters. In fact, by studying the four parameters, internal and external temperature, internal pressure and the yield stress together can be better analyzed on a functionally graded cylindrical tube, which has been shown at the end of the article.

A finite element model has been introduced for static bending analysis of thick plates based on mixed plate variational formulation. refined Reissner' s mixed variational theoy is employed to derive the governing equations in terms of the introduced transverse normal stress and displacement variables. The in-plane displacement components of the plate are described by combination of polynomial and exponential terms. Concerning the transverse displacement component, first-order polynomial is adopted. second-order expansion is considered for the variations of the transverse normal component of the stress tensor along the thickness direction of the plate. The boundary conditions of shear and normal tractions on the top and bottom surfaces of the plate are exactly satisfied. Based on the proposed mixed plate theory, 􀂃 four nodded compatible Hermitian rectangular element which ensures C1-type continuity of all unknown parameters of the plate along in-plane directions is employed. An arbitrary free parameter, called the splitting factor, appears in the functional of the proposed variational formulation. In the numerical part of the present paper, simple formulation has been proposed for selecting the splitting factor which leads to the results of higher precision. Comparison of present bending results for thin and thick plates with results of the three-dimensional theory of elasticity and other plate theories available in literature reveals efficiency of the proposed parametrized mixed plate theory. Moreover, the proposed model has high convergence rate and is computationally low cost.

In this article bubbly flow under the specified axial pressure gradient in a curved channel is studied numerically. To do so, a second order parallelized front-tracking/finite-difference method based on the projection algorithm is implemented to solve the governing equations including the full Navier-Stokes and continuity equations in the cylindrical coordinates system using a uniform staggered grid well-fitted to the geometry concerned. In the absence of gravity the mid-plane parallel to the curved duct plane, which is the symmetry plane in the single fluid flow inside the curved duct, separates the bubbly flow into two different flow regions not interacting with each other. Twelve bubbles with diameters of 0.125 wall units are distributed in equally spaced distances from each other. The numerical results obtained indicate that for the cases studied here, the bubbles reach the statistical steady state with an almost constant final orbital motion path due to the strong secondary field. Furthermore, the effects of different physical parameters such as Reynolds number, and curvature ratio on the flow field at the no-slip boundary conditions, are investigated in detail.

The aim of this study is numerical investigation of an evaporating and non-reacting diesel spray operating in a high pressure and high temperature constant volume combustion chamber, as an essential step in simulation of liquid fuels combustion. To this end, the impact of droplets diameter distribution on estimating two critical characteristic parameters, i.e., liquid and vapor penetration lengths is studied using the open-source OpenFOAM code. In order to determine droplets diameter distribution effect, three different distributions ranging from 0.25-100 micron are chosen and the liquid and vapor penetration lengths are individually calculated for each distribution. The results are validated against the experimental data published by Sandia National Laboratory. The results show that, while the droplets diameter distribution has a remarkable effect on the predicted value of the liquid length, it leads to overestimating liquid penetration lengths up to more than two times; its effect on the vapor length prediction is negligible. Also, assuming a nozzle diameter distribution leads to non-physical increase in the value of liquid length. This non-physical prediction may lead to the misleading prediction of spray impingement to piston and the cylinder walls resulting in an error in unburnt hydrocarbons concentration as well as the engine efficiency estimation.

Ultrasonic nondestructive testing is a powerful tool for detection of defects as well as characterization of various properties of materials. In ultrasonic testing, it is very important to know the exact wave velocity in the material because most measurements somehow depend on wave velocity. Many other characteristics of materials such as elastic constants also depend on wave velocity. While variations of wave velocity in ambient temperatures are very small, these variations could be noticeable at high temperatures. In this paper, a simple and innovative experimental method is proposed for measurement of ultrasonic wave velocities at high temperatures. To keep the ultrasonic probe far from the hot sample, a special waveguide is designed. The wave velocity measurements are performed by pulse-echo ultrasonic testing technique and variations of ultrasonic wave velocities at temperatures ranging from 40oC to 160oC are investigated. It is observed that the velocity of ultrasonic waves decrease with increase in temperature. Experimental results are compared with theory and measurement uncertainties are calculated. These uncertainties are ±0.01 m/s and ±0.003 m/s for longitudinal and transverse wave velocities, respectively. The theoretical and experimental results agree very well.

Increasing usage of magnesium and aluminum light metals in the transportation industry has made joining of these two metals one of the challenges for researchers and engineers. The aim of this study was to investigate the relationship between mechanical properties of lap friction stir welded Al-Mg plates and characteristics of the interface. Therefore, joining of aluminum and magnesium in various conditions was conducted. Optical and scanning electron microscopy analysis, micro-hardness test and tensile tests were performed on samples. The results showed that in the joints where Mg was on top, an approximately 10 micron thick layer of intermetallic compounds is created, while in the Al-top joints, approximately 1 mm thick intermetallic compounds with solidified microstructure were visible.Mechanical test showed Mg-top joint has higher strength in comparison with Al-top joint. On the other hand, hardness test of Mg-top joint showed more fluctuation than Al-top joint. Microstructural investigation also showed that in the Mg-top joint, formation mechanism of intermetallic compounds has occurred in solid state while in the Al-top joint, in addition to diffusion in solid state, eutectic formation in the molten state and solidification has occurred.

The purpose of this paper is to find the optimum design of a typical gas turbine exhaust diffuser. In order to access the maximum overall static pressure recovery at the condition of swirling flow, an evolutionary algorithm is used. The optimization process is studied in three independent cases. Firstly, the optimization is done for a single profile of strut cover from hub to shroud. Secondly, two profiles are selected for the strut covers, one in the hub section and the other in the shroud section. Finally, the optimization process is done for the strut cover and diffuser channel geometries simultaneously. In order to produce the strut cover profiles the PARSEC parameterization method is used. The turbulent 3D flow is solved using computational fluid dynamic (CFD). The optimization process starts with the initial sampling of solution domain and subsequently the genetic algorithm (GA) is used to find the global optimum. The swirling flow at the turbine exit with the Reynolds number of 1.7 ×105 based on the hydraulic diameter of the diffuser inlet is optimized. All steps of GA and corresponding processes of model creation, mesh generation by TurboGrid, flow simulation by ANSYS CFX and goal function calculation for all members of each generation are coded in the MATLAB platform. As a result of the optimization, the pressure recovery coefficients increased 1.94%, 3.1% and 7.42% in the first, second and third cases of the optimization process respectively.

In this paper, an innovative flexible sandwich structure is introduced which can be used in shape changing (morphing) aircrafts that adapt their external shape to different flight conditions. First, different ideas for achieving smart aircraft in the literature are briefly reviewed and then characteristics of the new deformable sandwich structure as well as its different features in comparison to other proposed structures are described. Moreover, fabrication details of deformable and load bearable sandwich panel are explained. In an aircraft with variable camber wings, deformable sections can be supposed as a cantilever beam. As a result, some specimens of new deformable sandwich structure are constructed and then tested as end-loaded beams. Since the numerical study of the new proposed structure requires an understanding of the mechanical behavior of components used, a comprehensive study about the mechanical behavior of individual components of structure is conducted. According to the observation of broken samples, a distribution of cavities resulting from the manufacturing process is supposed in one type of model to obtain more accurate numerical results. Finally, another example is analyzed with the same assumptions and it is shown that in the second example, the numerical results are close to the experimental data.

The stability analysis of workpiece in fixtures is considered as one of the stages of the fixture verification system. The stability of free-form workpieces in fixtures is affected by different agents including weight, locators, and clamps and machining wrenches. In this study, a mathematical model has been presented for part stability analysis based on the minimum norm principle that led to a nonlinear quadratic optimization problem. The solution to this problem is the reaction forces at the contact points between workpiece and locators. The study includes the workpiece stability analysis at the loading stages, determination of stability span for workpiece and investigating the effect of the base locator's distances on the workpiece stability through examples. A turbine blade model was incorporated as the case study to evaluate the suggested model capabilities in stability analysis. The loading procedure of this part into the fixture was categorized into sequential stages and its stability was investigated in contact with the locators. The results included the stability span of [22°-38°] for the workpiece on base locators, increased stability by the distanced base locators and the confirmation of the main locating plan through the stability verification at the loading stages. The results showed the model efficiency and accuracy in analyzing the free-form part stability in contact with the fixture elements. The proposed dexterous model can be integrated into the CAFD platform to be used at the early stages of locating and clamping system design applications.

Continuum and flexible manipulators have a special role in medical applications. One application is robotic manipulators used for surgery or endoscopic tool used for inspection of body parts like esophagus or colon. In addition to small size, better maneuverability increases the tool performance in real applications. One of the useful actuators in miniaturizing mechatronic systems is shape memory alloys. This material, typically used as a linear actuator, produces high forces in comparison to weight.In this paper, by embedding three shape memory springs inside the structure of a flexible module, a two-DOF mechanism is provided. The module has rigorous usage in modular robotic systems, especially flexible manipulators. The developed module produces large deflection in addition to covering a large workspace. Modeling of the module is discussed in this paper for extracting module parameters in design and implementing the simulation. Through the complex behavior of SMA and uncertainty in model, control of SMA is a challenge. In this paper using a novel algorithm, a desired shape for the module is provided. Using the new non model based control approach, the final shape or position is realized. The adequacy of introduced controller is verified through experiments. Large workspace and controllability of module make it feasible for real applications.

Condensing flow in nozzle and stationary blades of steam turbine has been the subject of many studies.Due to the lack of precise relationship between surface tension and small droplet radius, the radial dependence of surface tension has been ignored in calculations and instead of droplet surface tension, surface tension of flat surface is used. Gibbs-Tolman-Koenig-Buff equation express the radial dependence of surface tension that Kalova provides as a relationship of changes in surface tension versus radius of the surface by fitting response from the exact solution of GTKB equation. The aforementioned relationship is known as Kalova surface tension equation. The present study considers the effect of the Kalova surface tension correction on nucleation and droplet growth in condensing flows in an ultrasonic Laval nozzle. Since Tolman coefficient (d) is an important parameter in Kalova surface tension equation, by fitting response from Tolman equations a correlation for Tolman coefficient temperature changes is suggested for the first time. Kalova surface tension correction has a direct impact on the droplets crisis radius and droplets crisis free energy that the impact of both them in the modified classical nucleation equation have been studied for the first time. The results of analytical modeling of one-dimensional adiabatic supersonic flow by applying the Kalova surface tension correction and using the proposed equation for Tolman coefficient temperature changes indicate an improvement to the 12% in radius of the droplets and 5% in pressure distribution in the region of condensation shock.

Equal channel angular pressing (ECAP) is one of the most efficient techniques among severe plastic deformation (SPD) methods that enhance the mechanical properties of polycrystalline metals by refining subjected grains. In this article, equal channel multi angular pressing (ECMAP) process as one of the effective ECAP methods on production of ultra-fine grained (UFG) Al5754 strips is studied.Route C is considered as a multi pressing route and grey relational analysis is used as the optimization method. Geometrical parameters were taken as input variables and strain inhomogeneity index, equivalent plastic strain and required process load were taken as the objectives. The suggested tests by full factorial method were simulated by FEM. Finite element simulations were done by ABAQUS commercial software and obtained results were validated by analytical methods and experimental tests.Then considering all input and output parameters, optimization was done and optimum values of input parameters were elicited. Also, the effectiveness of each parameter on the objective parameters was obtained. It is concluded that, among geometrical parameters of route C, die channel angle (f_2) and die corner angle ( y_1) have the maximum and minimum effectiveness respectively..

A thermoacoustic refrigerator is a device that transfers heat from a low temperature reservoir to a high temperature reservoir by utilizing acoustic wave. Due to using no moving parts, no exotic and poisonous materials, Thermoacoustic refrigerators have been considered by many researchers. In this paper, the OpenFOAM package is used to simulate a thermoacoustic refrigerator. The unsteady compressible Navier-Stokes equations and equation of state are solved with PIMPLE algorithm. The effects of driver pressure ratio, frequency and heat exchangers temperature differencing for air and helium have been studied. Length of heat exchangers and stack remains constant throughout the analysis process. The results show that the coefficient of performance (COP) is decreased and cooling power is increased due to rise of driver pressure ratio. Helium cooling power is greater than air, but their COP is equal because of its need for greater input power. The cooling power for both air and helium are increased with the enhanced temperature difference of heat exchangers. Also, COP of air refrigerator is decreased, but COP of helium refrigerator is increased. The longer the device length, the smaller the COP and cooling power are a result of the reduction in driver frequency. When frequency is increased, the length of cold heat exchanger will be greater than gas particle displacement of air.Therefore, cold heat exchanger absorbs heat from the air instead of heat transfer to it and COP will be zero.

Reactant gases should be humidified before entering a polymer electrolyte membrane (PEM) fuel cell stack. Humidification of the gases can be performed by a membrane humidifier. In the present study, an analytical model has been proposed to investigate the performance of a water-gas membrane humidifier which is used in the fuel cell systems. At first, a set of nonlinear equations was obtained by applying the mass and energy conservation laws on the gas side of the humidifier. The temperature and the humidity ratio of the outlet gases from the humidifier are the unknowns of these nonlinear equations. The proposed model can evaluate the performance of the humidifier based on the temperature and relative humidity of the outlet gases from the humidifier. The effects of different parameters like: gas flow rate, length and depth of channel, temperature and pressure of the inlet gases on the performance of the humidifier were studied by the developed model. The results show that the channel depth does not have an effect on the temperature and humidity of the humidified outlet gases. In addition, increasing the channel length causes an increase on the dew point of the outlet gases but the relative humidity of the dry inlet gas does not have a noticeable effect on the dew point of the outlet gases. Increasing the temperature of the inlet gases does not improve the humidifier performance, considerably. The results of the model show that increasing the inlet pressure and using less air flow improves the humidifier performance.

Sedimentations and hard cakes formation of magnetic particles restrict magnetorheological fluid response to magnetic field and can cause the MR fluid containing device to collapse. Therefore, researches on MR fluids sedimentation reduction procedures and its effective factors are of considerable interest to improve magnetorheological applications. In this study, the effects of some parameters on typical MRF stability were investigated. For this purpose, at first, MRF samples were constructed and the effects of various factors including carrier fluid type, particles concentrations and MRF mixing methods on its stability were investigated and the importance of each factor was determined by Taguchi algorithm and the stable MRF sample for application of magnetorheological dampers was chosen. Next, by investigating the most stable MRF sample, based on the combination of stability and off-state viscosity factors, the relation for yield stress in various magnetic fields was presented. This relation was derived based on fitting the Herschel- Bulkley model with experimental data in conjunction with the existing relations of yield stress. As the results show, after 168 hours, sedimentation for the most stable sample is 7%. This sample consists of silicon oil and 70%wt iron powder which was prepared with mechanical stirrer. Adding 3%wt stearic acid to carrier fluid for increasing the stability results in increasing the viscosity of carrier fluid up to 39 times. In spite of this, an acceptable MR effect is presented so that, in magnetic field of 146 kA/m the sample yield stress is 15KPa.

General Purpose Graphics Processing Unite (GPGPU) allows the user to utilize GPU for general computing purposes. Using these processors can cause a great speedup in numerical calculations.Several studies have been performed to investigate the advantages of using the GPGPU in numerical calculations including solving tridiagonal set of equations. The main focus of the mentioned studies was on improving parallel methods, for example, CR and PCR algorithms. Although these algorithms are consistent with GPU architecture, they have higher arithmetic complexity compared with serial Thomas algorithm, they also have limitations in dimensions of the equations’ set. Therefore, in the present study, according to the advantages of Thomas algorithm compared with the parallel algorithms, a novel method entitled checkerboard Thomas has been developed to accommodate Thomas algorithm for running on GPU. This method has been used for solving 2D steady heat conduction problem and the results show an increase in the solution precision compared to Thomas and PCR algorithms. Also, the results indicate that the new algorithm can cause computing to increase in speedup between 5.7 to 22.2x, compared with Thomas algorithm. Furthermore, results show that the new method is about 2x faster than PCR algorithm. It has also been seen that speed decrement for uncoalesced access to global memory is 42.7% minimum and 81.9% maximum for128×128 and 1024×1024 grid size, respectively.

The linearity is a simplifying assumption in most vibration problems of real mechanical systems which may lead to a considerable error in predicting the system dynamic response. Determining a suitable mathematical model for a nonlinear vibrating system is an important step in order to analyze the structural dynamics behavior efficiently. When the amplitude of vibration is large, the system is said to be geometrically nonlinear. In this paper, the nonlinear identification of a cantilever slender beam undergoing large amplitude free vibration has been investigated. Because of no excitation force in this situation and lack of information about its response, the existing identification methods are not efficient.In present research a new approach based on optimum correction factor of terms having uncertainty is used and identification has been done using nonlinear free vibration decay. In order to solve the geometrical and inertial nonlinear terms, the method of modified differential transform according to Padé approximation was used and resonant frequency determined. Also, the resonant frequency of nonlinear system is calculated by generalized variational iteration method and compared with the obtained frequency from the modified differential transform method. Comparison of the current results with those of 4th order Runge-Kutta technique shows good agreement of the two approaches. Finally, obtained results compared with the experimental results showed good accuracy in identifying models for nonlinear beam.

According to the importance of the engine emission and because of the cost of the laboratory tests, it is necessary to simulate the engine via numerical methods. In this study a numerical simulation of single cylinder SI engine has been carried out to predict the internal combustion engine emission with the AVL BOOST software. The engine calibration has been performed at 2000 rpm engine speed and three loads (part load, mean load and WOT) and three compression ratios (12, 14, 16) with stoichiometric air fuel ratio. After the engine calibration, the Lambda value is changed in the range of 0.8 to 1.25 and the NOx and CO values are calculated.