Modares Mechanical Engineering
Year:2016 | Volume:16 | Issue:3|Papers Count:45

Widespread applications of machining in various industrial sectors as well as acute needs for optimization are among attractive research topics for both academic and industrial institutions. Finite element analysis based techniques are available to simulate machining processes. Success and reliability of numerical models are very much dependent to work material flow stress models in strain, strain rate and temperature functions. One of the most accurate and useful material model is the Johnson-Cook model. The basic equation for modeling the behavior of each material is needed to determine the equation coefficients. In this study, the model parameters are determined by fitting the data from both quasi-static compression tests at low strain rates and machining tests at high strain rates. Therefore; the experimental results were then compared with those obtained through simulation works by Abaqus code. Experimental results confirmed the capability of material equation to determine the dynamic behavior of 5083 alloy.

In this research, the use of the exact difference method forcing scheme in the pseudo-potential multiphase model is suggested for the simulation of a droplet impact on a thin liquid film at a density ratio of 1000, and the effect of inertia, surface tension, and gravity forces are considered by means of their corresponding non-dimensional numbers (i.e. the Reynolds, Weber, and Bond numbers). For this reason, the Palabos open source software is modified by implementing the exact difference method in it. The results of our simulations in different Reynolds and Weber numbers show that the Weber number has a slight influence on the crown layer radius, meanwhile, the Reynolds number has a direct effect on the crown radius. The crown height is increased with an increase in the Reynolds and Weber numbers. Furthermore, the comparison between the pseudopotential model simulations and the free-energy model show that the crown shape is related to the surface tension; in addition to the non- dimensional numbers and with a noticeable increase in surface tension the crown tip becomes bigger. The influence of the gravity force is investigated through the Bond number. According to the results, the crown height is noticeably affected by the Bond number. When the Bond number decreases, the crown radius and height increase. Therefore, the proposed model with the potential of being used for multiphase problems with large density ratios while producing a low spurious current could be utilized for a wide variety of other multiphase problems as well.

Oil well perforators (OWP) are explosive devices that are used in drilling industries of oil and gas wells to access the reservoir and increase the wells efficiency. The performance of oil well perforators is measured by the depth of penetration that can be achieved, for this reason depth as the main parameter must be examined. In this article, a complete report of numerical simulation and performance optimization of these devices, which are indeed small size shaped charges, is presented. To do this, the multi-material Eulerian and the Lagrangian methods are used for simulation of the jet formation and its penetration into underground rock processes, respectively. For solving the problem of large deformation elements in Lagrangian method, erosion criterion elements were used. Because the results of jet formation and penetration process are heavily influenced by the density of the mesh, in this study mesh sensitivity was examined. The described simulation is validated by the use of reliable results of some references and then, a new charge geometry is suggested which resolves the inhomogeneities in the distribution velocity of the tail and increases the effective length of jet in the penetration process.

Registration of point clouds is a key process in creating a digital model in reverse engineering. Registration is a complex and ill-conditioned problem and these impede fully automatic comprehensive algorithm. In this study a new method to improve automation level is proposed. In this method, at first surface features are extracted from point clouds and then these data are used for detecting correspondence points between point clouds. Registration process accuracy depends on carefully selected corresponding points between point clouds. In present research surface curvature and local shape are used for determining the correct correspondence points. For feature extraction, surface curvature for each point of point clouds is calculated by using umbrella curvature and also a new method for determining local shape is presented. For each point of point cloud a shape number is determined. Determination of shape number is done by neighbors’ coordinates of point of concern. In this method, the corrected corresponding points are points that have almost equal umbrella curvature and shape number. Rigidity helps the algorithm to find pairwise points. Analyzing the results shows that the proposed algorithm performs well and has appropriate abilities on fully automatic registration of point clouds.

In this paper, a new model called connective layer is developed for simulation of linear dynamical behavior of bolted lap joints and model updating in 3D models. Connective layer unifies neighboring zones on sides of common surfaces of substructures in joint region. The constitutive relation of connective elements is defined by decomposing it into its normal and shear components. Unknown and different elastic properties with respect to the neighboring solid elements are defined for connective layer and the unknown parameters of the model are identified by a finite element model updating technique using modal test data. The frequency response of the structure is measured by exciting the structure using an impact hammer. Using an optimization algorithm in ANSYS, the difference between the experimentally measured frequencies and the predictions of the parametric model is minimized as objective function. The connective element performance is demonstrated by application to an actual structure containing a single lap bolted joint coupling two identical aluminum alloy 7075-T651 beams and, finally, comparison of results to those of interface elements. The outcomes of presented model have good correlation with the experimental results. The proposed method predicts the higher mode frequencies which do not participate in model updating process with minimum error in comparison to those of interface element. Due to simplicity, accurate and computationally efficient manner, this model can be incorporated into commercial finite element codes to simulate bolted joints in large and complex structures.

Electrolyte type, due to the nature of its constituent ions, affects the reaction rate, the uniformity of the electric field and formation of the external layer on the workpiece surface in the machining area during the electrochemical machining process, as well as causing different dissolution behaviors of the workpiece to be created. Therefore, in this study the effect of sodium chloride, sodium nitrate, potassium chloride and hydrochloric acid electrolytes with different currents on the electrochemical machining characteristics of stainless steel 304, including material removal rate, side gap and surface roughness, has been investigated. The results showed that the formation of passive layer during the machining with sodium nitrate electrolyte reduces the material removal rate and side gap compared with sodium chloride and potassium chloride electrolytes. According to the experimental results the surface roughness in the sodium chloride and potassium chloride electrolytes is decreased by increasing the machining current, but increases in the sodium nitrate electrolyte. Also, the material removal rate is slightly increased and side gap increases with sodium chloride, sodium nitrate and potassium chloride when combined with hydrochloric acid. On the other hand, the surface roughness is reduced in combined sodium chloride and potassium chloride electrolytes, but increases in the combined sodium nitrate electrolyte.

In this paper, breakup of liquid jet is simulated using smoothed particle hydrodynamics (SPH) which is a meshless Lagrangian numerical method. For this aim, flow governing equations are discretized based on SPH method. In this paper, SPHysics open source code has been utilized for numerical solutions. Therefore, the mentioned code has been developed by adding the surface tension effects. The proposed method is then validated using dam break with obstacle problem. Finally, simulation of two dimensional liquid jet flow is carried out and its breakup behavior considering one-phase flow is investigated. Length of liquid breakup in Rayleigh regime is calculated for various flow conditions such as different Reynolds and Weber numbers and the results are validated by an experimental correlation. The whole numerical solutions are accomplished for both Wendland and cubic spline kernel functions and Wendland kernel function gave more accurate results. The results are compared to MPS method for inviscid liquid as well. The accomplished modeling showed that smoothed particle hydrodynamics (SPH) is an efficient method for simulation of liquid jet breakup phenomena.

In the present paper, a numerical study is performed to analyze the geometrical effects and operational parameters of viscous micropump based on the Entropy Generation Minimization by Lattice Boltzmann Method. Simultaneous examinations of the geometrical parameter L, and operational parameter ΔP^*, showed that in all applied ΔP^*, two ranges of 1.2<L<1.6 and 4.4<L<4.8 based on the entropy generation minimization and two ranges of 1.1<L<1.6 and 4.4<L<4.9 according to the minimum required power of rotors are identified as the optimum geometrical length scales. Due to the complete overlap of optimum ranges of the EGM minimization with that of the minimum power of rotors, the range associated with the EGM viewpoint is selected as the optimal range. Results of the effect of change in the geometric parameter L and operational parameters Re showed that in all considered Re, two ranges of 1.1<L<1.5 and 4.5<L<4.9 according to the EGM viewpoint and two ranges of 1.2<L<1.6 and 4.4<L<4.8 based on the minimum power of rotors viewpoint are introduced as the optimum ranges. Therefore, the common range of both viewpoints, namely 1.2<L<1.5 and 4.5<L<4.8 can be selected as the most optimal range. Regarding the effect of the geometrical variations of the parameter ε with operational parameters of Re and ΔP^* it is determined that in all considered Re and ΔP^*, the range of 0.1<ε<0.5 is selected as the optimum range according to the viewpoints of both the EGM and minimum power of rotors.

Static and dynamic stress intensity factors are important parameters in the fracture behavior of the cracked bodies. In the present study the displacement correlation technique (DCT) is presented to calculate static and dynamic stress intensity factors of functionally graded materials (FGMs). The displacement field is obtained using finite element method (FEM) and ABAQUS software. To consider the variation of material properties, a subroutine is prepared in the UMAT subroutine of the software. Eight-node singularity elements are used in the FEM. As ABAQUS software is not able to calculate stress intensity factors of FGMs, a MATLAB code is developed to obtain these factors. By analyzing an example under dynamic load, dynamic fracture behavior of orthotropic FGMs and effect of nonhomogeneity parameter are investigated for two cases of material properties variation directions which are perpendicular to each other. To verify presented method, a center crack in a plate of homogeneous and FGM materials are analyzed under static and dynamic loads, the results are compared with data from the literature. The results show that, if the material properties vary parallel to the crack direction, the mode I dynamic stress intensity factor at the crack tip located in the stiffer part increases with increasing of non-homogeneity parameter, while for variation in the normal direction to the crack, this factor first increases and then decreases.

In this paper, the effects of the addition of an active toe joint on a 2D humanoid robot with heel-off and toe-off motions are studied. To this end, the trajectories of joints and links are designed first. After gait planning, the dynamic model of the humanoid robot in different phases of motion is derived using Kane and Lagrange methods. Then, the veracity of the derived dynamic model is demonstrated by two different methods. The under-study model is in accordance with the features of SURENA III, which is a humanoid robot designed and fabricated at the Center of Advanced Systems and Technologies (CAST) located in University of Tehran. Afterward, the optimization procedure is done by selection of two different goal functions; one of them minimizes the energy consumption and the other maximizes the stability of the robot. At last, the obtained results are presented. According to the results, there is an optimum value for heel-off and toe-off angles in each velocity which minimizes the consumption of energy. The results also show that, the heel-off angle does not have any significant effects on the stability of the robot while increasing the toe-off angle improves the stability of motion. Finally, the effects of mass and length of the toe joint is inspected. These inspections suggest that heavier toe joints cause an increase in both energy consumption and stability of the robot while increasing the length of the toe joint does not have any effects on both goal functions.

In this paper, the effects of temperature and nanoparticles volume fraction on the viscosity of non-Newtonian hybrid nanofluid, containing water and ethylene glycol as a base fluid and multi-walled carbon nanotubes (MWCNTs) and silica (SiO2) as additives, have been investigated experimentally. The measurements have been carried out in temperatures range of 27.5oC-50oC by using a Brookfield DV-I PRIME digital Viscometer for different shear rates. The stable and homogeneous samples, with the solid volume fractions of 0.0625%, 0.25%, 0.5%, 0.75%, 1%, 1.5% and 2%, were prepared by dispersing equal volumes of dry MWCNTs and SiO2 nanoparticles in a specified amount of the binary mixture of water/EG (50:50 %vol.). The measurement results at different shear rates showed that the base fluid possessed Newtonian behavior, while all nanofluid samples exhibit a pseudoplastic rheological behavior with a power law index of less than unity (n<1). Moreover, the consistency index and power law index have been obtained by accurate curve-fitting for all nanofluid samples. The results also revealed that the apparent viscosity generally increases with an increase in the solid volume fraction and decreases with rising temperature.

In this study, the relative humidity of the gases in the PEM fuel cell was changed and its effect on electro-osmotic flow was investigated. By changing the humidity on both sides of the fuel cell and using the water balance equations, the values of the electro-osmotic flow, electro-osmotic coefficient and net drag in different humidity levels were found. Results showed that variations of the electro-osmotic flow changed linearly by anode and cathode humidity to the special humidity and after that little variation was observed. In addition, the results revealed that humidity change at anode had a more desirable effect than the cathode. For example, at 70% anode humidity and 35% cathode humidity with the current of 5A, the value of electro-osmotic flow was obtained as 2.66639E-06 mol/cm2.s, while in the former 35% and the latter 70% with the same current, this value was recorded as 2.56418E-06 mol/cm2.s. In addition, results showed that the variations of the electro-osmotic coefficient changed linearly by humidity. It was determined the current change of fuel cell does not have a good effect on the curves of electro-osmotic coefficient. The electro-osmotic coefficients varied between 0.636001 and 1.632476, which were in good agreement with the values obtained in other related papers. In addition, the variations of the net drag with respect to humidity were investigated, too. It was determined that the net drag changed linearly by the cathode humidity with positive slope, but its variations by the anode humidity were linear with negative slope.

A DC-DC buck converter is an electronic circuit with wide application in power electronics. This converter acts as a nonlinear system, so it is necessary to use a robust controller to control and regulate the output voltage under load changes, circuit elements and other disturbances. In this paper, a new fast terminal sliding mode control (FTSMC) using the property of the terminal attraction as a function of the inverse tangent for buck DC-DC converter is provided. The performance of this new controller is compared with FTSMC common type in terms of output voltage convergence time and input control function structure. The superior property of this controller is its low singular effect on the control function. Also, this controller has fast transient convergence in different situations for output voltage stability. Simulation results confirm the proper performance of the new proposed fast terminal sliding mode control method compared to traditional fast terminal sliding mode converter for DC-DC buck converter.

Due to increasing the heat transfer surface area and providing high capillary pressure with high permeability, porous structures play a key role in improving the performance of two phase heat transfer devices such as heat pipes. New porous structures (bi-porous structures) have two distinct size distribution of pores, of which the small pores provide the capillary pressure required for delivering liquid to the surface and large pores help vapor escape from the surface through increasing its permeability. The main goal is to gain a deeper understanding of the evaporator section of heat pipes and compare the performances of sample biporous structures. Towards this goal, first the Kovalev modeling technique is applied to determine the possibility of each phase’s existence in pores of different sizes throughout the computational domain. One dimensional heat transfer in a bi-porous wick is investigated. Inside the domain the conservation equations are solved for each phase and the results such as heat flux versus wall superheat are presented. Thermo-physical properties of the fluid and the matrix like the fluid properties, phase saturation and permeability and the conduction heat transfer coefficient are calculated from the geometry of the matrix and experimental relationships.

Pure mode II fracture toughness of polymethyl methacrylate and its components has been studied by essential work of fracture (EWF) approach via experimental and numerical methods. EWF fracture tests with double edge notched tension (DENT) were performed on the RT-PMMA specimens at room temperature. In this investigation, the mode II fracture of polymethyl methacrylate/graft-acrylonitrile butadiene styrene (PMMA/g-ABS) blends with different weight percentage of rubber (0, 10, 15, and 20) and the thickness of 0.8 and 4 millimeter samples was investigated. The results showed that the value for the specific essential work of fracture given by including lower ligament length may be more accurate because the ligament is completely yielded. The results also showed that for the loaddisplacement curves that have self-similarity in shape for the specimens with different rubber content, specimen thicknesses, and ligament lengths and the specific work of fracture (wf) increases significantly with the increasing of rubber content. The essential work (we) and the non-essential work (bwp) of fracture increase with the increased rubber for both thicknesses. The highest value of the specific essential work and the specific non-essential work of fracture belong to 20% composition in 0.8 mm specimen thickness 122.19 N/mm and 5.54 N/mm2 and in 4 mm specimen thickness 51.231 N/mm and 10.258 N/mm2, respectively. Significant changes were observed in the amount of essential and nonessential work of fracture when the thickness of the samples was changed.

In recent years, using the renewable energy resources has attracted the attention of researchers and automobile companies, because of limited fossil fuel resources, low efficiency of internal combustion engines and their contribution to environmental pollution. By using the fuel cell systems instead of internal combustion engines overcome these problems can be partially overcome. In this regard, the present article examines a PEM fuel cell system for use in an urban vehicle. In the first part of this article, by using the real component of system, the fuel cell system components including stack, membrane humidity of air and hydrogen, air compressor, water pump and pump cooler stack have been modeled in MATLAB Simulink environment. The mentioned model can evaluate the power consumption of system and its auxiliary components and also the required water, hydrogen and air for system. At the base case and the current density of 0.7A / cm2, 14% of power productions of stack are consumed by auxiliaries units. At this current density, the overall and net system efficiencies are 48.15% and 34.3%. Also, by increasing the air stoichiometric coefficient due to increased compressor power consumption, there is not a significant increase in output voltage. In the second part of this article, the system from the point of view of the first law of thermodynamics has been optimized with objective functions of maximum output power and maximum efficiency. The results indicate that first, the model search method is the best method for optimization, second, the optimization with the aim of maximum power, pure power and system efficiency are increased by 11.9% and 4% respectively and the power consumption by auxiliary unit is reduced about 42%.

Linear and angular displacement measuring encoders are the most important measuring tools in the industry. Linear encoders are widely used in various positioning applications, such as numerical controlled (NC) machine tools and factory automation, since they are essential for precision positioning systems. In this study, a capacitive-type linear encoder with un-tethered slider is designed. The main components are made of printed circuit films. Hence, the encoder can be set up in thin inter spaces or on curved surfaces. The encoder consists of a long receiver film and a short transmitter film, respectively containing four-phase and two-phase electrodes. The transmitter is used as a slider and the receiver as a stator. In order to designs an unconstrained slider; the encoder employs a unique approach. Electrical power is supplied to the transmitter film by electrostatic induction which removes electric wires from the slider. In this study the encoder was built using a new signal processing circuit and its performance was evaluated. The new signal processing circuit is more compact and facilitates using this encoder for trade purposes. The result of experimental evaluation shows that the encoder has ±20 micrometers error.

In this paper, the fracture trajectory in blunt V-notched specimens under mixed mode loading is investigated by using two numerical approaches: 1) the extended finite element method (XFEM), 2) the incremental method. The first approach is an extended form of the finite element method in which the fracture takes place and grows according to the cohesive zone model. The second one is also an increment approach in which the fracture initiation angle for notched specimen is first determined according to the concept of the maximum tangential stress (MTS) criterion. Afterward, a small crack is added to the notched specimen along the fracture direction and then a new cracked specimen is generated. In the next step, the fracture initiation angle is calculated from MTS criterion and another small crack is again added to the cracked specimen. These steps are continued until the crack reaches to the back boundary. To evaluate these methods, the fracture paths of the rounded-tip V-notched Brazilian disk (RV-BD) specimens under mixed mode loading are predicted by both XFEM and incremental method. It is shown that the incremental method can provide estimates more accurately than XFEM for the fracture initiation angle of the notched samples. It is also demonstrated that both methods can predict the fracture trajectory in good agreement with the experimental results.

In present study, heat transfer in double-tube heat exchanger filled with metal porous material has been investigated. In contrast to most of the previous studies, fluid flow is considered turbulent in heat exchanger which is in good agreement with the practical performance of these exchangers in the industry. Fluid flow and heat transfer equations have been discretized on a collocated grid by means of finite volume method with simple algorithm. Discretized equations are solved with a numerical program in FORTRAN language in order to study the effect of porous material parameters and Reynolds of fluid flow on the heat transfer in double-tube heat exchanger. According to the results and analysis of porosity in the range of 0.8 to 0.95 as well as pore diameter of 1 mm up to 6 mm and diverse types of porous material, it is observed that the decrease in porosity, the increase in pore diameter and the use of copper porous material (with high heat conduction coefficient), increase heat transfer. In the best case, overall heat transfer coefficient increases up to 7 times. Moreover, the results reveal that the heat transfer enhancement ratio has no distinct difference with changing Reynolds number of turbulent flow in the range of 10000 to 80000. Performance evaluation criteria, which investigate the effects of pump lost power and thermal power, shows that with using porous material the value of the pump lost power is of major importance, and can be decreased by increasing the porous pore diameter.

Due to the low formability of aluminum alloys at ambient temperature, forming of these alloys is performed at high temperature. Research has shown that the results of simple tensile test to predict the materials behavior at high temperatures are not sufficiently accurate to predict the formability of aluminum tubes at high temperature. The mechanical properties of the tube are very important at high temperatures. In this study the formability of 6063 aluminum alloy tubes are investigated by free bulging test at temperature range 430oC to 600oC. Then the mechanical properties including flow stress, strain rate sensitivity coefficient and strength constant are obtained using tube multi-bulge test at temperature range 530oC to 580oC. For this purpose, hot metal tube gas forming process is used and the effect of process parameters such as the effect of temperature, pressure and time on the expansion ratio and height of the bulge are studied. The results show that the maximum expansion ratio is 58% at 580oC. Bursting pressure decreases from 1.9MPa to 0.6MPa with temperature increasing from 430oC to 600oC. The bulge height increases with increasing forming time at constant pressure. Also, with increasing temperature in the temperature range 530oC to 580oC the flow stress and strength constant decrease and strain rate sensitivity coefficient increases.

In this research, the influence of adding carbon nanotubes on the tensile and the mode I interlaminar fracture of glass-fiber-epoxy laminated composite has been experimentally studied. For this purpose, the hybrid glass-fiber-epoxy-nanotube laminated composites which have 18 fiber-glass plain-weave layers were manufactured by hand lay-up method. The epoxy resin system is made of Epon828 resin with Epikure F205 as the curing agent. The multi-walled carbon nanotube (MWCNTs) modified with hydroxide (-COOH) is also dispersed into the epoxy system as a reinforcement in a 0%, 0.1%, 0.5% and 1% ratio in weight with respect to the matrix. In addition, the tensile nano-resin and hybrid nanocomposite specimen were produced. The results of the tensile test of nano-matrixes indicate that the maximum change in Young's modulus, ultimate strength and fracture toughness of the samples is in the 0.5% sample, with a 31.2%, 21.4% and 16.66% increase with respect to neat sample, respectively. Moreover, the results of the tensile test of hybrid nano-composites indicate that the maximum change in fracture toughness, ultimate strength and fracture strain and of the samples is in the 0.5% sample, with a 12.6%, 9.8% and 12.6% increase with respect to neat sample, respectively. The result of the mode I interlaminar fracture toughness test of hybrid nano-composites shows that the maximum change in value of the force (in force-displacement diagram) and value of the energy (of crack propagation in mode I interlaminar fracture), is in 0.5% sample, with a 24.4% and 24.15% increase respect to neat sample, respectively.

Research on microstructure of main engineering materials revealed that some of these materials exhibit similar microstructure patterns at different length scales. Since these patterns are replicated at different length scales the whole microstructure can be viewed as a set of periodic substructures. Homogenization technique for periodic microstructures has found many applications in simulation of composite materials by considering the geometry of fibers distribution. In this study a homogenization technique for periodic microstructures is developed. In this generalization a multi-step homogenization is being used. In each step of homogenization the geometry which is coincident with the true microstructure is produced to maintain the properties of the mechanical properties of the related cell. By using the presented method, effect of size and grading of each of the reinforcing phases and the interaction between fibers is taken into account. The results of the presented theory are compared with the existing experimental data on the particle reinforced composites. Good agreement between the presented theory and experimental data is found.

The purpose of this paper is to introduce a design and fabrication procedure for a solid propellant gas generator. Based on this procedure a gas generator was designed to supply the required operating fluid of a controllable flying object’s control surface gaseous actuator, the results of which are presented in this paper as well. Supplying required pressure during the mission and gas flow rate with expected chemical characteristics are requirements of the design. At first the necessary parameters like flow rate and pressure were specified. Then the design calculations were done according to the proposed approach. In order to evaluate the design process and achieved data, a full scale gas generator set was built. Since this study includes specifying a proper formula for solid propellant of gas generator, a lab scale motor was used to qualify the propellant’s characteristics experimentally. After doing tests and comparing the results with output data of gas generator design procedure, good agreement was observed. Besides, by doing several tests it was found that PSAN as oxidizer, HTPB as binder and chromium oxide as catalyst provide a proper composition for solid propellant of gas generator. Finally, covering basic requirements of a solid propellant gas generator such as uniform flow rate and less presence of solid phase and corrosive components in combustion products by means of designed gas generator are suitable to show the validity of presented design method.

Owing to direct contact with the machined surface, the flank surface can cause unfavorable effects on the surface integrity in high speed milling. Thus, in this study, the influences of flank wear width on the main characteristics of surface integrity like roughness, topography, microhardness and electrochemical corrosion resistance during high speed milling process is investigated. Milling tests were performed under constant cutting conditions with three repetitions and using 12 tools with flank wear widths on the AISI 4340 hardened steel. It was concluded that using the tool with flank wear width up to 0.4 mm increases roughness and microhardness, uniformly (95% for surface roughness and 6.3% for microhardness relative to new tool). However, using a tool with the flank wear of 0.6 mm increases these outputs up to 484% and 18.6%, respectively. Surface topography images also revealed that using the tool with the flank wear width of 0.6 mm can cause irregular forms of material flow on the surface. Using the tool with the flank wear of 0.4 mm or less had an insufficient effect on the in-depth microhardness distribution. In addition, electrochemical impedance spectroscopy of the milled surfaces showed that relative to new tool, using tools with 0.4 and 0.6 mm flank wear, reduces Rcorr up to 22% and 83%, respectively. It indicated lower electrochemical corrosion resistance of milled surfaces with 0.6 mm worn-out tools.

In this paper a mathematical model of pulsatile, unsteady and non-Newtonian blood flow through elastic tapered artery with overlapping stenosis is proposed. The blood flow has been assumed to be non-linear, fully developed, laminar, axisymmetric, two-dimensional. The non-Newtonian model chosen is characterized by Sisko model to describe the rheology of blood. The artery has been assumed to be elastic and time-dependent stenosis is considered. Because blood flow depends on the pumping action of the heart, the blood flow has been assumed pulsate. The stenosed artery is changed to a rectangular and rigid artery using a radial coordinate transformation on the continuity and the nonlinear momentum equations, and boundary conditions. The discretization of the continuity and the non-linear momentum equations and boundary conditions is obtained by finite difference scheme. The radial and axial velocity profiles are obtained and the blood flow characteristics such as resistive impedances and volumetric flow rate and the severity of the stenosis are discussed. The volumetric flow rate is minimum in the case of converging tapered arteries and the resistive impedance is maximum in the case of converging tapered arteries by effect of tapering angle.

This study presents a numerical investigation for laminar mixed convection flow of radiating gases in an inclined lid-driven cavity. The fluid is treated as a gray, absorbing, emitting and scattering medium. The governing differential equations consisting of the continuity, momentum and energy are solved numerically by the computational fluid dynamics (CFD) techniques to obtain the velocity and temperature fields. Discretized forms of these equations are obtained by the finite volume method and solved using the SIMPLE algorithm. Since the gas is considered as a radiating medium, besides convection and conduction, radiative heat transfer also takes place in the gas flow. For computation of the radiative term in the gas energy equation, the radiative transfer equation (RTE) is solved numerically by the discrete ordinate method (DOM). The effect of lid driven speed on the thermohydrodynamic behavior of two-dimensional cavity is carried out. Results are shown as contours of isotherms, streamlines and distributions of convective and total Nusselt numbers along the bottom wall of cavity. It is revealed that increase in Reynolds number causes almost uniform temperature distribution in cavity, especially for 30o and 60o inclination angles.

One way to control the position of a missile is to control thrust vector which moves with help of thrust due to exiting gas. All thrust vector control (TVC) methods are independent of aerodynamic forces of atmosphere and until the engine has thrust, maintain their performance. Secondary injection systems are one of the four major TVC methods. In this study, first the components are identified and conceptual design of system is drawn and the preliminary design of manifold of a type of thrust vector control system using a liquid injection thrust vector control (LITVC) is determined. Then the layouts of components on some parts such as injectors and reservoirs, as well as detailed design of the system are discussed. The numerical simulation of flow and the design and study of the sprayers in LITVC systems will be discussed. Also, numerical designing and simulation in two parts: injection system and the injector spray effects into the main flow are analyzed and the results are presented and validated. The results of this study can be used as a model for the design and analysis of various kinds of TVC systems with lateral fluid method on a variety of missiles with different launchers.

Aortic Valve simulation remains a controversial topic as a result of its complex anatomical structure and mechanical characteristics such as material properties and time-dependent loading conditions. This study aims to integrate physiologically important features into a realistic structural simulation of the aortic valve. A finite element model of the natural human aortic valve was developed considering Linear Elastic and Hyperelastic material properties for the leaflets and aortic tissues and starting from the unpressurized geometry. It has been observed that although similar stress-strain patterns were generated on Aortic Valve for both material properties, the hyperelastic nature of valve tissue can distribute stress smoothly and with lower strain during the cardiac cycle. The deformation of the aortic root can play a prominent role as its compliance changed significantly throughout cardiac cycle. Furthermore, dynamics of the leaflets can reduce stresses by affecting geometries. The highest values of stress occurred along the leaflet attachment line and near the commissure during diastole. The effects of high +G acceleration on the performance of valve, valve opening and closing characteristics, and equivalent Von Mises stress and strain distribution are also investigated.

Although it seems that two categories of robotic systems, the dynamic object manipulation and dynamic biped walking systems, are completely different in how they are dealt with at the first glance, we believe that there exists a strong relationship between these two types of robotic systems. This paper studies the correlation of the dynamic biped walking and dynamic object manipulation in all areas (passive, underactuated and fully-actuated classes). In this regard, the virtual problem of "the amputee walking robot" is defined to describe the relationship between these two kinds of robotic systems. From this viewpoint, the problem of dynamic biped walking is a special case of dynamic manipulation of multibody objects in which the multibody object has a structure similar to the structure of biped robots. In other words, we can say that the ground manipulates the biped robot which is considered as a multibody object. Then, the concept of correlation is investigated in three different classes of the problem: passive, underactuated, and fully-actuated systems. For each of three categories, appropriate examples are studied. It will be shown that the details of the walking problem could be extracted from the mother problem of dynamic manipulation of multibody objects in all its aspects.

Thermal Barrier Coatings (TBC) are used as thermal protective for components operating at high temperature. These coatings normally have three layers of: ceramic top coat, thermally grown oxide and the bond coat layer. Due to the manufacturing process and the special structure of a TBC system, failure mechanisms of these coatings are affected by both thermal and mechanical loads introduced to the coated part. In this paper delaminations of TBC layers subjected to the cyclic thermal loads have been numerically and experimentally studied. A rectangular specimen made of Inconel 617 coated by air plasma spray (APS) method was loaded in a specific design test setup with capability of simultaneously applying a constant four point bending load and thermal cyclic loadings. The thermal cycles were carried out until the coating was spalled out. The temperature of the specimen surfaces was measured and recorded during the test. Finite elements modeling was also performed using ABAQUS to simulate the the experiment. A cohesive zone model was used to simulate the top coat delamination. Finally, using a trial and error approach on the cohesive properties, finite elements results have been adapted on the experimental results and interfacial cohesive properties have been estimated.

In this paper, the internal inversion process of metallic cylindrical shells under dynamic axial loading is investigated experimentally and numerically. Experimental tests are performed on the steel tubes in a gas gun and the required force for internal inversion is obtained using the measurement system of impact loadings. Also, numerical analysis is carried out by the finite element software ABAQUS and the accuracy of simulated models is validated with the experimental results. In this paper, all geometrical properties of the tubes and die are assumed to be constant and the effect of the projectile mass and velocity is investigated on the shortening and energy absorption of the tubes which are affected by axial impact in the internal inversion process. Therefore the projectile is shot directly to the specimen with different masses and velocities. It is observed that if the projectile mass remains constant, increased impact velocity has almost no effect on the constant inversion load and just increases the tube displacement but if the impact velocity remains constant, increasing the amount of projectile mass causes increase in the constant inversion load besides increased tube displacement. Comparing the results of numerical simulations with the experimental results shows a good agreement between them.

Ultrasound test is a widely used non-destructive method for determining the mechanical and metallurgical properties of materials. In this method, ultrasonic wave velocity or attenuation coefficient is measured and measurement accuracy is very important. In this paper, variations of longitudinal wave velocity are studied in the presence of a thermal gradient both theoretically and numerically using a 2D model. A linear temperature distribution is assumed and the length of the work piece and the temperature of the hot side are considered as varying parameters. A new 2D theoretical model is developed for this problem. The test piece is made of st37 steel. To evaluate the proposed equation, we assume constant temperatures and the length of the work piece are varied in the range of 0.05-0.1 m. Then, we study the effect of the temperature of the hot side from 398-998 K. By ANSYS software, a novel two-dimensional finite element model (FEM) is developed in axisymmetric state for this problem. The results of the theoretical model are compared with those obtained from the numerical model and very good agreement is observed.

In this research, an intelligent method is introduced for prediction of remaining useful life of an internal combustion engine timing belt based on its vibrational signals. For this goal, an accelerated durability test for timing belt was designed and performed based on high temperature and high pre tension. Then, the durability test was began and vibration signals of timing belt were captures using a vibrational displacement meter laser device. Three feature functions, namely, Energy, Standard deviation and kurtosis were extracted from vibration signals of timing belt in healthy and faulty conditions and timing belt failure threshold was determined. The Artificial Neural Network (ANN) was used for predicting and monitoring vibrational behavior of timing belt. Finally, the ANN based on Energy, Standard deviation and kurtosis features of vibration signals could predict timing belt remaining useful life with accuracy of 98%, 98% and 97%, respectively. The correlation factor (R2) of vibration time series prediction by ANN and based on Energy, Standard deviation and kurtosis features of vibration signals were determined as 0.87, 0.91 and 87, respectively. Also, Root Mean Square Error (RMSE) of ANN based on Energy, Standard deviation and kurtosis features of vibration signals was calculated as 3.6%, 5.4% and 5.6%, respectively.

One important problem investigated in reverse engineering (RE) field is finding the best surface to approximate point cloud data. Swept surface is a surface type that in addition to various applications in CAD/CAM software, satisfies the whole standards required for use in RE software. The most important problem in utilization of swept surfaces for RE purposes is finding the areas belonging to it out of point cloud data. Through an algorithm presented in this paper, a method has been introduced to find these areas automatically. Currently, this process is performed by user intervention. In this paper, using kinematic surface formulation and slippable motion concept, a general method to find swept surfaces with any arbitrary central curve and profile is introduced. To this end, point cloud data are processed regarding slippable motion criterion using iterative segmentation algorithm, then by presenting an effective algorithm and employing the concept of hierarchical classification and drawing the dual graph, swept-surface-related areas are found. The introduced method is implemented in several models with different conditions for validation. It is observed that the results have good agreement with real model condition, showing the efficiency of this method in finding the swept surface.

In this study, optimum design of composite sandwich structures will be surveyed and presented using hybrid algorithm. Since, most modern payload fairings are constructed of a composite sandwich laminate, in this research the architecture of the fairing structure has been analyzed on the basis of the composite sandwich shell with a flexible core. However, from the geometrical point of view, fairing composed of two conical and cylindrical parts. Therefore, in the first phase, buckling analysis of conical composite sandwich shell has been done by using high-order theories and the obtained equations reduce to the governing equations of cylindrical sandwich shell when the semi-cone angle is set equal to zero. In the second phase, the obtained structure was optimized using hybrid algorithm. Due to the variety and complexity of design variables in composite sandwich structures, engineering design process leads to difficulties and obstacles in design optimization problems. Since, the most important selected discipline for improvement the mass specifications of launch vehicle is structure, therefore, relying on structural optimization, after optimization process, finally considerable mass reduction i.e. 40 percent comparing to the utilized fairing in this study (Fairing of Safir), will be concluded due to simultaneous changing of material and optimization.

In this study evaporation of biodiesel droplet under different operating conditions is investigated. The model is a common droplet vaporization model for multicomponent fuels. In this model, gas phase quasi-steady equations are solved analytically and energy and species transport equations in liquid phase are solved numerically. The sub-models are modified to consider high pressure effects. Peng-Robinson equation of state is used for gas phase and phase equilibrium is determined using fugacity. Effects of pressure on the thermophysical and transport properties of gas phase are considered. Five biodiesels with different composition are studied. These biodiesels have different composition of methyl esters. Biodiesel composition shows little effect on droplet lifetime and maximum difference is about 20%. It is observed that increasing ambient temperature leads to decrease in droplet lifetime and increases temperature gradient inside droplet. Ambient pressure has different effects on droplet vaporization behavior at different ambient temperature. At lower temperature environment, increasing pressure increases the droplet lifetime while at higher temperatures droplet lifetime first increases and then decreases with pressure. Increasing initial velocity of droplet reduces the droplet lifetime. Results show that at high pressures, droplet temperature reach to values near to critical temperature and accuracy of quasi-steady approximation decreases. Radius of vapor influenced sphere increases with temperature and decreases with pressure.

Time optimal trajectory planning of closed chain mechanisms has not been done by indirect method yet. In this paper, this problem is considered for a four bar mechanism and its solution is presented on the basis of the indirect solution of optimal control problem. To this end, the additional coordinates are omitted using the holonomic constraints, so the dynamic equation is obtained with respect to only one generalized coordinate. Then the necessary conditions for optimality are derived using Pontryagin's minimum principle by considering the constraint on the applied torque. The obtained equations lead to a two-point boundary value problem (BVP) the solution of which is the optimum answer. Unlike the direct methods that result in approximate solution, indirect method leads to an exact solution. But the main challenge in indirect method is solving the BVP. Solving this problem is sensitive to the initial guess. This problem is much more severe for time optimal problem which has a high nonlinear answer in bang-bang form. To overcome this problem an algorithm is proposed to solve the time optimal problem with any desired accuracy, and the initial solution can simply be zero at the start of the algorithm. But in the time optimal trajectory the large jerk occurs, due to control signals switching. In order to overcome this problem, another algorithm is presented to calculate the minimum time with bounded jerk. Finally, the simulation results show the performance of the proposed method in time optimal trajectory planning.

In this study, the method of molecular dynamics simulation is performed to investigate the shockwave propagation in a solid. The simulation cell contains 51840 atoms at 5 K interacting by means of a pairwise potential. The shockwave is generated using the motion of a piston with different velocities in the solid and the resulted shockwave velocity is in good agreement with the experimental data and the Hugoniot curve. The piston hit the sample from one side of the simulation box, at speeds ranging from 1.2 to 1.3 times the speed of sound in solid argon at the chosen density. Some thermodynamics properties such as density, temperature and pressure are measured during propagation of shockwave. It is found that those thermodynamics properties (density, temperature and pressure) remarkably and significantly increase when the shockwave passed through the solid. We also show that creating initial strain in the solid up to 6.5% can enhance the pressure increment in the solid up to 9%. The results can be useful in enhancing the shockwave power by giving a detailed microscopic description of the process.

In this article, the linear quadratic regulator method (LQR) for voltage control of a linear time-varying model of a robot is used to design an online adaptive optimal stable controller to trace the robot arm path. Normally, offline solving of Riccati differential equations in backward with final conditions for linear time-varying system or converting the Riccati differential equation to an algebraic one in linear time-invariant system is inevitable in LQR. However, in this paper, the differential Riccati equations are considered as the adaptation laws along with a voltage control strategy to be solved online in forward method with initial conditions. Choosing a proper Lyapunov function guarantees the asymptotic stability of the tracking. Furthermore, parametric model uncertainties such as mass parameter variation and external disturbances which affect the dynamics of the model are also taken into account. Simulation results show the energy used by dc motors of the voltage optimal control strategy is less than that of the torque control strategy and as good as the classical PID one. The superior performance of the voltage optimal control over torque control strategy is also shown in presence of disturbance.

In this paper, a combined cooling, heating and power system for using heat losses in PEM fuel cell has been proposed that can be used for residential application. This system consists of PEM fuel cell, heat storage tank, absorption chiller, hydrogen tank, air compressor and pump. Heat generated in fuel cell has been absorbed by a working fluid and a part of heat has been given to absorption chiller and another part to heat storage tank. Modeling of this system has been done from four energy, exergy, FESR and CDER perspective. Fuel cell of this CCHP system generates 38.63 kW electrical power and 39.17 kW heat power. Energy efficiency of fuel cell singly is 37.21% but when heat storage tank and absorption chiller have been used for recovering waste heat, energy efficiency reaches to 68%. Maximum irreversibility loss occurs in fuel cell which is calculated 47.21 kW and absorption chiller irreversibility has been calculated 5.94 kW. From the viewpoint of FESR and CDER in comparison with conventional systems, FESR and CDER are 34% and 25% respectively. Also, analysis has shown that with increasing fuel cell operating pressure energy and exergy efficiency increased.

steel. 16MnCr5 alloy steels have good wear resistance. For this purpose, pulse current and pulse time have been considered as variables in the process. The selected EDM parameters were pulsed current (8, 16 and 32A) and pulse time (25, 100 and 400μs). Tests were conducted in full factorial mode and the R. R. Moore fatigue test machine was used to determine the fatigue life of components. The results show that by increasing the spark current and pulse duration 16MnCr5 alloy steel fatigue life is reduced. Respectively, the greatest resistance to fatigue was achieved at current of 8A and pulse time of 25 microseconds and lowest resistance to fatigue achieved at pulse current of 32A and pulse time of 400 microseconds. Resistance to fatigue crack depends on cracks density on the surface of the workpiece and heat-affected zone, where the density of cracks increase resistance to fatigue will be reduced. Also, in the specimens that have low resistance to fatigue, fatigue cracks are initiated from multiple points of the cross-section. It seems the reason for this phenomenon is the high surface roughness in the samples. EDM machining with high energy sparks can decrease the fatigue strength of 16MnCr5 by as much as factors of 3-5.