Modares Mechanical Engineering
Year:2016 | Volume:16 | Issue:6|Papers Count:45

Accumulative press bonding (APB) is a novel variant of severe plastic deformation processes (SPD), which has been devised to produce materials with ultra-fine grain (UFG). In the present work, effect of APB technique on mechanical properties and microstructure of AA1100 alloy were investigated. The study of the microstructure of AA1100 alloy was performed via optical microscopy. This article revealed that the grain size of the produced samples decreased to 950 nm, after six passes of APB process. The yield strength of AA1100 alloy after six passes of the process increased up to 264 MPa, which is three times higher than that of the as-cast material (89 MPa). After six passes, microhardness values of AA1100 alloy increased from 38 to 61 HV. Furthermore, the results showed that the behavior of variations in mechanical properties is in accordance with the microstructural changes and it can be justified by using the Hall-Patch equation. Moreover, the rise in the yield strength can be attributed to the reduction of the grain size and strain hardening phenomenon.

Welding is one of the most popular methods of connection that is used across a variety of industries to join together materials. Nondestructive testing methods are commonly used to verify that welds are free of defects. Some limitations to common NDT techniques restrict their use. The new damage detection techniques are in demand. This paper presents a study on the new inspection method. The advantages of this method are cost and time effectiveness. This study was conducted to investigate the ability of Inner product vector (IPV) method to detect Lack of Root Penetration (LOP) and Lack of side-wall fusion (LOSWF) on 304 stainless steel beam. The IPV method was proposed as a damage detection algorithm which uses cross correlation functions between vibration responses under white noise excitation. The experimental method was the only method of previous research on the IPV method. This will be achieved by the use of finite element modeling combined with a modal dynamic analysis based vibration technique and MATLAB software is used to numerically implement the computational procedure. In this study, an ideal welding is intended and the effect of the heat-affected zone (HAZ) on the results is ignored. In order to verify the validity of the IPV method simulation, we referred to the results of previous experimental research. The results obtained from modeling are compared with experimental results which showed good agreement.

In this paper, a natural cooling system composed of domed roof and solar adsorption chiller is presented and its performance to provide the thermal comfort conditions in Yazd, Kerman, and Tehran is investigated based on ISIRI 14384 and ISO 7730. Furthermore, the effects of environmental parameters, wind speed, and geometric characteristics on the system performance are studied. To calculate the number of air changes of the room, Ansys Fluent software is used. Additionally, to estimate the room inlet temperature, the governing equations of the adsorption chiller and cooling channel are solved based on the fourth order Runge-Kutta and finite difference methods, respectively. The results show that increasing the incision diameter of the domed roof as well as the width of the cooling channel causes the number of air changes of the room to increase. Alternatively, increasing the width of the inlet air vent up to a threshold value will cause the number of air changes to increase. However, increasing beyond the threshold value has no significant effect on the number of air changes. Additionally, in the aforementioned cities, the room inlet air temperature is almost constant when the chiller operates. Finally, the environmental conditions for which the system is able to provide thermal comfort conditions, in the test room on July 15, are determined.

In this study, a simple and efficient closed form solution for bending and stress analysis of functionally graded circular and annular plates with elastic boundary conditions is presented based on the first order shear deformation theory (FSDT). By using the presented solution procedure, functionally graded plates subjected to arbitrary non-uniformly distributed normal and shear loads may be analyzed and all of the stresses components can be accurately achieved. Shear loads may be imposed on the top and bottom surfaces of plate. By using the constitutive equations based on the first-order shear deformation theory, the transverse shear stress components cannot be obtained correctly and constant through-the-thickness distributions will be extracted. So, to achieve the transverse normal and shear stress components in the proposed solution procedure, the three dimensional theory of elasticity is applied. To establish the accuracy and efficiency of the proposed approach, the obtained results are compared with other available published results and results of the three-dimensional theory of elasticity extracted from the ABAQUS software based on the finite element method (as the most exact method). Comparisons show that the obtained results are very accurate, while it is computationally much more economic than the three-dimensional elasticity approach. Also, transverse normal and shear stresses boundary conditions on the top and bottom surfaces of the plate are exactly satisfied, even for a complicated loading, when the non-uniform normal and shear loads are imposed simultaneously on the top and bottom surfaces of plate and transverse stresses boundary conditions on these surfaces are non-zero.

Rotary forging is a forming method in which the forces exerted on the work piece can be reduced by using an inclined forming tool. The final shape of work piece is formed gradually. The conventional machines used in this process typically have separate rotational and linear (feeding) motions. The rotational motion is applied by an eccentric mechanism; the feeding motion is exerted with a linear actuator. These machines follow forward kinematics which does not consider the geometry of the work piece to create motion profiles. Hence, having a special profile to be fully compatible with the piece is not possible. Such compatibility is beneficial to applying a more precise control on the material flow and achieving sound forgings. In this study, the feasibility of performing the rotary forging process on a hexapod table has been investigated. A hexapod machine available to the authors has been employed for this purpose. The hexapod table with six degrees of freedom is responsible for all shaping motions. This device can be used to produce different motion profiles for complex work pieces. The appropriate profiles are obtained through the inverse kinematics. The maximum force being applied on the hexapod actuators was calculated. Two circular and linear profiles were examined to practically shape cylindrical work pieces, and forming load was compared with conventional forging for producing lead cylindrical work pieces. Obtained results show that the linear profile is more desirable than the circular profile in terms of force analysis, and required force in orbital forging is far lower comparing to conventional forging.

Sandwich structures consist of two thin skins with high mechanical properties and a thick core with lower mechanical properties and weight. Due to high strength/stiffness to weight ratio, these structures are used extensively in engineering structures such as aerospace structures, ship hulls, turbines blades, etc. Skin/core debonding is one of the major failure modes in these structures. In this paper, debonding resistance of sandwich panels with composite skins and a core consisting of PVC foam and a corrugated composite laminate is investigated both experimentally and numerically. Square geometry is considered for corrugated composite laminate and obtained results are compared with reference specimen with simple core made of PVC foam. The three point bend test with attached ENS fixture is used to perform the standard experimental test. The results have shown that in square specimen with 3 and 6 layer skin before the separation between skin/core, the specimens fail from the middle of the upper skin, but for 8 layer skin, the skin/core debonding occurs before other modes of failure. The maximum skin/core debonding resistance for square specimen is increased 269.26 percent. Specimens are modeled in Abaqus and results show a reasonable agreement between experimental and numerical result.

Molecular Dynamics (MD) method is a computer simulation for studying the physical movements of atoms and molecules of an N-body system by solving classical equations of motion. Here, this method is used to investigate the structural changes of a vital molecular bond in the body. This bond is created by the interaction of P-selectin, expressed on activated endothelium, and its counterpart P-selectin glycoprotein ligand-1 (PSGL-1) which is expressed on leukocytes. Frequent association and dissociation of these bonds allow the leukocyte to roll on the endothelium layer which is a pivotal step in inflammatory responses. Understanding the mechanism underlying the dissociation process of this bond is helpful in pathological researches. Here this process is simulated with MD method using the program NAMD and Visual Molecular Dynamics (VMD). The results indicate that the hydrogen bonds between ion Ca2+ and residue fucose of glycan group of PSGL-1 and also between sulfated tyrosine residues are the most effective bonds in binding.

The cement paste properties as the matrix in concrete are of great importance in specifying its properties. Recent advances in concrete industry have highlighted the need for accurate knowledge about its nano-structure and components. To estimate the important mechanical properties such as Young's modulus, bulk modulus and Poisson’s ratio of the hardened cement paste at the nanoscale, a comprehensive set of crystalline structures that represent the main hardened cement paste constituents (CSH, CH, Ettringite, Hydrogarnet and monosulphoaluminate) is developed for Molecular Dynamics (MD) simulations. Five different force fields (COMPASS, COMPASS II, INTERFACE, Universal and Deriding) were used and compared with each other to be able to measure the mechanical properties of these compounds. Also, the properties of two types of C-S-H with high and low density were determined by using Mori-Tanaka method. Lastly, simulation results reported by the authors were compared with existing computational and experimental values. The results show that using molecular dynamics method was suitable in estimating mechanical properties of hardened cement paste. These findings might be applied on larger scales and also multi-scale simulations.

In this article, design and hardware development of a knee exoskeleton robot is discussed. The robot aims to help the individuals with lower extremity weakness or disability during the sit-to-stand movement. In the trajectory generation phase, a new method is proposed which uses a library of sample trajectories to predict the sit-to-stand movement trajectory based on the initial sitting conditions of the user. This method utilizes the theory of "dynamic movement primitives" to estimate the sit-to-stand trajectory. The trajectory generation method is tested on a library of human motion data which has been obtained in a laboratory of motion analysis. In the next step, an exponential sliding mode controller is used to guide the robot along the predicted trajectory. The controller and the trajectory generator are implemented on the exoskeleton robot. For the hardware development, the xPC Target toolbox of MATLAB software and a data acquisition card was used. Finally, the robot was tested on a male adult. The subjects were asked to wear the robot while doing several sit-to-stand movements from various sitting positions. According to the results, the average power which is required to be applied by the user’s knee is less when the exoskeleton assists him.

Friction stir welded butt joints were performed on sheets made of AA2024-T351 aluminum alloy at tool rotational speeds of 400, 630, 800 rpm and traverse speeds of 8, 16, 25 mm/min. The fatigue crack propagation rate was investigated according to standard ASTM-E647 in CT specimens. FE simulation of FSW process was implemented for different welding conditions and next the fatigue crack propagation was simulated using XFEM method. In this analysis, to assess the damage in the joints, maximum stress criterion is used. The maximum principal stress in element was the fracture criterion. Numerical results are in good agreement with the experiments so the simulation is reliable. The obtained results show that the tool rotational and traverse speed affect the fatigues crack growth rate. For all welded specimens crack propagation rate was slower than that of the base metal for low values of DK (DK£13 MPa) but is much faster at high values of DK. Furthermore fatigue properties of specimens that were welded with lower speeds are better than base metal and increase in rotational or traverse speeds of the tool will increase the crack propagation rate of the welded specimens.

Electrostatic micro-sensors as a part of microelectromechanical systems (MEMS) play an important role in modern technology. So, precise modeling and suitable solutions for solving the governing mechanical and vibrational equations of them are of great importance. Due to the nonlinear nature of the electrostatic excitation, numerical methods are used to solve the governing equations. This paper presents a comparison between two Galerkin-based approaches to solve them. In the first approach, as used by many researchers in the literature, both sides of the equations are multiplied with the denominator of the electrical force term and then the Galerkin method is applied, whereas in the second approach, we apply direct Galerkin method to solve the equation. As a case study the nonlocal elasticity theory has been used to obtain the governing equation. The results show that for a given beam, although the both approaches predict same pull-in voltage in most cases, but the first approach cannot predict the pull-in instability in some cases and also misses some fixed points. So, the bifurcation diagrams and phase portraits have different quality in the two approaches. Also, the results show that the singular point which is the position of the substrate plate, acts as a strong attractor in the phase diagrams which the first approach is unable to predict it.

Harvesting energy by piezoelectric materials is nowadays an efficient way for powering low-power electronic devices. Required energy for sensors, used in condition and health monitoring of bridges and other civil infrastructures, can be examples of the energy harvesters. This study aimed to improve the piezoelectric-based energy harvesting on civil infrastructures, especially on bridge structures. In this investigation, harvesting energy from the vibrations of a bridge under moving consecutive masses is studied. Harvesting energy is carried out through a cantilever beam with piezoelectric patch which is installed at the middle of a simply supported bridge. Governing equations for vibration of an Euler-Bernoulli beam under moving consecutive masses are derived. The inertial effects, including centrifugal and coriolis forces are considered. For verification, the results of the numerical solution of the moving mass problem are compared to the experimental tests data of the literature. The harvester is modelled by a cantilever beam with piezoelectric patch under base excitations which are calculated from vibrations of the bridge mid-point. The obtained equations are then solved in MATLAB environment by using the fourth order Runge-Kutta method. The calculated induced voltages are compared with those obtained from experiment. A good degree of accuracy is observed.

Hybrid ratio of each reinforcement phase in hybrid composite can be defined as proportion of its volume to total reinforcement volume of the composite. The hybrid ratio is an important factor which controls the participation extent of each reinforcement phase in overall properties of hybrid composites. Hence, in the present work, surface hybrid nano composites of Al2024, graphite average particle size of 100 mm and ZrO2 average particle size of 15 nm with different hybrid ratios were fabricated by friction stir processing method. For fabrication of nano composite the tool rotation rate was set to be 1000 rpm, and its advancing speed was 20 mm/min and tilt angle of 3 degrees were chosen. All samples were subjected to 2 passes of FSP to obtain more homogeneous dispersion of the reinforcements. Subsequently, effect of hybrid ratio on microstructural, mechanical and tribological properties was investigated. Optical microscopy and scanning electron microscopy were utilized to perform microstructural observation on the samples and showed that reinforcements are well dispersed inside the Nugget Zone. Hardness Vickers value measurements and pin on disk dry sliding wear tests were carried out to investigate effect of hybrid ratio on mechanical and tribological properties of the nano composites.

In this article, a form of Boltzmann-Hamel equations (Lagrange’s equations in terms of quasicoordinates), different from the latter’s standard form and avoiding its structurally inherent complexity, is derived based on which a general algorithm for the dynamics modeling of open-chain terrestrial and space robots with an arbitrary number of rigid elements is presented. This form of Boltzmann-Hamel equations is shown to be particularly advantageous in terms of not requiring the determination of the kinetic energy as a function of generalized coordinates and quasi-velocities, representing generalized forces in terms of body basis vectors and offering a panoramic view of the dynamics of the systems. In the act of developing the algorithm, three highly useful kinematic identities are derived via comparison between the single rigid body equations derived from both the standard and the proposed form of Boltzmann-Hamel equations. These identities are then used to considerably simplify the final dynamics model of both systems. Finally, the equations of motion for a two-link terrestrial robot is derived using the proposed algorithm and simulation results in MATLAB are compared with the model of the system in ADAMS to validate the model.

In the current work different organic rankine cycles (base and modified) coupled with proton exchange membrane presented to produce hydrogen and power. Organic rankine cycles used in this work are basic Organic Rankine Cycles (ORC), ORC incorporating regenerator, ORC incorporating feed fluid heater and ORC incorporating both the regenerator and feed fluid heater. ORC energy demand supplied by geothermal energy. A thermodynamic model (energy and exergy) of systems done. EES software used to model the systems. Also, a parametric study done to investigate the effects of the performance parameters (energetic and exergetic) of considered systems. The results showed that ORC incorporating both regenerator and feed fluid heater with PEM electrolyzer had the maximum energy (3.514%) and exergy (68.93%) efficiency in comparison with other systems. Also, it can be observed that evaporator and electrolyzer had the highest portion of exergy destruction of the system. Energy efficiency, exergy efficiency, hydrogen production and net power increased by pressure growth in all systems. The amount of exergy efficiency, energy efficiency, hydrogen production and net power increased by the evaporator temperature addition in ORC incorporating regenerator with PEM electrolyzer and ORC incorporating both regenerator and feed fluid heater with PEM electrolyzer, but their amount marginally decreased by the evaporator temperature addition in basic ORC incorporating with PEM electrolyzer and ORC incorporating feed fluid heater with PEM electrolyzer.

Simulation of the four degree of freedom parallel robot (Quattrotaar) is subjective of this paper. The mathematical model of the parallel robot is obtained too. The workspace is optimized for Non-singular kinematic type-2. Artificial Bees Colony algorithm and Particle Swarm Optimization algorithm as overall exploring algorithms are implemented and the results are compared to each other. In spite of any intrinsic complexity of the optimization problems, the result shows the capability of both methods for this robot parameters design. Comparison of the results indicates the Particle Swarm Optimization algorithm runs faster than Artificial Bees Colony algorithm. The exploring volume consists of a plan with 500 mm x 500 mm dimension which move in a vertical direction from 500 mm to 1000 mm. One of the important hints of the paper is a 90-degree rotation of end effector around vertical axis Z. This rotation is caused more flexibility and dexterity for the robot. A 3-D model of Quattrotaar parallel robot is created by Computer Aided Design software and finally Quattrotaar is fabricated in Human and Robot Interaction Laboratory (Taarlab).

In this paper, equations of motion for a horizontal axis wind turbine with movable base are extracted and natural frequencies and vibration of the system are studied. The wind turbine tower is assumed rigid while its blades are modeled as flexible beams. In-plane bending and twisting are considered as two degrees of freedom for blades motion. The shaft connected the tower to blades is assumed rigid and its rotational velocity is considered. In this paper, specifically, a 5-megawattfloating horizontal axis wind turbine, which it’s base has three angular velocities in different directions, is studied. Due to the complex shape and variation of the properties along the length, the turbine blade properties such as mass per lenght and geometric parameters are extracted by curve fitting in MATLAB. The equations of motion and boundary conditions are derived by Hamilton's principle and then are transformed to ordinary differential equations by Galerkin method. By setting the governing equations to standard form (space state), eigenvalues and frequencies are calculated. The numerical results are compared with published results and good agreement is observed. Then the effect of various parameters on turbine blades frequencies and time responses are demonstrated. Results show that the tower base angular velocity and blades rotational speed have considerable effects on turbine blades time response and vibration frequencies.

Poor lubrication is known as an important factor in the bearings failure. Therefore, it is very important to detect the lubrication condition. Hydrodynamic lubrication, mixed lubrication and boundary lubrication are the basic regimes of the fluid film lubrication. In a proper condition, development of hydrodynamic pressure is adequate to support the load and the bearings operate under hydrodynamic lubrication condition. However, in most situations, they operate in mixed lubrication or boundary lubrication regime and have metal-to-metal contact. To establish these regimes, employing the so-called Stribeck curve is a useful method. In this curve, the oil film thickness is proportional to the lubricant viscosity and sliding velocity and inversely proportional to the applied load. However, distinguishing the exact range of hydrodynamic lubrication regime from mixed and boundary regime using this curve and the relation related to the sliding bearings, due to high number of affecting design factors and operating parameters is difficult. The present study focused on the acoustic emission measuring method in order to monitor the lubrication conditions in a type of journal bearings. Thus, condition monitoring of the journal bearing lubrication is provided and the numerical value of operating variables of the bearing for lubrication regime change from hydrodynamic to mixed is achieved. Using wavelet method, frequency features for each regime are identified. Then, for each lubrication regime, metal-to-metal contact detection is performed.

Under a constant loading condition, use of a controller with constant coefficients can be acceptable for servo pneumatic systems. However, in variable loads with widespread changes, more advanced control methods should be considered to achieve desirable performance. In this paper, an adaptive controller is designed and implemented to a variably loaded servo pneumatic system with PWM driven switching valve. In the experimental setup, a pneumatic circuit is used which consists of one PWM driven fast switching valve instead of an expensive servo or proportional valve. In the designed adaptive controller, real time identification of system parameters is performed using input-output data and controller parameters are adjusted instantaneously. “Self-tuning regulators” algorithm in which the desired closed loop poles and zeroes are predefined, is applied to design the proposed controller. The designed controller is applied to the pneumatic actuator via an interface board and its results are compared to results of a PD and a multi model controller. Unlike the proposed method which varies the control parameters continuously according to load variations, in multi-model method the control law is selected among a number of fixed controllers. Experimental results demonstrate the high performance of the adaptive controller under variable loads.

Hartmann-Sprenger tube is a device in which an under-expanded jet enters a closed-end tube which is placed in a specific distance from the nozzle. By producing an intensive heat inside the tube in some specific modes of operation, the device could be used for some important engineering applications such as acoustic igniters. In present study, thermal performances of a set of six different case studies with conical tubes and different physical specifications are investigated. The variable parameters are the pipe material, the pipe length, the gap distance and the end wall condition which could be closed or perforated. The experimental tests in conjunction with numerical analysis are performed to evaluate the effect of changes in physical parameters on temperature rise inside the tube. The oscillatory flow with strong shock waves is the major reason of temperature rise inside the tube. As the gap distance changes, no oscillatory flow and no sensible temperature rise could be observed. Existence of a tiny hole on the tube end wall reduces the temperature rise, as the shorter tube does. The frequency of oscillations is near the tube resonance frequency for longer tubes. Tubes that are made of materials with lower thermal conductivity could produce higher temperatures.

Nano-fluids are prepared by suspending the ultrafine nanoscale particles in the base fluid and can be substantially enhanced by the heat transfer rate compared to pure fluids. Because of the great importance of dynamic viscosity applications in related applications of nanofluids in heat transfer and energy systems, experimental investigation of the effects of volume concentration and temperature on dynamic viscosity of MgO- MWCNTs/EG-water hybrid nanofluid has been presented in this study. The nanofluids were prepared with solid volume fractions of 0.025%, 0.05%, 0.1%, 0.2%, 0.4%, 0.6% and 0.8% and experiments were performed in the temperature range of 25 to 60oC. In addition, by investigating the rheological behavior of nanofluid against shear rate, the Newtonian behavior was observed. Regarding the weakness of the previous correlations for predicting the dynamic viscosity of this nanofluid and according to the experimental data, a new equation was proposed. The results showed that by increasing the solid volume fraction, the dynamic viscosity increased. This increase is more tangible at lower temperatures compared with higher temperatures. Moreover, at temperature of 60oC, the solid volume fraction does not have a significant effect on the dynamic viscosity of the nanofluid, an issue considered as an important achievement in the industrial and engineering applications.

In the present paper, from the results of an approximation method named structural index that was used as the first step, the process of design and optimization of stiffened panel with Gaussian type surrogate model are carried out. Modeling phase is based on the finite element analyses of the structure. Nonlinear buckling load is set as the design constraint. The surrogate model is employed to reduce the number of finite element analyses in the optimization process. Therefore time of design process is reduced. Using infill points in the modeling and optimization process, convergence to local optima is ensured. Introducing a novel technique, finding the global optimum of the surrogate model is guaranteed. The approach of surrogate based optimization is illustrated using two test problems. Also, the sensitivity of the response to the initial sampling plan is investigated. Convergence criteria usually used in surrogate based optimization is modified to speed the convergence but does not affect the quality of the response. Design optimization process is presented for two types of stiffened panels. In type 1 stiffened panel with 4 design variables, the initial training set is constructed using 55 points. The response is obtained after addition of 5 infill points. For type 2, the initial sampling plan is selected to be 58 points. The optimization process is stopped after adding 173 infill points. Finally, obtained results are compared with the results of structural index method and an approach toward global optimum of the compression stiffened panel is introduced with the characteristics of optimum structure.

Simultaneous localization and mapping (SLAM) is a fundamental problem in autonomous robotics. Many algorithms have been exploited to solve this problem, among these algorithms, Fast SLAM is one of the most widely used and Unscented Fast SLAM is one of the newest. Although in several scientific researches it is stated that Unscented Fast SLAM outperforms Fast SLAM, there are still unexamined potentials regarding Unscented Fast SLAM. Therefore, this paper seeks to improve the overall performance of Unscented Fast SLAM. Map accuracy and quality directly depend on the accuracy of localization and observations. In SLAM algorithms, robot pose is predicted using motion model, and then corrected using the difference between map features and recently observed features. Accuracy of pose estimation may improve by comparing two sequential observations and modifying robot pose to result in best match between them. This method is called scan matching and has been successfully combined with Fast SLAM algorithm and some other SLAM algorithms not including Unscented Fast SLAM. Therefore, this paper seeks to investigate the performance of Unscented Fast SLAM combined with scan matching. Simulation results show that combining Unscented Fast SLAM with scan match significantly improves accuracy of localization and mapping.

In this article the composite wing aeroelastic instability speed is optimized by genetic algorithm relative to fiber angle for different layers and follower forces. Aircraft wing is modeled as a beam with two degrees of freedom, which is a cantilever, with thrust as a follower force and mass of the engine. For structural modeling of composite wing the layer theory has been used and unsteady flow assuming subsonic and incompressible flow was used for aerodynamic model in the time domain. Using the assumed mode the wing dynamic equations of the motion were derived by Lagrange equations. Linear flutter speed according to the eigenvalues of the motion equations was calculated. The process of flutter speed calculation has been converted to computer code in which the number of layers, angle of fibers in each layer, the mass of the engine, and the thrust are input variables and the flutter speed is its output. Using Genetic Algorithm, optimum flutter speed was obtained by changing the angle of fibers. Finally, the impact of the number of layers, the mass of the engine, and thrust on optimum flutter speed has been investigated.

Steam jet ejectors are an essential part in refrigeration and air conditioning, desalination, petroleum refining, petrochemical and chemical industries. A greater understanding of flow physics inside an ejector plays an important role in improving its performance. In this study, analytical algorithm is developed for design of steam ejectors. The algorithm gives the flow ratio (motive to suction flow rate) as a function of the expansion ratio and the pressures of the entrained vapor, motive steam and compressed vapor. The compression ratio and back pressure variations were studied in ejector flow ratio with expansion ratio of 5 and 50. It was shown that compression ratio increases by increasing the flow ratio. Also, in a similar flow rate, compression ratio for ejector with expansion ratio of 50 is greater than compression ratio in the ejector with expansion ratio of 50, due to more vacuum in the case with expansion ratio 50. The code results were then compared with experimental results that were in agreement with other results.Finally, Mach number variations from nozzle exit to discharge diffuser were obtained by code. Results revealed that the pressurized condition causes the expansion angle to lessen, thus resulting in smaller jet core and larger effective area. The expanded wave is further accelerated at a lower Mach number. Therefore, the momentum of the jet core is reduced. However, the enlarged effective area allows a larger amount of secondary fluid to be entrained.

In this paper, a new inverse method has been presented for identifying the distribution of material properties and volume fraction index of rectangular functionally graded (FG) material plates. This method benefits from vibration analysis of FG plates accompanied by a novel and efficient metaheuristic optimization algorithm called Drops Contact Optimization (DCO) algorithm, being proposed for the first time in this article. The presented algorithm relies on the initial population and mimics the behavior of water drops in different level of contacting successively with a fluid surface. Through using the second shear deformation theory and applying the Hamilton principle, the motion equations are derived and, subsequently, the natural frequencies of the considered FG plates are obtained. The outcomes relevant to considered different material phases and various length to thickness ratios are achieved and compared with those available in the literature. Making a comparative study of the obtained results with five well-known optimization algorithms confirms that the proposed DCO algorithm produces better performance in convergent speed and accurate characterization of FG materials.

In the present study, effect of nanofluid aluminum oxide-water on heat transfer in a pipe partially filled with porous material in a turbulent flow is investigated numerically. In this regard, the heat pipe is studied in four structures: without porous material, filled with porous material, boundary and central arrangement of porous material. The results of numerical solution show that use of nano particles with changing thermo physical base fluid's properties, enhances heat transfer in all of the above structures. However, by using nanofluid, heat conduction enhancement ratio in porous medium is lower than clear medium (without porous material). As a result, heat transfer enhancement in boundary arrangement is less and in central arrangement it is more.

Engineering analyses of beams are based on the proper guesstimate of deformation fields. Up until now, the analyses of beams have been widely proposed and experienced in elastic region of materials behavior. This paper considers the elastoplastic engineering analysis of beams. In this regard, following the definition of a proper deformation pattern known as classical Euler- Bernoulli model and using the variational calculus principals the governing equations are extracted. In this analysis the behavior of material obeys the Romberg-Osgood model and yielding is based on the von Mises criterion. Different numerical solutions are suggested for the solution of these complicated equations in the literature. In this paper the exact solution is provided for a thin beam under the action of uniformly distributed load by using the two analytical methods of homotopy and Adomian for the clamped- clamped boundary conditions. In verification phase, the deformation of beam is compared with the results of Abaqus software. Different graphical representations are provided for the results of the analytical solutions and simulations. Using these data, the level of consistency between the simulated solutions on one side and the Adomian and homotopy techniques on the other side, are assessed. At the end, the validity of applying the classical engineering theory of beams in the elastoplastic analyses is discussed.

In this article, the structure of methods to produce ultrafine-grained (UFG) tubes is studied. The metallurgical and mechanical effects of these methods on the materials are fully investigated. Ultrafine grained materials have grains with an average size of 100-1000 nm. If the grain size is less than 100 nm, the material is classified as the nanostructure which has numerous applications in different industries such as aerospace, automobile, military and medical. Generally, the methods presented in this paper have been done on common materials like aluminum and pure copper and magnesium alloy AZ91. Generally, in severe plastic deformation (SPD) methods, very high strain applied to the material at low temperatures to change microstructure for ultrafine or nanostructure. Most severe plastic deformation methods are used for producing ultra-fine grain bulk, whereas the need for strong tube increased in the last decade. Therefore, types of researches were conducted to produce UFG tubes. Advances have been presented completely so that the advantages and disadvantages of each process are clearly comparable. Microstructural features, benefits ultrafine grained and nanostructured materials, improved mechanical properties will be discussed. Furthermore, this article reviews the refinement and deformation mechanisms, e.g. dislocation deformation mechanism, twin deformation mechanism, grain boundary sliding, etc. of SPD methods.

Heat and mass transfer in textiles are usually simulated using models that consider sorption and condensation. But in electrolyte solutions, ions existed in fluid passing the textile can cause a phenomenon called electric double layer. Charges on the textile pores will attract the ions with opposite charge which will affect the fluid flow. To investigate this effect, Poisson-Boltzmann equation is solved besides the other governing equations of the phenomenon. Net electric charge density is computed from this equation and is applied to liquid diffusion coefficient. In this research, the influence of electric double layer is shown and then the factors affecting the strength of this phenomenon have been studied. One side of the textile is thoroughly in contact with liquid and the other side is in contact with air. To validate the obtained results, temperature variations in the outer side of the textile are computed and compared with the available experimental works. There is good agreement between the results. According to the results, applying electric double layer effect in equations causes temperature difference of up to 20 percent in the outer surface of textile. In addition, time for textile full saturation when the electric double layer is considered increased more than fivefold. The results show that by reducing the viscosity of fluid the effect of electric double layer on the textile's outer surface temperature is increased. Porosity and zeta potential are other influential factors which, according to calculations, increasing each effect can accelerate the electric double layer.

In this paper, gas-liquid two-phase flow in the annulus of a real well during under-balanced drilling operations is simulated numerically. Oil and gas flow from the reservoir into the annulus is considered due to under-balanced drilling condition. A numerical code based on one-dimensional form of steady state single pressure two-fluid model in the Eulerian frame of reference is developed and its results are validated using experimental data from two real wells. The results of numerical simulation show better accuracy in comparison with other researches. Given the importance of prediction and control of the bottom-hole pressure and the amount of oil and gas production during the drilling operations, the effects of controlling parameters such as liquid and gas injection flow rate and choke pressure are discussed. Also, the effects of different controlling parameters on the characteristics of two-phase flow pattern, including liquid and gas void fractions, liquid and gas velocities and pressure distribution along with the annulus are discussed. According to the results, the effects of choke pressure and injected liquid flow rate on the production of the oil from the reservoir are independent of the values of each other and are dependent on the injected gas flow rate.

With the arrival of composite materials and because of their unique properties, ideas were presented in order to strengthen and improve their performance. The ideas were reason for building of Grid composite structures. These structures are widely used in the aerospace, missile and Marine industry because of their ideal mechanical properties: special stiffness and high strength against impact and fatigue. Grid composite plates are made from thin composite shell connected a series of composite ribs. Ribs network results in a significant increase in stiffness and strength of structure. In this research, experimental and numerical investigations of effect of Shape of ribs have been on flexural behavior of grid composite plates. For this purpose, three types of Grid plates were considered with triangle, square and rhombic ribs. For building these plates, silicone mold was designed and built and was also used for making plates from hand lay-up and hand-wound layer technique. Samples were subjected to three point bending test; for this purpose, the fixture was designed and built. From numerical solution of the problem and comparison of experimental results it was observed that there is very little difference between experimental and numerical results. Experimental results show that special flexural stiffness of plate with square rib is 1.92 and 1.88 plate with triangular and rhombic ribs, respectively. Also, the flexural strength of plate with square rib is 1.58 plate with triangular rib. Thus plate with square rib has the highest stiffness and bending strength.

Passive bipeds have become an interesting field of research for investigators. Probably all of the passive bipeds that have been modeled previously, are considered as rigid model with point-mass. In this paper, 2D planner compass-gait biped with elastic link is modeled and simulated and its period-one gait is investigated. The stance leg of the passive biped is modeled as an Euler-Bernoulli beam and its vibrations are modeled by using Assumed Modes Method and the equations of motion for the swing phase are developed. Then, behavior of the elastic biped is simulated by using numerical methods and the changes in leg angles and angular velocities of the biped are discussed. Computer simulations showed that when the vibrations of the stance leg are large, angular velocities become oscillating. Vibrations of the stance leg and the effects of Young's Modulus and damping coefficient on the motion of the elastic biped are discussed. Then model is simulated for the small vibrations of the stance leg and the results show that when the vibrations are small the elastic biped behaves like a rigid biped, which verifies our simulations. When the vibrations are small, period-one gait can be found for the elastic biped for the ramp slopes of 0<g£0.0328 rad. The split in the eigenvalues of the period-one gait happens at the ramp slope of g=0.029 rad.

In this paper, based on the concept of natural orthogonal complement, an algorithm is devised to analyze the inverse and forward dynamics and dynamic sensitivity of n-linkage planar serial robots. The first goal is to derive the governing dynamic equations of a planar serial robot systematically, more precisely, number of the linkages, mass, moment of inertia and the length of the linkages are the inputs of the algorithm and the output will be the dynamics equations of the robot. As a comparison study, a planar serial mechanism, namely dynamic modeling of 6R serial revolute manipulator is investigated and the results of the proposed algorithm are compared with other methods, i.e, Adams software and MatODE. In the next step, in order to develop a dynamic sensitivity analysis scheme, Sobol and EFAST methods are employed. By use of the dynamic equations of the robots, the sensitivity of the actuating torques to the design parameters such as mass and length of the linkages is analyzed. Dynamic sensitivity of three planar serial robots, namely 2R-PSM, 3R-PSM and 6R-PSM is studied in two different configurations such as singular and isotropic. Finally, the effects of various angular velocities on the sensitivity of actuated torques to the design parameters are investigated. The obtained results reveal that the tolerance of uncertainty in the design parameters of robot affects the actuating torques significantly and also the Sobol’s method predicts the sensitivity of the robot more precisely.

Thin sheets stiffened with lattice structures are used widely in many engineering industries. Investigation of stability behavior for the grid structures and determination of the buckling load under compressive loads is an issue that has attracted the attention of many researchers, and extensive studies have been done in this field. In this paper, a new grid called Diacube is introduced and its buckling load is examined. For this aim, first, the buckling behavior of 5 common types of stiffened flat lattice panels containing hexagonal, triangular, square, diamond and kagome grid are investigated under compressive axial load; and the results are compared with Diacube grid. The effect of network density used in each structure on the buckling of these structures is studied under different boundary conditions. Regarding the mass difference of samples, specific critical load parameter (the buckling load to mass ratio) is used for comparison between the structures. Using the finite element modeling and numerical analysis, the grid that has the highest buckling load in each boundary condition is determined. It is found that if unloaded edges in lattice panels are simply supported, the new Diacube grid will have the highest buckling load among all structures. Finally, validity of the numerical result obtained for two samples of the structures including hexagonal and Diacube grid is evaluated experimentally; and the numerical results are confirmed.

The laminated composites have many advantages such as high specific strength and specific stiffness. Despite these advantages, they are prone to different damage mechanisms. This paper focuses on quantification of damage mechanisms in standard Open-Hole Tensile (OHT) laminated composites using Acoustic Emission (AE) and Finite Element Method (FEM). These damages include three main mechanisms, matrix cracking, fiber/ matrix debonding and fiber breakage. To this aim, OHT tests were carried out. The specimens were fabricated from two types of glass/epoxy composite materials with [0]5S lay-up and [90]5S lay-up. AE accompanied with wavelet-based approach was then used to detect and quantify damage mechanisms of the specimens. FE analysis based on Hashin criteria was then utilized to simulate the damage mechanisms in the specimens and to validate the AE-wavelet based results. Comparison of the applied methods shows that the results of the AE-wavelet based approach are in very good agreement with the FEM results. Finally, it was concluded that the AE method has a good applicability to determine the damage mechanisms in laminated composite structures and to predict the remaining life-time of the structure.

Ultrasonic Phased Arrays are an emerging technology in nondestructive testing and evaluation. Some important factors affecting on the performance of these probes include, positioning elements in probe, number of elements, and distance between two elements, elements length, and time delays to excite probe elements. The type of linear phased array probe is a prevailing type in which elements are placed side by side and longitudinally. In this paper, based on analyzing the existent laws in design and performance of the phased array probes related to the propagation of ultrasonic waves, an improved dimensional design for ultrasonic linear phased array probes, as well as improvement of the sequence of time delays to excite the probe elements are done. In order to evaluate the performance of the probe with improved design in comparison with a similar ordinary probe, an ultrasonic phased array test is simulated using FEM-based ABAQUS software. By numerical simulations, the performance of the probe with improved design versus the ordinary probe for propagating the guided waves in a thin square aluminum plate is compared. In the first part, the attenuation coefficient of the received signals of reflected wave is evaluated, and in the second part, the performance of the probes for radial scanning is compared. Results of both simulations confirm that the performance of the probe with improved design is much better than the similar ordinary one. Especially, the probe with improved design propagates the ultrasonic waves with the maximum head wave energy, and steers them with higher accuracy towards a determined direction.

DNA is made up of molecules called nucleotides. There are four different nucleotides in DNA which are called Adenine, Guanine, Cytosine, and Thymine, or simply A, G, C and T. Determining the order of these bases is called DNA sequencing. This sequence determines the genes and these genes specify an individual’s unique traits. Therefore, the genetic research plays an important role in detection, prevention and treatment of diseases which are caused by genetic abnormalities and mutations. Common DNA sequencing methods are usually based on chemical reactions. These methods have some disadvantages, for example, they are expensive and also they cause DNA to be lost. So, in recent years the progress in molecular scale simulation methods has produced various approaches for DNA sequencing. In this paper, a suitable method for DNA sequencing has been presented and its accuracy is investigated by molecular dynamics simulations. In this method, DNA molecule passes through the carbon nanotube first, and then the graphene nanopore, with a specific speed. Different bases are determined by analyzing the required force for passing DNA. In this proposed method, the speed and cost of sequencing are improved.

When a cylindrical shell subject to a compressive load, because of various imperfections happened during processes as manufacturing, handling, assembling and machining, buckling occurs in loads lower than corresponding static failure load. Still many of cylindrical shell structures are designed against buckling based on experimental data introduced by NASA SP-8007 as conservative lower bound curves. In the manuscript, non-linear methods of Modified Linear Buckling Modeshaped Imperfections (M-LBMI) and Single Perturbation Load Imperfections (SPLI) for composite cylindrical shell with and without cutout are investigated. In order to evaluate the numerical results composite cylinder with stacking sequence of [90/+23/-23/90] are manufactured by using filament winding method and buckling tests are performed under axial loading. Non-linear numerical results in cylinder with and without cutout are close together and have good agreement with experimental data. It was concluded that buckling load predicted by SPLI and modified LBMI method on cylinder with cutout is close to result of case without apply geometric imperfections. In summary, it was concluded that cutout on the cylinder body act as an imperfection to trigger buckling of the structures so there is no need to apply geometrical imperfections.

Stability and transparency are both very important conditions in bilateral teleoperation systems. For the design of such systems, different methods have been suggested. Among the approaches presented, passivity framework is widely utilized in which human and environment is considered passive. The operator does not make the closed-loop system unstable. In addition, it is passive against an external input. Thus the adoption of this assumption is correct for the human. Nevertheless it is a conservative presumption for the environment and according to some modern applications of teleoperation systems such as cardiac surgery, it is absolutely not acceptable. In this paper a novel control structure for nonlinear bilateral teleoperation systems interacting with active environments is addressed. In this approach, first a criterion for measuring activity of the environment is presented. Then by developing a PD controller, an algorithm that guarantees master-slave position coordination and static force reflection is introduced. The overall stability of closed loop system is proved using passivity concept and Lyapunov-Krasovskii technique. Simulations are performed to verify the performance of the proposed bilateral teleoperation systems in contact with passive and non-passive environments. Experimental results were carried out to validate the theoretical consequences.

An Aerial Robot or Unmanned Aerial Vehicle (UAV) is an aerial vehicle that provides its flight condition using aerodynamic forces. This vehicle can be named as an autonomous robot. This robot is an under-actuated system and is inherently unstable. Thus, the control of this nonlinear system is a problem for both practical and theoretical interest. So, the goal of this research is to compare it with highly nonlinear dynamic system of Octorotor which is difficult to control in many cases and causes instability in this Unmanned Aerial Vehicle (UAV). At first, the structure of Octorotor is studied in this paper in order to increase power, better ability to carry a load and to increase resistance into the distribution. Also, the electronics and mechanics of this robot are studied in some sections. Then, in the following, in order to control attitude of robot with introduction of dynamic system, one of the most common implemented controllers is applied on this robot. Initially, this process is done on the dynamic model of robot by Matlab/Simulink software and finally, implementation of this controller is applied on a fabricated Octorotor during a real flight in autonomous trajectory tracking in outdoor environment. Finally, the study of sensors results is also shown.

Recognition of the dynamical behavior and vibrations of marine structures, submerged in vicinity of the water free surface, is one of the most important issues in the design of marine structures. It is obvious that physical properties of the ambient fluid have some influence on vibrational frequencies of the structures. For the structures that have been exposed under the influences of asymmetric environmental conditions, prediction of their dynamic behaviors is more complicated. In this paper the effects of immersion depth on first natural frequency of a bounded circular plate that was placed parallel in the vicinity of the water surface, are studied numerically and experimentally. The techniques used for exciting the plate and measurements of natural frequencies are innovations of this research. Numerical solutions are done using ABAQUS software. Comparison of the numerical and experimental results shows a good consistency. The investigations showed that increasing the immersion depth, the ratio of the depth to plate diameter reached to a certain value and the natural frequencies were also decreased. After that it remained constant while the immersion depths of plate were increased.