مجله مهندسی مکانیک
سال پنجاهم شماره 4 (پیاپی 93، زمستان 1399)

Musculoskeletal disorders caused by heavy workloads are very common in workers, which negatively affects workers' health and productivity. In this regard, the idea of designing an exoskeleton robot is proposed. This robot is similar to a dress which worn on the user's body and parallels the body, accompanying and helping the person. In this paper, the parallel distributed compensation control method is proposed to be applied to a four degrees of freedom exoskeleton waist robot. Initially, the dynamical equations governing the system are extracted. According to the nonlinear nature of the system, the parallel distributed compensator controller is designed with a Takagi-Sugeno linearization approach. The closed loop system response of this controller is investigated in the Simulink environment of the MATLAB software. The results show that the Takagi-Sugeno linearization approach accurately estimates the nonlinear system, and also the proposed controller perfectly intercepts the optimal closed loop response

The goal of the present study, has been to investigate one of methods for increasing the double absorption heat transformer’s (DAHT’s) productivity. For this propose, first, due to the special importance of the solution flow path shape on AHT’s operation, five novel schemes were proposed for DAHTs and then, their performance was compared numerically to the most efficient type of DAHTs as basic cycle in this research, through a parametric study. The operation of cycles has been studied from the energy coefficient of performance, exergy coefficient of performance and produced vapor amount, point of view. The effect of adding third heat exchanger to the cycles, is at least a minimum increment of vapor amount equal to 10.49% for basic DAHT, 8.6% for first scheme and 10.33% for second scheme. According to the optimization results, the third scheme in the higher and the fifth scheme in the middle range of absorber temperatures have the ability to produce the most water vapor amount, among all the schemes.

Abstract This paper has been devoted to the importance of considering the effects of nonlinear geometric and inertia terms and the effect of gyroscopic term on the forced vibrations of an unbalance rotating shaft. The nonlinear partial differential equations have been derived by using the extended Hamilton's principle and have been transformed to the ordinary differential equations by using the Galerkin method. Then the method of multiple scales has been applied to the discretized ordinary differential equations and the modulation equations describing interaction between modes have been obtained. By applying the solvability condition on the resulting modulation equations and considering the stable conditions in the vibration analysis, the frequency response diagrams have been plotted. The effect of the gyroscopic term, nonlinear geometric and inertia terms have been presented in the frequency response diagrams. In plotting the frequency response diagrams of rotating shaft, its rotating speed has been considered very close to the shaft’s forward natural frequency. Numerical results showed that the hardening type behavior has been observed in simulations due to nonlinear terms.

In this study, the simulation of water-titanium oxide nanofluid has been done in a double-tube counter flow heat exchanger with constant heat flux by Fluent software. The main purpose of this study is to compare single-phase and two-phase mixed model with experimental results to obtain the best simulation model. The results show that The Nusselt number and Heat transfer coefficient for a 0.2% volume fraction for Reynolds number more than 8000, the single-phase model is closer to the experimental results than the two-phase model, but for Reynolds lower than 8,000, two-phase mixed models are in good agreement with experimental results. The comparison of the dropping pressure shows that the faster the speed of Nanofluid is increased, the greater is the difference between the simulation and the experimental results. The results show that the highest increase in Nusselt number and Heat transfer coefficient is seen in higher Reynolds numbers, which indicates that in the high Reynolds numbers, the increase in volume fraction is more effective, one of the reasons is that homogeneity of Nanofluid in higher Reynolds is due to more turbulence. In general, it can be said that the results of the simulation are in good agreement with experimental results.

This article studies a class of non-minimum phase nonlinear systems with non-affine internal dynamics. The non-minimum phase feature, along with the non-affine internal dynamic, makes complexities in the process of controller design for this class of systems. In this paper, a new approach is presented for designing the control law for these systems. In the proposed method, the system’s equations, considering the system’s relative degree, are rewritten in the normal form. The design procedure of the control law, in the considered system, has three main steps. First, using the constraint optimization approach, the virtual control law is designed to stabilize the non-minimum phase and non-affine internal dynamics. Then, by designed virtual controller and combination with backstepping method, the proper sliding surface is extracted and, finally, based on the proposed sliding surface, a robust control law is designed to stabilize all of the system’s state variables. The simulation results on the TORA system show the efficiency of the proposed approach in the robust stabilization of this class of nonlinear systems.

The integrated gasification combined cycle power plant using hydrocarbon fuels produces cleaner and more efficient electricity through gasification, compared to direct fuel burning. In this paper, the power plant has been simulated and validated by Thermoflow software according power plant characteristics. The assumptions are applied and exergy analysis is done using EES software. The results of energy and power consumption analysis of the obtained components and exergy analysis are applied to find the values and locations of system irreversibility. The net output of the cycle and thermal efficiency are 234.89 MW and 30.69% respectively. The exergy efficiency of the power plant is 45.57%. Maximum amount of irreversibility is from the gasifire, gas cleanup system, and HRSG, respectively 216.600, 210.607 and 113.653 MW. Finally, the cycle is analyzed parametrically and the effects of the gas temperature change and pressure of the condenser on the efficiency and final power of the power plant have investigated.

A major problem in design of a controller is that some of the parameters of the governing dynamic equation might be unknown. The purpose of the present study is to provide a method for identification of the unknown parameters of the dynamic equations of the two-axis gimbal system and then applying a suitable controller to it. Among the uncertain parameters which may exist in the dynamic equations of a two-axis gimbal, the components of the inertia matrix, the coriolis matrix, and the friction matrix can be named. In this research, in order to estimate the unknown parameters of the system, the Gauss-Newton method which is a gradient-based inverse technique is used. Regarding the severe sensitivity of inverse problems to measurement errors, by using a proper smoothing technique, this undesirable effect is reduced. In the present work, for the first time, an accurate technique for computation of sensitivity coefficients of the dynamic equations of the gimbal is presented. After identification of the unknown parameters, the control of the two-axis gimbal is also performed by sliding mode control. The applied moments are designed by sliding mode control to stabilize the line of sight. Also, proof of stability is presented for the sliding mode control. Simulation of the laboratory tests in the identification section and also control results are obtained by modeling of the system in MATLAB.

Since 1990, data centers have taken on an increasingly central role in the progress of two parallel areas: Information technology (IT) services and telecommunications. Effective cooling of data centers is important for their proper operation and has a key part since the design of the earliest facilities. As computing capabilities advance, heat dissipations from IT servers have increased from a few kW to more than 20 kW within the past decade, particularly in high-performance server racks. Server rooms need significant advances in cooling technologies to enable the successful deployment of their hardware’s. This approach is about three dimensional CFD modeling of under flow air distribution (UFAD) cooling system in a sample data center with its total instruments. CFD results have been compared with the available experimental data, shown good agreement. Results show, CFD modeling of cooling patterns in a data center can help HVAC designers to decide how much cooling capacity needs to cool data center. Modeling pressures and velocities leads to know if there is recirculation or by-pass air around server racks.

Elastic analysis of a non-homogenous symmetric shells with arbitrary curvature and variable thickness is considered in the present research. First order Shear Deformation Theory and total potential energy approach is applied for obtaining the governing equations of non-homogenous symmetric shell. As a special case, the governing equations are rewritten for a cylinder with variable thickness. Analytical method for constant thickness and Perturbation method for variable thickness are used for solving the governing equations and obtaining results. The comparison between classical theory and First order Shear Deformation Theory (FSDT) are illustrated with some numerical results. Non-homogenous material, boundary condition effects, non-uniform pressure, arbitrary curvature with variable thickness are some advantages of current work. Functionally graded material is considered as non-homogenous material. Gradation is considered for all mechanical properties along the thickness direction based on a power function. Homogenous material and non-homogenous material by difference in non-homogeneity parameter can be studied with the present research. There is some applications of varying thickness shells in industrial like aerospace engineering. We can reach to optimum design of thickness and also some property like high temperature residence by applying non homogenous material.

The bobbin tool is a new design of the friction stir welding tool that provides the opportunity of the simultaneous double sided welding. The present research aims to design and manufacture a floating bobbin tool; the capability of which is examined through welding aluminum alloy 6061. The results indicated that the high quality and defect-free butt welds can be produced via this tool. Moreover, the strength of the welds is similar to the strength of welded samples done by the conventional friction stir welding method. Regarding the quality and strength, the obtained samples were investigated by mechanical tests including tensile strength test and hardness test. Obtaining 60.7% joint efficiency is indicative of success in the friction stir welding process; and, thus the efficiency of the designed tool. Besides, the second major characteristic of welding is the hardness, which is due to the dissolution of sediments, is decreased in the stirred area. This finding is confirmed by the behavior reported by the other investigators in the literature.

In this study, the effect of Al on the corrosion resistance of Hadfield steel weld joints’ fusion zone was investigated. For this purpose, 4 austenitized sheets (two sheets without Al and two sheets containing 1.7wt. %Al) with thickness 2mm prepared from Hadfield steel and then SMAW process was used for welding. Then potentiodynamic polarization and electrochemical impedance spectroscopy methods were used to evaluate corrosion behavior of welded joints’ fusion zone in the 3.5wt. %NaCl solution. The evaluation of the microstructures of weld metal in welding joints were conducted by optical microscopy and the corrosion mechanism were determined by scanning electron microscopy examination. The result of corrosion tests indicated that welded joints’ fusion zone of Al containing plates were more corrosion resistant. Because Al addition increased the solubility of carbon in austenite phase and as a result factors enhancing localized micro-galvanic corrosion such as grain boundaries and carbides were reduced. However as clearly was visible in scanning electron microscopy images, using joints design containing Al in comparison with joints design without Al led to changed weld metal corrosion mechanism from uniform corrosion to localized micro-galvanic corrosion in the Hadfield steel weld joints.

In this article, an analytical solution for the problem of time-dependent creep analysis of a functionally graded magneto-electro-elastic rotating disc is presented. The material properties are considered to be power function of radius through radial direction. Firstly, using the stress-strain relation and strain-displacement relation together with equilibrium equation and thermal equation in plane-stress condition, a differential equation containing creep strains is found. Then, eliminating creep strains, an analytical solution for the mentioned differential equation is obtained which is actually the response in time zero. Then, considering fixed temperature boundary conditions, creep strains are kept and creep stress rates are found by an analytical solution of the obtained differential equation. Lastly, the radial stress, hoop stress, radial displacement, electric potential and magnetic potential can be calculated using an iterative method in every desired time. In the numerical examples, the effects of creep evolution, temperature boundary conditions and speed of rotation are investigated comprehensively.

In this study, the effects of using multi-electrode spark plugs on the performance parameters of an internal combustion engine were investigated using experimental tests. In this regard, experimental tests have been carried out using two types of lead-free gasoline, as well as a combination of gasoline with commercial additive (Keropur-G). The results of experimental tests show that with the use of multi-electrode spark plugs in conventional fuel and fuel with additive, the braking torque and braking power are increased and in comparison the specific fuel consumption (SFC) is decreased. Also the use of multi-electrode spark plugs increases the emission of nitrogen oxides and reduces the emissions of unburnt hydrocarbons, carbon monoxide and carbon dioxide from the engine.

 In this paper, multi-objective optimization (MOO) of heat transfer and flow field in cross-flow heat exchangers with triangular and square arrangement is performed using Computational Fluid Dynamics (CFD) techniques and Non-dominated Sorting Genetic Algorithms (NSGA II). At first, fluid flow is solved numerically in 150 various heat exchanger using CFD techniques. Finally, the CFD data will be used for Pareto based multi-objective optimization of fluid flow in cross flow heat exchanger using NSGA II algorithm. In the MOO process there are two geometrical parameters and the conflicting objective functions are to simultaneously maximize the amount of heat transfer and minimize the pressure drop. The Pareto front including optimum design variables and objective functions for two triangular and square heat exchangers are shown in results. It is shown that the achieved Pareto solution includes important design information on fluid flow in the heat exchangers.

In this study, the sliding contact problem between a rigid cylindrical punch and a hard coating/ graded interlayer/ substrate system is considered. It is assumed that the hard coating and the substrate are both homogeneous- isotropic media. On the other hand, the interlayer exhibits the material orthotropy and even variable mechanical properties through the thickness by an exponential variation. The commercial finite element software ABAQUS is utilized to simulate the mentioned contact problem. The numerical solution is provided under the plane strain condition. A comprehensive sensitivity analysis is performed to explore the effect of the material orthotropy and the non-homogeneity parameters on the interior stress field as well as the interface stress distribution. The results indicate that the orthotropic graded interlayer can alter the stress field by appropriate adjustment of the material parameters. For an interlayer with a higher stiffness parallel to the contact surface with respect to the perpendicular direction, the maximum Von-Mises stress within the substrate can decreased a factor of 7%. Moreover, the bonding strength over the interfaces can be enhanced by increasing the thickness of the graded interlayer

In this paper, the integral backstepping sliding mode (IBSM) control with the extended Kalman-Bucy filter (EKBF) to control and state estimation of unmanned aerial vehicles (quadrotor) is provided. In the quadrotor system, the aerodynamic effects of the‏ dynamics of a system are considered, and dynamical equations are derived by the Newton-Euler method. Quadrotor's behavior, which is affected by forces and aerodynamic moments is non-linear, but the integral backstepping sliding mode control has been able to stabilize the system dynamically, without considering the system and measurement noises and assuming all the dynamic states of the system. Noise is immaterial in dynamic systems, and the measurement of all systems states in practice is very complex and expensive; for this purpose, the EKBF in the control structure is used as observer states of the system and noise reduction in these modes. Therefore, simultaneous use of the controller-observer is suggested for controlling and estimating quadrature states. The numerical simulation demonstrates the good performance of the proposed controller-observer so that they were able to estimate both the system's unobservable state and overcome system and measurement noise.

Numerical analysis of heat transfers and nanofluid flow structure inside a tube with multi-channel twisted tapes in a solar flat plate collector has been investigated. Al2O3/water nanofluid with a volume fraction of 3% was used as working fluid. Nanofluid with Reynolds number (Re) ranged from 4000 to 20000 was investigated. Effect of the number of channel (n=2, 3, 4, 5 and 6), twisting ratio (N=4, 5, 6 and 7) and diameter ratio (D*=0.1, 0.12 and 0.14) on heat transfer rate and friction loss have been presented separately. The results demonstrated that the insertion of multi-channel twisted tapes induce multi-swirling flows. Consequently, fluid mixing improves and heat transfer rate enhances. Augmenting number of channel, twisting ratio and diameter ratio leads to higher values of heat transfer and pressure loss. The maximum thermal performance factor is obtained by using twisted tapes with n=2, D*=0.14 and N=7 when Reynolds number has lowest value

Nowadays, simulation methods are used to obtain a preliminary estimate, in order to avoid many costing experiments. One of the common ways in which multiple commercial software has been developed based on it, is the finite element method. The present study has been tried by using the modeling of rubber in a finite element software, the patterns of propagation of longitudinal ultrasonic waves in rubber compounds are investigated. By definition of rubber as a hyperelastic material and by using of the Arruda-Boyce model for a rubber compound introduced into the finite element software, and the propagation of longitudinal ultrasonic waves was evaluated from two qualitative and quantitative perspectives. In order to quantitatively evaluate the proposed finite element model, a rubber compound was produced and the propagation velocity of the ultrasonic waves was measured. The comparison of the propagation velocity of longitudinal ultrasonic waves obtained from the finite element model and experimental data shows that the Arruda-Boyce model has a high accuracy in velocity estimation. Simulated proposed finite element is a very suitable method for interpreting and evaluating complex problems associated with the propagation of longitudinal ultrasonic waves in rubber materials, due to the accuracy of the results.

Recently, Numerous applications of nanofluid in various industries have led to the consideration of increasing the flow efficiency through nanofluid in nuclear reactors. One of the most important safety issues in the reactors is the safety margin of the cooling flow. For this purpose, the performance of flow without presence/presence of nanofluid is simulated numerically with the CFX software. The finite volume numerical method with the SST transition model for numerical analysis has been used, here. The flow in the pressurized pipe is analyzed using a fixed coordinate system. Four different volume fractions are numerically investigated in the nanofluid flow. To ensure the accuracy of the simulation and the setting of boundary conditions, a physical variable (outlet temperature from the pressure pipe) was monitored. The low numerical error has increased the confidence of the results. The results show that the 10% volume fraction has improved the performance of the flow.

The aim of this research is to redesign and optimize of a rotor blade in small size wind turbine. The objective of such optimization is both running time and power coefficient. The aerodynamic modeling of rotor has been presented based on blade element momentum theory. The output parameters have been calculated subsequently. For validation the model is being compared with NREL phase VI turbine data. The technique of optimization is NSGA-II which is appropriately cope with multi objective problems. Variable are assumed to be pitch or twist angle and chord length and the cost function or performance index is composed of drive time and power. The max tip speed ratio and the bounds of variables are constraints. Based on the Pareto diagram it can be deduced by increasing the power, time would be influenced and degraded. It is recommended 3 points and all the characteristics are being drawn for each one. By choosing the compromise point, it is achievable to enhance the power coefficient by approximately 9 percent and to reduce the time of reaching nominal speed by approximately 10 percent

In this paper, dynamic responses of a specific thruster used in Remotely Operated Vehicles (ROVs) have been extracted through experimental and laboratory tests. Based on the experimental results and dynamic analysis, thruster dominant dynamics have been determined and divided into saturation, and dead-zone effect dynamics. Then, elaborated dynamics have been mathematically modeled to simulate an appropriate dynamic model for thrusters. In the following, the effects of determined dynamics of thrusters have been studied on both open loop and close loop ROV dynamic analysis. Hence, by defining some scenarios based on practical ROV’s operations, its dynamic responses have been analyzed under both ideal and proposed thruster model. Simulation results and dynamic analysis elaborate that saturation effect confines generating force of thrusters yielding to decline ROV’s speed and decelerates its dynamic behavior response time. In addition, dead-zone effect dynamic causes a delay in the execution of applied commands. Therefore, it provides a gradual increase in dynamic response time of the ROV. Furthermore, it also declines the accuracy of the both depth and head regulators.

This study investigates the performance of Cylindrical- conical- and clover-shaped film cooling holes of gas turbine blades and the effect of transverse trench on film cooling hole efficiency. The purpose of the research was to achieve the optimal film cooling geometry in order to gain the highest film cooling hole of gas turbine blade efficiency. In this study, a three-dimensional blade simulation that based on k-e realizable turbulence model. Cooling fluid was injected into the mainstream at 30◦ on leading edge stagnation row, with BR=1, 1.5, 2. After injecting the cooling fluid into Cylindrical- conical- and clover-shaped film cooling holes geometries with and without the presence of transverse trench, it was observed that clover-shaped hole with transverse trench along, at BR=2 and film cooling efficiency of 0.147 offers the highest film cooling efficiency. Investigation on transverse trench effect for all geometries showed that the presence of trench leads to a higher film cooling efficiency.

Geometry change and thermal properties of coolant can increase the efficiency of solar collectors. In this study, solar flat plate collector which has rhombic geometry, with helical piping and without raiser is investigated. In order to increase performance, the SiO2/water nanofluid that is made by two-step methods and with 1% concentration is used. Collector is experimentally tested in Behbahan city in the south of Iran according to ASHRAE standard. Water and nanofluid (without surfactant) are tested in collector with similar conditions. Performance evaluation is consist of the effect of flow rate, temperature, solar radiation and ambient conditions. The results show that the rhombic collector without raiser has better performance than ordinary flat plate collector. The collector efficiency by using nanofluid is about 3% more than using net water and the maximum value of efficiency and outlet temperature by using SiO2/water nanofluid are about 66% and 780 C respectively.

This paper presents isogeometric analysis of free form shells based on Kirchhoff-Love and Reissner-Mindlin theories. The isogeometric approach utilizes Non-Uniform Rational B-Splines (NURBS) for different order approximation of the variables defining the geometry as well as the unknown functions. The geometry is defined for both theories by the NURBS technique for surface generation. In the employed Reissner-Mindlin shell theory, by making use of the anchor point concept the normal vector is calculated accurately. The Kirchhoff-Love shell theory uses three displacements degrees of freedom per node, but the Reissner-Mindlin theory uses five degrees of freedom, three displacements and two rotations. Also, for Kirchhoff-Love shell theory C1 continuity of the NURBS basis functions is needed, while for the Reissner-Mindlin shell theory C0 continuity is sufficient. Several standard benchmark examples with available analytical solutions are presented to demonstrate the performance and accuracy of the approaches. Also, a new benchmark problem is designed to study the performance of the methods as well as the convergence behavior of the presented approach when applied to completely free form shells.

To respond to the external forces, adherent cells are reoriented from random orientation to an ordered and homogeneous one, regarding the applied strain direction. Furthermore, in subcellular scale, cytoskeleton and stress filaments with polarization generate inner contractile forces leading to a reorientation of the cell at a specific angle. In the present study, morphological changes of mesenchymal stem cells under cyclic strain toward myogenic differentiation were analyzed. Simulation of stem cell morphological changes using a nonlinear mechanical model (viscoelastic) is the innovative aspect of this research. To do so, mesenchymal stem cells with real size by the use of finite element method were designed in the Comsol software. Furthermore, time-dependent mechanical properties were applied. The uniaxial cyclic strain is applied to the substrate, and the frequency of the strain was considered as one of the variables. The lying angle of the cell on the substrate was varied from 34 to 90°. Also, Young’s modulus of the substrate and the role of strain alteration were investigated. The results revealed that the stress distribution on the cell and the nucleus are strongly dependent on the lying angle. The approximate angle in which the stress and strain is minimum was calculated about 70 degrees.

 Chopped fiber composites have become attractive materials for many industrial applications due to simplicity of their manufacturing processes and uniform mechanical properties. The subject of this paper is investigation of damage progression in random chopped Eglass fiber/Epoxy composites and evaluation of Chow and Wang Model in simple tensile test. In the experimental section, digital
image correlation method was applied and variation curves of the Young's modulus and Poisson's ratio in tensile test of specimens were extracted until rupture. In theoretical section, for investigation of anisotropic damage and distinguishing between damage mechanism due to tensile and compressive stresses, the first part of the Chow and Wang criterion was employed. Growth of damage variables during the failure progression was characterized and anisotropic damage evolution in this composite was concluded. By means of linear approximation of damage dependency coefficient, validation of the model for prediction of mechanical behavior of the damaged composite was confirmed by comparing with experimental results.

Ejector is used as a key component in ejector refrigeration cycle. A supersonic ejector with optimal performance reduces energy consumption in refrigeration system and improves its performance. In this paper, the effect of using parallel flow primary nozzle on the performance of a supersonic ejector used in an ejector refrigeration cycle, with steam as working fluid, is numerically investigated. For this purpose, two primary nozzles, conical and parallel flow, with the same converging portion and the same ratio of exit surface to throat surface, have been used. The parallel-flow nozzle diverging curve is calculated using the characteristic method. Numerical simulations have been performed using the Ansys-Fluent software. The results show that by changing the primary nozzle diverging curve from conical nozzle to parallel flow nozzle, variation of entrainment ratio in the critical region will be negligible, but in the subcritical region, it is significant. The maximum relative variation of entrainment ratio in subcritical region of the parallel flow nozzle is +17.3%. Also, by changing the diverging curve of the primary nozzle, critical pressure increases by 1 mbar and the physics of the ejector internal flow changes.