مجله مکانیک سازه ها و شاره ها
سال یازدهم شماره 1 (فروردین و اردیبهشت 1400)

In the present study, an analytical model was presented for investigating the nonlinear aero-elastic response of cracked plate in supersonic flow. In this context, two-dimensional equations of cracked induced isotropic plate were proposed considering pure bending loading and simply support boundary condition. To form this equation, plate modeling based on the classical plate theory (CPT) and the von-Karman nonlinear relations. Also, the Line-Spring Model (LSM) and linear piston theory were considered for crack location and aerodynamic effect, respectively. Applying the Galerkin's method and plate assume modes, the partial differential equations (PDEs) are transformed into ordinary differential equations (ODEs). Then, by using Runge-Kutta numerical solution method, these equations (PDEs) were solved and the results were investigated. Eventually, some of effective aero-elastic parameters like flow condition and crack size were prescribed within the limit cycle oscillation (LCO) in flutter status. Results demonstrate that the presence of crack was leading early flutter, increasing the maximum amplitude of the limit cycle oscillations and aero-elastic instability that can ultimately reduce the structural performance.

In this paper, control of forced vibration amplitude of a rotating FGM conical shell is studied using FGPM patches. Four piezoelectric patches are placed inner and outer of the shell. In order to obtain the system dynamic equations, the energy method and the classical plate theory are used and simply supported boundary conditions are considered. Each system variable is considered as an expression by separation of variables with variable-time coefficients. Subsequently by substitution the considered responses in the energy functions and finally using the Lagrange equation, the governing equations of the system are obtained. Natural frequencies are compared with the result of previous researches in the case of non-rotating and rotating shell. Also to control vibration, velocity feedback is used so that sensor voltage, which is dependent on the surface, thickness, and location of each sensor, is calculated and used in the actuator voltage. In the following section, control of the system is done for first mode and then convergent system, which in both cases, Closed-loop system well dampens amplitude of forced vibrations.

The ultrasonic transducer transmits vibrations to the load carried by the piece called Horn. Depending on the application of the horn, vibrations can be concentrated or propagated. It is used in various applications such as ultrasonic welding. A limitation of this piece, the temperature rise during operation due to structural damping. Because this piece is vibrating at resonant frequency in the axial mode, the temperature increases and its mechanical properties change, the frequency of resonance and vibration efficiency decreases sharply. In this paper, a new optimized geometry is presented to reduce the horn temperature. The new geometry is optimal in terms of heat transfer and at the same time try to minimize lateral mode may be achieved. In order to improve this problem, a numerical model of the temperature rise for desired piece in steady-state is achieved. In addition, using the genetic algorithm, Horn thermal optimization is performed in two consecutive steps with the idea of fins. In the results section, the details of the new geometry are presented and discussed in detail in its various aspects.

In this experimental study, the forced convection heat transfer of thermally developing Fe2O3/water nanoparticles inside a copper tube under magnetic field created by a number of permanent flat magnets is investigated. Experiments have been carried out under laminar flow regime and uniform heat flux boundary condition and the influence of magnetic field position and intensity. The objectives of this study were to investigate the effect of using Fe2O3/water nanofluid, flow Reynolds number, intensity and magnetic field position on the thermal behavior of the flow. Validation of the test apparatus was performed using distilled water and compared with the theoretical results, which eventually achieved good agreement. The results show that due to the influence of the magnetic field on the Fe2O3/water nanofluid, secondary flow and boundary layer deformation occur, the heat transfer changes and its variation is dependent on the flow profile and intensity and position of the magnetic field. Increasing the intensity of the magnetic field perpendicular to the flow at a single point in the constant Reynolds causes a sudden increase in the local convection heat transfer coefficient but the mean convection heat transfer coefficient in the latter case is higher than in the former case.

In this paper, the capability of computational homogenization technique in the prediction of the macroscopic yield surface of polycrystalline materials at the presence of microscopic damage is investigated. In order to perform the computational homogenization, the irregular shape representative volume elements (RVE) composed of two dimensional grains is used. The grains are considered as an undamageable linear elastic material while the grain boundaries are modeled by a nonlinear cohesive interface law. To study the effect of initial microscopic damages, three different shapes of fully damaged cracks are inserted at the center of RVEs. The Drichlet’s boundary conditions extracted from a macroscopic strain tensor is imposed on the boundary of RVEs and gradually increased until the full failure of samples. The results show that initial damages can dramatically reduce the material strength and give rise to an asymmetric yield surface whereas they have no effect on the macroscopic modulus of elasticity.

In this article, a new combined approach including, the ring compression test and open die backward extrusion (ODBE) test, is presented to assess the friction coefficient in the extrusion process. It is assumed that the frictional behavior at the free surface of the die and the extrusion cavity is different. Firstly, the friction coefficient at the contact interface between the free surface of the die and the workpiece is obtained using the ring compression test. Then, to obtain the friction calibration curves at the contact interface between the workpiece and the extrusion channel, the finite element simulations of the ODBE test are performed for different friction coefficients in this area, and the friction coefficient is obtained comparing the experimental results and calibration curves. The comparison of the friction coefficients obtained from the three tests shows that the friction coefficient in the extrusion cavity is higher than that of the free surface.

The drop weight tear test has been used for nore than seven decades in oil and gas industries to measure fracture energy and to study the features of ductile and brittle fracture of energy trsnportaton pipelines. In this study, the fracture energy of API X65 steel was measured using the acceleration signal of the drop weight tear test equipped with accelerometer. The test sample was cut from the natural gas transmission pipe with outside diameter of 1219 mm and wall thickness of 14.3 mm. The test was carried out in accordance with API 5L standard on drop weight tear test apparatus with a 700 kg hammer equipped with accelerometer having 3 m drop. Using the obtained natural frequency and the low-pass Batterworth filter, the unwanted oscillations were removed from the high-velocity test data. The fracture energy (area under load-displacement record) obtained as 6791 J, which was around 4 percent less than the energy value of the drop weight tear test equipped with strain gauge in previous studies. Due to this small difference, the accuracy of the drop weight tear test equipped with accelerometer was confirmed.

Nowadays, damage models are used to simulate and predict the failure occurrence regions in manufacturing processes. One of the most famous models that is widely used in finite element commercial codes is Johnson-Cook damage model. The aim of the present research is to determine the constants of Johnson-Cook damage model for pure commercial copper sheet. For this, three different tensile testing specimens (a smooth (standard) specimen and two notched specimen with different notch radii of 10 and 2 mm) were separated along with the rolling direction and tensile tests were conducted on them. The stress triaxiality was determined for each of the specimens, by using the Bridgeman equation. Next, regards to the dependency of the fracture strain to the stress triaxiality, the constants of Johnson-Cook damage model was determined. Numerical simulation was used to evaluate the satisfaction of the method used to determine the constants of Johnson-cook damage model. For this, the error in fracture strain was used as the criterion and the value of it was calculated for each of the tensile testing specimens. The average value of error in fracture strain for three tested specimens was obtained as of 9.68 percent that showed the satisfaction of the method used in this research to determine the constants of Johnson-Cook damage model. The results of this study can be used to simulate the manufacturing processes of the studied copper sheet.

flushing of machining gap is one of the most crucial issues in the micro-EDM that not only removes debris from the machining gap, but it also increases the machining efficiency. Identification of dielectric flow pattern provides a good perspective for its user to enhance the flushing performance. In this paper, the effects of the hole depth and dielectric fluid parameters on the flushing in micro-EDM are studied on the flushing in micro-EDM. Three different levels of 5, 10 and 15 mm for depth are considered, while deionized water, kerosene, and EDM-30 oil are chosen as dielectrics. To find out the effect of these parameters on flushing, computational fluid dynamics (CFD) based on boundary volume method is applied to solve the fluid flow in the flushing gap. To verify the numerical simulation model, a comparison was conducted between the results obtained in this work and the experimental and numerical results of the available literatures. The maximum difference between the results of this work and the experimental results is about 10.81%, which indicates that these results are in good agreement with the results of the previous research. The simulation was in a close agreement with them. By investigating the numerical results, it is observed that decreasing the depth of the hole and using deionized water as dielectric fluid increase the velocity of the flow in the flushing gap and reduce the size of the stagnation region in the hole.

In this paper, a series of experiments were conducted aluminum plates to investigate the large plastic deformation of single, double, and triple-layered plates under repeated uniform loading up to five times. In order to perform experiments, plates were mounted onto a ballistic pendulum. Generally, the tested plates represented large plastic deformation of dome-like shape along with thinning or tearing occurring at the boundary due to the uniform impulsive loading. The experimental results showed that the maximum permanent delfection of single- and multi-layered plates increases by the increase of mass charge and number of blasts. Furthermore, the progressive deflection was decreased exponentially because the test plate material undergoes work hardening after each blast load. The results also indicated that triple-layered plates made of similar materials may have a better blast performance compared to double-layered at the first low-impulse blast but their resistance decreases as the number of blast increases. However, this trend was not observed for high-impulse blasts and multi-layered plates have a different behavior.

One of the common treatments in orthopedic surgery is machining of knee joints to attach the artificial joint. In this surgery preparation of tibia and femur surfaces is necessary to obtain proper joint kinematics and ligament balancing. For this purpose milling is involved in surgery requiring accurate machining of bone surfaces and creation of accurate slots. In this paper, milling process was carried out on polymethylmethacrylate as workpiece and suitable substitute for bone and for the first time, it is focused on modeling and optimizing the effective parameters in milling namely cutting speed, feed rate, tool diameter and cutting depth for analyzing the surface roughness using the surface response methodology. The second-order regression equation governing the model is drived and the influence of the input parameters and their interaction on the surface roughness as the output parameter is carefully investigated. Also, sobol statistical sensitivity analysis is used to ascertain the effect of process input parameters quantitatively. The results show that in order to achieve the desirable surface quality, minimum of feed, minimum rotational speed, smaller tool diameter and low cutting depth should be considered. Results show that among all effective input parameters tool rotational speed, feed rate, tool diameter and ctting depth have the highest influence on process roughness respectively. The behavior of each output parameters with variation in each input parameter is further investigated.

In the present paper, the optimal geometry of a two-dimensional multi-channel with one inlet and three outlets is investigated at Re=10. In this study, the topology optimization based on a porosity method using lattice Boltzmann simulation is adopted to find the optimal layout by computing the sensitivity analysis of an objective function. In contrary to previous studies of the channel with constant width of the outlets, the average velocity of the outlets is maintained as the same while the energy dissipation is reduced by 26.04% in comparison of the results, showing the adavntages of changing in width of ducts rather than the change in their average velocities. In this case, the geometry conditions for the inlet duct including width and position of the duct, play an important role in the final design as the main goal of this research. Assuming that the duct width in the outlets changes linearly, the numerical results showed that the equality of inlet width with the largest outlet caused the lowest power loss for the flow. The final geometry was getting much smooth when the inlet was located in front of the outlet with the largest width however it cannot be definitely optimum. In order to obtain the lowest energy loss, the inlet should be parallel to the space between the two outlets with medium and large widths. A reduction of 23.18% of loss was found in this case rather than the inlet was set in front of the duct having the smallest size.

In the turbines with a low mass-flow inlet, the partial-admission teqnique is used. Inorder to obtain a high specif work, it is necessary to use the turbine in supersonic flow regime. Firstly in this paper, the partially-admitted supersonic turbines and their applications are described. The main objective of the paper is to investigate the effects of the stator's nozzle divergent part profile on the turbine's performance. To this end, several different models are made using a nozzle profiling techniques. The flow field in the complete turbine pack is numerically simulated with the aim of computational fluid dynamics. The turbine's performance is investigated by examining the efficiency of the first law and the second law of thermodynamics, as well as the analysis of exergy and exergy degradation. The results show that the selection of suitable nozzle profile has an effective role in increasing the turbine efficiency and prevention of wasting energy.

The proper design of the stator turbine supercharger is of great help in increasing its performance and overall engine efficiency. In this research, with the help of FLUENT software, the local entropy generation and rate of exergy destruction of the turbine stator vane has been investigated. In the analysis, local entropy generation is divided into two sections: viscous entropy generation and thermal entropy generation, and the governing equations are defined with the help of UDF within the FLUENT software. After calculating the local entropy generation and determining the value of irreversibility quantities, According to the Gouy-Stodola equation, the amount of exergy destruction is calculated by numerical method. The results show that the two models of the Spalart Allmaras and k-ω(SST) have performed the best prediction wake of the vane. The amount of viscous entropy generation and thermal entropy generation is 69% and 31%, respectively. The values of the local entropy production calculated with the results of a stator turbine vane of authentic paper are validated with acceptable adaptation in it. The calculated exergy destruction is 281 kW, located in the range of modern gas turbines for a high-pressure turbine stage.

A two-dimensional numerical simulation of the spray flame has been carried out in a smooth counterflow configuration, and the region of formation of droplets group combustion by the flame index parameter for strain rate, equivalence ratio and the diameter of different droplets. n-decane (C10H22) is used as a liquid spray fuel, and a one-step global reaction is employed for the combustion reaction model. Fuel droplets are randomly injected using an UDF code in the air inlet and the droplet motion is calculated by Lagrange method. According to the results, in the ratio of high ratios, high strain rates and large droplet diameters, The temperature of the flame center decreases due to the suppression of the chemical reactions resulting from the reduction of oxidizing fractionation, and the dominant flame regime in these states is diffusion. The external group combustion of droplets occurs in the upper and lower layers of flame, and combustion of the internal group occurs at the center of the flame.

Nickel has great potential for engineering applications in the field of coating and exterior surface due to have good anti-wear and anti-corrosion properties with acceptable strength and ductility. In this study, molecular dynamics simulations of nano-indentation were performed using a single crystal of Nickel with the hemispherical shape of diamond tip. The substrate of workpiece and indenter was modelled using hybrid interatomic potentials including two-body and many-body potentials. Result of derived hardness in various indenter depths was validated with another research paper. At the atomic scale, the crystal structure has directional properties. When nanoindentation of one atomic layer is completed and it is the turn of the next layer, the mechanism of deformation, tip force and hardness can be changed. So in this paper, the effect of crystallography orientation was studied. According to the results, nickel at the crystalline surface (111) had the maximum hardness at depth of 1.5 nm. Also, simulations were conducted with rigid and non-rigid indenters to study the effect of tool deformation on derived hardness. The results show that due to the lack of change in the rigid shape of the tool, the tip force increased by 6.4% in nickel single crystal. Furthermore, due to the decrease of contact level and increase tip force, the hardness increased up to 3.6% in rigid tool compared with non-rigid indenter.

In this paper, theoretical and experimental simulations of the UAV hull’s structural strength in the landing phase on the water surface have been studied. In the seaplane design, calculating the strength of the hull structure against impact from the water surface is of special importance. Initially, the hull was numerically simulated using the finite element method, the interaction of the structure with the fluid (two-phase). Then, by performing an experimental study with variable speed, the results of numerical solution and experimental experiment are compared and evaluated. Comparing the results and graphs obtained from the numerical method with experimental experiments, it has been observed that throwing the seaplane free fall on the water surface can be a simulation of the most critical state for landing. The results show that increasing the descent height and velocity of the device when colliding with the water surface, causes the highest strain gradient along the axis perpendicular to the floor of the waterfowl and the least changes along the axis tangential to the water surface. According to the data obtained from the Tsai Hill and Tsai Wu rupture criteria for numerical solution and experimental testing, the values of stress and strength of the seaplane hull structure are within the allowable design range.

In the use of numerical control machines, offline setup has been possible by using a variety of software applications in the field of modeling and machining without the need to execute a process by the machine. Therefore, many decisions can be made before the machining process. The machine vision is considered as one of the technologies that can be used for offline setup of the workpiece which located on cnc machine tools before machining operations. In this research, this method has been used to determine the position of two casted part samples on a cnc milling machine. The mean error after 10 times testing in order to finding a circular center of Some kind of casted cap as workpiece origin includes 0.361 mm along the x axis and 0.372 mm along the y axis and the mean error of finding the workpiece origin of the casted pump impeller was 0.2 mm. Also, mean error for finding edge location of the molded cap in order to reduce the time for determining the movement direction of the tool relative to the workpiece which is clamped on the machine tool table, includes 0.25 mm in both x and y directions, and 0.4 mm along the z axis. The model error recovered from the extracted points were obtained by processing the images of the casted pump impeller to determine its position includes 0.363 mm in all three directions x, y and z.

In this paper, dynamic analysis of a microbeam conveying incompressible fluid flow and resting on a nonlinear viscoelastic-Pasternak foundation under the action of an axial load is investigated using the modified couple stress theory. Two boundary conditions including cantilever and clamped-clamped are considered for the microbeam with rectangular and circular hollow cross sections. The effect of changes of different parameters such as fluid flow velocity, the length scale parameter (size effect), linear and nonlinear stiffneses of foundation, foundation’s damping coefficient, shear layer stiffness of foundation and the value of axial force on the linear and nonlinear natural frequency of microbeam have been investigated. Increasing the axial force leading to tangible decreases of the natural frequencies for the microbeam. Moreover, the stability range of a cantilever microbeam is lesser than the one for microbeam with clamped-clamped boundary condition. Also, using the modified couple stress theory, the frequency and critical fluid velocity stability range for the microtube is greater than the one obtained using classical beam’s theory.

In this research, the dynamic analysis of composite functionally graded thick beam despite transverse cracking with the help of Tymoshenko theory and higher - order shear deformation theory has been studied. The governing equations and boundary conditions for composite functionally graded beam and thick using Timoshenko and Reddy theories and also, the principle of system energy minimization has been obtained. It is assumed that the inhomogeneous mechanical properties of the beam as a function of thickness as a function of power in which the thickness of the beam is variable. The boundary condition as clamp-clamp are also considered. According to these conditions, the closed form solution for natural frequencies of composite functionally graded beam despite the crack is obtained. Then, the results were analyzed and by those published in the literature and with the finite element results obtained by ABAQUS are validated. Finally, the results showed that the percentage of improvement of the answer obtained from Reddy's third order shear theory compared to Tymoshenko's theory for thick beam is about 24% and for thin beam the answers are close.

In this paper, the equations of motion are derived based on the improved third-order shear deformation theory with shear correction factors to investigate the free transverse vibration of rectangular sandwich panels having Levy boundary conditions. Reddy’s third-order theory devote a parabolic distribution for transverse shear stress along the thickness of plate and is suitable for thin to relatively thick single-layer plates. However, for the case of sandwich panels, in Reddy’s theory, the continuity condition of inter-laminar surfaces is not satisfied. This defect is improved by adding shear correction factors only in the energy viewpoint. Sandwich panel consist of isotropic facesheets and an auxetic honeycomb core making from the same facesheet’s material. The effective properties of honeycomb core were derived from the newest revised Gibson model based on Timoshenko beam theory. For validation, some comparison study is carried out to compare the current solution with the results reported in literature and also finite element ANSYS software.The results of validation indicates the important effect of suitable shear correction factors to decrease error percentage. Finally, the effect of boundary conditions, panel thickness to length ratio, core thickness to panel thickness ratio and geometrical parameters of the reentrant hexagon cell on the non-dimensional natural frequencies were investigated and the results were presented in some graphs.

Through experimental work and finite element simulations this paper investigates the effective geometrical parameters which absorb energy for structures with negative Poisson's ratio (auxetix structures). In the experimental section, the re-entrant auxetic structure is made from aluminum 1100 and subjected to quasi-static loading as well as stress-strain diagram. Then, the energy absorbing and specific energy absorbing values are calculated. Also, the amount of negative Poisson's ratio (NPR) is evaluated for every step of loading, and the results are compared with the finite element method. Good agreements are found between the experimental and FE results. Next, the effective parameters for energy absorbing including horizontal and oblique strut, initial angle, and structural thickness are investigated. The results show with the increase of the structural thickness and decrease of the initial angle, both horizontal and oblique strut length absorbing energy and specific absorbing energy increase. Finally, the results show, comparing to the honeycomb, the auxetic structure has higher ability to absorb energy.

In the present work, for the first time, using the finite - difference based lattice Boltzmann method, the convection heat transfer of the nanofluid in a cavity and channel is simulated. And also the effects of various factors such as Rayleigh number, nanofluid volume fraction and Reynolds number have been investigated. The Large Eddy Simulation (LES) method applied for modeling the turbulent flow. The natural convection heat transfer in the cavity for the Rayleigh range of 103 to 1010 and the volume fraction range of 0 to 1% has been evaluated. The forced convection heat transfer convection in the channel for the Reynolds number range of 50 to 3000 and the volume fraction range of 0 to 1% has been evaluated. The results show that the finite - difference based lattice Boltzmann method is able to simulate turbulent flows in different geometries. The results also show that natural convection heat transfer in the cavity, by enhancing Rayleigh number and the nanofluid volume fraction, the heat transfer rate increases. The forced convection heat transfer in the channel, enhancing the Reynolds number and enhancing the volume fraction of nanoparticles increases the heat transfer rate.

In this study, the effects of environmental conditions on solar chimney exergy have been investigated and a strategy has been proposed to select a suitable city for solar chimney utilization. The effect of parameters such as radiation intensity, ambient temperature and ambient humidity on the efficiency of the second law of solar chimney is investigated. Five different cities in Iran (Arak, Ahvaz, Tabriz, Rasht and Kerman) are considered. Then the exergy efficiency over a year is evaluated for all cities. Using the Analytical Hierarchy process and based on exergy efficiency and economic function, the studied cities in Iran have been ranked for installation of solar chimney. The results show that in the exergy analysis, the most effective parameters are radiation intensity, ambient temperature and ambient humidity, respectively. Also, the city of Ahvaz has the highest exergy efficiency during the year compared to other cities. Most of the cost function is related to Tabriz. Ahvaz city is ranked first with 0.3 and Tabriz with 0.16 is the last place to install solar chimney.

In this study, thermal modeling and optimization of combined production of cooling, heating, power (CCHP) and desalinated water system are carried out. Desalination system is coupled with CCHP plant which is supplied its required energy by prime mover and an auxiliary boiler. Three types of prime mover including gas turbine, gas engine and diesel engine are studied separately, to provide some of the power and heat required. Optimization is performed for each prime mover considering total annual cost as objective function. Developed code for modeling and optimization is written in MATLAB and optimized by genetic algorithm to reduce annual costs. The optimum results show that the diesel engine with total annual cost of 1.552271×106 $/year is the optimum prime mover compared with other studied prime mover. On the other hand gas turbine and gas engine are in the next ranking respectively with the 1.89904×106 $/year and 1.931765×106 $/year as annual cost. In other words, the annual cost of the system with diesel engine prime mover compared to gas engine and gas turbine engine have decreased by 19.64% and 18.26%, respectively.