مجله مکانیک سازه ها و شاره ها
سال ششم شماره 2 (تابستان 1395)

Choosing the appropriate end supports has been great importance in rotating machinery. The mechanical bearings (ball and journal) are more popular types of supports that used in rotating systems. However, the rub-impact between the rotor and bearing is main disadvantage of these types of bearings. Whereby, the active magnetic bearings have been developed recently that removes the rub-impact, but induces new nonlinear factors that affect the dynamical behavior of system. An unbalanced disk is mounted on the shaft. The rotor is modeled as three masses and 8 D.O.F. The governing equations are extracted in form of nonlinear coupled ordinary differential equations. The influence of magnetic bearing stiffness on the chaotic behavior of a flexible rotor supported by active magnetic bearings is investigated. The bifurcation diagrams, phase planes, power spectra, Poincare map and maximum lyapanov exponents are used to analyze the response under different operational conditions. The numerical results shows a rich variety of nonlinear behavior including periodic, sub-periodic, quasi-periodic and chaotic vibration due to active magnetic bearing stiffness. Also the results reveals the significant changes in the chaotic regions in 8 D.O.F model.

One of the main challenges in the design of the flexible flying vehicle controller is large parametric variation in flight. Problems to control the vehicles arise due to long and slender body and disturbance forces and forces generated by moving control surfaces causing the properties aeroelastic in these vehicles. The effect of flying vehicle's elastic behavior appears as vibration and error creation in measurement sensors and due to the interaction of each components on the other, it will have undesired effects on control system. In this paper, taking into account the different conditions of flight, L1 adaptive control performance has been studied. The results show that L1 adaptive controller with guaranteed stability and robustness can satisfactory be controlled undesirable effects of low-frequency modes of structural in a short time and in the presence of dynamic uncertainties, such as unexpected structural failures, time-varying disturbances and uncertainties and time delay in actuators the designed controller have very desirable performance.

In this paper the optimal values of effective parameters on the stress distribution around polygonal cutouts in isotropic plates are calculated. To achieve this goal, the complex variable method and PSO algorithm have been used. The expansion of the Muskhelishvili’s method are used to analyze the stress distribution in infinite isotropic plates containing various cutouts. By using conformal mapping, the area outside the non-circular cutout is mapped to the area outside of unit circle. The effective parameters on stress distribution around the cutout as design variables include: cutout shape, cutout orientatin and bluntness. The proper selection of these parameters leads to achieve minimum stress around the cutout and result in the load-bearing capacity of structures increases. The goal function in this problem is the maximum stress created around the cutout calculated by the analytical solution method. The results presented in this study shows that by choosing the appropriate shape of cutout and the optimal effective parameters, stress concentration factor can be significantly reduced and lowest stress concentration factor rather than the value of stress concentration corresponding to circular hole can be achieved.

In this paper, adaptive fuzzy integral sliding mode controller for controlling the position of the mobile robot on wheels in presence of motor’s dynamic, structural and un-structural uncertainties, existing in equations of mobile robot is designed. In the proposed controller, based on kinematic controlling of the back stepping method, the sliding surface dynamic controller the value of integral sliding mode controller is defined by a new method. Furthermore, to overcome undesired chattering phenomena in control input by using the fuzzy logic, a SISO fuzzy approximator is designed in a way to eliminate the chattering phenomena. Then to reduce the tracking error and to prevent, increasing of fuzzy system computational load, the adaptive fuzzy approximator will be presented, to approximate structural and un-structural uncertainties’ bound. Mathematical proof shows that the closed-loop system of kinematic control with adaptive fuzzy integral sliding mode control in the presence of all the uncertainties has global asymptotical stability. To show the performance of proposed controller, a case study of the wheel mobile robot in presence of DC servo motors is performed. Simulation’s results show the desired performance of the proposed controller.

In this study, conventional methods of the groove test on an incremental forming process by finite element method have been investigated in order to analyze the forces imposed by the spindle of CNC machine. Initially, the finite element method analysis using ABAQUS software was validated by empirical research in the published paper. In this analysis, tool and clamping are supposed rigid body and the sheet is deformable body. When using the side fixture for constraining of side edge of sheet was investigated, it was found that the use of the side fixture increase force between 2 to 2.5 times. However the number of tool sweeps would be halved and groove test would be completed with 1 tool sweep. Also by smoothly graded loading for prevention of traumatic force peaks to the machine, this test can be done at a lower cost. For example, for AA7075-0 sheet with 80mm forming path between two different steps 0.25 with 2 sweeps and 0.5mm with 1 sweep, using lower slope and 2 tool sweeps, the maximum vertical force peaks decrease and process is very proper for low load capacity CNC machine.

In this article, the vibration analysis of parallel Timoshenko beams connected by flexible connections is studied in which a moving mass passes from one or a number of the beams. In this system, the number of beams and flexible connections is arbitrary. The moving mass is considered to travel with a constant velocity or acceleration. All forces between the beam and the moving mass are considered such as gravity, coriolis, centrifugal, inertia and mass acceleration. The method involves a change of variables and modal analysis to decouple and to solve the governing differential equations, respectively. The eigenvalues and eigenfunctions of the system are obtained adopting transfer matrix method, andthe method of auxiliary functions is applied to separate the coupled second order differential equations. The displacements and the bending moments of the system subjected to the moving mass will be examined. The moving force formulation can be achieved by considering only the gravitational force. The system response by considering the moving mass is obtained and compared with the moving force problem and the effects of the inertia, coriolis, centrifugal and mass acceleration, is investigated, separately.

Recently, tube hydroforming had been developed in many industry. Prediction and prevention of possible defects such as wrinkling and failure are the major issues when it comes to hydroforming technique. These flaws are directly dependent on the initial and ultimate pressures and axial feed in this method. Thus, selecting an appropriate pressure path in a proper correlation with axial feed is crucial. In this paper, a new die capable of producing two-step box shaped pure copper specimens is presented by experimental test and finite element simulation. Compared with conventional hydroforming dies, it consists of four moving bushes to produce completely filled steps. Moving bushes can be considered as one of the advantages of the new die. When feeding, the movement of the specimen is accompanied by the movements of the bushes which results in elimination of friction. Also, sliding between the die and the specimen decreases to zero, so the produced specimen has filled and complete corners. Besides low forming pressure, simple die structure and cheaper machining costs as compared with conventional hydroforming dies can be considered as other advantages of the proposed die.

Nonlinear electro-magneto-thermo-mechanical analysis of a rotating disk made of piezoelectric polymeric material reinforced with multiwalled carbon nanotubes has been considered in this study. This material has viscoelastic behavior andthe elasticity modulusis calculated from Maxwell elasticity modulus inBurger’s viscoelastic model whichtime, stress and temperature dependent.The disk has been placed in an axisymetric distributed temperature and magnetic field and is subjected to an axial centrifugal body force. A non-homogeneous differential equation with variable coefficients is derived using stress-strain relation for plane-stress condition, strain-displacement relation, the equilibrium equation, charge equation of electrostatics and the Maxwell’s equation due to variation of the elasticity modulus with respect to stress and temperature.A numerical method in conjuction with a semi analytical solution has been developed to obtainelasticity modulus, displacement, stresses, strains and electric potentialdistributions.Applying magnetic field and electric potential reduced displacement and the deformation of thedisk can becontrolled by applying a suitableelectric potential and magnetic field. An electric potential distribution due to piezoelectric property of the polymeric material will be distributed throughout the disk.

In this paper, the Element-Free Galerkin (EFG) method is employed to obtain three dimensional static behavior of thick functionally graded plates. The Poisson’s ratio is taken to be constant and the Young’s modulus is considered to be graded through the thickness of plate by an exponential function. The shape function is calculated using the 3D moving least squares (MLS) approximation. Because the MLS approximation lacks the Kronecker delta function property, therefore the constrained Galerkin weak-form is used. The Lagrange multiplier method is employed to enforce the essential boundary condition. Effects of weight functions, nodal density and the dimensionless size of the support domain are investigated and favorable value for the dimensionless size of the support domain is calculated. Also a new trigonometric weight function is introduced. Effects of functionally grading index, dimensionless thickness and boundary conditions on the stress and deformation of the plate are investigated. Several examples are presented for thick functionally graded plates under static load. Also in order to verify the obtained results, they are compared with the results of other data reported in the literature. Numerical results indicate that the rate of convergence of the proposed method is higher than that of finite element method especially for stress calculation.

In the present study, A finite element method based on the mixed theory is developed for the coupled-field analysis of sandwich and laminated plates containing piezoelectric layers. The concept of mixed theory is that the mechanical component is modeled by the global–local theory which satisfies the free surface conditions and the geometric continuity conditions at interfaces, whereas the electric field is modeled with layerwise theory. In the present theory, The global displacement components are assumed to be linear and the local components are considered to be piecewise linear. The transverse shear stresses are derived based on the three-dimensional theory of elasticity instead of the constitutive equations.
Accuracy of the present approach has been verified by comparing the present results with those of the three-dimensional theory of elasticity , the higher-order global–local and layerwise shear deformation theory available in the literature. The numerical examples analysis show that the present theory is suitable in predicting coupled behaviors of sandwich and laminated plates containing piezoelectric layers under mechanical and electric loadings.

Multiple diaphragm couplings are a special type of flexible couplings of rotary equipments. These couplings are designed in multiple thin diaphragms instead of thick one to reduce the loads transmitted to the equipments and stresses induced to the diaphragms in misalignment. Because of need for different analyzes, such as static and dynamic analysis, modal, thermal, creep, fatigue and combination of these, and low stresses and flexibility, the design process of diaphragm coupling are complex, time consuming and usually performed iteratively. In many of these analyzes, the finite element solution is practically the only solution method. However, if we give analytical solutions for some of the analysis, the design process will be simpler and finding the optimum design is much faster. In this paper the exact analytical solutions for calculation of deformations and stresses in the diaphragms under axial and angular misalignment are presented and close-form solutions for axial and angular stiffness are obtained. In the end we confirm accurate analytical solutions by comparing the finite element results and determine the most critical point in the fatigue design of diaphragms.

In many applications such as marine and nuclear industries, laminated composite plates are subjected to the hygrothermal environment which developed expansion strain in the plate; in such that the expansion strain influence on vibrational behavior of the plate. So in this paper vibration of symmetric laminated composite plates under strain due to moisture and temperature are studied. For analysis, first using the classical theory of plate (thin plate theory) and Newton’s method which the influence of the rotary inertia and shear stresses in the thickness direction are ignored. equations of motion of plate are extracted and then using the energy method, eigenvalue problem is achieved. Then the eigenvalue problem is solved using trigonometric Ritz method and frequencies and mode shapes of plate are calculated. For validation of solving method, the results are compared with results that are presented in some papers by others. Finally the effect of different parameters such as boundary conditions, thermal gradient and percent of moisture, geometrical dimensions of plate and fiber orientation on modal behavior of plate are investigated and the results are presented in some tables and graphs.

Present study describes the approach of applying Multi-Objective optimization method to optimizing of sheet metal forming Die. In many studies, Finite element analysis and optimization technique have been integrated to solve the optimal process parameters of sheet metal forming by transforming multi-objective problem into a single-objective problem. This paper aims to minimize the objective functions of fracture and wrinkle simultaneously. Design variables are blank-holding force and draw-bead geometry. Response surface model has been used for design of experiment and finding relationships between variables and objective functions. In designing of experiments v-optimal design has been used which minimizes the average prediction error variance, to obtain accurate predictions. Forming Limit Curve has been used to define the objective functions. Finite element analysis applied for simulating the forming process. Proposed approach has been investigated on a drawing part and experimentally verified. The optimal design showed a good agreement with experimental species. It has been observed that proposed approach provides an effective solution to design of process parameters without a the ‘trial and error’ procedure.

Thin-walled structures have been extensively usedas energy absorbers in automobile and aerospace industries.This paper treats the collapse behaviour and energy absorption response of brass cylindrical tubes subjected to axial loading, using experiment and non-linear finite element models. In experimental approach, brass cylindrical samples were made by the process of extrusion. These samples are compressed between two rigid platens under quasi-static loading conditions and the collapse mechanism, the variations of crushing load and absorbed energy are determined. A numerical model is presented based on finite element analysis to simulate the collapse process considering the non-linear responses due to material behaviour, contact and large deformation. The comparison of numerical and experimental results showed that the present model provides an appropriate procedure to determine the collapse mechanism, crushing load and the amount of energy absorption. Numerical simulation techniques validated are used to carry out a parametric study of brass cylindrical tubes. In the following, influence of important parameters such as geometry imperfection (wall thickness gradient and wave formation), boundary condition, semi-apical angle, multi-cell columns reinforace and velocity impact was investigated. The results of this paper highlight the advantages of using brasscylindrical tubes as energy absorber.

Today, gas turbines are considered as one of the most important equipment in energy industry and with a lot of applications in other industries. So far, very few CFD studies on the effect of roughness on turbine performance have been performed. This paper presents numerical study of roughness which affects performance of a single stage axial flow turbine. Numerical calculations have been performed using an in-house developed software in the C environment employing Roe scheme to solve governing equations in generalized coordinates. In order to simulate the turbulent flow the Baldwin-Lomax model and for simulating the roughness the Cebeci-Chang model is used. To realize the roughness effects in a turbine stage, several roughness heights in transitionally rough and fully rough flow regimes on the stator and rotor blades have been simulated. Results show that the efficiency is reduced with increasing roughness height. Also, with increasing the surface roughness of the stator and rotor, deviation angle will increase and thus the work coefficient will reduce. Moreover, the loss coefficient in both stator and rotor blades is increased especially, suction surfaces are faced with more losses.

One of the most important issues of applied Hydrodynamics is Analysis of Moving hydrofoil near the free surface. In this paper attention is being paid to the analysis of hydrofoil near the free surface. For this simulation, an iterative method based on Green’s theorem is employed, and the problem is divided to hydrofoil and free surface and the effects on each other is calculated, and then perturbation potential on hydrofoil and free surface are acquired. Next, the values of these potentials are modified with an iterative algorithm until the results converge to real values. Then by having these potentials, Pressure distribution on hydrofoil surface and curve wave on free surface are obtained. Having validated this method, various factors on the hydrofoil performance such as thickness, camber, angle of attack, the Froud number, distance from the free surface, and distance from depth are surveyed. It can be observed that the results of boundary element method with good approximation predict the flow performance. However, the existence of an ideal fluid is assumed.

In this article, laminar mixed convection heat transfer of CuO-water nanofluid flow has been investigated in a vertical square duct. Constant Wall heat flux condition is considered for the walls. Governing equations are solved three dimensionally in steady state. Two phase Euler-Lagrange approach is employed to simulate nanoparticles behavior. Governin equations was discretized using control volume based finite element method (CVFEM). Effect of different parameters such as nanoparticles concentration, Reynlods number and Grashof number on heat transfer coefficient, nusselt number, velocity and temperature profiles, friction factor and particles diffusion is presented and discussed. Due to the importance of Brownian and Thermoforetic forces in nano scale analysis, effect of these forces on heat transfer rate, analyzed by Euler-Lagrange approach, is presented. Results show increasing nanoparticles concentration improves convective heat transfer coefficient, while has no significant effect on friction factor. As an example for φ=2%, enhancement of heat transfer coefficient exceed to 25%. Moreover increasing Grashof number decreases heat transfer improvement and increases skin friction on the walls.

In this study, the centrifugal fan of a cooling turbine is numerically investigated and is then compared with the experimental results at a performance point. The experimental performance characteristics of the fan are as: rotational speed and flow parameters at the inlet and outlet of the fan. In this cooling turbine, the fan is used for cooling of the turbine inlet air flow. The aims of this study are obtaining the fan performance characteristics curve and 3D numerical simulation inside the fan. Rotor and case modeling is respectively conducted using Blade Gen and Catia software, the grid generation of the rotor domain and fan casing is respectively accomplished by Turbo Grid and Ansys Mesh software and finally, 3D numerical solution of flow inside the fan is done by CFX software. In the CFX software, compressible flow equations are solved using pressure based method with SST turbulence modeling. To verify the numerical results, grid independency is investigated. Finally, the numerical results are compared with the experimental results that shows a good agreement.

Today, the development of unmanned aircrafts with specific performance characteristics, Including UAVs which capable to fly at high altitude and long endurance is very regarded. In this paper, we have analyzed a nonlinear aeroelastic wing with a high aspect ratio as well as a store (tank) which attached to the wing. Also, the aerodynamic model and structural model have been coupled in steady states. The equations of motion have obtained from Hamilton's principle and the Lagrange equations have acquired from three modes of “bending outside the plate", "bending inside the plate" and” torsion". First of all, we have examined the aeroelastic stability analysis “k” approach. Then, by considering the non-linear terms in equations by using fourth order Rung -Kutta approach, we have studied the results of the simulation and noticed to some phenomena like limit cycle oscillations and bifurcation. The Nonlinear terms are structure and store types and aerodynamics flow have been studied in the linear modes. For solving the equations we have used Galerkin method. Also, the equations have governed in the domain time. By Comparing the results, acceptable accuracy of our modeling and undertaken analysis has observed.

This paper studies numerically the problem of flow control on NACA0012 airfoil with porous surface in transonic flow. The flow is assumed turbulent and stationary. Results show that the normal shock intensity on airfoil surface is weak and consequently, the drag coefficient decrease 21 percent. This passive method also postpones the separation point. The penalty of this inappropriate effects reduction is lift reduction. The effects of this flow control method on lift and drag coefficients are investigated in this paper via geometric modeling of the porous surface. The efficiency of this method depends on various factors like as geometric parameters. For this purpose, we investigate the effects of cavity depth, cavity location, cavity length, number of porous surface holes and cavity shape in this parametric study. Finally the results of the parametric study show the optimum of cavity depth is 0.7 of the airfoil thickness, the optimum location of cavity is then 75% of the cavity length is located in upper side of shock wave, and the optimum length of cavity is 0.2 of chord. Study of geometric shapes of different cavities shows that optimum shape of cavity is ramp.

Non-destructive tests can identify and investigate the defects and properties of the test piece without changing the physical and mechanical properties of the sample. In this study, the non-destructive inspection method of ultrasonic waves was used to investigate the rubber formulation. In this method, the time between the transmission and the reflection of ultrasonic waves is measured and using this time propagation velocity is calculated. As components percent of the rubber are changed the physical and mechanical properties of rubber are altered and as a result, the velocity of ultrasonic propagation is changed. To investigate the rubber formulation at first 12 samples with different formulations were prepared and for each of the samples the propagation velocity of the longitudinal sound waves was measured. In order to establish the mathematical model between used elements percent in the rubber formulation and longitudinal wave velocity the multiple linear regression was used. To evaluate the accuracy another sample with a new formulation was developed and longitudinal wave velocity was measured. The comparison between the results of the test and those of the regression model showed a low error in the predicted result by the proposed model; therefor, the ultrasonic waves can be used to investigate the rubber formulation and the rubber production lines can use this non-destructive test to control tire quality online.

In this study, effects of various boundary conditions on the free vibration of double layer graphene nanoribbons (DLGNRs) are investigated by considering both of tensile-compressive and shear effects of van der Waals (vdWs) interactions. Sandwich beam theory is used to model the DLGNRs. In the references, only one of tensile-compressive or shear effects of vdWs interactions have been considered. Based on sandwich beams theory, vdWs interactions are equivalent to the sandwich core and are modeled in a way that they can withstand the tensile-compressive and shear forces simultaneously. Hamilton’s principle is employed to extract governing equations of motion and boundary conditions. Harmonic differential quadrature method is utilized to investigate natural frequencies and related mode shapes of DLGNRs. In order to verify, results are compared to other literatures in a condition that one of the vdWs effects to be neglected. The effect of boundary condition and interlayer shear direction on the mode shape, sequence and value of DLGNRs natural frequencies are investigated.

This study investigates the feed water heating repowering of Isfahan Montazeri Steam Power Plant is. To do this, three designs have been suggested; low pressure heat exchangers, high pressure heat exchangers and both of heat exchangers respectively are replaced by the new heat recovery heat exchangers. Energy and exergy efficiencies are used as goal functions. Cycle-tempo software is used to simulate. As it shown in results, in the method of replacing all heat exchangers with two heat recovery heat exchangers, the maximum exergy efficienciy is reached. In this case, The gas turbine westinghause-401 is the best choice that energy and egergy grass efficiencies and power production of the cycle increase 12.8, 13.1 and 42 percents, respectively.
This study investigates the feed water heating repowering of Isfahan Montazeri Steam Power Plant is. To do this, three designs have been suggested; low pressure heat exchangers, high pressure heat exchangers and both of heat exchangers respectively are replaced by the new heat recovery heat exchangers. Energy and exergy efficiencies are used as goal functions. Cycle-tempo software is used to simulate. As it shown in results, in the method of replacing all heat exchangers with two heat recovery heat exchangers, the maximum exergy efficienciy is reached. In this case, The gas turbine westinghause-401 is the best choice that energy and egergy grass efficiencies and power production of the cycle increase 12.8, 13.1 and 42 percents, respectively.

In the present study, the blade shape of a new kind of VAWT that has a different structure in comparison with other drag-based wind turbines, hunter turbine, is investigated and then optimized. The purpose of this study is designing a blade with the greatest drag coefficient in order to increase the power coefficient of the turbine. In this study 4 kinds of flat blades, including squared, circular, semi-circular and compound flat plates, have been investigated in static state experimentally and numerically. The numerically simulation has been carried out by assuming the in compressible, unsteady two-dimensional and steady three-dimensional flow. Flow simulation has been accomplished by discretizing and solving the Navier-Stokes equations with shear-stress transport (SST) k-ω turbulence model. This model simulates the near-wall flow by directly solving Navier-Stokes equations. In experimental method, the drag force is measured by a load-cell and the drag coefficient has been calculated by using the blade area. The results show a great agreement between the experimental and numerical data. It is concluded that the squared blade has the greatest drag coefficient among the considered cases, which is 1.18.

In this paper, the effect of non-uniform magnetic field on temperature and velocity profile for a boundary layer flow over a moving and horizontal flat plate by using of a self-similar method is studied. A variable and non-uniform magnetic field is considered and it is assumed that there is suction by the wall with a variable velocity.It is assumed that The velocity out of boundary layer is varying with x. For solving, governing partial differential equations by help of similarity parameter and similarity solution were changed into ordinary differential equations and then by using of shooting method and forth-order range-Kutta method, all equations are solved. At the end, the effect of dimensionless parameters on the velocity and temperature profiles, are given. It is revealed that by increasing the magnetic field parameter, the velocity inside the boundary layer decreases and the temperature increased since the fluid particles in the boundary layer are ionized.