مجله مکانیک سازه ها و شاره ها
سال چهارم شماره 4 (زمستان1393)

Shape design problems، in general، and inverse design problems، in particular، are often solved via optimization techniques. Evolutionary algorithms provide robust and efficient solution methods for such problems. This paper focuses on the application of genetic algorithms (GA)، particle swarm optimization (PSO)، and two hybrid variants of GA and PSO. Performance of these optimization methods in the solution of inverse design problems is examined and it is shown that hybridization of GA and PSO can be used to improve the convergence rate of the iterative design procedure. Global Minimums of a number of well known optimization test functions are found by the proposed hybrid algorithms and the solutions of both internal and external flow inverse design problems are discussed. Up to 30% speed up is observed in the numerical test cases when the hybrid methods are employed and it is also shown that hybrid methods can get closer to the optimum solution as compared to either GA or PSO.

This paper presents a novel model-free control for an active suspension system to achieve vehicle riding and passenger comfort in the presence of road disturbances. Compared with the previous designs، the proposed design does not employ the force control and uses only the position control. As a result، its design is simpler and less computational. The controller is designed for a quarter car active suspension system driven by hydraulic actuator with a nonlinear model. To obtain the proposed controller، the feedback linearization control and estimation of uncertainty and then compensation by time-delay approach are used. Therefore، it can overcome nonlinearity and uncertainty associated with model. It uses feedbacks of displacement، speed and acceleration of sprung mass. A state space model for the hydraulic suspension system is derived and the system stability is proven. Simulation results show the desired performance of the suspension system to achieve the convergence of body to the equilibrium point with an ignorable error. Comparing performance of the active suspension system with the passive suspension system shows the efficiency and superiority of the proposed controller in providing the passenger comfort and ride handling.

The use of Ti-64 titanium alloy has been expanded in the aerospace industry because of its unique properties such as lightness and stability at high temperatures. Unusual mechanical behaviors such as plastic anisotropy and asymmetry of tension and pressure of Ti-64 Titanium alloy is observed in different directions. In this paper، by choosing the appropriate yield criterion، isotropic work hardening and return mapping numerical method، elastic-plastic model is implemented using finite element analysis in Abaqus software and prediction of forming limit diagram at 400C° is done using Hill-Swift elastic-plastic theory and random finite element analysis. Material inhomogeneities are considered as random plate thickness field in the stochastic finite element analysis. Considering the effect of step size on accuracy of solution، appropriate step size is applied to prediction of forming limit diagram. Selecting 99% safety percentage in stocastic finite element analysis، the appropriate variance of the random field is considered and is used in predicting forming limit diagram. Also، effects of plastic anisotropy of Ti-64 titanium alloy sheet have been studied in forming limit diagram.

In this research، the general form of the governing equations that could be accounted as an essential prerequisite for simulation of a body is derived for an underwater varying mass body. In this study، the appropriate form of the aforementioned equations in the state space is presented and the procedure of motion simulation is illustrated. The hydrodynamic forces and moments، which could be viewed as the main inputs for simulation process، are generally nonlinear functions of system variables and are calculated with their respective coefficients. These coefficients، in turn، are associated with different system variables (such as control variables) and are calculated utilizing the MISSILE DATCOM code. Comparison of results associated to the current work and those of existing simple cases quantifies the accuracy of simulation procedure. The definition of body shape، simulation process، post processing and animating the body trajectory are all carried out in a general prepared software which is capable of simulating arbitrary underwater bodies consist of fixed wings and moving flaps.

The behavior and modeling of circular concrete sections confined by Fiber Reinforced Polymer (FRP) wraps have been recently studied extensively while non-circular sections have received much less attention. This study focuses on FRP-confined specimens with square sections under axial loading. A fuzzy logic based model is developed to predict the compressive strength of the FRP-confined square sections. The explicit formation of the proposed fuzzy model for the compressive strength of FRP-confined concrete is presented as a function of the section width، corner radius، compressive strength of unconfined concrete، thickness of FRP sheets، tensile strength and elastic modulus of FRP materials. The results indicated that the accuracy of the proposed fuzzy model is quite satisfactory as compared to a large database containing experimental results of 182 specimens assembled from the open literature. Moreover، the proposed fuzzy model is compared with the existing nine models developed by various researchers and is found to be more accurate.

In a forging process، a preform dies shape plays an important role in the improving the quality of forging parts and decreasing the cost of production. Some important defects in the forging process are die filling، folding، nonhomogeneous Metallurgical properties and large amount of flash. A proper preform die can reduce or eliminate these defects. In the present research، shape optimizing of preform dies in the two-stage forging process is investigated. A code is developed in ABAQUS commercial software in order to simulate the forging process. The optimization method is based on Genetic Algorithm (GA) supported by an elitist strategy which is developed by authors. To verify the proposed method، two examples have been studied. In the first example، folding defect will occur in the inner surface of a hollow cylinder if forging is done in a single-stage. In order to eliminate folding defect، forging process is done in two stages and the preform die shape is determined by optimization. In the second example the amount of flash is reduced from 13. 9% to 3. 5% in forging of a sliding cluster gear3rd and 4th of Peugeot 405 vehicles. For validation of optimization results the gear has forged in Ahangari Company by optimum preform die shape.

In this paper، a generalized modified molecular structure mechanics (MSM) model for analysis of carbon nanotubes (CNTs) is put forward. In this method، the interactions between carbon atoms of CNT are modeled using an equivalent beam element of general section. A systematic approach is presented to specify the local directions of each of the beam elements. In contrast to the original MSM method، the contributions from bond angle bending and inversion interactions are distinguished in the present model. For deformation modes corresponding to bond stretching and bond angle bend، nonlinear behaviors according to the modified Morse potential are considered، and for torsion and inversion a linear form is adopted. The proposed model suggests a size and chirality dependence of elastic moduli for CNTs. In particular، with increasing the tube diameter، the Poisson’s ratio of the nanotube decreases and approaches a value of 0. 2. Moreover، the nonlinear response of armchair and zigzag CNTs subject to tensile load up to the fracture point is studied. The present model enables capturing the progressive bond breaking and failure of CNTs with Stone-Wales and vacancy defects. The predicted elastic properties and tensile behaviors of nanotubes are found to be very similar to the results reported from the time consuming molecular dynamics simulations.

Many parameters affect the stress distribution around holes in perforated plates under tensile load. Rotation Angle (RA)، Fiber Angle (FA)، Load Angle (LA)، bluntness (w)، and shaped cut out (c) are design variables which influence the stress distribution. In this paper، the interaction effects of these parameters on the normalized stress are discussed. The optimum design of the perforated orthotropic plate is attempted in the presence of these parameters by Genetic Algorithm (GA) method. Based on the fitness function obtained from Lekhnitskii theory، the optimization model of stress distribution parameters was created using GA. Optimum parameters based to achieve possible minimum stress are predicted. Interaction of parameters affect the stress distribution around hole were investigated. Also، results showed that the type of material is an important factor influencing on minimum stress. The results show that stress distribution depends on the varied parameters and correct selection of these parameters causes the possible minimum stress.

The main effort of this work is on acquiring the Campbell diagram of a helicopter main rotor by processing the signals measured for the bending and torsional structural moments of its blade. These moments are gathered by some sets of strain gauges while increasing the blade rotational speed. Because of the non-stationary nature of these signals، the Short Time Fourier Transform (STFT) is chosen as the relevant processing method and applied to the moment signals. The result is the moment''s spectral content variation versus time. Combining the measured rotational speed signal with the results of time-frequency transform، moment spectral content variation versus rotational speed is acquired. By determining the local maxima of this diagram، it is possible to determine the resonance frequencies which are in fact the natural frequencies. Natural frequencies of the rotor under the test have been calculated using computational Holtzer-Myklestad method which then used to find the assignment of some of natural frequencies to corresponding natural modes.

In this research، the transient stress distribution in an adhesive joint due to fiber breakage has been investigated. Transient stress is a dynamic response of the joint to the fiber discontinuities till their static equilibrium state. To study this behavior، equations governing the motion of fibers in the matrix، due to their breakage، are derived and the effect of number of broken fibers and their location is studied on transient response of the structure. Shear lag model is used to extract fiber displacement. The equilibrium equations are solved using the explicit finite difference method. The effect of fiber material and adhesive thickness is also studied on stress distribution. Results show that for an increase in the number of broken fibers، and for locations of fiber breakage closer to the edges of the joint، the stress concentration in the composite structure increases. Moreover، the shear stress created in the matrix and the adhesive layer is reduced with an increase in fiber elastic modulus، such thatfor glass and graphite fibers (E=74 and 130 GPa respectively)، the maximum shear stresses in the adhesive are 0. 861 and 0. 461 MPa، while in the adherends، they are equal to 3. 192 and 2. 409 MPa respectively.

In this article، nonlinear axisymmetric bending analysis of sandwich circular plates with homogeneous face sheet and functionally graded core subjected to thermo - mechanical loads is presented. The formulations are based on first-order shear deformation plate theory (FSDT) and large deflection von Karman equations. The nonlinear equilibrium equations are solved using the dynamic relaxation (DR) method combined with the finite difference discretization technique. Material properties are compare to be temperature-dependent and temperature-independent. In order to verify the current work some obtained results are compared with Abaqus finite element method. Finally، The influences of material grading index، boundary conditions،tempresure، core-to-face sheets thickness ratio on the results are studied in detail. Also، in order to investigate the differences between the linear and nonlinear solutions some results are presented based on both linear and nonlinear analysis. Some results indicate that variation of core-to-face sheets thickness ratiohas greater effect on the results of annular geometry compared to circular one.

This paper studies novel structure of communication disturbance observer for teleoperation systems. In time-delayed systems that time delay is unknown and unpredictable، stability of system by using time delay compensation method based on the concept of network disturbance and communication disturbance observer (CDOB)، can be obtained. Since this method works without time-delay model، it can be implemented in teleoperation systems that contain communication channel with time-varying delay. So، in this paper، first، time varying delay compensation by conventional communication disturbance observer is studied and design condition of conventional communication disturbance observer by dynamic property of network disturbance is explained. However، the system model error and disturbance seriously affects the system performance and causes steady-state error. Hence، in this paper، by considering the effects of model uncertainty and disturbance on steady state characteristics، the structure of conventional communication disturbance observer is modified and another structure for it in teleoperation systems has been proposed. By using slave''s delayed torque instead of its delayed position as one of the observer''s input، steady state error is eliminated. Also، simulation results are presented to demonstrate the effectiveness of the proposed structure.

In the current study، the non-Newtonian fluid flow over a circularcylinder has been simulated using the Immersed Boundary – lattice Boltzmann method. In order to couple the Lagrarigian nodes on Immersed Boundary and Eulerian nodes in the fluid domain، the direct forcing method based on sharp interface scheme is employed. The split-forcing lattice Boltzmann method is used to apply the effects of boundary force on non-Newtonian power-law fluid which leads to more monotonic force implantation in times interval. The impact of numerical parameters on the accuracy of the introduced method has been investigated in details. The different fluid regimes consist of steady and unsteady flows in different Reynolds numbers and power-law indices has been investigated. The results show that the immersed boundary – lattice Boltzmann method can completely captured the properties of non-Newtonian fluids in presence of Immersed Boundary. In future،this algorithm can be used for modeling of moving bodies in non-Newtonian fluids.

In recent years، the lattice Boltzmann method (LBM) has become an alternative and promising computational fluid dynamics approach for simulation of complex fluid flows. Despite its huge success in many practical applications، the conventional (standard) LBM is restricted to the lattice uniformity in the physical space. This is an important drawback of the standard LBM for the application to flow problems with complex geometry. Currently there are several ways to remove this drawback of standard LBM. One of these methods is the Taylor series expansion and least squares-based LBM (TLLBM). This method is based on the standard LBM with introduction of the Taylor series expansion and the least squares approach. The salient feature of the TLLBM is the fact that the final equation is an explicit form and essentially has no limitation on the mesh structure and lattice model. In the present work، the TLLBM with D2Q9 lattice model is used to simulate 2-D steady incompressible viscous external flow on non-uniform meshes. Two test cases are studied: a) flow past a circular cylinder with a non-uniform O-type mesh; b) flow past a NACA0012 airfoil at different attack angles with orthogonal and non-uniform C-type mesh. It was found that this method can give very accurate results.

The study of an external magnetic field effects on fluid flow and heat transfer characteristic has direct applications to various physical phenomena. In this paper، flow and heat transfer of an electrically conducting fluid around a cylinder wrapped with a porous ring and under the influence of A constant magnetic field has been studied numerically. The range of Stuart (N)، Reynolds (Re) and Darcy (Da) numbers are 0-5، 1-40، and 10-8-10-1 respectively. The Darcy–Brinkman–Forchheimer model has been used for simulating the flow in porous medium. Governing equations provides the coupling between the flow field and the magnetic field. These equations with the relevant boundary conditions are solved numerically using Finite Volume Method (FVM). The effects of varying Stuart، Reynolds، and Darcy numbers on flow patterns and heat transfer rate are explored. Finally، two overall expressions for the average Nusselt number are proposed in which effects of Magnetic field and Darcy number are considered. It was found that in the presence of a magnetic field، the drag coefficient and the critical radius of insulation increases، while the wake length and Nusselt (Nu) number decreases.

In the large scale pipe systems the use of pump group connected in series is evitable to meet the desired pressure head. If one or more of the pumps shuts down، water hammer event occurs which manifests in pressure and rare faction wave، along the pipeline these waves result in huge stresses on the system. In this regard، this paper aims at investigating the pump stations performance during steady and transient flows، in which the focus is placed on various possibility of pump failure in a system of pumps in series. The mathematical model to carry out this research includes continuity and momentum equations (transformed to two characteristic equations) as well as equations to govern mechanical behavior of pump stations. These governing relations are numerically solved and programmed in a MATLAB code. The results reveal that the use of the same pumps leads to larger efficiency compound to using different pumps. Furthermore، if the pumps shut down one after other، the water hammer pressures remarkably alleviate. The result for this sequential pump station failure are quantitatively presented and discussed.