مجله مکانیک سازه ها و شاره ها
سال یکم شماره 1 (تابستان 1390)

Isogeometric Analysis is a newly developed approach for analysis of engineering problems. Due to some interesting features of this method it can be a potential substitute for other numerical techniques such as finite elements and meshfree methods. Derivation of the formulation and application of the approach to analysis and shape optimization of axisymmetric problems is the subject of this paper. To evaluate the activity of the method, an example with available analytical solution is considered and solved. Also, two optimization examples are presented and solved by using the SQP optimization algorithm. The obtained results show that by using the isogeometric analysis method, the optimum shape can be achieved by a smaller number of design variables. Also, due to elimination of several mesh generations from the process, the computational cost is considerably decreased.

The concept of random tessellation is extensively used in wide area of natural sciences, especially material sciences. In this paper a simple but complete explanation of the random tessellation and mathematical tools requirements is presented. Then introducing the algorithm and the program for display random tessellation diagram was written. This program, with high speed and simple algorithm for random tessellation has the ability to change the level of statistical parameters such as number, mean, variance of the area of the grain. The ability to model microstructures of metals and grains for mechanical application, such as estimation of mechanical properties and crack propagation model at microstructure scale in FEM software is very important. More, an application of random tessellation for finding of equivalent mechanical properties was mentioned. In this paper by increasing the number of grains, in other words, decreasing the size of the grain, property’s values were changed and approached to the experimental global values and isotropic properties.

Automatic crack propagation and stable fatigue crack growth as complicate problems in fracture mechanics are studied. In this study The Extended Finite Element Method (XFEM) is utilized because of several drawbacks in standard finite element method in crack propagation modeling. Estimated Crack paths are obtained by using Level Set Method (LSM) in coupling with XFEM for 2D mixed mode crack propagation problems. Stress intensity factors for mixed mode crack problems are numerically calculated by using interaction integral method completely based on familiar path independent J-integral method. Different crack path growths are shown for different types of boundary conditions. Also, stable fatigue crack growth due to numerous cycles of loading using Paris law is presented. Finally, some experimental results and analytical solutions for stress intensity factors are concerned to guarantee the numerical estimated crack path growths and stress intensity factors respectively.

Motion and deformation of a sequence of drops rising through an immiscible viscous fluid (water) were studied experimentally using an image analysis method. In order to have a complete investigation between the behavior of a drop moving in the wake region of another one and a single drop moving freely in a quiescent fluid, we compared the shape evolution and the drag coefficient related to a single drop and a distinct drop in a sequence. Also, the effect of non-Newtonian characteristics on the size variation and motion of drops was studied. Results show that addition of viscoelastic characteristics to a fluid causes a more stable shape in the drop and its deformation is closer to a single drop during motion in the wake region. Besides, in order to find a reasonable correlation between drag coefficient and Reynolds number for a drop moving in sequence, we used a two dimensional model based on drop trajectory. In addition, a theoretical correlation in the form was proposed by neglecting the effect of size variations of a single drop and it caused an error lower than 25% in comparison to real values. Comparison between drag coefficients of a single drop and a sequence of drops revealeddrops experience higher values of drag coefficient during the motion in sequence.

Full Load Thermoeconomic and environmental optimization of a hybrid Solid oxide fuel cell-Micro gas turbine system for use in distributed power generation has been investigated in this paper. For this reason, a multi objective optimization approach based on genetic algorithm has been incorporated. The hybrid system has been simulated in a computer code and all performance related results for assumed Decision parameters has been validated using available literature data. Decision parameters are calculated in the optimization procedure with respect to the system constraints, to reach the optimum criteria for both Exergetic and Economic Objective functions while the Environment damage penalty is also added to the system total Cost. Effects of fuel unit cost, capital cost and System output size on optimum results has been considered. it is clear from the results that the most sensitive and important design parameter in this system is the fuel cell's current density which it's careful chose has significant effect on the balance between cost and performance of the system.

Inverse design is one of the design methods of aerodynamic ducts such as S-duct intakes. In these problems, the duct geometry is unknown but the pressure distributions along the walls are given. In this paper, a new inverse design method called “flexible string method” is introduced. In this method, the duct walls are modified from initial guess to final shape based on the flexible string movement algorithm according to pressure distribution. At first, in order to practical design of a 2D jet engine S-shaped air intake according to flight Mach No. and required conditions in the jet engine intake, a preliminary duct is designed using an inverse design algorithm and an Euler flow solver (with no considering the jet engine nose effects). Design of the lips engine is the second step. At the third step, the effect of the jet nose is considered. Finally the design of S-shaped duct is analyzed with a viscous code to study the performance of the designed duct.

This article presents a numerical investigation of turbulent flow in an axisymmetric separated and reattached flow over a longitudinal blunt circular cylinder. With respect to the complexity of the simulation of the separation phenomenon, some numerical techniques such as discretization methods and pressure-velocity coupling algorithms were compared to reach a proper numerical method to solve the considered flow case. Further, the results of the standard k-ε and shear stress transport (SST) turbulence models were used to predict the effects of turbulence were investigated. Besides, the effects of using the steady and pseudo-transient equations were discussed. Furthermore, a multi-grid method was employed to accelerate the convergence speed. All the numerical techniques were coded and compiled with Fortran 90 compiler. The code and computational method were then validated against experimental data, with good agreement. Finally, the structure of separation and reattachment of the considered flow case was investigated with the numerical code. Some characteristics of the flow such as reattachment length, velocity distributions, turbulent kinetic energy and pressure were discussed.

In most of the researches which has been conducted until now on the robot manipulator tracking control, it has been presumed which the kinematic of robot manipulator or Jacobian Matrix of robot from the joint space to the task space is quite known. However, none of the physical parameters in robot manipulator equations can be calculated by high accuracy. In addition, when the robot manipulator moves something, uncertainties occur in length, direction and point of contact of end-effector. Hence, uncertainties occur on the kinematic of robot manipulator either, and due to the different functions of the robot manipulator, its kinematic is changed as well. Therefore, ensuring stability of the closed-loop system in the presence of uncertainties in the dynamics and kinematics of robot manipulator is quite challenging. In this paper, for overcoming on these uncertainties, we present our simple robust control for tracking the position of robot manipulator in the presence of uncertainties in the dynamics kinematics and Jacobian Matrix of the robot manipulator. The proving of the stability shows the closed-loop system has Global asymptotic stability. Then, to overcome the practical problems of proposed control, amendments will be provided. The closed loop system with improved control has limited uniform stability. Since in many of the operations has been performed by the robot manipulator, transient error plays an important role. Due to this reason, the modified control structure is a changed in a way which could improve the transient error. Finally, for displaying the function of the final controller, a case study is implemented on a two-link elbow robot. Mathematical proof and simulation results confirm the good performance of the proposed control.

Free surface sloshing of liquid in a rectangular tank induced by lateral excitation has been studied numericallyand experimentally in this paper. The numerical simulation is carried out based on the volume of fluid (VOF) method that provides free surface extraction in such problem.A tank with transparent walls is used to free surface tracking in experimental study while the movement amplitude and frequency of periodic motion is controlled by a reciprocating designed mechanical system. This mechanical device provides a pure lateral periodic motion for liquid tank exciting. Free surface of sloshing liquid in rectangular tankobtained by image processing. Also a test with a vertical plate in the middle of the tank is carried out to study of the solid baffle effects on sloshing mode shapes and especially on maximum and minimum ofliquidfree surface displacement. Comparison of numerical results and similar experimental tests shows good agreement for low amplitude and low frequency of tank excitations in maximum and minimumof free surface displacement. The average of relative errors estimated less than 10% while they increase by increasing amplitude and frequency excitations due to nonlinear behavior of liquid free surface.