فصلنامه مکانیک هوافضا
سال چهارم شماره 3 (پیاپی 13، پاییز 1387)

Because of unsuitable surface quality resulted from wire electrical discharge machininig (WEDM), this method is not used for manufacturing of accurate parts, such as blades of gas turbine engines. Therefore, a post treatment is needed after WEDM. In this study, the application of sand blasting as a post treatment is investigated by design of experiment (DOE) technique. In this method, pressure, duration of the process, and the type of particles used in sand blasting process as input variables, as well as roughness and thickness of removed layer as response variables are considered. After performing the experiments and measuring the responses, the effects of inputs on response variables are investigated and the the optimal amount of inputs are obtained. The results show that sand blasting under specific condition can remove the recast layer with suitable roughness and hardness.

A non-linear model of a drill string system in an inclined well with axially moving motion and loading is established and analyzed by perturbation technique. The drillstring is modeled as a simply supported axially moving rotor. Based on Hamilton's principle, the dynamic model developed consists of kinetic and potential energy. Non-linear coupling terms are kept in the formulation, which leads to full coupling between the axial and the transverse vibrational modes. The method of multiple scales is employed to solve the non-linear equations, to analyze the steady state response, and to find the stability region of the system. Analytical and numerical results reveal the non-linear phenomena, such as primary and parametric resonance that have not been previously reported. Our results are compared with some existing experimental date showing relatively good agreements.

In this paper, an integral model, based on momentum balance, is developed for analyzing a gas- liquid two phase flow in a vertical pipe. This model can predict liquid flow rate and pressure distribution, using both empirical correlation governing on specific regime for two phase flow friction and numerical analysis on equation achieved from establishing equilibrium in each cell. The presented model can either predict the water or air flow rate for a given airlift system. Moreover, this method can optimize pipe diameter, volumetric flow rate, and efficiency of airlift pumps. Accuracy of this approach is greater than that of the mean void fraction model and removes some of its limitations. Results were compared with experimental data showing relatively good agreements.

In the present research, a non-linear controller is designed for the control of an active suspension system for a half-model vehicle, using a Fuzzy Neural Network (FNN) along with Feedback error learning. The purpose in a vehicle suspension system is reduction of transmittance of vibrational effects from the road to the vehicle chassis, hence providing ride comfort. This requires a minimum reduction in road contact along rough roads. In addition, the role of the suspension system in vehicle control along a curved route and in accelerating and braking is quite evident. To accomplish this, one can first design a PD controller for the suspension system, using a classic control method and use it to train a fuzzy controller. This controller can be trained using the PD controller output error on an online manner. Once trained, the PD controller is removed from the control loop and the neuro-fuzzy controller takes on. In case of a change in the parameters of the system under control, the PD controller enters the control loop again and the neural network gets trained again for the new condition. Important characteristics of the proposed controller is that no mathematical model is needed for the system components, such as the non-linear actuator, spring, or shock absorber, and that no system Jacobian is needed. The performance of the proposed FNN controller is compared with that of the PD controller through simulations. The results show that the proposed controller is indeed capable of meeting the stated control requirements.

In this paper, a reformulation for equations of motion for transversely isotropic plates has been obtained based on first shear deformation theory. Unlike the three coupled original equations, this new reformulation consists of two decoupled partial differential equations which can be solved much easier. For free vibration analysis of transversely isotropic plates, the mentioned equations have been solved for two cases. In the first case, using double Fourier series, closed-form solution has been achieved for natural frequencies of simply supported plates. In the second case, free vibration of a plate with arbitrary boundary conditions in two parallel edges has been considered. In order to satisfy the arbitray boundary conditions, some relations have been presented for unknown functions of the original system of equations in terms of the unknowns of the decoupled equations. Finally, numerical results for six possible cases of classical boundary conditions have been presented. Besides, some important results of the presented analysis have been presented for transversely isotropic plates. The results show that despite of the small differences between the natural frequencies of the thin isotropic and transversely isotropic plates, the differences are more significant for the natural frequencies of the thick plate. Also, effects of increasing thickness on the value of natural frequencies for transversely isotropic plates are more than the isotropic ones.

In this paper, temperature and residual stress fields due to multi-pass welding of AISI 304 stainless steel pipe-flange joints are investigated by both finite element method and experimental measurements. First, a two-dimensional axisymmetric finite element model has been developed, using ABAQUS software and transient temperature fields and residual stresses are computed. Then experimental measurements have been performed to verify welding parameters, such as welding arc, total heat transfer coefficient, and triangular heat input function. Residual stresses are measured using hole-drilling method. The obtained numerical results agree well with experimental measurements. The results indicate that 2D axisymmetric model can not only reduce the computational time, but also gives reasonable results.

Traditional robot manipulators that have large links need powerful actuators and their massive structures strongly limit their operating speed. Flexible manipulators having lightweight links are designed to overcome these disadvantages and in this case their flexibility is an important and unavoidable characteristic. In this paper, a new approach to finite element modeling of flexible manipulators, using Hamiltonian mechanics and simulation of their dynamic behavior is presented. The finite element model includes all non-linear terms, such as dynamic interactions between linkages. A computer program is developed in the MATLAB medium to simulate the effects of flexibility on the robot’s motion quality. Our results indicate the importance of flexibility and existence of considerable errors in the end-effector's positions.