فصلنامه مکانیک هوافضا
سال دوم شماره 3 (پیاپی 6، اسفند 1385)

The procedure of modeling+of shape memory alloy (SMA) dynamics and the corresponding computerized algorithm are described extensively in this paper. In shape memory alloys, due to the existence of the Martensite and Austenite phases in the microscopic structure of the alloy and also conversion between the phases, the elasticity modulus of the material shows a time varying behavior, while the structures of the equations describing the dynamics of the system remain similar to the ones with constant elasticity modulus. To evaluate the effect of conversion between phases on the elasticity modulus, the Martensite to Austenite volume fraction is of the main concern. Therefore, the mentioned elasticity modulus can be modeled as an oscillating spring-damper-mass system with variable parameters. Then, a new chattering alleviation algorithm for sliding mode control (SMC) of non-linear systems is presented. In the new method, a mathematical function for regulating parameter is introduced. Due to the initial choice of this parameter by designer, the state vector of the system approaches the sliding surface with any arbitrary speed and enters the boundary layer surrounding the sliding surface, while the state vector does not include fast behaviors through the boundary layer. To show the abilities and the merits of the aforementioned regulating routine on the vibration control of a SMA system, simulations are carried out within Simulink environment. The results show that our proposed algorithm has positive effects on the control system performance, while the 5% uncertainity in the elasticity module has no effect on it.

Shape memory Alloys (SMA) are widely used as new materials with higher potential in implants, self expanding NiTi stents, etc. for medical applications and in actuators, micro actuators, components for isolation of vibrations, etc. for non-medical applications. In this study, shape memory effect, superelasticity, and excellent damping capability, three special characteristics of this material, are discussed and an extended one-dimensional constitutive model for prediction of superelastic behavior is proposed. The model is based on strain as control variable (strain driven). Stress induced austenit-martensite evolutionary equations are first proposed and time-discrete model is then obtained through backward Euler scheme. The solution of the time-discrete model is approached by a modified return map algorithm for phase transition. Then, tangent modulus is used for the quadratic convergence of the Newton method in the phase transition conditions. Muller model helps us to predict material behavior in the pseudo-elastic hysteresis. In the next step, extended solution algorithm is proposed and different experimental test results are discussed. Finally, the proposed model results are compared with the result of some experimental studies, which shows good agreements.

In this research, a finite volume numerical approach was developed to simulate physical details of convection-dominated water solidification in pipes. Our approach was based on the enthalpy-porosity method, which is used to track the motion of liquid-solid fronts and to obtain freezing length and time of solidifications. The Navier-Stokes equations were solved on a staggered grid, using a pressure-based implicit procedure. The results, which were validated against experimental data, show a remarkable quality of our simulation.

The main aim of this work is to present an algorithm to find the minimum weight for the composite high-pressure vessels with non-metallic liner. Composite pressure vessels are widely used in aerospace and commercial industries such as in rocket motor cases, in portable oxygen storage, and in fuel tanks. Weight minimization in these vessels (in addition to large savings in amount and cost of material) results in reducing fuel consumption and transportation costs; and increasing the ratio of travel range to weight. Previous studies [1] show that geodesic head has the best performance among different known head shapes. The stacking sequence optimization at each winding angle is performed for different number of layers. This is done through using genetic algorithm and finite element analysis. Finally, the composite shell with minimum number of layers whose Tsai-Wu failure criterion is bellow one is selected as the laminate with minimum weight. A comparison between different winding angles shows that winding angle of 9 degrees has better performance than 20 and 30 degrees. With performing a comparison between existing vessel and optimized 16 layered vessels, it is seen that, in addition to maximum strength, manufacturing conditions should be considered for the best stacking sequence selection.

In this work, the ball-end mill machining has been analyzed and a model based on mechanics of metal cutting is developed. This model is used for prediction of the cutting forces in ball-end milling from fundamental data. The cutting edge in this model is considered as a series of infinitesimal elements and the oblique cutting process with these elements has been analyzed as orthogonal cutting process. Cutting forces result from the summation of cutting forces on these elements. The results have been compared with the experiments on machining 1045 steel and Al4 Ti6V and show relatively good agreements.

An automatically controlled variable converging-diverging supersonic nozzle with high dependability is required for wide range of Mach numbers in a supersonic wind tunnel. In this work, the performance of a converging- diverging nozzle of the ST2 supersonic wind tunnel was evaluated and analyzed. Based on our results, an automatic control system for adjustment of the nozzle was designed and implemented. Then, a fuzzy control system was used to optimize the wind tunnel’s performance and behavier. The results show that, it is supprior in speed, precision, stability, and reliability. In addition, the new system has caused better wind tunnel overall performance, enhancing its reliability and reducing its cost.

In present work viscous flow over a remotely operated vehicle (ROV) was investigated numerically using the FLUENT CFD software. In this simulation, the ROV hydrodynamic forces and dimensionless force coefficients are estimated in defined motions. These motions include horizontal and vertical movement with and without acceleration. Hydrodynamic forces in accelerating motion include two parts; actual and virtual. The Morrison’s equation was used to separate these two parts. Results show that the Morison equation is capable of estimating forces in such accelerating motions. In addition, the type of motion does not seem to have considerable effects on virtual force coefficients.

Non-linear characteristic of tire forces is the main cause of vehicle lateral dynamics instability, while direct yaw moment control is an effective method to recover the vehicle stability. In this work, an optimal non-linear controller for yaw dynamics to track a linear model, limited by road friction coefficient, is developed. The yaw rate response of an extended 2DOF non-linear model is first predicted by Taylor series expansion and then a control law is introduced by minimizing the local differences between the predicted and the desired responses. Here, the main properties of the proposed control law and its advantages over the other conventional control methods are discussed. The derived control law has an analytical form which is easy to apply and handles the control input saturation explicitly. The effectiveness of the designed controller is investigated through simulations of severe lane change maneuvers, using a developed non-linear full vehicle dynamic model. The simulation results show that the vehicle stability can be remarkably improved when the optimal non-linear controller is applied.

In this paper, the effects of angle of attack on flow behavior in the near wake of a flat plate, using a hot-wire anemometer are reported. The symmetric flat plate used has a rounded leading edge and a sharp trailing edge with a sweep angle of. All measurements were carried out in a wind tunnel with a cross sectional area of 305×305 mm and a maximum velocity of 40. The Reynolds number, based on the chord length, was. The model was placed at angles of attack 5, 10, 15, and 20 degrees. It was concluded that, although it has been shown that the logarithmic law for angle of attack of zero is validated up to distances in near wake, its validity there is decreased by increasing the angle of attack. However, this law will not be validated by going away from the trailing edge. Also, the effect of angle of attack on turbulence kinetic energy in shear layer and on drag coefficient has been investigated. The results show that the turbulent kinetic energy is enhanced by increasing the angle of attack in a certain distance from the trailing edge and its value is reduced by going away from the trailing edge at a particular angle of attack. Also, the results show how the drag coefficient is increased when angle of attack gets larger.