Computational Methods in Engineering

---

This paper is concerned with development of a technique for performance prediction of current transformers (CT), accurately. In this technique, a new optimized model from view-point of both computation time and memory is introduced in order to simulate the current transformers on digital computers. The introduced software in this paper in combination with relay tester equipments can be used instead of the actual CT to test different types of protection relays.

---

In this paper, computer simulation of a three-phase synchronous motor drive is described. This drive/system is supplied with a new type of pulse-width modulated voltage source inverter (PWM VSI) whose amplitude of the first harmonic, compared to a conventional sinusoidal PWM VSI, is increased by about 17% and whose amplitude of higher harmonics is also reduced inversely proportional to their orders. This system has low torque pulsations and approximately has a unity power factor at operation below the base speed. In addition, it has a dynamic performance very close to DC drives with quick dynamic response.

---

In this paper, a new method is proposed for the speed control of a three-phase current forced synchronous machine drive. This metohd is based on the use of an approximate constant commutation safety margin angle in the output thyristor bridge of the drive system as well as on the compenastion of stator magnetization reaction. The commutation process in the output converter of the machine drive is naturally performed by means of the rotor induced emfs in the stator winding. In comparsion with other speed control methods reported for these types of drives, this method is more stable and robust. In additon, higher efficency and power factor can be achieved for the synchronous machine drive through this method. The impacts of rotor damper windings on the drive system performance are also explained in the paper.

---

A wind tunnel investigation was performed to study the flow field over a 70° swept sharped edge delta wing model at high angles of attack. The experiments were conducted in the subsonic wind tunnel at the Department of Mechanical Engineering, Sharif University of Technology. Velocity profiles have been measured using a special pitot tube and hot wire anemometer at angles of attacks of 10 to 35 degrees and Reynolds numbers between 1.5 to 5´105 over half and a full model. From these studies the shape of the leading edge vortices as well as the turbulence intensity inside the vortices were obtained and analyzed. This study revealed a region of increased velocity highly tubulent flow at the vortex core. As a result, the lift will increase nonlinearly with angle of attack. Keywords: Leading edge vortex, Turbulence Intensity, Delta Wing, Vortex Bursting, Vortex core

---

An extensive experimental investigation to understand the aerodynamic behavior of wrap around fin (WAF) missile configuration has been conducted. Various tests using at first a standard model (TTCP) in the trisonic wind tunnel of Imam Hossein University has been performed. The tunnel has attest section of 60×60 cm2 and can operate at Mach numbers of 0.4≤M∞≤2.2 and at attack angles of -4˚≤α≤10˚. Experimental longitudinal results are compared with those of NASA and an engineering code. The results for TTCP model are in good agreement. After gaining confidence on the TTCP results, a new model of WAF rocket was designed, built and tested. This paper compares the results of two models tested under the same conditions. Keywords: Wind tunnel, Wrap around fin, Missile, Balance WAF-TTCP

---

In this paper, using a personal computer (PC), the practical implementation of scalar and vector control methods on a three–phase rotor surface- type permanent magnet synchronous machine drive is discussed. Based on the machine dynamic equations and the above control strategies, two block diagrams are presented first for closed-loop speed controlling of the machine drive/system. Then, the design and implementation of hardware circuits for power, insulating, and signal matching stages are explained along with a description of the written software program for the servo drive system control. These circuits are used to produce the drive inverter switching pulses. To supply the machine drive, the sinusoidal, uniform sampling and step-trapezoidal PWM voltage source inverters are examined. For closed loop speed control of the drive system, the stator currents and rotor speed signals (in scalar control method only the rotor speed) are sampled on-line. After filtering, buffering and matching operations, these signals are transferred to a personal computer port via a high frequency sampling and high resolution A/D converter. It is worth mensioning that both methods of controlling mathematical calculations is done by computer. Finally, the practical and computer simulation results obtained are demonstrated. Keywords: Machine Drive, Synchronous Machine, Permanent Magnet, Rotor Surface Type, Scalar and Vector Control, Voltage – Source Inverter, Control by PC.

---

In this paper, based on feedback linearization control method and using a special PI (propotational integrator) regulator (IP) in combination with a feed-forward controller, a three-phase induction servo-drive is speed controlled. First, an observer is employed to estimate the rotor d and q axis flux components. Then, two input-output state variables are introduced to control the dynamics of torque and the magnitude of the rotor flux independently. In addition, based on the model refrence adaptive system (MRAS) and the recursive least square (RLC) error techniques, the rotor time constant and the mechnical parameters (J, R) are simultaneously estimated. Finally, the efficiency of the proposed method is confirmed against results from computer simulation. Keywords: Adaptive speed ontrol, Inducation servo-drive, Feedback linearization, IP controller, Model reference, Adaptive system, Recursive least square.

---

Several experiments involving two-dimensional and axisymmetric bodies have been carried out in a Trisonic wind tunnel at supersonic speeds to investigate and analyse the measured values of base pressure and to compare them with those from the theoretical methods. The objective of the experiments was to obtain an appropriate method for processing the results of wind tunnel tests on rockets or aircraft having base area, hence, base drag. Among numerous semi-empirical methods available for two-dimensional and axisymmetric geometry, the methods presented by Chapman, Korst and Tanner have been utilized to calculate values of base pressure for comparison with the experimental findings for the same model. The results indicate that the pressure increase for the 2-D model with a steplike base is greater that of the axisymmetric model with the same conditions. The experimental results for all cases considered in this investigation compare well with those from existing the theoretical method developed by Tanner

---

A series of supersonic wind tunnel tests on an ogive-cylinder body were performed to investigate the pressure distribution, the boundary layer profiles, and the flow visualization at various angles of attack. All tests were conducted in the trisonic wind tunnel of the Imam Hossein University. The theoretical shock angle at different model positions compared well with those we obtained via Schilerian results. The static surface pressure results show that the circumferential pressure at different nose sections vary significantly with angle of attack. However, minor changes in the circumferential pressure signatures along the cylindrical part of the body were observed. The total pressure measurements in the radial direction, perpendicular to the incoming flow, vary significantly both radially and longitudinally (along body length). The boundary layer thickness increases along the body. At the beginnig and at the midle part of the cylinderical portion of the body, the boundary layer thickness increases uniformly with increasing angle of attack. However, this situation differs near the end of the body. Our measurements indicated a turbulent boundary layer along the model, which is probably due to the high turbulence level in the tunnel test section.

---

An intensive experimental investigation was conducted to study the effect of vertical tail, single and twin (with different cant angles) on the flow field and the corresponding aerodynamic forces and moments of a model of a fighter A/C. Aerodynamic forces under different flight conditions and different vertical tail settings were measured in a supersonic wind tunnel. Furthermore, effects of vertical tail on the model wake at subsonic speed were investigated. In addition to the force and pressure measurements, schlieren system was used to visualize the shock formation and movement oat various locations on the model. The results show existence of a pair of symmetric vortices for the model equipped with a 22 degree vertical tail cant angle. The vortices burst symmetrically at moderate angle of attack. The drag coefficient increases with increasing cant angle at low to moderate alpha and decreases when alpha is further increased.

---

The blades of wind turbines are the most important parts in producing power output. In this study, a section of a 660 KW wind turbine blade will be installed in Iran in near future was tested in a wind tunnel. In addition to steady tests, various unsteady tests including the effects of reduced frequency, mean angle of attack, and amplitudes were carried out. The preliminary results show strong effects of reduced frequency on the aerodynamic coefficients of the airfoil. Moreover, increasing the reduced frequency delays dynamic stall angle of attack but increases lift and drag coefficients compared to the static results. Further, the values of the aerodynamic coefficients in the upstroke motion (increasing angle of attack) are different from their corresponding values in the downstroke (decreasing angle of attack). These differences create a hysteresis loop where its width and shape are strong functions of reduced frequency, mean angle of attack, and amplitudes.

---

Utilizing one of the mesh free methods, the present paper concerns static analysis of thin plates with various geometric shapes based on the mindlin classical plate theories. In this numerical method, the domain of issue is solely expressed through a set of nods and no gridding or element is required. To express the domain of issues with various geometric shapes, first a set of nodes are defined in a standard rectangular domain , then via a three-order map with, these nodes are transferred to the main domain of the original issue; therefore plates of various geometric shapes can be analyzed. Among meshfree numerical methods, Element Free Galerkin method (EFG) is utilized here. The method is one of the weak form integral methods that uses MLS shape functions for approximation. Regarding the absence of Delta feature in MLS functions, boundary conditions cannot be imposed directly; hence the Lagrangian method is utilized to impose boundary conditions. At the end, our outputs are compared with those of analytic and finite element methods for plates, in order to validate the exactness of our solution method, and then after reliability is established, a few new examples will be solved.

---

In present study, dynamic modeling and control of a tethered space robot system in trajectory tracking of its end effector is investigated. Considering variation of the tether length in the model, dynamics of the system is modeled using Lagrange’s method. Librational motion of the tether is controlled by adjusting the tether length similar to conventional manipulators,control of the robot is performed by its motors. It is clear that, in the trajectory tracking of the end effector, the tether length should be kept more or less constant, keeping them in a stable position. Limiting the tether length variation while using it as a tool for controlling the tether librational motion, is the main challenging part of the control system. To deal with this problem, a hybrid control  system is proposed to control the system. A nonlinear model predictive control approach (NMPC) is utilized to control the tether librational motion and a modified computed torque method (CTM) is used to control the manipulator motion. Initially the NMPC controller is developed for a simple tethered satellite system. Then it is combined with the CTM controller. The proposed controller is employed to control motion of a space robot’s end effector on a predefined trajectory. Performance of the controller is then evaluated by numerical simulations.