Computational Methods in Engineering

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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

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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.

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Soft Switcing techniques have recently been applied in the design of dc-ac converters, in order to achive better performance, higher efficiency, and power density. One of the soft switching techniques uesd in inverters is resonant dc links. These topologies have some disadvantages such as irregular current peaks, large voltage peaks, uncotrollble pulse width, etc. Another soft switching method in inverters is using Quasi –resonant links, which have PWM modulation capability. Inverters with series or parallel Quasi-resonant dc links use several quasi-resonant current or voltage pulses, respectively, to produce PWM modualation. In this paper an inverter with a novel Quasi-resonant series dc link is introduced. This topology enables current source inverters to have characteristics such as resonant pulse peak limition and pulse width controllability. This circuit provides the inverter with two to three ranges of PWM control capability which increases the switching time control in a larger range. Various operational modes of this novel Quasi-resonant dc link is analyesed and then the circuit losses is calculated. Finally, simulation results by PSPICE software is presented to justify the circuit operation. Keyword: Inverter, Soft switching, Novel quasi-series resonant link, increasing control areas, Losses

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Pulse Width Modulation (PWM) techniques are commonly used to control the output voltage and current of DC to AC converters. Space Vector Modulation (SVM), of all PWM methods, has attracted attention because of its simplicity and desired properties in digital control of Three-Phase inverters. The main drawback of this PWM technique is its complex and time-consuming computations in real-time implementation. The time-consuming calculation as well as software and hardware complexities of the network grow dramatically as the number of inverter levels increases. Therefore, it is necessary to develop an exact, fast, and general computation SVM algorithm for multi-level converters. This paper introduces such an algorithm. Specifically, the SVM computation algorithm based on a vector classification technique, introduced for 2-level inverters in 1996, is developed and generalized to be applicable in determining the switching sequences and calculating the switching instants in m-level inverters. The proposed technique reduces hardware and software complexities, decreases the computation time, and increases the accuracy of the positioning of the switching instants when compared with the conventional implementation of the SVM in multi-level converters

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Similar to random sea waves, forces on the offshore structures due to waves are random. These forces can be mainly divided into two components, namely, inline forces and transverse or lift forces. The random nature of lift forces is more complicated than that of inline forces and both should be combined for design purposes. In the present paper, two different approaches have been used to determine time series of lift forces. Along these lines, the determination of lift coefficients is discussed which have then been used to obtain transverse forces and compared with experimental data. The experimental data used in this study were collected at Delft Hydraulics Laboratory on a full-scale rough vertical cylinder.

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One of the difficulties with PWM switching converters is high switching loss and electromagnetic interference due to switching at non-zero voltage and current, which limits the operating frequency. In order to reduce the converter volume and weight (by increasing the frequency) and reducing switching losses, zero voltage and current switching methods are recommended. In this paper, four main zero voltage switching (ZVS) methods in full bridge converters are introduced and compared. These four methods are compared on dead times required to obtain the ZVS, load range at ZVS condition, circulating energy in the switch anti parallel diodes during freewheeling periods and voltage oscillations on rectifying diodes. Finally, the results of a 3 KW prototype full bridge ZVS converter with a clamp circuit for rectifier diodes oscillations are presented and analyzed

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In this paper, the Radiation Transfer Equation(RTE) for a non-gray gas between two large parallel planes has been solved and the temperature distribution obtained. With the RTE, solution heat fluxes are also determined. Since and are two components of most combustion products, the problem has been solved for these two gases. The results were, whenever possible, compared with data reported elsewhere. Since the simulation of exact absorbing bands has been used, it can be claimed to be relatively close to exact solution. From the results otained, it can be maintained that treating, the above mentioned gases as a gray gas could cause considerable errors in the determination of temperature distribution and heat fluxes. The error would be more for water vapour than for carbon dioxide.

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In this study, two CCCT diagrams are drawn to be compared with a CCT diagram. The CCCT diagrams represent continuous cooling transformations in stress assisted state. The increased Md and Bd temperatures of CCCT diagrams were also compared with those of the CCT diagrams and the cause was investigated from both thermodynamic and metallurgical viewpoints. Thermodynamic examinations revealed that stress causes the mechanical driving force to increase but the total free energy of transformation to decrease hence, Md and Bd will rise. Metallurgical investigations showed that if deformation temperatures are selected in a manner to increase the structural strength of the original austenite grains prior to deformation, the shear force required for martensite and bainite transformations will arduously obtain hence, Md and Bd will fall. However, if recrystallization or full recovery occurs during or after deformation, interior grain structure softens and the shear force required for martensite and bainite transformations will readily obtain hence, Md and Bd will rise. It was also found that the nose in CCCT curves are shifted to the left as compared to that of CCT curves. This indicates that deformation of steel enhances bainite formation more readily than that of the martensite phase.

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A rectangular rod is placed in a flow field flowing parallel to a flat plate. Effect of chord-thickness ratio of rectangular rod on developing vortex shedding downstream to the rod is studied. Then, for each one of the aspect ratios, the distance of the rod from the neighboring flat plate is reduced until the rod sticks to the flat plate. In each case, the effect of the flat plate boundary layer on Strouhal number and the contrary effect of the boundary layer on vortex shedding from the rectangular rod are studied. Results show that as the rectangular rod enters into the flat plate boundary layer, vortex generation from the closest side of the rod reduces, thereby reducing the Strouhal number as well. Finally, when the rectangular rod sticks to the flat plate, a stationary wake forms downstream the rod and sticks to the flat plate. Meanwhile, the boundary layer over the flat plate is disturbed effectively and heat transfer coefficient from the flat plate is enhanced by an average of 50% and up to 200% in some places, locally.

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One of the most important issues in engineering is to find the optimal global points of the functions used. It is not easy to find such a point in some functions due to the reasons such as large number of dimensions or inability to derive them from the function. Also in engineering modeling, we do not have the relationships of many functions, but we can input and output them as a black box. Therefore, the meta-heuristic algorithms are presented.
In this paper, a meta-heuristic algorithm based on the behavior of vortices in fluid physics is presented. Technically, the algorithm is made up of vortices. Each vortex contains some particles. The particles move by the presented rotation matrix. This movement causes the local search. Also by selecting another vortex through the selection algorithm, each vortex attempts to escape the local optima and reach the global optima. The algorithm will explore and exploit the given function using its operators. Another innovation of this paper is the introduction of two new evaluation criteria for optimization algorithms. These two criteria show the behavior and convergence of algorithms along the way to reach the global optimal point or fall into the local optima. The proposed algorithm has been implemented, evaluated and compared with the numerical optimization state of the art algorithms. It was observed that the proposed method was able to achieve better results than most of the other methods in the major of twenty-four standard functions in different dimensions.  (All codes available at http://web.nit.ac.ir/ h.omranpour/.).