Computational Methods in Engineering

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Using the highly recommended numerical techniques, a finite element computer code is developed to analyse the steady incompressible, laminar and turbulent flows in 2-D domains with complex geometry. The Petrov-Galerkin finite element formulation is adopted to avoid numerical oscillations. Turbulence is modeled using the two equation k-ω model. The discretized equations are written in the form of a set of nonlinear equations by block implicit method and are then linearized by the Newton-Raphson method. The set of linearized equations are, finally, solved Through Frontal method. This generates a full implicit solution. A few laminar and turbulent flow sample problems are solved using the code. Results obtained are in perfect agreement with those obtained from numerical and experimental works reported in the literature.

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In this paper an algorithm is presented for the regularization of singular integrals with any degrees of singularity, which may be employed in all three-dimensional problems analyzed by Boundary Elements. The integrals in Boundary Integrals Equations are inherently singular. For example, one can mention the integrals confronted in potential problems to evaluate the flow or the gradient of the flow or the integrals employed to determine the stress or the deformation in elastic problems. Having only the numerator functions and their derivatives derived either explicitly or implicitly, this algorithm may be employed to evaluate the strongly, hyper or supersingular integrals with a satisfactory of accuracy. To regularize the integrals, some functions are either added or subtracted successively so as to differentiate the singular and nonsingular terms. Three examples with their numerical solutions are included which show the efficiency and the accuracy of the proposed algorithm.

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The useful and efficient method of Finite Element (FEM) has a drawback for dynamic analysis of complex structures, especially in the medium and high frequency range. To overcome this fundamental difficulty, application of Statistical Energy Analysis (SEA) and power flow technique has been suggested. As the SEA is based on the average response of structure and statistical properties of its resonant modes, SEA is found to be effective in cases like high modal density, high modal overlap and in short wave length. In these cases, the average response is found to be both real and an acceptable value, contrary to the FEM which is not accurate enough. In this study, the fundamental difficulties of FEM in the high frequency range are explained and then the advantages and applications of SEA and power flow technique are presented. Moreover, a description of SEA basis, fundamental concepts of General Energy Method (GEM) are explained and formulated to be used in the longitudinal and transversal vibrations of beams. Finally, by satisfying the energy boundary conditions, the energy flow analysis of rods and beams with elastic support and under harmonic excitation are investigated.

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In this paper the changes in the heat capacity of cooling towers has been investigated as a result of injecting combustion products of boilers. In the tower, the air is re-heated through mixing with hot flue gas after passing through the heat exchangers. As a result of this process, the draft of the cooling tower is increased. The additional produced draft depends on the ambient temperature and the amount of heating. The results of the study show that the heating process is more effective in the hot weather than in the cold weather.

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In this paper, collapse behavior of simple and reinforced thin-walled channel- section beams, subjected to three- point bending, is investigated. Many simple channel- section beams of different geometry and some reinforced ones were tested, and their strengths after collapse were obtained thoroughly. Since the available theories in this area are complicated and little attention has been paid to the case of reinforced thin-walled beams, simple models were used and further developed to give an account of the collapse load and the deformation energy of such beams. The method proposed in this paper is valuable both to the further studies of the symmetric thin-walled beams with arbitrary cross-section and to its practical application of such beams as energy absorbers.

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Fuel oil used in power plants contain metal impurities like Vanadium. After combustion, this metal remains in boiler fuel ash in the form of oxide. In this research, extraction of Vanadium oxide from fuel ash has been investigated. Two processes were used for this purpose. A pyro- hydrometallurgy and a hydrometallurgy process. In the pyro-hydrometallurgy process, using sodium carbonate, salt roasting of the ash followed by water leaching was carried out. Vanadium was then precipitated as ammonium vanadates which on heating decomposed and vanadium pentoxide (V2O5) was obtained. In the hydrometallurgy process, the ash was dissolved in sodium hydroxide. Vanadium oxide was then recovered from solution. Effects of various parameters in each case were investigated and the optimum condition for maximum recovery was determined.

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In this study carburizing and boronizing processes were applied to powder metallurgy steel specimens and the mechanical and tribological properties of the substrate and coatings were evaluated under various process conditions. The specimens, made from industrial test pieces, were carburized in a powder pack for a duration of 2-5 hrs at 850-950 ˚C. Similar specimens were pack boronized for 4 hrs at 950 ˚C. The effect of austenitization-quench treatment was also investigated on some specimens. The wear tests were carried out by means of a pin-on-disc tribotester against ball bearing steel. The results indicate that by appropriate selection of process parameters it is possible to obtain high wear resistance together with moderate toughness. Boride layers with hardness values of 1700HV are properly formed on PM samples. The wear resistance, therefore, is significantly increased with practically no reduction in impact resistance. It is concluded that boronizing treatment can be more suitable for some PM parts under tribological conditions.

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The proposed algorithm suggests a new method for determination of K/S value of primaries based on linear least Squares Technique. By applying the matrix pseudoinverse, a modification is introduced to eliminate the limitation on the numbers of applied dyes in one – constant Kubelka-Munk theory. The selection of dyes for tristimulus matching are also done on the basis of the initial spectrophotometric results. The applicability of suggested methods are tested through a computer colour matching attempt with more/less than three primaries.

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In this paper it is attempted to predict the off-design performance of a jet engine. After a review of the governing equations, the off-design performance is investigated by two methods. In the first method, the component characteristic curves of the Gas Turbines are used. In the second method some design point parameters and the reference state conditions are employed. The results obtained by this two methods fairly agree, and therefore, the second simple method which is independent of the component characteristics are recommended.

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This paper deals with the design and evaluation of a robust controller for static VAR compensator (SVC) in remote industrial power systems to enhance the voltage profile for three-phase single cage induction motor (SCIM) loads. The controller design is based on H∞ theory to deal with uncertainties arising in industrial network modelling. The performance of the H∞ controller has been evaluated extensively through non-linear time domain simulation. It is concluded that the robust controller (RSVC) enhances the voltage profile for SCIM loads compared with the optimal (OSVC) type which consists of optimal state feedback (LQR).

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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.

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Storaging or transmission of speech signals at very low bit rate is a hot area in the field of speech processing. We used stochastic inter-frame interpolators and vector quantization (VQ) as a new method for developing a high quality 1200 BPS speech vocoder. The objective and subjecgtive test results show that performance of the new vocoder is compairable with 4800 BPS standard vocoders (as CELP).

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During the last two decades, Maximum Likelihood estimation (ML) has been used to determine Direction Of Arrival (DOA) and signals propagated by the sources, using narrowband array signals. The algorithm fails in the case of wideband signals. As an attempt by the present study to overcome the problem, the array outputs are transformed into narrowband frequency bins, using short time Fourier transform together with ML, to estimate DOAs and the signals. The effect of window parameters (i.e, type, length and decimation factor) on the bias and variance of estimation of DOAs and signals is investigated. The algorithm robustness and convergence in presence of low SNR and coherent signals is illustrated. It is also shown that the local optimal problem encountered in the narrowband case is resolved for the wideband signals.

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This paper considers the Economic Lot Scheduling Problem, that is, the problem of scheduling several products on a single facility so as to minimize holding and setup costs. Combination of frequency and timing as well as production quantity make this problem Np-hard. A heuristic is developed to obtain a good solution to ELSP. The proposed heuristic makes use of the Simulated Annealing Technique. This heuristic gives a sharper upper bound upper bound to holding and setup costs.

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In this research, 210 concrete cube specimens (15cm) were made replacing 0, 5, 10, 15, 20, 25, 30, 35, 40, 50, and 60 percent of slag powder from Isfahan Steel Mill for cement. The control specimens were made with 350 kg/m3 cement content and the water cement ratio in all specimens was 0.51. The Slump value and compressive strength of concrete specimens in 7, 28 and 90 days were obtained and the results were compared. The test results show that replacement of 25 to 35 percent or slag powder for cement gives satisfactory results in obtaining concrete with suitable compressive strength and slump value. Also the results indicate that by increasing the percentage of slag powder replaced for cement, the shrinkage of the concrete and its resistance against sulphates will increase.

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A knowledge of rainfall distribution over a finite area is needed for small bydraulic structures design. Hydrologists have always been concerned about Depth- Area- Duration (DAD) for a specific storm in a watershed. An intense storm occurred on June 6, 1992 which claimed some lives in parts of Mashhad, Khorasan Province, Iran. Data of rainfall depth at 29 raingauge stations around the city and its suburbs were gathered. There also exist three automatic raingauges at Mashhad airport, the College of Agriculture, and Khorasan Water Authority and one at Toroq Dam in Mashhad suburb. A survey of hyetographs for June 6, 1992 of these stations revealed that the airport raingauge did not work well while dimensionless mass curves for other raingauges were quite similar. Two rain centers at the College of Agriculture (with 50 mm rainfall) and Wheat Silage (with 53 mm rainfall) were detected. The DAD relationships for this storm were derived. Intensity- Duration- Frequency (IDF) curves for the airport station were derived and were compared with those of the College of Agriculture and Khorasan Water Authority. In the absence of an abnormal topography, the Theissen method was utilized for deriving a regional IDF for Mashhad City. Intensity- Duration- Frequency- Area (IDFA) curves, a new concept in hydrometeorology, has been initiated for the first time for Mashhad. Combination of regional IDF with DAD of the most intense storm in Mashhad was a key to represent such a new concept.

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Two-dimensional Euler equations have been solved on an unstructured grid. An upwind finite volume scheme, based on Roes flux difference splitting method, is used to discretize the equations. Using advancing front method, an initial Delaunay triangulation has been made. The adaptation procedure involves mesh enrichment coarsening in regions of flow with high low gradients of flow properties, according to an introduced adaptation criteria. To validate the procedure, a couple of internal and external steady flows are solved. One may see the effectiveness of introducing relatively few cells and the local adaptation algorithm on accuracy. Solution dependency on grid is also studied.

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An optical system has been redesigned and used to measure detonation velocity as a function of condensed explosive density for RDX, HMX, PETN. These measurements are then used to determine and fine tune parameters appearing in the BKW (Beker-Kistiakowsky-Wilson) equation of state for gases at extremely high pressures. Steady state form of conservation equations along with the BKW state equation and the Chapman-Jouquer hypothesis are used to determine the detonation velocity and the thermodynamic state of the gas behind the detonation front. An iterative scheme is used to adjust the BKW equation parameters such that the calculated detonation velocities, obtained from the solution of the above set of equations, match the measured values.

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In this paper a novel numerical approach is presented for solving the transient incompressible fluid flow problems with free surfaces in generalized two-dimensional curvilinear coordinate systems. Solution algorithm is a combination of implicit real-time steps and explicit pseudo-time steps. Governing fluid flow equations are discretized using a collocated finite-volume mesh. Convective terms are approximated with an accurate monotonicity preserving upwind scheme. Free surfaces are first approximated by lines of constant slope and then convected using the volume-of-fluid (VOF) technique. A number of problems, both with and without free surfaces, have been solved to demonstrate the ease and usefulness of the scheme. Accuracy of the results thus obtained is assessed by comparison with other numerical as well as analytical results in the literature.

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A novel neuro-based method is introduced to solve the laminar boundary layer and the turbulent free jet equations. The proposed method is based on cellular neural networks, CNNs, which are recently applied widely to solve partial differential equations. The effectiveness of the method is illustrated through some examples.