Showing 306 results for Co
Sh. Samavi and R. Kelishadi,
Volume 20, Issue 1 (7-2001)
Abstract
Trigonometric functions are among the most useful functions in the digital signal processing applications. The design introduced in this paper computes the trigonometric functions by means of the systolic arrays. The method for computing these functions for an arbitrary angle, , is the CORDIC algorithm. A simple standard cell is used for the systolic array. Due to the fixed inputs, in some cases, a number of the cells are optimized. The control unit and a read only memory are the essential parts of any CORDIC implementation. The introduced hardware does not use any of these two structures, which makes it a simple and expandable design
J. Soltani and S. Sadr Arhami,
Volume 20, Issue 1 (7-2001)
Abstract
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.
B. Mirzaeian, M. Moallem, V. Tahani and Caro Lucas,
Volume 20, Issue 1 (7-2001)
Abstract
In this paper, a new method based on genetic-fuzzy algorithm for multi-objective optimization is proposed. This method is successfully applied to several multi-objective optimization problems. Two examples are presented: the first example is the optimization of two nonlinear mathematical functions and the second one is the design of PI controller for control of an induction motor drive supplied by Current-Source-Inverter (CSI). Step response of the system is considered and controller parameters are designed based on multi-objective optimization technique. Rise-time, maximum over-shoot, settling time and steady state error are considered as objective functions. The simulation results of the new method for induction motor speed control and optimization of two nonlinear mathematical functions are compared with the results obtained from other methods [4,14,15], which shows better performance.
S. A. Shojaosadati and A. Seifi,
Volume 20, Issue 1 (7-2001)
Abstract
In order to evaluate the performance of biofilter for H2S removal from contaminated air, a 120×14 cm column biofilter was constructed using clear polyacrylic material in conjunction with H2S production and control systems. The column was divided into four stages using perforated plates. The column was packed with a mixture of mushroom compost and snail shell (4:1).
The performance of biofilter was evaluated during 4 months of operation under various flow rates and H2S concentrations in ambient temperature. According to the results, the removal effeciency of H2S in polluted air for concentrations of up to 150 ppm, average temperature of 26.9°C and under flow rates of 6 and 12 l.min-1 was more than 95% the results for average temperature of 20.5°C and the flow rates of 19.5 and 26 l.min-1 but constant H2S concentration was more than 85%. The maximum removal rate (Vm) was measured as 0.075 g S/kg-dry compost.h and average saturation constant (Ks) was 32.5 ppm
D. Mostofinejad,
Volume 20, Issue 1 (7-2001)
Abstract
The paper presents the results of casting and testing of 264 GFRC specimens. The glass fibers were 25 mm long, with the aspect ratio (L/D) ranging between 1250 and 3570. The parameters studied were the ratio (by weight) of fibers to cement, i.e. F/C=0%, 1.5%, 3%, and 4.5%, and the ratio of coarse to fine aggregates (gravel to sand), i.e. G/S=1.1, 0.7 and 0.2. In total, 12 mix designs were selected for GFRC specimens while the water-cement ratio was constant and equal to W/C=0.4. The balling of glass fibers in the mix was overcome by using adequate and sufficient antistatic agents. The specimens were tested under compressive, tensile and flexular loading at the ages of 7 and 28 days. Furthermore, the modulus of elasticity and the absorption of the concretes were determined. Finally, the mechanical and physical properties of the GFRC specimens were analysed and an empirical expression describing the modulus of elasticity of the GFRC was proposed.
S. Ketabi,
Volume 20, Issue 1 (7-2001)
Abstract
In this paper the problem of minimum cost communication network design is considered where the costs are piecewise linear concave. Several methods are compared: Simulated Annealing method, a heuristic based on the method proposed by Minoux, and a lagrangian method based on lower bounding procedure.
A. Rezvani, G. Karami and M. Yaghoubi,
Volume 20, Issue 1 (7-2001)
Abstract
One of the great enemies of rubber compounds is heat. Heat will cause chemical and physical degradation of vulcanized rubber as well as a considerable loss in its strength. A major source of heat generation in a tire is due to internal friction resulting from the viscoelastic deformation of the tire as it rolls along the road. Another source of heat generation in a tire is due to its contact friction with the road. Prediction of the temperature rise at different parts of the tire will help to detect the behavior of the tire as regards its strength and its failure.
In the present work, initially the data required for the thermal analysis of the tire are determined which include: the thermal conductivity of rubber compounds, the tire rolling resistance and its heat build-up rate. The thermomechanical analysis of a typical tire then follows based on the thermodynamics of an irriversible process. The mechanical dissipatives, i.e. the hystersis losses are assummed to be the major source of heat in the mathematical formulation.
A finite element code is developed for two-dimensional heat transfer analysis of the tire. The results obtained show that the highest temperature rise will occur on the carcass-tread interface in a tire specially at heavy loading and under high speed conditions.
Keywords: Heat Generation, Rubber, Contact Friction, Design, Finite Element, Viscoelastic Deformation
M. H. Fathi, Gh. Feizi, Sb. Moosavi, Gh. Gahanshahi, M. Salehi, A. Saatchi and V. Mortazavi,
Volume 20, Issue 1 (7-2001)
Abstract
Hydroxyapatite coatings have been used on metallic substrates in a variety of applications, including modifying the surface of human implants, bone osseointegration and biological fixation. In this paper, the effects of various kinds of metallic substrate on clinical and pathological results of in vivo tests are presented. Four kinds of endodontic implants i.e, stainless steel, cobalt base alloy, plasma sprayed hydroxyapatite coated stainless steel, plasma sprayed hydroxyapatite coated cobalt base alloy were prapared and implanted in mandibular canine of
cats. After a healing period of 4 months, investigation by SEM and histopathological interpretation and evaluation showed significant differences in tissue response and osseointegration between coated and non-coated metallic implants. It was concluded that the results were affected by the kind of metallic substrate .
Keywords: Hydroxyapatite coating, Dental endodontic implant, Osseointegration, Corrosion, Stainless steel,
Cobalt base alloy
S.samavi, V. Tahani and P. Khadivi,
Volume 20, Issue 2 (4-2001)
Abstract
Routing is one of the basic parts of a message passing multiprocessor system. The routing procedure has a great impact on the efficiency of a system. Neural algorithms that are currently in use for computer networks require a large number of neurons. If a specific topology of a multiprocessor network is considered, the number of neurons can be reduced. In this paper a new recurrent neural network and its energy function is introduced. This network requires a significantly smaller number of neurons compared to its counterparts. Also presented is the performance of this neural network.
Keywords: Routing, Multicomputer systems, Recurrent neural networks, Mesh interconnection networks.
M.e. Hamedani Golshan, H. Ghoudjehbaklou and H. Seifi,
Volume 20, Issue 2 (4-2001)
Abstract
Finding the collapse susceptible portion of a power system is one of the purposes of voltage stability analysis. This part which is a voltage control area is called the voltage weak area. Determining the weak area and adjecent voltage control areas has special importance in the improvement of voltage stability. Designing an on-line corrective control requires the voltage weak area to be determined by a sufficiently rapid and precise method. In this paper, a new algorithm based on assigning a vector to
each power system bus is presented. These vectors indicate buses conditions from the viewpoint of voltage stability. In this new method, using the clustering methods such as kohonen neural network, fuzzy C-Means algorithm and fuzzy kohonen algorithm, voltage control areas are determined The proposed method has advantages such as determining PV and PQ buses which belong to the weak area simultanously, under all operating conditions and without a need to system model. Also by comparing the results of applying clustering methods, it has been observed that, due to simplicity of implementation and precision of the results, the two dimensional kohonen neural network is a more suitable tool for clustering power system to voltage control areas than the fuzzy C-Means and fuzzy kohonen methods.
Keywords: Voltage stability, Voltage weak area, Voltage control area, Corrective control, Pattern recognition, Kohonen neural network, Fuzzy C-Means algorithm, Fuzzy Kohonen algorithm.
K. Eshghi and S. Pasalar,
Volume 20, Issue 2 (4-2001)
Abstract
Multicommodity distribution problem is one of the most interesting and useful models in mathematical programming due to its major role in distribution networks. The purpose of this paper is to describe and solve a special class of multicommodity distribution problems in which shipment of a commodity from a plant to a customer would go through different distribution centers. The problem is to determine which distribution centers to use so that all customer demands are satisfied, production capacities are not exceeded, and the total distribution cost is minimized.The proposed problem is formulated as a mixed integer linear program and a solution technique based on tabu search is developed, implemented and successfully applied to the test problems.
Keywords: Commodity Distribution Systems, Tabu Search, Mathematical Programming
A. Shadaram, H. Khaleghi and M.s. Sadeghipour,
Volume 20, Issue 2 (4-2001)
Abstract
The present work introduces a modified scheme for the solution of compressible 2-D full Navier-Stokes equations, using Flux Vector Splitting method. As a result of this modification, numerical diffusion is reduced. The computer code which is developed based on this algorithm can be used easily and accurately to analyze complex flow fields with discontinuity in properties, in cases such as shock wave boundary layer interactions. This scheme combines advantages of both Advective Upstream Splitting (AUSM) and Low Diffusion Flux Vector Splitting (LDFVS) Methods. To increase accuracy and monotonicity, the conservative variables are extrapolated at the cell interfaces by using the MUSCL approach with limiter. This algorithm has been used to solve four sample problems. It has been shown that the numerical diffusion has been reduced and the results are in good agreement with published numerical and/or experimental data.
Keywords: Compressible Navier Stokes Equations, Flux Vector splitting, Advective upwind, Numerical diffusion
M. R. Soltani and A. R. Davari,
Volume 20, Issue 2 (4-2001)
Abstract
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
M. R. Modarres- Razavi, H. Niazmand and S. A. Mirbozorgi,
Volume 20, Issue 2 (4-2001)
Abstract
In this paper, the flow-field of an incompressible viscous flow past a solid-sphere at low Reynolds numbers (up to 270) is investigated numerically. In order to extend the capabilities of the finite volume method, the boundary (body) fitted coordinates (BFC) method is used. Transformation of the partial differential equations to algebraic relations is based on the finite-volume method with collocated variables arrangement. For solving the obtained algebraic relations, the TDMA in periodic state is used. To approximate the convective fluxes, the differencing scheme of Van leer is used and SIMPLEC handles the linkage between velocities and pressures. The verification of the code is checked by the analysis of flow past a solid sphere at low Reynolds numbers of 20 to 210. A good agreement is obtained between the present results and the available experimental and numerical data. The flow-field past a sphere at low Reynolds numbers of 210 to 270 shows that the steady non-axisymmetric regime is going to build up at the Reynolds number of 211.
Keywords: Solid-Sphere, Wake, Three Dimensional Analysis, Boundary Fitted Cordinates
V. Tahani, S. M. Saghaeian Nezhad and G. R. Arab,
Volume 21, Issue 1 (7-2002)
Abstract
Because of extreme local saturation at pole tips of excited phase and uncircular shape of rotor and stator, a Swithed Reluctance Motor (SRM) does not have a simple and accurate mathematical model. Therefore, the output control of this motor requires a robust controller which is not based on an accurate model of the process. Fuzzy controllers, to some extent, will satisfy these requirements. Teta-on and teta-off are controller outputs. The output of teta-off controller is a Variable Structure Controller (VSC) which contains two parts: coarse controller which is used when the speed error is large and its output causes large changes in teta-on angle. This part of the controller is similar to a fuzzy PI controller. The other part of the controller is a fine controller and is used when the speed error is low. The fine controller increases the speed of response and reduces the speed error to zero. This part is similar to a fuzzy I or PI controller. Finally, experimental results of no-load and underload speed controls are demonstrated. The fuzzy controller robustness to measurement noise and parameter uncertainty is also studied.
Keywords: Fuzzy Controller. SRM Variable Structure Controller
F. Mokhatab Rafiee,
Volume 21, Issue 1 (7-2002)
Abstract
A Two-phase model for configuring a cellular manufacturing system is proposed. In phase (I), for the first time, number of cells is considered as a decision variable. In phase (II), pursing two different objectives, one minimization of underload and the other, maximization of similarity of parts within a group, the design procedure is performed. As one cannot have these two objects together, a heuristic algorithm based on cellular similarity coefficient and integration of two objects is proposed. The results confirmed that the proposed heuristic procedure has reasonable outcomes.
Keywords: manufacturing cell, cellular similarity coefficient, two-phase model, heuristic algorithm
M. Rabbani,
Volume 21, Issue 1 (7-2002)
Abstract
In most stochastic inventory models, such as continuous review models and periodic review models, it has been assumed that the stockout period during a cycle is small enough to be neglected so that the average number of cycles per year can be approximated as D/Q, where D is the average annual demand and Q is the order quantity. This assumption makes the problem more tactable, but it should not be adopted when the beck order and lost sales penalty costs are relatively small. In this paper, considering a continuous review inventory model, we relax the above assumption and we explicitly take into account the stockout period when computing the expected cycle length. Further, we consider the effect of using exact number of cycles rather than using approximate of cycles in a continuous review inventory model.
Keywords: Inventory control, Stochastic demand, Continuous review, Inventory cycle
D. Mostofinejad and M.davoodnabi,
Volume 21, Issue 1 (7-2002)
Abstract
In most cases, structural engineers assume a concrete floor to be a rigid diaphragm. Although this simplification is in most cases acceptable, it should be noted that such an assumption may be distrusted due to certain problems. Concrete structures with staggered shear walls are among those whose analysis should be conducted with special concern for the behavior of their floor diaphragms. However, in the structures with staggered shear walls, the horizontal shear due to lateral loads is transmitted to the lower stories through the floor diaphragm since the walls are not usually located over each other in consecutive stories. Therefore, the rigidity of the floor diaphragm is of great importance. In the present study, a parametric analysis was performed to investigate the effect of the rigidity of the floor diaphragm on the load-carrying procedure of the structures with staggered shear walls. The investigated parameters were the number of stories, the ratio of length to width of the plan, and the thickness of walls and diaphragms. Furthermore, the study was carried out for both rectangular and I-shaped plans. All analyses were dynamically performed by ANSYS 5.4 using acceleration spectrum recommended by Iranian Building Code Standard No. 2800. Finally, the behavior of these structures and comparison of the frequencies, the maximum lateral displacements and the shear in the walls and columns as the responses of rigid and flexible diaphragms were highlighted and outlined.
Keywords: Reinforced concrete, staggered shear wall, load carrying, floor diaphragm, rigidity.
M. Ghafoori- Ashtiani and A. Foyooz,
Volume 21, Issue 1 (7-2002)
Abstract
The importance of the equipment and secondary systems in seismic design and performance evaluation is well recognized and has been the subject of many studies. In all of these studies, earthquake is considered as a single component, and in most of them the primary system is considered as shear building. Most attention has been concentrated on the response of secondary system and its response spectrum. In this paper, the transfer function for absolute acceleration of the secondary system is obtained.
The squared modulus of transfer function relates the power spectral density function of the input (excitation) to the output (response), which is useful in the study of the various dynamic parameters of the system. In addition to transfer function, the autocorrelation and power spectral density function of absolute acceleration of the secondary system are obtained. Earthquake is considered as a multi-component system and the necessary formulation is developed for the calculation of these functions as well as the critical angle with and without interaction between the two systems. The damping of the system is considered as proportional in the decoupled analysis, and nonproportional in the coupled analysis. The formulation developed has been illustrated by considering a ten-story torsional builing. Various parameters such as eccentricity, correlation between components, tuning interaction and nonproportional damping are studied. Results show that eliminating the effect of multicomponentness of earthquake can cause large errors especially at large eccentricities.
Keywords: transfer function, Random vibration, secondary systems, critical angle, interaction, nonproportional damping
M. R. Soltani, H. Fazeli, B. Farahanieh and A. R. Davari,
Volume 21, Issue 1 (7-2002)
Abstract
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
