Showing 238 results for Gh
M. Golafshani and H. Ghassemi,
Volume 19, Issue 2 (1-2001)
Abstract
There are several sources for pressure oscillations in solid propellant rocket motors. Oscillatory flow field is one of them. Free shear layers in motor flow field cause vortex shedding. End edges of propellant grains and baffle edge in two-segmented motors are samples of such zones. These vortices move from their forming points and strike the field walls. The kinetic energy of vortices change to pressure, forming acoustical pressure oscillations. Acoustical characteristics of pressure oscillations such as frequency and amplitude change with the gradual change in the internal geometry of the motor. In this paper, the interaction between mean flow and acoustic field in a solid propellant rocket motor is studied numerically. Roe’s flux function in an unstructured grid strategy for solving compressible viscous flow equations show large changes in frequency of pressure oscillations in motor. Six different motor geometries are used for simulation of motor internal geometry at different burning times and grain configurations. Using this methodology, the frequency and intensity of pressure waves are well predicted. It is also shown that frequency jump from second longitudinal mode to the first is formed as a result of changes in the internal geometry.
A. Nasirahmadi, L. Estehghari, and A. A. Soudi,
Volume 19, Issue 2 (1-2001)
Abstract
The compounds, lanthanium chromite, lanthanium magnetite and lanthanium ferric oxide were prepared through sol-gel methods in powdered form, then calcinated, peletized, and sintered at various temperatures. Electrical conductivity of these compounds was measured at various temperatures. The effectiveness of strontium as a dopant on electrical conductivity was also investigated. Electrical conductivity enhancement was observed in dopped compounds
M. Ghodsi and K. Bazargan,
Volume 20, Issue 1 (7-2001)
Abstract
In this paper
, a fast method for automatic generation and scientific design of Persian letters is proposed. Scientific typeface design is an approach in which fonts are described by mathematical curves with well-defined parameters, where these parameters can be automatically tuned. METAFONT is a language suitable for the type of design used in this work. This language is particularly useful in designing Persian fonts because it can be used to simulate the pen movements of a calligrapher through automatic conversion of the scanned bitmap image of a font into a METAFONT program, which can in turn, produce the font at a high quality. A complete software has been implemented based on these algorithms that works interactively with the user to facilitate the font design.
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. S. Saidi and M. Saghafian, ,
Volume 20, Issue 1 (7-2001)
Abstract
In this paper, the oscillating two-dimensional laminar flow about a cylinder and the oscillation of a cylinder in still water are studied. A finite volume method is applied to solve the Navier Stokes equations using SIMPLEC algorithm on a body fitted co-located O-type grid. In this study, the non-dimensional flow numbers, Keulegan-Carpenter and Stokes’ numbers are chosen over a range where different laminar flow regimes are normally three-dimensional. The results of this simulation and comparison with numerical and experimental works indicate the good capability of this two-dimensional model in showing the various regimes of flow patterns and vortex shedding. Considering the forces exerted on the cylinder, this study shows that in cases where the flow is of a
regular type, there is a good match between longitudinal force presented by this work and the one calculated through Morrison’s equation. But for irregular flows where the flow pattern changes in each cycle, there is less overlap and the accuracy of Morrison’s equation is reduced. Studying the time variation of the transversal force gives accurate information about the vortex shedding and its frequency in each cycle and mode changing. Since the flow mode changes continuously with time, the average of transversal and longitudinal forces on consecutive cycles is not a good representation of the force exerted on the cylinder. On the other hand, the model has satisfactorily reproduced the time variation of the tranversal and longitudinal forces of a pure mode, matching the experimental results.
Keywords: Oscillating flow, Laminar flow about a cylinder, Numerical solution
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. Mirzaei, M. Saghaein - Nejad, V. Tahani and M. Moallem,
Volume 20, Issue 2 (4-2001)
Abstract
This paper introduces a novel passive suspension system for ground vehicles. This system is based on a flexible Electromagnetic Shock Absorber (EMSA). In the proposed system, efforts are made to a) select a high damping coefficient usable in a car b) determine Physical dimensions and geometry not much different from those of the mechanical shock absorbers and c) seletct EMSA weight and volume low enough for the core not to be saturated. A model is designed and developed followed by determining the dynamic equations for the model. The results from the simulation in a quarter car model are then compared with those from passive and active suspension systems.
Keywords: Active Suspension Systems, Electromagnetic damper, Finite Element method
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.
F. Nateghi-A and N.a. Hosseinzadeh,
Volume 20, Issue 2 (4-2001)
Abstract
This paper presents a methodology for the assessment of ductility and strength capacities in low-rise buildings. This method utilizes the characteristics of force-displacement for the lowest story level or considers the weakest story in any given low-rise building for its primary analysis. Calculations are based on two levels of earthquake motions, namely strong earthquakes (PGA=0.3 g), and very strong earthquakes (PGA=0.45). Failure mechanism for the structure is established based on three criteria which are: a) bending mode, b) shear mode, and c) shear-bending mode. Evaluation is then performed using a five step procedure starting with a: modeling the building, b) developing the non-linear properties of the model, c) strength calculations, d) ductility calculations, and finally, e) assessing the safety of the building under consideration. All these evaluations are performed based on a matrix format, which simplifies the whole procedure. Developed equations and step-by-step procedure are presented and described in this paper Satisfactory results are obtained from the use of the method developed.
Keywords: Strength, Ductility, Failure mechanism, Low-Rise R. C. Buildings
M. Saffarzadeh and Gh. Masoumi,
Volume 20, Issue 2 (4-2001)
Abstract
In the process of the optimum design of aprons, solutions should be found for problems and such issues as the optimum area and dimensions of the apron, including the passenger and the cargo the number and dimensions of the gates on the basis of different types of aircraft parking configuration aircraft simulation and arrangement in different time periods of the given day at the airport. In this research, a mathematical model was developed for the analysis and design of airport aprons based on minimum transportation cost. Some of the parameters of transportation cost include user, capital, and operational costs. Moreover, based on the fundamentals of the mathematical model, a computerized simulation model was developed taking into consideration the actual parameters of design of airport aprons such as stochastic demand, passenger behaviour, and evaluation of analytical model. The results obtained from the computerized simulation model indicate that policies of the airport authorities and air carriers such as flight schedules, gate use strategy, the mix of aircraft fleet during the planning horizon, operational conditions, and economic cosiderations have significant impacts on the design of the aprons.
Keywords: Airpornt, Apron, Optimization, Design.
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
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
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
B. Ghasemi,
Volume 21, Issue 1 (7-2002)
Abstract
In this paper, mixed forced and natural convection heat transfer in a rectangular cavity has been numerically studied. the cavity receives a uniform heat flux from one side and is ventilated with a uniform external flow. The external flow enters the cavity from the heated side and leaves the cavity from the opposite side. The velocity and temperature fields and heat transfer rate are determined by solving the two-dimensional continuity, momentum and energy equations. In this research, steady-state flow with constant Reynolds number, Re=100, is considered. Rayleigh number is in the range of 0≤Ra≤107. First, the results are presented for a cavity with constant aspect ratio, AR=2, and four different inlet and exit opening positions. Then cases with a fixed opening position and different aspect ratios including 0.1, 0.25, 1, 4 and 10 are modeled. In the cavities with opening in the bottom or cavities with aspect ratios less than one, the results show weak effects of natural convection on heat transfer. This research has been done for air as a working fluid (Pr=0.71). In some cases, the results are compared with those from previous studies.
Keywords: Convection, Natural, Forced, Cavity, Rayleigh, Ventilate
S. Yaghmaei, A. A. Seifkordi and H. Shirzadi,
Volume 21, Issue 1 (7-2002)
Abstract
A mathematical model has been analyzed for in-situ bioremediation with the purpose of remediating organic contaminated soil. Oxygen rich water when passed through the porous media of soil activates the aerobic microorganisms, leading to the biodegradation of the organic content. The model equations comprise three convection-dispersion partial differential solution of these equations has been conducted using the finite difference method. The effects of insufficient oxygen supply, growth of biomass and resistance to contaminant migration on the rate of biodegradation have been analyzed by numerically solving the equations.
The results from the numerical simulation indicate that the rate of biodegradation of contaminants in soil may be constrained not only by insufficient oxygen supply, but also by resistance to contaminant migration within the pore network.
Keywords: Bioremediation, Soil, Porous media, modeling
Gh. Yousefi, H.seifi and M. S. Ghazi-Zadeh,
Volume 21, Issue 1 (7-2002)
Abstract
In a de-regulated open access environment, reactive power is one of the ancillary services which must be provided by an Independent System Operator (ISO). In this paper, a new algorithm is proposed in which reactive power resources are initially so tuned that optimum security in terms of voltage profile and voltage stability are achieved while at the same time, the system losses are minimized. The resulting optimization case is solved as an Extended Multi-objective Optimal Power Flow (EMOPF) problem using Lexico Graphic Method (LGM). Thereafter, using the concept of Fair Resource Allocation (FRA), the reactive powers generated are distributed among existing transactions so that the costs incurred are properly and fairly recovered. The algorithm is successfully tested on a typical power system.
Keywords: Reactive Power, Reactive Power Management, Reactive Power Pricing, Voltage Profile, Voltage Stability, Deregulated Environment, Open Access
A. Hagilouy, M. Ghelich Khani and S. Ghasemi,
Volume 21, Issue 2 (1-2003)
Abstract
In order to carry out experimental investigations on radial inflow gas turbine, a special test laboratory was designed and constructed at Sharif University of Technology. This laboratory is introduced in the present paper and experimental procedures are elaborated on. Then, some test results are presented and discussed. The trends of performance characterisitics match our expectation and show good agreement with the published research results in this field.
Keywords: Radial inflow gas turbine, Experimental, Laboratory, Performance characteristics
M. Oliazadeh, M. Noaparast and R. Dehghan Simakani,
Volume 21, Issue 2 (1-2003)
Abstract
Application of gravity and magnetic separation methods to upgrade low grade Manganese ores from Venaj Mine has been reported elsewhere. This paper discusses the results of flotation tests, as well as combination of flotation and magnetic method to concentrate fine particles (less than 150 microns) of manganese ore. Results obtained from various direct and reverse flotation tests, using different types of reagents, indicated that manganese fines cannot easily be concentrated by flotation.
In this investigation, combination of direct flotation and magnetic separation for fine particles (finer than 150 microns) with 8.36% Mn, 34.11% SiO2, 23.05% Fe2O3 yielded a manganese concentrate with 26.78% Mn, 11.64% SiO2, 20.37% Fe2O3 and 56% recovery. Desliming 10-15 micron particles prior to flotation tests improved product quality and the recovery.
Keywords: Mineral Processing, Flotation, Manganese, Vanarj Mine
H. Farzanehfard and A. Pakizeh Moghadam,
Volume 22, Issue 1 (7-2003)
Abstract
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
