فهرست مطالب

  • Volume:15 Issue: 2, 2002
  • Transactions : A : Basic
  • 210 صفحه،
  • تاریخ انتشار: 1381/03/01
  • تعداد عناوین: 13
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  • S.Yaghmaei Pages 105-114
    A mathematical model has been simulated to describe natural in situ bioremediation of contaminated soil. The model equations consist of a system of three non-linear partial differential equations. Sensitivity analysis conducted by numerically solving them, has demonstrated the effect of initial contaminant concentration on the time and mechanism of remediation. The result of simulation indicates that microbiological degradation of contaminant within the soil aggregate may play a significant role in remediation so that most of contaminants, which are initially present within the aggregates, are consumed before they reach the outer surface.
  • S.A.Sadrnezhad Pages 115-124
    A constitutive multi-laminate based elastic-plastic model developed to be capable of accounting induced anisotropic behavior of granular material such as sand. The fabric feature or grain orientation characteristic effects through medium are considered in a rational way under any complex stress path, including cyclic loading. The salient feature of the developed model is a non-associative on plane plasticity with biaxial hardening as a function of plastic shear strain components. Generalized form of multi-laminate framework employed to sum up the non-symmetric plastic compliance matrices of sampling planes to build up the main compliance matrix. Two normal translation rules of yield boundary are specified upon the components of plastic shear strain on every sampling plane. The constitutive model is capable of describing expansion of two yield functions upon two predefined coordinate axes. The hardening parameters affect the plastic strain tensor uponthe contribution of twenty-six different sliding orientations through any point in medium. This contribution makes a powerful representation of strain hardening due to fabric effects in behavior of material. The comparison of experimental test results with model results represents that this model is more capable in cyclic behavior of porous media such as sand.
  • M.M.Toufigh Pages 125-134
    In seepage problems, the coefficients of permeability in Laplace equation are usually assumed to be constant vs. both space and time; but in reality these coefficients are variable. In this study, the effect of material deformation due to external loads (consolidation) and variation of head in the consolidation process are considered. For the first case, formulation of kx and ky can be defined by a second order binominal equation in order to take into account the material changes due to volume changes. For the second case, kx and ky can be defined as a function of unknown total head. The solution of the resulting non-linear differential equation is found using the Least Square Finite Element formulation. In order to increase the accuracy of the solution, eight nodal (isoperimetric) elements were obtained. This method was used satisfactorily to solve several seepage problems and to examine the accuracy and convergence of the results. The effect of a variable coefficient of permeability may not be significant on small dams, but as the height of the dam increases, the effect becomes more considerable. It is believed that a variable permeability analysis such as the one described in this paper should be taken into account.
  • H.R.Bakhshi, M.H.Ghassemian, M.Kahrizi Pages 135-140

    In recent years, there has been considerable interest in the use of CDMA in mobile communications. Bit error rate is one of the most important parameters in the evaluation of CDMA systems. In this paper, we develop a technique to find an accurate approximation to the probability of bit error for asynchronous direct–sequence code division multiple–access (DS/CDMA) systems by modeling the multiple access interference (MAI) as an improved Gaussian process. The channel is modeled as a multipath flat Rayleigh fading channel. Numerical results are obtained by using the improved Gaussian approximation (IGA) and compared to the standard Gaussian approximation (SGA).

  • J.Chitizadeh R.Ghazizadeh Pages 141-146
    In Local Area Networks (LAN) connected together by bridges, flow control and smooth traffic in the network is very important. However, congestion at bridges can cause intensive loss of received frames. In addition, the received frames are thrown away and have to be retransmitted by the source station, which causes more congestion and massive reduction in the overall network throughput. The network has a series topology in which all the LANs use Token Ring protocol at the presence of voice and data services. In the simulation voice traffic has the priority to access the network over the data traffic. In this paper techniques are introduced to reduce congestion at the bridges and improve the overall performance in the LANs connected through bridges. In the first technique data transmission is allowed during voice transmission by bridges. In the second technique the bridges have higher priority of access. The results show that using these techniques significantly improves network throughput.
  • N.Komjani Barchloie, M.Solaimani Pages 147-156

    The frequency-dependent characteristics of microstrip planar circuits have been previously analyzed using several full-wave approaches. All those methods directly give characteristic of the circuits frequency by frequency. Computation time becomes important if these planar circuits have to be studied over a very large bandwidth. The transmission line matrix (TLM) method presented in this paper is another independent approach for obtaining the frequency-domain results for microwave circuits through time-domain results, followed by Fourier transform. The advantage of this method is obtaining characteristics versus frequency over the whole band with a single simulation. The method is shown to be an efficient tool for modeling microstrip planar circuits as well as rectangular microstrip antennas and filters. The frequency characteristics calculated with the 3D-TLM method show excellent agreement with measured values and results obtained by other methods.

  • R.P.Ghimire M.Jain Pages 157-160
    This paper reports on the minimization of the average waiting time of the customers in the M/G/1 queue with vacation. Explicit formula for the unknown service parameter of a particular customer has been obtained by considering the exhaustive service discipline in the case of multi-user with unlimited service system. Moreover, results in case of partially gated and gated service disciplines under limited/unlimited service systems have been provided. Some particular cases such as M/M/1 and M/D/1 models with and without vacation have also been discussed.
  • M.Jain A.Kumar G.C. Sharma Pages 161-168
    With the evolution of technology, the maintenance of sophisticated systems is of concern for system engineers and system designers. The maintenance cost of the system depends in general on the replacement and repair policies. The system replacement may be in a strictly periodic fashion or on a random basis depending upon the maintenance policy. At failure, the repair of the system may be performed perfectly or minimally associated with some probability. When perfect repair is done, it makes the system as good as the new one. In case of minimal repair, it returns to the working condition of the system at the time of failure. In the present paper, we study the replacement policies for the system wherein minimal or perfect repair is done at the time of failure. The expressions for sexpected cost for the system with replacement and minimal or perfect repair are evaluated. The maintenance costs are discussed for various policies. Numerical simulation is performed to validate the analytical results.
  • M.Jain P.Singh Pages 169-178
    In this paper, we consider a multi-server queuing system in which additional servers are allowed for a longer queue to reduce the customer’s balking and reneging behavior. The customers are assumed to arrive in Poisson fashion and their service- times follow exponential distribution. The number of servers in long run depends on the queue-length. We obtain equilibrium queue size distribution. The expression for expected number of customers in the system in the long run has been obtained. Some other performance measures have also been provided. In order to match our results with earlier work, special cases are discussed. The numerical results are presented to verify the validity of the proposed analytical method.
  • M.Modarres E.Taimury Pages 179-190
    We develop a method to obtain an optimal solution for a constrained distribution system with several items and multi-retailers. The objective is to determine the procurement frequency as well as the joint shipment interval for each retailer in order to minimize the total costs. The proposed method is applicable to both nested and non-nested policies and ends up with an optimal solution. To solve this large nonlinear and integer problem, a two-level algorithm is proposed. In the first level, the functional constraints are relaxed and a solution is obtained by taking advantage of its special structure. Then, we apply separable programming technique for finding the optimal solution of the original problem. To decrease the size of the problem, some appropriate bounds on variables are introduced. We will show that under some conditions, the optimal solution of the original problem is proportional with the solution of its unconstrained problem.
  • S.J.Sadjadi Pages 191-196
    This paper presents a practical and efficient method to solve large-scale nonlinear equations. The global convergence of this new trust region algorithm is verified. The algorithm is then used to solve the nonlinear equations arising in an Expanded Lagrangian Function (ELF). Numerical results for the implementation of some large-scale problems indicate that the algorithm is efficient for these classes of problems
  • M.Hemayati G.Karami Pages 197-204
    Abstract A formulation and an implementation of two-dimensional Boundary Element Method (BEM) analysis for steady state, uncoupled thermoelastic problems is presented. This approach differs from other treatments of thermal loads in BEM analysis in which the domain integrals due to the thermal gradients are to be incorporated in the analysis via particular-integrals. Thus unlike Finite Elements or Field Boundary Elements algorithms the domain discretization becomes unnecessary. The algorithm and the formulation are implemented in a general purpose, multi-region twodimensional analysis. Isoparametric quadratic elements are employed to represent the geometry and the field variables. Examples are presented to demonstrate the accuracy and versatility of the method.
  • A.Adami Pages 205-210

    A two-dimensional transient axi-symmetric model was developed to study the effects of various thermal and geometric parameters on cyclic heating and cooling modes of a phase-change thermal energy storage system. The high-temperature thermal energy storage device utilizes LiH for heat sink applications to store the waste heat generated during power-burst periods. The stored heat is then discharged into space through radiators during the off-peak periods. The enthalpy method is developed to analyze a shell and tube configuration where the phase-change material is contained in an annulus by an inner tube and an outer shell. The development ignores the energy source, pressure variation, and external work done. The Gauss-Seidel iterative method with successive over-relaxation is used to solve the non-linear simultaneous difference equations. The charging period is about 1000 seconds (0.27777 hours). Various effective liquid thermal conductivities have significant effects on heating periods and various inner tube and outer shell radius have significant effects on cooling periods.