AMIRKABIR
Year:2008 | Volume:18 | Issue:67-B|Papers Count:7

An accurate and efficient computational method is proposed for the time-optimal switching control problem of non linear systems. The method is based on spectral collocation technique using the Legendre-Gauss nodes. Spectral method is used to discretize the problem in a sequence of time subdomains separately and form a non linear programming problem. By solving this NLP problem, the optimal control in each subdomain and the joining times of the subdomains will be obtained simultaneously. The method is computationally attractive and applications are demonstrated through illustrative examples.

The ever-changing economic turns the decision-making on financing a project into a crucial for banks. There are many cases in developing countries in which projects failed either by completing far later than their dues or by not meeting their minimum economic and managerial requirements.There are many techniques for project evaluation, but it is obvious that the criteria being used depend upon the specific conditions like economic, infrastructure, technical ability, government policy, etc. In this research, a decision-making model for project evaluation is developed using expert systems and AHP technique. AHP technique is used to determine the weights of each criterion these results of evaluation by use of AHP and some other managerial criteria like honesty of the manager and previous history forms the knowledge base for evaluation. VP-Expert shell is used to model the expert system for evaluation. The model is tested in 10 cases and the results confirmed the validating of the developed model.

A mobile ad hoc network is a wireless network, which supports mobile nodes without any fixed infrastructure. Multicast routing is one of the main challenges in mobile ad hoc networks. There is no fixed router in MANETs, so nodes act as routers and determine required routes. However, the route once found is not necessarily valid after movement of nodes. Since there is no centralized information, finding stable routes satisfying global constraints is challenging. In this paper, a method is proposed for sharing nodes information to form a virtual infrastructure. A mathematical formulation of maximum stable multicast tree construction problem with delay constraint is developed and a heuristic algorithm is proposed to solve it. By simulation, we evaluate the performance of proposed algorithm.

This paper describes a theoretical study to investigate the heat transfer characteristics of porous radiant burners. A one dimensional model is used to solve the governing equations for porous medium and gas flow before the premixed flame to the exhaust gas. Combustion in the porous medium is modeled as a spatially dependent heat generation zone. The homogeneous porous media, in addition to its convective heat exchange with the gas, may absorb, emit and scatter thermal radiation. In the computation, the radiation effect in the gas flow is neglected but the conductive heat transfer is considered into account. In order to analyze the thermal characteristics of porous burners, the coupled energy equations for the gas and porous medium based on two-flux radiation model are solved numerically and the effect of various factors on the performance of porous radiant burners are determined. Comparison between the present results with those obtained by other investigators shows a good agreement.

In the metal forming processes, the process variables (such as forming speed) have significant effects on the forming parameters. In this study, using the motion equations (instead of the equilibrium equations) in the biaxial stretching analysis of the sheet, the evolution of initial in homogeneity has been simulated. Adopting work hardening and strain rate hardening characteristic for material, the effects of forming speed on the shape and position of the forming limit diagram (FLD) are investigated theoretically over a wide range of strain rate. Furthermore, the combined effects of the strain rate and material intrinsic parameters (density, strain hardening component and strain rate sensitivity index) on the critical strain are obtained in the case of plane strain loading. The analysis indicates that after a certain forming rate, by increasing the strain rate, localized necking is retarded and formability of sheet improves monotonically because of inertia effects.

In this paper, a finite element model is formulated for dynamic analysis of beams containing two parallel and unequal edge-cracks. First, by using the compliance matrix concept, which accounts for the effect of local flexibility due to the presence of crack in the structure, the stiffness matrix is calculated for a beam element with one edge-crack. The effects of crack depth and its location on the first and third natural frequencies of the beam are investigated. The same model is then extended for an element with a double edge-crack of unequal lengths. Considering the interaction between cracks, the influence of crack depth on free vibration of the beam is investigated. Finally, a new model is developed for a beam element containing two parallel edge-cracks. It is shown that the vibrational behavior of beam is significantly influenced when the second crack approaches the first one.

The stress intensity factor (SIF) of an internal surface straight or curve fronted crack (semi-elliptical) of an internally pressurized thick cylinder at its deepest point is an essential parameter for calculation of fatigue life and crack behavior prediction. The weight function method is a widespread method for SIP determination in fracture mechanics. It was shown that; if for one loading system we have a known complete solution for a crack problem (crack face displacement & SIF) then for any other loading system the SIF will be solved directly. In this paper, a new method using FEM based on the crack face displacement applied to evaluate a weight function to a specific geometry. Comparison of the result with other approved data shows this method is efficient.