سال:1383 | دوره: | شماره: |تعداد مقالات:10

Application of the computer simulation for solving the incompressible flow problems motivates developing efficient and accurate numerical models. The set of Inviscid Incompressible Euler equations can be applied for wide range of engineering application. For the steady state problems. The equation of continuity can be simultaneously solved with the equations of motion in a coupled manner using the Artificial Compressibility Technique. This technique helps coupling the pressure and the velocity fields during the explicit computation procedure of the incompressible flow problems and therefore speeds of the convergence of the solution. The discrete form of the two-dimensional flow equations are formulated using the Xell Vertex Finite Volume Method for triangular unstructured meshes. Using triangular unstructured meshes provides great flexibility for modeling the flow in geometrically complex domains. Some numerical techniques adopted for the unstructured meshes are used to stabilize and accelerate the explicit solution procedure without degradation of accuracy. In order to verify the accuracy of the numerical model, computed results are compared with the analytical solutions of potential flow problems.

An SPICE compatible model for multiple coupled nonuniform lossless transmission lines (TLs) is presented. The method of the modeling is based on the steplines approximation of the nonuniform TLs and quasi-TEM assumptions. Using steplines approximation the system of coupled nonuniform TLs is subdivided into arbitrary large number of coupled uniform lines (steplines) with different characteristics. Then using modal decomposition method the system of coupled partial differential equations for each step is decomposed to a number of uncoupled ordinary wave equations describing uncoupled uniform TLs in each step. To satisfy the boundary conditions at the discontinuities a new model is developed. Therefore each step of the system can be modeled in SPICE using a set of ideal delay lines representing uncoupled TLs and some linear-dependent voltage and current sources. Finally some examples are given to show the validity and usefulness of the model.

In recent years, nonlinear circuit analysis techniques have been extensively investigated. One of the most important reasons is the application development of solid-state devices at microwave frequencies. Different methods have been used to analysis large signal behavior of these devices. In this paper load-pull curves (one of design requirement) are obtained using Volterra series. The main advantage of this technique is shortening the time of computations for weak nonlinear analysis in cases such as Class A power amplifiers. The proposed procedure has been coded in MATLAB. Using iterative methods, loads with constant output power have been obtained. Finally the results are compared to experimentally measured values and a fair degree of calculation between them is observed.

The purpose of this paper is to present the possibility of replacing physical unit cost in transportation or distribution problems by an aggregate coefficient, getting qualitative and subjective considerations involved. The model for constructing aggregate cost is a two stage multiple attribute decision-making problems. In the first stage supply points, demand points and routes of transportation are alternatives and have to be weighted against their own attributes. In the second stage, the alternatives are placed as attributes in a new matrix and the unit aggregate costs will be the new alternatives. Some heuristic techniques are developed for tradeoffs between attributes. Experts and decision-makers do tradeoff. The results are compared with the simple physical costs.

This paper presents a mathematical model for designing cellular manufacturing systems (CMSs) solved by genetic algorithms. This model assumes a dynamic production, a stochastic demand, routing flexibility, and machine flexibility. CMS is an application of group technology (GT) for clustering parts and machines by means of their operational and / or apparent form similarity in different aspects of design and production. Most previous researches carried out in CMSs have been embodied in static production and deterministic demand states. Due to real situations of a CM model, if includes a great number of variables and restrictions requiring a long period of time, memory, and process power in order to be solved using available software packages and current optimal methods. Therefore, most researchers pay attention to novel methods. One of these methods is genetic algorithms (Gas). GA is a class of stochastic search techniques used for solving the NP-complete problems, such as CMSs. In this paper, a nonlinear integer model of CMS is designed in dynamic and stochastic states. Then, genetic algorithm is used to solve the problem and finally computational results are compared to existing optimal solutions in order to validate the efficiency of the proposed algorithm.

We develop a bicriteria model for the resource allocation problem in PERT networks, in which the total direct costs of the project as the first objective, and the mean of project completion time as the second objective are minimized. The activity durations are assumed to be independent random variables with either exponential or Erlang distributions, in which the mean of each activity duration is a non-increasing function of the amount of resource allocated to it. The direct cost of each activity is assumed to be a non-decreasing function of the amount of resource allocated to it. Finally, we use the goat attainment method to solve the related bicriteria optimal control problem numerically, by converting this problem to a related bicriteria nonlinear programming, and obtain the optimal values of the resources allocated to the activities.

This paper deals with machine repair problem with balking and reneging. There is provision of mixed standby (warm and cold) components to replace the failed machines. The lifetime and repair times are assumed to have exponential distribution. Birth-death technique is suggested to obtain the queue size distribution in explicit form. A repair facility of C permanent repairmen is facilitated to repair the failed machines in FCFS basis. Some special cases are deduced which tally with earlier results. To find out the optimal number of spares and repairmen, a cost function is also mentioned.

A packed bed thermal storage has several desirable characteristics to be energy storage. The behavior of packed bed is predicted by set of differential equations. A numerical solution is developed for packed bed storage tank accounting to the secondary phenomena of thermal losses and conduction effect. The effect of heat loss to surrounding (k__1), conduction effect (k__2) and air capacitance (k__3) are examined in the numerical solution. It is found that the values of (k__1) and (k__2) are small and can practically be neglected in the solution. The solution indicates the profiles of air and rock bed temperatures with respect to time and length of the bed.

Optimal load of mobile robots, carrying a load with predefined motion precision, is an important consideration regarding their applications. In this paper a general formulation for finding maximum load carrying capacity of flexible joint mobile manipulators is presented. Meanwhile, overturning stability of the system and precision of the motion on the given cnd-effector trajectory are taken into account. The main constraints applied for the presented algorithm are torque capacity of actuators, limited error bound for the end-effector and overturning stability during the motion on the given trajectory. In order to verfy the effectiveness of the presented algorithm a simulation study considering a compliant joint two-link planar manipulator mounted on a differentially driven mobile base is explained in details.

The effect of the asymmetric water entry over submerged part of a ship on the hydrodynamic impact is investigated numerically. A wedge body is considered to study and the problem is assumed to be two-dimensional. The Results of symmetric and asymmetric impacts are compared together. The effect is found significant in the numerical simulation. The maximum hydrodynamic pressure at a heel angle of 10 degrees becomes about 95% more than that of the symmetric entry. The result of the present work proves the importance of asymmetrical hydrodynamic impact loading for structural design of a ship. Besides, the numerical procedure is not limited to a wedge type cross section and it is possible to apply it for any real geometry of ships and high-speed crafts.