AMIRKABIR
Year:2005 | Volume:16 | Issue:62-B|Papers Count:13

This study aims at flutter characteristics of orthotropic plates subjected to supersonic flow. The plate is subjected to uniform in-plane forces (Nx, Ny) and its edges are assumed to be elastically restrained (with various degree) against rotation. This type of boundary conditions covered a wide range of B.C's from simply support to clamped supports. The analytical solution is obtained using classical plate theory. Frequency equations of these systems are very large and difficult to solve. A simple method of solving this type of equations has been used in order to investigate the flutter characteristics of 011hotropic plates. The effect of plate geometry, material propel1ies, in-plane load and boundary condition are presented and discussed.      

Leveling process has been widely used for decreasing and improving the flatness strip defect which occurred after rolling or opening coil. In this paper, a two-dimensional Numerical analysis method for analysis of elastic-plastic behavior of strip in leveling process is presented. By using the method, effect of different variables such as intermesh, roll diameter and number of rolls on residual curvature and residual stress in final product are investigated. Moreover, experiments with connecting the process for decreasing curvature of strip are carried out. The theoretical predictions are compared with results of finite element method (ANSYS) and experiment. Good agreements between the analysis and experimental results show the validity of the method. The method has the advantages of saving both the trial and error time and decreasing the production cost.      

One of the important JIT assumptions is the setup time reduction so that an ideal lot size of one unit is possible. Setup time reduction is rather a technical problem than a production planning problem and it is not readily possible. Therefore if maximum effort to reduce set-up time has resulted in a fixed batch size in accordance with the EOQ model, batch manufacturing occurs. This paper is to obtain JIT scheduling of parts while minimizing the overtime cost of resource in the multi-product batch manufacturing. The problem is formulated as two models. From the first model, a mixed integer programming with minimum WIP cost objective, just-in-time production scheduling of batches is obtained. This inventory minimizing approach produces the right parts, in the right quantity and at the right time. However the achievement of minimum inventory may be at the expense of significant workload variability. Since the first model provides multiple optimum solutions, the second model allows us to select the best solution with regard to minimum overtime resource cost. Hence JIT production with minimum resource overtime cost is realized.     

A new technique for reducing the drag of either existing or new aircraft configuration was investigated. Heating the surface under a turbulent boundary layer reduces the turbulent skin friction. In present paper, friction drag reduction in axisymmetric bodies is investigated numerically, applying heating the surface in the turbulent boundary layer. For numerical analysis, an axisymmetric model, nearly like an airplane body, with a length of 0.5 m and taper ratio of 10 is considered which, quarter length of its tip and end has elliptical form and the others is flat. Also, axisymmetric form of the thin layer Navier Stokes equations using Baldwin-Lomax turbulent model is applied for the numerical simulation. Detecting the best of the heating area is performed by dividing the flat section of the model to five equal segments and therefore, effects of temperature variation in various segments and changing number of heated segments are investigated. Numerical results show that warming the first segment to 100°c, causes approximately 25 percent reduction in drag force.      

Iran Khodro Company is a leading auto maker in Iran which holds about 65% of the market share and hence the share holders show great interests in predicting its stock exchange price. On the other hand due to the chaotic behavior of share price in Tehran Stock Exchange the classical models such as ARIMA and ARCH would not be efficient models to represent the dynamics governing the share price. However, neural network (NN) models are proposed to predict Iran Khodro Stock Exchange Price (IKSEP). Several neural network models such as MLP, ELMAN, CASCADE, GRNN and RBFN were examined. Because of serious volatility in IKSEP, special method was proposed for testing and training the data which considerably improved the results. Extensive tests have been curried out to choose the most suitable feature such as, the type of transfer function, the number of hidden and output layers, the training algorithm, and the technical and fundamental variables. Some fundamental variables such as oil price, P/E and volume of stock exchange were introduced in the model and showed to be considerably effective in the accuracy of forecast. The best results obtained from NN models were compared to those obtained by using exponential smoothing and Box-Jenkns models. The results showed the NN forecasts were superior to those of the time series model.        

Microcellular foams are thermoplastic foams containing a large number of small cells on the order of 10 μm in size. As blowing agent, an inert gas (CO2 or N2) is used. Because of very tiny cells, these foams exhibit improved mechanical and physical properties. For instance, tensile-to-density, toughness, thermal properties and electrical properties are improved. In this paper, structural and mechanical properties of microcellular foams of ABS and its composite (mineral filled) are studied. Gas absorption. Gas desorption, density, size and number of microcells and also ultimate tensile strength and ultimate strain of foams are compared. Results show that a sound structure and a high tensile-to-density can be obtained with regulating time and temperature just above glass transition temperature. The results also revealed that although mechanical properties are improved, the enhancement is not as it has been commonly asserted.    

At present, "computer aided fixture design" (CAFD) systems designs fixtures based on locators and clamps. But under some complex machining conditions supports should be added to the fixture system as well as locators and clamps. This paper presents rule based, analytical and numerical methods for selecting support parameters. In addition, the algorithm for using these methods in a CAFD system has been suggested to develop the present systems. For verification of the presented methods, the results are compared with experimental results and the results obtained in the previous works.      

In this paper, we consider a typical supply chain where a single supplier produces multiple components on a flexible job shop (FJS) and delivers them directly to an assembly facility (AF). It is assumed that demand rates for these components are deterministic and constant over a finite planning horizon. The objective is to find a common cycle lot production and delivery schedule that minimizes the average of holding, setup, and transportation costs per time unit for the supply chain. This problem consists of a combinatorial part (machine assignment and sequencing sub-problems), and a continuous part (common cycle duration and scheduling sub-problems). To account for these two elements, a new mixed integer nonlinear program (MINLP) is developed which simultaneously determines machine allocation, sequencing, lot sizing and scheduling decisions. In order to reduce computational complexity, instead of solving this MINLP directly, we propose an efficient enumeration method to determine the optimal solution of the model. The performance of the proposed method is evaluated by some numerical experiments. Another applicable case (Lot streaming) is also studied and required modifications in the model formulation and the solution procedure are described. Moreover, a numerical example is presented to illustrate applicability of the proposed mathematical models and the solution methods.      

In this paper, a new method is proposed for trip distribution forecasting in multi-commodity transportation networks. In this new approach, real trip distributions at present are modeled by approximation functions for each commodity. In large-scale multi-commodity transportation networks, these functions could be obtained by parts of network as training data, without losing generality of the algorithm. Of course, it will have an influence on the approximation. Then, a fuzzy model for trip distribution forecasting calibrates these functions. This model considers all adjustments simultaneously. In order to show the application of the new method, a numerical example is given and solved.      

The main purpose of this paper is developing a computer program for prediction of ship maneuverability in deep water in early stage of ship design. The authors have chosen Kijima's model for prediction of ship maneuverability together with some approximate formula for estimating hydrodynamics forces acting on hull, propeller, and rudder in deep water. A computer program has been developed where the results of program show satisfactory agreement with the model tests result. The conclusion is that the calculation method of the present study is very useful and powerful for prediction of ship maneuverability at the initial design stage, when the principle particulars of ship hull, propeller, and rudder are known.