A. Azaron , S.m. Fatemi Ghomi,
Volume 18, Issue 3 (11-2007)
Abstract
Abstract : In this paper , we apply the stochastic dynamic programming to approximate the mean project completion time in dynamic Markov PERT networks. It is assumed that the activity durations are independent random variables with exponential distributions, but some social and economical problems influence the mean of activity durations. It is also assumed that the social problems evolve in accordance with the independent semi-Markov processes over the planning horizon. By using the stochastic dynamic programming, we find a dynamic path with maximum expected length from the source node to the sink node of the stochastic dynamic network. The expected value of such path can be considered as an approximation for the mean project completion time in the original dynamic PERT network.
S.m. Seyed-Hosseini, M. Sabzehparvar, S. Nouri ,
Volume 18, Issue 3 (11-2007)
Abstract
Abstract: This paper presents an exact model and a genetic algorithm for the multi-mode resource constrained project scheduling problem with generalized precedence relations in which the duration of an activity is determined by the mode selection and the duration reduction (crashing) applied within the selected mode. All resources considered are renewable. The objective is to determine a mode, the amount of continuous crashing, and a start time for each activity so that all constraints are obeyed and the project duration is minimized. Project scheduling of this type occurs in many fields for instance, predicting the resources and duration of activities in software development projects. A key feature of the model is that none of the typical models can cope with the continuous resource constraints. Computational results with a set of 100 generated instances have been reported and the efficiency of the proposed model has been analyzed.
M. Kargari, Z. Rezaee, H. Khademi Zare ,
Volume 18, Issue 3 (11-2007)
Abstract
Abstract : In this paper a meta-heuristic approach has been presented to solve lot-size determination problems in a complex multi-stage production planning problems with production capacity constraint. This type of problems has multiple products with sequential production processes which are manufactured in different periods to meet customer’s demand. By determining the decision variables, machinery production capacity and customer’s demand, an integer linear program with the objective function of minimization of total costs of set-up, inventory and production is achieved. In the first step, the original problem is decomposed to several sub-problems using a heuristic approach based on the limited resource Lagrange multiplier. Thus, each sub-problem can be solved using one of the easier methods. In the second step, through combining the genetic algorithm with one of the neighborhood search techniques, a new approach has been developed for the sub-problems. In the third step, to obtain a better result, resource leveling is performed for the smaller problems using a heuristic algorithm. Using this method, each product’s lot-size is determined through several steps. This paper’s propositions have been studied and verified through considerable empirical experiments.
P. Akhavan, M. Fathian, M. Jafari ,
Volume 18, Issue 3 (11-2007)
Abstract
Abstract: Nowadays knowledge is recognized as an important enabler for competitive advantages and many companies are beginning to establish knowledge management systems. Within the last few years many organizations tried to design a suitable knowledge management system and many of them were successful. This paper is to discover critical success factors (CSF) of knowledge management (KM) and their relationships in an effective way. A qualitative case study technique has been used in this paper for data collection and analysis. In this way, grounded theory (GT) research approach has been selected .The collected data are categorized and analyzed through specific stages of GT. A semantic network has been developed by categorized data showing the relationships between the extracted CSFs and finally a theory has been emerged. The semantic network and the emerged theory show the roadmap of success in KM area for the organizations.
A. Arefmanesh, M. Najafi, H. Abdi ,
Volume 18, Issue 4 (12-2007)
Abstract
Abstract : The meshless local Petrov-Galerkin method with unity as the weighting function has been applied to the solution of the Navier-Stokes and energy equations. The Navier-Stokes equations in terms of the stream function and vorticity formulation together with the energy equation are solved for a driven cavity flow for moderate Reynolds numbers using different point distributions. The L2-norm of the error as a function of the size of the control volumes is presented for different cases and the rate of convergence of the method is established. The results of this study show that the proposed method is applicable in solving a variety of non-isothermal fluid flow problems.
M. Shishehsaz ,
Volume 18, Issue 4 (12-2007)
Abstract
Abstract : The effect of a bond failure and its extent is studied on stress concentration in long fibers as well as stress distribution in short fibers and their surrounding matrix bays. The material is assumed to be a finite width hybrid composite lamina which is subjected to a tensile load of magnitude "P" at infinity. The surrounding matrix is assumed to take only shear (shear-lag theory). The bay adjacent to the first intact filament is allowed to experience a bond failure of size 2d . This failure is due to excessive shear load in the matrix which exceeds the fiber-matrix bond strength. The matrix at this zone may or may not experience yielding. The short fibers are simulated by assuming two successive breaks along each filament. The effect of bond failure length on short fiber load bearing capability, as well as stress concentration in the first intact filament is fully investigated. The effect of hybridization, in presence of bond failure is also examined on short fiber load bearing behavior.
H. Ahmadian, S. Nazari , H. Jalali ,
Volume 18, Issue 4 (12-2007)
Abstract
Abstract: The governing equations of motion for a drill string considering coupling between axial, lateral and torsional vibrations are obtained using a Lagrangian approach. The result leads to a set of non-linear equations with time varying coefficients. A fully coupled model for axial, lateral, and torsional vibrations of drill strings is presented. The bit/formation interactions are assumed to be related to the following parameters: bit motion, effects of gyroscopic moments, contact with the borehole wall, axial excitation due to bit/formation interactions, and hydrodynamic damping due to the presence of drilling mud. Simulation results show that parametric resonance and whirling may occur simultaneously within the range of operating conditions of drilling. The contact force between collar and borehole wall is calculated and its behavior is investigated. The dynamic behavior is quite complicated and may become non-periodic, suggesting a chaotic behavior.
H. Golestanian ,
Volume 18, Issue 4 (12-2007)
Abstract
Abstract: This paper presents the results of experimental determination of fiber bed permeability variation with porosity. Flow measurement experiments were designed to measure fiber mat permeability for fiber beds with various fiber volume fractions. Woven fiberglass, chopped fiberglass, and carbon fiber mats were used as reinforcements. The effects of reinforcement type and porosity on fiber bed permeability were investigated. Fiber mat permeability of woven mats showed large degrees of anisotropy, whereas chopped fiberglass mats had isotropic permeability. In all cases perform permeability increased with fiber bed porosity. Fiber mat permeability of woven carbon was found to be about four times lower than that of woven fiberglass mats at the same porosity. This lower permeability results in longer injection time and higher manufacturing cost for composite parts made with carbon fiber mats. The results of this investigation could be employed in process/product optimization in Resin Transfer Molding (RTM) processes.
K. Farhadi ,
Volume 18, Issue 4 (12-2007)
Abstract
Abstract: This paper presents the results of an experimental examination of the effect of non-uniform wall temperature on local heat transfer coefficient in a rotating smooth-walled square channel. Three different thermal boundary situations were investigated: (a) even and odd (four) wall uniform temperature, (b) even and odd (four) wall uniform heat flux, and (c) even (leading and trailing) walls hot with two side walls kept cold. It is demonstrated that the local heat transfer coefficients on the trailing edge are much higher than that of the leading edge. For situation (a) of even (leading and trailing) walls with two sides uniform temperature, the leading edge heat transfer coefficient decreases and then increases with increasing rotational numbers. And the trailing edge heat transfer coefficient increases monotonically with rotational numbers increasing. However, the trailing edge as well as the side walls heat transfer coefficient for situation (b) is higher than situation (a) and the leading edge local heat transfer coefficients for situations (b) and (c) are significantly higher than situation (a). The obtained results suggest that the local non-uniform wall temperature creates the local buoyancy force that diminishes the effect of the Coriolis force. Consequently, the local heat transfer coefficients on leading, trailing, and side edges are affected by the wall non-uniform temperature.
R. Farnoosh, B. Zarpak ,
Volume 19, Issue 1 (3-2008)
Abstract
Abstract: Stochastic models such as mixture models, graphical models, Markov random fields and hidden Markov models have key role in probabilistic data analysis. In this paper, we used Gaussian mixture model to the pixels of an image. The parameters of the model were estimated by EM-algorithm.
In addition pixel labeling corresponded to each pixel of true image was made by Bayes rule. In fact, a new numerically method was introduced for finding the maximum a posterior estimation by using EM-algorithm and Gaussians mixture distribution. In this algorithm, we were made a sequence of priors, posteriors were made and then converged to a posterior probability that is called the reference posterior probability. Maximum a posterior estimated can determine by the reference posterior probability which can make labeled image. This labeled image shows our segmented image with reduced noises. We presented this method in several experiments.
R. Tavakkoli-Moghaddam, M. Aryanezhad, H. Kazemipoor , A. Salehipour ,
Volume 19, Issue 1 (3-2008)
Abstract
Abstract : A tandem automated guided vehicle (AGV) system deals with grouping workstations into some non-overlapping zones , and assigning exactly one AGV to each zone. This paper presents a new non-linear integer mathematical model to group n machines into N loops that minimizes both inter and intra-loop flows simultaneously. Due to computational difficulties of exact methods in solving our proposed model, a threshold accepting (TA) algorithm is proposed. To show its efficiency, a number of instances generated randomly are solved by this proposed TA and then compared with the LINGO solver package employing the branch-and-bound (B/B) method. The related computational results show that our proposed TA dominates the exact algorithm when the size of instances grows.
A. Seifoddin, H.a. Salimi , A. Seyed Esfahani ,
Volume 19, Issue 1 (3-2008)
Abstract
Abstract: Innovations, commercialized by new or old established firms, located at the core of industrial renewal process. The innovation concept has suffered transformations, along with the evolution of the models that try to explain and understand the innovation process. The innovative process corresponds to all activities that generate technological changes and the dynamic interaction between them, not necessarily being novelties. Linier model, Chain-Linked Model and National Innovation Systems (NIS) Approach, are three models that have developed for innovation process. Innovation process can be viewed as evolutionary process. One can recognize some mechanism for innovation evolution. These are grouped into two classes those that increase configurations variation and those that decrease it. Emergence of knowledge, knowledge flow and recombination are the mechanism to increase variation of configuration. Internal and external selections are the mechanism to selecting. Innovation operators are evolutionary operators that create new combinations of configuration and increase variation. This paper develops an evolutionary cycle in innovation process and extends evolutionary mechanisms of innovation.
F. Sanati , S.m. Seyedhoseini,
Volume 19, Issue 1 (3-2008)
Abstract
Abstract: At the last decade of the 20th century, Womack et. Al introduced Lean concept to the industrial world. Since 1990 up to now, existed studies mostly have focused on lean production in the step of manufacturing, but in this research leanness concept has developed in the plant life cycle. In this paper leanness concept will be described as elimination of wastes in the phases of investment, plant design & construction(hardware), organization & systems design (software) and these three steps will be added to, elimination of previously described seven wastes in production step. For this purpose at first, the types of wastes in the above mentioned phases are defined by using Axiomatic Design methodology. After defining the types of wastes, a model for assessment of leanness is submitted. In this quantitative model, amount of leanness in each phase will be determined and combined to make a unique measure for total leanness. Dimensions of leanness are shown for quick understanding, by using a spider diagram. In the last section of the paper, the results of an example of the application of this model in fan industry are brought. This example shows the simplicity and powerfully of the model to determine the leanness in before production phases. © 2008 Authors all rights reserved.
M.b Aryanezhad , A. Roghanian ,
Volume 19, Issue 1 (3-2008)
Abstract
Abstract: Bi-level programming, a tool for modeling decentralized decisions, consists of the objective(s) of the leader at its first level and that is of the follower at the second level. Three level programming results when second level is itself a bi-level programming. By extending this idea it is possible to define multi-level programs with any number of levels. Supply chain planning problems are concerned with synchronizing and optimizing multiple activities involved in the enterprise, from the start of the process, such as procurement of the raw materials, through a series of process operations, to the end, such as distribution of the final product to customers. Enterprise-wide supply chain planning problems naturally exhibit a multi-level decision network structure, where for example, one level may correspond to a local plant control/scheduling/planning problem and another level to a corresponding plant-wide planning/network problem. Such a multi-level decision network structure can be mathematically represented by using “multi-level programming” principles. This paper studies a “bi-level linear multi-objective decision making” model in with “interval” parameters and presents a solution method for solving it this method uses the concepts of tolerance membership function and multi-objective multi-level optimization when all parameters are imprecise and interval .
M. Alaeiyan,
Volume 19, Issue 2 (7-2008)
Abstract
Abstract : We call a Cayley graph Γ = Cay (G, S) normal for G, if the right regular representation R(G) of G is normal in the full automorphism group of Aut(Γ). In this paper, a classification of all non-normal Cayley graphs of finite abelian group with valency 6 was presented.
A. Neisy ,
Volume 19, Issue 2 (7-2008)
Abstract
Abstract: Determination of the diffusion coefficient on the base of solution of a linear inverse problem of the parameter estimation using the Least-square method is presented in this research. For this propose a set of temperature measurements at a single sensor location inside the heat conducting body was considered. The corresponding direct problem was then solved by the application of the heat fundamental solution.
H. Tavallaee , R. Varmazyar ,
Volume 19, Issue 2 (7-2008)
Abstract
Abstract: Let be a commutative ring and be a unitary module. We define a semiprime submodule of a module and consider various properties of it. Also we define semi-radical of a submodule of a module and give a number of its properties. We define modules which satisfy the semi-radical formula and present the existence of such a module.
M. Hadian Dehkordi , A. Cheraghi ,
Volume 19, Issue 2 (7-2008)
Abstract
Abstract : A visual cryptography scheme based on a given graph G is a method to distribute a secret image among the vertices of G, the participants, so that a subset of participants can recover the secret image if they contain an edge of G, by stacking their shares, otherwise they can obtain no information regarding the secret image. In this paper a maximal independent sets of the graph G was applied to propose a lower bound on the pixel expansion of visual cryptography schemes with graph access structure (G ). In addition a lower bound on the pixel expansion of basis matrices C5 and Peterson graph access structure were presented .
A. Allahverdi, K. Mehrpour , E. Najafi Kani,
Volume 19, Issue 3 (7-2008)
Abstract
Abstract: In recent years, many research works have been done to investigate the possibility of utilizing a broad range of materials as raw materials in the production of geopolymer cements. The use of artificial pozzolans or aluminosilicate-type industrial waste materials such as granulated blast-furnace slag and fly ash has been reported in many research works. Natural pozzolans are also aluminosilicate-type materials which can be activated with solutions of NaOH and Na2SiO3. Using a pumice-type natural pozzolan from Taftan Mountain located at the south east of Iran and different alkali-activators based on combinations of Na2SiO3 and NaOH, a number of natural-pozzolan-based geopolymer cement systems were designed and prepared. Final setting time, workability, and 28-day compressive strength of the systems were studied. The results obtained reveal that Taftan pozzolan can be activated using a proportioned mixture of Na2SiO3 and NaOH resulting in the formation of a geopolymer cement system exhibiting suitable workability and relatively high 28-day compressive strengths up to 63 MPa.
M. Parvari , M. Moradi,
Volume 19, Issue 3 (7-2008)
Abstract
Abstract : In this study, we prepared a LaNi0.3Al0.7O3 perovskite catalyst using a sol-gel related method (with prop ionic acid as a solvent) for use in the methane dry reforming reaction to produce synthesis gas. We defined the catalyst structure on the basis of X-ray diffraction analysis and measurements of the specific surface area and particle size distribution. The mixed oxide structure was shaped into a cylindrical pellet before being measured for its mechanical strength. The shaped perovskite catalyst was then tested in the methane dry reforming reaction to produce synthesis gas at atmospheric pressure. The results are compared with the predictions of a mathematical model that is used to estimate the concentration profile within the pellet. The outlet concentration of the reactants and conversion products calculated by the mathematical model has been consistent with the results obtained by experiments conducted in a fixed bed reactor.
