Scopus İndeksli Yayınlar / Scopus Indexed Publications

Permanent URI for this collectionhttps://hdl.handle.net/11413/6358

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On Scheduling Project Activities With Variable Expenditure Rates
(KLUWER ACADEMIC PUBL, SPUIBOULEVARD 50, PO BOX 17, 3300 AA DORDRECHT, NETHERLANDS, 1998) Özdamar, Linet
Commercial projects are undertaken with the expectation of maximizing financial returns. There are a variety of cash flow models in the literature in which payments, whose amounts depend on the type of contract quoted between the contractor and the client, are tied to the project events or to the completion times of activities. These models are of deterministic nature with respect to cash inflows, that is, the amount and timing of the payments, once determined, are fixed. However, there are situations specifically in the housing industry where the contractor is the owner of the project. In this case, the contractor starts with an initial capital to cover the activity expenditures and then, capital is augmented by the sale of flats which take place randomly over the progress of the project. In this risky environment, the contractor has to decide on the rate of expenditure at each decision time in order to maintain a positive cash balance. Hence, activities are represented by multiple performing modes with different activity durations and the same total cost. A heuristic to construct and re-construct schedules during the progress of the project is proposed here with the aim of maximizing the project Net Present Value while completing the project on time. The heuristic incorporates dynamic mode selection criteria which change adaptively according to the current status of the project. Computational experiments with the heuristic demonstrates that it provides satisfactory results regarding the feasibility of the schedules with respect to the project due date and the nonrenewable resource constraints.
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Electrochromic Properties of Sol-gel Deposited Ti-doped Vanadium Oxide Film
(ELSEVIER SCIENCE SA, PO BOX 564, 1001 LAUSANNE, SWITZERLAND, 1999-01) Özer, Nilgün; Sabuncu, Sevsen
Titanium-doped vanadium pentoxide, (100 - x)V2O5.xTiO(2) films with compositions ranging from 0 to 20 mole % titanium were prepared by using sol-gel dip coating techniques. Highly transparent (100 - x)V2O5.xTiO(2) films With properties superior to vanadium oxide were formed. The coating solutions were prepared by reacting vanadium tri-isopropoxide oxide and titanium tetra-isopropoxide as precursors and anhydrous isopropanol as solvent. The films were formed on transparent conducting substrates by a dip coating technique and heat treated at 300 degrees C for 1 h. The films were characterized by cyclic voltammetry, optical spectroscopy, scanning electron microscopy +/- SEM, and X-ray diffractometer (XRD). XRD results show that the films heat treated at temperatures below 400 degrees C were amorphous. The electrochemical lithium insertion shows that (100 - x)V2O5.xTiO(2) films exhibited good recharge ability of Li+/e(-) insertion/extraction process. Spectroelectrochemical measurements from 300 to 800 nm show that (100 - x)V2O5;O-5.xTiO(2) films exhibit weak cathodic coloration in the near uv and weak anodic coloration from 500 to 800 nm. It was observed that the addition of titanium resulted in marked improvement in the electrochromic behavior. This was especially seen for vanadium pentoxide doped with 5 mole % titanium. (C) 1999 Elsevier Science S.A. All rights reserved.
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A Hierarchical Planning System for Energy Intensive Production Environments
(ELSEVIER SCIENCE BV, PO BOX 211, 1000 AE AMSTERDAM, NETHERLANDS, 1999-01-15) Özdamar, Linet; Birbil, SI
This paper describes a hierarchical production planning approach with decision support features for energy intensive industries with particular reference to a tile manufacturing factory. In the tiling industry, the facilities which contribute most to the consumption of energy (and, hence, to the production costs) are usually the kilns where the curing operation is carried out. Frequently, the kilns are also the bottleneck in terms of capacity utilization. Thus, in order to save on energy costs, a planning approach which aims at minimizing the number of active kilns throughout the year is needed besides optimizing the process design in the curing department. To achieve the latter goal, it is necessary to take into account demand fluctuations as well as detailed capacity restrictions while deciding on the lot sizes of the products and the kilns on which the products are loaded. Rather than adopting a monolithic mathematical model for developing a desirable production plan, a hierarchical approach which decomposes the problem into two sub-problems is preferred. In the first level, products and capacity are aggregated over the planning horizon to achieve an overall consideration of demand fluctuations over time. Then, the solution provided by the aggregate solution for the current planning period is disaggregated into a detailed lot sizing and loading solution. The disaggregated problem is difficult to solve and hence, a heuristic is proposed here. This planning approach is sustained by a Decision Support System which enables the elimination of possible inconsistencies in the production plan by providing an effective interaction with the decision maker. (C) 1999 Elsevier Science B.V. All rights reserved.
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A Genetic Algorithm Approach to a General Category Project Scheduling Problem
(IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC, 345 E 47TH ST, NEW YORK, NY 10017-2394 USA, 1999-02) Özdamar, Linet
A genetic algorithm (GA) approach is proposed for the general resource constrained project scheduling model, in which activities may be executed in more than one operating mode and renewable as well as nonrenewable resource constraints exist, Each activity operation mode has a different duration and requires different amounts of renewable and nonrenewable resources. The objective is the minimization of the project duration or makespan, The problem under consideration is known to be one of the most difficult scheduling problems, and it is hard to find a feasible solution for such a problem, let alone the optimal one, The GA approach described here incorporates problem-specific scheduling knowledge by an indirect chromosome encoding that consists of selected activity operating modes and an ordered set of scheduling rules, The scheduling rules in the chromosome are used in an iterative scheduling algorithm that constructs the schedule resulting from the chromosome. The proposed GA is denoted as a hybrid GA (HGA) approach since it is integrated with traditional scheduling tools and expertise specifically developed for the general resource constrained project scheduling problem. The results demonstrate that HGA approach produces near-optimal solutions within a reasonable amount of computation time.
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FRACTOP: A Geometric Partitioning Metaheuristic for Global Optimization
(KLUWER ACADEMIC PUBL, SPUIBOULEVARD 50, PO BOX 17, 3300 AA DORDRECHT, NETHERLANDS, 1999-06) Demirhan, M; Özdamar, Linet; Helvacıoğlu, L; Birbil, SI
We propose a new metaheuristic, FRACTOP, for global optimization. FRACTOP is based on the geometric partitioning of the feasible region so that search metaheuristics such as Simulated Annealing (SA), or Genetic Algorithms (GA) which are activated in smaller subregions, have increased reliability in locating the global optimum. FRACTOP is able to incorporate any search heuristic devised for global optimization. The main contribution of FRACTOP is that it provides an intelligent guidance (through fuzzy measures) in locating the subregion containing the global optimum solution for the search heuristics imbedded in it. By executing the search in nonoverlapping subregions, FRACTOP eliminates the repetitive visits of the search heuristics to the same local area and furthermore, it becomes amenable for parallel processing. As FRACTOP conducts the search deeper into smaller subregions, many unpromising subregions are discarded from the feasible region. Thus, the initial feasible region gains a fractal structure with many space gaps which economizes on computation time. Computational experiments with FRACTOP indicate that the metaheuristic improves significantly the results obtained by random search (RS), SA and GA.
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Hybrid Heuristics for the Multi-stage Capacitated Lot Sizing and Loading Problem
(STOCKTON PRESS, HOUNDMILLS, BASINGSTOKE RG21 6XS, HAMPSHIRE, ENGLAND, 1999-08) Linet, Özdamar
The multi-stage capacitated lot sizing and loading problem (MCLSLP) deals with the issue of determining the lot sizes of product items in serially-arranged manufacturing stages and loading them on parallel facilities in each stage to satisfy dynamic demand over a given planning horizon. It is assumed that regular time capacity decisions have already been made in the tactical level and allocated to the stages, but it is still an important decision problem whether to augment regular time capacity by overtime capacity. Each item may be processed on a technologically feasible subset of existing facilities with different process and setup times on each facility. Since the solution of the MCLSLP requires the design of a powerful algorithm, simulated annealing (SA) and genetic algorithms (GA) are integrated to enhance their individual performances. Furthermore, these global optimisation methods are incorporated into a Lagrangean relaxation scheme, hence creating a hybrid solution methodology. Numerical results obtained using these methods confirm the mutual benefits of integrating different solution techniques.
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A Note On the Use of a Fuzzy Approach in Adaptive Partitioning Algorithms for Global Optimization
(IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC, 445 HOES LANE, PISCATAWAY, NJ 08855-4141 USA, 1999-08) Demirhan, Melek; Özdamar, Linet
In global optimization, adaptive partitioning algorithms (APA) operate on the basis of partitioning the feasible region into subregions, sampling and evaluating each subregion, and selecting one or more subregions for repartitioning, The purpose of the repartitioning process is to locate a narrow neighborhood around the global optimum. In this correspondence, He propose to use a fuzzy approach in the assessment of subregions using random samples taken from these subregions. We discuss different types of uncertainties involved in APA and ne conclude that the use of a fuzzy approach in the assessment of subregions is in concurrence with APA's convergence property, We provide numerical results for the fuzzy approach on 13 test functions from the literature.
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A hierarchical planning approach for a production-distribution system
(TAYLOR & FRANCIS LTD, ONE GUNPOWDER SQUARE, LONDON EC4A 3DE, ENGLAND, 1999-11-10) Özdamar, Linet; AKTİN, AYŞE TÜLİN; 57877; 109203
A production-distribution model involving production and transportation decisions in a central factory and its warehouses is developed. The model is based on the operating system of a multi-national company producing detergents in a central factory from which products are distributed to geographically distant warehouses. The overall system costs are optimized considering factory and warehouse inventory costs and transportation costs. Constraints include production capacity, inventory balance and fleet size integrity. Here, a hierarchical approach is adopted in order to make use of medium range aggregate information, as well as to satisfy weekly fluctuating demand with an optimal fleet size. Thus, a model which involves an aggregation of products, demand, capacity, and time periods is solved. In the next planning phase, the aggregate decisions are disaggregated into refined decisions in terms of time periods, product families, inventory and distribution quantities related to warehouses. Consistency between the aggregate and disaggregation models is obtained by imposing additional constraints on the disaggregation model. Infeasibilities in the disaggregated solution are resolved through an iterative constraint relaxation scheme which is activated in response to infeasible solutions pertaining to different causes. Here, we investigate the robustness of the hierarchical model in terms of infeasibilities occurring due to the highly fluctuating nature of demand in the refined time periods and also due to the aggregation process itself.
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Some radius problem for certain families of analytic functions
(TÜBİTAK, 2000) Bolcal, Metin; POLATOĞLU, YAŞAR; 199370
The aim of this paper is to give bounds of the radius of α-convexity for certain families of analytic functions in the unit disc. The radius of α-convexity is generalization of the radius of convexity and the radius of starlikeness, and introduced by S.S.Miller; P.T.Mocanu and M.O.Reade [3,4]
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A Fuzzy Areal Assessment Approach for Potentially Contaminated Sites
(PERGAMON-ELSEVIER SCIENCE LTD, THE BOULEVARD, LANGFORD LANE, KIDLINGTON, OXFORD OX5 1GB, ENGLAND, 2000-04) Özdamar, Linet; Demirhan, Melek; Özpınar, Alper; KILANÇ, BURAK
This article describes a new fuzzy areal site assessment approach in the detection of contaminated zones within a site which is under environmental investigation. Conventional techniques utilized in this held are mostly interpolation based. These methods interpolate the contaminant concentration values at unobserved locations using observed values. Unlike these interpolation techniques, the motivation of the approach introduced here consists of partitioning a given site into smaller sections. Partitioning is carried out iteratively where subregions of interest are divided into further smaller regions. This re-partitioning scheme leads to a dynamic grouping of observations, since areas of differing sizes and boundaries contain different sets of samples. The potential of an area to contain contaminated zones is assessed by a fuzzy measure which converts the data values in that area into an aggregate grade normalized on [0, 1]. Thus, this approach does not assume concentration values at unobserved locations, rather, an areal potential is evaluated.
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Analysis of Solution Space-Dependent Performance of Simulated Annealing: The Case of the Multi-level Capacitated Lot Sizing Problem
(PERGAMON-ELSEVIER SCIENCE LTD, THE BOULEVARD, LANGFORD LANE, KIDLINGTON, OXFORD OX5 1GB, ENGLAND, 2000-08) Barbarasoğlu, G; Özdamar, Linet
This study describes an analysis of different neighbourhood transition schemes and their effects; on the performance of a general purpose simulated annealing (SA) procedure in solving the dynamic multi-level capacitated lot sizing problem (MLCLSP) with general product structures. The proposed neighbourhood transition schemes are based on relaxing different types of constraints, each of which defines a different solution space. The experiments assess the influence of expanding the search space which includes infeasibilities arising from the elimination of different model restrictions. The results indicate that the performance of SA is highly dependent on the definition and the tightness of the search space. Furthermore, the increase in the number of search moves carried out by SA is shown to significantly improve the results with linearly increasing computational times.
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Experiments with New Stochastic Global Optimization Search Techniques
(PERGAMON-ELSEVIER SCIENCE LTD., 2000-08) ÖZDAMAR, LİNET; Demirhan, Melek
In this paper several probabilistic search techniques are developed for global optimization under three heuristic classifications: simulated annealing, clustering methods and adaptive partitioning algorithms. The algorithms proposed here combine different methods found in the literature and they are compared with well-established approaches in the corresponding areas. Computational results are obtained on 77 small to moderate size (up to 10 variables) nonlinear test functions with simple bounds and Is large size test functions (up to 400 variables) collected from literature.
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A Decision Support Model for Customer Value Assessment and Supply Quota Allocation
(TAYLOR & FRANCIS LTD, 11 NEW FETTER LANE, LONDON EC4P 4EE, ENGLAND, 2000-09) Barbarasoğlu, G; Yazgaç, Ayşe Tülin; AKTİN, AYŞE TÜLİN
The aim of this study is to develop a decision support tool for a supplier in a value-chain environment. The supplier under consideration is assumed to provide a strategic product to a number of customers and needs to allocate his capacity among them in a way to maximize his business value. First, an analytic hierarchy process (AHP) structure is designed to represent the criteria which are identified from the supplier's point of view to assess customer performance, and customer priorities are obtained by using the AHP composition principle. Then, these are deployed in numerical algorithms which aim to allocate the total supply capacity among customers as supply quotas. The approach is implemented in an electric motor manufacturer in Turkey, which possesses high competitive power with advanced manufacturing technology.
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Comparison of Partition Evaluation Measures in an Adaptive Partitioning Algorithm for Global Optimization
(ELSEVIER SCIENCE BV, PO BOX 211, 1000 AE AMSTERDAM, NETHERLANDS, 2001-01-01) Özdamar, Linet; Demirhan, M
An adaptive partitioning algorithm with random search is proposed to locate the global optimum of multimodal functions. Partitioning algorithms divide the feasible region into nonoverlapping partitions in order to restrict and direct the search to the most promising region expected to contain the global optimum. In such a scheme a partition evaluation measure is required to assess sub-regions in order to re-partition the most promising sub-region and intensify the search within that area. This study provides computational results on several classes of partition evaluation measures used in the assessment of samples taken from all partitions. Among the partition evaluation classes used in our comparison are fuzzy, statistical, and deterministic interval estimation measures. Performance in terms of solution quality is reported on an extensive set of 77 test functions collected from the literature.
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θ-Euclidean L-fuzzy Ideals of Rings
(TÜBİTAK, 2002) Balkanay, Erol; KOÇ, AYTEN; 112205; 10199
The concept of fuzzy ideals is extended by introducing θ-Euclidean L-fuzzy ideals in rings. In particular, some structural theorems for a θ-Euclidean L-fuzzy ideal of R are proved.
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Placing Democratization within the Global Frame: Sociological Approaches to Universalism, and Democratic Contestation in Contemporary Turkey
(BLACKWELL PUBL LTD, 108 COWLEY RD, OXFORD OX4 1JF, OXON, ENGLAND, 2002-05) Rumford, C
The process of democratization in Turkey is enhanced by both proximity to the European Union's enlargement process and universalistic discourses of personhood rights. and. at the same time, compromised by a nationalistic rejection of global human rights and democratic norms and state-led resistance to political pluralism. One key feature of the democratization process is the way in which contending parties - the Kemalist elite, religious and ethnic minorities, the European Union attempt to legitimise their claims by appeals to universal principles. The paper examines three sociological/social theory approaches to universalism (Beck, Laclau. Robertson) and demonstrates their usefulness for an understanding of political contestation in contemporary Turkey. It is argued that the work of these theorists allows us to move beyond a simplistic polarisation of the universal and the particular, where the state represents the universal and minorities the particular. The conceptualisation of universalism advanced by Beck. Laclau and Robertson points to the need to understand the processes of democratization within a sociologically informed globalization framework.
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The Influence of Tl4Bi2S5 Precipitates on the Crystalline TlBiS2 Properties
(WILEY-V C H VERLAG GMBH, PO BOX 10 11 61, D-69451 WEINHEIM, GERMANY, 2002-09-16) Özer, Mehmet; Kalkan, N; Kyritsi, KG; Paraskevopoulos, KM; Anagnostopoulos, AN; Stergioudis, G; Polychroniadis, EK
Crystalline TlBiS2 having homogeneously distributed Tl4Bi2S5 precipitates was grown by the vertical Bridgman technique in a two-step procedure. X-rays and Transmission Electron Microscopy (TEM) studies were used to identify the as-grown material whereas a Scanning Electron Microscopy (SEM) examination revealed its layered structure. The electrical conductivity a was measured both along (sigma(parallel to)) and across (sigma(perpendicular to)) its layers. IR reflectivity measurements were also performed and the plasma minimum was determined. Both electrical and optical characterization show properties similar to those of TlBiS2 but a reduced N/m* ratio and an irregular voltage oscillation in the I-U characteristics were deduced. Combining the results from these investigations a possible explanation was proposed.
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Discussions On Suitable Modulation Schemes for a Bi-directional Hot Electron Light Emitter and Absorber
(IEE-INST ELEC ENG, MICHAEL FARADAY HOUSE SIX HILLS WAY STEVENAGE, HERTFORD SG1 2AY, ENGLAND, 2002-10) Wah, JY; Yenidünya, R; Boland-Thoms, A; Demirer, R. Murat; Balkan, N
The hot electron light emitting and lasing semiconductor heterojunction (HELLISH) device is a novel emitter that utilises hot electron/hole transport parallel to the layers of an AlxGa1-xAs p-n junction containing GaAs quantum well(s) in the depletion region. The structure, nicknamed top-hat HELLISH, presented provides a new functionality of the device where n- and p-layers of different lengths are contacted separately but are biased with the same voltage longitudinally. In this configuration carrier injection into the active region is further enhanced in the vicinity of the cathode due to an effective forward biasing as well as the longitudinal biasing of the junction. Therefore, the emission intensity is increased compared with the conventional HELLISH device. Near the anode, however, there is an effective reverse biasing of the junction and in this region the top-hat device acts as an absorber. As a result of these two features, the device can be operated as a wavelength converter or amplifier. The intensity of the emitted light is independent of the polarity of the applied voltage. However, the positions of the absorber and emitter depend on the polarity. Therefore, the device can be operated as a bidirectional wavelength converter and amplifier. The possible modulation scheme and application circuit for this device are discussed.
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Energy dispersive X-ray fluorescence and neutron activation analyses of sediments from the Turkish coast of the west Black Sea
(PARLAR SCIENTIFIC PUBLICATIONS (P S P), ANGERSTR. 12, 85354 FREISING, GERMANY, 2003) Mukhamedshina, N. M.; Mirsagatova, A. A.; AKYÜZ, SEVİM; AKYÜZ, ZEKİ TANIL
Some surface sediment samples, collected from 10 sampling sites of the southwest part of the Black Sea in 1998, were analysed quantitatively by radioisotope energy dispersive X-ray fluorescence (EDXRF) and instrumental neutron activation analysis (INAA) methods. The concentration of U, Th, Sc and some other rare earth elements (REEs) and important heavy metals in the sediment samples are as follows: U (1.5-5.7 mg/kg), Th (1.9-11 mg/kg), Sc (1.7-13 mg/kg), La (7.8-30 mg/kg), Ce (12-68 mg/kg), Nd (9-20 mg/kg), Sm (1.5-5.3 mg/kg), Th (0.18-0.82 mg/kg), Lu (0.06-0.44 mg/kg), Cd (0.10-0.26 mg/kg), Sb (0.036-2.30 mg/kg), Hg (0.02-0.08 mg/kg), and Pb (2-35 mg/kg). The results investigated indicate the presence of correlations between the concentrations of some elements of the sediment samples. The results are also compared to those of other marine sediments.
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FT-IR spectroscopic investigation of adsorption of pyrimidine on sepiolite and montmorillonite from Anatolia
(Springer, Van Godewijckstraat 30, 3311 Gz Dordrecht, Netherlands, 2003-06) AKYÜZ, SEVİM; AKYÜZ, ZEKİ TANIL; 10127
The adsorption of pyrimidine ( PM) on natural montmorillonite and sepiolite from Turkey was investigated by FT-IR spectroscopy. The intercalation of PM within montmorillonite has been shown by X-ray diffraction to increase the interlayer spacing. The spectroscopic results indicate that PM molecules adsorbed on sepiolite are coordinated to Lewis acidic centers or surface hydroxyls by H-bonding interaction through one of the pyrimidine ring nitrogen lone pairs. Moreover, some of the adsorbed PM molecules may enter the interior channels of the sepiolite structure and replace zeolitic water. The intercalated PM molecules within montmorillonite are coordinated to exchangeable cations directly or indirectly through water bridges.

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