itüdergisi (E-Journals - İstanbul Teknik Üniversitesi, Istanbul Teknik Uniersity., İTÜ)
itüdergisi (E-Journals - İstanbul Teknik Üniversitesi, Istanbul Teknik Uniersity., İTÜ)Not a member yet
1059 research outputs found
Sort by
Sayısal bir yöntem ile podlu pervane analizi
Get PDFIn this study, the flow around the pod unit (propeller+axisymmetric pod+taşıyıcı eleman) are analysed and the performance characteristics of the propeller on the pod are investigated. The main objective of the present work is to establish a numerical method for the prediction of performance of podded propeller. The flow domain around the podded propeller is mainly divided into three parts; i) the axisymmetric pod part, ii) the strut part and iii) the propeller part. While the pod and strut parts are modelled by a low-order boundary element method (BEM), the propeller is represented by a vortex lattice method (VLM). A vortex lattice (lifting surface) method is developed and used to calculate the propulsive performance and induced velocities due to propeller blades. This model is based on appropriate vortex and source-sink distribution. The singularities are distributed on the mean lines of the propeller blade sections. Those vortices are divided into two parts; bound and trailing vortices. The bound vortices, located in a radial direction, are to simulate the load distribution on the propeller blade. The trailing vortices are placed in the direction of the flow, obtained from the different intensities of adjacent bound vortex elements. A number of source elements are taken at adjacent bound vortex to simulate the thickness of the blade. The vortex strengths are calculated by solving a set of simultaneous equations which satisfy the flow tangency condition at the blade control points. Induced velocities due to vortex elements of the lifting surface are calculated using Biot-Savart Law. Once the bound vortex elements intensity is solved, then the velocity induced by the propeller in any point in space can be computed using five angular position of the propeller blade. Finally, the arithmetic average of these five values becomes induced velocity at the corresponding point. During the calculation of induced velocities, nine (9) span-wise and twelve (12) chord-wise elements are taken into account while high number of elementary vortex lines (450) are located on the blade surface to discrete the blade. The BEM, which is based on Green’s Third Identity, allows the separation of axisymmetric pod and the strut problems. Those two (axisymmetric pod and strut) sub-problems are solved separately, with the effects of one on the other being accounted for in an iterative manner. The integral equations (both for axisymmetric pod and strut) which are obtained from Green’s Third Identity are discretized using quadrilateral panels. In order to achieve this, the axisymmetric pod surface and the strut surface are modelled with constant strength dipole and constant strength source panels. The discretized integral equations that form the linear algebraic equation systems can be solved for unknown potential values. Coupling of the BEM and the VLM is, on the other hand, carried out in an iterative manner to incorporate the effect of the pod unit on the propeller, and vice versa. Initially, the VLM is applied for the propeller in the absence of pod and strut and calculates the perturbation velocities on the control points of pod unit (including strut). Then, the BEM is applied to the axisymmetric pod problem, including the effect of propeller and later, the BEM is used for the strut problem with the effects of propeller and axisymmetric pod. The induced velocities due to pod unit (pod + strut) at the propeller disk plane are calculated by corresponding BEMs and the VLM is re-applied to the propeller problem with the modifed right hand side. All those sub-problems (axisymmetric pod + strut + propeller) interact with eachother in terms of induced velocities and induced potential values and the rigth hand sides of BEMs and VLM are modified according to those induced velocities and induced potential values. The iterative steps between BEMs and VLM are repeated until the results are converged. This iterative numerical method is applied to two different podded propellers to compare the results with those of experimental measurements and other numerical methods. It was found that the results by the present numerical method were in good agreement with those of experimental measurements and other numerical methods. The effect of pod unit on the propeller and vice versa are discussed for different type of podded propulsor with zero yaw angle and with a yaw angle. Some preliminary results on the effect of yaw angle on pressure distribution on pod and strut are shown in the paper as well. Keywords: Podded propulsors, vortex-lattice technique, iterative boundary element method.Bu çalışmada, podlu pervane birimlerinin (pervane + eksenel simetrik "pod" + "taşıyıcı eleman") etrafındaki akımın sayısal analizi yapılmış ve podlu pervane performans karakteristikleri incelenmiştir. Temel amaç, podlu pervanelerin performans analizini yapan sayısal bir yöntem geliştirmektir. Ayrıca, pod ve taşıyıcı elemanların, hem beraber hem de ayrı ayrı pervane üzerindeki etkilerinin ve podlu pervane birimi üzerine gelen kuvvetlerin ve torkların sayısal olarak hesaplanması amaçlanmıştır. Bunun için, podlu pervane birimi etrafındaki akım üç bölgeye ayrılmıştır: 1) Eksenel simetrik pod bölgesi, 2) Taşıyıcı Eleman bölgesi ve 3) Pervane kanatları bölgesi. Pod ve taşıyıcı eleman bir panel yöntemiyle (BEM, Boundary Element Method) modellenirken pervane kanatları etrafındaki akım alanı ve kanatlara gelen kuvvetler bir girdap-ağ yöntemiyle (Vortex-Lattice Method) hesaplanmıştır. Panel yöntemi uygulanırken pod ve taşıyıcı eleman dörtgen (ve/veya üçgen) panellere bölünerek herbir panel üzerinde sabit şiddetli kaynak ve duble (dipol) dağılımı olduğu düşünülmüştür. Pervane kanatları girdap-ağ tekniği ile modellenirken de kanatlar üzerinde hem kaynak hem de girdap dağılımı yapılmıştır. Pod ve taşıyıcı elemanların pervane kanatları üzerindeki ya da pervane kanatlarının pod ve taşıyıcı eleman üzerindeki etkileri iteratif bir şekilde, bu iki yöntem kullanılarak gerçekleştirilmiştir. Bu önerilen iteratif sayısal yöntemin doğruluk ve duyarlılığını test edebilmek için, literatürde verilen diğer sayısal ve deneysel çalışmalarla yöntem karşılaştırılmalıdır. Elde edilen sonuçlar, literatürde verilen diğer sayısal ve deneysel sonuçlarla karşılaştırılmıştır. Yine, pod açısının sonuçlar üzerindeki etkileri de tartışılmıştır. Anahtar Kelimeler: Podlu pervane, girdap-ağ tekniği, iteratif sınır elemanları yöntemi. 
Radyo Frekanslı Tanıma (RFID) teknolojisinin tedarik zinciri üzerindeki etkileri
Get PDFRadio frequency identification (RFID) technology introduces the opportunity for increased visibility by facilitating easy tracking and identifying of goods, assets and even living things. However, high investment cost and inadequate technical capability remain as challenges for RFID system implementations. That being the case, fair evaluation of savings associated with increasing performance and investment costs has a great role in the success of RFID projects. RFID is an Automatic Identification (Auto-ID) system consisting of a microchip with a coiled antenna (tag) and a reader. Data and energy are transmitted without any contact between the tag and the reader. The reader sends out electromagnetic waves that form a magnetic field so the microchip's circuits are powered. The chip modulates the waves and sends back to the reader. The reader converts the new waves into digital data. Radio Frequency Identification (RFID) is regarded as the future technology for the optimization of supply chain processes. It can improve manufacturing and retail operations, from forecasting demand to planning, managing inventory and distribution. Real time inventory and logistics information is shared at any stage of the supply chain. The supply chain processes gain many benefits from RFID technology. These are better managing and tracking of the supply chain, less time and lower cost for inventory management, lower labor requirement, less inventory shrinkage, improved customer service and better modeling of the customer behavior. RFID provides efficiency, accuracy, visibility and security on the supply chain. Real time inventory and logistics information is shared at any stage of the supply chain by the supplier, manufacturer, distributor and retailer. In this study using a simulation model, the cost differences between an RFID applied and non-applied system are examined on a supply chain consisting of a retailer, distributor and producer. Considering misplacement, shipment error, theft and damage ratios, physical and system inventory levels are determined. The expected benefits obtained through the performance increase of efficiency, accuracy, visibility and security levels are calculated on RFID integrated supply chain considering the lost sales, theft, inventory, order and labor cost factors. The influences of the product value, lead time and demand uncertainty factors are examined on an RFID integrated supply chain by using experimental design. According to the research results, the total supply chain cost savings are increased through the raising product prices. This increase is predominantly based on the theft and lost sales cost savings due to the increase of the unit cost; however the savings at the retailer are higher than the savings at the distributor and producer. Despite the decrease of the inventory level on the supply chain, the total inventory cost savings are increased due to the raising product prices. The gap between the system and physical inventory level is decreased, thus the loss of the order costs are reduced. The total retailer cost savings are increased through the increased lead time due to the decrease of the lost sales cost savings. Raising the lead time also increases the total theft cost savings. Through the increase of the inventory level on the supply chain, the total inventory and process cost savings are increased. The loss of the order costs are reduced by raising lead times due to the ascending inventory level. The total supply chain cost savings are decreased through the raising demand uncertainty due to the increased lost sales. The decrease of the cost savings are noticed at each level of the supply chain. The increase of demand uncertainty causes the reduction of the inventory cost savings. The influence of the product value on the total supply chain cost savings is higher than the influence of the demand uncertainty. The influence of the demand uncertainty on the total supply chain cost savings is higher than the influence of the lead time. However, the critical and sensitive dependent cost factor is the lost sales cost savings on the whole supply chain. The total cost savings at retailer are higher than cost savings at distributor and producer. In the end of the study, an RFID investment evaluation model is proposed which is considering the sharing of the total cost of the tags according to the expected benefits of each member of the supply chain. Keywords: Radio Frequency Identification (RFID), supply chain, simulation model, investment evaluation.Radyo Frekanslı Tanıma (RFID) teknolojisi, etrafında anten sarılı olan bir mikroçip (etiket) ve bir okuyucudan oluşan otomatik tanıma sistemidir (Auto-ID). Veri ve enerji transferi, etiket ve okuyucu arasında herhangi bir temas olmadan sağlanmaktadır. Okuyucunun yaydığı elektromanyetik dalgalar antenle buluşmakta ve etiket içindeki devreleri harekete geçirmektedir. Etiket dalgaları modüle ederek okuyucuya geri göndermekte ve okuyucu da yeni dalgayı dijital veri haline dönüştürmektedir. RFID teknolojisi, tedarik zincirinde süreç verimliliğini, doğruluk, güvenlik ve görünürlük seviyelerini artırmaktadır. Tedarik zincirinin her aşamasında gerçek zamanlı stok ve lojistik bilgisi, üretici, dağıtıcı ve perakendeci tarafından paylaşılabilmektedir. Yatırım değerinin yüksekliği ve süreçlerin yeniden yapılandırılması, RFID yatırımlarının, firmalar açısından stratejik bir karar olarak değerlendirilmesi gerektiğini göstermektedir. RFID uygulama başarısının sağlanması için öncelikle kapsamlı bir değerlendirme analizinin yapılması, teknoloji yatırımının oluşturduğu maliyetlerin ve sağladığı kazanımların en detaylı şekilde incelenmesi gerekmektedir. Tedarik zinciri üzerinde gerçekleşecek RFID yatırımı için, zincir üyeleri arasındaki etkileşimin de değerlendirme aşamasında göz önüne alınması gerekmektedir. Bu çalışmada perakendeci, dağıtıcı ve üreticiden oluşan tedarik zincirinde gerçekleşecek RFID yatırımının etkilediği maliyet faktörleri belirlenmekte, maliyet değişiminden kaynaklanan kazanımlar benzetim modeli yardımı ile ortaya koyulmaktadır. Ürün değeri, temin süresi ve talep belirsizliği bağımsız değişkenlerinin maliyet faktörleri üzerindeki etkisi araştırılmaktadır. Nihai olarak tedarik zinciri kazanım oranına dayalı etiket maliyet paylaşımını esas alan RFID yatırım değerlendirme modeli önerilmektedir. Anahtar Kelimeler: RFID, tedarik zinciri, maliyet, benzetim modeli, yatırım değerlendirme
Müşteri memnuniyet indeks modelinde yapay sinir ağları kullanımı
Get PDFThe concept of customer satisfaction has attracted much attention in recent years. A key motivation for the growing emphasis on customer satisfaction is that higher satisfaction can lead to a stronger competitive position resulting in higher market share and profit, reduce price elasticity, lower business cost, reduce failure cost, and reduce the cost of attracting new customers. The purpose of customer satisfaction index (CSI) models is to measure the quality of the goods and services as experienced by the customers that consume them. The independent and uniform measurement characteristics of the CSI model provide a useful tool for tracking performance and systematic benchmarking over time. A major advantage of the measurement model is the use of generic questions, which are sufficiently flexible to be used across a wide variety of products and services. In this study, a new customer satisfaction index model is developed considering the previous CSI models such as American Customer Satisfaction Index and European Customer Satisfaction Index. The proposed model was applied for Turkish mobile phone sector since the competition in this industry results a dynamic product development and an increasing demand for that products. CSI models are designed as a structural equation model (SEM) which consists of well established theories and approaches in customer behaviour. The constructs of the CSI models are latent variables indirectly described by a block of measurement variables. The structural model of the proposed CSI consists of 6 latent variables with their 23 observable variables. The latent variables of the model are company image, customer expectations, perceived quality, perceived value, customer satisfaction and customer loyalty. A survey instrument, developed to measure the manifest variables, was conducted to 700 mobile phone users. Besides the model questions, some demographic questions (e.g. age, gender, education level etc.) are also included in the survey. The structural model of the present model is analyzed using variance based Partial Least Squares (PLS) method. The main concern of the PLS is related to the explanatory power of the path model along with the significance level of standardized regression weights. An iterative scheme of simple and/or multiple regressions contingent on the particular model is performed until a solution converges on a set of weights. The general applicability of a SEM model depends on the reliability and validity of the modelling results. Reliability and validity of the proposed CSI model were assessed by checking unidimensionalty of the blocks, individual item reliability, convergent validity and discriminant validity. All test results satisfy the crucial requirements for validity and reliability of structural model. In this study, a feed forward neural network model is proposed as an alternative to simple or multiple regression methods for the inner model estimation of the CSI. The use of artificial neural networks (NN) gained popularity in different fields, and some studies have demonstrated the superiority of NN over multiple regression. NN simulates human cognition by modelling the inherent parallelism of neural circuits in the brain using mathematical models of how the circuits function. However, the NN approach has been applied more recently to customer satisfaction and loyalty analysis. In this study there are 5 different inner models estimating 5 different endogenous latent variables of the CSI model. Each inner model was estimated using three-layer feed forward neural networks. A sigmoid function is used in the hidden layer, and a linear function in the output layer. The data was divided into two sets, 75 percent for training, and 25 percent for testing. Training is performed using the Levenberg Marquardt back propagation algorithm, and the weights are initialized using Nguyen Widrow algorithm. After the NN is trained it was evaluated for the test data. Root mean squared error and R square scores were used as performance criteria. The use of NN provides a powerful estimation for the inner models used in the CSI. The results of the CSI model as a whole can be a valuable guide for the managers in formulating competitive marketing strategies. Considering the results of the CSI model, the limited resources of the firms can be allocated for critical factors which have important impacts on satisfaction. In conclusion, the CSI model provides important information for the purchase decisions of the customers and lead to improvements in the quality of goods and services they consume. Keywords: Customer satisfaction index, partial least squares, neural networks. Müşteri memnuniyet indeks modelleri son yıllarda birçok ülkede yaygın olarak uygulanmaktadır. Müşteri memnuniyet indekslerinin en büyük özelliği yapısındaki ölçüm faktörlerinin ürün ve hizmet sektöründe rahatlıkla kullanılabilmesine imkan sağlamasıdır. Bu sayede güvenilir bir ölçekle firmalar arasında, sektörler arasında ve ülke çapında karşılaştırmalı bir memnuniyet ölçümü mümkün olmaktadır. Müşteri memnuniyet indeks modelleri, müşteri memnuniyeti ile ilişkili gizli (latent) değişkenler ve bu gizli değişkenleri ölçen ölçüm değişkenleri arasındaki ilişkilerden oluşan yapısal eşitlik modelleridir. Bu çalışmada diğer ülkelerde kullanılan müşteri memnuniyet indeks modellerinden yola çıkarak ülkemiz şartlarında kullanılabilecek bir müşteri memnuniyet indeks modeli geliştirilip test edilmiştir. Model, 6 gizli değişken ve bunlara bağlı toplam 23 ölçüm değişkeninden oluşmaktadır. Modele uygun olarak hazırlanan anket formu kullanılarak, cep telefonu sektöründe 700 kullanıcı ile yüz-yüze anket uygulaması yapılmıştır. Müşteri memnuniyet indeks modelindeki gizli değişkenler ve ölçüm değişkenleri arasındaki ilişkiler kısmi en küçük kareler yöntemiyle tahmin edilmiştir. Müşteri memnuniyet indeks modelinin güvenilirlik ve geçerlilik test sonuçları modelin genel uygulanabilirliğini göstermektedir. Çalışmada ayrıca, gizli değişkenler arası ilişkilerin oluşturduğu iç modellerin tahmininde Yapay Sinir Ağları (YSA) metodu önerilmiştir. Yapısal modeller için tasarlanan YSA modeli 3 katmanlı ileri beslemeli ve geri yayılımlı bir modeldir. Gizli katmanda sigmoid transfer fonksiyonu, çıkış katmanında ise doğrusal transfer fonksiyonu kullanılmıştır. Bu şekilde 5 farklı iç modelin tahmini için yapay sinir ağları metodunun kullanımı modellerin açıklayıcılık gücünü artırmıştır. Anahtar Kelimeler: Müşteri memnuniyet indeksi, kısmi en küçük kareler, yapay sinir ağları. 
Dynamical modelling of kinematically deficient cooperating manipulators
Get PDFBir manipülatörün kinematik yeterliliği, o manipülatörün uç noktasının erişebildiği toplam küme olan çalışma uzayında keyfi seçilen her yönde öteleme ve yine keyfi seçilen her yönde dönme hareketi yapabilmesidir. Diğer bir değişle kinematik yetersiz manipulatörler, çalışma uzaylarında geçerli bütün konfigürasyonlara ulaşmak için gereken serbestlik derecelerinden (SD) daha azına sahip olan manipülatörlerdir. Üç boyutlu çalışma uzayı için bu durum, bir manipülatörün serbestlik derecesinin altıdan daha küçük olmasına karşı düşer. Çünkü üç boyutlu çalışma uzayına sahip kinematik yeterliliği olan bir manipülatörün uç noktası, üç boyutta dönme ve üç boyutta öteleme olmak üzere toplam altı boyutlu bir manifold tanımlar. Birlikte çalışan manipülatörlerden oluşan bir sistem üzerindeki kuvvet ve moment dağılımlarını hesaplayabilmek için sistemin Jakobiyen matrisinin sütunlarının bütün kombinasyonları bu manifoldu tamamıyla tarayabilmelidir. Bundan dolayı literatürde genellikle manipülatörlerin kinematik yeterliliği ve tekil durumda olmamaları bu problemin çözümüne ön koşul olarak getirilmektedir. Birlikte çalışan manipülatörlerin dinamik analizinde kinematik yeterlilik ön koşulunun kaldırılması amacıyla bu çalışmada, manipülatörlerin taşıdığı yükü bir mobil platform olarak modelleyerek sisteme altı serbestlik derecesi kazandırmak ve bunun yeterli olmadığı durumlarda sisteme “sözde eklem” eklemek olarak özetlenebilecek bir yöntem tanıtılmaktadır. Bu yöntemin holonomik olmayan sistemlere uygulaması da araç dinamiğinde tekerlek ve yol arasındaki kuvvetlerin hesaplanabilmesi anlamına geldiği için otomotiv endüstrisinde ayrıca önem taşır. Anahtar kelimeler: Birlikte çalışan manipülatörler, kinematik yetersiz manipülatör, ortak yük yerine mobil platform, sözde eklem. Precision and load capacity are among the reasons why more than one manipulator may be used to perform a common task. In such cooperation, propagating force and torque within and among manipulators cannot be computed unless the columns of the System-Jacobian matrix span the operational space completely. This can be made sure as long as the end-effector of each manipulator in the system independently move and rotate at any direction in the operational space. In order to have a three-dimensional task space for rotation and translation, a manipulator needs to have at least six Degrees of Freedom (DOF). Then the Jacobian of such manipulator is required to have columns that span a six-dimensional manifold provided that it is not at a singular configuration. Many industrial applications do not require the full kinematic capability to move and rotate the tip point of the manipulator at any direction. In regards to cost, manufacturing, and compactness, any DOF not necessary for the task should be avoided unless redundancy is needed for both operational space and joint space controls such as obstacle or joint limit avoidance problems. With this motivation, kinematic deficiency is defined as the lack of ability of the end-effector to independently move towards and rotate around any vector in the operational space. Therefore, kinematically deficient manipulators are those that have fewer degrees of freedom than necessary to achieve any admissible configuration in their operational space. This paper addresses the challenges associated with the computation of constrained forces at a cooperating manipulator system in the presence of kinematic deficiency. One may suggest eliminating the deficiency by reducing the size of the task space by removing the directions towards which the end-effector of the manipulator cannot move from the Jacobian. Considering that the range space of the Jacobian represents the space in which the end-effector of the manipulator is free to move. The space in which the tip of the manipulator cannot move is the one perpendicular to the range space. Therefore, it is the null space of the transpose of the Jacobian. The drawback of this method is that the computation of null space requires Singular Value Decomposition (SVD), which introduces instability due to the fact that singular vectors are not unique, and may introduce discontinuity. This drawback alone makes this methodology impractical, not to mention the cost associated with the numerical computation of SVD. These are the disadvantages of the numerical methods in general addressing this issue. Consequently, there is a need for an analytical approach on which our work particularly concentrates. We suggest two methods; "common load as mobile platform" and the "pseudo joint." Common-load-as-mobile-platform method is essentially a modeling strategy. The basic idea behind it is to model the load carried by the manipulators as if it were the mobile platform carrying the manipulators. Hence, we need to re-label the body frames to reflect the new assignments of the end-effector and the base. The reason of this is to gain 6 DOF from the platform. In many cases, this additional 6 DOF together with appropriate constraint equations yield the constraint forces. For the other cases we propose the pseudo-joint method, which may be used in conjunction with the common-load-as-mobile-platform method. The pseudo-joint method is used at a kinematically deficient system where additional 6 DOF introduced by the "common load as mobile platform" method is not sufficient. In this case, we first augment the system with additional joints called pseudo-joints and then remove them from the system by computed constraints, which actually correspond to stress along the pseudo-joints. Therefore, for the price of stress computation, we obtain full propagation of forces and torques including the constrained ones throughout the system. These techniques can be applied to nonholonomic systems where the constraints are not integrable. We demonstrate the use of the algorithm on wheeled systems and show the constrained forces. In many applications it is critical to know the tire-road interaction forces, which determine how a vehicle turns, brakes and accelerates. Future work of this study includes multitrailer vehicle dynamics and control. Keywords: Cooperating manipulators, kinematically deficient manipulators, common-load-as-mobile-platform, pseudo-joint.
Düzensiz talep koşullarında melez üretim sistemlerinin aksiyomlarla tasarımı
Get PDFToday, product demands have become erratic due to increase in product variety, and competitiveness among companies. Manufacturing systems must be redesigned in order to adapt to these changes, and respond quickly to erratic demand. Lead time of the manufacturing system should be decreased in order to achieve these goals. Therefore, one-piece flow is employed during design, at this study. Moreover, as the companies produce wide range of products, these products should be in different stages of their life cycle. For instance, products that are at their birth phase must be produced in functional layout. Sample parts or prototypes are examples to these part types. However, parts with relatively stable design and high demand must be produced in cells. This results in coexistence of both cells and functional layout at the same factory. These systems are called hybrid cellular manufacturing systems (HMS). These systems are met frequently in real world. However, solution approaches that focus on erratic demand conditions and HMS design are very limited. For these reasons, a road map for the design of HMS in erratic demand conditions using Axiomatic Design (AD) was proposed at this study. Exceptional operations causing inter-cell traffic should be eliminated, so as to implement one-piece flow. For this purpose, a one-piece flow algorithm where alternative machines are utilized is developed at this study. As technology improved, machines have become able to carry out operations of other machines, and this caused them become alternative for each other. At our one-piece flow algorithm, alternative machines are evaluated, and exceptional operations are tried to be eliminated by reassigning them to alternatives systematically. This algorithm is one of the unique components of our study, because it utilizes alternative machines both in cells and in functional layout in order to eliminate exceptional operations. After the elimination of exceptions, unidirectional flow must be established in each cell. This enhances one-piece flow in cells, and decreases production lead time of the parts. Simulation technique is used so as to check sufficiency of system capacity, and bottlenecks are identified on the basis of simulation results. Bottleneck identification is also a rule-based approach using AD principles. Afterwards, bottlenecks must be eliminated. For this purpose, a systematic procedure is developed and pursued. In order to eliminate bottlenecks, some sequential solutions are proposed using AD. Solutions must be implemented starting from the simplest to the hardest. The hardest solution is purchasing machines. This approach was applied on a small hypothetical example, to reflect most of the possible situations. First, part-machines matrix of the problem was clustered using Average Linkage Method, and some exceptional operations were found. The improved machine-part matrix was established, including alternative machines for each operation. Exceptional operations were tried to be eliminated by reassigning them to alternatives based on the rules. By pursuing these rules, all exceptional operations were eliminated in the problem. Afterwards, unidirectional flow in each cell was achieved by using the heuristic of Aneke and Carrie (1986). The resulting system consists of cells with one-piece flow and functional layout with batch flow, without exceptions. The sufficiency of the resources was checked by using the simulation technique. Bottlenecks were identified by evaluating the lead times and queue lengths. Addition of new machines were also tried to remove the bottlenecks. Moreover, the systems before and after the implementation of the proposed algorithm were modeled, and were compared. It is clear that the proposed algorithm decreased the production lead time of the parts produced. Finally, the model explained above was applied at the factory of a Turkish automotive-parts manufacturer. The system was designed as a HMS where partial one-piece flow is achieved through the proposed model and the algorithm. The lead time performance of the proposed design where one-piece flow is applied within the cells was compared with that of the former design with batch flow, using simulation models. Based on simulation results, the proposed design had superior performance. Moreover, existence of bottleneck machines was analyzed using the simulation model. This analysis showed that machines were not bottleneck resources. Keywords: One-piece flow, hybrid cellular manufacturing system design, erratic demand.Çalışmamızda kısmî tek parça akışının uygulandığı melez üretim sistemlerinin tasarımı için Aksiyomlarla Tasarım (AD) yöntemini kullanan bir yol haritası önerilmiştir. Tek parça akışının uygulanabilmesi için parçaların hücreler arası hareketlerine neden olan istisnai operasyonlar yok edilmelidir. Çalışmamızda alternatif makineler incelenerek, geliştirilen kurallar çerçevesinde istisnai operasyonlar sistematik biçimde alternatiflere atanmaya çalışılır. Tek parça akışına yönelik tasarım algoritması çalışmanın özgün yönlerinden biridir. Çünkü istisnai operasyonları yok etmek için alternatif makineleri kullanan, fonksiyonel alandan da yararlanmaya olanak tanıyan sistematik bir yaklaşım önerilmiştir. İstisnai operasyonların yok edilmesinin ardından hücre içerisinde tek yönlü akışın sağlanması hücrelerde tek parça akışını mümkün hale getirir. Bu aşamadan sonra benzetim tekniğinden yararlanarak darboğaz kaynakları belirleme ve yok etme prosedürü uygulanır. Bu prosedür de AD'yi temel alan sistematik bir yaklaşımdır. Darboğazlar belirlenirken hücredeki hangi iş merkezinin veya iş merkezlerinin darboğaz olduğu kurallar yardımıyla belirlenir. Daha sonra iş merkezinde hangi kaynağın buna neden olduğu sistematik biçimde araştırılır. Belirlenen darboğazların yok edilmesi için de AD kullanılarak bir dizi çözüm önerilmiştir. Son olarak sistemin yerleşim planı, melez yerleşime uygun biçimde hazırlanır. Önerilen yol haritası düşünsel bir örnek üzerinde uygulanmıştır. Ayrıca istisnai operasyonları içeren sistem ile TPA algoritması uygulandıktan sonra elde edilen sistem Arena 10.0 benzetim yazılımı ile ayrı ayrı modellenmiştir. Geliştirilen algoritmanın, düşünsel örnekteki sistemin parça imalat temin süresini çarpıcı biçimde azalttığı gösterilmiştir. Anahtar Kelimeler: Tek parça akışı, melez üretim sistemi tasarımı, düzensiz talep
Affective components of organizational commitment: A comprehensive structural equations model in a public sector organization
Get PDFÖrgütsel bağlılık, yazında bugüne dek çok araştırılmasına rağmen hem bağlam duyarlılığı hem de belirleyicileri ve sonuçları açısından karmaşıklığı nedeniyle hala ilgi toplayan bir araştırma başlığıdır. Çalışmamızın amacı, örgütsel bağlılığın duygusal bileşenlerinin belirleyicilerinden sayılan “iş tatmini” ve “genel yaşam tatmini” gibi faktörler ve sonucu sayılabilecek “örgütsel performans algısı” ve “ayrılma niyeti” gibi diğer faktörlerle eş anlı etkileşimini bir model çerçevesinde sınamaktır. Öncelikle yazın incelemesinden duygu tabanlı örgütsel bağlılığın belirleyicileri ve sonuçları sentezlenerek bir kuramsal model geliştirilmiş, geliştirilen bu modeldeki nedensellik ilişkileri önsavlara dönüştürülmüştür. Bu kuramsal modelin tamamını ve faktörler arası etkileşimleri eş anlı olarak test edebilmek için yapısal denklem modelleme yönteminden faydalanılmış ve AMOS 4.0 yazılım programı kullanılarak test modelleri geliştirilmiştir. Geliştirilen modeller bir kamu kuruluşunda çalışan 752 memur personelden soru formu yoluyla toplanan veriler üzerinde sınanmıştır. Kuramsal modelde duygusal tabanlı örgütsel bağlılık için öngörülen kompozit faktör yapısı veri setine tam uymayınca iki faktörlü çözüme geçilmiş, elde edilen iki faktör “duygusal aidiyet” ve “sadakat” olarak adlandırılmıştır. Çift faktörlü yapı belirleyicilik ve sonuçlar açısından test amaçlı yapısal denklem modellerine dahil edildiğinde bulgular bazı önsavları tamamen, bazılarını kısmen desteklemiş, bazılarını ise hiç desteklememiştir. “İş tatmini”, “duygusal aidiyet” ve “sadakat” arasındaki ilişkilerin yön ve şiddeti konusunda bu kavramlar arasındaki yoğun çoklu bağlantılar nedeniyle berrak bir değerlendirme yapmak zorlaşmıştır. Buna rağmen denilebilir ki “iş tatmini” genel olarak “duygusal aidiyet” ve “sadakat”i, bu ikisinin iş tatminini belirleme düzeyinden çok daha fazla belirliyor görünümdedir. İşinden tatmin olan bir kamu çalışanı özellikle örgüte karşı duygusal aidiyet geliştirmekte ve bu duygusal aidiyet sadakate yol açmaktadır. Ayrıca, yazındaki bulgulara benzer şekilde sadakatin ayrılma niyetini azalttığı, iş tatmininin ise örgütsel performans algısını arttırdığı gözlemlenmiştir. Anahtar Kelimeler: Örgütsel bağlılık, iş tatmini, ayrılma niyeti, yapısal denklem modelleri.Organizational commitment construct has attained widespread attention from the organizational scholars during the last three decades. Yet, its close relations to other organizational attitudes and behaviors have rendered its causal identification quite complicated. Besides, many recent studies have indicated that the concept might reveal different characteristics in different contexts. Thus, this study attempts to identify and explain the affective component of organizational commitment with regard to a set of correlated organizational attitudes in a Turkish public sector organization. In order to achieve a reliable and causally robust analysis only the affective component of organizational commitment was analyzed. A thorough literature survey was performed to synthesize a theoretical model about the causal linkages of the construct. According to this survey, one of the most important correlates of organizational commitment is job satisfaction and organizational performance. Therefore, job satisfaction and perceived organizational performance was included in the model with two-way causal paths. Life satisfaction was found to be strongly correlated to job satisfaction and thus integrated to the theoretical model. The last construct included in the model was intent-to-leave. Five testable hypotheses were constituted to test the causal paths between the main constructs of the model. As method, we designed self-administrative questionnaires to measure these organizational attitudes. Questionnaire items were developed based on reliable scales for each construct but adapted to the Turkish and public sector domain. We ran a pilot sample and rechecked validity and reliability of our measures. Then we proceeded to apply our questionnaire to the whole civil servant population of a public sector organization. The total of returned questionnaires were 752 (approx. 90% of population), and due to missing values we ended up with 704 usable observations. Before testing our hypothesis, we started to examine the reliability and validity of our constructs. We used Exploratory Factor Analysis (EFA) method and Cronbach's Alpha for testing construct reliability. EFA did not load on a composite factor for affective organizational commitment scale but supported a two-factor solution. Since this was not expected, a confirmatory factor analysis was run to verify this solution. We found considerable support for a two-factor solution of affective organizational commitment construct, which we named "affective belongingness" and "loyalty" respectively. Having confirmed our measures, as a next step, the theoretical model was moved to AMOS 4.0 program to test the whole model simultaneously. Structural Equations Modeling (SEM) technique enables testing of multiple latent (unobserved) constructs and their interactions, using simultaneous structural equations derived from observed variables. Using SEM, we tested our theoretical model but due to the complex two-way interactions between key constructs, two test models were needed to account for all hypothesized relations. The first test model's overall fit, determined by 2 and GFI, was deemed acceptable, but some of the causal paths were observed as insignificant. The second test model fitted poorly to the data and again some paths were found to have insignificant scores. An optimum model with all paths significant and a good overall fit was produced and designated. Test models using SEM revealed that job satisfaction determined affective belongingness and by being emotionally attached to their organizations, public sector civil servants became more loyal. On the other hand, as predicted, loyal civil servants became less inclined to leave their organization. Yet, there were no statistically significant causal relations between affective based commitment and perceived organizational performance. However, test models indicated that the more satisfied the employees became the more inclined they were to rate their organization as performing well. Also, job satisfaction was found to determine life satisfaction more, even though the reverse is also statistically true. Thus, we conclude that job satisfaction has become even more critical a factor for public organizations to create loyal and emotionally charged employees. To increase satisfaction not just tangible categories such as pay system count but also good administrative capabilities matter. Keywords: Organizational commitment, job satisfaction structural equations model, intent-to-leave
Spatial informatics in the cultural heritage sector
Get PDFGünümüzde bilişim ve iletişim teknolojileri, hemen hemen her alanda olduğu gibi, kültürel miraslarla ilgili projelerde de önemi giderek artan bir rol oynamaktadır. Sözü edilen bu rol kültürel miraslar alanında, insanoğlunun kendi kültürel ve doğal mirasının değerini anlamasında ve arttırmasında, bu mirasları korumasında ve bunlara erişimin kolaylaştırılmasında önem kazanmaktadır. Bugün kültürel miraslarla ilgilenen kuruluşların çoğu projeleri ile ilgili mekansal ve öznitelik verilerinin toplanmasında, düzenlenmesinde, yorumlanmasında ve internet üzerinden paylaşımında bilişim ve iletişim teknolojilerinden yararlanmaktadır. Bu teknolojilerle çeşitli değerlendirme, analiz ve karar-verme sistemlerine yönelik olarak geliştirilen uygulamalar ile internet servisleri genellikle farklı zamanlarda tasarlanmakta ve değişik biçimlerde oluşturulmaktadır. Bu nedenle, sözü edilen kuruluşlar sürekli yenilenen akademik ve/veya ticari hedeflere ulaşabilmek için, farklı kullanımlara yönelik uygulamaların ve teknolojilerin, birlikte aynı amaca yönelik hizmet verebilmelerini sağlayacak etkin, ucuz ve hızlı yöntemleri ortaya çıkarma problemi ile karşı karşıya kalmaktadır. Bu çalışma kapsamında geliştirilen ve Kültürel Miras Projelerinin Mekan Bazlı Girişim Mimarisi Çatkısı olarak adlandırılan “GeoCHEAF” belirtilen probleme çözüm getirmektedir. Değişik amaçları olan farklı kullanıcılar uyarlanabilir yapıdaki bu mimari çatkıyı kendi gereksinimlerine göre kültürel miraslarla ilgili araştırmalarına, projelerine, karar-verme süreçlerine katabilir ya da farklı çözünürlükte çözümlerin üretilmesinde kullanabilir. Bu mimari çatkı kültürel miras projelerinde en üst düzeyde açıklık, birlikte işlerlik, genişletilebilirlik, esneklik, etkileşim, sürdürülebilirlik ve ölçeklenebilirlik elde edebilmek amacıyla geliştirilmiştir ve çatkı tasarımının ana fikri bu çalışmada açıklanacaktır. Anahtar Kelimeler: Kültürel miraslar, girişim mimarisi, bilişim ve iletişim teknolojileri, mekânsal bilişim.Information and Communication Technology (ICT) plays an increasingly critical role in almost everything these days as well as in the work of the heritage sector, which acts to understand, promote, present, preserve, and improve access to humanity's cultural and natural heritage. Many of today's cultural heritage (CH) organizations rely on digital information and communication technology to gather, organize, interpret, and disseminate data relating to their various projects. In many cases, this involves applications and services that were created at different times and designed for different computing platforms. The challenge now faced by these organizations is to provide efficient and effective methods by which these disparate technologies can work together to achieve academic and/or commercial objectives that are constantly evolving. The Geo-enable Cultural Heritage Enterprise Architecture Framework (GeoCHEAF), a spatially-enabled open enterprise architecture framework for the cultural heritage domain, has been developed in response to this challenge in the scope of the study. 'GeoCHEAF' is the result of rigorous analysis of the CH domain operations and data sharing requirements and consideration of the ways in which modern internet technologies can support the CH projects workflows. It explores and captures business and technical requirements of the cultural heritage domain and elaborates design principles for the requirements. These principles have been intended to deliver enterprise solutions and to provide decision-making activities, which needs a modeling and design approach. Therefore, in this study, the modular design process of the comprehensive and sophisticated framework 'GeoCHEAF' has been explained first as a development and then, as a customization framework for the cultural heritage projects to make them more effective in almost every sense. The crucial point of the design is to pinpoint web based-Spatial Information System at the center of the enterprise architecture framework and integrate it with information and communication technologies in a coherent conceptual model along with an executable architecture for optimized use. While 'GeoCHEAF' delivers an innovative road map for the cultural heritage community, it provides an opportunity for the cultural heritage domain to share rethinking on the direction of the cultural heritage resource management and research, and on future cultural heritage strategies for improving and evaluating the impact of these on the cultural heritage sector. 'GeoCHEAF' demonstrates how a cultural heritage enterprise works within a geo-enable enterprise architecture and the relationship between the parts (systems) of the whole by capturing the structure and the behavior of the systems in the enterprise. The conceptual framework 'GeoCHEAF' is comprised of many other sub-frameworks, such as an enterprise-wide business framework, web computing framework, geoprocessing framework, run-time framework for visualization. The theoretical concepts underlying the approaches to be advocated in this article concerned the cultural heritage domain-driven science-oriented technology-enabled business-sensitive cultural heritage studies. Under this approach 'GeoCHEAF' seeks to reach out across the CH domain for partnering with CH enterprises' research, development and solutions. In the context of this study, this cultural heritage strategy proposed describes its design rationale in attempting to achieve maximum openness, interoperability, extensibility, flexibility, interactivity, maintainability and scalability for projects of cultural significance. Effective seamless integration of technology based upon the open standards and semantic technologies provides an interoperable architecture which expands communication and dissemination of the cultural heritage data and information to the public. 'GeoCHEAF' uses this approach not only to create a better modularity for user and functional requirements of the enterprise, but also to create the most productive enterprise computing architecture for the cultural heritage domain.Finally, this article examines not only how modern-day spatially-enabled open enterprise architecture framework can be configured to embrace cultural heritage projects, but also addresses how this cultural heritage strategy proposed can drive positive change within the cultural heritage domain by simply adopting elements of the enterprise architecture process. Keywords: Cultural heritage, enterprise architecture, information and communication
Probabilistic dependency parsing of Turkish
Get PDFBu çalışma, Türkçe için geliştirilmiş ilk istatistiksel bağlılık ayrıştırıcısının sonuçlarını sunmaktadır. Türkçe, tümce içi öğe dizilişleri serbest, karmaşık bir çekimsel ve türetimsel biçimbirime sahip olan bitişken bir dildir ve bu özellikleri ile istatistiksel ayrıştırma konusunda ilginç sorunlar ortaya koymaktadır. Türkçe’de, bağlılık ilişkileri “çekim kümesi” adı verilen sözcük parçacıkları arasında kurulmaktadır. Bu bağlılıkların bulunması amacı ile Türkçe’nin karmaşık yapısının ayrıştırma sırasında nasıl modelleneceğinin irdelenmesi gerekmektedir. Bu çalışmada, ayrıştırma için farklı gösterim birimleri kullanan olasılık tabanlı modeller incelenmiştir. Başlangıç olarak biri kural tabanlı bir ayrıştırıcı olmak üzere üç dayanak model geliştirilmiştir. Gerçekleştirilen üç olasılık tabanlı modelin, dayanak modellere ve birbirlerine oranla başarımları değerlendirilmiştir. Ayrıştırıcının eğitimi ve sınaması için Odtü Sabancı Türkçe ağaç yapılı derlemi kullanılmıştır. Çalışma ayrıca bu derlem üzerinde sınanmış ve sonuçlaı raporlanmış ilk çalışmadır. Bu ilk incelemede, derlemin sadece sağa bağımlı (iye sözcüklerin uydu sözcüklerin sağ taraflarında yer aldığı) türde ve kesişmeyen bağlılıklar içeren bir alt kümesini ayrıştırmaya odaklanılmıştır. Eldeki derlemin boyutu nedeni ile görünüm bilgisi (sözcüğün tümünün veya gövdesinin ayrıştırma birimi gösterimlerinde bir özellik olarak kullanılması) kullanmayan ve sadece birimler arası etiketsiz bağlılıkları bulmaya yönelik incelemeler yapılmıştır. Sonuçlarımız, çekim kümeleri arasındaki doğru bağlıkların bulunma başarımı gözönüne alındığında, ayrıştırma birimi olarak çekim kümelerinin kullanıldığı ve bağlam bilgisinden yararlanan modelin en yüksek başarımı sağladığını göstermektedir. Anahtar Kelimeler: Bağlılık ayrıştırması, doğal dil işleme, ayrıştırma, sentaks analizi.This paper presents results from the first statistical dependency parser for Turkish. Turkish is a free-constituent order language with complex agglutinative inflectional and derivational morphology and presents interesting challenges for statistical parsing, as in general, dependency relations are between "portions" of words called inflectional groups. We have explored statistical models that use different representational units for parsing. We have used the Turkish Dependency Treebank to train and test our parser but have limited this initial exploration to that subset of the treebank sentences with only left-to-right non-crossing dependency links. Our results indicate that the best accuracy in terms of the dependency relations between inflectional groups is obtained when we use inflectional groups as units in parsing, and when contexts around the dependent are employed. Turkish shows very different characteristics from the well-studied languages in parsing literature. Many of these characteristics are common for all agglutinative languages such as Basque, Estonian, Finnish, Hungarian, Japanese and Korean. It is a flexible constituent order language. Even though in written texts, the constituent order of sentences generally conforms to the SOV or OSV structures, the constituents may freely change their position depending on the requirements of the discourse context. From the point of view of dependency structure, Turkish is predominantly (but not exclusively) head final. Furthermore, Turkish morphotactics is quite complicated: a given word form may involve multiple derivations and the number of word forms one can generate from a nominal or verbal root is theoretically infinite. Derivations in Turkish are very productive, and the syntactic relations that a word is involved in as a dependent or head element, are determined by the inflectional properties of the one or more (possibly intermediate) derived forms. In this work, we assume that a Turkish word is represented as a sequence of inflectional groups (IGs hereafter), separated by ?DBs, denoting derivation boundaries. A sentence would then be represented as a sequence of the IGs making up the words. When a word is considered as a sequence of IGs, linguistically, the last IG of a word determines its role as a dependent, so, syntactic relation links only emanate from the last IG of a (dependent) word, and land on one of the IGs of a (head) word on the right (with minor exceptions). And again with minor exceptions, the dependency links between the IGs, when drawn above the IG sequence, do not cross. We implemented three baseline parsers: 1. The first baseline parser links a word-final IG to the first IG of the next word on the right.2. The second baseline parser links a word-final IG to the last IG of the next word on the right. 3. The third baseline parser is a deterministic rule-based parser that links each word-final IG to an IG on the right based on the approach of Nivre (2003). The parser uses 23 unlexicalized linking rules and a heuristic that links any non-punctuation word not linked by the parser to the last IG of the last word as a dependent. In addition to these, we implemented three probabilistic models:1. 'Unlexicalized' Word-based Model, where the words are represented as the concatenation of their IGs and are used as the parsing unit during the parsing. 2. IG-based Model, where each word is splitted into its IGs and then the IGs are used as the smallest parsing unit. 3. IG-based Model with Word-final IG Contexts, where the IGs are again used as the parsing unit. This model differs from the previous one in the way it uses the contextual units and calculates the distances between units. Our results indicate that all of our models perform better than the three baseline parsers, even when no contexts around the dependent and head units are used. We get our best results with Model 3, where IGs are used as units for parsing and contexts are comprised of word final IGs. The highest accuracy in terms of percent of correctly extracted IG-to-IG relations excluding punctuations (73.5%) was obtained when one word is used as context on both sides of the the dependent. We also noted that using a smaller treebank to train our models did not result in a significant reduction in our accuracy indicating that the unlexicalized models are quite effective, but this also may hint that a larger treebank with unlexicalized modeling may not be useful for improving link accuracy. Keywords: Dependency parsing, natural language processing, parsing, syntax analysis.
Ocean wave characteristics prediction by using triple diagram method
Get PDFDalga enerjisinin tahmini için belirgin dalga yüksekliği ve ortalama dalga periyotlarının bilinmesi gereklidir. Bu iki değişken aslında feç uzunluğu, rüzgâr esme süresi ve rüzgâr hızına bağlı olarak değerler almaktadır. Belirli bir feç mesafesi boyunca ve belirli bir süre zarfında sabit bir hızda esen rüzgarın ortaya çıkardığı dalga yüksekliği ve periyodunu tahmin etmek için bir çok yaklaşım geliştirilmiştir. Burada ilk defa üçlü diyagram modeli yardımıyla rüzgâr hızı, feç mesafesi ve esme süresi gibi büyüklüklerden dalga yüksekliği ve periyodu tahmin edilmeye çalışılacaktır. Üçlü diyagram yönteminin esası jeoistatistiksel ilkelere dayanmaktadır. Jeoistatistikte yatay ve düşey eksenler konumu belirtirken bunları dik kesen üçüncü eksen ise bölgesel değişkeni göstermektedir. Benzer olarak bu çalışmada yatay ve düşey eksenler modele ait girdi değişkenlerini temsil ederken, bunlara dik olan eksende ise elde edilmek istenen çıktı değişkeni yer almaktadır. Daha sonra bu tahminler kullanılarak dalga enerji miktarları elde edilecektir. Bu yöntem girdi ve çıktı değişkenleri arasında doğrusal olmayan ilişkiler kurabilirken tahmin sonuçlarını oldukça geliştirebilmektedir. Elde edilen sonuçlar literatürde mevcut bulunan JONSWAP formülleri ile karşılaştırılmıştır. Yöntemin uygulanması için Ontario gölünde ölçülmüş saatlik rüzgâr ve dalga verileri düzenlenerek kullanılmıştır. Veriler eğitim ve test verisi olarak iki kısma ayrılmıştır. Önerilen yöntemin dalga karakteristiklerini tahmin etmekte klasik formüle göre çok daha iyi olduğu çeşitli grafik ve nümerik gösterim şekilleri ile ortaya konulmuştur. Anahtar Kelimeler: Dalga enerjisi, belirgin dalga yüksekliği, ortalama dalga periyodu, Kriging, jeoistatistik, optimum enterpolasyon, tahmin.A geostatistical approach was used to express correlation structure between dependent and independent variables of waves and to model prediction uncertainties. This correlation structure is represented by the variography of predicted values in which the distance variable in the variogram is determined by measuring the distance between predictors. This variogram produced from the predictors makes it possible to predict variables as unmeasured points while considering the historic information as measurement points of the field. In fact the method is based on Kriging principles. In order to make optimum interpolation these principles are required. The most important feature is to be applicable for unstationary data. It can relate input variables to output variables in a nonlinear manner. In this study, the method was employed to predict significant wave height and wave period for fetch limited and duration limited cases. Scatter diagram of variables was plotted prior to application of triple diagram method. These diagrams were called as templates where it can be seen that variables are independent from each other. The variation of third variable considered as output variable was investigated in this scatter area. The variogram based on variation of predictors is characterized the features of scatter area. The semi-variograms (SV) were obtained for each case and output variable. The best theoretical SV which fit to experimental SV were searched. The Gaussian type SV was found to be more appropriate for fetch limited case. The variogram value which does not change with distance anymore is called as sill value. This means that variance is fixed after range value that corresponds to sill. It can be seen that nugget effect is zero which means there are no variations for little distances. If there is rapid variation in the SV values while the distances increase very slowly, then it can be concluded that the variation is very high for closer points. The Gaussian type SV was fitted for the duration limited case. It was shown that sill and range values also exist for the duration limited case. This indicates that concerned event is stationary and has constant variance which also means that it has significant covariance. There is a consistency and continuity in data since nugget value is close to zero. The slow variation in the SV values while distances increase shows that points show similar properties with each other. After obtaining the theoretical SV, now one can make optimum interpolation by using Kriging principles. In order to make predictions significant wave height and wave period contour maps were prepared. For fetch limited case while the horizontal axis and vertical axis represent the fetch length and wind speed, the third dimension that perpendicular to those two axis exhibits the wave parameters such as wave height and period. Likewise for duration limited case, blowing duration and wind speed put into horizontal and vertical axes, respectively, as a third axis wave parameters are taken. Triple diagram method outperforms the classical JONSWAP method for both fetch limited and duration limited cases. It is possible to arrive some useful interpretations by using contour maps. The application of this methodology was achieved by using measurements from Ontario Lake. The data obtained from lake was divided into two parts, namely training and testing. Training data were used to set up the model and testing data were employed for validation. Moreover in order to develop the prediction results, adaptive triple diagram method was proposed. This developed method makes it possible to predict errors which come from the triple diagram method and to add the predicted errors to the previous step results. This procedure can be repeated many times until reaching to correct results. However, here satisfactory results were obtained after one step. It is apparent that results were improved much more comparing the triple diagram method. After predicting the wave parameters correctly, these parameters were used to obtain wave power values. Wave power contour maps were constructed using wind properties. It is possible to predict wave power from fetch length, blowing duration and wind speed by employing these maps. It is also concluded that wind speed has a significant effect on the wave power comparing to fetch length and blowing duration. For the small values of fetch length and blowing duration, these parameters loose their effects on wave power. Keywords: Wave energy, significant wave height, average wave period, Kriging, geostatistics, optimum interpolation, prediction.
Autogenous deformation and fracture properties of cementitious materials: Effect of size and type of aggregates
Get PDFYüksek performanslı çimentolu malzemelerde erken yaşlarda oluşan otojen deformasyonu önlemek için önceden suya doyurulmuş hafif agregalar su rezervuarı olarak kullanılabilir. Sabit bir düşük su/çimento oranına sahip silis dumanı katkılı çimento hamurunun otojen deformasyonunu önlemek için, hacimce %30’u normal veya hafif agrega ile yer değiştirildi. Kullanılan agrega cins ve tane boyutunun oluşturulan kompozitlerin kırılma enerjisi, karakteristik boy ve kiriş açıklık ortası en son sehime olan etkileri incelendi. Sonuçlar, normal agregaların rijitliğinin çimento hamuruna göre yüksek olması sayesinde otojen deformasyonu bir miktar kısıtlarken, hafif agregaların çimento hamuruna su sürükleyerek otojen deformasyonu önlediğini gösterdi. Çimento hamuruna hafif agrega ilave edilmesiyle hidratasyon derecesi özellikle erken yaşlarda arttı. İnce hafif agrega ilavesiyle su rezervuarları hamur fazı içerisinde daha yakın dağılarak iyi bir içsel kürleme sağladı. Böylece, ince hafif agrega ile çimento hamurunun hidratasyon derecesi iri hafif agrega ilavesine göre daha yüksek bulunmuştur. Aynı zamanda içsel kürlemenin çimento hamurunun hidratasyon derecesini de artırdığı bulundu. Ayrıca, ince hafif agrega kullanımının iri hafif agregaya göre belirgin şekilde çimento hamurunun otojen deformasyonunu azalttığı belirlendi. İnce hafif agregalı kompozitlerin kırılma enerjisi, kirişin açıklık ortasındaki son sehim değeri, yarılmada çekme dayanımı ve basınç dayanımının iri hafif agregalı kompozitlere kıyasla daha yüksek olduğu görüldü. Normal agrega kullanımı ile kompozitlerin kırılma ve mekanik özelikleri artarken hafif agrega kullanımı ile azalmalar görüldü. Diğer yandan hafif agregalı kompozitlerin sünekliği çimento hamuruna göre daha yüksek bulundu. Anahtar Kelimeler: Agrega tane boyutu, hafif agrega, içsel kür, kırılma enerjisi, otojen deformasyon, süneklik. Since high strength/high performance concretes (HSC/HPCs) are produced using rich mixes with high cement contents and low water/cement ratios, they are denser and more homogeneous than normal strength concretes. Low porosity and non-connected capillary pore structures are the most important properties of these concretes. There is an insufficient amount of water in concrete to complete the hydration process, and also the water income is prevented because of the impermeable character of concrete. Concurrent with the production of hydrates, after de-moulding (also before) time dependent volume changes of cement paste occur. Development of the empty pores due to chemical shrinkage is the main reason for diminishing the radii of menisci. This decrease causes self-desiccation by increasing capillary stress in the pore water. The absorption of free water into pores by the hydrates and formation of fine pores is driven by the chemical shrinkage. At later ages, however, self desiccation becomes the major drive mechanism. The development of empty pores continues during self-desiccation and it affects the hydration kinetics. This effect limits the ultimate degree of hydration and consequently the strength of cement paste. Since the tensile strain capacity of concrete at early ages is not sufficient enough to resist stresses caused by the external and internal restraints, autogenous deformation leads to the formation of cracks. These early age cracks may induce durability problems. Conventional curing techniques are not effective in mitigating this decrease of relative humidity (RH) in the hardened cement paste. Using pre-soaked fine lightweight aggregate (LWA) for preventing the decrease of RH in the hardened cement paste matrix was first recommended by Philleo (1991). It is known that the bond between lightweight aggregate and matrix is stronger than that of normal aggregate. This enhanced interfacial zone prevents the length changes developed under RH and/or thermal conditions. In application, concrete is usually designed on the basis of compressive strength theory, in which the brittleness of concrete is ignored. For this reason, fracture parameters of concrete have to be defined along with its brittleness. Because the LWAs produce more microcracks, the concrete containing these aggregates behaves more ductile (i.e., less brittle) than normal cementitious materials. On the other hand, the use of LWA instead of dense aggregate is expected to result in a lower strength. It is obvious that the amount of LWA for internal curing must be optimized according to the mechanical and fracture properties of cementitious materials. This study presents the results of experimental studies conducted for determining the restraining effect of both LWAs and normal weight aggregates (NWAs) on autogenous deformation and fracture properties of composites containing 70% paste and 30% aggregate by volume. It has been shown that the size of NWAs has no significant effect on autogenous deformation at 28 days, while the autogenous deformation of cement paste with 0-2 mm size fraction of NWA has been found higher than that with 2-4 mm and 4-8 mm at the age of 360 days. At early ages, the finer NWAs (0-2 mm) restrained the autogenous deformation of cement paste, because the modulus of elasticity of cement paste was not too high. On the other hand, addition of coarser size NWAs easily restrained the shrinkage of cement paste at later ages. The fine size LWAs, as expected, has been found to be more effective in mitigating the autogenous deformation of cement paste than coarse LWAs. The addition of LWAs to paste phases increased the degree of hydration, especially at early ages. The use of fine LWAs causes the water reservoirs to be distributed in proximity and results in a better internal curing, hence the degree of hydration of cement paste with finer LWAs is greater compared to that with coarser LWAs. While the compressive strength of composites slightly increases with the increasing size of NWA, the size of LWAs has no significant effect on compressive strength. Also, the moduli of elasticity along with the bending and splitting tensile strengths were not affected significantly by the size of aggregates (both NWA and LWA). Addition of fine sand aggregate at the size fraction of 0-2 mm increased the characteristic length significantly compared to the cement paste. Although the fracture energies of cement pastes with LWAs were found smaller than that of reference cement paste the addition of LWAs caused an increase in the characteristic length of cement paste. Keywords: Autogenous deformation, fracture energy, internal curing, lightweight aggregate, particle size of aggregate, toughness.
Data sources:
Access Repository Dashboard
Do you manage itüdergisi (E-Journals - İstanbul Teknik Üniversitesi, Istanbul Teknik Uniersity., İTÜ)? Access insider analytics, issue reports and manage access to outputs from your repository in the CORE Repository Dashboard!