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The evaluation of strategic decisions with real options
Get PDFOpsiyon seçenek anlamına gelmektedir, organizasyonun sahip olduğu seçenekler firmaların daha da başarılı olmalarını sağlarken başarısızlık riskini de ortadan kaldırmalıdır. Firma yöneticilerinin görevi bu seçenekler kümesini en etkin değeri sağlayacak şekilde yönetmektir. Stratejik opsiyon kümesinin en etkin şekilde yönetilebilmesi için stratejik opsiyonların değerini etkileyen unsurların neler olduğunun belirlenebilmesi gerekmektedir. Stratejik opsiyonların değeri tek bir unsurdan oluşmamaktadır. Firmanın sahip olduğu stratejik seçenekler kümesinin değerli olması stratejik opsiyondan elde edilen değerin bir göstergesidir ancak değerin tamamını açıklamamaktadır. Firma yöneticileri bu stratejik opsiyon kümesinden bazılarını kullanmaya karar vermekte, çevre koşulları, firma yöneticisinin özellikleri ve benzeri unsurlar kullanılan bu opsiyonun değerini etkilemektedir. Bu sebeple sorgulanması gereken bir unsurda firmanın kullanmayı seçtiği stratejik opsiyonların değeridir, ancak firmaların değerli stratejik opsiyon seçmeleri firma performansına her zaman katkı sağlamamaktadır. Stratejik opsiyonların etkin yönetimi için göz önüne alınması gereken bir diğer unsur, stratejik opsiyonların kullanımının etkin bir şekilde yönetilmesidir. Firmalar değerli bir stratejik opsiyon kullanmayı seçmiş olsalar bile uygulamadan kaynaklanan hatalar bu değerin elde edilmesini engelleyebilmektedir. Stratejik opsiyonlar değerlendirilirken, stratejik opsiyonların firmanın yetkinlik geliştirme sürecine ve ardıl stratejik opsiyonlara olan etkisinin ve bu ilişkinin firmaya katkısının da incelenmesi gereklidir. Bu çalışma kapsamında stratejik opsiyonların değerini etkileyen tüm bu unsurlar göz önüne alınmış ve stratejik opsiyonların değerini etkileyen unsurların neler olduğu belirlenmeye çalışılmıştır. Bu amaçla İMKB'ye bağlı firmalarda uygulama yapılmıştır. Anahtar Kelimeler: Stratejik opsiyonlar, firma performansı, yetenek geliştirme, çok ölçütlü istatistiksel karar verme.The developments in the recent years such as removal of cross border barriers in Europe; the globalization of product markets, the technology developments, and the change in the organization forms increased the competition among firms. This increased competition caused high level of changes in the firms' environment and uncertainty for organizations of all types. The high level of competition and uncertainty forced firms to make right and on time strategic decisions in order to survive. The management practices for the stable environments are not suitable for the new dynamic environment. In order to respond to this dynamic and uncertain environment, managers and academicians focused on the organization's strategic flexibility. According to the strategic options view the main property of strategic flexibility is the ability to access to the resources and capabilities when they are needed (Sanchez, 1993). Strategic flexibility is defined as "condition of having strategic options that are created through combined effects of an organization's coordination flexibility in acquiring and using flexible resources" (Sanchez, 1993). According to this definition in order to have strategic flexibility, firms need to have strategic options and the main task of the managers is to define, develop, acquire and coordinate the resources and competencies that will optimize the value of strategic options cluster. Therefore managers should answer the question "exercising what kind of options result will optimize the value created and captured through the strategic options and what are the factors affecting this process"? In literature several researchers emphasized the value of strategic options (Reuer and Leiblein, 2000; Tong and Reuer, 2006; Vassolo et al., 2004; Sanchez, 2003). According to McGrath et al. (2004) the link between real options, the factors that are effecting these options and the performance is largely unexamined. There is a need to understand the factors that affect the value created and captured through the strategic options. The objective of this study is to reveal this relation and the effectiveness of it. In the study the factors affecting the value crated and captured through strategic options are classified as firms' internal factors (firm size, management practices etc) and environmental factors (uncertainty, munificence). On the other hand Coff and Laverty (2007) claimed that strategic options differ from financial options: in strategic options, significant uncertainty remains at the exercise decision, and the value of knowledge depends on its transfer and integration within the firm. Even if managers are able to create valuable options, the value captured from a firm's options may still be low. The management of strategic options therefore includes both creating valuable strategic options and capturing the value of a firm's strategic options. Based on the main question "what are the factors affecting the value created and captured through the strategic options" five research questions are evaluated: i) What are the factors effecting the value of strategic options portfolio? ii) What are the factors that affect a firm's ability to choose an optimal portfolio of strategic options to develop (e.g., strategic planning processes, strategic decision processes), iii) what is the relation between competence building process and exercising strategic options, iv) what is the value of the strategic options that trigger the sequential strategic options? v) What are the factors that affect management's ability to capture value from its strategic options (e.g., operational capabilities important in implementing options)? In order to answer the research questions a five period model is developed. The market value of a firm is the sum of the market value of these two components. Assets-in-place refers to particular allocations of a firm?s resources already made. The value of this component is derived from the stream of cash-flow generated over time. Rights to decide the allocation of resources, have value to the extent that they affect future cash-flow, and their valuation is the principal object of the real options approach. In judging decisions undertaken by managers, investors consider not only the effects of managerial decisions on the amount, time and risk of a firm's expected cash flow, but also on the variables that determine the value of its real option portfolio. Therefore in order to evaluate the value created through strategic options the market reaction is considered. The event study analysis is utilized to measure the market reaction. The multi-attribute statistical models are utilized in order to evaluate the factors affecting the value created and captured through strategic options. Keywords: Strategic options, competence, firm performance, multi attribute statistical analysis
The impact of technological capabilities on knowledge transfer from foreign direct investments
Get PDFDoğrudan Yabancı Yatırım (DYY) literatüründe, doğrudan yabancı sermaye yatırımlarının yeni istihdam imkanları yaratmak, sermaye birikimini sağlamak, üst düzey teknoloji ve bilginin transferine katkıda bulunmak yoluyla ev sahibi ülkelerin ekonomik gelişmesine yapmış oldukları olumlu etkiler sıklıkla vurgulanmaktadır. Özellikle sınırlı endüstriyel ve teknolojik yetkinliklere sahip az gelişmiş ülkeler için üst düzey teknoloji bilgisinin transferi, ulusal teknolojik yetkinliklerin artırılması için kritik önem taşıyan bir etkendir. Bu sebepten, az gelişmiş ülkeler için, daha fazla DYY çekmeyi amaçlamaları önerilmektedir. Buna karşın, farklı ülkeler bağlamında yapılan çalışmalar, üst düzey teknoloji bilgisinin transferinin DYY'lerin doğal bir sonucu olmadığını ve "kendiliğinden" gerçekleşmediğini ortaya koymaktadır. Gerekli yerel koşulların ve yetkinliklerin olmadığı durumlarda, DYY’den teknoloji bilgisinin transferi sağlanamadığı gibi, DYY’lerın ev sahibi ülkeye uzun dönemli etkilerinin de beklendiğinin aksine, olumsuz olduğu gözlemlenmiştir. Bu çalışma, çok uluslu şirketlerin ev sahibi ülkedeki yabancı şubeleri ile yerel sermayeli şirketler arasındaki tedarikçi-müşteri ilişkilerinde, yerel şirketlerin teknoloji yetkinliklerinin bilgi transferi süreci üzerindeki etkilerini araştırmaktadır. Yerel sermayeli parça ve sistem tedarikçisi 7 şirket ve yabancı sermayeli otomotiv üreticisi 7 şirketin Türkiye'deki yabancı şubeleriyle yapılan vaka çalışmaları, yerel şirketlerin üretim ve inovasyon yetkinliklerinin, müşterilerinin tedarik zincirleri içindeki konumlarını, müşterileriyle olan ilişkilerinin yapısını, bilgi akışının yoğunluğunu, içeriğini ve kanallarını önemli ölçüde etkilediğini ortaya koymuştur. Anahtar Kelimeler: Doğrudan Yabancı Yatırım, bilgi transferi, teknolojik yetkinlikler.It is commonly emphasized in the international policy agenda as well as in the "FDI spillover" literature that the inward Foreign Direct Investment (FDI) flow has positive impacts on the growth of host countries by creating new employment opportunities, accumulation of capital stock, and "spillovers" of superior technology and knowledge to host economy and to local firms. Especially for less developed countries which are generally associated with limited capital accumulation, low level of industrial and technological capability and insufficient human capital, transfer of advanced technological knowledge is regarded a critical factor to upgrade their national technological capabilities. Thus, in order to benefit from technology spillovers that occur "autonomously" as a result of quantitative accumulation of FDIs in the national economy, it is generally suggested for less developed countries to improve their national investment climate to attract more Foreign Direct Investments (FDIs). However, previous studies in different country contexts showed that in cases where the necessary local conditions and capabilities were lacking, the transfer of technological knowledge from FDIs did not occur, and on the contrary to what was expected, the long-term effects of FDIs to the host countries have been negative. Moreover, even the existence of the business linkages between MNCs' foreign affiliates and local firms themselves can not guarantee the flow of technological knowledge. Transfer of high-value added knowledge requires certain level of capabilities and efforts of both MNC foreign affiliates and local firms, and certain level of collaboration among them. This research aims to explore how the technology using and technology creating capabilities of local firms affect the knowledge transfer content, intensity and channels from the foreign affiliates of multinational corporations to local firms in supplier-buyer relationships. The methodological approach of this research is based on qualitative analysis of case studies and face-to-face in-depth interviews with the respondents of 7 local supplier firms and 7 foreign affiliates of MNCs. The findings of case studies with local supplier firms showed that the production scales, production capabilities and design capabilities of local firms have significant impacts on their positions in the supply chain of their customer firms, on the nature of their relationships with their customer firms, and on the channels, content and intensity of knowledge transfer from the foreign affiliates of multinational customer firms. However, above all other factors, the vision, motivations, investments and self efforts of local firms are the most important factors that determine to what extent they gain a strategic position in the supply chain of their customer firms. The policy implications of the findings of this research are at two levels. At the first level, it is crucial to support Especially Small and Medium Sized Enterprises (SMEs) that constitute significantly large part of Turkish industry and automotive sector in order to make necessary investments for the modernization of manufacturing technologies and to create collaboration opportunities among those firms. It is believed that the improvements in the production capabilities of automotive supplier industry would contribute to the attraction of new FDIs, to encourage current foreign investors for enlarging their production scales, and for expanding their trade volumes with the local supplier firms. At the second level, the local supplier firms that possess advanced production capabilities and innovative vision should be supported by providing funding opportunities to allow them making further investments in R&D and to increase their design and innovative capabilities. Using the funding incentives provided by the government in a selective way to support local firms with high innovative potential would generate an efficient solution. It is expected that the increase in the innovation capabilities of local firms would contribute to the attraction of new R&D intensive investments, and to encourage current foreign investors to increase their design and R&D activities in Turkey. Moreover, organization of R&D incentives in a way to support long-term product design and technology development collaborations between local supplier firms and MNCs foreign affiliates would provide long-term benefits to the local firms. Keywords: Foreign direct investments, knowledge transfer, technological capabilities
Geometric accuracy assessment of high resolution satellite imagery
Get PDFYüksek çözünürlüklü uydu görüntülerinin son yıllarda eriştiği üstün kalite düzeyi, bu verilerin ortofoto görüntüler ve farklı haritacılık üretimleri konusunda iyi bir veri kaynağı olabileceğini kanıtlamıştır. Ancak günümüzde halen, yüksek çözünürlüklü uydu görüntülerinin büyük ölçekli harita üretim çalışmalarında kullanılabilmesine ilişkin tartışmalar sürmektedir. Bu nedenle, yüksek çözünürlüklü uydu görüntülerinin geometrik doğruluk yönünden incelenmesi amacıyla, 10 km x 10 km boyutlarında bir alana ve 440 m yükseklik farkına sahip olan Ankara yakınlarındaki Gölbaşı bölgesinde bir çalışma gerçekleştirilmiştir. Bu çalışma için 12 adet Yer Kontrol Noktası (YKN) tespit edilmiş ve görüntü alımından önce bu noktalar işaretlenmiştir. Daha sonra stereo Ikonos (4 Ağustos 2002 tarihli) ve mono Quickbird görüntüleri ile 1/16000 ölçekli stereo hava fotoğrafları (29 Ağustos 2002 tarihli) temin edilmiştir. Geometrik doğruluk çalışması için öncelikle fotogrametrik ve kartografik yöntemler ile Ikonos uydu görüntülerinden Sayısal Yükseklik Modelleri (SYM) elde edilmiş ve farklı sayı ve dağılımda bulunan YKN kullanılarak değişik özelliklere sahip ortofotolar üretilmiştir. Arazide belirgin olarak tespit edilebilen 27 adet bağımsız kontrol noktası (test noktası) seçilmiş ve bunların koordinatları GPS ile arazide ölçülmüştür. Bağımsız kontrol noktalarının GPS koordinatları, aynı noktaların tüm ortofoto görüntüler üzerinde okunan koordinatları ile karşılaştırılmış ve aradaki farklar belirlenmiştir. Geometrik doğruluk araştırmaları sonucunda yapılan değerlendirmeler, yüksek çözünürlüklü uydu görüntülerinin orta/büyük ölçekli (1/6000–1/10000) harita üretim çalışmalarında kullanılabileceğini göstermektedir. Anahtar Kelimeler: Yüksek çözünürlük, ortofoto, kontrol noktası, Sayısal Yükseklik Modeli (SYM).The high quality level that high resolution satellite images have reached in the last years has proved that these images could be a useful data source for the production of orthophoto images and different mapping products. Specifically, it is very important in areas where aerial photogrammetry is not possible or feasible due to political reasons / restrictions and mapping frequency constrained by the limits of flight planning. Research activities about high resolution satellite images have been mainly focusing on the geometric accuracy aspects in ortho-image generation and Digital Elevation Model (DEM) extraction. In addition to these activities, some researches are increasingly concentrating on feature detection, recognition and reconstruction studies like automated mapping of roads, extraction of 3D buildings and cadastral mapping. There are two important criteria for quality assessment of the images; geometric accuracy and object definition. When we take the mapping processes into consideration, these criteria are basically dependent on the relation between pixel size and the map scale, contrast information (spectral range and colour), atmosphere and the sun elevation, the printing technology and the resolution of the human eye in a usual reading distance. A study has been implemented in Gölbaşı near Ankara, which has 10 km X 10 km area and a height difference about 440 m, for geometric accuracy assessment of high resolution satellite images. 12 ground control points were revised and signalized before the recording of images. Stereo-pair Ikonos (dated August 4th, 2002) and mono Quickbird (dated May 26th, 2002) images and 1/16000 scaled aerial photographs (dated August 29th, 2002) have been provided by Inta (Turkey), Eurimage (Italy) and General Command of Mapping (Turkey) free of charge. For geometric accuracy assessment; different digital elevation models (DEM’s) have been created using photogrammetric and cartographic methods. Photogrammetric DEM has been produced using 1/16000 scaled stereo aerial photographs and has accuracy about 1 m. From the other side, cartographic DEM has been produced digitizing map contours and has accuracy about 5 m. And lastly, Ikonos DEM has been produced from stereo-pair Ikonos images automatically and has accuracy about 2 m. With these models, various orthophoto and mosaic images have been produced using ground control points. Used ground control points have had different distributions and number of points between 0 and 11. Well-recognized 27 checkpoints such as road junctions and field corners have been selected and measured with GPS. Planimetric coordinates of the checkpoints were measured on all orthophoto, mosaic and stereo images. These coordinates have been compared with GPS coordinates. From the comparison of these measurements, the differences and also the errors have been obtained. Afterwards the gross error positions have been evaluated according to ground control points’ network. The geometric accuracy results show that IKONOS - DEM can be used instead of photogrammetric DEM, the quality of GCP’s is as important as the number of GCP’s, systematic errors have occured in the easting / north easting (across track) direction and IKONOS images have better accuracy than Quickbird images when using direct sensor orientation parameters. Also we can say that the high resolution satellite images can be used in mid / large also from 1/6000 to 1/10000 scale map productions and these results are harmonious with the examples given in the literature. Actually there is nearly a consensus among photogrammetry and remote sensing society that some mapping applications from high resolution space imagery can be realized in mid / large scale. In addition, in spite of increasingly concentrating by some researches on feature detection, recognition and reconstruction studies in high resolution satellite images, it can not be said that these images couldn’t reach to the level of aerial images in determining and identifying of small features in big scale maps (e.g. 1/5000) yet. Therefore the discussions about usage of high resolution space imagery in big scaled map production are still lasting. Keywords: High resolution, satellite images, orthophoto, control point, Digital Elevation Model (DEM)
A novel neural network model for linearly nonseparable classification problems
Get PDFParça Parça Sabit (PPS) ve Parça Parça Doğrusal (PPD) sistem modelleri, çoğunlukla bu türde yapılar olduğu açıkça belirtilmese de Yapay Sinir Ağları (YSA) alanında sıklıkla kullanılmaktadır. Bilimsel yazında oldukça iyi bilinen bir çok model PPS veya PPD işlevler gerçeklerler. Diğer yandan, bazı karmaşık modeller, PPS/PPD yaklaşıklıkları oluşturularak basitleştirilebilir. PPS ve PPD modeller basitlik, hızlılık ve verilen herhangi bir fonksiyona istenen duyarlılıkla yaklaşabilme özelliklerine sahip olmakla, YSA literatüründe ayrı bir yere sahiptir. Bu çalışmada, yeni bir PPD YSA yapısı olarak 'girişe bağlı eşikli ayrık algılayıcı' önerilmektedir. Klasik ayrık algılayıcının doğrusal olarak ayrıştırılamaz kümeleri sınıflandıramadığı gayet iyi bilinmektedir. Bu sorunun aşılması için, algılayıcının eşik değeri, girişin PPS bir fonksiyonu olarak değişecek şekilde yeniden düzenlenmiştir. Önerilen model böylece PPD bir ayrım yüzeyi oluşturur ve doğrusal ayrıştırılamayan problemlerin bir alt kümesi için çözüm üretebilmektedir. Önerilen modelin etkinliği, tüm bağlantı ağırlıklarının algılayıcı öğrenme kuralı kullanılarak öğrenilebiliyor olmasına dayanmaktadır. Modelin eğitimi için algılayıcı öğrenme kuralının üç aşamalı uygulanmasından oluşan bir algoritma önerilmiştir. Bu algoritmanın işleyişiyle problem kümesinin doğrusal ayrıştırılabilir olup olmadığı, doğrusal ayrıştırılamaz giriş vektörlerinin hangileri olduğu ve doğrusal ayrıştırılamaz vektörlerin kümenin diğer elemanlarından doğrusal ayrıştırılabilir olup olmadığı test edilmektedir ve yöntemin son adımında belirli şartların sağlanması halinde ağın tüm ağırlıkları belirlenmiş olur. Modelin ve yöntemin etkinliği, doğrusal ayrıştırılamayan bir sınıflandırma problemi olan siyah beyaz görüntüler üzerinde kenar belirleme problemi üzerinde gösterilmiştir. Anahtar Kelimeler: Ayrık algılayıcı, doğrusal ayrıştırılamaz kümeler, parça parça doğrusal ayırım yüzeyleri.Piecewise Constant (PWC) and Piecewise Linear (PWL) models are commonly used in a diverse field including circuit theory (Chua etal. 1987, Güzeliş and Göknar 1991, Lin and Unbehauen 1995, Julian etal. 1998). Altough it is not declared that they are such models, PWC and PWL models are common also in the field of Artificial Neural Networks (ANN or simply Neural Networks – NN henceforth). At a first glance, discrete Multilayer Perceptron (MLP), Cellular Neural Networks (CNNs), Discrete Hopfield Network (D-HN), Discrete Time CNNs (DT-CNNs), Brain State in a Box (BSB) are examples for PWC or PWL artificial neural network models. Due to their generality, namely their capability of representing all known complex functions, despite their simplicity, PWL models can be used not only for synthesis and analysis purposes but also to simplify some complex models. Despite the fact that PWL and PWC structures are simple and able to approximate to any function, there is not enough emphasis on these subjects on the NN literature. One of the most important works in PWL literature is by Lin and Unbehauen (1995). Lin and Unbehauen (1995) presents some modifications and applications of classical canonical PWL functions to supply mathematical background for mapping networks. It also studies the canonical PWL feature of the popular Multilayer Perceptron-Like networks. It is shown that generalized canonical PWL representation is suitable for neural network applications. In this work, as a new PWL NN model, a special discrete perceptron whose threshold is a function of input variables is considered (Genç and Güzeliş 1998b). It is well known that discrete perceptron cannot classify linearly nonseparable sets. To overcome this problem, perceptron can be modified as its threshold depends on its input, and threshold value is a PWC function of input as given below where is the unit step function and are the weight vectors Thus, the proposed model realizes a PWL separating hyperplane and it can be used to classify some kind of linearly nonseparable sets. What is more important is that all connection weights can be learned using a three stage application of the perceptron learning rule. Proposed learning algorithm defined here can be applied to the linearly nonseparable set if the complement of the largest linearly separable subset, contains elements from only either or and this linearly separable with. By explaining verbally, linearly nonseparable vectors are all from the same class and these are linearly separable from the rest of the set. The 3-stage algorithm does, in fact, test of the input set for linear separability, define nonseparable vectors and test the linear separability of nonseparable vectors from other vectors of the set. Meanwhile, it finds the resulting weight vectors and thresholds if the cases are admissible with the prerequisites of the method. Justification of the design method is demonstrated by computer simulations done on a specific example, namely the edge detection, since binary edge detection can be considered as a pixel classification problem where for each pixel, it can be decided whether it is an edge pixel or not, by investigating the pixel's and its neighboring pixels' values. The definition of the ideal edge class stated as “The Pixel belongs to edge class if it is black and at least one of its east, west, south and north neighbors is white but not all of its 8 neighbors are white”. With this definition, a classification problem set is obtained containing 512 binary 9-dimensional vectors and corresponding desired outputs for a 3´3 neighborhood. In the example work, after posing the binary edge detection task as such a linearly nonseparable but piecewise-linearly separable threshold function, the modified perceptron is trained to learn this task. Then the performance of this perceptron on the edge detection of some binary images is examined. Keywords: Discrete perceptron, linearly nonseparable sets, piecewise linear separating plane
Investigation of the flashover process under fast transient voltages
Get PDFNanosaniyelik darbeler, EMP (ElektroManyetik Darbe) yoluyla oluşabilir. EMP uzayda doğal yolla meydana gelebileceği gibi yapay yolla da oluşturulabilir. Yüksek irtifada meydana gelen bir EMP, çok geniş alanları etkisi altında bırakabilir. EMP’nin en önemli bozucu etkisi, elektronik sistemlerin ve cihazların iletkenlerine girişimde bulunarak bunların içine ulaşması ve sistem veya aygıtların bozulmasına ya da yanlış çalışmasına neden olmasıdır. EMP benzeri darbe gerilimleri genel olarak nanosaniye süreli darbelerdir. Cihazların böyle darbeler karşısındaki davranışlarını incelemek için yapay EMP üreteçleri kullanılır. Bu üreteçler EMP’nin oluşturduğu elektrik ve manyetik alanları üretirler. Bu çalışma kapsamında EMP benzeri nanosaniyelik darbelerin aygıtların iletkenlerine ve elektronik kartlarına ulaşmaları durumunda meydana gelebilecek olaylar değerlendirilmiştir. Bu amaçla, yaklaşık 50 nanosaniyede tepe değerine ulaşan ve toplamda yaklaşık 1000 nanosaniye süren 10 kV’a kadar darbe gerilimi üreten bir üreteç tasarlanmış ve gerçeklenmiştir. Üreteç tek katlı Marx devresi ilkesi ile çalışmaktadır. Elektronik aygıtların devre kartlarını benzetmek için elektronik devrelerde sıkça karşılaşılan yedi farklı geometride akım yolu FR-4 malzemeden yapılmış devre kartına hazırlanmıştır. Darbe gerilimi üretecinden elde edilen gerilimlerle akım yolları arasındaki boşalma olayları incelenmiştir. Numune akım yollarında geometriye bağlı olarak boşalma noktaları belirlenmiş, uygulanan gerilimler ile elektrotların yüzde elli atlama eğrileri çıkarılmıştır. Yapılan deneysel çalışma, teorik hesaplamalar ile desteklenmiştir. Bunun için sonlu elemanlar yöntemi ile bilgisayar analizi yapılmıştır. Bu çalışma ile nanosaniyelik darbe gerilimine maruz kalan elektronik kartlarda geometriye bağlı olarak etkilenme üzerine yorum yapılmıştır. Anahtar Kelimeler: EMP, hızlı geçici darbe gerilimi, yüzeysel boşalma, akım yolları. Improvements in technology had widened the use of electricity. The process beginning with direct current continued with alternative current. Wide usage had brought different requirements with. The natural effect of lightning and the disturbance effect of switching in electrical installation necessitated research on this subject. It?s necessary to have an idea on the behavior of electrical systems facing such an effect, before they are put into operation. This necessity had triggered generation of pulse voltages in laboratories. Pulse voltages generated gave the chance of performing tests on systems and components forming systems. In the course of time, pulse voltages have been generated not only for experimental objective, and also directly for industrial, medical, military etc. objectives. The microsecond and millisecond pulses reduced to nanosecond even picosecond pulses with the development in technology. Nanosecond pulses may be formed by EMP (ElectroMagnetic Pulse). EMP can be originated naturally in space; beside it can be formed artificially. A high altitude EMP can effect a wide area on earth. The most important disturbance effect of EMP is coupling to the conductors of electronic systems and equipments, entering the device and causing a breakdown or malfunction. EMP like pulses are usually nanosecond pulses. To examine the behavior of devices facing such a pulse, artificial EMP simulators are used. These simulators generate the electric and magnetic fields simulating EMP. In this study, the possible events that may take place when the EMP like nanosecond pulses reaches to the conductors and circuits of electronic devices are evaluated. In this study it is intended to perform conducted EMP tests directly on to the conductors. For this aim, a pulse generator with an output up to 10 kV peak value, generating a pulse with a risetime of about 50 ns and total duration of about 1000 ns is designed and constructed. The realized generator operates according to principle of a single stage Marx circuit. Accurate and reliable measurement of the fast voltage pulse is an important point to be considered. Different ways of measurement of the high voltage pulse is experimented. An oscilloscope with capability of sampling 4 GSa/s is used to scope the pulse generated. The high speed and the other technical benefits of the oscilloscope brought many advantages with. For simulating the circuit of electronic device, widely used seven different geometrical current ways are prepared on to the board made of Fr4. Fr4 is a fire rated electrical-grade, dielectric fiberglass laminate epoxy resin system combined with a glass fabric substrate. Series of pulses of different voltage level is applied by the generator on the sample board, and the flashovers between current ways are observed. Number of flashover occurred over the applied pulses is noted as a table. Data on this table versus the voltage applied is used to draw graphics, which can be evaluated as the fifty percent discharge curves of the electrodes. Another work performed on the sample board is the determination of the flashover points for different geometries. Arc points of the electrodes were observed visually during the tests; besides, the electrodes were investigated by an electron microscope. The experimental work performed is tried to be supported with theoretical analysis. For this aim, electrical field intensity and field distribution of the electrodes under the voltage applied is numerically analyzed. Analysis is performed on computer using FEMM program based on finite elements method. The expected point of flashover is the point where the maximum electrical field observed. Both the experimental study and the computer analysis gave the similar results. Current way electrodes set straight parallel has lower fifty percent discharge level when compared with parallel electrodes having angles or arcs. Among the seven different geometries, electrodes having 135° mutually has the minimum fifty percent discharge voltage level. On parallel electrodes, flashover takes place on the edges however electrodes with arcs or angular in shape flashes at these angle points. The electromagnetic interference radiated from the test setup is determined. For this aim, the emission of the system is measured by biconical antennas during the test for all seven geometrical current ways. One step further, the current induced by the emission coupled to a power cord 1 m away from the setup is measured and evaluated. Keywords: EMP, fast transient pulse voltage, surface flashover, current ways
Production of glass, glass-ceramic and ceramic materials from industrial wastes
Get PDFBu çalışmada, termik santral uçucu külü ile alüminyum üretimi sırasında açığa çıkan kırmızı çamurdan cam ve cam-seramik üretimi gerçekleştirilmiştir. Öncelikle, cam üretimini gerçekleştirebilmek amacıyla, Çayırhan uçucu külüne %5 oranında atık kırmızı çamur ilave edilmiştir. Kissinger yöntemi kullanılarak, üretilen cam numunelerinin kristalizasyon aktivasyon enerjisi 370 kJ/mol olarak tespit edilmiştir. Cam numunelerine, cam-seramik malzeme üretebilmek amacıyla sinterleme yöntemi uygulanmıştır. Cam numunelerine; diferansiyel termal analiz sonucunda elde edilen bilgiler ışığında, çekirdeklenme ve kristalizasyon ısıl işlemleri uygulanarak cam-seramik malzemeler üretilmiştir. Kristalizasyon sıcaklığında bekleme süresinin üretilen cam-seramik numunelerinin mikroyapısal, kimyasal ve fiziksel özelliklerine olan etkisi incelenmiştir. Kristalizasyon sıcaklığında süresinin artması ile birlikte cam-seramik numunelerde oluşan kristal fazın oranı artmıştır. Ayrıca cam-seramik numunelerinin iyi yönde geliştiği gözlenmiştir. Sinterleme yöntemiyle üretilen cam-seramik numunelerde oluşan kristalin fazın diopsid (Ca(Mg,Al)(Si,Al)2O6) olduğu belirlenmiştir. Ayrıca, uçucu külden herhangi bir katkı maddesi ilavesi olmadan, dört farklı sinterleme sıcaklığında ısıl işlem uygulanarak seramik malzemeler üretilmiştir. Üretilen seramik numunelerde oluşan fazların kuvars (SiO2), mullit (Al6Si2O13), anortit (CaAl2Si2O8) ve diopsid (Ca(Mg,Al)(Si,Al)2O6) olduğu saptanmıştır. 1348 K’in üzerindeki sinterleme sıcaklığında üretilen seramik malzemenin, daha düşük sinterleme sıcaklıklarında üretilen numunelere oranla daha üstün özelliklere sahip olduğu tespit edilmiştir. Üretilen cam-seramik malzemelerin mikroyapısal ve fiziksel özelliklerinin, seramik malzemelere kıyasla çok daha iyi olduğu gözlenmiştir. Anahtar Kelimeler: Uçucu kül, cam, aktivasyon enerjisi, cam-seramik, sinterleme ve seramik.In the near future the development of new recycling technologies is getting more important and the recycling of by-products and industrial waste materials will dramatically increase. Fly ash, a waste product of coal combustion in thermal power plants, is produced in large quantities and thus is a major source for environmental pollution. In Turkey, considerable amounts of coal fly ash is generated daily in the thermal power plants, due to the high coal consumption and the high mineral matter content of Turkish lignites. The management of coal fly ash is a major problem in Turkey since only a small amount of it has been utilized. Fly ash contains various valuable oxides such as SiO2, Al2O3, CaO, Fe2O3, and other oxides. These oxides have been mainly considered as a low cost material resource for the glass, glass-ceramic and ceramic industry. Red mud is another waste material generated by alumina production from bauxite.In this study, it is aimed to use coal fly ash, belonging to SiO2-Al2O3-Fe2O3 ternary system and red mud, in the development of low-cost glass, glass-ceramic and ceramic materials. For this purpose, all glass and glass-ceramic materials were firstly produced from coal fly ash with the addition of 5% red mud. The crystallization activation energy of glass sample was determined as 370 kJ/mol by using Kissinger equation. To produce the glass-ceramic samples, sintering method was applied to the powder glass samples. glass samples were milled until they passes through a sieve of 180 mm. Five wt % polyvinil alcohol (PVA) water solution was added to the grounded powder to determine the effect of the binder on the properties of sintered glass-ceramics. Glass powders were than cold pressed using 40 tons in a disc shape (10 x 5 mm). Cylindrical samples were dried in an electric oven at 383 K for 2 h. Finally, both bulk and pressed glass samples were crystallized by suitable nucleation and crystal growth heat treatments on the basis of DTA results. All the glass samples were heat treated at the crystallization temperature for 15, 30 and 60 min to investigate the effect of holding time at the crystallization temperature on the properties of glass-ceramic samples. X-ray diffraction (XRD) analysis of the sintered glass-ceramic materials revealed that the main crystalline phase was diopside (Ca(Mg,Al)(Si,Al)2O6). Scanning electron microscopy (SEM) investigations showed that tiny crystallites homogeneously dispersed in the microstructure of the produced glass-ceramic samples. It was observed that addition of polyvinyl alcohol (PVA) as a binder and the increasing of holding time at the crystallization temperature improved the properties of the sintered glass-ceramic materials. Ceramic materials were also produced from coal fly ash without any additives and binders using classical sintering technique. In sample preparation, a small amount of water was used to humudify the fly ash before compaction. The circular pellets of 10 mm diameter were uniaxially pressed at 40 MPa to achieve a reasonable strength. The sintering temperature varied between 1298 K and 1373 K to determine the effect of firing temperature on the properties of sintered fly ash samples. The heating rate was 10 K/min and the sintering time was 120 min for all samples. XRD analysis revealed that quartz (SiO2), mullite (Al6Si2O13), anorthite (CaAl2Si2O8), hematite (Fe2O3) and enstatite ((Mg,Fe)SiO3) phases detected in the fly ash sample disappeared; diopside (Ca(Mg,Al)(Si,Al)2O6) phase occurred in the ceramic sample at the sintering temperature of 1373 K. SEM investigations were conducted on the produced ceramic materials to investigate the microstructural evolution of the samples. Inspection of crystalline structures of ceramic samples indicated that the spherical crystallites dispersed randomly in the microstructure of the samples. The number of the crystallites and the average crystalline size increased with the increase in sintering temperature. It was observed that the crystal size and morphology appeared to play an important role in affecting physical and mechanical properties of ceramic materials. The density and the hardness values of the ceramic samples increased significantly with increasing sintering temperature while the water absorption and porosity values decreased. Overall results showed that it is possible to produce glass, glass-ceramic and ceramic materials from coal fly ash. It was also concluded that produced glass-ceramic materials had superior properties than the obtained ceramic materials. Keywords: Coal fly ash, glass, activation energy, glass-ceramic, sintering, ceramic
Effects of the axial flux modes on the parallel channel instablities in BWR's
Get PDFKaynar sulu reaktörlerdeki kaynama olayı bu sistemlerin, kararlılık açısından zayıf noktasını oluşturur. Nötronik ile kuple olan termalhidrolik olaylar problemi daha karmaşık yapar. Genelde BWR (kaynar sulu reaktör) kararsızlıkları güç salınımlarının şekline göre global ve yerel (zıt-fazlı) olarak sınıflandırılırlar. Global salınımlarda bütün bölgesel güç monitörlerinden alınan sinyaller hep birlikte salınım yapmaktadırlar; halbuki zıt-fazlı güç kararsızlıklarında monitörlerin kaydettiği sinyaller birbirilerine göre belirli bir faz ilişkisi içinde iken; ortalama güç monitörleri büyük boyutlardaki bu sapma olaylarını tespit edemezler. Reakör akısının temel mod dışındaki tüm uzaysal harmonikleri kritik altıdır. Temel prensiplerden başlayarak modal nokta kinetik denklemleri türetilebilir. Bu denklemler termalhidrolik pertürbasyonların bölgesel veya üniform olmayan davranışları nedeniyle kupledir. Reaktör operatörleri tarafından rapor edilen, çok sayıda beklenmedik güç salınım olayı vardır. Güncel reaktör kodları ile kararsızlıkların benzeşimleri iyi bir şekilde yapılmaktadır; ancak analitik modeller olayın fiziksel yorumunda daha yararlı olmaktadır. Reaktör kararlılık analizi için önerilen en eski modellerden biri paralel kanal kararsızlığıdır; bu isim uygulanan hidrolik sınır koşullarından kaynaklanır ve global kararsızlık analizinde kullanılabilir. Bu çalışmada sözü edilen model, temel akı moduna birinci eksenel harmoniği de ilave ederek geliştirilmiş, böylece yeni bir kararsızlık modu önerilmiş ve analizi yapılmıştır. Analiz lineer denklemlerle frekans domeninde gerçekleştirilmiştir. Kaynama sınırının hareketi, modal nokta kinetik denklemler üzerinden reaktivite geri besleme etkisi yaratmaktadır. Sonuçlar, net bir şekilde, akının doğal birinci eksenel harmoniğinin reaktör kararlılığını etkilediğini göstermiştir.Anahtar Kelimeler: BWR kararlılığı, zıt fazlı güç salınımları, modal kinetik denklemler.Thermalhydraulic stability is one of the major and extensively studied problems of systems operating under two-phase flow conditions; especially BWRs, steam generators and drum boilers are of concern due to safety issues involved. In BWRs there is an additional fact that complicates the problem: thermalhydraulic and neutronic instabilities are strongly coupled and feed each other. Existence of nonuniform boiling phenomenon in boiling water reactors makes these systems susceptible to instabilities. Customarily, BWR instabilities are classified as global (in-phase) and local (out-of-phase) according to the spatial dependency of the power fluctuations. In global oscillations, the output signals from all local power range monitors fluctuate in unison; whereas in out of phase power oscillations they record signals which are in a certain phase relation to each other while the average power range monitors maybe missing to detect any such anomaly of gross dimensions. There are several unexpected power oscillation events reported by the operators. Present numerical codes can simulate these instabilities rather well on event basis; but more general physical insight can be gained readily through analytical modeling. Two-phase flow is inherently unstable due to its thermal-hydrauIic nature. In BWR such instability is expected to be modified for the worse by the void feedback to reactivity mechanism. One of the earliest models applied to the problem is the parallel channel instability so called because of the applied hydraulic boundary conditions, which is used to account for global instability. In this case, the boiling boundary of a single hot channel begins to move, as affected by any perturbation. This action in turn will modify the reactor power which further provides the system with a feedback of reactivity. Under certain circumstances, if overlooked, this chain of events may lead towards an episode of growing instability resulting in a scram. On the other hand out-of-phase oscillations are usually thought to be linked with the spatial harmonics of the fundamental flux mode and can easily pass undetected. All spatial harmonics save the fundamental mode are subcritical. Starting from basic principles modal point kinetics equations can be derived. These equations are coupled due to the local or nonuni-form behavior of the thermal-hydraulic perturbations. When the diameter of the core is close to its height, the first azimuthal mode is the least subcritical flux harmonic, whereas in a slender core the first axial mode dominates the first azimutal mode with respect to criticality. The motion of the boiling boundary is quite likely to trigger the axial mode in any case. In this work parallel channel model is extended by including the effect of the first axial harmonic of the fundamental flux mode, and hence a new type of instability is proposed and studied. In other words this study constitutes a second order perturbation analysis of the parallel channel model. Thermal-hydraulics is represented with two-phase homogeneous equilibrium model equations. Modal point kinetics equations are derived allowing for modal interactions through the motion of the boiling boundary which in fact separates the core into two nodes, one being single phase subcooled region and the other the two-phase region where nucleate boiling takes place. In this way the modal point kinetics equations attain a nodal character as well. Analysis is performed with linearized equations in frequency domain. The motion of the boiling boundary explicitly feeds back to the modal point kinetics equations. The Froude number Fr and fuel-time constant are important parameters that strongly affect the density-wave instability. The Froude number effect is stronger in the case of with boiling boundary feedback than in the case of without the feedback The Ledinegg instability is not likely to occur due to the coupling of the boiling boundary feedback and the thermalhydraulic feedback. In general, a positive boiling boundary feedback coefficient yields a less stable system to the density-wave instability and vice versa. For a very small fuel-time constant, however a positive boiling boundary feedback coefficient would stabilize the system. This is explained by the fact that an increase in void fraction would cause a decrease in power that would suppress further increase in void fraction. Results clearly indicate that the inclusion of the axial flux harmonic narrows the region of stability on the appropriate parameter space, therefore our approach results in more conservative prediction regarding the core stability.Keywords: BWR stability, out-of-phase power oscillations,modal kinetics equations
Surface salinity determination for the Bosphorous with nuclear technique, a new analytical model and evaluation
Get PDFİstanbul Boğazı, Türk Boğazlar Sistemini oluşturan Marmara Denizi ve Çanakkale Boğazı’ndan daha özel bir önem taşımaktadır. Karadeniz’den gelen nispeten az tuzlu su üst akış olarak ve Marmara Denizi’nden gelen daha tuzlu su alt akış olarak İstanbul Boğazı boyunca iki katmanlı akışı oluşturmaktadır. Yoğunluk farkından dolayı iki katmanlı akış halinde boğaz boyunca akan tuzlu suda farklı akış rejimlerinden dolayı karışımlar meydana gelmektedir. Önemli bir dar su yolu olan İstanbul Boğazı’nın yüzey tuzluluk değerleri farklı lokasyon ve mevsimlerde nötron aktivasyon analiziyle tayin edilmiştir. Günümüzde çok çeşitli tuzluluk tayin yöntemleri kullanılmakla beraber literatürde, nötron aktivasyon analiziyle tuzluluk tayinine ilişkin bir yayına rastlanmamıştır. Nötron aktivasyon analizi element analizinde sıkça kullanılan bir nükleer teknik durumundadır. Tuzluluk tayini için kullanılan nötron akivasyon analizi yönteminde sodyum konsantrasyonu elde edilerek tuzluluk değerlerine gidilmiştir. Nötron aktivasyon analizi tuzluluk sonuçlarını kıyaslamak için konvansiyonel bir tuzluluk tayin tekniği olan uçurma yöntemi seçilmiştir. Nötron aktivasyon analizi tuzluluk sonuçlarının uçurma sonuçlarına yakınsadığı gösterilmiştir. Nötron aktivasyon analizi tuzluluk değerleri kullanılarak yere ve zamana bağlı matematik bir model geliştirilmiştir. Bu yeni model İstanbul Boğazı’na özgü parametreleri barındırmaktadır. Deneysel ve matematik model sonuçları FEMLAB 3.1 programında tek boyutlu difüzyon denkleminin çözülmesiyle oluşan simülasyon sonuçlarıyla kıyaslanmıştır. Deneysel ve teorik sonuçların iyi bir uyum içinde olduğu ortaya çıkmıştır. Böylelikle nötron aktivasyon analizi ile deniz suyu tuzluluk tayininin yapılabileceği ve önerilen analitik modelin güvenilirlikle kullanılabileceği gösterilmiştir.Anahtar Kelimeler: İstanbul Boğazı, tuzluluk, Na-24, uçurma yöntem, nötron aktivasyon analizi. Turkish Strait System consists of the Bosphorous, Marmara Sea and Dardanelles. The Bosphorous is the most interesting one because of its strategic role. The Bosphorous is a very important channel in which there are two flows with different salinities. The seawater of Black Sea passes by as the top layer flow and the seawater of Marmara Sea as the bottom layer because of the density difference. For years, the oceanographers work on the Bosphorous two- layer flow. This two-layer flow mixes along the Bosphorous causing salinity dispersion. To have an idea about the surface salinity dispersion, the salinity levels should be determined along the channel. The main aim of the experimental work is to measure the surface salinity level with neutron activation analysis. There are many studies to determine the salinity with different methods which are primarily based on physical and chemical principles. Physical salinity determination methods depend on refractive index, density and electrical conductivity measurements. On the other hand chemical methods are based on chlorinity determination. Beyond the physical and chemical methods there are alternative salinity measuring systems as well. These can be called as "alternative methods". In this study for the salinity determination, neutron activation analysis has been chosen as the experimental method but in the literature there is no study that deals with salinity determination by neutron activation analysis. Neutron activation analysis is a nuclear technique that has widely been used for the element analysis. In neutron activation analysis the sample is irradiated and the isotopes of the mother elemental is formed. The activated samples are usually gamma-active and detecting the gamma rays, the concentration and the element identification can be performed Neutron activation analysis can be named as the "new alternative method for salinity measurement". Initially, surface salinity of the Bosphorous was determined by the neutron activation analysis. On the other hand. evaporation technique which is a routine salinity determination method has been chosen. From the specified sample locations along the Bosphorous, the seawater samples have been collected and stored in polymer bottles. In order to determine the salinity of the samples, small amounts of samples were prepared. First of all, the water of the small samples about 100 cc. were evaporated and from the salt remaining after the water evaporation, again small samples have been prepared for the irradiation. The Bosphorous samples were irradiated with a reference salt sample. The salinity of the Bosphorous samples were determined with an equation which allows a comparison of the reference results with those of the irradiated samples. The results of neutron activation analysis have been compared with those of evaporation and literature. Neutron activation analysis results were in good accuracy with those of evaporation and the literature. With the neutron activation analysis results, a model which depends on position and time has been offered. The offered model was a product of two independent functions. The function depending on the position is an exponential and the time dependent one is a sinusoid. The parameters of the independent functions have been determined by neutron activation salinity results. They have been compared with those of evaporation. The model results were in good agreement with the evaporation ones. A theoretical comparison has also been chosen to be done with the results of neutron activation analysis which have been compared in many ways. For this comparison, a widely used computer programme. FEMLAB 3.1 (Finite Element Method Laboratory). has been selected. This programme has widely been used in many different studies where finite element method is valid. The suitable geometry for the Bosphorous has been formed and one dimensional diffusion equation has also been solved with the parameters of the Bosphorous. The values obtained from one dimensional diffusion equation that has been solved for the Bosphorous have been used in comparison with those of neutron activation analysis, evaporation and the mathematical model and it has been concluded that neutron activation results have been in good accuracy with those of the programme. It is also concluded that neutron activation analysis can be used to determine the salinity and mathematical model as well in relevance. A different study has been done in terms of the experimental work and the analytical model. Keywords: Bosphorous, salinity, Na-24, evaporation method, neutron activation analysis
Dağınık kontrol sistemleri için bir ağ tasarımı
Get PDFIn this study, modular, scalable and flexible control system architecture around a serial communication bus is proposed to fulfill control tasks. This bus is named K-Bus. One of the differentiating features of the proposed architecture is its capability to access different data on distributed modules in a single network transaction. Accessing multiple modules in a single transaction can be achieved both while data writing and data reading. Second differentiating feature of K-Bus is its capability to access predefined data without using module or data addresses. Network data collision avoidance is obtained by adding hardware components around RS?485 electrical layer to detect and avoid data collision. Package priorities are obtained with sender's identification number in the protocol, which is also used by the hardware layer to decide the winner in case of a collision. Two timer based locks are provided by the hardware layer blocks accessing to the bus during a transaction. One of the timers measures the time to end a whole transaction, while other timer keeps the time between different module or data cycles within the transaction. Modules replying to a high priority master will be able to produce their response in the same package cycle, without the need to wait for the next available slot. This allows high priority master modules to rapidly access to low priority modules within master module's priority. This feature enables predictable access times to resources in multi mastered networks. K-Bus also allows acknowledgement to be transferred in a single network transaction. High real time performance and relatively less processor resource consumption are key design requirements. Signals between K-Bus hardware and microprocessor are designed to allow interrupt driven software for network interfacing. Hardware layer is designed to transfer minimal load to the processor. It is capable of blocking interrupts to the processor for data packages that are not targeted to processor's module. An analyze have been performed to show that, in the given example, using hardware provided method to block interrupts for a transaction that does not involve the module would reduce the protocol related CPU load from 18.75% to 3%. Proposed architecture enables K-Bus to be used in different applications with variety of modules. K-Bus has following properties: different data from multiple modules can be accessed in a single transaction; multiple master modules can be present on the same data bus; hardware priority is given for data packages; data package collisions are detected at hardware level; erroneous or collided data packages are automatically resend by software; one data package can be broadcasted to more than one module; by the help of standard data concept, fundamental parameters can be reached without using module address; by the help of block data transfer, more than one data can be transferred in single K-Bus package and the overhead coming from package structure is reduced. The paper is organized as follows: first the underlying electrical layer of K-Bus, RS-485, is briefly compared to other real-time network electrical layers. In the following section, logical components of K-Bus are described. These are given in Figures 1 and 2. An analysis is done in this section to demonstrate the CPU savings using K-Bus?s interrupt disabling capability for packages that are not targeted to a module. In the next section a sample read package was shown in Figure 3. This sample read package demonstrates interrupt blocking capability and multiple data read capability. All K-bus packages have 9 fields: sender's identification, SID; receiver's identification, RID; command, CMD; package length, LEN; data fields DTA; optional acknowledge or not acknowledge, ACK/NAK; optional command data, CD; checksum CSM; and end of package, EOF. All K-Bus fields, sections except DTA, are of fixed length of 1 byte. Figure 3 shows all the sections except optional ACK/NAK. All K-Bus protocols start with sender identifier (SID) field. This field is used by the hardware layer to determine the priority of the sender. In case of data collision this field is used to decide the winner. Figure 3 also shows two hardware timers, HM and HM2, which are used to control transaction. In the later section a detailed electrical design of the given K-Bus interface is given and is shown in Figure 4. Last section discusses trade off of performance for real time capabilities in K-Bus design. Keywords: Real-time buses, distributed control systems, unmanned aerial vehicles.Bu çalışmada, K-Bus olarak adlandırılan bir seri iletişim yolu çevresinde ölçeklenebilir, esnek ve modüler bir kontrol mimarisi önerilmiştir. RS–485 elektrik seviyesine donanım ilavesi yapılarak güvenli ve yazılım yükü çok olmayan veri çarpışma önleme sistemi kurulmuştur. Verilen donanım modeli üzerinde bir veri paketi iletim protokolü tanımlanmıştır. Yüksek gerçek zaman performanslı, göreli olarak az işlemci gücü gerektiren bir mimari anahtar tasarım istekleridir. Önerilen mimarinin düşük işlem güçlü mikroişlemcilerde kullanılabilmesine imkân tanımak için K-Bus ilgilenilmeyen veri paketlerini belirleyerek işlemciye iletimi kesme ve bir paket iletimi boyunca hatta erişimi engelleme kabiliyetine sahiptir. Bu şekilde işlemciye yüklenen protokol ile ilgili yazılım yükü azaltılmıştır. Önerilen mimarinin ayırt edici özelliklerinden birisi, tek çevrim içerisinde farklı ağ modüllerinde bulunan verilere ulaşabilme kabiliyetidir. Bu özellik hem modüllere yazarken hem de modüllerden okurken kullanılabilir. K-Bus’ın diğer ayırt edici özelliği ise modül ve veri adresi kullanmadan, dağınık gerçek zamanlı ağ üzerinden, verilere ulaşabilme kabiliyetidir. Bu ayırt edici özelliklerinin yanı sıra, K-Bus, CAN ve Profibus gibi diğer modern gerçek zamanlı veri yollarında bulunan kabiliyetlere de sahiptir. Birçok konuşucu modül aynı veri yolu üzerinde bulunabilir. Bir veri paketi yayın olarak birden çok modüle iletilebilir. Veri paketleri için öncelik verilebilir. Paket çarpışmaları donanım düzeyinde önlenir. Hatalı veya çarpışan paketler yazılım tarafından tekrar yollanır. Anahtar Kelimeler: Gerçek zamanlı ağlar, dağınık kontrol sistemleri, insansız hava araçları
Investigation of hydraulic jump self-aeration process
Get PDFDoğal havalandırma, havadaki oksijenin suya olan transferi olarak tanımlanmaktadır. Doğal havalandırmayı sağlayan üç tane hidrodinamik süreç vardır: (1) hidrolik sıçrama, (2) su düşümü, (3) basamaklı kanallardan ilerleyen akım. Bu yapıların bir diğer ortak özelliği hidrolik yapılarda enerji kırıcı olarak kullanılmalarıdır. Bu noktadan hareketle, doğal havalandırma verimliliğiyle enerji kırılması arasında pozitif bir ilişkinin bulunması beklenmektedir. Bu çalışmada, bu hidrodinamik süreçlerden, hidrolik sıçrama olayı deneysel olarak incelenmiştir. Doğal havalandırmayı sağlayan bu hidrodinamik süreçlerde, termodinamiğin 1. prensibine dayanılarak, kaybolan enerjiden büyük ölçekli çevrilerin sorumlu olduğu ve bu çevrilerin yaptığı işin suya oksijen transferini sağlayan mekanizma olduğu yaklaşımında bulunulmuştur. Bu hipotezi desteklemek için laboratuvarda sistematik deneyler yapılmıştır. Deneyler, genişliği 0.5 m olan bir açık kanalda gerçekleştirilmiştir ve deneylerde arasında, ve arasında değerler almıştır. Transfer olan oksijen miktarıyla, kırılan enerji miktarı arasında kuvvetli bir ilişki bulunmuştur. Deneysel verilerin analizi sonucu, hidrolik sıçramanın havalandırma verimliliğini yük kaybının ve birim genişlikten geçen debinin fonksiyonu olarak tahmin eden bir formül geliştirilmiştir. Havalandırma verimliliği yük kaybının ¾’üncü, birim genişlikten geçen debinin ise ¼’üncü kuvvetiyle ilişkilendirilmiştir. Hidrolik sıçrama boyuncaki akım doğrultusundaki türbülans şiddeti eksponansiyel olarak azalan, türbülans kinetik enerjisi ise lineer olarak azalan bir fonksiyon sergilemiştir ve bu gidiş hidrolik sıçramadaki hava konsantarasyonu dağılımıyla uyuşmaktadır. Elde edilen deneysel bulgular, hidrolik sıçramdaki yük kaybının, ortamdaki türbülans kinetik enerjisini temsil ettiğini işaret etmektedir ve bu süreç yüzey yenilenme teoremini desteklemektedir. Anahtar Kelimeler: Doğal havalandırma, hidrolik sıçrama, su kalitesi, türbülans.Hydraulic jump takes place at the transition from supercritical regime to subcritical regime and characterized by highly turbulent flow, macro-scale vortices, kinetic energy dissipation and bubbly two-phase flow due to air entrainment (Leutheusser et al. 1973; Hager 1992). In the literature, studies about hydraulic jump aeration efficiency are very limited (Avery and Novak 1978; Wilhelms et al. 1981). The term self-aeration means transfer of oxygen from air towards water (Gulliver 1990) and it has important environmental and ecological implications for polluted streams. Aeration efficiency is calculated with a formula suggested by Gamenson (1957): where, E denotes aeration efficiency which has a range between 0, for no aeration, and 1, for total downstream saturation. Cu and Cd are dissolved oxygen concentrations at the upstream and downstream of a hydraulic structure, respectively and Cs is the mass concentration of dissolved oxygen under saturated conditions. Although, most of researches emphasized that turbulence structure is the main mechanism promoting to oxygen transfer, (Kobus 1991; Sene et al. 1994; Chanson 1996; Ervine 1998, El-Kamash et al. 2005) simultaneous measurements of turbulence quantities and dissolved oxygen are very limited. Indeed, hydrodynamic processes which ensure the self-aeration mechanism such as: (1) hydraulic jump, (2) plunging jet or water fall, and (3) stepped channels, have other common property: they are also used as energy dissipaters at hydraulic structures. The aim of this experimental study is to investigate aeration performance of hydraulic jumps in terms of energy loss approach and to reveal the turbulence?s important role in the process. Accordingly, experiments were carried out in a horizontal rectangular flume of glass sidewalls and concrete bottom 0.5 m wide, 0.45 m deep and 12.30 m long. Hydraulic jumps were generated downstream of a sluice gate placed 6.5 m distance from the beginning of the channel and location of the jumps were controlled by a tail gate installed at the outlet of the flume. Hydraulic jumps were grouped at seven different unit discharges with a range of and F1 took values between 2.1-6.4. During the experiments, a digital video camera was employed to determine the hydraulic jump and roller lengths. Dissolved oxygen (DO) measurements were conducted simultaneously at the upstream and downstream of the hydraulic jump with handheld oxygen meters (Model WTW Oxi 330i) which has a an accuracy of ±0.5% of oxygen concentration and ± 0.1% of temperature in the range of 0-19.99 mg/L DO and -5 to +50 oC of temperature. The air calibration technique was used in the experiments and the sampling rate of the oxygen probe was 1 Hz. In order to drop the DO content of the approaching flow, sodium sulfide and cobalt chlorite as a catalyst were added into chamber. Turbulence quantities were collected by a Nortek 10 Mhz type Acoustic Doppler Velocimeter (ADV) at 25 Hz frequency for two minute sampling time. The root mean squares (RMS) of the turbulent fluctuation velocities in the longitudinal, vertical and lateral directions, respectively, were determined by using WinADV32 software. Hydraulic jump turbulence structure was investigated at low Froude numbers: F1=1.9 and F1=3 because at higher Froude numbers, ADV does not make accurate measurements due to bubbly two-phase flow (Liu et al. 2004). Aeration efficiency took values between 0.01-0.1 and a strong correlation was found between the aeration efficiency and head loss indicating the large-scale surface renewal eddy's dominant role in the process. It was found that aeration efficiency is in proportion to the ¾ power of head loss and ¼ power of unit discharge. Turbulent characteristics exhibit decreasing exponential function through the hydraulic jump and this trend matches with the air concentration distribution. Moreover, the data reveals that head loss term represents the degree of turbulence related with the macro-scale vortices occurred in the roller length region. These findings are in agreement with the surface renewal theory, which states that the rate of mass transfer is a function of the surface renewal which is ensured by large-scale organized eddy motions near the free surface. Keywords: Self-aeration, hydraulic jump, energy dissipation, turbulence.
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