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Novel high precision current mode multiplier/divider circuit based on the translinear principle
Get PDFHer geçen gün kullandığımız elektronik cihazların daha da küçülmekte olduğu, daha küçük ve çok fonksiyonlu elektronik cihazlara olan ihtiyacın artarak devam ettiği göz önüne alındığında, devre tasarımcılarının devreyi oluşturan MOS tranzistorlardaki ikincil etkileri daha çok dikkate alması gerekliliği ortaya çıkmaktadır. Buradan hareketle gerçekleştirilen bu çalışmada, karesel işlemlerde kullanılan MOS translineer devrelerinin çıkış akım fonksiyonlarındaki küçülen boyutlar nedeniyle oluşan hatanın azaltılmasına yönelik önerilen yöntemle, karekök alıcı ve kare/bölücü devreler tasarlanmıştır. Tasarlanan bu devrelerin birlikte kullanılmasıyla yeni bir çarpıcı/bölücü devre önerilmiştir. Önerilen bu devrenin çıkış akım fonksiyonu, aynı MOS tranzistor boyutlarıyla kurulan klasik yapının çıkış fonksiyonuyla ve ideal fonksiyon ile karşılaştırılmıştır. Önerilen devrenin yüksek doğruluklu olduğu ve devrenin avantajları sunulmuştur. Ayrıca önerilen çarpıcı/bölücü devreyi oluşturan karekök alıcı ve kare/bölücü devrelerin bir VC kontrol gerilimiyle ayarlanabilir olması, önerilen devrenin çıkış akım fonksiyonunun da elektronik olarak ayarlanabilir olmasını beraberinde getirir. Bunun yanında önerilen çarpıcı/bölücü devreyle yüksek doğrulukta çıkış akım fonksiyonu elde edilebilmenin yanı sıra bu devrede emsallerine göre daha küçük boyutlarda MOS tranzistor kullanımı mümkün olmaktadır. Böylece kırmık üzerinde çok daha az yere ihtiyaç olacak ve yüksek frekanslarda çalışmaya olanak yaratılmış olacaktır. Önerilen çarpıcı/bölücü devrenin uygulaması olarak bir frekans katlayıcı devre de gerçekleştirilmiş ve önerilen devrenin, klasik yapılarla kurulacak olan devreye göre çok daha yüksek doğrulukla çıkış akımı ürettiği gösterilmiştir. Tüm analizler SPICE benzetim programında gerçekleştirilmiştir. Anahtar Kelimeler: MOSFET ikinci dereceden etkilerin azaltılması, karekök alıcı devre, kare/bölücü devre, çarpıcı devre, frekans katlayıcı.Electronic devices we use in our daily life are getting smaller from day to day and the need for smaller and more functional devices is increasing. This leads researchers to reduce chip area and power consumption, to develop new design techniques and to use smaller devices on the circuits. It is well known that the decrease in dimensions of MOS transistor in IC fabrication technology affects MOS transistor performance and the MOS transistor voltage current relationship changes from square law to linear. Therefore errors may occur in the output current function of the current mode circuits employing small sized MOS transistors. Nowadays, due to the decrease in dimensions of MOS transistors in IC fabrication, second order effects cause more errors in the MOS transistor performance. Actually drain current expression includes effects of W, L and VDS on effective threshold, velocity saturation effects, effective mobility dependence on normal field and channel length modulation. Especially, the short channel effects become more important in MOS transistors at channel lengths of about 1 m or less and require modifications to the MOS models as well as the circuits that are designed using these MOS transistors. Due to the second order effects small sized MOS transistors do not operate properly and therefore errors may occur in the output current function of the current mode circuits employing these transistors. The multiplication of two current signals is one of the most important operations in analog signal processing. Recently several CMOS multipliers have been reported and some of them are based on MOS translinear principle. Translinear circuit principle which was originally formulated for loops of bipolar transistors is generalized and the MOS translinear loop (MTL) principle is derived by Seevinck in 1991. MTL circuits are designed by applying MTL principle and used in synthesizing many nonlinear signal processing functions such as square rooting. Square root and squarer/divider circuit are two important structures of the MTL circuits and a multiplier/ divider circuit can be obtained by using both square root and squarer/divider circuit. In this paper, a method is proposed to reduce the errors generated by the second order effects in the current mode circuits employing MOS translinear loop. This method is applied to the current mode the square root and squarer/divider circuit. Thus high precision square root circuit and high precision squarer/divider circuit are proposed. Furthermore multiplier/divider circuit which contains these proposed circuits is designed and presented. Proposed circuits have been simulated with SPICE simulator using 0.35 µm CMOS technology parameters. The small signal and DC transfer characteristics of the current functions of the proposed circuit and the conventional circuit are observed. The DC transfer characteristics of the output current function of the proposed multiplier/ divider circuit shows approximately the ideal current function and the output current function of the conventional circuit is very different from the ideal curve. The output current function of the proposed circuits can be controlled by a control voltage. The small signal bandwidth of the proposed multiplier/divider circuit is about 50 MHz. Analog building blocks such as analog modulator, frequency doubler and etc. can be easily obtained using the proposed multiplier/divider circuit. The frequency doubler circuits are used in microwave, radar and wireless communication circuits frequently. So a frequency doubler circuit is designed as an application of the proposed multiplier/divider circuit and the transient analysis of this circuit is given in this study. Absolute error of the current function of the frequency doubler circuit is observed. The proposed method enables the use of much smaller transistors and the circuits to be designed are smaller than their counterparts. Thus they may be operated at much higher frequencies. The main advantages of the proposed circuit are reduced errors of the output current function, a smaller area on the chip, possibility of controlling the output current with the control voltage, operation at higher frequencies and more efficient power consumption. The proposed circuit is appropriate to be used for filtering in square root domain, fuzzy logic controllers, artificial neural networks, modulators, phase discriminators, adaptive filters, RMS-DC converters, sine/cosine synthesizers, cryptography systems etc. Keywords: reduction MOSFET's second order effects, square root circuit, squarer/divider circuit, multiplier circuit, frequency doubler
Determining the factors affecting E-reverse auction technology acceptance
Get PDFElektronik-açık eksiltme (e-açık eksiltme), satın alma yapacak organizasyonla iki veya daha fazla tedarikçinin davet edilmesi ile gerçek zamanlı ve çevrim içi olarak gerçekleştirilen elektronik ihale türüdür. E-açık eksiltme ile ilgili yapılan literatür taraması bu teknolojinin kullanımının gerek satın alma yapan firmalara gerekse de tedarikçi firmalara çok önemli avantajlar sağladığını ortaya çıkarmıştır. E-açık eksiltme teknolojisinin birçok avantajına rağmen bazı kullanıcılar tarafından bu sistem kullanılmamaktadır. Bu çalışmanın amacı e-açık eksiltme teknolojisinin kullanımına etki edecek faktörlerin belirlenmesidir. Bu amaçla oluşturulan araştırma modelinde, Teknoloji Kabul Modeli’nden algılanan fayda ve algılanan kullanım kolaylığı, Planlı Davranış Teorisi’nden sübjektif normlar ve algılanan davranışsal kontrol ve Yenilik Yayılma Teorisi’nden uyumluluk değişkenleri bir araya gelerek entegre bir model oluşturulmuştur. Modeli test etmek için Yapısal Eşitlik Modellemesi kullanılmış ve 40 farklı ülkeden e-açık eksiltme teknolojisini kullanan kişilerden toplam 156 anket toplanmıştır. Sonuçlara göre e-açık eksiltme teknolojisi kullanım niyetini sübjektif normlar, algılanan fayda ve algılanan davranışsal kontrol değişkenleri etkilemektedir ve kullanım niyeti açıklanma yüzdesi %76 olarak bulunmuştur. Sübjektif normların algılanan fayda ve algılanan davranışsal kontrol değişkenlerine göre daha güçlü bir etkisi vardır. Gerçek kullanım ise sadece kullanım niyetinden etkilenmektedir. Algılanan kullanım kolaylığı hem algılanan davranışsal kontrol hem de uyumluluk değişkeninden etkilenmesine rağmen algılanan fayda sadece uyumluluk değişkeni tarafından açıklanmaktadır. Anahtar Kelimeler: E-açık eksiltme, teknoloji kabul modeli, planlı davranış teorisi, uyumluluk.E-reverse auction is an online- and real-time auction between a buying company and two or more suppliers (Carter et al., 2004). Use of the e-reverse auction tool was first offered by FreeMarkets in 1999 and has been progressively adopted more intensively by firms since then. Using electronic-reverse auction (e-reverse auction) offers many advantages to buyers and companies such as price reduction, decrease in cycle time, standardization, increase in buyer productivity, creation of a more competitive environment and transparency in purchasing process (Carter et al., 2004; Bartezzaghi and Ronchi, 2003). E-reverse auction has benefits not only for buyers but also for suppliers. These are growing markets, accessed by system users all over the world, who are enabled to compare their own competitiveness in the market and follow up auctions by potential customers on the Internet. Besides, they can estimate their customers' needs and market trends by checking the e-reverse auctions' specifications and conditions for the products and services. Thus, suppliers can see not only areas for improvement but also their own needs for improvement (Emiliani, 2000; Mullane et al., 2001). Despite its great advantages, some users do not want to use e-reverse auction technology for the procurement of products or services. This study aims to understand factors affecting e-reverse auction usage in companies by integrating important theories. Up to now, so many models have been developed to explain acceptance of a technology. Technology Acceptance Model (TAM) is one of the most effective and widely used model. Although TAM is a useful model to understand and explain user behavior in information technology use, it is recommended to be extended by including human and social factors. Theory of Planned behavior (TPB) takes into consideration these factors. Therefore, TAM is integrated with TPB to create an integrated model which includes subjective norms as a social factor. Several studies have integrated TAM with TPB successfully. Besides, studies in the literature show that compatibility in Innovation Diffusion Theory (IDT) is an important variable to explain human behavior in technology acceptance. Moreover, technology compatibleness may expand the rate of adoption. Therefore in this study, compatibility is also integrated with TAM and TPB to form an extended model. In conclusion, to explain behavioral intention and actual use of e-reverse auction technology, two factors (perceived usefulness, perceived ease of use) from TAM, two factors (subjective norms, perceived behavioral control) from TPB and one factor (compatibility) from IDT are integrated to form an extended model. In this model, in addition to the current relations defined in TAM and TPB, new relations are added to the integrated model according to the literature. Data collected from 156 employees working in the procurement department of companies in 40 different countries were used to test the proposed research model. The model was tested using LISREL 8.54 with SIMPLIS project. According to the results, both measurement model and structural model fitted well to the data and all the model fit indices were within acceptable limits. In addition, convergent- and discriminant validities of the measurement model were also verified. The results of the structural model show that behavioral intention to use e-reverse auction is explained by subjective norms, perceived behavioral control, and perceived usefulness with an explanation rate of 76%. Among them, subjective norms have the highest impact on behavioral intention to use this technology. However the effect of perceived ease of use on behavioral intention to use is found to be insignificant. Another result of our study was that actual use of e-reverse auction technology is directly affected by behavioral intention to use. The effect of perceived behavioral control on actual use is found to be insignificant. The explanation rate of actual use is 14%. The lower explanation rate of actual use with respect to behavioral intention to use shows that potential users do not act according to their intentions. In addition, 69% of perceived usefulness is explained by compatibility. However, the effects of subjective norms and perceived ease of use on perceived usefulness were found to be insignificant. Another finding of this study is that a high proportion of perceived ease of use is explained by perceived behavioral control and compatibility. Among them, perceived behavioral control is the strongest predictor. We conclude with managerial implications and recommendations for possible future research. Keywords: Technology acceptance, electronic-reverse auction, theory of planned behavior, compatibility
Examination of welding distortion via applying thermal tensioning technique on austenitic stainless steels
Get PDFTürk Deniz Kuvvetlerinin modernizasyonu kapsamında İstanbul Tersanesi Komutanlığı/Pendik’te, Alman Abeking&Rasmussen ve Lürssen firmalarının konsorsiyumu ile Aydın Sınıfı mayın avlama gemileri inşa edilmektedir. Aydın Sınıfı mayın avlama gemilerinin, genel olarak dünya bahriyesindeki mayın avlama gemilerinden farkı, bu sınıf gemi inşasında kullanılan ostenitik paslanmaz çeliklerdir. Hali hazırda bu çeliği, Alman, İtalyan ve Kanada donanmaları kullanmaktadırlar. Anılan çeliğin, mayın avlama gemilerindeki kullanım amacı, manyetik olmamaları, deniz suyu korozyonuna dayanımlı olmaları ve yüksek mukavemete sahip olmalarıdır. Ancak manyetik olmayan ostenitik paslanmaz çeliklerin, ısı iletimlerinin kötü, buna mukabil ısıl genleşmelerinin yüksek olması, bu çeliklerin kaynak sonrası distorsiyonlarının, diğer çeliklere oranla, daha fazla olmasına neden olmaktadır. Bu durum, kaynaklı parçaların montajları esnasında, birleştirme problemlerine yol açtığından, her kaynak işleminden sonra kaynaklı parçalar, alevle düzeltme işlemine tabi tutulmaktadır. Bu çalışmanın amacı, İstanbul Tersanesi Komutanlığı/Pendik’te imali devam eden Aydın Sınıfı mayın avlama gemilerinin kaynak işlemi sonrasında meydana gelen distorsiyonlarını en aza indirmek amaçlı, kaynak esnasında uygulanan ilave ısı girdisinin bir yöntem olarak uygulanabilirliğinin ispat edilmesidir. İskoçya Strathclyde Üniversitesi’nde yapılan deneyler sonucunda, kaynak esnasında parçaya ilave ısı girdisi verilerek, Isıl Gerilme Tekniği uygulanmasının, T kaynaklı parçalarda, ondüle şeklindeki (buckling) distorsiyonlarını önlediği ve açısal çarpılmaları azalttığı ispatlanmıştır. Anahtar Kelimeler: Ostenitik paslanmaz çelik, ondüle distorsiyon, ısıl gerilme tekniği.Under the construction of German Mine Hunter Consortium (Abeking&Rasmussen and Lürssen), Istanbul Shipyard/Pendik is building Aydın Class Mine Countermeasures (MCM) Vessels for Turkish Navy. Aydın Class Mine Countermeasures (MCM) Vessels distinct from the other world navies because of their hull made of austenitic stainless steel. Nowadays, German, Italian and Canadian navies use this type of steel for ship hulls. Austenitic stainless steel has many advantages, like resistant to the corrosion of sea water, high yield point and non-magnetic structure. But these metals are having also disadvantages, like poor heat conductivity and high thermal expansivity. These disadvantages create more distortions than normal steels in welding conditions. Fittings of welded structure are very important section in shipbuilding, so geometry of welded structure need to be accurate. For straightening of distortions, flame straightening method is widely using in shipbuilding industry. Nowadays for quality products it is necessary to reduce price. Straightening of distortions via flame straightening method is more time, manpower and material consuming. So improving of straightening of distortions will be beneficial for budget of government. The aim of this thesis is to show benefits of usage of Thermal Tensioning Technique as a method of straightening of distortions for Aydın Class Mine Countermeasures (MCM) Vessels which are built at the Istanbul Shipyard/Pendik. Straightening of distortions via Thermal Tensioning Technique is examined for ship building industry as a preventing method with many academic researches. The stiffened panel made of austenitic stainless steel is used in the experiment of the Strathclyde University/Scotland. In specific condition under gas protection weld (MIG), thermal camera is used for distribution of heat, thermo couples are used for temperature measurement and laser appliance is used for distortion measurements. This paper focuses on the transient temperature fields created by the large heat inputs from arc welding which is the driving force for distortion in the plates. The use of a thermal imaging camera allows the thermal fields to be viewed on a whole-field basis. The use of a welding rig in this kind of investigation is essential as it allows parameters to be kept constant throughout. The moveable track is controlled by stepper motors such that the travel speed of the weld can be adjusted to a tenth of a millimeter per second. The torch is held static while the plate is pulled along at a constant speed by a series of pulleys and small stepper motors. The welding machine was connected to the computer and so all parameters could be entered via the computer so that all the variables could be controlled. The camera was held in place with scaffolding and the tripod for the camera fixed to a bracket that allowed movement in all directions. The CCD thermal imaging camera used here was the ThermaCam SC500 from FLIR systems. The camera is able to provide thermo graphic images in the infrared spectrum from 3 to 12μm. Following initial tests with austenitic stainless steel plates the amount of background radiation reflected from the plate to the camera was seen to be high. This led to the painting of the plates with a high emissivity, two-part zinc silicate primer made up in two parts. To verify thermal images thermocouples were also present at distances of 10, 20, 30, 40, 50, 60, 70, 80, 90 and 100mm. The laser appliance is used for distortion measurements. All of austenitic stainless steel plates are scanned before welding and again after welding. The data was used to compare the distortion effects under different restraint conditions. The distortion analysis was carried out using an automated laser scanner which was programmed to cover the whole plate recording data at intervals throughout the scan. The surface deformation data obtained from Labview program was evaluated on the graphics. Austenitic stainless steel plates are tested under four conditions. These are free welding, restrained welding, restrained and additional heat input welding, free and additional heat input welding. As a result, the experiments in Strathclyde University proved that Thermal Tensioning Technique prevent buckling distortions and improves angular distortions on stiffened welded plates. Usage of Thermal Tensioning Technique for Aydın Class Mine Countermeasures (MCM) Vessels and also other type of ship building industry is additional benefit and assistance for economy. Keywords: Austenitic stainless steels, buckling distortions, thermal tensioning technique
Betonun zamana bağlı deformasyonlarının tahmini
Get PDFIn this study, the main objective is to analyze the creep, the basic creep and the shrinkage deformations of normal and low strength concretes with or without mineral admixture and to investigate of the agreement between the results that have been gained using creep and shrinkage prediction methods and the experimental values of creep and shrinkage. Eight concrete mixtures at two water/binder ratios (0,55 and 0,70) were produced by using either only cement as a binder or by partially replacing it with silica fume, fly ash or ground granulated blast furnace slag. As the experimental values taken from concretes with and without mineral admixtures are compared, it was found that the mineral admixtures did not have a significant effect on the time dependent deformations of low strength concretes, whereas in normal strength concretes they caused the creep and shrinkage values to decrease. On the other hand, comparison was made between the experimental results and the creep and shrinkage values that were obtained by applying creep and shrinkage prediction models; ACI, CEB-FIP, B3, GL2000. The results show that the differences between the experimental values of shrinkage and the values that were predicted by B3 and GL2000 models were within the accepted intervals (±100 D) whereas the values predicted by CEB-FIP and ACI 209 were outside those intervals. The deviations of the values of creep function (elastic deformation + creep deformation divided by the applied stress) from the experimental results were found to be outside the accepted intervals (±33 D/MPa). Keywords: Creep, shrinkage, time dependent deformations, elastic deformation.Bu çalışmada, normal ve düşük dayanımlı, katkısız ve mineral katkılı betonların sünme, temel sünme ve rötre deformasyonları incelenmekte, rötre ve sünme tahmin modelleri kullanılarak elde edilen sonuçların, deneysel rötre ve sünme değerlerine uygunluğu belirlenmektedir. Mineral katkılar olarak silis dumanı, uçucu kül ve öğütülmüş yüksek fırın cürufu, rötre ve sünme deformasyonlarının deneysel değerlerinin tahmin edilmesi için ise ACI, CEB-FIP, B3 ve GL2000 modelleri kullanılmaktadır. Mineral katkılı ve referans betonlardan elde edilen deneysel sonuçlar karşılaştırıldığında, mineral katkıların düşük dayanımlı betonların zamana bağlı deformasyonları üzerinde belirgin bir etkisi olmadığı, buna karşın, normal dayanımlı betonlarda sünme ve rötre değerlerinde düşüşe neden olduğu belirlenmektedir. Rötre ve sünme tahmin modelleri ile elde edilen değerler analiz edildiğinde, B3 ve GL2000 modelleri yardımıyla elde edilen rötre tahmin sonuçları ile deneysel sonuçlar arasındaki farkların kabul edilen sınırlar içerisinde kaldığı; sünme fonksiyonu tahmin değerlerinin kabul edilir sınırları aştığı görülmektedir. Anahtar Kelimeler: Sünme, rötre, zamana bağlı deformasyonlar, elastik deformasyon
HZSM-5 destekli katalizörler ile metanın aromatizasyonu reaksiyonu
Get PDFConversion of methane into useful chemicals has been and still is a subject of worldwide renewed efforts. Several routes have been explored over the previous two decades. In addition to methane conversion into syngas, which is of real industrial significance, more ambitious routes such as direct conversion into oxygenates, oxidative coupling of methane and conversion of methane into benzene and hydrogen were explored. The latest developments were concerned with the conversion of methane into benzene and hydrogen in the presence of various transition metal oxide catalyst precursors. Mainly molybdenum and rhenium based active components deposited over HZSM-5 were investigated. In the case of molybdenum-based catalysts, the activation procedure was scrutinized, and it was generally agreed that molybdenum was converted, in the presence of methane, under the reaction conditions, into molybdenum carbide. The reaction proceeded via the formation of C2H2 (or C2H4) over the molybdenum carbide species and further cyclization and aromatization would occur over the acid sites. The present study was conducted to understand the different aspects of the aromatization reaction of methane under non-oxidative conditions. In an effort to understand the specific effect of external acid sites on the catalytic performance, the activities of catalysts with molybdenum supported on HZSM-5 and surface dealuminated HZSM-5 were compared under two different reaction conditions. Molybdenum carbide associated with a surface dealuminated HZSM-5 appeared to be more active and more stable than when associated to the parent HZSM-5, contrary to the expectation, in the conversion of a 1:1 mixture of methane and argon at atmospheric presssure and low flow rate. By contrast, under a low methane partial pressure and a high flow rate, the expected order of activity was found; the carbide associated with the parent zeolite showed a significantly higher activity. The discrepancy shown by the results of the two sets of experiments was interpreted in terms of a more rapid deactivation of the most active catalyst under a high methane load, resulting in a lower apparent activity. The higher stability of the carbide associated with the surface dealuminated zeolite resulted from the elimination of the surface sites which produce bulky aromatics with low vapor pressure leading to a rapid clogging of the pore mouths of the zeolite, in line with the general background on zeolite catalysis. Treatment with CaC2,, a possible carburization agent, in addition to carburization with a CH4/H2 mixture as was routinely carried out in the previous studies, enhanced the activities of the catalyst. The higher dispersion of Mo2C species in the zeolite framework by preferred reaction of CaC2 with (MoO2)2+ or (Mo2O5)2+ species was proposed as a plausible explanation for the higher activities. Induction period seen in the case of the sample prepared without precarburization decreased with CaC2 treatment This result shows that carburization of MoO3/HZSM-5 catalyst took place to a considerable extent prior to the reaction with CaC2 treatment. All samples, precarburized with CaC2 or CH4/H2 mixture or carburized in the course of the reaction, deactivated in a similar manner after the variable induction period. In order to study the influence of the catalyst support on the growth and location of the molybdenum carbide and coke precursors in this reaction, the activities of molybdenum catalysts supported on three HZSM-5 samples, which differ in their morphology and aluminum content, were compared and it was seen that the rates of conversion of methane into hydrocarbons appeared not to vary simply with the number of acid sites as usually expected. Turnover frequencies of the surface acid sites, calculated using the Si/Al ratios determined from XPS measurements, were observed to vary linearly with the external surface area, indicating that transport phenomena were determining the overall rate. In an effort to see the possible effect of the acidity of the inorganic carrier, calcium and hydrogen forms of MFI were used as support for rhenium and molybdenum catalysts. HZSM-5-supported molybdenum and rhenium catalysts were observed to be more active for the aromatization of methane, compared to those supported on CaZSM-5. As evident from the formation of aromatics, the reaction also proceeded, but to a lower extent on CaMFI-supported molybdenum and rhenium catalysts, which were shown to have only residual acidity. Although highly dispersed rhenium is expected to form over CaZSM-5, lower conversion values obtained with CaZSM-5-supported rhenium catalyst underline the importance of acid sites on the formation of aromatics, in line with the commonly accepted bifunctional mechanism. Keywords: Aromatization, methane, molybdenum, rhenium, ZSM-5-supported catalyst, activity. Bu çalışmada ZSM-5 destekli molibden ve renyum katalizörler üzerinde ve oksijensiz ortamda metanın aromatizasyonu reaksiyonu araştırılmıştır. Katalizörlerin deaktivasyondan en az etkilendiği koşullarda (düşük metan kısmi basıncı, yüksek akış hızı) elde edilen aktivite, gerçek aktivitedir ve katalizörler bu koşullar altında incelendiklerinde, seçici yüzey dealüminasyonu ile katalizörün aktivitesinin azaldığı gözlemlenmiştir. Bunun nedeni dış yüzey dealüminasyonu sonucunda katalizörün aktif merkez sayısının azalmasıdır. Yüzeydeki aktif merkezlerin eliminasyonu, gözenek ağızlarının tıkanmasına neden olan yüksek aromatik bileşiklerin oluşumunu kısıtladığından, yüzey dealüminasyonu sonucunda katalizörün kararlılığı artmıştır. CaC2 ile ön işlem sonucu elde edilen düşük indüksiyon ve tmaks süreleri, CaC2 ile işlemin katalizörü reaksiyon öncesi karbürize ettiğinin göstergesidir. CaC2 ile hazırlanan örnek ile elde edilen yüksek aktivitenin, (MoO2)2+/(Mo2O5)2+ ile CaC2’ün seçimli reaksiyonu sonucunda zeolitin iskelet yapısında daha iyi dağılmış Mo2C’ün oluşumundan kaynaklandığı düşünülmektedir. Hangi yöntem ile karbürize edildiğine bağlı olmaksızın tüm örnekler, benzer şekilde deaktive olmuştur. ZSM-5’in dış yüzey asit merkezi başına düşen maksimum reaksiyon hızları zeolitin dış yüzey alanı ile doğrusal olarak artmıştır. Bu durum, ZSM-5 morfolojisine bağlı olarak değişen kütle taşınım sınırlamalarının katalitik performans üzerinde önemli bir etkisi bulunduğunu göstermiştir. Asit merkez içeren hidrojen formundaki Re/Mo-ZSM-5 katalizörlerine oranla kalsiyum formundaki Re/Mo-ZSM-5 katalizörleri ile çok daha düşük aktivite değerleri elde edilmiştir. Kalsiyum formundaki ZSM-5 örneğinde C2 bileşenlerinin aromatizasyonunu gerçekleştirebilecek asit merkezler bulunmamaktadır. Metanın aromatizayonu reaksiyonunda yüksek aktivitede bir katalizör hazırlanabilmesi için ZSM-5’in yeterli miktarda asit merkezlere de sahip olması gerektiği belirlenmiştir. Anahtar Kelimeler: Aromatizasyon, metan, molibden, renyum, ZSM-5, aktivite. 
Investigation of wake characteristics of a fishing boat suitable for Turkish waters
Get PDFPervane düzlemindeki iz katsayısı dağılımının tespiti; kavitasyon olayı, titreşim ve gürültü bakımından önemlidir. Bir geminin pervane düzlemindeki iz katsayısı dağılımı, model iz deneyleri yapılarak belirlenebilir. İz deneylerinde, gemi model deney laboratuvarlarının sahip olduğu teknolojiye göre Kempf halkaları, Pitot tarağı, "Hot Wire (sıcak tel)-Hot Film (sıcak film)" ve lazer anemometrelerinin kullanıldığı yöntemler uygulanmaktadır. Bu yöntemlerden en çok Pitot tarağı (iz tarağı) yöntemi kullanılmaktadır. Bu yöntemde, Pitot tüplerinden oluşan bir iz tarağı kullanılarak, pervane düzleminin belirli noktaları için pT toplam basınç ve pS statik basınç değerleri ölçülmektedir. Sonra, ölçüm yapılan her bir nokta için pD dinamik basınç değerleri elde edilmektedir. Dinamik basınç değerlerinden ise Bernoulli denklemi kullanılarak eksenel su hızları hesaplanmaktadır. Bu çalışmada, Türkiye sularına uygun balıkçı gemilerinden 148/1 kodlu ana balıkçı gemisinin 148/1C kodlu modelinin; yüklü (1.00xT) durumda, üç farklı model hızında gerçekleştirilmiş olan iz deneylerinin bilgisayar destekli analizleri yapılmıştır. Söz konusu model, 3/25 ölçek değerinde ve ahşap malzemeden inşa edilmiştir. Deneyler, ana geminin 8, 10 ve 12 knot hızlarına karşılık gelen model hızlarında ve model; takıntısız, meyilsiz ve trimsiz olarak gerçekleştirilmiştir. Model ile gemi arasındaki Reynolds benzerliğinin sağlanabilmesi için, modelin 9.5'inci postasına 2 mm çapında bir tel türbülans yapıcı olarak yerleştirilmiştir. Yapılan analizler sonucunda, üç farklı hız için iz karakteristikleri belirlenmiştir. Ayrıca, nominal iz katsayısının hıza göre değişimi elde edilmiştir. Son olarak, elde edilen bulgular maddeler halinde sıralanmıştır. Anahtar Kelimeler: İz deneyi, nominal iz katsayısı, balıkçı gemisi.The wake field of a ship is determined with wake measurement tests. It is so important to know the distribution of wake coefficient on the propeller disk plane in point of cavitation, vibration and noise. Wake field measurements on the propeller disk plane by means of Pitot tubes or electronic pressure transducers are to investigate wake quality and hull efficiency. The Pitot tube (named after Henri Pitot in 1732) measures a fluid velocity by converting the kinetic energy of the flow into potential energy. The method of Pitot tubes or the method of wake comb is used widely. In this method, by using a wake comb consists of Pitot tubes, the values of total pressure pT and static pressure pS are measured at the certain points of propeller disk plane. Later, the values of dynamic pressure are obtained for these points. By using the values of dynamic pressure in the Bernoulli equation, the axial velocities of water flow are calculated at all the points. In this study, a computer aided analysis of the wake measurement tests for the model 148/1C of the parent fishing boat 148/1 was performed. All the tests had been carried out at Ata Nutku Ship Model Testing Laboratory operating within the Faculty of Naval Architecture and Ocean Engineering at Technical University of Istanbul (Soygur and Taseli, 1981). In the tests, a model 148/1C of the parent fishing boat 148/1 had been used. The model 148/1C is made of wood and its scale value is 3/25. This model had been towed in the loaded condition for the situations of no appendages, no inclined and no trim. The wake measurement tests had been carried out at the model speeds corresponding three different ship velocities in 8, 10 and 12 knots. Thus, the distribution of wake coefficient according to ship velocity was studied. In the system of manometer range and wake comb, Pitot tubes connected to a shaft with a hydrofoil. This part is called "Wake Comb". The pressures on Pitot tubes are evaluated with a manometer range. The wake comb is connected to the manometer range with the elastic and transparent pipes. The levels of the manometer range are immobilized with the electromagnetic valves in the testing region. Thus, it is possible to have enough time for measurement. Air in the system is emptied with a vacuum container and a small pump and also the manometer range is calibrated at zero situations. In the tests, two different types of the wake comb had been used. Each wake comb consists of eight tips. Total pressure is measured with four tips. Static pressure is measured with other tips too. Thus, by carrying out the measurements at the 456 points, the values of total pressure and static pressure had been obtained. Then, the values of dynamic pressure were calculated as follows: r: Radial coordinate, q: Angular coordinate By using the values of dynamic pressure in Bernoulli's equation, the axial velocities of water flow were also calculated as follows: g: Acceleration of gravity Later, the wake coefficients were obtained as follows: vm: Model speed, vA: Axial velocities of water flow on the propeller disk plane. The manometer range had been fixed with an angle of 508 in order to read the values of pressures in enough sensitivity. Then, the values for vertical situation had been calculated by multiplying by sin (508) (0.76604). The obtained results are as follows: 1. The values of wake coefficient in a big region of the propeller disk plane are less than 0.1 for each velocity. 2. The values of wake coefficient increase suddenly, in other words the water velocities decrease suddenly, in a region of the propeller disk plane near the aft body for each velocity. 3. The region in which the values of wake coefficient increase suddenly becomes narrow if velocity increases. 4. The nominal value of wake coefficient decreases if velocity increases. 5. Some solutions must be obtained to accelerate water flow in the region defined with q = [1508 - 2108]. The optimum one from these solutions must be selected. 6. The wake measurement tests according to block coefficient, the ratio of length to beam and the ratio of beam to drought must be carried out systematically to determine the wake characteristics of the Fishing Boats Series of ITU. Keywords: Wake measurement test, nominal wake coefficient, fishing boat.
Enzymatic polymerization and biodegradation of polylactic acid
Get PDFBu çalışmada, enzimatik halka açılımı polimerizasyon yöntemi kullanılarak polilaktik asit sentezi gerçekleştirilmiş ve elde edilen polimerin proteaz enzimi ile biyobozundurulması incelenmiştir. Candida cylindracea lipaz katalizörlüğünde laktitten halka açılımı reaksiyonunun optimize edilmesi amacıyla, reaksiyon şartları değiştirilerek farklı sıcaklıklarda gerçekleştirilen polimerizasyon işlemlerinde, reaksiyona giren Candida cylindracea lipaz konsantrasyonuna bağlı olarak oluşan polilaktik asitin molekül ağırlığı, polidispersitesi ve dönüşüm verimi değerleri tespit edilmiştir. Elde edilen polilaktik asit, proteaz DSM enzimi ile biyobozundurma işlemine tabii tutulmuş ve polimerin yapısındaki ester grupları ile biyobozundurma mekanizması takip edilmiştir. Biyobozunma süresince molekül ağırlığı kaybı izlenmiş ve orjinal polimerin morfolojik ve termal geçiş özelliklerinin değişimi incelenmiştir. Candida cylindracea lipaz kullanılarak laktitin halka açılımı polimerizasyonunda, yüksek molekül ağırlıklı polimer sentezinde, en uygun reaksiyon şartı, sıcaklığın 80ºC ve lipaz konsantrasyonunun ağırlıkça %4 olduğu durum olarak belirlenmiştir. Bu sıcaklık değerinde, sabit enzim konsantrasyonunda, sürenin artmasıyla birlikte molekül ağırlığı ve monomer dönüşümü belirgin bir şekilde artış göstermiş, polidispersite değerleri yaklaşık 1.5 civarında olan polimerler sentezlenmiştir. Proteaz DSM ile yapılan biyobozundurma işlemi sonucunda ise, ilk 15 günlük süre içersinde, polimerin bozunmasına ilişkin molekül ağırlığı kaybı son derece düşük seviyede olduğu, bu süreden sonra molekül ağırlığı kaybında belirgin bir artış gözlenerek 90 günlük süre sonunda %23’lük bir kayıp meydana geldiği saptanmıştır. Biyobozundurma işlemine tabii tutulan polimerin çeşitli özellikleri incelenerek orijinal polimer ile arasındaki farklar tespit edilmiştir. Anahtar Kelimeler: Lipaz, halka açılımı polimerizasyonu, biyobozunma.Because of its biocompatibility and degradability to non-toxic products, polylactic acid (PLA) based polymers and copolymers have been employed in novel applications, such as absorbable bone plates, artificial skin, tissue scaffolds, and carriers of drugs for controlled release systems. Enzymatic polymerization using lipase has been receiving much attention as one of the new methodologies that provide biodegradable polymer synthesis without toxic catalysts. Lipases are the most versatile biocatalysts because they can be applied to the synthesis of a wide range of substrates with a high stereospecificity and enantioselectivity. In general lipases used in polyester synthesis are of mammalian (Porcine pancreatic lipase (PPL)), fungal (Candida antarctica lipase B (CAL)), or bacterial origin (Pseudomonas cepacia (PCL)). The hydrolytic degradation of PLA polymers has been extensively investigated. It is now well know that degradation of macroscale PLA devices is heterogeneous: it is faster inside than at the surface. This phenomenon has been assigned to an internal autocatalytic effect of carboxyl end groups. Biodegradation of PLA polymers in soil under natural conditions has also been examined. It was reported that degradation by-products resulting from the hydrolytic degradation can be totally assimilated by microorganisms such as fungi, bacteria, or earthworms, thus indicating that PLA polymers are environmentally benign materials. Hydrolysis reactions may be catalyzed by enzymes known as hydrolases, which include proteases, esterases, glycosidases, and phosphatases, among others. This class of enzymes comprises cell-derived proteins that are responsible for the catalysis of several reactions in the human body. The degradation of PLA in the presence of enzymes has been investigated by many groups. Early in 1981, it was found that proteases showed strong effects on PLLA degradation. In this work, polylactic acid was prepared by lipase catalyzed ring opening polymerization of lactide. Enzymatic polymerization was performed with Candida cylindracea lipase, enzyme and the effect of enzyme concentration, temperature and time parameters on polymerization mechanisms were investigated in detail. A typical procedure for the polymerization of lactide using Candida cylindracea lipase is described as follow. To a mixture of lactide (2.50 mmol) and toluene (0.72 mL) at 80 and 100oC was added Candida cylindracea (2 and 4 wt.-%) under nitrogen atmosphere at different time (1-7 days). After the reaction, the enzyme was removed by filtration and the polymer was isolated by precipitation in choloform. Then degradation mechanisms were examined. PLLA films were prepared using a solvent casting method. One gram of PLLA was dissolved in 20 mL of chloroform while mixing vigorously at room temperature. The dissolved solution was poured onto a leveled Teflon film coated glass plate, spread evenly with a bent glass rod and then allowed to dry for about 24 h at room temperature. The resultant film was peeled from the casting surface. Each of polymeric films were placed in test tubes and dipped in 5 mL of DSM protease solution (50 mM Tris-HCl buffer, pH 8.6). The test tube was sealed and kept constant at 37°C for a predetermined period and enzyme solutions were added every 48 h so that enzyme activity remained at a desired level throughout the experiment. The time course of the weight loss and enzymatic degradation were evaluated and the appearance of the samples was examined. It was found that the L-lactide was polymerized in bulk by Candida cylindracea lipase between 3 and 7 days, in a temperature range 80-100oC to yield the polylactide with an Mw of up to 69800 g.mol-1. Both the conversion and Mw of the resultant polymeric fraction increased with increasing reaction time 3 days to 7 days respectively at constant lipase concentrations. The chemical structures of the degraded polymer samples were characterized using DSC, FTIR and XRD. The molecular weight of the polymer samples after enzymatic degradation process with protease DSM, decreased 25% after 2 months degradation period. PLA polymers investigated in this study showed degradation as illustrated by the decrease in molecular weight, disappearance of amorphous phase and formation of micro fractures and acicular structures. After detailed evaluation of biocompatibility and toxicity of PLAs, it is expected that they will take the petroleum-based plastics' place in future. Keywords: Lipase, ring opening polymerization, biodegradation
Discrimination of extra virgin olive oil with chemometric methods according to their aroma profiles
Get PDFBu çalışmada, ülkemizin kalite ve ekonomik açıdan en önemli yağlık zeytin çeşitleri olan Ayvalık ve Memecik tipi zeytinlerinden elde edilen naturel sızma zeytinyağlarının aroma bileşenlerine göre karakterizasyonu ve sınıflandırılması kemometrik tekniklerden PCA (temel bileşen analizi) ve LDA (doğrusal ayırma analizi) yardımıyla gerçekleştirilmiştir. Zeytinyağlarında, lipoksigenaz metabolik yolu ile meydana gelen hekzanal, E-2-hekzenal, hekzanol, Z-3-hekzenol ve E-2-hekzenol bileşenlerinin aroma profilini oluşturan başlıca uçucu bileşenler olduğu saptanmıştır. Ayvalık tipi zeytinyağlarında terpenlerden limonen ve undekan tespit edilirken Memecik tipi zeytinyağlarında α-kopaen tespit edilmiştir. Ayvalık ve Memecik tipi zeytinyağlarının, hekzanal, E-2-hekzenal, E-2-hekzenol, 3-etil-1,5-oktadien ve linolenik asitten meydana gelen C6 bileşikleri toplamı pik alanı yüzdeleri arasındaki fark istatistiksel olarak önemli bulunmuştur. İki hasat sezonundan alınan örnekler ile yapılan çalışmada, hasat sezonları arasında E-2-hekzenal, Z-3-hekzenol, Z-3-hekzenil asetat, hekzil asetat ve linolenik asitten meydana gelen C6 bileşikleri toplamı pik alanı yüzdeleri arasındaki farklılıkların istatistiksel olarak önemli olduğu saptanmıştır. Memecik tipi zeytinyağlarının C6 bileşiklerinden E-2-hekzenal, hekzanol ve E-2-hekzenol pik alanı yüzdelerinin Ayvalık tipi zeytinyağlarına göre daha yüksek olduğu tespit edilmiştir. Temel bileşen skor grafiğinde, Ayvalık ve Memecik tipi zeytinyağlarının iki ayrı grup oluşturduğu gözlenmiştir. Doğrusal ayırma analizi sonuçlarına göre zeytinyağlarının doğru gruplara atanma oranı % 100 olarak bulunmuştur. Anahtar Kelimeler: Zeytinyağı, aroma, Ayvalık zeytinyağı, Memecik zeytinyağı, kemometrik analiz.Olive oil is obtained from the fruits of olive trees (Olea europaea L.) by mechanical procedures. In terms of value, it accounts for 30 % of the world trade in edible vegetable oils. Mediterranean members of the EU produce approximately 75 % of the world’s olive oil. Turkey, not member of EU, is the world’s 5th largest producer of olive oil (% 5). The consumption of olive oil, the main component of the Mediterranean diet, is increasing because of its nutritional quality and health effects. The beneficial effects of olive oil are attributed to its high content of monounsaturated fatty acids and antioxidants. Olive oil consists of triacylglycerols (98 %) and minor compounds (2 %). Tocopherols, phenols, sterols, hydrocarbon and flavor compounds are minor compounds found in olive oil.The quality of monovarietal olive oil varies on environmental (climate and soil), agronomic (irrigation and fertilization), cultivation (harvest and ripeness) and technological (storage and extraction) factors.The EU allows labelling of food products with growing areas, which could claim a typical quality based on geographical origin of product. Oils are certified with Protected Designation of Origin (PDO) label improving the commercial value of products. Since PDO olive oils have a much higher market price, they are subjected to frauds. To prevent frauds, the reference database are needed. Therefore, characterization of olive oils with respect to cultivar and geographical origin becomes more important in olive oil industry. The chemometric methods such as principal component analysis (PCA) and linear discriminant analysis (LDA) are the methods utilized for the characterization and discrimination of olive oils according to the cultivar and geographic origin. In recent years, the numerous studies have conducted on the chemometric characterization of olive oils using physical and chemical parameters such as fatty acids, sterols, phenols and volatiles. Aegean Region, Marmara Region, Mediterranean Region and Southeasth Region are the olive growing region in Turkey. Aegeon region provides around 75-80 % of total olive oil production. Memecik and Ayvalık are the major cultivars grown in Aegean Region. In literature, the number of studies regarding the characterization of Turkish olive oils are limited. Only few chemical parameters (fatty acids and sterols etc.) were evaluated. Moreover, the chemometric classification of Turkish olive oils were performed only with respect to fatty acid composition.The characteristic aroma of olive oil depends on the volatile compounds. The C6 compounds (aldehydes, alcohols and esters) are enzimatically produced from linoleic and linolenic acids through the so-called lipoxygenase (LOX) pathway. Hexanal, hexanol and hexyl acetate derive from linoleic acid, whereas Z-3-hexenal, E-2-hexenal, Z-3-hexenol and Z-3-hexenyl acetate derive from linolenic acid. The C6 compounds are responsible for the “green” note. The profile of volatiles varies according to the enzymatic activity, which depends on the external factors such as environmental, cultivation and technological ones. To evaluate olive oil aroma, several methods are developed. Solid phase microextraction (SPME) is simple, fast and solvent-free technique integrating sampling, extraction and sample introduction into a single run. A short, thin, solid rod of fused silica, coated with adsorbent polymer is called SPME fiber, which is the same type of chemically inert fused silica used to make capillary GC columns.Various studies have been carried out on the characterization of volatile profile of virgin olive oils. Previous studies were mainly done on Spanish, Italian and French cultivars. However, there is a lack of data defining the volatile profile of Turkish olive oils. The aim of this study was to characterize the profile of volatiles of Turkish olive oils obtained from Ayvalık and Memecik cultivars. The objective of this study was also to classify olive oils with respect to cultivar using chemometric methods. Keywords : Olive oil, aroma, Ayvalık olive oil, Memecik olive oil, chemometric analysis
Interrelationship between the variability of Northern Hemispheric total ozone, temperature and eddy heat flux
Get PDFToplam ozonun en yoğun olarak bulunduğu 15km ile 55 km arasındaki orta atmosfer, dinamik etkilerin de en fazla görüldüğü bölgedir. Brewer-Dobson sirkülasyonu, orta atmosferdeki ozon taşınımını etkileyen temel çevrimdir. Girdap ısı akısı, Brewer-Dobson sirkülasyonu olarak da bilinen, kalan sirkülasyonun, düşey bileşenidir ve dinamik bir bakış için kullanılan rezidüel ortalama teorinin temelini oluşturmaktadır. Bu çalışmada, toplam ozon değişiminin orta atmosferdeki sıcaklık ve girdap ısı akısı ile etkileşimi dinamik olarak incelenmiştir. Brewer-Dobson sirkülasyonunun matematiksel olarak modellenmesinde, dönüştürülmüş Eulerian ortalama akış denklemlerinden yararlanılmıştır. Çalışmada sirkulasyonun hesaplanmasında 1998-2007 yılları arasındaki ECMWF-ERA- Interim re-analiz verilerinden, toplam ozon değerleri için ise TOMS, OMI uydu verileri ile ECMWF ozon karışma oranı verilerinden yararlanılmıştır. 1998-2007 yılları arasında Kuzey yarıküre ozon değişimleri incelendiğinde, yukarı orta enlemlerde ve kutup bölgesinde ozon miktarının tropiklerden daha fazla olduğu görülmüştür. Orta ve yukarı enlemlerdeki toplam ozonun değişmini de etkileyen birincil mekanizma olan sirkülasyonun, yıl içinde en kuvvetli olduğu dönem Ocak ayı, etkilerinin en az görüldüğü dönem ise Temmuz ayıdır. Gözönüne alınan periyotta toplam ozon değeri ortalama olarak tropiklerde 240DU değerinden kutup bölgelerinde 420 DU değerine kadar değişmektedir. Orta enlemlerde ise 260-360DU arasında değişmektedir. Planeter aktivitenin değişiminin en fazla görüldüğü 100mb seviyesi için orta enlemlerdeki toplam ozonla, sıcaklıklar arasında yakın bir ilişki elde edilmiştir. Benzer şekilde, tropiklerdeki girdap ısı akısının orta enlemlerdeki sıcaklıkları, orta enlemlerdeki girdap ısı akısının da kutuplardaki sıcaklıları kontrol ettiği görülmüştür. Anahtar Kelimeler: Stratosferik ozon, Brewer-Dobson sirkülasyonu, ozon azalması.Ozone levels in the atmosphere have significantly decreased over the last three decades not only over Polar Regions but also over mid-latitudes. This finding led to an intense debate on the characteristics of the stratospheric dynamics and atmospheric transport. Since most of the atmospheric ozone is found in the stratosphere and the vertical motion in the stratosphere is quite slow so the dynamics in this layer becomes of great significance to our climate. Evidently dynamical changes in the circulation contribute to observe ozone trends at northern mid-latitudes. Total ozone trends have been directly connected to changes in various dynamical parameters such as temperature, so thus eddy heat flux. The eddy heat flux is the product of meridional wind departures and temperature departures. Dynamically eddy heat flux is the vertical component of Brewer-Dobson circulation. Brewer-Dobson circulation which is also known as residual circulation has a strong influence on stratospheric ozone variability and transport processes. Brewer-Dobson circulation is very strong during winter (January) and it increases the total ozone over high latitudes. In summer (July) the circulation is weak. The global distribution of atmospheric heat transport is supported by zonally averaged parameters namely eddy heat flux and temperature. Since eddy heat flux transports heat, it has been obtained that dynamical temperature change is highly affected by the eddy heat flux. Physically temperature and eddy heat flux are strongly related to each other. Evidently the strong correlation between eddy heat flux and temperature interprets a relationship between total ozone and pronounced parameters. Dynamical features of Northern Hemispheric ozone transport, temperature and eddy heat flux and their relationships for the specified time interval (1998-2007) makes up the basics of the study. The dynamical processes produced by eddies or wave motion in the stratosphere and mesosphere give rise to the meridional circulation in the middle atmosphere that closely balances the force caused by the temperature deviation from the radiative equilibrium. Dynamic features affecting ozone include especially the atmospheric transport temperature variability and changes in eddy heat flux. Ozone transport and variability of eddy heat flux and temperature are defined by zonally averaged parameters. It is easier to examine atmospheric variability through zonal mean fields which is also mathematically less complicated than using 3 dimensional flow equations.. In order to view the residual atmospheric circulation and describe zonally averaged structure of the atmosphere, a mathematical formalism is used for the eddy-mean flow cancellation which provides a separation between eddies and the mean meridional circulation. The formulation defines the transformed Eulerian mean (TEM) or residual circulation. Residual circulation calculation is given by the sum of the Eulerian mean circulation components and the eddy terms. In the study eddy heat flux and temperature are separately processed to be linked with total ozone transport. In the present research total ozone data from Total Ozone Mapping Spectrometer (TOMS) (1998-2004), Ozone Monitoring Instrument (OMI) (2005-2007) and European Centre for Medium-Range Weather Forecasts (ECMWF) ERA-Interim Re-Analysis data (1998-2207) have been used. The selected period for Northern Hemisphere and 180°W- 180°E is compiled for the middle atmosphere which includes the region from 100mb in the lower stratosphere and up to 10mb in the stratosphere. Middle atmosphere is a dynamically driven region compared to the troposphere, which is more sensitive to thermal driven heat engine. Relationship between mid-latitudinal ozone, eddy heat flux and temperature is chosen as the starting point to create an emphasis on the area around this region. It is confirmed that tropical eddy heat flux highly controls the mid-latitudinal total ozone and mid-latitudinal lower stratospheric temperature. Additionally polar ozone is strongly affected by polar eddy heat flux. Eddy heat flux at the mid-latitudes is highly correlated with the polar temperatures. Keywords: Stratospheric ozone, Brewer-Dobson circulation, ozone depletion
Physically based deformable object modeling and soft tissue deformation
Get PDFBilgisayar destekli sanal ameliyat benzetimleri ve farklı alanlardaki uygulamaları, birçok parametrenin olası en doğru biçimde belirlendiği ve uygulandığı matematiksel modellerle mümkündür. Bu parametreler benzetimi yapılacak biyolojik yapıların geometrik sınırlarının, organ özelliklerinin, farklı kuvvetler altındaki davranışlarının ve en önemlisi dokuya ait malzeme parametrelerinin olası en doğru biçimde belirlenmesi ile elde edilir. Bu çalışma yumuşak dokuların doğrusal olmayan, viskoelastik davranışlarının matematiksel olarak modellenmesi ve önerilen bu modelin sonlu elemanlar yöntemi kullanılarak sayısal olarak çözülmesine odaklanmıştır. Örnek organ olarak seçilen insan karaciğerine ait malzeme katsayıları, hastane ortamında canlı dokular üzerinde yapılan deney sonuçları kullanılarak doğrusal olmayan eniyileme yöntemi ile elde edilmiştir. Uygulanan genel yaklaşımlardan farklı olarak benzetim sonuçlarının çözümden önce yani modelleme aşamasında yapılan ön basitleştirmelerden etkilenmemesi ilkesi benimsenmiştir. Bu nedenle, modelleme aşamasında uygulanan ve gerçeklikten uzaklaşılması ile sonuçlanan geleneksel basitleştirme adımları uygulanmamış, bu yaklaşım yerine tam çözümü veren benzetim sonuçları üzerinde model indirgeme teknikleri uygulanmıştır. İlk aşama olarak yumuşak dokuların önemli özelliklerini içeren bir model önerilmiş, sonrasında oluşturulan bu model sonlu elemanlar yöntemi kullanılarak analiz edilmiştir. Analiz sonrası elde edilen sonuçlar bir sonraki aşamada kullanılmak üzere saklanmıştır. İkinci adımda saklanan sonuçlar üzerinde Karhunen Loeve model indirgeme yöntemi kullanılarak, ilk adımda elde edilen sonuçlara yakın değerleri gerçek zamanlı sunabilen bir çözüm elde edilmiştir. Son adımda ise basitleştirilmemiş model ile indirgenmiş model sundukları hızlar ve sonuçların doğrulukları açısından karşılaştırılmıştır. Anahtar Kelimeler: Yumuşak dokular, sonlu elemanlar yöntemi, model indirgeme, viskoelastisite.As indicated by The Institute of Medicine Report, 44,000 to 98,000 people die annually as a result of procedure mistakes in surgery. The main reason cited was the young surgeons' insufficient experience of new techniques before facing real-world surgical operations. With the use of virtual surgery, surgeons have the opportunity to test different critical surgical procedures in a low- cost, ethically sound environment. The virtual surgery simulators equipped with physically based modeling engines are outstanding candidates for the simulation of deformable human organs. The real time soft tissue simulation has gained a great interest recently as a result of advancements in areas such as surgery planning and the surgical simulations. Linear deformation models may not provide the required accuracy in such areas whilst nonlinear models do not serve the real time needs. Therefore, there is a common need for a computationally simplified yet accurate, nonlinear, large deformable viscoelastic model of soft tissues to be used in these real time applications. Computer aided surgery and surgery simulation applications require accurate form and feature description as well as proper material and behavior descriptions of the biological tissues in a mathematically formulated model. This study focuses on mathematical formulation and numerical implementation of a nonlinear viscoelastic model of soft tissues using Finite Element Method (FEM). As an object, a human liver is selected in our study. The necessary material parameters are extracted from results of in vivo material tests on human liver using a nonlinear optimization method. Due to the technology limitations, today's physics based surgery simulators are forced to use different simplification methods to obtain satisfactory visual effects and response time in delivering an acceptable visual and haptic user experience. Thus, previous approaches to this problem involve techniques in the simplification of the mathematical models prior to obtain the necessary numerical solutions. The model simplification is necessary to achieve the required computational speeds however; these simplifications will inevitably result in reduced accuracy. Unlike the above mentioned approaches, our research is based on the opposite view that accuracy should not unduly influenced due to premature model simplifications prior to the analysis. Therefore, simplification step that is applied in the modeling phase, resulting in inaccurate outcome, is displaced to be applied after the solution phase. This aims to limit the degradation of already calculated values. As the first step, a complete soft tissue model is created. Subsequently this model is analyzed using Finite Element Method (FEM) and the results are stored and are used as the input to the next stage. Then, the Karhunen-Loeve decomposition technique is used to simplify the previously obtained data resulting in the final simulated model. Simplification technique is necessary to achieve acceptable real-time update rates due to the computing power available. Finally, a complete (unsimplified) model is compared with the simplified model in terms of accuracy and speed. Since soft tissues undergo large deformations under applied load and required volume preservation behavior, the use of linear strain tensor is not suitable for the accurate modeling of the resulting large deformations involved. Limitation of using a linear strain tensor is overcome by the use of nonlinear Green strain tensor in our customized FEM code. Standard material tests on human liver reveal the material nonlinearity relationship between applied forces and the resulting displacements. This nonlinear behavior is necessitated the use of hyperelastic strain energy density function in our FEM implementation. Viscoelastic behavior is also a predominant feature of soft tissues that must be included in any soft tissue deformation simulation. Therefore, the quasi-linear viscoelastic behavior of a human liver is added to our implementation to provide for this need. To implement time dependent viscoelastic behavior in improved the reduced order model, the surface nodes and its neighboring nodes are determined inside the Radius of Influence (ROI). Then, a constant unit force is applied to each of surface nodes for a period of 20 seconds. It is assumed that the creep response of the soft tissue lasts for 20 seconds due to the limitations of data storage and higher computational costs. Our proposed model, when employed, results in a final constitutive equation that successfully produces accurate results while catering for the whole previously mentioned phenomenon namely; geometric and material nonlinearity, volume preservation, viscoelasticity. Keywords: Real-time simulation; deformable models; model order reduction; nonlinear simulations; soft tissue; viscoelasticity
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