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Effect of surface potential upon the crystal growth and dissolution rates of potassium chloride
Get PDFBir maddenin aynı boyuta sahip kristallerinin aynı ortam ve şartlarda birbirlerinden farklı hızlarda büyüdükleri uzun zamandır bilinmektedir. Bu olay, büyüme hızındaki saçılma (Growth Rate Dispersion) olarak tanımlanır. Çözeltide bulunan anyon ya da katyonların kristal yüzeyine seçimli adsorpsiyonları nedeniyle kristaller farklı yüzey potansiyelleri kazanırlar. Bu olay büyüme hız saçılımının nedenlerinden bir tanesidir. Bu çalışmada büyümede saçılım gösterdiği bilinen KCl kristallerinin büyüme ve çözünme hızları üzerine yüzey potansiyelinin etkisi durgun ortam tek kristal ölçüm sisteminde incelenmiştir. Deneylerde, elektrostatik ayırıcıda sırasıyla 1, 3, 5, 9 ve 16 kV ayırma voltajları uygulanarak yüzey potansiyellerine göre sınıflandırılmış olan (-425+300) μm boyut bölgesindeki KCl aşı kristalleri kullanılmıştır. Saf ortamda gerçekleştirilen deneylerde, büyüme ve çözünmenin olmadığı bir ölü bölgenin varlığı tespit edilmiştir. Deneylerde elde edilen bir diğer sonuç, büyümenin başladığı duruma göre nispeten düşük sayılabilecek aşırı doygunluklarda dentritik büyüme görülürken, yüksek aşırı doygunluklarda çok farklı bir büyüme mekanizmasının ortaya çıkmasıdır. Yüksek aşırı doygunluklarda dentritik büyüme yanında yüzeye dik olarak fışkıran uzun ve çok ince iğnecikler zamanla büyüyerek çok farklı büyümüş kristal görünümünü ortaya çıkartmaktadır. Bu davranışla farklı voltajlarda ayrılan tüm kristallerde karşılaşılmıştır. Bu tip büyüme davranışının, artan ayırma voltajının artışına bağlı olarak azalma gösterdiği görülmüştür. Yapılan bu deneysel çalışma ile, KCl kristallerinin sahip oldukları yüzey potansiyellerinin büyüme ve çözünme hızları üzerinde önemli rol oynadıkları gösterilmiştir. Anahtar Kelimeler: Kristalizasyon, yüzey potansiyeli, büyüme hız saçılımı, elektrostatik ayırma, KCl.Crystallization is a very complex operation. The main reason of this complexity is the number of mass transfer steps involved in the process. In a supersaturated solution, the first step is the creation of a new surface by nucleation; then solvated growth units diffuse to the surface and are adsorbed onto it. Surface migration then occurs followed by desorption back into solution or incorporate into the crystal lattice (Davey, 1976). These steps are governed by different physical laws which are not explained completely. Growth rate dispersion (GRD) is one of the phenomena which is difficult to explain. Growth rate dispersion can be described as the difference in the growth rate of crystals which are of the same size at the time when the growth process was started (Ulrich, 1989). Explanation of this difference is based on a well-known BCF theory developed by Burton et al., (1951). They suggested that the different growth rates were resulted from the distribution of dislocation sites on the crystal surfaces. Dislocation sites which have low attachment energy lead to the formation of the specific ion adsorption on the surface. The probable reason of specific ion adsorption is the different hydrated diameters of the ions. Details of this double layer formation can be found elsewhere (Kuşkay and Bulutcu, 2003). The surface potential resulting from electrical double layer formation dominates the growth and dissolution behavior through two different mechanisms. The first mechanism was explained by Knapp (1922). According to this theory, the higher the surface potential, the lower the equilibrium concentration. For this reason, crystals having higher surface potential will grow relatively in higher supersaturated conditions. The second mechanism was related to electrical double layer. The surface potential acts as an additional resistance to mass transfer, so that it will reduce the growth rate of the crystals. Surface charge can be measured easily in the case of sparingly soluble substances. But in the case of highly soluble salts it is difficult or sometimes impossible to measure it by the present technologies. But its effect can be still detected by indirect methods; such as changing the crystal growth rates in the presence of polyelectrolytes in different charge density (Sayan, 1995; Titiz, 1995). Measuring the crystal growth rates of crystals separated with respect to their surface potential is another indirect method. Şahin and Bulutcu (2001) and İlyaskarov (2002) made experiments systematically using different types of crystals which were separated in an electrostatic separator and the crystal growth /dissolution rates were determined in each crystal group having different surface potential. KCl is one of the crystalline material showing surface potential distributions originating from specific ion adsorption (or desorption) to defected sites on the crystal surface, in its own solution. The aim of this work is to investigate the effect of surface potential on the growth rate and growth behavior of KCl in pure KCl solution. Closely sized KCl crystals were separated to sub fractions in an electrostatic separator at different voltages in the range of 0-1, 1-3, 3-5, 5-9 and 16 kV and separation voltages were taken as a measure of surface potential of each sub fraction. Crystal growth and dissolution rates of these crystal fractions were measured at around 20 oC in a stagnant type single crystal cell using an image analyzer system. Experiment showed that growth and dissolution rates versus supersaturation plots had a dead zone in which no detectable growth was observed. Widths of dead zone were slightly different for the crystals having different surface potential. During the growth of KCl crystals surface nucleation was always dominant, even at very low supersaturations. At moderate and high supersaturation levels very unusual growth behavior were observed. In this region growth of crystals were dominated by the very thin but very long rod-like crystal branches growing perpendicular to the surface and their separately growing products on the brances. For this reason, seed crystals lost their original shape during the growth process. Frequency of the formation of branches from growing surface was the function of supersaturation and surface potential. From the experimental data the role of surface potential was shown to explain the reason of growth and dissolution rates dispersions of KCl. Keywords: Crystallization, surface charge, growth rate dispertion, electrostatic separation, KCl
TiO2 photocatalysts
Get PDFSon yıllarda, titanyum dioksit (TiO2) üzerinde, fotokatalitik aktivite özelliğinden dolayı yoğun olarak çalışılmaktadır. TiO2, UV ışığı ile uyarıldığı zaman fotoaktif özellik gösteren ve organik grupları parçalayabilen yarıiletken bir malzemedir. TiO2, ışığa maruz bırakıldığında, suyun arıtılmasında, kendi kendini temizleyebilen, buğulanmayan yüzeylerin elde edilmesinde, fotokimyasal olarak kanser tedavisi uygulamalarında, havanın arındırılmasında kullanılabilir. TiO2 filmler, kimyasal buhar biriktirme, sıçratma, elektron demeti ile buharlaştırma, iyon ışını destekli biriktirme ve sol-jel gibi yöntemlerle değişik yüzeyler üzerine kaplanabilirler. TiO2, anataz, rutil ve brukit olmak üzere üç farklı kristal yapıya sahiptir. Birçok uygulamada TiO2’in anataz formu en iyi fotoaktivite özelliği göstermektedir. TiO2’in, solar spektrumun çok az bir bölümünü oluşturan UV ışığı ile aktive edilebiliyor olması bu malzemenin pratik uygulamalardaki kullanımını sınırlandırmaktadır. Bundan dolayı, pratik uygulamalar için, TiO2’in fotoaktivitesinin geliştirilmesi gerekmektedir. Fotoaktiviteyi arttırmanın bir yolu, TiO2’in geçiş metalleri veya soy metallerle katkılandırma işlemi yaparak soğurma (absorption) bandının UV bölgesinden, görünür bölgeye kaydırılmasıdır. Literatürde, titanyum oksit filmlere gümüş, tungsten ve molibden katkılandırılmasına yönelik çalışmalar yapılmış ve üç katkılandırmanın da, titanyum oksit filmlerin fonksiyonalitesine farklı mekanizmalar üzerinden ciddi katkılar yapacak nitelikte olduğu belirtilmiştir. Bu çalışmada, TiO2’in fotokatalitik aktivite mekanizması tartışılmış ve TiO2’in kullanım alanları anti-bakteriyel özelliklerine odaklanarak özetlenmiştir. Buna ek olarak, gümüş, tungsten ve molibden katkısının TiO2’in, anti-bakteriyel aktivitesine olan etkileri tartışılmıştır. Anahtar Kelimeler: TiO2, sol-jel, anti-bakteriyel, fotokatalitik aktivite.Recently, titanium dioxide (TiO2) has been studied extensively due to its high photocatalytic activity for handling of several types of environmental problems. Major areas of activity in TiO2 photocatalysis are; water purification, photochemical cancer treatment, air purification, self-sterilizing, fog-proof and self-cleaning surfaces. Photocatalysis can be defined as ?acceleration of a photoreaction by the presence of a photocatalyst?. Photocatalytic reactions necessitate a photocatalyst that absorbs the phonons and drives the redox reactions. TiO2 is a semiconductor and it can be chemically activated by UV light. TiO2 has three different crystal structures which are anatase, brookite and rutile. TiO2 in the anatase form is the most efficient of photocatalysts for many applications. The band gap energy of anatase TiO2 is 3.2 eV and it can be only activated by UV light. Although UV light is present in the solar spectrum it is only a very limited part. For practical applications the photocatalytic activity of TiO2 needs further improvement. Doping TiO2 with transition metals or noble metals is an effective way to improve photocatalytic activity. When TiO2 is exposed to UV light, electron-hole pairs are created. The photogenerated holes in the valence band, which has strong oxidizing power, diffuse to the surface and react with adsorbed water in order to produce hydroxyl radicals (?OH). These hydroxyl radicals participate in oxidizing organic molecules. On the other hand, electrons in the conduction band react with molecular oxygen in the air to produce the superoxide radical anion (O2-?), which also participates in further oxidation processes. The photocatalytic efficiency of TiO2 strongly depends on surface area and electron-hole recombination rate. The surface area of the photocatalyst increases with a smaller particle size and the active surface sites increase. For improving photocatalytic efficiency, electron-hole recombination rate should be reduced. An effective way to seperate electron-hole pairs is to introduce foreign materials into TiO2 matrix. As mentioned above, TiO2 can be used in different application areas. One of the remarkable property of TiO2 is its self-cleaning effect. The surfaces of glasses, ceramic tiles can be contaminated by organic particles such as smoke residue, oil and dirt. TiO2 thin films can be applied to these surfaces in order to decompose those organic species. Another excellent property of TiO2 photocatalysts is their anti-bacterial effect. TiO2 can decompose bacteria and virus when it is exposed to UV light. TiO2 has advantages over conventional self-sterilizing surfaces. For instance, in the case of E.coli, TiO2 decompose both the living cells as well as the endotoxin released from these cells during their death. TiO2 photocatalysts can also be used for cancer treatment. TiO2 particles which are injected to the tumor clearly inhibit the tumor growth. In literature, there are several studies which are related to the doping effect of silver. Studies which are performed on the effect of silver dopant are focused on the change of optical and electronical properties of TiO2. Moreover, since silver itself is known as strong anti-bacterial agent it is used as dopant for improving anti-bacterial properties of TiO2. Doping silver can give rise to the separation of electron-hole pairs and can accelerate the formations of oxidative species. In addition to this, silver can reduce particle size which is needed for increasing surface area of TiO2. In order to obtain anti-bacterial effect in the dark, energy storage photocatalyst can be produced by doping TiO2 with tungsten. TiO2-WO3 photocatalyst can be photo-charged by irradiating their surfaces with UV light. Photo-charged tungsten doped TiO2 films are able to show anti-bacterial effect when the light is turned off. Molybdenum also is an energy storage material and it can be used as an alternative to those of tungsten. In this study, the mechanism of photocatalytic activity is discussed and the application fields of TiO2 photocatalysts were summarized by focussing on the bacterial activity of TiO2. Moreover, the effect of silver, tungsten and molybdenum dopants on the bacterial activity of TiO2 were discussed. Keywords: TiO2, sol-gel, photocatalyst, anti-bacterial, photocatalytic activity
A price-sensitive quantity-flexible supply chain contract model
Get PDFTedarik zinciri bir ürünün tasarım aşamasından tüketicinin eline ulaşıncaya kadar geçireceği ve gerekli olan tüm aşamaları kapsar. Bu çalışmamızda tedarik zinciri sözleşmelerinin bir performans geliştiricisi olarak tedarik zinciri katma değerini en üst düzeye çıkartmada nasıl kullanılabileceği araştırılmış ve iki sözleşme modeli incelenmiştir. İlk sözleşme modeli olarak, ürüne olan talebin satış fiyatı ile bağlantılı olduğu bir ortamda, üreticinin satıcıya belli bir miktarda ürün alma garantisi karşılığı önerdiği indirimler ele alınmıştır. İkinci sözleşme modeli olarak ise, üreticinin toplam tedarik zinciri katma değerini arttırmak için satıcıya önerdiği satılamayan ürünü geri alma ve satın almada miktar esnekliği sağlama sözleşmeleri incelenmiştir. Birinci modelde, görüleceği üzere, her ne kadar talep, satış fiyatı ile bağlantılı ise de, sonuç yalnız satıcı açısından değerlendirildiğinden, modelin tedarik zinciri toplam katma değeri üzerindeki etkisi belirsizdir. Diğer yandan, ikinci model tedarik zincirinin toplam katma değerini arttırdığı halde, talebin fiyat duyarlılığı göz önüne alınmamıştır. çalışmamızda geliştirdiğimiz modelin özgün yanı, yukarıdaki iki modelin zayıf noktalarına cevap vermesi ve talebin fiyata duyarlı olduğu bir ortamda üretici satıcı arasında miktar esnekliği sağlayan bir sözleşmenin tedarik zinciri katma değerini en üst düzeye çıkarmasıdır. çalışmamızda ayrıca sözleşmeden kaynaklanan bu ek katma değer artışının her iki tarafın da kazanması için nasıl paylaştırılabileceğini öneren iki yöntem geliştirilmiştir. Anahtar Kelimeler: Tedarik zinciri, esnek miktar sözleşmesi, talep esnekliği.A supply chain consists of all stages involved, directly or indirectly, in fulfilling a customer request. The objective of every supply chain is to maximize the overall value generated. The value a supply chain generates is the difference between what the final product is worth to the customer and the effort the supply chain expends in filling the customer's request. For most commercial supply chains, this value will be strongly correlated with the supply chain profitability, the difference between the revenue generated from the customer and the overall cost across the entire supply chain. The objective of maximizing this supply chain surplus can be achieved by improving the supply chain performance in terms of efficiency and responsiveness using the four supply chain drivers: inventory, transportation, facilities, and information. In this dissertation, we discussed these drivers and introduced supply chain contracts as a new driver to maximize supply chain profitability. Effective use of the contracts can substantially increase the overall supply chain profitability and its competitive advantage by forcing the companies into an intercompany interfunctional scope of strategic fit to evaluate every action in the context of the entire supply chain. This broad scope increases the size of the surplus to be shared among all stages of the supply chain. A contract specifies the parameters within which a buyer places orders and a supplier fulfills them. It may contain specifications regarding quantity, price, time, and quality. As contracts change, the risk different stages of the supply chain bear changes, which affects the buyer's and supplier's decisions and supply chain profitability. By entering into such a contract, the buyer often stands to gain guaranteed delivery of the product, which is very useful in times of scarcity, shorter delivery times, lower purchasing price, and a lower safety stock level. The supplier will also benefit with a better production plan, reduced variance of demand, economies of scale, and less paperwork. Harder to measure, but also important, is the increased level of trust and cooperation which can develop between a buyer and a supplier who decide to engage in such a contract. Of particular interest here are contracts that specify the parameters within which a buyer places orders and a supplier fulfills them in order to maximize the total supply chain surplus. We presented two supply chain contract models. First, where a retailer facing price sensitive demand may obtain a discount by committing a fixed quantity over a finite horizon, and second where a manufacturer offering buyback or quantity flexibility contracts may increase the total supply chain profit. We concluded that the first model incorporates demand as a function of the selling price but does not address the crucial issue of total supply chain surplus maximization. On the other hand, the second model, although it increases the total supply chain surplus, does not incorporate the demand elasticity. We then developed a model to address the individual weaknesses of the models discussed by incorporating the price sensitive demand into quantity flexibility contracts by determining the optimal level of product availability, as a function of the selling price, which maximizes the total supply chain profit. We also proposed two solutions to the issue of profit sharing related to the distribution of the additional supply chain profit generated by using the contracts. Furthermore, through numerical experiments, we showed that our model maximizes total supply chain surplus by incorporating demand elasticity and profit sharing into quantity flexibility contracts. We also developed a computer program to help simulate the system to find optimum contract parameters. We then summarized the strength and weaknesses of the models discussed with respect to our model and showed that our model, Maximize Supply Chain Profit with Buyback and Quantity Flexibility Contracts and Profit Sharing with Demand as a Function of the Selling Price Model, combines the strength of the models discussed and addresses their main weaknesses. It is our belief that the supply chain contract model developed in this dissertation can be an integral part of any Advanced Planning and Scheduling (APS) system. Keywords: Supply chain, quantity flexibility contracts, demand elasticity.
An incremental method for non-linear static analysis of reinforced concrete structures
Get PDFBu çalışmada, betonarme uzay çubuk sistemlerin dış yükler altındaki doğrusal olmayan davranışlarının incelenmesi, taşıma kapasitelerinin bulunması ve deprem performanslarının belirlenmesi amacıyla bir artımsal analiz yöntemi geliştirilmiştir. Yöntemde betonarmenin doğrusal olmayan davranışı ve geometri değişimlerinin denge denklemlerine etkisi gözönüne alınmaktadır. Geliştirilen artımsal analiz yönteminde, doğrusal olmayan şekildeğiştirmelerin plastik kesit adı verilen ve sınırlı plastik şekildeğiştirme kapasitesine sahip olan belirli kesitlerde toplandığı, bunların dışındaki bölgelerde sistemin doğrusal-elastik davrandığı gözönünde tutulmaktadır. Böylece, basit eğilme etkisindeki sistemlere uygulanmakta olan plastik mafsal hipotezi, bileşik iç kuvvet durumunu da kapsayacak şekilde genelleştirilmiş olmaktadır. Ayrıca, plastik kesitlerdeki rijit-plastik davranış modeli, pekleşme ve gevşeme (ani dayanım kaybı) etkilerini içerecek şekilde genişletilmiştir. Doğrusal olmayan analizde, düşey işletme yüklerinin bu yükler için öngörülen bir yük katsayısı ile çarpımından oluşan belirli değerleri altında, aralarındaki oran sabit kalacak şekilde değişen yatay yüklere göre hesap yapılmaktadır. Uygulanan yöntemde, her plastik kesitin oluşumundan sonra o kesitteki plastik şekildeğiştirmeleri ifade eden plastik şekildeğiştirme parametresinin yeni bir bilinmeyen olarak alınması ve kesitteki iç kuvvet durumunun değişimi ile plastik şekildeğiştirme parametresi arasındaki bağıntıyı ifade eden yeni bir denklemin denklem takımına ilave edilmesi öngörülmüştür. Önerilen yöntemin pratik uygulamaları için bilgisayar programları hazırlanmıştır. Bu bilgisayar programlarından yararlanarak, betonarme yapı sistemlerinin malzeme ve geometri değişimleri bakımından doğrusal olmayan analizi etkin bir şekilde yapılabilmektedir.Anahtar Kelimeler: Doğrusal olmayan analiz, deprem performansı, artımsal analiz, pekleşen ideal elasto-plastik, gevşemeli ideal elasto-plastik. The use of elastic-plastic analysis and design methods, which consider the non-linear behavior of reinforced concrete as well as the non-linearity caused by geometrical changes, may result in both more realistic and more economical solutions for reinforced concrete structures. Furthermore, by the use of these methods, the earthquake performance of existing reinforced concrete structures can also be evaluated. In this study, an incremental method is developed for the analysis of materially and geometrically nonlinear reinforced concrete structures subjected to factored constant gravity loads and proportionally increasing lateral loads. The classical plastic hinge concept, applicable to the case of simple bending, is extended to cover the case of combined biaxial bending. Also, strain-hardening and strain-softening behaviors are considered. The investigation covers the actual internal force-deformation relationships, the yield conditions and the piecewise linear idealization of non-linear behavior of R/C structural elements. The non-linear behavior models for reinforced concrete and the idealized models used in this study are explained in detail. This investigation is based on three basic assumptions made for reinforced concrete, such as plane sections remain plane after bending, full bond exists between concrete and reinforcing steel, tensile strength of concrete is negligible after cracking. In the incremental analysis method developed herein, the structure is analyzed under factored constant gravity loads and monotonically increasing lateral loads. At the end of the analysis, the lateral load-lateral displacement relationship (capacity curve) is determined under factored gravity loads. When the gravity loads are known, the member axial forces can be easily estimated through the equilibrium equations. Thus, the second-order effects are linearized by calculating the elements of stiffness and loading matrices for the estimated constant axial forces. In this method, the structure is analyzed for successive lateral load increments. At the end of each load increment, the state of internal forces at a certain critical section reaches the limit state defined by the yield condition, i.e. a plastic section forms. Since the yield vector is assumed to be normal to the yield surface, the plastic deformation components may be represented by a single plastic deformation parameter which is introduced as a new unknown for the next load increment. Thus, an equation is added to the system of equations to express the relationship between the change of the state of internal forces and the plastic deformations developed in the last formed plastic section. This equation is linear, because the yield and ultimate capacity surfaces are idealized as composed of planes. Since the system of linear equations corresponding to the previous load increment has already been solved, the solution for the current load increment is obtained through the elimination of the new unknown. In this study, two computer programs MKAPA IMNOSA have been developed and coded in Fortran programming language. For the numerical illustration of the method, the non-linear behaviors of two reinforced concrete structural systems are examined in detail. The reinforced concrete frames are analyzed by the second-order, elastic-plastic theory under constant factored gravity loads and monotonically increasing lateral forces. The analyses are carried out by means of the above computer programs. In each analysis, several numerical results, such as the second-order limit load (ultimate load carrying capacity), base shear versus lateral top displacement diagram, total number of plastic sections and the plastic hinge pattern, lateral load parameters for the first plastic sections developed in beams and columns are obtained.Keywords: Non-linear static analysis, earthquake performance, incremental analysis, strain-hardening,strain-softening
Effect of singular vegetative elements on open channel flow characteristics
Get PDFSon yirmi yıldır artan çevre bilinci ile birlikte akarsu yatağındaki bitkilerin akım alanı üzerine etkisini araştıran çalışmalara olan ilgi oldukça artmıştır. Farklı türde bitki topluluklarının akım alanı üzerine etkisini araştıran pek çok araştırma yapılmış olmasına rağmen geniş gövdeli tekil ağaçlar gibi bitki türlerinin akım alanı üzerine etkisi henüz yeterince anlaşılmış değildir. Bu çalışmada bir akım ortamındaki tekil doğal bitkilerin akımın hız ve türbülans karakteristikleri üzerine etkisini araştırmayı amaçlayan iki boyutlu deneyler gerçekleştirilmiştir. Deneylerin tamamı 26 m uzunluğunda, 0.98 m genişliğinde ve 0.85 m derinliğindeki akım kanalında gerçek bitki fidanları kullanılarak gerçekleştirilmiştir. Taşkın yataklarında sıkça rastlanan bu doğa olayını analiz etmek amacıyla, geniş gövdeli ağaçlar hacim yükseklik değişimleri gözönüne alınarak başlıca üç sınıfa ayrılmıştır. Hız ölçümlerinde üç adet akustik Doppler hız ölçer kullanılmıştır. Analiz aşamasında akım doğrultusundaki ve düşeydeki zamansal ortalama hız bileşenleri, akım doğrultusundaki ve düşeydeki türbülans bileşenleri ve türbülans kinetik enerjileri araştırılmıştır. Buna ilave olarak bitkinin mansab tarafında, bitkiden belirli bir mesafede, bitkinin yapısal özelliklerinin bir fonksiyonu olarak hız profilini veren bir eşitlik elde edilmiştir. Elde edilen eşitliğin geçerliliği deney verileri aracılığı ile sınanmıştır. Ayrıca, deneyler sonucunda geçirimli yapılarına rağmen akım ortamındaki bitkilerin akımı kayda değer ölçüde etkilediği ve neden oldukları türbülans ile önemli miktarda enerjiyi kırdıkları görülmüştür. Anahtar Kelimeler: Açık kanal, hidrolik, bitki, taşkın yatağı, hız profili.In the last two decades with the increasing environment awareness there is a growing interest on studies which attempt to understand the impact of vegetation on flow field in river and flood plain systems. Although in the past great attention has been devoted to explore the impact of vegetation community on flow pattern, the effect of singular vegetative element, such as trees with large trunk, on flow and turbulence pattern is not yet known. The primary aim of the study was to explore the impact of presence of natural singular vegetative elements (trees with large trunk) in flood plains on velocity and turbulence characteristics. In order to achieve the objective stated above two dimensional experimental measurements were conducted in controlled laboratory conditions. All the experiments were performed in the flume, which is capable of supplying steady flow and regular wave, located in Hydraulic Laboratory of Istanbul Technical University. The size of the flume is 26 m in length, 0.98 m in width and 0.85 m in depth. Tree saplings were utilized to represent the vegetative effect on flow field. In order to analyze this commonly observed nature phenomenon, trees with large trunk were classified into three groups on the basis of their volume versus height relation. The primary difference between those introduced three types was the volume increment gradient along the height. For given successive height intervals, the volume of vegetation pertain to any interval is always larger than the volume of the interval belong to the one closer to bed for Type 1, approximately constant for Type 2, smaller for Type 3. In nature, since each species is unique in terms of architectural and structural properties, three representative species were selected which could characterize those three types mentioned above. Those selected species were Pinus Pinea, Thuja Orientalis, and Cupressus Macrocarpa for Type 1, Type 2 and Type 3, respectively. Throughout the velocity measurements three Acoustic Doppler Velocimeters, two with maximum 200 Hz sampling frequency, one with maximum 25 Hz sampling frequency, were employed. During the data analysis in order to further understand of the impact of presence of vegetation on flow, the time averaged velocity and turbulence kinetic energy characteristics were examined. The experiments that aim to examine the More specifically, the experiments on velocity profile measurements along the flume were planned to be run for the combination of 2 depth values, 5 discharge values and 3 vegetation species. At the 10 locations velocity profiles were obtained by measurements along the centerline of the flume at the downstream of vegetation. In overall, 15 velocity profiles were acquired for each test condition. At the design stage of some water resources projects, estimating the flow conditions in open channels with an adequate accuracy is important. However, the gap was detected in the literature that there is still no any method or formulation which gives the velocity profile at the downstream of vegetation depending on the vegetative characteristics. In this context, it was considered that generating a formulation which gives the velocity profile at the downstream of vegetation is necessary for river engineers. Based on these facts a formulation which gives the velocity profile at a certain distance from vegetation was introduced. The validity of the proposed formulation was tested with experimental data for verification. It was seen that the velocity results obtained by introduced formulation are in well agreement with the experimental data. It should be kept in mind that the formulation is a function of vegetative characteristics such as volume, projected area, and submergence degree. Hence approximate calculation of the volume of related vegetation in the field with acceptable accuracy is crucial for the success of the application. The similarity between whole vegetation and its branch of related vegetation can be employed as a reference during the calculation of volume of vegetation. Experimental findings revealed that despite their porous structures, the presence of vegetation considerably disturbs the flow field and dissipate a remarkable amount of energy by turbulence. Furthermore experiments showed that sub-canopy flow occurs for three types of vegetation at the downstream of vegetation in the region close to bed. With increasing compactness of vegetation the magnitude of sub-canopy flow increases. In overall assessment it was concluded that presence of any type of tree with large trunk leads to 0-70 % extra sub-canopy flow at the bottom; 30-110 % retaining effect in the region close to the water surface.Keywords: Open channel, hydraulics, vegetation, flood plain, velocity profile
Numerical investigation of film cooling effectiveness on the curved surface
Get PDFGaz türbin kanatlarının soğutulmasında film soğutması yaygın olarak kullanılmaktadır. Bu makalede, dört farklı eğrisel yüzey ile bir düz yüzey üzerinde film soğutma etkenliği sayısal olarak incelenmiştir. Modeller bir sırada onbir enjeksiyon deliğinden oluşmuştur. Delik geometrileri tüm modellerde dikdörtgendir ve aynı kesit alanına sahiptir. Üfleme oranları 0.5 ile 2.0 arasında ve delik enjeksiyon açıları ana akış yönü ile 30° alınmıştır. Modellerde rüzgâr tüneli ve delikler için sırasıyla hexahedral ve tetrahedral elemanlar kullanılarak ağ yapısı oluşturulmuştur. Sayısal modellerde optimum ağ kullanılmıştır. Enjeksiyon delik kesitlerinde ve jetler ile ana akışın karıştığı bölgelerde diğer bölgelere göre daha sık ağ aralığı kullanılmıştır. Modellerimizde k-e türbülans modeli, enerji denklemi, ana akış ve soğutucu akışkan olarak hava ve duvarlar için standart duvar fonksiyonları seçilmiştir. Ayrıca sistemin kararlı, duvarlarda ısı kaybının olmadığı ve havanın ideal gaz olduğu kabulleri yapılmıştır. Düşük üfleme oranlarında ana akış, jeti daha kolay bükebilmekte ve böylece jet yüzey üzerine yapışabilmektedir. Bu nedenle düşük üfleme oranlarında film soğutma etkenlikleri yüksek elde edilmiştir. Delik geometrisi, eğrisel yüzeyin eğimi ve üfleme oranının film soğutma etkenliğine önemli etkileri vardır. Sonuçlar, verilen bir yüzey eğriliğinin hem ana akış hemde ana akışa dik yönde film soğutma etkenliğinin, yukarıda ifade edilen parametrelerin uygun seçimine bağlı olduğunu göstermiştir. İyi bir film soğutma için optimum yüzey eğriliği yada gaz türbini kanat eğriliğine uygun enjeksiyon delik geometrisi ve üfleme oranı seçilmelidir. Anahtar Kelimeler: Film soğutma, eğrisel yüzeyler, film soğutma etkenliği, sayısal inceleme.Gas turbine designers want the gas turbines to have high efficiency. High efficiency is one of the reasons for choosing gas turbines. High turbine inlet temperatures are required to obtain high cycle efficiency in modern gas turbines. The turbine inlet temperature is limited by current blade and vane materials. Some damages and deformations are observed in the blade and vane materials at high temperature. Thus, blade and vane materials should be resistible to such high temperatures. The resistible production of blade and vane materials at high temperatures is related with the improvements of material technology. The current blade and vane materials are made of special alloys and coatings. Moreover, the manufacturing of new blade materials takes a very long time. Therefore, the resistible manufacturing of blades at high temperature is an expensive method. Cooling with air is a less expensive method for the protection of blade and vane materials from high temperatures. Therefore, air is used for cooling on gas turbine vanes and blades as commonly. Film cooling is one of the most commonly used blade and vane cooling methods. Although the film cooling is used commonly it has lots of unknown aspects. Therefore, many researchers are interested in this cooling method. In film cooling, cooling air is injected from holes which are opened with a certain angle to the blade surface and the film layer appears on the blade surface. In this paper, effectiveness of the film cooling on four different curved surfaces and a flat surface was investigated numerically. Models consist of eleven injection holes aligned in a single row. The hole geometries are rectangle and they have the same cross-sectional area in all models. The blowing ratios vary between 0.5 and 2.0 and the injection angle is 30o. The main flow velocity value is 15 m/s and the injected fluid velocities are calculated from the blowing ratio equation. The main flow temperature is 288 K in the wind tunnel. The injected cooling air temperature is selected as 328 K. The flat plate, curved surfaces and wind tunnel are modeled in 3-D by using GAMBIT and these models were solved with the FLUENT CFD (computational fluid dynamics) software programme. In models, the wind tunnel and the holes are meshed with hexahedral and tetrahedral map respectively. Optimum mesh has been used in computational models. For the injection hole sections, a finer mesh structure has been used in comparison with that of the other sections. Standard k-e turbulence model with standard wall function for walls have been selected. The standard k-e model is a semi-empirical model based on model transport equations for the turbulent kinetic energy (k) and its dissipation rate (e). The model transport equation for k is derived from the exact equation, while the model transport equation for e was obtained using physical reasoning and bears little resemblance to its mathematically exact counterpart. In the derivation of the k-e model, it was assumed that the flow is fully turbulent, and the effects of molecular viscosity are negligible. A wall function is used between the turbulent zone and the wall,. Therefore, the distance from the wall at the cells adjacent to the wall is determined by considering the range over which the log law is valid. Steady state solutions have been obtained assuming no heat loss at the injection hole surfaces. Furthermore, it has been accepted that the system is determined, there is no loss of heat in the walls and the air behaves like an ideal gas. The main flow bends the jet at the low blowing ratios easily and so, the jet gets stuck on blade surface. At high blowing ratios, jet goes in the main flow more easily. This shows that the momentum of jet is better than the momentum of main flow. In that case, the separations from the blade surface have appeared. The hole geometry, the slope of the curved surface and the blowing ratio have important effects on the film cooling effectiveness. The results show that the film cooling effectiveness of a given curved surface both along the mainstream and spanwise direction depends on the optimum selection of the parameters mentioned above. The best surface is C and the best blowing ratio is 0.5 in the mainstream and in the lateral direction in this study. Either optimum surface curvature or appropriate injection hole geometry and blowing ratio for the blade curvature of gas turbine need to be selected for a good film cooling effectiveness. Keywords: Film cooling, curved surfaces, film cooling effectiveness, numerical investigation
Speed sensorless direct vector control of the IM
Get PDFBu çalışmada, Sincap Kafesli Asenkron Motorların (SKASM’lerin) algılayıcısız kontrolünde başarımı olumsuz yönde etkileyen modele ait elektriksel ve mekanik yana ilişkin parametre belirsizliklerini çözmek üzere Genişletilmiş Kalman Filtresi (GKF) tabanlı gözlemleyici algoritmaları tasarlanarak gerçeklenmiştir. Doğrudan vektör kontrol sistemi ya da vektör kontrollü a.a sürücüsü ile başarımları test edilen bu algoritmalar ile herhangi yüksek frekanslı bir işaret eklemeksizin ve başlangıç değerleri bilinmediği varsayımıyla algılayıcısız kontrol için gerekli tüm durumlar, sabit sürtünme –viskoz– terimini de içeren yük momenti ve değeri bilinmeyen rotor direnci eş zamanlı olarak kestirilebilmiştir. Benzetim/deney sonuçları önerilen yöntemlerin oldukça etkin ve dayanıklı olduğunu göstermiştir. Bu yönleriyle, literatürde bilinen ilk çalışmadır.Anahtar Kelimeler: Genişletilmiş Kalman filtresi, gözlemleyici, kestirici, hız algılayıcısız vektör kontrol, asenkron motor.This paper offers a solution to the performance deteriorating effect of uncertainties in the sensorless control of induction motors (IMs). The major contribution of the study is the development and implementation of a Extended Kalman Filter (EKF) algorithms that take electrical and mechanical uncertainties into account. Also, unlike previous EKF based estimation studies taking the angular velocity, wm, into consideration as a constant parameter, in this study, wm, is estimated as a state with the utilization of the equation of motion. In this regard, this is the first known study to estimate the mechanical uncertainties together with the estimation of the rotor resistance, without injecting high frequency signals. The EKF algorithms also estimate the rotor flux, angular velocity and stator currents with no apriori knowledge on the states and initial values taken as zero. Experiments performed under unknown load torque and with rotor resistance variations up to twice the rated value, demonstrate the good performance and robustness of the estimation methods. The algorithms also estimate the mechanical uncertainties as a constant state to capture the unknown viscous and Coulomb friction in steady state, therefore, could be used to improve the performance of the velocity or position control of IM's, if utilized in combination with a compensation scheme.Keywords: Extended Kalman filter, observer, estimator, speed sensorless vector control, induction motor
Time series analysis at GPS permanent stations
Get PDFIGS’e bağlı GPS sabit istasyonlarından Anadolu plakası ve yakın ve uzak çevresindeki noktalar seçilerek tasarlanan ağa ait günlük veriler 3 yıl için değerlendirilmiştir. Ağ çözümleri, daha başka ağ çözümleri ile birleştirilmiş ve seçilen noktalar kullanılarak datum tanımlaması yapılmıştır. Farklı fonksiyonel model ve datum kullanılarak iki ayrı çözüm elde edilmiştir. Bu çözümler sıfır çözümü olarak kabul edilen ve datum noktalarının global olarak dağıldığı çözüm ile karşılaştırılmıştır. Lineer regresyon kullanılmak suretiyle istasyonların Kuzey-Güney (KG) ve Doğu-Batı (DB) hız bileşenleri bulunmuştur. Zaman serilerinin güç spektrumu kullanılmak suretiyle doğrulukları hakkında çıkarımlar yapılmıştır. Serilerin Wavelet transformasyonu kullanılarak seçilen bazı istasyonlarda analizleri de yapılmıştır. Datum seçiminin, serilerden elde edilecek nokta hızlarını etkilediği gösterilmiştir. Ayrıca zaman serilerinden gerçekçi doğruluklar elde edebilmek için alternatif pratik bir yöntem önerilmiştir.Anahtar Kelimeler: GPS, zaman serileri, doğruluk analizi, wavelet analizi. A network of 24 IGS permanent stations in the near and far neighborhood of the Anatolian Plate has been designed. The stations? daily GPS observations covering three years are analyzed. The solution is then combined with different network solutions and the datum of the combined solution has been fixed via appropriate pre-defined datum points. Two different solutions are obtained using diverse functional models and datum definitions. These solutions are compared to the so-called zero solution in which datum points are globally distributed. Only North-South (NS) and East-West (EW) components of the station coordinate time series are anticipated in the study. The height component is not included in the analysis. NS and EW velocity components are computed applying least squares linear regression to the station time series. Blunders in the time series are removed via robust estimation strategy. Spectral index values are estimated from the spectra of the residuals after least squares regression. Spectral indices are found to be close to 1. This confirms that the adjacent solutions consist of repeated signals and are also correlated. Time-frequency analysis or multi-resolution analysis at selected locations is attempted. It has been shown that datum definition affects site velocities estimated from the time series. An alternative approach is suggested to deduce realistic uncertainty values from the series. Keywords: GPS, time series, accuracy analysis, multi-resolution analysis
Integrated optimum design of structures and LQR control systems
Get PDFBu çalışmada, yapının ve kontrol sisteminin birleşik optimum tasarımı çalışılmıştır. Yapı sonlu elemanlar metodu kullanılarak modellenmiş ve kontrolcü “linear quadratik regulatör (LQR)” formülasyonu kullanılarak tasarlanmıştır. Yapı ve kontrolcü eşzamanlı ve ardışık olarak optimize edilmiştir. Yapıların ve LQR formülasyonları kullanılarak kontrolcülerin eş zamanlı optimizasyonunun, yapıların yapısal tekil değer şekillendirmesi ile optimize edildiği ve başarılı bir şekilde herhangi bir ilgili kontrol kuralının tasarlanabileceği bir eşdeğer ayrık optimizasyon problemine indirgenebileceği gösterilmiştir. Yapıların ve kontrolcülerin ayrık optimizasyonu özellikle çok serbestlik derecesine sahip yapılar için belirli üstünlüklere sahiptir. Yapıların ve kontrolcülerin eş zamanlı ve ardışık optimizasyonları bir kafes yapı ve kontrolcüsüne uygulanmıştır. Birleşik LQR tasarım problemi için ardışık optimizasyon yaklaşımının eş zamanlı optimizasyon yaklaşımından çok daha hızlı global minimumu verdiği sonucuna varılmıştır. Anahtar Kelimeler: Yapısal optimizasyon, optimum kontrol, tekil değer ayrıklaştırması, sonlu elemanlar metodu. In this study, integrated optimum design of structures and control systems is studied. The structures are modeled by the use of finite elements method, and the controllers are designed by the use of linear quadratic regulator (LQR) formulation. The structures and controllers are optimized simultaneously and successively. It is shown that simultaneous optimization of structures and controllers by using LQR formulations can be achieved by an equivalent decoupled optimization problem where structures are optimized by shaping the structural singular values and succeedingly any control law of interest can be designed. Decoupled optimization of structures and controllers has certain advantages, especially for structures having large degrees-of-freedom (DOF). Computational cost of the associated singular value shaping problem is very low since it is only necessary to compute the largest and smallest singular values that can be computed by using selective eigenvalue solvers. Model order reduction techniques should be employed at every design iteration for simultaneous optimization approach in order to design LQR laws; however, successive optimization approach developed in this paper does not need any model order reduction technique during structural optimization and can easily be implemented into problems having large DOF. Then, simultaneous and successive optimization of structures and controllers are applied to a truss structure and its controller. it is concluded that successive optimization approach to integrated LQR design problem yields the global minimum much faster than the simultaneous optimization approach. Keywords: Structural optimization, optimum control, singular value decomposition, finite element method
Yeni bir fakoemülsifikasyon cihazı tasarımı ve uygulaması
Get PDFGünümüzde, modern katarakt cerrahisi fakoemülsüfikasyon cihazları ile gerçekleştirilmektedir.Bu çalışma ile ilk Türk malı ve benzerlerinden üstün özellikleri olan ama onlardan daha ucuz bir fakoemulsifikasyon sisteminin tasarlanması, gerçeklenmesi ve kataraktın sertliğine göre minimum fakoemulsifikasyon(ultrason) enerjisi kullanılmasına yönelik çalışmalar yapılması hedeflenmiştir. Bunun nedeni gereğinden fazla uygulanan fakoemulsifikasyon enerjisinin, özellikle korneanın endotel hücreleri başta olmak üzere duyarlı tüm göz içi dokularına geri dönüşümsüz zarar verebilmesidir. Uygulanan fako enerjisinin oluşturduğu ısı miktarlarının çeşitli modlara göre farklı olmasından hareketle, özgün deneysel araştırmalar yapılmış ve prototip sistem geliştirilerek kataraktın sertliğine göre minimum fakoemulsifikasyon enerjisi kullanılmasını sağlayabilen özgün modlar (UPM-User Pulse Modulation) geliştirilmiştir. Anahtar Kelimeler: Fakoemülsifikasyon, katarakt, sistem tasarımı. This study covers design, realization and experimental applications of a new phacoemulsification apparatus for cataract surgery. The aim of this study was to design and test a competitively priced Turkish made phacoemulsification system with superior specifications when compared to commercially available ones. The new phacoemulsification system consists of a main unit controlling the whole system and providing communication via a 7.7? colored touch screen operator panel, a phaco unit, irrigation and aspiration (I/A) units, vitrectomy and diathermy units, custom designed footswitch pedal. This system was designed to function adaptively with respect to hardness of cataracts so that minimum phacoemulsification energy is used, because excessive phacoemulsification energy (ultrasound) may cause irreversible damage to cornea?s endothelial cells and to the sensitive inner organs of the eye cambium. The amount of heat produced in the eye by ultrasound depends on the way the ultrasound energy is delivered, in other words, on different modes of phacoemulsification energy. Therefore, experimental research was carried out to improve a prototype system. Unique modes (UPM-User Pulse Modulation) were designed to minimize phacoemulsification energy. Other unique properties of this system include multifunctional foot-switch pedal, user-friendly pre-operation test menu, aspiration totalizer, and 3 different choices for vitrectomy probe. In conclusion, a unique and Turkish made phacoemulsification system was developed for ophthalmic surgeons for clinical use and research.Keywords: Phacoemulsification, cataract, system design
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